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Mechanical testing of articular cartilage yields highly variable results, posing challenges for tissue characterization. Many factors cause variability, one is sample geometry. Using in-situ phase-contrast enhanced synchrotron micro-tomographs of cartilage samples while tested in unconfined compression (stress relaxation) our group found high variability in the mechanical response. Since all samples originated from a single bovine knee, they were assumed to share mechanical properties. Microscale tomography images showed geometric irregularities in samples that were not accounted for in the often assumed intended cylindrical shape. We aimed to determine the influence of sample shape on mechanical response in unconfined compression and how sample geometry affects identified mechanical properties. Using a parametric FE model incorporating geometric irregularities in a Design of Experiments approach, results were analysed with 2-way ANOVA. Furthermore, a material parameter fitting was done with multiple segmented sample-specific finite element models simultaneously to assess the influence of sample geometry on material parameters. Results revealed that the average inclined sample surface (4°) caused a 15 % decrease in reaction forces compared to the intended cylinder. Fitting multiple sample-specific geometries simultaneously altered material parameters between −70 to +159 % compared to the average model. Strikingly, initial fibril stiffness and permeability increased by 137 % and 159 %, while the root-mean-square error of the fit was reduced by ∼2/3 compared to using parameters from a cylindrical shape model. In conclusion, minor variability in sample geometry affects property characterization and can account for some of the inter-sample variability in the mechanical data for cartilage. [ABSTRACT FROM AUTHOR]

The purpose of this study was (1) to determine the utility of shear wave elastography as a predictor for the mechanical failure of superficial knee ligaments and (2) to determine the viability of shear wave elastography to assess injury risk potential. Our hypothesis was that shear wave elastography measurements of the anterolateral ligament and medial collateral ligament would directly correlate with the material properties and the mechanical failure of the ligament, serving as a prognostic measurement for injury risk. 8 cadaveric specimens were acquired, and tissue stiffness for the anterolateral ligament and medial collateral ligament were evaluated with shear wave elastography. The anterolateral ligament and medial collateral ligament were dissected and isolated for unilateral mechanical failure testing. Ultimate failure testing was performed at 100 % strain per second after 50 cycles of 3 % strain viscoelastic conditioning. Each specimen was assessed for load, displacement, and surface strain throughout failure testing. Rate of force, rate of strain development, and Young's modulus were calculated from these variables. Shear wave elastography stiffness for the anterolateral ligament correlated with mean longitudinal anterolateral ligament strain at failure (R2 = 0.853; P <0.05). Medial collateral ligament shear wave elastography calculated modulus was significantly greater than the anterolateral ligament shear wave elastography calculated modulus. Shear wave elastography currently offers limited reliability in the prediction of mechanical performance of superficial knee ligaments. The utility of shear wave elastography assessment for injury risk potential remains undetermined. [ABSTRACT FROM AUTHOR]

Abstract This study combines non-invasive mechanical testing with finite element (FE) modelling to assess for the first time the reliability of shear wave (SW) elastography for the quantitative assessment of the in-vivo nonlinear mechanical behavior of heel-pad. The heel-pads of five volunteers were compressed using a custom-made ultrasound indentation device. Tissue deformation was assessed from B-mode ultrasound and force was measured using a load cell to calculate the force – deformation graph of the indentation test. These results were used to design subject specific FE models and to inverse engineer the tissue's hyperelastic material coefficients and its stress – strain behavior. SW speed was measured for different levels of compression (from 0% to 50% compression). SW speed for 0% compression was used to assess the initial stiffness of heel-pad (i.e. initial shear modulus, initial Young's modulus). Changes in SW speed with increasing compressive loading were used to quantify the tissue's nonlinear mechanical behavior based on the theory of acoustoelasticity. Statistical analysis of results showed significant correlation between SW-based and FE-based estimations of initial stiffness, but SW underestimated initial shear modulus by 64%(±16). A linear relationship was found between the SW-based and FE-based estimations of nonlinear behavior. The results of this study indicate that SW elastography is capable of reliably assessing differences in stiffness, but the absolute values of stiffness should be used with caution. Measuring changes in SW speed for different magnitudes of compression enables the quantification of the tissue's nonlinear behavior which can significantly enhance the diagnostic value of SW elastography. [ABSTRACT FROM AUTHOR]

Abstract Lumbar intervertebral body fusion devices (L-IBFDs) are intended to provide stability to promote fusion in patients with a variety of lumbar pathologies. Different L-IBFD designs have been developed to accommodate various surgical approaches for lumbar interbody fusion procedures including anterior, lateral, posterior, and transforaminal lumbar interbody fusions (ALIF, LLIF, PLIF, and TLIF, respectively). Due to design differences, there is a potential for mechanical performance differences between ALIF, LLIF, PLIF, and TLIF devices. To evaluate this, mechanical performance and device dimension data were collected from 124 Traditional 510(k) submissions to the FDA for L-IBFDs cleared for marketing from 2007 through 2016. From these submissions, mechanical test results were aggregated for seven commonly performed tests: static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077 , and subsidence testing per ASTM F2267. The Kruskal-Wallis test and Wilcoxon signed-rank test were used to determine if device type (ALIF, LLIF, PLIF, TLIF) had a significant effect on mechanical performance parameters (static testing: stiffness and yield strength; dynamic testing: runout load; subsidence testing: stiffness [Kp]). Generally, ALIFs and LLIFs were found to be stiffer, stronger, and had higher subsidence resistance than PLIF and TLIF designs. These results are likely due to the larger footprints of the ALIF and LLIF devices. The relative mechanical performance and subsidence resistance can be considered when determining the appropriate surgical approach and implant for a given patient. Overall, the mechanical performance data presented here can be utilized for future L-IBFD development and design verification. [ABSTRACT FROM AUTHOR]

This work describes the design and validation of a novel device, the High-Throughput Degradation Monitoring Device (HDD), for monitoring the degradation of 24 soft tissue samples over incubation periods of several days inside a cell culture incubator. The device quantifies sample degradation by monitoring its deformation induced by a static gravity load. Initial instrument design and experimental protocol development focused on quantifying cartilage degeneration. Characterization of measurement errors, caused mainly by thermal transients and by translating the instrument sensor, demonstrated that HDD can quantify sample degradation with <6 μm precision and <10 μm temperature-induced errors. HDD capabilities were evaluated in a pilot study that monitored the degradation of fresh ex vivo human cartilage samples by collagenase solutions over three days. HDD could robustly resolve the effects of collagenase concentration as small as 0.5 mg/ml. Careful sample preparation resulted in measurements that did not suffer from donor-to-donor variation (coefficient of variance <70%). Due to its unique combination of sample throughput, measurement precision, temporal sampling and experimental versality, HDD provides a novel biomechanics-based experimental platform for quantifying the effects of proteins (cytokines, growth factors, enzymes, antibodies) or small molecules on the degradation of soft tissues or tissue engineering constructs. Thereby, HDD can complement established tools and in vitro models in important applications including drug screening and biomaterial development. [ABSTRACT FROM AUTHOR]

The effects of the rib cage on thoracic spine loading are not well studied, but the rib cage may provide stability or share loads with the spine. Intervertebral disc pressure provides insight into spinal loading, but such measurements are lacking in the thoracic spine. Thus, our objective was to examine thoracic intradiscal pressures under applied pure moments, and to determine the effect of the rib cage on these pressures. Human cadaveric thoracic spine specimens were positioned upright in a testing machine, and Dynamic pure moments (0 to ±5 N·m) with a compressive follower load of 400 N were applied in axial rotation, flexion - extension, and lateral bending. Disc pressures were measured at T4-T5 and T8-T9 using needle-mounted pressure transducers, first with the rib cage intact, and again after the rib cage was removed. Changes in pressure vs. moment slopes with rib cage removal were examined. Pressure generally increased with applied moments, and pressure-moment slope increased with rib cage removal at T4-T5 for axial rotation, extension, and lateral bending, and at T8-T9 for axial rotation. The results suggest the intact rib cage carried about 62% and 56% of axial rotation moments about T4-T5 and T8-T9, respectively, as well as 42% of extension moment and 36–43% of lateral bending moment about T4-T5 only. The rib cage likely plays a larger role in supporting moments than compressive loads, and may also play a larger role in the upper thorax than the lower thorax. [ABSTRACT FROM AUTHOR]

