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Objectives The relationship between radiographic grayscale and elastic modulus was determined using mandibular cortical bone of colobine monkeys. Quantification of this relationship is critical for establishing absolute measures of structural rigidity of skeletal elements. Materials and methods We determined the Vickers microhardness values in mandibular bone from two species of African colobine monkeys and related these values to elastic modulus through an empirically determined correlation. We also determined radiographic grayscale values from microcomputed tomographic scans of the mandible in the same regions in which microhardness was sampled. We then correlated modulus to grayscale with a power law relationship. Results We found that elastic modulus scaled with negative allometry with respect to grayscale with an exponent of 0.77. Discussion Our results suggest a single exponent can effectively capture the relationship of grayscale to elastic modulus and facilitate development of heterogeneous structural models for use in comparative and computational biomechanical studies.

Primate canines function in social displays but are also recruited for biting in agonistic encounters. Although the precise nature of the loads such behavior places on the canine crown is unknown, it is thought that bending is a major component of such loads. To date, modeling of canine bending strength has relied on idealized geometric representations. Accounting for the tapering of the crown as well as shape changes along an apical-basal axis provides a more realistic model for assessment of bending stress in canines. We provide such an accounting and evaluate the hypothesis that the morphology of the cercopithecoid canine represents a structural solution for maintaining constant maximum bending stress under apical or distributed loading in a parasagittal plane. This isostress hypothesis is analogous to a design criterion of minimum mass for a given structural requirement. Examining permanent maxillary canines from males and females representing eight West African cercopithecoid species, we reconstructed crown geometry from apex to base using microcomputed tomography. From reconstructed cross-sections, we determined section moduli about a buccolingual centroidal axis. We then determined what the taper should be for a variety of parasagittal loading distributions if the isostress hypothesis were true and compared these theoretical tapers to actual crown geometry. We found that a variety of loading distributions can be accommodated by the canines, particularly among males. These results suggest that our sample of canines are not optimized for resisting particular biting loads, but effectively limit stress gradients associated with a range of behaviors.

Researchers have increasingly recognized the need to quantify the material properties of primate food items, particularly hardness (H) and stiffness (E), which is measured as elastic modulus. Assessing E in the field is particularly difficult because the typical equipment needed to perform the requisite analyses is expensive and cumbersome. Alternatively, researchers can use hand-held, relatively inexpensive, portable durometers that measure H on Shore scales. Shore-D durometers show a reliable ability to characterize H in harder-stiffer materials, and Shore-D measures in these materials can be reliably converted to E. Shore-A durometers-employed in past field studies of food properties-do not accurately characterize the properties of harder-stiffer materials, which are likely to be those materials exerting the greatest mechanical demands on primate masticatory morphology. We offer recommendations for Shore-D durometer usage in the field.

Alveolar bone is functionally involved with tooth support and must also be competent in accommodating masticatory forcess. Data from a variety of perspectives – including observations of anatomy, metabolic activity and determination of mechanical properties – suggest that alveolar bone is more compliant than the more inferiorly situated basal bone of the mandibular corpus. The nature of stiffness variation within the alveolar process, however, remains largely unexplored. The technique of microindentation is used to estimate bone stiffness variation in the cortical alveolar bone of a sample of eight adult mandibles from two species of West African colobus monkeys. The null hypothesis under investigation is that bone stiffness variation in the alveolar process is random with respect to species, sex, and location. Alternative hypotheses are evaluated with respect to species, sex and locality effects, including those associated with cortical thickness variation and masticatory loading. Microindentation data do not indicate species or sex differences. Spatial variation in alveolar bone, however, is nonrandom. The stiffest bone is localized in the superior alveolar process, and significant differences between buccal and lingual cortical plates are found in five of eight specimens. Evidence for a superoinferior stiffness gradient is found in half the specimens but is confined to either the buccal or lingual cortical plate. Covariation between bone stiffness and bone thickness is weak. Relative compliance of the alveolar process represents a biomechanical solution for enhancing toughness of cortical bone. The relationship of the spatial patterning of bone stiffness to the masticatory loading environment is unclear.

The masticatory loading regime of lateral transverse bending (wishboning) is hypothesized to be instrumental in the evolution of symphyseal form among primates. The biomechanics of wishboning have largely been inferred by assuming that the mandible behaves as a curved beam under this load; however, the characterization of stress and strain in the anthropoid symphysis has been interpretively challenging. This is due, in part, to both limitations of sampling strain in an in vivo context and the incongruence of beam theory assumptions on the one hand, and the anatomical complexity of mandibular morphology on the other. Utilizing three-dimensional (3D) Digital Image Correlation (DIC), we employ an in vitro approach to characterize the strain field in a sample of colobine mandibles under simulated wishboning loads in order to assess the utility of idealized curved beam models for characterizing strain gradients in symphyseal bone. Conventional theory of curved beams suggest that colobine mandibles should exhibit reduced disparity of labial and lingual stresses relative to papionin primates given differences in overall mandibular architecture. This prediction is borne out by our analysis: whereas macaques experience lingual:labial strain disparities of 3.5:1, the colobine mandibles exhibit ratios on the order of 2-3:1. However, despite the fact that wishboning loads represent a case of asymmetric bending, details of the wishboning strain field do not conform to expected stress distribution under this model.

The anterior mandibular corpus of anthropoid primates is routinely subjected to masticatory loads that result in relatively high local levels of stress and strain. While structural morphological responses to these loads have been extensively explored, relatively little is known about material property variation in mandibular bone of nonhuman primates. Consequently, the role of regional and local variation in bone stiffness in conditioning stress and strain gradients is poorly understood. We sampled elastic modulus variation in the bone of the anterior mandibular corpus in two species (N = 3 each) of sympatric colobine monkeys, Procolobus badius and Colobus polykomos. These monkeys were chosen for comparison owing to their distinctive dietary regimens, as P. badius rarely includes hard objects in its diet while C. polykomos habitually processes obdurate items during feeding. Elastic modulus is determined through bone hardness data obtained via microindentation, which enables the description of stiffness variation on sub-millimeter scales. Labial bone stiffness exceeds that of lingual bone in the sample overall. Female mandibular bone is generally stiffer than that found in males, and overall Procolobus mandibular bone is stiffer than that in Colobus. These results, interpreted collectively, suggest that the material response to elevated masticatory stress is increased compliance of the affected bone.

Traditional measures of structural stiffness in the primate skeleton do not consider the heterogeneous material stiffness distribution of bone. This assumption of homogeneity introduces an unknown degree of error in estimating stiffness in skeletal elements. Measures of weighted stiffness can be developed by including heterogeneous grayscale variations evident in computed tomographic (CT) images. Since gray scale correlates with material stiffness, the distribution of bone quality and quantity can be simultaneously considered. We developed weighted measures of bending resistance and applied these to CT images at three locations along the mandibular corpus in the hominoids Gorilla, Pongo, and Pan. We calculated the traditional (unweighted) moment of inertia for comparison to our weighted measure, which weighs each pixel by its gray-scale value. This weighing results in assignment of reduced moment of inertia values to sections of reduced density. Our weighted and unweighted moments differ by up to 22%. These differences are not consistent among sections, however, such that they cannot be calculated by simple correction of unweighted moments. The effect of this result is that the rank ordering of individual sections within species changes if weighted moments are considered. These results suggest that the use of weighted moments may spur different interpretations of comparative data sets that rely on stiffness measures as estimates of biomechanical competence.

