Biomedical subjects

B W Cunningham

Publications and source records attributed to B W Cunningham.

At least 37 records · Page 2Linked to original sources

PubMed1998

Video-assisted thoracoscopic surgery versus open thoracotomy for anterior thoracic spinal fusion. A comparative radiographic, biomechanical, and histologic analysis in a sheep model.

STUDY DESIGN: In this in vivo investigation, a sheep model was used to compare the efficacy of a video-assisted thoracoscopic approach and a traditional thoracotomy in promoting a successful interbody spinal arthrodesis. OBJECTIVES: To compare the incidence of successful anterior spinal arthrodesis among three stabilization techniques-iliac crest, Bagby and Kuslich device, and Z-plate--performed using a video-assisted thoracoscopic approach and conventional open thoracotomy approaches. SUMMARY OF BACKGROUND DATA: A clinical outcome study on open versus endoscopic spinal fusion is not yet available. Moreover, no basic scientific investigations have been conducted to determine whether the success of an endoscopic arthrodesis is comparable to that of a conventional open procedure. METHODS: Fourteen Western Crossbred sheep underwent three identical destabilization procedures at T5-T6, T7-T8, and T9-T10, in which the anterior and middle osteoligamentous columns of the spine were resected, followed by three randomized reconstruction procedures using iliac autograft alone, and Z-plate stabilization with iliac autograft. In seven sheep, the entire destabilization-reconstruction procedure was performed using a video-assisted thoracoscopic surgical approach. In the remaining seven, the procedure was performed by conventional open thoracotomy. RESULTS: Histomorphometric and biomechanical evaluation demonstrated that the video-assisted thoracoscopic approach and open thoracotomy arthrodesis had comparable bone formation and biomechanical properties (P > 0.05). However, the Z-plate fusions, as a group, demonstrated increased flexion-extension stiffness properties and trabecular bone formation compared with the autograft and Bagby and Kuslich device fusions (P < 0.05). CONCLUSIONS: Thoracic interbody spinal fusions performed by thoracoscopy have demonstrated histologic, biomechanical, and radiographic equivalence to those performed by a thoracotomy approach. However, in the endoscopy group, intraoperative complications causing longer operative times, higher estimated blood loss, and increased animal morbidity indicated a substantial learning curve associated with the adoption of this surgical technique.

Analysis of Variance↗

PubMed1998

The effects of rigid spinal instrumentation and solid bony fusion on spinal kinematics. A posterolateral spinal arthrodesis model.

STUDY DESIGN: Spinal kinematics after the implementation of rigid spinal instrumentation or the achievement of a solid fusion was studied using a sheep posterolateral spinal arthrodesis model. OBJECTIVE: To investigate the effects of rigid spinal instrumentation or solid fusion on spinal kinematic parameters. SUMMARY OF BACKGROUND DATA: Numerous studies have attempted to define spinal instability in terms of kinematics. Recent in vitro studies have documented the neutral zone, or a measure of spinal laxity, as more sensitive to spinal instability than the range of motion. METHODS: Seven skeletally mature sheep underwent a single-level posterolateral lumbar arthrodesis using autologous bone graft augmented with transpedicular screw fixation. The animals were killed 4 months after surgery. The identical surgical procedures were performed in seven sheep cadaveric spines, which served as acute postoperative controls. Each functional spinal unit was tested biomechanically before and after hardware removal. The experimental control groups consisted of destabilized spines and spines that underwent transpedicular screw fixation alone, whereas the fusion groups included spines that underwent posterolateral fusion alone or posterolateral fusion with instrumentation. RESULTS: Rigid instrumentation and solid fusion significantly decreased the neutral zone and range of motion in all testing modes. In axial rotation and lateral bending, solid fusion reduced the range of motion significantly more than transpedicular screw fixation alone. However, in all testing modes, the neutral zones showed no statistical difference between transpedicular screw fixation alone and fusion groups. CONCLUSIONS: The range of motion was an equivalent or better indicator of fixation or fusion stability compared with the neutral zone. Moreover, the immediate postoperative fixation stability, even if using transpedicular screw fixation, was less than the stability present after a solid fusion.