Metatarsal stress fracture is a common injury observed in athletes and military personnel. Mechanical fatigue is believed to play an important role in the etiology of stress fracture, which is highly dependent on the resulting bone strain from the applied load. The purpose of this study was to validate a subject-specific finite element (FE) modeling routine for bone strain prediction in the human metatarsal. Strain gauge measurements were performed on 33 metatarsals from seven human cadaveric feet subject to cantilever bending, and subject-specific FE models were generated from computed tomography images. Material properties for the FE models were assigned using a published density-modulus relationship as well as density-modulus relationships developed from optimization techniques. The optimized relationships were developed with a ‘training set’ of metatarsals (n = 17) and cross-validated with a ‘test set’ (n = 16). The published and optimized density elasticity equations provided FE-predicted strains that were highly correlated with experimental measurements for both the training (r 2 ≥ 0.95) and test (r 2 ≥ 0.94) sets; however, the optimized equations reduced the maximum error by 10% to 20% relative to the published equation, and resulted in an X = Y type of relationship between experimental measurements and FE predictions. Using a separate optimized density-modulus equation for trabecular and cortical bone did not improve strain predictions when compared to a single equation that spanned the entire bone density range. We believe that the FE models with optimized material property assignment have a level of accuracy necessary to investigate potential interventions to minimize metatarsal strain in an effort to prevent the occurrence of stress fracture. [ABSTRACT FROM AUTHOR]

Cervical intervertebral body fusion devices (IBFDs) are utilized to provide stability while fusion occurs in patients with cervical pathology. For a manufacturer to market a new cervical IBFD in the United States, substantial equivalence to a cervical IBFD previously cleared by FDA must be established through the 510(k) regulatory pathway. Mechanical performance data are typically provided as part of the 510(k) process for IBFDs. We reviewed all Traditional 510(k) submissions for cervical IBFDs deemed substantially equivalent and cleared for marketing from 2007 through 2014. To reduce sources of variability in test methods and results, analysis was restricted to cervical IBFD designs without integrated fixation, coatings, or expandable features. Mechanical testing reports were analyzed and results were aggregated for seven commonly performed tests (static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077 , and subsidence testing per ASTM F2267 ), and percentile distributions of performance measurements were calculated. Eighty-three (83) submissions met the criteria for inclusion in this analysis. The median device yield strength was 10,117 N for static axial compression, 3680 N for static compression-shear, and 8.6 N m for static torsion. Median runout load was 2600 N for dynamic axial compression, 1400 N for dynamic compression-shear, and ±1.5 N m for dynamic torsion. In subsidence testing, median block stiffness (Kp) was 424 N/mm. The mechanical performance data presented here will aid in the development of future cervical IBFDs by providing a means for comparison for design verification purposes. [ABSTRACT FROM AUTHOR]

Mechanical properties of biological tissues are of key importance for proper function and in situ methods for mechanical characterization are sought after in the context of both medical diagnosis as well as understanding of pathophysiological processes. Shear wave elastography (SWE) and accompanying physical modelling methods provide valid estimates of stiffness in quasi-linear viscoelastic, isotropic tissue but suffer from limitations in assessing non-linear viscoelastic or anisotropic material, such as tendon. Indeed, mathematical modelling predicts the longitudinal shear wave velocity to be unaffected by the tensile but rather the shear viscoelasticity. Here, we employ a heuristic experimental testing approach to the problem to assess the most important potential confounders, namely tendon mass density and diameter, and to investigate associations between tendon tensile viscoelasticity with shear wave descriptors. Small oscillatory testing of animal flexor tendons at two baseline stress levels over a large frequency range comprehensively characterized tensile viscoelastic behavior. A broad set of shear wave descriptors was retrieved on the unloaded tendon based on high frame-rate plane wave ultrasound after applying an acoustic deformation impulse. Tensile modulus and strain energy dissipation increased logarithmically and linearly, respectively, with the frequency of the applied strain. Shear wave descriptors were mostly unaffected by tendon diameter but were highly sensitive to tendon mass density. Shear wave group and phase velocity showed no association with tensile elasticity or strain rate-stiffening but did show an association with tensile strain energy dissipation. The longitudinal shear wave velocity may not characterize tensile elasticity but rather tensile viscous properties of transversely isotropic collagenous tissues. [ABSTRACT FROM AUTHOR]

The clinical relevance of mechanical testing studies of cadaveric human thoracic spines could be enhanced by using follower preload techniques, by including the intact rib cage, and by measuring thoracic intervertebral disc pressures, but studies to date have not incorporated all of these components simultaneously. Thus, this study aimed to implement a follower preload in the thoracic spine with intact rib cage, and examine the effects of follower load, rib cage stiffening and rib cage removal on intervertebral disc pressures and sagittal plane curvatures in unconstrained static conditions. Intervertebral disc pressures increased linearly with follower load magnitude. The effect of the rib cage on disc pressures in static conditions remains unclear because testing order likely confounded the results. Disc pressures compared well with previous reports in vitro , and comparison with in vivo values suggests the use of a follower load of about 400 N to approximate loading in upright standing. Follower load had no effect on sagittal plane spine curvature overall, suggesting successful application of the technique, although increased flexion in the upper spine and reduced flexion in the lower spine suggest that the follower load path was not optimized. Rib cage stiffening and removal both increased overall spine flexion slightly, although with differing effects at specific spinal locations. Overall, the approaches demonstrated here will support the use of follower preloads, intact rib cage, and disc pressure measurements to enhance the clinical relevance of future studies of the thoracic spine. [ABSTRACT FROM AUTHOR]

The choice of coordinate system and alignment of bone will affect the quantification of mechanical properties obtained during in-vitro biomechanical testing. Where these are used in predictive models, such as finite element analysis, the fidelic description of these properties is paramount. Currently in bending and torsional tests, bones are aligned on a pre-defined fixed span based on the reference system marked out. However, large inter-specimen differences have been reported. This suggests a need for the development of a specimen-specific alignment system for use in experimental work. Eleven ovine tibiae were used in this study and three-dimensional surface meshes were constructed from micro-Computed Tomography scan images. A novel, semi-automated algorithm was developed and applied to the surface meshes to align the whole bone based on its calculated principal directions. Thereafter, the code isolates the optimised location and length of each bone for experimental testing. This resulted in a lowering of the second moment of area about the chosen bending axis in the central region. More importantly, the optimisation method decreases the irregularity of the shape of the cross-sectional slices as the unbiased estimate of the population coefficient of variation of the second moment of area decreased from a range of (0.210–0.435) to (0.145–0.317) in the longitudinal direction, indicating a minimisation of the product moment, which causes eccentric loading. Thus, this methodology serves as an important pre-step to align the bone for mechanical tests or simulation work, is optimised for each specimen, ensures repeatability, and is general enough to be applied to any long bone. [ABSTRACT FROM AUTHOR]

As research progresses to find a suitable knee meniscus replacement, accurate in vitro testing becomes critical for feasibility and comparison studies of mechanical integrity. Within the knee, the meniscus is bathed in synovial fluid, yet the most common hydration fluid in laboratory testing is phosphate buffered saline (PBS). PBS is a relatively simple salt solution, while synovial fluid is a complex non-Newtonian fluid with multiple lubricating factors. As such, PBS may interact with meniscal tissue differently than synovial fluid, and thus, the hydration fluid may be an important factor in obtaining accurate results during in vitro testing. To evaluate these effects, medial porcine menisci were used to evaluate tissue mechanics in tension ( n =11) and compression ( n =15). In all tests, two samples from the same meniscus were taken, where one sample was hydrated in PBS and the other was hydrated in synovial fluid. Statistical analysis revealed no significant differences between the mean mechanical properties of samples tested in PBS compared to synovial fluid; however, compressive testing revealed the variability between samples was significantly reduced if samples were tested in synovial fluid. For example, the compressive Young׳s Modulus was 12.69±7.49 MPa in PBS versus 12.34±4.27 MPa in synovial fluid. These results indicate testing meniscal tissue in PBS will largely not affect the mean value of the mechanical properties, but performing compression testing in synovial fluid may provide more consistent results between samples and assist in reducing sample numbers in some experiments. [ABSTRACT FROM AUTHOR]