Osteonal bones are crack-tolerant due to their repair activity and special tissue structures formed by interstitial material and osteons. Crack-resistant engineering structures may be developed by mimicking bones. In this paper, bone-like composite materials were modeled by using circular inclusions as osteons and their cement lines. A two-inclusion model was investigated to see how crack location and inclusion size and property variations affect the propagation of an initial crack between the inclusions. A four-inclusion model was studied to determine conditions for attracting cracks to inclusions for arrestment. The four-inclusion model was optimized in order to arrest any crack initiated within the interstitial material between the inclusions. The optimized model was verified with the FRANC2D software. The results showed that inclusions with effective elastic moduli smaller than that of the interstitial material always tend to attract cracks. On the contrary, an inclusion with effective modulus greater than that of the interstitial material has a tendency to repel cracks. A crack’s propagation direction can be changed by varying inclusions’ sizes and elastic properties. At optimal point, any crack initiated within the interstitial material between the four inclusions can be attracted to two inclusions for arrestment.

Finite-element modeling provides a full-field method for describing the stress environment of the skull. The utility of finite-element models, however, remains uncertain given our ignorance of whether such models validly portray states of stress and strain. For example, the effects of boundary conditions that are chosen to represent the mechanical environment in vivo are largely unknown. We conducted an in vitro strain gauge experiment on a fresh, fully dentate adult mandible of Macaca fascicularis to model a simplified loading regime by finite-element analysis for purposes of model validation. Under various conditions of material and structural complexity, we constructed dentate and edentulous models to measure the effects of changing boundary conditions (force orientation and nodal constraints) on strain values predicted at the gauge location. Our results offer a prospective assessment of the difficulties encountered when attempting to validate finite-element models from in vivo strain data. Small errors in the direction of load application produce significant changes in predicted strains. An isotropic model, although convenient, shows poor agreement with experimental strains, while a heterogeneous orthotropic model predicts strains that are more congruent with these data. Most significantly, we find that an edentulous model performs better than a dentate one in recreating the experimental strains. While this result is undoubtedly tied to our failure to model the periodontal ligament, we interpret the finding to mean that in the absence of occlusal loads, teeth within alveoli do not contribute significantly to the structural stiffness of the mandible.

Holes in engineering structures cause stress concentrations that often lead to failure. In nature, however, blood vessel holes (foramina) in load-bearing bones are not normally involved in structural failures. It has been found that this behavior is linked to the material distribution near the hole. In the present paper, we have investigated the effectiveness of optimizing the radial distribution of the isotropic elastic modulus around a circular hole to increase load-carrying capacity. Bezier curves were used to describe the radial distribution of the elastic modulus. Since changing the elastic modulus usually affects the strength, the ratio of maximum principal stress to strength was chosen as the objective function for optimization. Using non-dimensional analysis of the 2-D elasticity equations, we identified three parameters that govern the optimum design and are applicable to a wide range of materials, loading, and geometries. The first is a material parameter that describes the relationship between the strength and elastic modulus, the second is the geometric parameter given by the ratio of the optimized field to the hole radius, and the third is the biaxial load ratio. The effect of failure criterion choice on the optimum elastic modulus distribution is also investigated. Optimum elastic modulus distributions for materials whose strength increases faster than the stiffness, as density and/or composition is varied, completely eliminated the effect of the hole by locally stiffening areas that experience high stresses. When the strength lagged behind the stiffness, optimum designs were similar to those found in bones, and relied on modulus distributions that direct the loads away from the hole.

Objective— To compare anatomic reduction and the biomechanical properties of a circular external skeletal fixator (CESF) construct to pin and tension band wire (PTBW) fixation for the stabilization of olecranon osteotomies in dogs. Study Design— Cadaveric study. Animals— Forelimbs from 12 skeletally mature mixed-breed dogs, weighing 23 to 28 kg. Methods— An olecranon osteotomy was stabilized with either a CESF construct or PTBW fixation. A single distractive load to failure was applied to each specimen through the triceps tendon. Osteotomy reduction and biomechanical properties were compared between fixation groups. Results— Reduction was not significantly different (gap: P= .171; malalignment: P= .558) between fixation groups. Osteotomies stabilized with the CESF had greater stiffness (P < .0001) and maximum load sustained (P < .0001) compared to PTBW fixation. There was no significant difference for yield load (P= .318) or for load at 1 mm of axial displacement (P= .997) between fixation groups. Failure of fixation occurred by bending of the intramedullary Steinmann pin and the fixation wires in the CESF specimens and by untwisting of the tension band wire knot with pullout and bending of the Kirschner wires in the PTBW specimens. Conclusions— Specimens stabilized with the CESF construct had similar reduction and yield load, greater stiffness and maximum load sustained, and less elastic deformation than specimens stabilized with PTBW fixation. Clinical Relevance— The CESF construct may provide a biomechanically favorable alternative to PTBW fixation for stabilization of olecranon osteotomies in dogs, and its application warrants clinical investigation.

The objective of this cadaveric biomechanical study was to establish further bovine spines as models for evaluating lumbar interbody allografts and to provide guidance for their use in pediatric humans. It is unknown whether interbody allografts can be used in the pediatric spine without failure of the host vertebral bone. Allografts were placed in cow and calf spines and loaded in compression. The cow spines were much stronger and stiffer than the calf, but moderate in vivo activities were estimated to result in loads on the allograft constructs that would result in host bone failure. Bovine spines were established as suitable models for the compressive behavior of interbody allografts in the human spine, when bone density is considered. Interbody allografts should continue to be used with adjunctive instrumentation so as to preclude host bone failure.

Bones do not normally fracture near blood vessel holes. This in part may be related to spatial distribution of porosity, mineralization, and microstructure observed near the holes, as these parameters are known to affect bone stiffness and strength. Material parameters representing physical variables are optimized in this paper using a simple two-dimensional model of the bone in an attempt to duplicate the observed material distributions. The objective is to minimize the failure index defined (stress-strength ratio) under a given uniaxial load. In addition to tailoring the elastic properties (by changing porosity and mineral density), and orthotropy (via optimizing material orientation) the study also investigates the effect of the shape of the hole. Of the three variables, shape was the most effective in minimizing the failure index, followed by tailoring of the elastic properties and lastly material orientation. The elastic modulus distribution in the optimum design was similar to that observed around foramen holes in bones. The contributions of the physical variables in minimizing the failure index were almost independent of each other. The optimization was able to reduce the failure index compared to a hole in a homogeneous plate but was not sufficient to prevent failure at the hole. This can be attributed to the limitations of the shape functions used to describe the spatial distribution of mineralization and volume fraction and limitations in models used to relate composition and microstructure to elastic properties. The shape functions used to describe the spatial distribution of mineral mass fraction and volume fraction and the constraints imposed on them resulted in a non-convex design space.

