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The goal of this study was to develop a nondestructive radial compression technique and to investigate the viscoelastic behavior of the rat tail disc under repeated radial compression. Rat tail intervertebral disc underwent radial compression relaxation testing and creep testing using a custom-made gravitational creep machine. The axisymmetric viscoelasticity and time-dependent recovery were determined. Different levels of hydration (with or without normal saline spray) were supplied to evaluate the effect of changes in viscoelastic properties. Viscoelasticity was found to be axisymmetric in rat-tail intervertebral discs at four equidistant locations. Complete relaxation recovery was found to take 20 min, whereas creep recovery required 25 min. Hydration was required for obtaining viscoelastic axisymmetry and complete viscoelastic recovery. [ABSTRACT FROM AUTHOR]

BACKGROUND: No data exist on the cutting efficiency of monopolar versus bipolar radiofrequency energy application systems on human meniscal tissue. PURPOSE: To compare the effects of monopolar and bipolar thermal energy systems on human meniscal tissue. STUDY DESIGN: Controlled laboratory study. METHODS: Fresh-frozen menisci were cut in cross-section into 180 pie-shaped specimens. A specially designed jig was used to consistently apply radiofrequency energy to the tissue under a constant 30-g force. Three different systems were tested at the low, middle, and high ranges, with application times of 1 and 3 seconds. Thermal effects were measured by image analysis microscopy. RESULTS: No significant differences in thermal effects were found with respect to energy output for each system. Both the individual system tested and the application time had statistically significant effects on thermal damage, with the individual system tested having a greater effect. The mean depths of thermal change produced by the Mitek (bipolar) device were 564 and 648 microm at 1 and 3 seconds applications, respectively. The Arthrocare device (bipolar) produced depths of 1444 and 1697 microm at 1 and 3 seconds. The Oratec device (monopolar) produced depths of 895 and 1057 microm, respectively. CONCLUSIONS: A differential thermal effect was created in the meniscal tissue by three commercially available radiofrequency systems. Within the parameters of the experiment, all three systems limited thermal damage to a depth of less than 2 mm. The results appeared to depend more on the particular system used, not whether it had monopolar or bipolar electrodes. CLINICAL RELEVANCE: These data imply reasonably safe (less than 2 mm) thermal changes in the meniscus after radiofrequency energy application from these three systems. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the effects of laser-induced collagen shortening on the biomechanical properties of the inferior glenohumeral ligament complex. Fifty-seven bone-ligament-bone specimens underwent uniaxial tensioning to 10% strain. Approximately half of the specimens then underwent 10% shortening by lasing using a holmium:yttrium-aluminum-garnet laser. Both groups were again tensioned to 10% strain, and then loaded to failure. Ultimate strain and yield strain were significantly higher in the lased specimens than in the nonlased specimens. No significant difference was found for ultimate stress, yield stress, or elastic modulus between the two groups. Failure of the ligament did not appear to occur in the lased areas. The load-to-failure results suggested that the strength of the ligament complex was not significantly compromised by this lasing protocol. [ABSTRACT FROM AUTHOR]