Animals↗

PubMed1998

The effects of spinal fixation and destabilization on the biomechanical and histologic properties of spinal ligaments. An in vivo study.

STUDY DESIGN: An animal study was conducted to assess whether different surgical procedures of spinal fixation and destabilization would influence the biomechanics and histology of lumbar spinal ligaments. OBJECTIVES: To investigate the effects of spinal fixation and destabilization as well as surgical intervention itself on the biomechanical and histologic properties of lumbar spinal ligaments. SUMMARY OF BACKGROUND DATA: Although several investigators have reported normal biomechanical properties of different spinal ligaments, there have been no studies in which changes in spinal ligament properties, secondary to the altered biomechanical environment provided by such surgical procedures as spinal fixation and destabilization, have been investigated. METHODS: Thirty-six mature sheep were divided into four groups: Group I: nonsurgical control: Group II: sham operation consisting of bilateral posterolateral exposure at L4-L5; Group III: spinal fixation using transpedicular screws and plates and bilateral posterolateral bone graft at L4-L5; and Group IV: spinal destabilization consisting of bilateral facetectomy and anterior discectomy at L4-L5. Four months after surgery, the biomechanical analysis included destructive tensile testing of four different bone-ligament-bone complexes at the operative and proximal adjacent levels: anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, and supraspinous and interspinous ligaments combined. Histomorphometric analyses of the vertebral body and spinal ligaments were performed histomorphometrically. RESULTS: Biomechanical analysis results demonstrated remarkable changes in the structural and mechanical ligament properties at the operative level. The fixation group's ligaments showed consistent decreases in the ultimate load and elastic modulus compared with those parameters in the control group (P < 0.05). Histologically, the fixation group's ligamentum flavum showed marked vacuolation in the ligament substance, whereas the interspinous ligament exhibited significant insertion changes compared with little change in substance. In all eight sheep in Group IV, unintentional bilateral facet fusions were obtained; and in all eight animals in Group III with pedicle instrumentation and posterolateral fusion, solid arthrodesis was exhibited. This allowed a distinction to be made between the stress-shielding effect of spinal instrumentation and arthrodesis (Group III) versus spinal fusion alone (Group IV) on both spinal ligament and vertebral body. Group II (sham) had a significant decrease in supraspinous and interspinous ligaments, but nonsignificant decreases in the stress-shielding effect of 10-12% in other ligaments. CONCLUSIONS: Posterior spinal instrumentation and fusion led to decreased biomechanical properties of the ligamentum flavum, posterior longitudinal ligament, and interspinous and supraspinous ligaments. The stress-shielding effects were ligament dependent and were most pronounced on the posterior side. The altered biomechanical environment produced by spinal fixation, surgical intervention itself, or nonphysiologic mobilization can affect the ligamentous properties in vivo, possibly serving as the impetus for low back pain.

Animals↗

PubMed1997

The effect of spinal destabilization and instrumentation on lumbar intradiscal pressure: an in vitro biomechanical analysis.