The larger natural holes in bone are not involved in failure because of the special material distribution in their vicinity. In this paper, a procedure was developed that designs the nonaxisymmetric functionally graded material distribution around a hole in a plate in an attempt to mimic bone for increased strength at small weight increase by minimizing a failure index (stress/strength ratio). Nonaxisymmetric graded material distributions were represented by a generic continuous function of eight design variables. The results show that the optimal nonaxisymmetric functionally graded plate with a hole has a material distribution pattern similar to that found in bone, and the load-carrying capability of the plate can be greatly increased at small hole weight ratio#.

We discovered that the foramen—a natural hole in bone—is a structurally optimized feature. Through a heterogeneous arrangement of its composition, the foramen has shifted the highest stress away from the hole edge into a region of higher strength. We used these natural principles to design plates with holes, and demonstrated such increased performance in actual test articles. Most recently, we found the foramen to be tougher and we are currently applying this knowledge in tougher designs of fibrous materials. Our work is beneficial for the design of engineering structures with discontinuities.

SummaryThis study evaluated the effects of fixation wire tension (0 kg, 30 kg, 60 kg, and 90 kg) on the biomechanics of a four ring asymmetric circular external fixator construct in four testing modes (axial compression, medio-lateral bending, craniocaudal bending, and torsional loading) using a gap fracture model. Wire tension had a significant direct effect on gap stiffness in all of the testing modes. Axial compression load-deformation curves exhibited non-linearity, characteristic of the self-tensioning effect observed with fine wire fixation. Bending loaddeformation curves had two discrete linear segments, attributable to slipping of the bone models on the fixation wires, once a critical bending moment was exceeded. Torsional loading caused a gradual ‘windup’ of the construct which was followed by a linear load-displacement curve. Increasing wire tension had a small but significant effect on gap stiffness, which generally diminished as higher tensions were applied, thus demonstrating the need for adequate initial construct design to achieve sufficient stability during fracture healing.

Objective—To compare radius of curvature along the ulnar trochlear notch of Rottweilers and Greyhounds to determine whether morphologic differences exist that may contribute to the cause and pathogenesis of fragmented coronoid process in Rottweilers. Sample Population—Paired elbow joints from 13 Rottweilers and 14 Greyhounds. Procedure—Elbow joints were radiographically scored on the basis of severity of osteoarthritic lesions. The articular contour of each ulnar trochlear notch was digitized. The radius of curvature at defined points along the ulnar trochlear notch was compared between breeds. Results—Radius of curvature of the ulnar trochlear notch was not a constant function of arc length in either breed but had a consistent characteristic appearance in both breeds. Radius of curvature was greatest at each end of the ulnar trochlear notch and had 2 peaks in the midportion of the notch in both breeds. These peaks occurred farther distally in the notch and were larger in Rottweiler ulnae than Greyhound ulnae. A significant difference in mean radius of curvature was detected between breeds at these peaks. Greyhounds had significantly greater mean radius of curvature at the end of the medial coronoid process, compared with Rottweilers. Conclusions and Clinical Relevance—Radius of curvature of the ulnar trochlear notch is a complex function of arc length in Rottweilers and Greyhounds. The waveform has a consistent characteristic appearance in both breeds. Although significant differences were identified between breeds, associations between these differences and cause or pathogenesis of fragmented coronoid process in Rottweilers were not apparent. ( Am J Vet Res 2001; 62:968–973)

Human cadaveric occipitocervical specimens were implanted with three types of instrumentation. The devices were tested biomechanically under three modes of loading to determine the stiffness of spinal constructs and the failure mechanisms of the constructs under extreme flexion. The devices tested were the AXIS Fixation System (with custom plate), the Y-Plate, and the Luque rectangle. No significant differences in stiffness among the devices were found under compression and flexion. The stiffnesses of the plate systems were statistically higher than the Luque rectangle in extension and torsion. In extreme flexion, the plate systems failed by fracture of the C2 pedicles. Modern plate systems, for occipitocervical fixation, provide more stiffness and stability than traditional wiring techniques. This study provides surgeons with information on the relative merits of modern plate and screw systems compared with traditional rod and wire constructs.

A novel approach is developed for mathematically modeling the variability observed in experimentally determined elastic moduli of longitudinally oriented fibrous tissues such as ligaments and tendons. The elastic modulus of these tissues is modeled with a rule of mixtures (ROM) where each parameter (fibril and matrix moduli and fibril volume fraction) is assumed to be an independent random variable. A joint density function formed from the independent densities results in a probabilistic ROM (pROM). This pROM is used to generate a distribution of moduli which agrees well with moduli determined from tests of rabbit medial collateral ligaments (Woo and Ohland, 1994, Unpublished experimental data as gift). Minimizing the error between the pROM and experimental distributions resulted in an integrated error of 9% for a constrained set of independent distribution parameters derived from the literature. This pROM thus incorporates microstructural observations (fibril and matrix moduli and fibril volume fraction) to partially explain the experimentally observed variability in a macroscopic property (tissue modulus).

Study design Cadaveric human and bovine lumbar spine models simulating the acute postoperative period were used to compare the biomechanical properties of two designs of intervertebral body threaded fusion cages. The instrumented spines were compared with intact spines and with spines with resected posterior elements, representing a revision case. Objective To determine the relative biomechanical performance of these competing devices. Summary of background data These cages are currently under clinical investigation, and basic biomechanical data are needed. Methods Insertion torques and maximum pushout loads were measured for each cage. Intact spines, posteriorly instrumented spines (posterior lumbar interbody fusion), and spines with resected posterior elements were loaded in axial compression, flexion and extension bending, and axial torsion. Stiffness comparisons were made between the different configurations. Results Insertion torques and pushout loads were similar for the cages. Both cages significantly increased stiffnesses above those of the intact spines and the resected spines. The BAK-instrumented spines were more stiff in axial compression, while the Threaded Interbody Fusion Device spines were more stiff in extension. Conclusions This study revealed the two cages to have similar biomechanical characteristics immediately after posterior insertion and warrant further clinical studies.