Objective Lumbar fusion performed through lateral approaches is becoming more common. The interbody devices are generally supported by supplemental posterior fixation implanted through a posterior approach, potentially requiring a second incision and intraoperative repositioning of the patient. A minimally invasive lateral interspinous fixation device may eliminate the need for intraoperative repositioning and avoid disruption of the supraspinous ligament. The objective of this in vitrobiomechanical study was to investigate segmental multidirectional stability and maintenance of foraminal distraction of a lateral interspinous fixation device compared to commonly used pedicle screw and facet screw posterior fixation constructs when combined with lumbar interbody cages. Methods Six human cadaver lumbar spine specimens were subjected to nondestructive quasistatic loading in the following states: (1) intact; (2) interspinous fixation device alone and (3) with lateral interbody cage; (4) lateral lumbar interbody cage with bilateral pedicle screws; (5) lateral lumbar interbody cage with unilateral pedicle screws; and (6) lateral lumbar interbody cage with facet screws. Multidirectional pure bending in 1.5 Nm increments to 7.5 Nm, and 7.5 Nm flexion-extension bending with a 700 N compressive follower load were performed separately with optoelectronic segmental motion measurement. Relative angular motions of L2-L3, L3-L4, and L4-L5 functional spinal units were evaluated, and the mean instantaneous axis of rotation in the sagittal plane was calculated for the index level. Foraminal height was assessed during combined flexion-extension and compression loading for each test construct. Results All implant configurations significantly restricted flexion-extension motion compared with intact (p < 0.05). No significant differences were found in flexion-extension when comparing the different posterior implants combined with lateral lumbar interbody cages. All posterior fixation devices provided comparable neuroforaminal distraction and maintained distraction during flexion and extension. Conclusions When combinedwith lateral lumbar interbody cages, the minimally invasive lateral interspinous fixation device effectively stabilized the spine and maintained neuroforaminal distraction comparable to pedicle screw constructs or facet screws. These results suggest the lateral interspinous fixation device may provide a favorable alternative to other posterior systems that require patient repositioning during surgery and involve a greater disruption of native tissues., Competing Interests: The authors have declared financial relationships, which are detailed in the next section., (Copyright © 2023, Hedman et al.)

Exogenous crosslinking has been shown to have potential for treating disc degeneration and back pain due to its ability to increase the strength and toughness of the annulus fibrosus, increase intervertebral joint stability, decrease intradiscal pressure, and increase fluid flow through the disc. Some results imply that crosslink augmentation may also lead to changes in the compressive load sharing properties of the disc. The objective of the present study was to evaluate directional stress distribution changes of the disc following genipin crosslinking treatment. Bovine lumbar motion segments were randomly divided into control and crosslinked groups. Annular strains were determined from simultaneous deformation measurements at various time points during compressive creep testing. Four stress components of the annulus were then calculated according to the previously measured modulus data. Immediately after the application of a 750-N compressive load, mean axial and radial compressive stresses in the crosslinked group were twofold higher than control means. Conversely, mean lamellae-aligned and circumferential tensile stresses of the crosslinked discs were 8- and threefold lower, respectively, compared to control means. After 1-h creep loading, the two compressive mean stresses in both the control and genipin-crosslinked specimens increased approximately threefold from their initial 750-N-loaded values. The two tensile mean stresses in the crosslinked group remained lower than the respective levels of the control means after creep loading. A greater proportion of annular compressive load support under compressive creep loading, with a commensurate decrease in both tensile stresses and strains, was seen in the discs following exogenous crosslink augmentation.

Study Design: In vivo examination of intradiscal pressure by quantitative discomanometry (QD)., Objective: To determine whether an injectable, exogenous crosslinking could acutely restore intradiscal pressure of stab-injured discs in vivo by short-term treatment., Summary of Background Data: Disc biomechanical performance depends on its integrity associated with the intradiscal pressure and mechanical properties. Genipin crosslink augmentation has demonstrated the in vitro biomechanical capability to improve intervertebral joint stability and increase mechanical properties of the annulus fibrosus., Methods: 4 lumbar discs on each of 8 swine were randomly assigned to 4 groups: intact, injured, untreated, and crosslinked. A 16G needle was stabbed into the annulus fibrosus to create the disc injury model. An injection of 0.33% genipin solution was delivered into the annulus to treat the injury. QD technique was performed to examine the intradiscal pressure for the intact and injured discs at the time of surgery, while untreated and crosslinked discs were measured 1-week postsurgery. 4 QD parameters were analyzed and compared across the 4 groups: leakage pressure and volume, and saturation pressure and volume., Results: The leakage and saturation pressures of the injured group were significantly lower than those of the intact group (P = 0.004 and P = 0.01, respectively). The leakage and saturation pressures of untreated discs were statistically equivalent to the injured levels, but with a 2-times higher saturation volume. Relative to the untreated group, the leakage pressure and saturation pressure of genipin-crosslinked discs had a 617% (P = 0.008) and a 473% increase (P = 0.007), respectively., Conclusion: A large disc injury produced by annular puncture immediately lowered intradiscal pressure when left untreated. Genipin crosslinking can restore intradiscal pressure acutely in vivo without any obvious morbidity associated with the injection.