Successful healing of long bone fractures is dependent on the mechanical environment created within the fracture, which in turn is dependent on the fixation strategy. Recent literature reports have suggested that locked plating devices are too stiff to reliably promote healing. However, in vitro testing of these devices has been inconsistent in both method of constraint and reported outcomes, making comparisons between studies and the assessment of construct stiffness problematic. Each of the methods previously used in the literature were assessed for their effect on the bending of the sample and concordant stiffness. The choice of outcome measures used in in vitro fracture studies was also assessed. Mechanical testing was conducted on seven hole locked plated constructs in each method for comparison. Based on the assessment of each method the use of spherical bearings, ball joints or similar is suggested at both ends of the sample. The use of near and far cortex movement was found to be more comprehensive and more accurate than traditional centrally calculated interfragmentary movement values; stiffness was found to be highly susceptible to the accuracy of deformation measurements and constraint method, and should only be used as a within study comparison method. The reported stiffness values of locked plate constructs from in vitro mechanical testing is highly susceptible to testing constraints and output measures, with many standard techniques overestimating the stiffness of the construct. This raises the need for further investigation into the actual mechanical behaviour within the fracture gap of these devices. [ABSTRACT FROM AUTHOR]

In recent years, an increasing number of studies reporting on meniscal root tears have been published. While the meniscus and its ligamentous meniscal attachments have been studied before, little is known about the transitional zone between these two structures. Therefore, the aim of this study was to mechanically and morphologically characterize the transitional zone between meniscus and its meniscal attachments. Dumbbell-shaped specimens were obtained from the transitional zone between meniscus and its meniscal attachments of 6 knee joints. Samples were divided into tibial and central layers of the anterior lateral (AL), anterior medial (AM), posterior lateral (PL) and posterior medial (PM) transitional region. Testing was performed to obtain the dissipated energy during hysteresis as well as the linear modulus (Elin), the maximum strain (εmax), the maximum engineering stress (σmax,eng) and location of rupture during tensile test to failure. Two additional knee joints were used to investigate morphological differences between meniscus, transitional zone and meniscal attachments in 8 mm transverse slices. The central layer of the AL, AM and PL dissipated up to 48% less energy than the tibial layer. Elin was highest in the tibial layer of the PM with 107.4761.1 MPa and lowest in the central layer of the PL with 56.0720.5 MPa. The maximum strain was higher in the central layer than in the tibial layer at the AL, AM, and PL locations. The average σmax,eng was 12.779.9 MPa over all location and layers. 78% of the samples ruptured during tensile test to failure in the transitional zone. The morphological evaluation showed a smooth transitional zone with a transitional curve which was either linear or bell-shaped. The strength found in the transitional zone was lower than in the meniscus and the meniscal attachments, which corresponds well to clinical findings. [ABSTRACT FROM AUTHOR]

Quantitative computer tomography (QCT)-based finite element (FE) models of vertebral body provide better prediction of vertebral strength than dual energy X-ray absorptiometry. However, most models were validated against compression of vertebral bodies with endplates embedded in polymethylmethal-crylate (PMMA). Yet, loading being as important as bone density, the absence of intervertebral disc (IVD) affects the strength. Accordingly, the aim was to assess the strength predictions of the classic FE models (vertebral body embedded) against the in vitro and in silico strengths of vertebral bodies loaded via IVDs. High resolution peripheral QCT (HR-pQCT) were performed on 13 segments (T11/T12/L1). T11 and L1 were augmented with PMMA and the samples were tested under a 4° wedge compression until failure of T12. Specimen-specific model was generated for each T12 from the HR-pQCT data. Two FE sets were created: FE-PMMA refers to the classical vertebral body embedded model under axial compression; FE-IVD to their loading via hyperelastic IVD model under the wedge compression as conducted experimentally. Results showed that FE-PMMA models overestimated the experimental strength and their strength prediction was satisfactory considering the different experimental set-up. On the other hand, the FE-IVD models did not prove significantly better (Exp/FE-PMMA: R=0.68; Exp/FE-IVD: R2=0.71, p=0.84). In conclusion, FE-PMMA correlates well with in vitro strength of human vertebral bodies loaded via real IVDs and FE-IVD with hyperelastic IVDs do not significantly improve this correlation. Therefore, it seems not worth adding the IVDs to vertebral body models until fully validated patient-specific IVD models become available. [ABSTRACT FROM AUTHOR]

Articular cartilage enables efficient and near-frictionless load transmission, but suffers from poor inherent healing capacity. As such, cartilage tissue engineering strategies have focused on mimicking both compositional and mechanical properties of native tissue in order to provide effective repair materials for the treatment of damaged or degenerated joint surfaces. However, given the large number design parameters available (e.g. cell sources, scaffold designs, and growth factors), it is difficult to conduct combinatorial experiments of engineered cartilage. This is particularly exacerbated when mechanical properties are a primary outcome, given the long time required for testing of individual samples. High throughput screening is utilized widely in the pharmaceutical industry to rapidly and cost-effectively assess the effects of thousands of compounds for therapeutic discovery. Here we adapted this approach to develop a high throughput mechanical screening (HTMS) system capable of measuring the mechanical properties of up to 48 materials simultaneously. The HTMS device was validated by testing various biomaterials and engineered cartilage constructs and by comparing the HTMS results to those derived from conventional single sample compression tests. Further evaluation showed that the HTMS system was capable of distinguishing and identifying 'hits', or factors that influence the degree of tissue maturation. Future iterations of this device will focus on reducing data variability, increasing force sensitivity and range, as well as scaling-up to even larger (96-well) formats. This HTMS device provides a novel tool for cartilage tissue engineering, freeing experimental design from the limitations of mechanical testing throughput. [ABSTRACT FROM AUTHOR]

Nonlinear, linear and failure properties of articular cartilage and meniscus in opposing contact surfaces are poorly known in tension. Relationships between the tensile properties of articular cartilage and meniscus in contact with each other within knee joints are also not known. In the present study, rectangular samples were prepared from the superficial lateral femoral condyle cartilage and lateral meniscus of bovine knee joints. Tensile tests were carried out with a loading rate of 5 mm/min until the tissue rupture. Nonlinear properties of the toe region, linear properties in larger strains, and failure properties of both tissues were analysed. The strain-dependent tensile modulus of the toe region, Young's modulus of the linear region, ultimate tensile stress and toughness were on average 98.2, 8.3, 4.0 and 1.9 times greater (p < 0.05) for meniscus than for articular cartilage. In contrast, the toe region strain, yield strain and failure strain were on average 9.4, 3.1 and 2.3 times greater (p < 0.05) for cartilage than for meniscus. There was a significant negative correlation between the strain-dependent tensile moduli of meniscus and articular cartilage samples within the same joints (r =-0.690, p=0.014). In conclusion, the meniscus possesses higher nonlinear and linear elastic stiffness and energy absorption capability before rupture than contacting articular cartilage, while cartilage has longer nonlinear region and can withstand greater strains before failure. These findings point out different load carrying demands that both articular cartilage and meniscus have to fulfil during normal physiological loading activities of knee joints. [ABSTRACT FROM AUTHOR]