Study design To evaluate the fatigue strength and stiffness of four anterior thoracolumbar fixation devices using a corpectomy model without load-sharing bone graft to test the devices under the worst case scenario of instability. Objectives To gain a more thorough understanding of the biomechanical qualities of anterior fixation devices to improve clinical application and design. Summary of background data For many surgeons, the anterior approach has become the treatment of choice for patients with compression of the spinal cord, whether it is caused by trauma, tumor, or infection. When stabilization is needed, anterior fixation devices have been advocated for many years to avoid the additional approach required for posterior fixation. Many of these devices, however, have an unacceptably high rate of hardware failure. Recently, several new devices for anterior fixation have been marketed with purported advantages in fatigue life and ease of use. Methods Four implants, the Synthes Anterior Thoracolumbar Locking Plate, the Kaneda device, a Texas Scottish Rite Hospital anterior construct, and the Z-Plate were attached to vertebral models and tested for stiffness in multiple planes on a modified Materials Testing System machine. They then were fatigued to failure on an Instron testing machine. Results The Anterior Thoracolumbar Locking Plate was the stiffest in axial compression, lateral flexion, and torsion. The Texas Scottish Rite Hospital anterior construct was the least stiff in flexion-extension, with no significant differences in the stiffness of the anterior thoracolumbar locking plate, that of the Kaneda device, and that of the Z-Plate. Fatigue life exceeded 80,000 cycles for the anterior thoracolumbar locking plate and averaged 26,472 cycles for the Z-Plate, 6915 cycles for the Teas Scottish Rite Hospital construct, and 4419 cycles for the Kaneda device. Conclusions The significantly greater fatigue life of the Anterior Thoracolumbar Locking Plate and the Z-Plate may predict a lower incidence of hardware failure than with previous anterior devices. This has been confirmed in preliminary clinical studies with the Z-Plate. Further clinical studies are needed to show if these lower failure rates will continue over a long-term period.

Primate canines function in social displays but are also recruited for biting in agonistic encounters. Although the precise nature of the loads such behavior places on the canine crown is unknown, it is thought that bending is a major component of such loads. To date, modeling of canine bending strength has relied on idealized geometric representations. Accounting for the tapering of the crown as well as shape changes along an apical-basal axis provides a more realistic model for assessment of bending stress in canines. We provide such an accounting and evaluate the hypothesis that the morphology of the cercopithecoid canine represents a structural solution for maintaining constant maximum bending stress under apical or distributed loading in a parasagittal plane. This isostress hypothesis is analogous to a design criterion of minimum mass for a given structural requirement. Examining permanent maxillary canines from males and females representing eight West African cercopithecoid species, we reconstructed crown geometry from apex to base using microcomputed tomography. From reconstructed cross-sections, we determined section moduli about a buccolingual centroidal axis. We then determined what the taper should be for a variety of parasagittal loading distributions if the isostress hypothesis were true and compared these theoretical tapers to actual crown geometry. We found that a variety of loading distributions can be accommodated by the canines, particularly among males. These results suggest that our sample of canines are not optimized for resisting particular biting loads, but effectively limit stress gradients associated with a range of behaviors.

This study compared three methods of gluteus medius tendon attachment to an allograft/endoprosthetic composite of the proximal 25% of the femur in a canine model. The three methods were bone to bone, tendon to bone, and tendon to tendon attachment. In an in vivo study, 24 dogs were assigned to three groups of eight dogs each, and serial radiography and weight-bearing analyses were performed throughout the study. The dogs were killed at 6 months, and the specimens were tested in tension to failure and were analyzed histologically. In an in vitro study, each repair was done on six limbs, with a contralateral limb serving as a control for each. In these specimens, the bone to bone attachments were significantly stronger (99.1% of the controls) than the tendon to bone attachments (71.8% of the controls) and the tendon to tendon attachments (40.0% of the controls); there were no differences in tensile stiffness among the three types of attachment. By 6 months, the tensile strength of the tendon to tendon attachments increased significantly and that of the tendon to bone attachments decreased significantly. There were no significant differences in tensile strength among the three types of attachment. The tensile stiffness of the bone to bone attachments (91.0% of the controls) was significantly greater than that of the tendon to bone attachments (40.8% of the controls) but not significantly different from that of the tendon to tendon attachments (63.2% of the controls). The bone to bone attachment was associated with increased bone resorption, bone remodeling, and bone porosity, accompanied by thinner allograft cortices, when compared with the other types of attachment. In dogs with a bone to bone attachment, weight-bearing increased more slowly than in dogs with either of the other two attachments. These changes associated with the bone to bone attachment may merely be secondary to healing of the bone to bone attachment to the greater trochanter; therefore, they may only be temporary phenomena or they may be the portents for long-term complications. Longer term studies of at least 1-2 years must be performed before these questions can be answered.

Study design An in vitro biomechanical study using a simulated anterior cervical discectomy and interbody fusion model to compare the load sharing properties of two semiconstrained cervical (Premier and Zephir) plates. Objectives To determine the percent load transmission through these plates and grafts under simple axial compression. Summary of background data No published data exist as to the load transmission through these semiconstrained plates. Methods Cadaveric calf spines were subjected to axial compression loading while instrumented with an interbody graft and with the graft plus one of the plates. Load transmission was computed through an analysis of the load-displacement data. Results A mean load transmission of 23% was shared by the Premier plate. The Zephir, a more constrained plate but still semiconstrained, shared a mean of 32% of the load. Conclusions The semiconstrained plates tested allow more graft loading than some previously tested constrained plates. However, there are differences between the research methods used in these studies that provide a less than satisfactory comparison.

Alveolar bone has several mechanical functions, including tooth support and accommodation of occlusal and other masticatory forces. Its unique functional-mechanical environment is reflected by its structural characteristics, but whether alveolar bone is materially distinct from bone elsewhere in the primate facial skeleton is uncertain. This uncertainty is attributable not only to a limited amount of data but also to conflicting findings among these data. We evaluated elastic modulus variation in the mandibular corpus of eight adult specimens of the monkeys Procolobus badius and Colobus polykomos via microindentation to evaluate whether alveolar bone is more compliant than basal bone and to quantify patterns of variation between sexes and species. We sampled Vickers hardness from six serial transverse sections and one coronal section from both the alveolar process and the basal corpus. Hardness values were converted to elastic modulus via regressions specific for bone tissue. Analysis of variance reveals that a plurality of variation is found on a regional scale; i.e., alveolar bone is more compliant than adjacent basal bone. Species affiliation and sex are not significant sources of variation. These findings support a hypothesis that compliance of alveolar bone represents a material solution for avoiding large stress concentrations arising from occlusal loads. Other comparative data suggest important differences between colobine and cercopithecine mandibles in terms of bone stiffness, both overall and in terms of relative stiffness of alveolar and basal cortical bone.

We characterized the heterogeneous ani- sotropic elastic properties of mandibular bone in an adult female specimen of Macaca fascicularis using the technique of microindentation. This approach used an in- denter of known mass and geometry to sample bone hardness at a spatial resolution in the order of 100 lm. Hardness values were converted to elastic modulus using empirically derived regression. We determined properties in alveolar, midcorpus, and basal regions of coronal and transverse sections taken from multiple locations along the corpus and ramus. Within sections, we determined properties from endosteal, midcortical, and periosteal regions. We found regional variations in bone structure, including bands of orthotropic circumferential lamellar bone at the endosteal and periosteal corpus base, angu- lar region, and ramus. Transversely isotropic osteonal bone characterizes the midcortices of alveolar and basal regions, with many resorption spaces in alveolar regions restricting sampling opportunities. Regional variations in elasticity include relatively compliant bone in the an- terior corpus and ramus. Basal cortical bone is stiffer longitudinally than transversely or superoinferiorly, while the evidence for directional dependence in alveolar bone is equivocal. Alveolar bone appears to be relatively compliant with respect to bone found in midcorpus or ba- sal regions. Considerable variation exists in structure and material properties on a highly localized scale, more so than is discernible through conventional approaches for determining material property variation. Am J Phys Anthropol 135:100-109, 2008. V C 2007 Wiley-Liss, Inc.