Background Context: Intervertebral disc and facet joints are the two primary load-bearing structures of the lumbar spine, and altered loading to these structures may be associated with frontal plane spinal deviations., Purpose: To determine the load on the lumbar facet joint and intervertebral disc under simulated frontal plane pelvic obliquity combined loading, an in vitro biomechanical study was conducted., Study Design/setting: An in vitro biomechanical study using a repeated-measures design was used to compare L4-L5 facet joint and intervertebral disc loading across pure moment and combined loading conditions., Methods: Eight fresh-frozen lumbosacral specimens were tested under five loading conditions: flexion/extension, lateral bending, axial rotation using pure moment bending (±10 Nm), and two additional tests investigating frontal plane pelvic obliquity and axial rotation (sacrum tilted left 5° and at 10° followed by a ±10-Nm rotation moment). Three-dimensional kinematics, facet load, and intradiscal pressures were recorded from the L4-L5 functional spinal unit., Results: Sagittal and frontal plane loading resulted in significantly smaller facet joint forces compared with conditions implementing a rotation moment (p<.05). The facet joint had the highest peak load during the 10° combined loading condition (124.0±30.2 N) and the lowest peak load in flexion (26.8±16.1 N). Intradiscal pressure was high in lateral flexion (495.6±280.9 kPa) and flexion (429.0±212.9 kPa), whereas intradiscal pressures measured in rotation (253.2±135.0 kPa) and 5° and 10° combined loading conditions were low (255.5±132.7 and 267.1±127.1 kPa, respectively)., Conclusions: Facet loading increased during simulated pelvic obliquity in frontal and transverse planes, whereas intradiscal pressures were decreased compared with sagittal and frontal plane motions alone. Altered spinopelvic alignment may increase the loads experienced by spinal tissue, especially the facet joints., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Purpose: Quantify changes in the flexion--extension neutral zone of the intervertebral disc with injections of increasing genipin concentration., Methods: Bovine motion segments were treated with varying concentrations of genipin using bilateral injections of constant volume. After overnight static compression loading of the treated segments, anterior-posterior offset loading was used to simulate flexion-extension motion. Range of motion, neutral zone length, neutral zone stiffness, and an instability score were measured., Results: Injection of the disc annulus with increasing concentrations of genipin resulted in corresponding changes in flexion-extension neutral zone. A minimum concentration of 40 mM was needed to observe a significant change. The largest changes were observed with the 400 mM injection. Netural zone stability was the most sensitive of the metrics with a percent change of 48% at 40 mM and over 200% at 400 mM., Conclusion: This study establishes the efficacy of using injection delivery to affect disc joint mechanics and quantifies the dose response between injected genipin and the flexion-extension stability of the disc.