Standard in-vitro spine testing methods have focused on application of isolated and/or constant load components while the in-vivo spine is subject to multiple components that can be resolved into resultant dynamic load vectors. To advance towards more in-vivo like simulations the objective of the current study was to develop a methodology to apply robotically-controlled, non-zero, real-time dynamic resultant forces during flexion-extension on human lumbar motion segment units (MSU) with initial application towards simulation of an ideal follower load (FL) force vector. A proportional-integral-derivative (PID) controller with custom algorithms coordinated the motion of a Cartesian serial manipulator comprised of six axes each capable of position- or load-control. Six lumbar MSUs (L4-L5) were tested with continuously increasing sagittal plane bending to 8Nm while force components were dynamically programmed to deliver a resultant 400N FL that remained normal to the moving midline of the intervertebral disc. Mean absolute load-control tracking errors between commanded and experimental loads were computed. Global spinal ranges of motion and sagittal plane inter-body translations were compared to previously published values for non-robotic applications. Mean TEs for zero-commanded force and moment axes were 0.7±0.4N and 0.03±0.02 Nm, respectively. For non-zero force axes mean TEs were 0.8±0.8 N, 1.3±1.6 Nm, and 1.3±1.6 N for Fx, Fz, and the resolved ideal follower load vector FLR, respectively. Mean extension and flexion ranges of motion were 2.6°±1.2° and 5.0°±1.7°, respectively. Relative vertebral body translations and rotations were very comparable to data collected with non-robotic systems in the literature. The robotically coordinated Cartesian load controlled testing system demonstrated robust real-time load-control that permitted application of a real-time dynamic non-zero load vector during flexion-extension. For single MSU investigations the methodology has potential to overcome conventional follower load limitations, most notably via application outside the sagittal plane. This methodology holds promise for future work aimed at reducing the gap between current in-vitro testing and in-vivo circumstances. [ABSTRACT FROM AUTHOR]

Robotic methods applied to in-vitro biomechanical testing potentially offer more comprehensive evaluations however, standard position control algorithms make real-time load control problematic. This paper describes and evaluates a novel custom developed Cartesian force controlled biomechanical testing system with coordinated 6 degree of freedom (DOF) real-time load control. A custom developed 6-DOF serial manipulator with cascaded force over position control algorithms was designed, assembled, and programmed. Dial gauge tests assessed accuracy of custom linear axes. Standard test input and tuning procedures refined control performance. Two single motion segment units (L4-L5) and lumbar (L1-S) spine segments were tested under continuous pure moment application in flexion-extension, left-right lateral bending and axial rotation to 8 Nm under full 6-DOF load control. Mean load control tracking errors between commanded and experimental loads were computed. Global spinal ranges of motion were compared to previously published values for standard non-robotic protocols. Individual linear and rotational axis position control accuracies were equal to or less than 6.35 m and 0.0167° respectively. Pilot pure bending tests demonstrated stable load control performance, as well as load rates, rotational velocities, and ranges of motion comparable to those for standard non-robotic invitro tests. Tracking errors for zero commanded forces and all moment controlled axes were less than 0.81 7 0.68 N and 0.18 7 0.19 Nm over all tests, respectively. The Cartesian based system simplified control application and demonstrated robust position and load control that was not limited to single axis or zero commanded loads. In addition to emulating standard biomechanical tests, the novel Cartesian force controlled testing system developed is a promising tool for biomechanical assessments with coordinated dynamic load application and coupled motion response in 6DOF. [ABSTRACT FROM AUTHOR]

Ex vivo mechanical testing is an essential tool for study of vertebral mechanics. However, the common method of testing vertebral bodies in the absence of adjacent intervertebral discs (IVDs) may limit the physiological relevance of the results. The goal of this study was to determine the influence of IVDs on vertebral mechanical properties and failure mechanisms. Rabbit thoracic vertebral bodies were tested with and without IVDs in a stepwise fashion that incorporated a micro-computed tomography scan at each loading step. The image sequences were analyzed using digital volume correlation to quantify deformations throughout the vertebral body. The observed deformation patterns differed substantially between the groups. Specimens tested with IVDs exhibited a slow increase in strain in the inferior and posterior regions, followed by a sudden increase in strain in the anterior cortex right at the yield point. In contrast, the highest strains in the isolated vertebral bodies were in the posterior regions throughout the test. Specimens tested with IVDs had lower stiffness (507.49 7 184.73 N/mm vs. 845.61 7 296.09 N/mm; p=0.044), higher ultimate displacement (2.00 7 0.68 mm vs. 1.17 7 0.54 mm; p=0.043), and higher maximum shear strains (e.g. top 25th percentile: 0.19 7 0.11 vs. 0.06 7 0.07 mm/mm; p o 0.0458), and tended to have lower ultimate force (690.28 7 160.25 N vs. 873.81 7 131.48 N; p=0.056). Similar work to failure (648.157 317.86 N-mm vs. 603.497 437.95 N-mm; p = 0.844) was observed between the two groups. These results indicate that testing vertebral bodies in the absence of IVDs can elicit artifactual failure mechanisms. These artifacts may be more prominent than the effects on vertebral strength and toughness. [ABSTRACT FROM AUTHOR]

Individuals with spinal cord injury experience a rapid loss of bone mineral below the neurological lesion. The clinical consequence of this bone loss is a high rate of fracture around regions of the knee. The ability to predict the mechanical competence of bones at this location may serve as an important clinical tool to assess fracture risk in the spinal cord injury population. The purpose of this study was to develop, and statistically compare, non-invasive methods to predict torsional stiffness (K) and strength (Tult) of the proximal tibia. Twenty-two human tibiae were assigned to either a "training set" or a "test set" (11 specimens each) and mechanically loaded to failure. The training set was used to develop subject-specific finite element (FE) models, and statistical models based on dual energy x-ray absorptiometry (DXA) and quantitative computed tomography (QCT), to predict K and Tult; the test set was used for cross-validation. Mechanical testing produced clinically relevant spiral fractures in all specimens. All methods were accurate and reliable predictors of K (cross-validation r²>0.91; error<13%), however FE models explained an additional 15% of the variance in measured Tult and illustrated 12-16% less error than DXA and QCT models. Given the strong correlations between measured and FE predicted K (cross-validation r²=0.95; error= 10%) and Tult (cross-validation r²=0.91; error=9%), we believe the FE modeling procedure has reached a level of accuracy necessary to answer clinically relevant questions. [ABSTRACT FROM AUTHOR]

Abstract: Tensile testing of repaired tendons has been used to assess the efficacy of repair techniques. However, individuals flex and extend fingers at rates higher than those typically used for testing. This study characterized the effect of loading rate on the failure strength of repaired canine flexor tendons. Thirty-six canine flexor digitorum profundus tendons were lacerated, repaired, and tested at three displacement rates: 0.33mm/s; 84mm/s; and 590mm/s. Peak force and stiffness of the repairs were evaluated. Peak force was significantly greater (p<0.05) for tendons distracted at 590mm/s than at 0.33mm/s. Crosshead stiffness was significantly greater for tendons distracted at 590mm/s than at either 84mm/s or 0.33mm/s. The predominant failure mode was core suture knot untying. Distracting tendons at slow loading rates provides a conservative assessment of tendon repair strength. Additionally, an estimate of the failure load of this repair for different clinical events has been identified. [Copyright &y& Elsevier]

Abstract: In attempting to develop non-invasive image based measures for the determination of the biomechanical integrity of healing fractures, traditional μCT based measurements have been limited. This study presents the development and evaluation of a tool for assessment of fracture callus mechanical properties through determination of the geometric characteristics of the fracture callus, specifically along the surface of failure identified during destructive mechanical testing. Fractures were created in tibias of ten male mice and subjected to μCT imaging and biomechanical torsion testing. Failure surface analysis, along with previously described image based measures was calculated using the μCT image data, and correlated with mechanical strength and stiffness. Three-dimensional measures along the surface of failure, specifically the surface area and torsional rigidity of bone, were shown to be significantly correlating with mechanical strength and stiffness. It was also shown that surface area of bone along the failure surface exhibits stronger correlations with both strength and stiffness than measures of average and minimum torsional rigidity of the entire callus. Failure surfaces observed in this study were generally oriented at 45° to the long axis of the bone, and were not contained exclusively within the callus. This work represents a proof of concept study, and shows the potential utility of failure surface analysis in the assessment of fracture callus stability. [Copyright &y& Elsevier]