STUDY DESIGN: In vitro biomechanical testing was performed in human cadaveric lumbar spines, using pressure needle transducers to analyze the effects of spinal destabilization and instrumentation on lumbar intradiscal pressures. OBJECTIVES: To quantify changes in lumbar intradiscal pressures at three adjacent disc levels under conditions of spinal reconstruction, and to evaluate the possibility of pressure-induced disc pathology secondary to spinal instrumentation. SUMMARY OF BACKGROUND DATA: Lumbar intradiscal pressures under in vivo and in vitro conditions and the use and development of spinal instrumentation have been investigated comprehensively. However, the effects of spinal destabilization and instrumentation on lumbar intradiscal pressure have not been delineated clearly. METHODS: In 11 human cadaveric lumbosacral specimens, specially designed pressure needle transducers quantified intradiscal pressure changes at three adjacent disc levels (L2-L3, proximal; L3-L4, operative; and L4-L5, distal) under four conditions of spinal stability: intact, destabilized, laminar hook and pedicle screw reconstructions. Biomechanical testing was performed under axial compression (0-600 N), anterior flexion (+12.5 degrees) and extension (-12.5 degrees), after which the level of degeneration and disc area (cm2) were quantified. RESULTS: In response to destabilization and instrumentation, proximal disc pressures increased as much as 45%, and operative pressure levels decreased 41-55% (P < 0.05), depending on the instrumentation technique. Linear regression and correlation analyses comparing intradiscal pressure to the grade of disc degeneration were not significant (r = 0.24). CONCLUSIONS: Changes in segmental intradiscal pressure levels occur in response to spinal destabilization and instrumentation (P < 0.05). Intradiscal cyclic pressure differentials drive the metabolic production and exchange of disc substances. Conditions of high or low disc pressure secondary to spinal instrumentation may serve as the impetus for altered metabolic exchange and predispose operative and adjacent levels to disc pathology.

Adult↗

PubMed1997

Maturation of the posterolateral spinal fusion and its effect on load-sharing of spinal instrumentation. An in vivo sheep model.

UNLABELLED: We investigated the temporal relationship among the biomechanical, radiographic, and histological properties of a posterolateral spinal fusion mass to elucidate the changes in load-sharing of the spinal instrumentation and that of the fusion mass throughout the healing process. Destabilization of the posterior spinal column and transpedicular screw fixation at the segments between the third and fourth and the fifth and sixth lumbar vertebrae was performed in twenty-four sheep. A posterolateral spinal arthrodesis with use of autologous corticocancellous bone graft was done randomly at one of the two segments; the other segment (without bone graft) served as the instrumented control. Six animals each were killed at four, eight, twelve, and sixteen weeks postoperatively. Biomechanical testing showed that the posterolateral fusion mass had increased mechanical stiffness after the fourth week. The strain on the hardware, measured with use of rods instrumented with strain-gauges, decreased significantly (p < 0.01) beginning at eight weeks. Radiographically, three independent observations of each of the six animals at each time-period showed that, although all of the fusion masses were considered solid unions at sixteen weeks, bridging of trabecular bone was noted during only ten of eighteen observations at twelve weeks, three of eighteen observations at eight weeks, and none of eighteen observations at four weeks. Computerized tomography and histomorphometric analyses demonstrated that mineralization in the fusion mass increased in a linear fashion even after eight weeks. Histologically, the fusion mass consisted predominantly of woven bone at eight weeks; thereafter, it was gradually trabeculated. CLINICAL RELEVANCE: We found a great discrepancy between biomechanical stability and histological maturation of the posterolateral fusion mass. The biomechanical properties of a stable spinal fusion preceded the radiographic appearance of a solid fusion by at least eight weeks, suggesting that immature woven bone provided substantial stiffness to the fusion mass. The spinal instrumentation was subjected predominantly to bending stress rather than to axial stress, and the load-sharing of the spinal instrumentation decreased concurrently with the development of the spinal fusion.

Animals↗

PubMed1996

In vitro biomechanical comparison of multistrand cables with conventional cervical stabilization.