A new sensor array intended to accurately and directly measure spatial and time-dependent pressures within a highly curved biological intra-articular joint was developed and tested. To evaluate performance of the new sensor array for application within intra-articular joints generally, and specifically to fit within the relatively restrictive space of the lumbar spine facet joint, geometric constraints of length, width, thickness and sensor spatial resolution were evaluated. Additionally, the effects of sensor array curvature, frequency response, linearity, drift, hysteresis, repeatability, and total system cost were assessed. The new sensor array was approximately 0.6mm in thickness, scalable to below the nominal 12 mm wide by 15 high lumbar spine facet joint size, offered no inherent limitations on the number or spacing of the sensors with less than 1.7% cross talk with sensor immediately adjacent to one another. No difference was observed in sensor performance down to a radius of curvature of 7 mm and a 0.66±0.97% change in sensor sensitivity was observed at a radius of 5.5mm. The sensor array had less than 0.07 dB signal loss up to 5.5 Hz, linearity was 0.58±0.13% full scale (FS), drift was less than 0.2% FS at 250 s and less than 0.6% FS at 700 s, hysteresis was 0.78±0.18%. Repeatability was excellent with a coefficient of variation less than 2% at pressures between 0 and 1.000 MPa. Total system cost was relatively small as standard commercially available data acquisition systems could be utilized, with no specialized software, and individual sensors within an array can be replaced as needed. The new sensor array had small and scalable geometry and very acceptable intrinsic performance including minimal to no alteration in performance at physiologically relevant ranges of joint curvature., (Copyright © 2011 IPEM. Published by Elsevier Ltd. All rights reserved.)

Study Design: In vitro study of genipin crosslinking effect on disc water content changes under compressive loading and unloading., Objective: To investigate the influence of collagen crosslinking on hydration and fluid flow in different regions of intact discs, and to evaluate the nutritional implications., Summary of Background Data: Age-related reductions of nutrient supply and waste product removal are critically important factors in disc pathogenesis. Diffusion and fluid flow are blocked by subchondral bone thickening, cartilaginous endplate calcification, loss of hydrophilic proteoglycans, and clogging of anular pores by degraded matrix molecules. Previous studies demonstrated increased hydraulic permeability and macromolecular transport through crosslinked collagenous matrices. Genipin has also demonstrated the capability to increase retention of proteoglycans., Methods: A total of 57 bovine lumbar motion segments were divided randomly into phosphate buffered saline and 0.33% genipin-soaked treatment groups. Water content changes were measured using a mass-loss technique in 3 intervertebral disc regions following successive stages of compressive loading and unloading (post-treatment, after 1 hour 750 N compression, and after a subsequent 24-hour period of nominal loading). Net flow of fluid into or out of a region was determined from the percentage change in mean water content from successive groups., Results: Fluid flow to and from the nucleus doubled with genipin crosslinking. Relative to the buffer-only controls, overall net fluid flow increased 103% in the nucleus pulposus, 36% in the inner anulus, and was 31% less in the outer anulus of genipin treated discs., Conclusion: The effects of genipin crosslinking on matrix permeability and proteoglycan retention can alter hydration levels and fluid flow in the intervertebral disc. Resulting increases in fluid flow, including a doubling of flow to and from the nucleus, could lead to enhanced nutritional inflow and waste product outflow for the disc, and may have implications for emerging cell-based therapies.

This study involves a biomechanical evaluation of a prospective injectable treatment for degenerative discs. The high osmolarity of the non-degenerated nucleus pulposus attracts water contributing to the hydrostatic behavior of the tissue. This intradiscal pressure is known to drop as fluid is exuded from the matrix due to compressive loading. The objective of this study was to compare the changes in intradiscal pressure in control and genipin cross-linked intervertebral discs. Thirty bovine lumbar motion segments were randomly divided into a phosphate-buffered saline control group and a 0.33% genipin group and soaked at room temperature for 2 days. A needle pressure sensor was held in the center of the disc while short-term and static creep compressive loads were applied. The control group demonstrated a 25% higher average intradiscal pressure compared to genipin-treated discs under 750 N compressive load (p=0.029). Depressurization during static compressive creep was 56% higher in the control than in the genipin group (p=0.014). These results suggest cross-linking induced changes in the poroelastic properties of the involved tissues affected the mechanics of compressive load support in the disc with lower levels of nucleus pressure, a corresponding decrease in the elastic expansion of the annulus, and an increased axial compressive loading of the inner and outer annulus tissues. It is possible that concurrent changes in hydraulic permeability and proteoglycan retention known to be associated with genipin cross-linking were also contributors to poroelastic changes. Reduction of peak pressures and moderation of pressure fluctuations could be beneficial relative to discogenic pain.