Abstract: A distal radius fracture in middle-age and older adults is often considered a sentinel indicator of osteoporosis. Mechanical testing of cadaveric specimens is often used to quantify bone strength and develop insight for relating in-vivo measures to fracture force. Mechanical testing protocols using an intact forearm have been successful at replicating a Colles fracture, however, excised isolated radius protocols based on the intact forearm testing protocol have not been as successful. One protocol originally designed to replicate the physiological condition of a fall on an outstretched hand was reproduced in our laboratory, yet surprisingly the produced distal radius fracture patterns were not consistent among specimens nor was dorsal angulation of the distal fragment that is characteristic of a Colles fracture observed. The purpose of this study was to perform a mechanics-based analysis of the excised radius loading protocol in order to quantify the imposed and internal forces on the radius. An idealized beam model of the excised radius revealed that in the area of the distal radius where Colles fractures occur, 99.99% of the maximum strain on the bone outer surface was the result of pure compressive loading. This loading condition is in direct contrast to the accepted mechanics of a Colles fracture, which is characterized as a metaphyseal bending fracture with the volar cortex failing due to tensile stresses and the dorsal cortex exhibiting compression and comminution. The results suggest that additional research, particularly related to overcoming the difficulties of reliably supporting and applying a force to the distal end of the radius, is necessary for clinical fracture patterns to be reliably reproduced with an excised radius mechanical testing protocol. [Copyright &y& Elsevier]

Abstract: Purpose: To test the hypothesis that ruptured abdominal aortic aneurysms (AAA) are globally weaker than unruptured ones. Methods: Four ruptured and seven unruptured AAA specimens were harvested whole from fresh cadavers during autopsies performed over an 18-month period. Multiple regionally distributed longitudinally oriented rectangular strips were cut from each AAA specimen for a total of 77 specimen strips. Strips were subjected to uniaxial extension until failure. Sections from approximately the strongest and weakest specimen strips were studied histologically and histochemically. From the load-extension data, failure tension, failure stress and failure strain were calculated. Rupture site characteristics such as location, arc length of rupture and orientation of rupture were also documented. Results: The failure tension, a measure of the tissue mechanical caliber was remarkably similar between ruptured and unruptured AAA (group mean±standard deviation of within-subject means: 11.2±2.3 versus 11.6±3.6N/cm; p=0.866 by mixed model ANOVA). In post-hoc analysis, there was little difference between the groups in other measures of tissue mechanical caliber as well such as failure stress (95±28 versus 98±23N/cm2; p=0.870), failure strain (0.39±0.09 versus 0.36±0.09; p=0.705), wall thickness (1.7±0.4 versus 1.5±0.4mm; p=0.470) , and % coverage of collagen within tissue cross section (49.6±12.9% versus 60.8±9.6%; p=0.133). In the four ruptured AAA, primary rupture sites were on the lateral quadrants (two on left; one on left-posterior; one on right). Remarkably, all rupture lines had a longitudinal orientation and ranged from 1 to 6cm in length. Conclusion: The findings are not consistent with the hypothesis that ruptured aortic aneurysms are globally weaker than unruptured ones. [Copyright &y& Elsevier]

Abstract: Finite element (FE) models of long bones constructed from computed-tomography (CT) data are emerging as an invaluable tool in the field of bone biomechanics. However, the performance of such FE models is highly dependent on the accurate capture of geometry and appropriate assignment of material properties. In this study, a combined numerical–experimental study is performed comparing FE-predicted surface strains with strain-gauge measurements. Thirty-six major, cadaveric, long bones (humerus, radius, femur and tibia), which cover a wide range of bone sizes, were tested under three-point bending and torsion. The FE models were constructed from trans-axial volumetric CT scans, and the segmented bone images were corrected for partial-volume effects. The material properties (Young’s modulus for cortex, density–modulus relationship for trabecular bone and Poisson’s ratio) were calibrated by minimizing the error between experiments and simulations among all bones. The R 2 values of the measured strains versus load under three-point bending and torsion were 0.96–0.99 and 0.61–0.99, respectively, for all bones in our dataset. The errors of the calculated FE strains in comparison to those measured using strain gauges in the mechanical tests ranged from −6% to 7% under bending and from −37% to 19% under torsion. The observation of comparatively low errors and high correlations between the FE-predicted strains and the experimental strains, across the various types of bones and loading conditions (bending and torsion), validates our approach to bone segmentation and our choice of material properties. [Copyright &y& Elsevier]

Abstract: Excessive axial tension is very likely involved in the aetiology of tendon lesions, and the most appropriate indicator of tendon stress state is the true stress, the ratio of instantaneous load to instantaneous cross-sectional area (CSA). Difficulties to measure tendon CSA during tension often led to approximate true stress by assuming that CSA is constant during loading (i.e. by the engineering stress) or that tendon is incompressible, implying a Poisson''s ratio of 0.5, although these hypotheses have never been tested. The objective of this study was to measure tendon CSA variation during quasi-static tensile loading, in order to assess the true stress to which the tendon is subjected and its Poisson''s ratio. Eight equine superficial digital flexor tendons (SDFT, about 30cm long) were tested in tension until failure while the CSA of each tendon was measured in its metacarpal part by means of a linear laser scanner. Axial elongation and load were synchronously recorded during the test. CSA was found to linearly decrease with strain, with a mean decrease at failure of −10.7±2.8% (mean±standard deviation). True stress at failure was 7.1–13.6% higher than engineering stress, while stress estimation under the hypothesis of incompressibility differed from true stress of −6.6 to 2.3%. Average Poisson''s ratio was 0.55±0.12 and did not significantly vary with load. From these results on equine SDFT it was demonstrated that tendon in axial quasi-static tension can be considered, at first approximation, as an incompressible material. [Copyright &y& Elsevier]

Abstract: In the 1920s and 1930s, Terzaghi and coworkers realized that the failure of various porous geomaterials under internal pore pressure is given through evaluating the failure function for the same materials at zero pressure, with ‘total stress plus pore pressure’ instead of ‘total stress alone’ as argument. As to check, probably for the first time, the relevance of this (‘Terzaghi''s’) failure criterion for trabecular bone, a series of poromechanical and ultrasonic tests was conducted on bovine and human trabecular bone samples. Evaluation of respective experimental results within the theoretical framework of microporomechanics showed that (i) Terzaghi''s effective stress indeed governs trabecular bone failure, (ii) deviatoric stress states at the level of the solid bone matrix (also called tissue level) are primary candidates for initiating bone failure, and (iii) the high heterogeneity of these deviatoric tissue stresses, which increases with increasing intertrabecular porosity, governs the overall failure of trabecular bone. Result (i) lets us use the widely documented experimental results for strength values of bone samples without pore pressure, as to predict failure of the same bone samples under internal pore pressure. Result (ii) suggests a favorable mode for strength modeling of solid bone matrix. Finally, result (iii) underlines the suitability of microfinite element simulations for trabecular bone microstructures. [Copyright &y& Elsevier]

Abstract: Mechanical characterization of cartilage, other soft tissues and gels has become a ubiquitous and essential aspect of biomechanics and biomaterials research. Current progress in theoretical modeling and tools for data analysis often exceed what is required for routine mechanical characterization assays in experimental studies, making selection of methodologies difficult for the nonspecialist. We have therefore developed an approach for measurement of confined compression modulus and hydraulic permeability based on simple poroelasticity theory and requiring only linear regression tools for data analysis. This technique involves a new application of an early-time solution for creep combined with stress relaxation measurements to characterize soft tissue mechanical parameters as a function of compressive strain or water content. This combined methodology allows measurement of hydraulic permeability by two different techniques with only a modest increase in experimental duration, providing a more precise assessment of permeability and associated measurement error. [ABSTRACT FROM AUTHOR]

Abstract: The assessment of repair site gap formation during cyclic loading of reconstructed flexor tendons provides important data on the performance of repair techniques in the early postoperative period. This study describes our cyclic testing protocol and evaluates the relationship between changes in optical gap and grip-to-grip displacement. Sixteen sheep hind limb deep flexor tendons were randomized into four repair groups (n=4 per group): a 2-strand repair (modified Kessler) and 4-strand repair (Adelaide), both with and without a simple running peripheral suture. Repaired tendons were cycled for 1000 cycles at appropriate rehabilitation loads for the reconstruction. Tendons were paused at 18 pre-determined cycle points to measure gap and displacement. A strong positively linear relationship between gap and displacement was demonstrated for all repair groups (R 2>0.90). An initial non-linear region during the first 10 cycles was noted with some combined core and peripheral repairs. Although trends in displacement after 10 cycles can be used to reflect gapping behaviour, direct optical measurement of gap remains preferable. We hypothesized that the adjustment of suture strands and equilibration of forces within the reconstruction occurs mostly during the initial 10 cycles. Gap–cycle curves provide a good illustration of dynamic changes at the repair site, and should be added more frequently to cyclic testing studies. [Copyright &y& Elsevier]