STUDY DESIGN: The biomechanical stability of six different methods of cervical spine stabilization, three using multistrand cables, were evaluated in a bovine model. OBJECTIVES: To quantify and compare the in vitro biomechanical properties of multistrand cables used for posterior cervical wiring to standard cervical fixation techniques. SUMMARY OF BACKGROUND DATA: Fixation of the posterior cervical spine with monofilament stainless steel wire is a proven technique for stabilization of the cervical spine. Recently, multistrand braided cables have been used as a substitute for monofilament stainless steel wires. These cables, made of stainless steel, titanium, or polyethylene, are reported to be stronger, more flexible, and fatigue resistant than are monofilament wire based on mechanical testing. However, no in vitro biomechanical studies have been performed testing a standard posterior cervical wiring technique using multistrand cables. METHODS: Thirty-six fresh frozen cervical calf spines consistent in size and age were mounted and fixed rigidly to isolate the C4-C5 motion segment. Six different reconstruction techniques were evaluated for Rogers' posterior cervical wiring technique using: 1) 20-gauge stainless steel monofilament wire, 2) stainless steel cable, 3) titanium cable, 4) polyethylene cables, 5) anterior locking plate construct with interbody graft, and 6) posterior plate construct. Six cervical spines were included in each group (n = 6), with each specimen statically evaluated under three stability conditions: 1) intact, 2) reconstructed, and 3) postfatigue. The instability model created before the reconstruction consisted of a distractive flexion Stage 3 injury at C4-C5. Nondestructive static biomechanical testing, performed on an material testing machine (MTS 858 Bionix test system, Minneapolis, MN), included axial compression, axial rotation, flexion-extension, and lateral bending. After reconstruction and static analysis, the specimens were fatigued for 1500 cycles and then statically retested. Data analysis included normalization of the reconstructed and postfatigue data to the intact condition. The calculated static parameters included operative functional unit stiffness and range of motion. RESULTS: Posterior cervical reconstruction with stainless steel monofilament wire proved inadequate under fatigue testing. Two of the six specimens failed with fatigue, and this construct permitted the greatest degree of flexion-extension motion after fatigue in comparison with all other constructs (P < 0.05). There were no significant differences in flexural stiffness or range of motion between stainless steel, titanium, or polyethylene cable constructs before or after fatigue testing. The posterior cervical plate constructs were the stiffest constructs under flexion, extension, and lateral bending modes, before and after fatigue testing (P < 0.05). CONCLUSIONS: Multistrand cables were superior to monofilament wire with fatigue testing using an in vitro calf cervical spine model. There were no failures or detectable differences in elongation after fatigue testing between the stainless steel, titanium, and polyethylene cables, as shown by the flexion-extension range of motion. The posterior cervical plate construct offered the greatest stability compared with all other constructs.

Animals↗

PubMed1996

The role of spinal instrumentation in augmenting lumbar posterolateral fusion.

STUDY DESIGN: Using a sheep model, clinically practical posterolateral intertransverse process fusion was successfully achieved and biomechanically tested to determine the load-sharing environment provided by spinal instrumentation and posterolateral fusion mass following solid arthrodesis. OBJECTIVES: To quantify the in vivo load-sharing capacity of spinal instrumentation on augmenting the posterolateral intertransverse fusion. The hypothesis was that transpedicular screw fixation maintains the biomechanical contribution to the posterolateral fusion stability even after successful arthrodesis because of its providing anterior and middle column support. SUMMARY OF BACKGROUND DATA: Although many previous studies have documented the biological and biomechanical advantages of posterolateral fusion, it is known that posterolateral fusion without spinal instrumentation allowed significant remaining motion at the fused segment even after the solid arthrodesis. Whether spinal instrumentation, especially transpedicular screw fixation, augments in vivo posterolateral fusion stability after solid arthrodesis has not been previously investigated. METHODS: Radiographic, macroscopic, and biomechanical analyses of a posterolateral intertransverse process fusion model were performed on 18 sheep at 4 months postoperatively. The load-sharing contribution of the spinal instrumentation was calculated based on the stability with or without spinal instrumentation tested in five loading modalities. Histomorphometry of the vertebral body spanned by spinal instrumentation provided the information regarding the biological effect of the load-sharing capacity of spinal instrumentation on bone remodeling. RESULTS: All sheep who received posterolateral intertransverse process fusion demonstrated successful solid arthrodesis and high biomechanical quality of the posterolateral fusion mass when compared to previous posterolateral fusion models. The significant difference in stiffness between fixation and subsequent fixation removal was observed in flexion, despite maintaining high lateral bending stiffness equivalent to the fixation (with instrumentation) level. This significant load-sharing contribution of spinal instrumentation detected in flexion corresponded to 27% when compared to the fixation level. The qualitative and quantitative bone histology showed 64% of the volumetric density of bone in the fixation group when compared to that of the sham group as well as narrow trabeculae and reduced connection of trabeculae. CONCLUSIONS: The continuance in support offered by transpedicular screw fixation was assured in vivo after the solid posterolateral intertransverse process fusion. This was clearly demonstrated under eccentric loads in a sagittal plane, suggesting that transpedicular screw fixation was able to provide anterior and middle column support and resist eccentric loads.