The goal of this study was to develop a nondestructive radial compression technique and to investigate the viscoelastic behavior of the rat tail disc under repeated radial compression. Rat tail intervertebral disc underwent radial compression relaxation testing and creep testing using a custom-made gravitational creep machine. The axisymmetric viscoelasticity and time-dependent recovery were determined. Different levels of hydration (with or without normal saline spray) were supplied to evaluate the effect of changes in viscoelastic properties. Viscoelasticity was found to be axisymmetric in rat-tail intervertebral discs at four equidistant locations. Complete relaxation recovery was found to take 20 min, whereas creep recovery required 25 min. Hydration was required for obtaining viscoelastic axisymmetry and complete viscoelastic recovery.

Background: Collagen crosslinks may play a vital role in preventing ongoing disc degeneration. Age-accumulating crosslinks have been thought to increase brittleness and reduce fatigue resistance. However recent studies have demonstrated increases in fatigue resistance, joint stability and nutritional flow properties resulting from crosslink augmentation. In this study, multi-directional moduli of bovine lumbar intervertebral discs were measured in vitro, including circumferential tension, radial compression, axial tension, and axial compression in control and crosslinked specimens., Methods: Four types of annulus fibrosus specimens were dissected from control and crosslinked discs. Cross-sectional areas were measured using a non-contact laser measurement system and then four separate mechanical tests were conducted using a materials testing machine with custom-made loading fixtures., Findings: The circumferential specimens demonstrated the highest moduli in both low stiffness and linear elastic regions. After a crosslink treatment, the modulus increased more in circumferential tension compared to axial tension and more in axial compression compared to radial compression. Other tensile properties had higher increases in circumferential tension compared to axial tension after crosslinking., Interpretation: Assuming form follows function, circumferential tension is the predominant type of stress experienced by non-degenerated annulus fibrosus. The anisotropic mechanical properties of the annulus fibrosus is non-uniformly affected by crosslink augmentation. Dominant effects were in the directions with greater inherent stiffnesses. These results suggest some beneficial effects of crosslink augmentation on the mechanical properties of the annulus fibrosus: increase in ultimate strength, yield strength, toughness, and modulus in the principal stress directions.

Study Design: The mechanical stability of cross-linked and control spinal motion segments was evaluated using neutral zone, range of motion (ROM), and instability score metrics., Objective: To determine if exogenous cross-linking could increase the stability of spinal motion segments., Summary of Background Data: The microstructure of the anulus fibrosus extracellular matrix can affect the stability of the intervertebral joint. Parallel testing in our laboratory has shown that exogenous cross-linking can improve the fatigue resistance of anulus fibrosus., Methods: There were 3 separate experimental protocols conducted. The first study used calf lumbar intervertebral joints randomly divided into a genipin cross-linked group and phosphate buffered saline-soaked controls. After 2 days of soaking, flexion-extension ramp cycles were applied to the specimens. The second study repeated the test protocol using 22 moderately and severely degenerated human lumbar intervertebral joints. The third experiment compared the effect of cross-linking treatment on human discs with known degrees of preexisting mechanical instability. Each data set was used to assess joint instability by 3 calculations: ROM, neutral zone, and an instability score. Joint instability for each data set was evaluated using 3 calculations: ROM, neutral zone, and a novel instability score., Results: These results show that cross-link augmentation can effectively reduce instability of intervertebral discs. The stabilizing effect was observed to be higher in the more mechanically unstable discs. However, cross-linking did not appear to affect the total range of sagittal motion., Conclusions: By reducing the neutral zone, exogenous cross-linking may help combat the progression of instability in degenerative disc disease.