Objective— To determine the change in tibial plateau angle (TPA) during healing after tibial plateau leveling osteotomy (TPLO) performed for cranial cruciate ligament insufficiency in dogs and to examine factors that may be associated with the change. Study Design— Retrospective study. Study Population— One hundred and forty-nine canine stifles after TPLO procedure. Methods— Records of dogs that had TPLO were reviewed. Patient age, weight, sex, breed, pre- and postoperative TPA, recheck TPA, time to recheck, type of implant used, and radiographic evidence of healing were analyzed. Results— Mean time to recheck evaluation was 46 days (range, 28–65 days). Mean difference between immediate postoperative and recheck TPA measurements was 1.5° (range, −3 to 9°). Recheck TPA was a significantly greater (numerically higher) than immediate postoperative TPA (P

An orthotropic index (OI) is proposed to indicate the existence of a preferred material direction in each of the symmetry planes of an orthotropic material such as bone. Currently, this function is performed by the anisotropy ratio (AR) of any two Young's moduli or compressive (A(c)) and shear (A(s)) anisotropy factors comprised of complicated functions of the elastic constants. The OI incorporates the four independent engineering constants (the shear modulus and Poisson's ratio in addition to the two Young's moduli) in each symmetry plane into a single index. The OI thus improves upon the AR by reflecting orthotropy in a more holistic sense and upon the AR, A(c) and A(s) by taking on a unique value (zero) only when the material is in fact isotropic.

Microindentation is an investigational tool often used to determine hardness and other derived material properties of the material bone. This study explored the variation of microindentation hardness results with five independent variables. The variables were: applied mass, dwell time, drying time, time between indentation and measurement, and distance between the center of an indentation and the edge of other indentations and pores. These variables were selected because they represented a reasonable range of specimen investigational steps. We also investigated the cross sections of typical indentation residual impressions to determine the degree of material pile-up at the edges of the impressions. We found that microindentation hardness varied with applied mass and with distance between the indentation and neighboring indentations and pores but not with the other variables. Our recommended minimum applied mass is 0.10 kg versus a previously published value of 0.05 kg. We also found no discernable material pile-up at the residual impression edges, in contrast to reports of others.

Objective—To evaluate mediolateral, axial, torsional, and craniocaudal bending behavior of 6 distal ring-block configurations commonly used to stabilize short juxta-articular bone segments in small animals.Sample Population—8 circular external skeletal fixator constructs of each of 6 distal ring-block configurations. The distal ring-block configurations were composed of combinations of complete rings, incomplete rings, and drop wires.Procedure—Constructs were nondestructively loaded in axial compression, craniocaudal bending, mediolateral bending, and torsional loading by use of a materials testing machine. Gap stiffness was determined by use of the resultant load displacement curve.Results—Circular external skeletal fixator configurations and constructs significantly affected gap stiffness in all testing modes. Within each loading mode, gap stiffness was significantly different among most configurations. In general, complete ring configurations were significantly stiffer than similar incomplete ring configurations, and addition of a drop wire to a configuration significantly increased stiffness of that configuration.Conclusions and Clinical Relevance—When regional anatomic structures permit, the use of complete ring configurations is preferred over incomplete ring configurations. When incomplete ring configurations are used, the addition of a drop wire is recommended. (Am J Vet Res2004;65:393–398)

This study was designed to compare the biomechanical and functional characteristics of allograft/endoprosthetic composites of the proximal 25% of the femur repaired with either a transverse or a step-cut osteotomy, using a canine model (10 dogs, five with each type of osteotomy). Serial radiography and weight-bearing studies were performed monthly, and mechanical testing was done 6 months after surgery. The femora were tested in torsion and compared with the contralateral control (insertion of a femoral component but no osteotomy). At 6 months, the composites with a step-cut osteotomy had 36% greater structural stiffness than the composites with a transverse osteotomy (p < 0.005) and 121% greater maximum torque at failure than the controls (p < 0.005), without greater structural stiffness. Evaluation of peak vertical ground reaction forces revealed significantly greater weight-bearing on the experimental limb in dogs with a transverse osteotomy. The results of this relatively short-term study were mixed. Despite the increased structural stiffness of the allograft/endoprosthetic composite with a step-cut osteotomy, the dogs with this type of reconstruction had decreased weight-bearing throughout the course of the study. The step-cut osteotomy may augment the stability of the allograft/endoprosthetic composite, allowing faster healing (as demonstrated by the results of mechanical testing), but in some way, not understood, may cause pain in the reconstructed limb. Longer term studies are needed to answer these questions and to determine whether alteration of the traditional transverse osteotomy has any advantage.

We investigated the microstructural basis of a reduced stress concentration around the primary nutrient foramen of the equine third metacarpus. We quantified the spatial variations of compositional parameters (mineral content, volume fraction, histological architecture, and osteonal trajectories) from microradiographs and polarizing microscopic images of thin sections. These variations in composition and organization in turn cause variations in mechanical properties of cortical bone. We modeled the spatially inhomogeneous anisotropic elastic properties based on the measured compositional parameters and used the properties as inputs to a finite element model of the bone containing the foramen. This model, spatially constructed solely from the microscopic images, was subsequently validated by our mechanical test results. We found that: (1) a primary mechanism for stress concentration reduction appears to be due to an increased compliance near the foramen: the sharp discontinuity represented by the hole is softened by embedding it in a compliant region; (2) a reinforcing ring of increased stiffness exists at some distance from the foramen; and (3) a ring of lamellar bone exists along the foramen inside edge, which might serve to reduce the chance of cracks forming there. Our work is allowing us to design biomimetic structures with holes by mimicking the microstructure near the nutrient foramen.

Unlike in engineering structures, in nature, holes in bones are not normally involved in failure. This may be partly due to special material orthotropy distribution pattern in the vicinity of the hole. Material orthotropies have a dramatic influence on the strength of composite plates with holes. In this paper, a procedure for optimally determining the material orthotropies around a hole in a composite plate is presented. The material is assumed to have constant transversely isotropic elastic properties. Piecewise bilinear interpolation functions formed by quadrilateral elements are used to represent continuous material orthotropy distribution. Normalized Tsai-Wu failure index is minimized through genetic algorithm-based two-step optimization technique. The results show that the strength reduction due to hole in a composite plate can be dramatically alleviated by material orthotropy optimization near the hole. The optimized material orthotropies are similar in pattern to those found in bone around a foramen (blood vessel hole).