Abstract: Vertebral compression fracture is a common medical problem in osteoporotic individuals. The quantitative computed tomography (QCT)-based finite element (FE) method may be used to predict vertebral strength in vivo, but needs to be validated with experimental tests. The aim of this study was to validate a nonlinear anatomy specific QCT-based FE model by using a novel testing setup. Thirty-seven human thoracolumbar vertebral bone slices were prepared by removing cortical endplates and posterior elements. The slices were scanned with QCT and the volumetric bone mineral density (vBMD) was computed with the standard clinical approach. A novel experimental setup was designed to induce a realistic failure in the vertebral slices in vitro. Rotation of the loading plate was allowed by means of a ball joint. To minimize device compliance, the specimen deformation was measured directly on the loading plate with three sensors. A nonlinear FE model was generated from the calibrated QCT images and computed vertebral stiffness and strength were compared to those measured during the experiments. In agreement with clinical observations, most of the vertebrae underwent an anterior wedge-shape fracture. As expected, the FE method predicted both stiffness and strength better than vBMD (R 2 improved from 0.27 to 0.49 and from 0.34 to 0.79, respectively). Despite the lack of fitting parameters, the linear regression of the FE prediction for strength was close to the 1:1 relation (slope and intercept close to one (0.86kN) and to zero (0.72kN), respectively). In conclusion, a nonlinear FE model was successfully validated through a novel experimental technique for generating wedge-shape fractures in human thoracolumbar vertebrae. [ABSTRACT FROM AUTHOR]

Tendon injuries are extremely common, resulting in mechanically weaker tendons that could lead to tendon rupture. Dry needling (DN) is widely used to manage pain and function after injury. However, DN is invasive and high inter-practitioner variability has led to mixed success rates. Focused ultrasound (fUS) is a non-invasive medical technology that directs ultrasound energy into a well-defined focal volume. fUS can induce thermal ablation or mechanical fractionation, with bioeffect type controlled through ultrasound parameters. Tendons must withstand high physiological loads, thus treatments maintaining tendon mechanical properties while promoting healing are needed. Our objective was to evaluate mechanical effects of DN and 3 fUS parameter sets, chosen to prioritize mechanical fractionation, on Achilles and supraspinatus tendons. Ex vivo rat Achilles and supraspinatus tendons (50 each) were divided into sham, DN, fUS-1, fUS-2, and fUS-3 (n = 10/group). Following treatment, tendons were mechanically tested. Elastic modulus of supraspinatus tendons treated with DN (126.64 ± 28.1 MPa) was lower than sham (153.02 ± 29.3 MPa; p = 0.0280). Stiffness and percent relaxation of tendons treated with DN (Achilles: 114.40 ± 31.6 N/mm; 49.10 ± 6.1%; supraspinatus: 109.53 ± 30.8 N/mm; 50.17 ± 7.6%) were lower (all p < 0.0334) than sham (Achilles: 141.34 ± 20.9 N/mm; 60.30 ± 7.7%; supraspinatus: 135.14 ± 30.2 N/mm; 60.85 ± 15.4%). Modulus of Achilles and supraspinatus tendons treated with fUS-1 (159.88 ± 25.7 MPa; 150.12 ± 22.0 MPa, respectively) were similar to sham (156.35 ± 23.0 MPa; 153.02 ± 29.3 MPa, respectively). These results suggest that fUS preserves mechanical properties better than DN, with fUS-1 performing better than fUS-2 and fUS-3. fUS should be studied further to fully understand its mechanical and healing effects to help evaluate fUS as an alternative, non-invasive treatment for tendon injuries. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a novel application of a velocity-based force control routine used for robotic biomechanical testing. The routine employs a jog function, available from the robot''s motion commands, that permits easy adjustment of velocity on each axis. Force and moment targets are achieved by adjusting jog velocities in proportion to force or moment errors while limiting the maximum velocity of the system. The force control jog routine does not require specimen stiffness values and is inherently stable. The performance of the method was shown to be suitable for unconstrained in vitro spine testing in a rabbit model where extremely small motions are necessary to maintain the target force values. The jogging feature on which this work is based is a feature available on most robots and is equally applicable to a serial robot. The simplicity, stability, and performance of this method warrant its consideration for other robotic biomechanical testing applications where force control is required. [Copyright &y& Elsevier]

Abstract: In total joint arthroplasty, third body particle access to the articulating surfaces results in accelerated wear. Hip joint subluxation is an under-recognized means by which third body particles could potentially enter the otherwise closely conforming articular bearing space. The present study was designed to test the hypothesis that, other factors being equal, even occasional events of femoral head subluxation greatly increase the number of third body particles that enter the bearing space and become embedded in the acetabular liner, as compared to level-walking cycles alone. Ten metal-on-polyethylene hip joint head-liner pairs were tested in a multi-axis joint motion simulator, with CoCrMo third body particles added to the synovial fluid analog. All component pairs were tested for 2h of level walking; half were also subjected to 20 intermittent subluxation events. The number and location of embedded particles on the acetabular liners were then determined. Subluxation dramatically increased the number of third body particles embedded in the acetabular liners, and it considerably increased the amount of scratch damage on the femoral heads. Since both third body particles and subluxation frequently occur in contemporary total hip arthroplasty, their potent synergy needs to be factored prominently into strategies to minimize wear. [Copyright &y& Elsevier]

Abstract: Total variation in any measured variable, in conjunction with expected treatment effect, defines the minimum sample size (minSS) required to detect the expected effect with statistical confidence should the effect truly exist. A comprehensive literature survey of 3472 original studies was carried out to identify studies with biomechanical testing of whole bones. Total variation in common biomechanical traits and expected treatment effects in typical interventions were statistically determined. According to this survey, total variation in biomechanical traits between different species of experimental animals was similar, justifying the use of rat femur as a model in further analyses. Due to poorer precision, stiffness and energy absorption assessment require substantially larger sample size than breaking load. Due to same reason, minSS for femoral neck compression test is considerably larger than for femoral shaft three-point bending test. For the bending test, minSS to show a 10% treatment effect in the breaking load with 80% statistical power is 11rats/group, while corresponding minSS is 23 for the stiffness, and 53 for the energy absorption. For the femoral neck compression test, minSSs are 16, 51, and 134rats/group, respectively. Among the reviewed studies, the mean sample size was 11animals/group. This underscores the need for considerably larger sample sizes in experimental bone interventions which employ mechanical traits as primary outcome variables. In particular, poor precision and generally small expected treatment effects compromise the utility of stiffness and energy absorption assessments in experimental bone interventions. [Copyright &y& Elsevier]

Abstract: Spinal segments show non-linear behavior under axial compression. It is unclear to what extent this behavior is attributable to the different components of the segment. In this study, we quantified the separate contributions of vertebral bodies and intervertebral discs to creep of a segment. Secondly, we investigated the contribution of bone and osteochondral endplate (endplates including cartilage) to the deformation of the vertebral body. From eight porcine spines a motion segment, a disc and a vertebral body were dissected and subjected to mechanical testing. In an additional test, cylindrical samples, machined from the lowest thoracic vertebrae of 11 porcine spines, were used to compare the deformation of vertebral bone and endplate. All specimens were subjected to three loading cycles, each comprising a loading phase (2.0MPa, 15min) and a recovery phase (0.001MPa, 30min). All specimens displayed substantial time-dependent height changes. Average creep was the largest in motion segments and smallest in vertebral bodies. Bone samples with endplates displayed substantially more creep than samples without. In the early phase, behavior of the vertebra was similar to that of the disc. Visco-elastic deformation of the endplate therefore appeared dominant. In the late creep phase, behavior of the segment was similar to that of isolated discs, suggesting that in this phase the disc dominated creep behavior, possibly by fluid flow from the nucleus. We conclude that creep deformation of vertebral bodies contributes substantially to creep of motion segments and that within a vertebral body endplates play a major role. [Copyright &y& Elsevier]