Analysis of Variance↗

PubMed1996

Biomechanical assessment of a new tenodesis for correction of hallux varus.

Each of six below-the-knee amputation specimens were transfixed to a wooden block and mounted to a jig on an amputee testing device preloaded with 5 N applied to the proximal phalanx and displaced at a constant rate of 2 mm/min. Load displacement curves were generated for the intact joint and after sequential incisions of the lateral capsule, the adductor hallucis, and the lateral slip of the flexor hallucis brevis tendon, which caused varus dislocation of the hallux. An extensor hallucis brevis tenodesis was performed after the varus dislocation. Division of the lateral capsule, the adductor, and the flexor brevis reduced the force required to displace the hallux by 42.2%, an additional 25.2%, and a further 14.2%, respectively. Use of the extensor hallucis brevis tenodesis restored the load displacement curves to that of the normal joint. We conclude that the extensor hallucis brevis tendon may be useful as a tenodesis for reconstructing the deformity of acquired hallux varus.

Biomechanical Phenomena↗

PubMed1994

Biomechanical analysis of cervical stabilization systems. An assessment of transpedicular screw fixation in the cervical spine.

STUDY DESIGN: The biomechanical stability of seven cervical reconstruction methods including the transpedicular screw fixation was evaluated under four instability patterns. These four modalities, based on the range and grade of instability, allowed a reproducible biomechanical assessment to establish the in vitro role of internal fixation in the cervical spine. OBJECTIVES: This study biomechanically investigated the stability of seven reconstruction methods in the cervical spine as influenced by four instability patterns and assessed whether three-column fixation for the cervical spine using transpedicular screw fixation would provide increased stability over that of conventional cervical fixation systems. METHODS: A total of 24 calf cervical spine specimens were divided into four experimental groups. The spinal constructs including seven reconstruction techniques--the posterior AO titanium reconstruction plate, Bohlman's posterior triple-wiring, transpedicular screw fixation, anterior iliac bone graft, anterior AcroMed plate, anterior AO titanium locking plate, and combined fixation with the AO anterior plate and posterior triple-wiring--were tested under four loading modes. RESULTS: Anterior plating methods provided less stability than that of posterior constructs under axial, torsional, and flexural loading conditions. Exclusive posterior procedures provided increased stability compared with the intact spine in one level fixation, however, did not sustain the torsional stability when the anterior and middle column was eliminated in two-level fixation. The stabilizing capabilities of both the combined fixation and transpedicular screw fixation were clearly demonstrated in all loading modes, however, those of the latter were superior in multilevel fixation. CONCLUSION: Front and back approaches, employing the anterior plate and posterior triple-wiring, and transpedicular screw fixation demonstrated clear biomechanical advantages when the extent of instability increased to three-column or multilevel. Three-column fixation for the cervical spine using transpedicular screw fixation offers increased stability over that of conventional cervical fixation systems, particularly in multiple level constructs.

Animals↗

PubMed1994

Experimental study of thoracolumbar burst fractures. A radiographic and biomechanical analysis of anterior and posterior instrumentation systems.