Objective— To compare the safety and accuracy of Steinmann pin placement in vertebral bodies T10 through L7 using either an open or closed fluoroscopic method. Study design— In vitro radiographic and anatomic study. Animals— Ten medium-sized canine cadavers. Methods— Cadavers were randomly assigned to 2 groups: open and closed. Steinmann pins were placed in vertebral bodies through a standard dorsal incision in the open group and percutaneously with the aid of fluoroscopy in the closed group. Pins were placed bilaterally in vertebral bodies T10 through L7 at approximately 30° from horizontal and driven to a uniform depth. Necropsies were performed to examine potential pulmonary, vascular, or neurological trauma as a result of pin placement. Spines were cross-sectioned through intervertebral disc spaces, and radiographs were performed to evaluate accuracy of pin placement. Descriptive statistics were determined for pin angle, percentage of bone purchase, and penetration length. Means of interest between groups were compared using a Student t test. Complication incidence was compared using χ2 analysis. Significance was P < .05. Results— Mean pin insertion angle was significantly different than 30° for the open group in thoracic and lumbar vertebrae and for the closed group in thoracic vertebrae. Mean pin insertion angle for all vertebrae was significantly greater than 30° for the open group. Mean pin penetration distance in each vertebra was significantly different between groups with the closed group having less penetration and lower variance. Both groups were significantly different from the ideal penetration distance. The mean percentage of bone purchase was greater in the closed group for all vertebrae except T10 and T11. The complication incidence was significantly greater in the open group for thoracic vertebrae. Conclusion and clinical relevance— The results of this study suggest that a closed technique for placement of Steinmann pins in lumbar vertebrae for use in external skeletal fixation is a reasonable and safer alternative to the traditional open technique. Use of either technique in thoracic vertebrae should be avoided.

Holes in engineering structures cause stress concentrations that often lead to failure. In nature, however, the holes (foramina) in load-bearing bones are not normally involved in structural failures. In previous work, this was linked to the material distribution near the hole. In the present paper, a design procedure was developed which designs the elastic modulus distribution around a hole in an inhomogeneous isotropic plate in an attempt to mimic a bone with a foramen. Bezier curves were used to model inhomogeneous material distributions. A non-dimensional normalized failure index was minimized. Different materials, load conditions, near field widths, and failure models were considered. The results show that the optimal inhomogeneous plate with a hole has a material distribution pattern similar to that of a bone with a foramen, the stress varies smoothly, and the reduction of the strength due to the hole can be completely eliminated.

In this paper, we address the problem of maximizing the strength of a plate in which a circular hole is placed within a homogeneous material. We mimicked in a polyurethane plate the modulus distribution we found around a naturally occurring hole in bone by discretizing the panel into rings and matching modulus ratios (ring to given homogeneous material) between the bone and available materials. This biomimetic inhomogeneous plate (IP) was then fabricated, tested under uniaxial tension, and compared to a homogeneous plate (HP) of the same geometry. Results from moire interferometry were in good agreement with our finite element (FE) analyses of both plates. With confidence in our FE models, we found that we could decrease the maximum failure index (roughly, the ratio of local stress to allowable stress) from near 3 in the HP to 1.5 in the IP (or, in other words, double the strength of the plate). We were able to decrease the weight of the IP by 45% compared to the HP of identical strength. INTRODUCTION. Holes are well known to be a serious design problem and a frequent site of fracture in structures. Traditional engineering solutions to this problem either involve massive reinforcements around the hole (often expensive in terms of weight and cost), or shape contouring to alleviate the effects of the hole (which is not always possible, e.g., most holes must be round). Some manufacturing techniques have met with limited success due to their difficulty in implementation in a production environment. Specifically, tow placement machines allow the variation of fiber orientations of composite materials in a continuous manner, which has already been shown to be a powerful way of alleviating stress concentrations near holes. New manufacturing technologies allow the production of functionally graded materials (FGMs), such as foamed metals with density profiles tailored to functional requirements. Natural holes (foramina) exist in practically all bones to allow blood vessels to pass through their hard outer shells. Foramina form at a very early fetal age, which gives the bone much time to adapt to their presence by arranging the local material microstructure. This adaptation was hypothesized by us to be the reason why foramina never appear as fracture sites clinically or in laboratory tests of whole bones. As part of our ongoing biomimetic structures research, we studied a naturally occurring hole in a horse bone in detail to characterize the material microstructure and spatial gradation in elastic properties around the hole[5]. We found that the foramen is embedded within a compliant zone of material with a bracket shaped stiffer region approximately two foramen diameters away from its edge. This functional gradation in elastic modulus was shown to reduce the stress concentration at the hole when compared to a hole in a homogeneous medium. Further optimization studies by us using the observed material microstructure and composition variables showed that spatial variations in microstructure in addition to reducing stress concentrations also increased the strength[6]. We recently tested beams made from bone containing a foramen and a manufactured hole of identical shape under four point bending. The manufactured holes were adjacent to the foramen, but far enough away to preclude interaction. The bending moments at the foramen and manufactured hole were identical. Failure occurred through the manufactured holes and not the natural hole, offering further evidence that the material design around the foramen makes it a stronger hole. Future investigations will include fatigue tests on similar beam specimens to explore the damage resistance and tolerance characteristics of the foramen. Given these encouraging results, the natural solution offered by the foramen is clearly worth investigating for use in engineering design. To that end, we constructed an inhomogeneous plate containing a hole with similar functional gradation in elastic modulus properties as observed in the bone. The elastic modulus variation was simplified to an axisymmetric design and was obtained by means of discrete rings of varying modulus. We also fabricated a homogeneous plate of the same geometry. The plates were tested under uniaxial tension and full field displacements acquired using moire interferometry. The strains obtained from the experiment were compared with finite element models (FEMs) of the plates and analytical solutions. The verified FEM was then used to calculate a stress failure index (akin to a safety factor) for the plate and compare it to that of a homogeneous plate with a hole. The objective of this paper is to describe the design, analysis, and results from mechanical tests of our inhomogeneous plate, and compare the results to the homogeneous plate. MATERIALS AND METHODS. The functionally graded plate made of discrete rings (Figure 1, left) was ordered from a commercial vendor (Pacific Research Laboratories; Vashon, WA). We also purchased bulk foam material of uniform density for fabricating the homogeneous plate (HP; Figure 1, right). The functionally graded biomimetic inhomogeneous plate (IP) is 10 cm wide by 17.5 cm long and 1 cm thick with a centrally located 2 cm diameter hole. The base material is polyurethane foam. The IP consists of six annular rings surrounding the central hole, each of a different apparent density (porosity) and, hence, modulus and strength. Each ring is 5 mm wide from its inside to outside edge. The ring next to the hole is the same material as that of the far field, and the other ring densities are such that the modulus and strength distribution matches what we observed in bone as well as possible given the available materials (Figure 2). The rings are bonded within the plate with the same polyurethane resin that is used to make the foam. The HP consists of the same material as the far field material of the IP. A diffraction grating was applied and then end tabs were glued to each plate for mechanical testing. The moire interferometry technique provides very accurate full field measurements of in plane displacements by use of optical interference principles. The interference pattern produced by a reference grating and deformed grating applied to the surface of the specimen is used to measure the strains on the surface of the specimen with high spatial resolution. The maximum resolution in strain measurements is a function of the grating pitch and wavelength of light used. We used a grating that could produce 2,400 fringes/mm with an illumination source of 632.8 nm wavelength. Details of moire interferometry for high accuracy strain measurements are presented elsewhere [4]. We used this technique to acquire displacement fields from which we derived the longitudinal strain distribution along a line through the hole in each plate. We needed to develop a methodology for applying a compliant diffraction grating to the plates to use the moire technique. This was challenging due to the porous nature of the foams used in manufacturing our plates. Beginning with techniques developed by others[3] and after much experimentation on spare material, we settled on the following process. First, we lightly sanded the plate surface using 300 grit and 600 grit sandpaper. After forcing dust out of the pores with compressed air, we applied a silicon rubber primer (GE Silicone SS4120) to the sanded surface and left it to cure overnight. This was followed by an application of a two part silicone rubber (GE Silicone Rubber RTV615). A glass square with a sheet of polytetrafluoroethylene (PTFE) was placed on the plate with the silicon rubber mixture. To ensure flatness and to speed cure time, the plates were then sealed in an airtight bag, attached to a vacuum line and placed in an oven. Finally, we applied a dental impression material (Reprosil Type I Low Viscosity; DENTSPLY International; Milford, DE) to receive the replication grating. The grating covered one quadrant of the plate around the hole, as this was the biggest field of view possible within the interferometer. Testing was performed within 24 hours, before the specimen grating warped due to hygral effects. Uniaxial tension tests were performed on an IP and a HP using a custom fixture designed to provide stress free boundary conditions everywhere except in the vicinity of the grips. Uniaxial tension was applied to each plate in displacement control, incrementally to 76 N. This small load was chosen to keep the maximum stress below the yield stress of the materials in the plate and produce a far field strain on the order of 1,000 μe. The uniaxial load was monitored with an in-line load cell. Photographs of the fringe patterns were obtained at various load levels. A plane stress FEM was created for verification of analysis models and material properties. Due to the two fold symmetry in loading and geometry, only one quarter of the plate was modeled with symmetry boundary conditions. The Figure 1. Inhomogeneous plate (IP) containing a hole (left). The hole diameter is 2 cm, and the width of each ring is 5 mm. The homogeneous plate (HP) is of same dimensions as the IP and made of same material found Figure 2. Comparison between longitudinal modulus distributions as designed in our inhomogeneous plate (IP) and as found near the natural hole in bone. Plotted is the ratio of the local modulus to the modulus at the hole edge versus the normalized distance from the hole. 0 2 4 6 8 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Distance from Hole Edge R at io o f L oc al to H ol e Ed ge M od ul us