Purpose: Currently, two different methods of applying radiofrequency (RF) energy (monopolar and bipolar) are available to the surgeon for thermal shortening of the shoulder capsule. The objective of this study was to investigate the temperature changes and the thermal conduction across the human inferior glenohumeral ligament (IGHL) during radiofrequency energy application., Methods: Thermistors were secured onto both the intra-articular and extra-articular surfaces of human IGHL. Monopolar RF energy and bipolar RF energy were delivered to the intra-articular surface at the manufacturer's recommended settings. Pre-treatment and post-treatment ligament lengths, widths, heating times, and temperatures were measured and compared., Results: For the monopolar devices, temperature spikes to 89 degrees C were recorded for the set temperature of 67 degrees C, averaging 77 degrees C +/- 10 degrees C. Temperatures across the ligament averaged 48 degrees C +/- 3 degrees C. For both devices, the IGHL became thicker with higher RF settings. Recorded temperatures decreased as distance increased from the point of application. Maximum temperatures occurred at least 6 to 7 seconds after cessation of energy application., Conclusions: The bipolar and monopolar devices had similar conduction times across the ligament, suggesting that this occurs by simple diffusion of heat. Bipolar and monopolar devices were equally efficacious for capsular shrinkage if the extent of the shortening is tightly defined., Clinical Relevance: The thermal probe should not rest in one position for an extended period of time during RF energy application because, as our study showed, the monitoring of temperature or the visualization of tissue change is not efficacious for determining the end point of thermal shrinkage of the shoulder capsule.

Purpose: The purpose of this study was to evaluate the thermal effects produced in meniscal tissue with different radiofrequency (RF) energy levels and exposure times using a bipolar device., Type of Study: An anatomic in vitro analysis of the thermal effects of an RF device., Methods: A specially designed jig was used to apply RF energy under a constant force to cadaveric menisci. Three different energy levels were applied for 4 different contact times., Results: The overall mean depth of thermal change was 547 microm (range, 468 to 650 microm). There were no significant differences or trends when contact time and energy were varied., Conclusions: These data suggest that increased contact times and energy outputs are not associated with increased thermal change in the meniscus. The submillimeter thermal denaturation was consistent with published reports using other thermal devices, such as laser.

A slipped capital femoral epiphysis was created in 12 matched pairs of immature bovine femora using an anterior-to-posterior-directed shear force. All soft tissues, with the exception of the perichondrial ring, were removed before testing. One specimen from each pair was fixed with a single cannulated screw in standard fashion, whereas the contralateral specimen was fixed with a single screw that compressed the physis. The amount of compression achieved was quantified using Fuji film. Standard fixation yielded 1.4 MPa of pressure across the physis; compression fixation yielded 3.2 MPa, a 2.3-fold difference (p = 0.0001). The compression fixation was 47% more stiff than standard technique (p = 0.030), yet the differences in ultimate strength (p = 0.180) and energy absorbed at failure (p = 0.910) were not statistically significant. The stiffness of the compressed specimens remained less than that of the intact femora. Single-screw compression fixation of in vitro bovine femora was significantly more stiff than the current, widely used noncompression fixation technique, yet does not compromise the ultimate strength, energy absorbed, or the technical ease of single-implant fixation.

Purpose: To outline the development of electrosurgical (radiofrequency) devices, explain the basic principles, and review the current orthopaedic literature regarding the application of electrosurgery to arthroscopic partial meniscectomy., Materials and Methods: The history and principles of electrosurgery were obtained from various pertinent texts and journal articles. A literature search was performed using MEDLINE; reviewed articles consisted of articles in the English language cataloged between 1966 and January 1999., Results: The history and principles of electrosurgery are reviewed. The articles pertaining to arthroscopic electrosurgical meniscectomy are discussed with a separate discussion on the potential complications of using radiofrequency energy for meniscal ablation, including articular cartilage damage, osteonecrosis, and damage caused by irrigant., Conclusion: Electrosurgery has been shown to be an effective tool in arthroscopic meniscectomy. Further research and refinement is warranted because it may show superiority to other methods in certain situations.