STUDY DESIGN: The efficacy of posterior instrumentations for treating thoracolumbar burst fractures to restore spinal alignment and indirectly reduce intracanal bone fragments was investigated. Also, a biomechanical study was performed to compare the mechanical stability of anterior and posterior instrumentations. METHODS: Twenty-four fresh human cadaveric thoracolumbar spine segments were used. After clinically identical L1 burst fractures were created, two posterior instrumentations were used to restore spinal alignment: 1) Harrington dual distraction rods with sleeves and 2) AO internal fixator. Radiographs and computed tomography scans were obtained to assess spinal alignment and canal dimensions. Biomechanical testing was performed in axial compression, rotation, and flexion-extension on all constructs, including anterior reconstruction with the Kaneda device. SUMMARY OF BACKGROUND DATA: Kyphosis averaged 14 degrees in the injured specimens and was corrected to 1 degree of lordosis after posterior reduction, and vertebral body height was restored to normal in most of the specimens. Reduction rate of canal compromise was 12.3% for Harrington instrumentation and 18.5% for AO internal fixator. Anterior reconstruction with the Kaneda device was more stable than the posterior instrumentation systems in all loading conditions. RESULTS: The posterior reduction and stabilization with posterior instrumentation provided effective restoration of the sagittal alignment. However, the reduction capability of the intracanal bone fragments was distinctly limited. CONCLUSIONS: The anterior reconstruction method permits effective decompression of the spinal canal and offers superior mechanical stability compared with the indirect decompression and stabilization of posterior instrumentation.

Biomechanical Phenomena↗

PubMed1994

Interbody lumbar fusion using a carbon fiber cage implant versus allograft bone. An investigational study in the Spanish goat.

STUDY DESIGN: A carbon fiber-reinforced polymer implant, designed to aid interbody lumbar fusion, was tested biologically in an experimental surgical model. Twenty-seven Spanish goats had interbody lumbar fusion surgery in a randomized protocol. Seventeen goats were implanted with the carbon fiber-reinforced polymer cage packed with autologous bone, and 10 goats were implanted with ethylene oxide-sterilized allograft bone. OBJECTIVES: To determine fusion success, biocompatibility of the carbon polymer material, and possibility of carbon wear debris at intervals after surgical implantation. METHODS: Goats were killed at 6 months, 12 months, and 24 months and full-body autopsies were done. Spine specimens were studied by plain radiography, three-dimensional reformatted computed tomography studies, and histology. RESULTS: At 6 months, one of three allograft implantations showed histologic and radiographic fusion, whereas five of five carbon fiber-reinforced polymer cage fusions showed at least partial fusion. At 12 months, two of three allograft implantations and five of five carbon fiber-reinforced polymer cage fusions were solidly fused. At 24 months, five of five allograft implantations and three of three carbon fiber-reinforced polymer cage implantations were solidly fused. CONCLUSIONS: Interbody fusion using a carbon cage implant packed with autologous bone achieved a quicker and more reliable fusion compared with ethylene oxide-sterilized allograft bone. There were no adverse effects from the implant material.

Animals↗

PubMed1994

Proximal femoral fractures: a biomechanical study to compare intramedullary and extramedullary fixation.

This study assesses the rigidity and strength of fixation provided by intramedullary and extramedullary devices for proximal femoral fractures. Stable and unstable intertrochanteric fractures were studied in paired femora after internal fixation with the Gamma nail and Richards 135 degrees classic hip-screw implants; in subtrochanteric fractures, the 95 degrees Richards condylar screw was studied in addition. Subsidence of the sliding screw within the plate and nail constructs was measured. Fixation of subtrochanteric fractures with the intramedullary Gamma nail was significantly stronger and more rigid than that with the extramedullary screw plate devices. Under conditions of simulated protected weight bearing, the 95 degrees condylar screw plate provided more rigid fixation than did the 135 degrees hip screw. There was no significant difference in the strength of fixation of stable and unstable intertrochanteric fractures between the Gamma nail and the hip screw, although the Gamma nail provided more rigid fixation.