Thoracic spine stabilization after trauma or in tumor reconstruction cases frequently is performed with hook and rod internal fixation systems, the use of which is not always possible. Pelvic reconstruction plates with pedicle screw fixation offer an alternative to hooks and rods. In this study, we biomechanically compared a plate construct with a hook and rod system in an acute postoperative, unstable thoracic spine model. We found that the hook and rod system offered more resistance to flexion and extension bending than the plate construct; the opposite was true for lateral bending and axial torsion. We further determined that the addition of pars interarticularis screws to the plate construct provided increased resistance to all loading modes. Our study indicates that plate constructs can effectively stabilize the thoracic spine.

This is a retrospective study of 83 consecutive adult patients with isthmic spondylolisthesis who underwent identical decompressive surgery combined with posterolateral spine fusion. We sought to determine factors that affect the fusion rate and clinical outcomes for adult patients with isthmic spondylolisthesis. The outcome of operative treatment for isthmic spondylolisthesis in adults has been poorly documented, as opposed to the treatment of children and adolescents. From 1989 to 1994, 83 consecutive adult patients (age 19-66 years; average, 38 years) underwent surgical treatment consisting of the Gill procedure and posterolateral fusion for isthmic lumbosacral spondylolisthesis. Seventy-three patients (46 men and 27 women) were available for an average of 3.8 years' follow-up (1.0-7.4 years). Thirty-eight underwent one-level fusion, and 35 underwent two-level fusions. Pedicle screw instrumentation was performed in 69 patients. A postoperative questionnaire including the Roland index, clinical charts, and radiographs were reviewed by an independent observer to assess the postoperative course, clinical results, and fusion status. Twenty-five variables were evaluated to determine which affected the fusion and success rates. Primary radiologic fusion and clinical success rates were 78 and 71%, respectively. There was a strong positive correlation between radiologic fusion and clinical success. Overall, single-level fusions showed an 82% fusion rate, and two-level fusions, a 74% rate. For two-level fusions, a significantly higher fusion rate was achieved with a rigid pedicle screw-fixation system than a semirigid system (79 vs. 57%). For smokers, cessation from smoking postoperatively did not increase the fusion rate, and patients who continued to smoke after surgery showed a significantly higher rate of pseudarthrosis. Worker's compensation status did not affect clinical results significantly. Patients who continued to take nonsteroidal antiinflammatory drugs (NSAIDs)3 months postoperatively showed significantly lower fusion and success rates (44 and 37%). Single-level lumbar fusion for isthmic spondylolisthesis was equally effective with either rigid or semirigid pedicle screw instrumentation. For multilevel spine fusion in isthmic spondylolisthesis, rigid pedicle screw-fixation systems resulted in a high fusion rate. A smoking history or NSAIDs use postoperatively had strong negative influences on the fusion and clinical success rates.

STUDY DESIGN: The Alpine goat model for multilevel anterior cervical discectomy and fusion was used to analyze the use of an intervertebral fusion device to promote an arthrodesis after anterior cervical discectomy. Comparisons were drawn with biomechanical, histologic, and radiographic data. OBJECTIVES: To analyze the use of an intervertebral fusion device, with and without a bone graft substitute, to promote an arthrodesis anterior cervical discectomy. SUMMARY OF BACKGROUND DATA: In previous studies, the goat cervical spine has proven to be an excellent model for examining the healing of fusions using bone grafts, instrumentation, or bone substitutes. METHODS: Three-level anterior cervical dissectomies were performed on 21 mature Alpine goats. Three treatment groups of seven goats each were used. Group I used a standard titanium cervical BAK device filled with autogenous bone graft. Group II used a hydroxyapatite-coated BAK device filled with autogenous bone graft. Group III used a BAK device filled with recombinant human bone morphogenetic protein-2. RESULTS: Radiographically, no cages became displaced. Lucencies were seen around 3 of the 21 cages in Group 1, 4 cages in Group II, and none in Group III. Fluorochrome analysis revealed that the recombinant human bone morphogenetic protein-2-filled cages had an accelerated rate of bone growth around and through each cage-vertebral body interface at 3 weeks. A successful arthrodesis was also more likely with a recombinant human bone morphogenetic protein-2-filled cage (95%) than the hydroxyapatite-coated (62%) or the standard (48%) cage. Biomechanical stiffness testing did not reveal any statistically significant differences between the three groups. There was a tendency for successfully arthrodesed interspaces to be stiffer than those that were not. CONCLUSIONS: The use of a threaded intervertebral fusion cage, with or without hydroxyapatite coating, filled with autogenous bone graft provides a fusion rate that is slightly better than those previously reported using autogenous interbody bone grafts with or without plate stabilization. Recombinant human bone morphogenetic protein-2-filled cages resulted in a much higher arthrodesis rate and accelerated bone formation compared with either autogenous bone-filled BAK devices, or autogenous interbody bone grafts with or without plate stabilization.