Twenty-six paired, fresh-frozen cadaveric feet were disarticulated at the ankle joint, and the dome of the talus was potted. Stress-risers were placed along the medial, lateral, and posterior aspects of the calcaneus, and the specimen was loaded rapidly to failure in a testing machine to produce a type-IIB displaced intra-articular fracture according to the classification system of Sanders et al. One specimen of each pair was treated with standard internal fixation with bone-grafting (the control group), and the other was treated with similar fixation but with SRS (Skeletal Repair System) calcium phosphate bone cement placed in any osseous defect. All of the specimens were cured for twenty-four hours in a bath of saline solution at 37 degrees Celsius. The specimens were tested cyclically for ten cycles from zero to 100 newtons at one hertz and for 1010 cycles from zero to 350 newtons at one hertz. The deformation per cycle (millimeters per cycle), first-cycle deformation (millimeters), number of cycles to failure, and number of specimens withstanding the cyclical testing were calculated. The specimens were examined radiographically before and after fracture and after reconstruction and testing. A large difference in the results of the cyclical testing was noted. The specimens that had been augmented with the SRS bone cement had an average deformation of 0.00195 millimeter per cycle compared with 1.013 millimeters per cycle in the control group (p < 0.005). A similar magnitude of difference was noted when the results were stratified for good and poor-quality bone. Visual examination and radiographs demonstrated that a type-IIB displaced intra-articular fracture had been created reproducibly, and computed tomographic scans showed that nearly anatomical reconstruction had been achieved in all of the specimens. The computerized tomographic scans revealed good filling of the osseous voids and no evidence of failure of the cement after cyclical loading.

Simulated syndesmosis injuries were created in 12 fresh-frozen, below-knee cadaver specimens. Six specimens were repaired with a 4.5 mm stainless steel screw, and six were repaired with a 4.5 mm polylactide screw. Three specimens of each group were tested in load to failure by axially loading with 1400 N and externally rotating to 90 degrees. Three specimens in each group underwent fatigue testing by axially loading with 700 N and applying 2.5 N-m of torque for 57,700 cycles. Radiographs and computed tomography scans were evaluated. None of the screws broke or failed. Similar load to failure was noted in polylactide and control groups. Fatigue testing revealed no significant change in stiffness. No significant screw damage was evident on radiographic or computed tomography evaluation. The data suggest that a polylactide screw has sufficient fatigue and failure strength to allow for healing of this injury in a clinical situation.

Study Design: In vitro force and deformation measurements formed the basis for determinate, quasistatic analysis of principal forces in the seated lumbar spine., Objectives: To explore the relationship between seated postures and the mechanical response in component tissues of lumbar intervertebral joints., Summary of Background Data: Despite the high prevalence of low back pain syndrome, the precise mechanisms relating specific mechanical loads to spinal degeneration are not well understood. Simultaneous, time-dependent measurement of anterior column forces and articular facet forces has not been presented previously. consequently, a determinate analysis of principal component forces has not been possible., Methods: Twelve lumbar spines (L1-S1) were subjected to constant loading conditions while in flexed and extended seated postures. Time-dependent forces were measured in the anterior column at the L4 and L5 superior endplates and in the four facets of the L3-L4 and L4-L5 motion segments. A quasi-static analysis of sagittal plane forces was used to compute the remaining principal joint forces, including ligament, disc shear, and facet impingement forces., Results: Component forces changed under static loading in both postures. There were significant differences between the mechanical responses of the two postures. Although the vertical creep displacement was greater in the extended seated posture (3.22 mm versus 2.11 mm), the escalation of forces was more severe in the flexed posture., Conclusions: The results suggest a mechanism of force balancing in lordotic postures under static loads, whereas flexed postures produce large increases to the tensile forces in the region of the posterior anulus.