Abstract: Human cancellous bone is a heterogeneous material. Despite this, most of the published studies report correlations between mechanical properties and morphometric parameters averaged on the whole specimen. This work investigated whether local variations in morphometric parameters were linked to the localized failure regions of cancellous bone. Additionally, it was examined whether local values of morphometric parameters can predict the ultimate stress better than the average bone volume fraction (BV/TV). Cylindrical cancellous bone specimens extracted along the primary compressive group of human femoral heads were studied. These were microCT-imaged to assess the morphometric parameters, compressed to determine the ultimate stress, and rescanned by microCT to visualize the failure region. Failure involved slightly less than half of the free height of the specimens. Significant differences were found in the morphometric parameters calculated in the failure and in the non-failure regions. The cross-sections containing minimum BV/TV values were those most often located inside the failure region (83%, p<0.001). Regression analysis confirmed that variations in BV/TV best describe variations in ultimate stress (R 2=0.84) out of the averaged morphometric parameters. The prediction of ultimate stress increased when minimum or maximum values of the morphometric parameters were taken, with the highest prediction found by considering the minimum BV/TV (R 2=0.95). In conclusion, due to the heterogeneity of cancellous bone, there may exist regions characterized by a different microarchitecture, where the bone is weaker and consequently is more likely to fail. These regions mostly contain minimum values in BV/TV, which were found to predict ultimate stress better than average BV/TV. [Copyright &y& Elsevier]

Abstract: The aim of this study was to verify whether a misalignment between the testing direction and the trabecular main direction has a significant effect on the compressive behaviour of cancellous bone. Ten cylindrical specimens were extracted from femoral heads with a misalignment to the trabecular main direction of approximately 20°. Each specimen was paired with a specimen extracted aligned with the main direction of the trabeculae on the basis of the closest bone volume fraction, obtaining two groups, one ‘aligned’ and one ‘misaligned’. The average off-axis angle was 6.1° and 21.6° for the ‘aligned’ and ‘misaligned’ group, respectively. The specimens underwent micro-tomographic analysis, compressive testing, micro-indentation testing and ashing. No significant differences were found in histomorphometric parameters, hardness and ash density between the two groups, whereas significant differences were found in Young''s modulus and ultimate stress: both parameters, measured for the ‘misaligned’ group, were about 40% lower than those measured for the ‘aligned’ group. These results demonstrate a great effect of the angle between the testing direction and the main direction of the trabecular structure on the compressive behaviour of cancellous bone. This angle should be reduced as much as possible (in the present work the average value was 6.6±3.3°), in any case measured, and always reported together with the mechanical parameters of cancellous bone. [Copyright &y& Elsevier]

Abstract: This paper describes the design, evaluation, and preliminary results of a specialized testing device and surgical protocol to determine translational stiffness of a rabbit knee, replicating the clinical anterior drawer test. Coronal-plane transverse pins are inserted through the rabbit leg, two in the tibia and one in the distal femur, to hold and reproducibly position the leg in the device for tests at multiple time points. A linear stepper motor draws the tibia upward then returns to the home position, and a load cell measures the resisting force; force–displacement knee stiffness is then calculated. Initial evaluation of this testing device determined the effects of preconditioning, intra-operator repeatability, rabbit-to-rabbit variability, knee flexion angle (90° vs. 135°), and anterior cruciate ligament (ACL) sectioning (0%, 25%, 50%, 75%, 100%). Knee stiffness generally decreased as ACL sectioning increased. This testing device and surgical protocol provide an objective and efficient method of determining translational rabbit knee stiffness in vivo, and are being used in an ongoing study to evaluate the effect of knee instability (via partial to complete ACL sectioning) on the development of post-traumatic osteoarthritis. [Copyright &y& Elsevier]

Abstract: In vivo studies on disc mechanics show loss of fluid from the intervertebral disc (IVD) during loading and full recovery during rest. Previous work indicated that in vitro recovery is hampered after static loading. The aim of the present study was to investigate the role of the endplate after dynamic and static loading on mechanical recovery in vitro. Lumbar spines (caprine) were obtained from the local slaughterhouse and stored frozen. Twenty-four intervertebral discs were thawed and subjected to a compression test in a saline bath (37°C). The discs were pre-loaded at 20N for 15min. Three 15-min loading cycles (static: 2.0MPa or dynamic: average load 2.0MPa at 0.5Hz) were applied, each followed by a 30-min period of unloading (20N). After this protocol, the endplates of half of the discs were blocked with silicone paste and the long-term recovery protocol was applied; the discs were subjected to a single loading cycle (15min of static or dynamic loading) followed by 10h of unloading at 20N. All specimens showed a net loss of height and a gain in stiffness during the first part of the test. Eventually, height and stiffness were restored during a long-term recovery test. The difference in recovery between blocked and free endplates was marginal. If fluid flow plays a role during recovery in vitro, the role of the endplate appears to be limited. Our findings show no influence of loading type on recovery in vitro. [Copyright &y& Elsevier]

Abstract: Many musculoskeletal tissues exhibit significant anisotropic mechanical properties reflective of a highly oriented underlying extracellular matrix. For tissue engineering, recreating this organization of the native tissue remains a challenge. To address this issue, this study explored the fabrication of biodegradable nanofibrous scaffolds composed of aligned fibers via electrospinning onto a rotating target, and characterized their mechanical anisotropy as a function of the production parameters. The characterization showed that nanofiber organization was dependent on the rotation speed of the target; randomly oriented fibers (33% fiber alignment) were produced on a stationary shaft, whereas highly oriented fibers (94% fiber alignment) were produced when rotation speed was increased to 9.3m/s. Non-aligned scaffolds had an isotropic tensile modulus of 2.1±0.4MPa, compared to highly anisotropic scaffolds whose modulus was 11.6±3.1MPa in the presumed fiber direction, suggesting that fiber alignment has a profound effect on the mechanical properties of scaffolds. Mechanical anisotropy was most pronounced at higher rotation speeds, with a greater than 33-fold enhancement of the Young''s modulus in the fiber direction compared to perpendicular to the fiber direction when the rotation speed reached 8m/s. In cell culture, both the organization of actin filaments of human mesenchymal stem cells and the cellular alignment of meniscal fibroblasts were dictated by the prevailing nanofiber orientation. This study demonstrates that controllable and anisotropic mechanical properties of nanofibrous scaffolds can be achieved by dictating nanofiber organization through intelligent scaffold design. [Copyright &y& Elsevier]

Abstract: The nasofrontal suture links the nasal complex with the braincase and is subject to compressive strain during mastication and (theoretically) tensile strain during growth of nasal soft tissues. The suture''s ability to transmit compressive and tensile loads therefore affects both cranioskeletal stress distribution and growth. This study investigated the in vitro viscoelastic and failure properties of the nasofrontal suture in the pig, Sus scrofa. Suture specimens from two ages were tested in compression and tension and at fast and slow rates. In additional specimens, strain gauges were applied to the suture and nasal bone for strain measurement during testing. Relaxation testing demonstrated higher elastic moduli in tension than compression, regardless of test rate or pig age. In contrast, maximum elastic moduli from failure tests, as well as peak stresses, were significantly higher in compression than in tension. Sutures from older pigs tended to have higher elastic moduli and peak stresses, significantly so for tensile relaxation moduli. Strain gauge results showed that deformation at the suture was much greater than that of the nasal bone. These data demonstrate the viscoelasticity and deformability of the nasofrontal sutural ligament. The suture achieved maximal resistance to tensile deformation at low loads, corresponding with the low tensile loads likely to occur during growth of nasal soft tissues. In contrast, the maximal stiffness in compression at high loads indicates that the suture functions with a substantial safety factor during mastication. [Copyright &y& Elsevier]