By use of wire ropes as the transosseous component, an external skeletal fixator for the repair of long bone fractures in horses and cattle has been designed and tested in axial compression. Theoretical methods were used in the design process to size fixator components; however, our results suggest that conventional methods of analyzing the displacement of the transosseous component may not apply to wire ropes. Large pretensions in the wire ropes are necessary to obtain functional stiffnesses for fracture fixation. Therefore, a method was sought for terminating the ropes so that an appropriate pretension could be introduced into the rope through its interface with the fixator rings. Ropes were terminated by use of 5 methods and were tested in axial tension to failure. These methods included 3 copper sleeve arrangements, welded ends, and drum sockets. The drum sockets (57.6% of rope breaking strength) far exceeded the strengths provided by the copper sleeves (8.5 to 26.6%) and the welded ends (44.3%). Using the drum sockets, 5 rope configurations were assembled to the fixator, using wood blocks to simulate bones with a gap defect. The fixator was loaded in axial compression for each of the rope configurations, and stiffnesses were determined from measured axial displacement and applied load. The 4-ring fixator configuration, with 2 ropes at 60 degrees angular separation/ring, was the stiffest. In a worst case (gap) model, a mean axial compression load of 1,730 N was observed at 2 mm of displacement for a 4-ring fixator configuration.(ABSTRACT TRUNCATED AT 250 WORDS)

Basic research in canine mechanics must be performed to better understand the forces and moments the appendicular skeleton must withstand. This type of research may allow surgeons to make substantial advances in total joint replacement and fracture fixation design and may enhance our understanding of bone remodeling and fracture occurrence in relation to exercise and trauma. In our study, craniocaudal bending stiffness, mediolateral bending stiffness, axial compressive stiffness, and torsional stiffness of the humerus, femur, radius, and tibia of dogs was determined, using nondestructive bending, compression, and torsional tests. Entire diaphyseal and middiaphyseal properties of these long bones were evaluated. Bones also were tested to failure in torsion to quantify the failure properties of these long bones. Left to right variability was examined to validate the use of contralateral limbs as the control condition for experimental studies. There were no significant right to left differences in entire diaphyseal mechanical properties for any of the long bones, except for compressive stiffness of femurs. Homotypic differences in entire diaphyseal mechanical properties, if present, ranged from 8.0 to 35% for the 4 long bones (power = 0.8). For middiaphyseal mechanical properties, there were no significant right to left differences in the 4 long bones, except for craniocaudal bending stiffness of tibias. The homotypic differences in middiaphyseal mechanical properties, if present, ranged from 7.2 to 62% for the 4 long bones (power = 0.8). In all bones and loading modes, middiaphyseal stiffness was greater than entire diaphyseal stiffness (P0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Thoracic spine stabilization after trauma or in tumor reconstruction cases frequently is performed with hook and rod internal fixation systems, the use of which is not always possible. Pelvic reconstruction plates with pedicle screw fixation offer an alternative to hooks and rods. In this study, we biomechanically compared a plate construct with a hook and rod system in an acute postoperative, unstable thoracic spine model. We found that the hook and rod system offered more resistance to flexion and extension bending than the plate construct; the opposite was true for lateral bending and axial torsion. We further determined that the addition of pars interarticularis screws to the plate construct provided increased resistance to all loading modes. Our study indicates that plate constructs can effectively stabilize the thoracic spine.

Anterior discectomy and fusion with an interbody bone graft and anterior plate is a common procedure in cervical spine surgical management. However, the graft may be shielded from some mechanical loading by the plate. Mechanical testing was performed on six cadaveric calf spines that were subjected to a simulated anterior cervical discectomy and fusion with an interbody bone graft alone and with an anterior plate to determine the amount of load sharing between the graft and plate. The load-displacement data were used to compute the amount of load sharing between the graft and the plate as a continuous function of the applied axial compression load. Although the percent load transmitted through the graft decreased (53 to 41%) as the axial load increased (45 to 90 N), the magnitude of load transmitted through the graft increased (24 to 37 N), with corresponding intervertebral strains

Summary This is a retrospective study of 83 consecutive adult patients with isthmic Spondylolisthesis who underwent identical decompressive surgery combined with posterolateral spine fusion. We sought to determine factors that affect the fusion rate and clinical outcomes for adult patients with isthmic spondylolisthesis. The outcome of operative treatment for isthmic spondylolisthesis in adults has been poorly documented, as opposed to the treatment of children and adolescents. From 1989 to 1994, 83 consecutive adult patients (age 19–66 years; average, 38 years) underwent surgical treatment consisting of the Gill procedure and posterolateral fusion for isthmic lumbosacral spondylolisthesis. Seventy-three patients (46 men and 27 women) were available for an average of 3.8 years' follow-up (1.0–7.4 years). Thirty-eight underwent one-level fusion, and 35 underwent two-level fusions. Pedicle screw instrumentation was performed in 69 patients. A postoperative questionnaire including the Roland index, clinical charts, and radiographs were reviewed by an independent observer to assess the postoperative course, clinical results, and fusion status. Twenty-five variables were evaluated to determine which affected the fusion and success rates. Primary radiologic fusion and clinical success rates were 78 and 71%, respectively. There was a strong positive correlation between radiologic fusion and clinical success. Overall, single-level fusions showed an 82% fusion rate, and two-level fusions, a 74% rate. For two-level fusions, a significantly higher fusion rate was achieved with a rigid pedicle screw-fixation system than a semirigid system (79 vs. 57%). For smokers, cessation from smoking postoperatively did not increase the fusion rate, and patients who continued to smoke after surgery showed a significantly higher rate of pseudarthrosis. Worker's compensation status did not affect clinical results significantly. Patients who continued to take nonsteroidal anti-inflammatory drugs (NSAIDS) > 3 months postoperatively showed significantly lower fusion and success rates (44 and 37%), Single-level lumbar fusion for isthmic spondylolisthesis was equally effective with either rigid or semirigid pedicle screw instrumentation. For multilevel spine fusion in isthmic spondylolisthesis, rigid pedicle screw-fixation systems resulted in a high fusion rate. A smoking history or NSAIDs use postoperatively had strong negative influences on the fusion and clinical success rates.