Eight adult below-knee cadaver specimens were placed in a testing machine and loaded to 350 newtons according to a strict protocol. Arch height and length measurements were obtained in each specimen with the toes resting on the foot plate, dorsiflexed to 30 degrees, and maximally dorsiflexed manually. The plantar fascia was then divided from medial to lateral in one-quarter increments, and the effect on arch height and length measurements was assessed using the same loading protocol. A consistent decrease in the arch-supporting function on sequential sectioning of the plantar fascia was encountered. A less consistent decrease in the arch-supporting function was reflected by the increase in the height of the arch with sequential sectioning of the plantar fascia. The study demonstrates that partial plantar fasciotomy decreases the arch-supporting function of the plantar fascia in addition to weakening the structure. Strict surgical indications for this type of procedure should be maintained.

We investigated the relative importance to stability of the four component ligaments of the distal tibiofibular syndesmosis. Eight fresh-frozen cadaver specimens were tested on a hydraulic test system during sequential cutting of the ligaments. The percentage resistance to 2 mm of diastasis was measured for the four ligaments. The anterior inferior tibiofibular ligament provided 35%, interosseous ligament 22%, superficial posterior inferior tibiofibular 9%, and deep posterior inferior tibiofibular 33%. These results have clinical implications with regard to injury. Damage to the syndesmosis should be assessed anteriorly and posteriorly at the time of examination. The interosseous ligament can be visualized arthroscopically.

Sixteen human anatomic specimen ankles underwent an arthrodesis using two (medial and lateral) or three (additional anterior) screw fixation. It was shown that significantly greater compression was achieved with three screws (p = 0.001). The order of screw fixation was important--better compression was achieved by inserting the lateral screw first. The resistance to torque was measured on a materials testing system, and the three screw fixation technique was significantly better (p = .001). The recommended fixation is one screw medially, one laterally, and one anterior from the tibia to the talus.

Two long-term wear behavior studies used in part of the process of selecting the best materials for a new all-metal orthopaedic spinal implant are presented. The relative and absolute wear volume rates of candidate metals were investigated through simulation of two bearing regions of an intervertebral disc prosthesis. It was determined gravimetrically that hot isostatically pressed Co-Cr-Mo alloy provided the lowest Co-Cr-Mo alloy wear debris generation. It had an average wear volume rate of 0.093 mm3/million cycles from a spring-in-pocket simulation and 0.126 mm3/million cycles from a hinge (pin-in-slot) simulation. The estimated total wear volume of a hot isostatically pressed intervertebral disc prosthesis with titanium-6%Al-4%V alloy springs was 2.9 mm3/million cycles.

A new transducer capable of direct measurement of time-dependent loads in human lumbar facet joints was developed and tested. The transducer was comprised of a force-sensitive resistor (FSR) in series with a pressure-sensitive film. A wide range of experiments revealed the performance attributes and limitations of the FSR. The output signal of the FSR is actually sensitive to both force and area of contact independently. Therefore, a pressure-sensitive film was used to quantify the contact area. At least two transformation equations were calculated for each FSR corresponding to known contact areas. Each equation was a linearization of the log of the FSR output vs the log of the applied ramp loads. Coefficients of determination (CD) were calculated for small (21 mm2) and large (32 mm2) contact areas, and were found to exceed 0.900 for all data. The average of nine cycles was nearly linear for some FSRs (CD of 0.999). FSR output signal and contact area were recorded in cadaveric lumbar facets under ramp load. The appropriate transformation equation was determined by a linear interpolation between benchmark equations based on the contact area measured in vitro. Facet force measurements compared well with those of other researchers. The transducer was found to be quite easy to use.

This article presents criteria for the design of a lumbar intervertebral disc prosthesis that take into consideration issues of endurance, materials behavior, geometry, kinetics, motion constraints, fixation to bone, and safety. The criteria and design philosophies discussed are generally applicable in the synthesis of any new implant system. Specifications of the design of a disc prosthesis are presented along with an evaluation of the design based on some of the preliminary test data.