Abstract: Visible light combined with naphthalimides has previously been shown to catalyze formation of physical bonds in avascular meniscal tissue. The first objective was to modify the existing in vitro testing method (i.e., adhesion testing using lap-jointed slices) to gain more sensitivity in detecting relative bonding strengths among candidate bonding agents. A repeated measures experimental design (RMED) was used to account for variability in properties among bovine menisci and was achieved by testing all treatments/controls on slices from each meniscus. Additionally, to make the method more clinically relevant in modeling a bucket-handle tear, the bovine meniscal slices were cut with collagen fibers parallel to the test slice''s length. Peak stress was greater for the complete treatment group (light plus naphthalimide) than for the control or incomplete treatment groups (light only or napthalimide only). The second objective was to perform concentration and photoactivation time dose-response studies. In the concentration dose-response study, peak stress was greater for all treatments when compared with the control but not different among treatment groups; however, there was a trend of increased bonding strength with increased concentration. In the photoactivation time dose-response study, peak stress was greater for all treatments when compared with the control and greater for the 3-min treatment vs. the 6- and 9-min treatments. Peak stress was not different between the longer treatments. The RMED provided increased reproducibility and statistical sensitivity for detecting differences among treatments and will be used to test candidate bonding agents prior to in vivo testing. [Copyright &y& Elsevier]

Abstract: The material properties of articular cartilage in the rabbit tibial plateau were determined using biphasic indentation creep tests. Cartilage specimens from matched-pair hind limbs of rabbits approximately 4 months of age and greater than 12 months of age were tested on two locations within each compartment using a custom built materials testing apparatus. A three-way ANOVA was used to determine the effect of leg, compartment, and test location on the material properties (aggregate modulus, permeability, and Poisson''s ratio) and thickness of the cartilage for each set of specimens. While no differences were observed in cartilage properties between the left and right legs, differences between compartments were found in each set of specimens. For cartilage from the adolescent group, values for aggregate modulus were 40% less in the medial compartment compared to the lateral compartment, while values for permeability and thickness were greater in the medial compartment compared to the lateral compartment (57% and 30%, respectively). Values for Poisson''s ratio were 19% less in the medial compartment compared to the lateral compartment. There was also a strong trend for thickness to differ between test locations. Similar findings were observed for cartilage from the mature group with values for permeability and thickness being greater in the medial compartment compared to the lateral compartment (66% and 34%, respectively). Values for Poisson''s ratio were 22% less in the medial compartment compared to the lateral compartment. [Copyright &y& Elsevier]

Abstract: The effectiveness of the cupping technique, a treatment modality in Traditional Chinese Medicine, in stimulating acupuncture points for pain relief was examined in this paper from a biomechanical perspective. Parametric studies including the effects of vacuum pressure, loading rate, friction coefficient at the cup–skin interface, and size and shape of the cup were carried out using a model based on the finite-element method. The anatomical structures of skin, fat, and muscle were modelled. All the soft-tissue layers were assumed to be nonlinearly elastic and viscoelastic. The rim of the cup was also modelled to study the interaction between cup and skin; the cup rim was assumed to be rigid. The simulation results showed that the stresses in the soft tissue were increased for increasing applied vacuum pressures and that the effects of cupping were mostly limited to the region enclosed by the cup. The simulations also indicated that the magnitude of the applied vacuum may have had direct implications for the severity of bruising of the skin following cupping treatment. Most significantly, the simulation results contradicted the established practice of cup size selection according to the depth of the disorder. Experimental verification of the proposed multi-layered finite-element model is presented. The nature of the bruising inherent to the cupping treatment is also explained by the proposed model. [Copyright &y& Elsevier]

Abstract: Microdamage induced by falls or other abnormal loads that cause shear stress in trabecular bone could impair the mechanical properties of the proximal femur or spine. Existing microdamage may also increase the initiation and propagation of further microdamage during subsequent normal, on-axis, loading conditions, resulting in atraumatic or “spontaneous” fractures. Microdamage formation due to shear and compressive strains was studied in 14 on-axis cylindrical bovine tibial trabecular bone specimens. Microdamage was induced by a torsional overload followed by an on-axis compressive overload and quantified microscopically. Fluorescent agents were used to label microdamage and differentiate damage due to the two loading modes. Both the microcrack density and diffuse damage area caused by the torsional overload increased with increasing shear strain from the center to the edge of the specimen. However, the mean microcrack length was uniform across the specimen, suggesting that microcrack length is limited by microstructural features. The mean density of microcracks caused by compressive overloading was slightly higher near the center of the specimen, and the diffuse damage area was uniform across the specimen. Over 20% of the microcracks formed in the initial torsional overloading propagated during compression. Moreover the propagating microcracks were, on average, longer than microcracks formed by a single overload. As such, changes in loading mode can cause propagation of microcracks beyond the microstructural barriers that normally limit the length. Damage induced by in vivo off-axis loads such as falls may similarly propagate during subsequent normal loading, which could affect both remodeling activity and fracture susceptibility. [Copyright &y& Elsevier]

Biomechanical testing of the spine has traditionally been performed to help understand the normal function of the spine as well as to evaluate the effects of injury and surgical procedures on spinal behaviour. The overall objective of this investigation was to compare traditional stepwise loading with the recently introduced continuous loading protocol, determining the effect of loading protocol on the mechanical behaviour of the spine. For all tests, a custom spine testing machine was used to apply pure moments of flexion extension, axial rotation, and lateral bending to a maximum of 2 N m, using six porcine cervical spine specimens (C2–C4). Motions of C2 with respect to C4 were measured with an optoelectronic camera system. Motion parameters calculated were range of motion (ROM), neutral zone (NZ), and the ratio of NZ and ROM. The continuous loading protocol had smaller values for all motion parameters in each loading direction (p<0.05). ROM for the continuous test ranged between 88% and 93% of that of stepwise for the three loading directions. The continuous protocol NZ was 56–75% of that of the stepwise test. The findings of the study demonstrate that the two loading protocols provide differing spinal behaviours. [Copyright &y& Elsevier]

Contemporary computational models potentially allow the practical incorporation of the effects of a joint capsule on both motion and the loads transmitted to the other parts of the joint. However, the required material properties have not been available for this purpose. To determine these properties we took both hip joints from five fresh-frozen, nondiseased cadavers. Following dissection and potting of the hemi-pelvis, distraction of the intact joint was conducted to measure the structural tangent stiffness of the joint capsule. Anatomical insertion points of the hip capsule were then recorded, and a complete capsulectomy was performed. Once excised, the capsule was sectioned into eight, approximately even sectors, and initial geometrical measurements were recorded for material property calculations. Material properties (i.e., structural tangent stiffness, failure load, ultimate strength, tangent modulus) were calculated using the load-displacement and geometric data collected for each of the sectors. This specimen-to-specimen thickness variability reveals significantly lower (p<0.01) average tangent structural stiffness values in the posterior-inferior portion of the capsule. Explorations of hip stability using numerical models can now be enhanced by incorporation of these experimental capsule data. [Copyright &y& Elsevier]

Changes in the fibril-reinforced poroelastic (FRPE) mechanical material parameters of human patellar cartilage at different stages of osteoarthritis (OA) are not known. Further, the patellofemoral joint loading is thought to include more sliding and shear compared to other knee joint locations, thus, the relations between structural and functional changes may differ in OA. Thus, our aim was to determine the patellar cartilage FRPE properties followed by associating them with the structure and composition. Osteochondral plugs (n = 14) were harvested from the patellae of six cadavers. Then, the FRPE material properties were determined, and those properties were associated with proteoglycan content, collagen fibril orientation angle, optical retardation (fibril parallelism), and the state of OA of the samples. The initial fibril network modulus and permeability strain-dependency factor were 72% and 63% smaller in advanced OA samples when compared to early OA samples. Further, we observed a negative association between the initial fibril network modulus and optical retardation (r = -0.537, p < 0.05). We also observed positive associations between 1) the initial permeability and optical retardation (r = 0.547, p < 0.05), and 2) the initial fibril network modulus and optical density (r = 0.670, p < 0.01).These results suggest that the reduced pretension of the collagen fibrils, as shown by the reduced initial fibril network modulus, is linked with the loss of proteoglycans and cartilage swelling in human patellofemoral OA. The characterization of these changes is important to improve the representativeness of knee joint models in tissue and cell scale. [ABSTRACT FROM AUTHOR]