Aged↗

PubMed1994

Anatomical restraints to dislocation of the second metatarsophalangeal joint and assessment of a repair technique.

The aim of this study was to determine the anatomical restraints to dislocation of the second metatarsophalangeal joint and to assess the biomechanical efficacy of a technique that is commonly used to stabilize this joint. Cadaveric feet were disarticulated at the ankle, with preservation of the long flexor tendons at the medial malleolus. The hindfoot was transfixed to an aluminum jig, and a contoured nylon block was secured to the dorsum of the second metatarsal. A Kirschner wire was passed transversely through the proximal phalanx and was attached to a wire loop through which a constant vertical displacement was applied with a universal testing machine. A preload of five newtons was applied, followed by a constant displacement of two millimeters per minute, and the load-displacement curves were measured. The volar plate and the collateral ligaments were divided in five feet each. In another ten feet, both of these structures were divided simultaneously. Each load cycle was repeated four times. The force required to dislocate the joint, in the position in which testing was performed, was reduced by a mean of 30 per cent when the volar plate was divided and by a mean of 46 per cent when the collateral ligaments were divided. Division of both of these structures created an unstable joint, which dislocated at an applied load of five to ten newtons. The metatarsophalangeal joint was then repaired with use of a flexor tendon transfer in all twenty feet. This repair technique restored the load-displacement curves to that of the normal toe.

Cadaver↗

PubMed1993

The pathomechanics of compression injuries in the cervical spine. Nondestructive and destructive investigative methods.

Biomechanical analysis using nondestructive and destructive investigative methods was performed to evaluate the mechanisms of cervical compression injuries. These injuries produce two basic modes of failure: 1) anterior dislocation; and 2) rupture of the anterior ligamentous complex of the vertebral body. Distribution of these two failure patterns was determined by the initial cervical spine position; translational alignment did not have a significant effect. Different results were observed between spines positioned in flexion and extension, indicating that the most important factor determining the mode of failure was rotational alignment in the sagittal plane.

Animals↗

PubMed1993

Static and cyclical biomechanical analysis of pedicle screw spinal constructs.

Biomechanical evaluation of twelve different spinal devices in vitro employing pedicle screws was performed using static (n = 5) and cyclical testing (n = 3) parameters. In general, the rank order of implant failures was similar between static and cyclical tests, performed at 600 N compressive load, 5 Hz, and 1 million cycles. The mean number of cycles to failure was higher for spinal instrumentation systems employing longitudinal rods than those using plates (ANOVA F = 16.94, P < .001). At 600 N, the compact Cotrel-Dubousset, TSRH, and ISOLA rod systems demonstrated mean cycles to failure ranging from 200,000 to 800,000 cycles. The remaining devices including Dyna-lok, Kirschner plate, and VSP devices had failures ranging from 50,000 to 210,000 cycles. Polyethylene cylinders representing vertebral bodies were used to eliminate the problems of biologic deterioration encountered with cadaveric spines (a full cyclical test to 1 million cycles required 56 hours), and thus to provide analysis of the weak portion of each spinal system. The failure of eleven of the twelve spinal systems occurred by fracture of a pedicle screw, most commonly at the junction of the upper screw thread and the collar (Kirschner, AO fixator, standard CD, ISOLA, and TSRH). However, in Dynalok and VSP systems, fracture of the threaded portion of the screw just posterior to the integral nuts was the most common screw fracture location. The compact CD system was the only spinal implant that consistently failed by fracture of the longitudinal spinal member (rod). The fatigue life of rod based systems was longer than plate based systems. These studies confirm the importance of anterior column load sharing (vertebral body, corpectomy bone graft) as the mean bending strength demonstrated by these implant systems was not inordinately high using this "worst case scenario" model.

Biomechanical Phenomena↗

PubMed1993

Compression strength of donor bone for posterior lumbar interbody fusion.

Forty-three blocks of allograft bone used clinically for posterior lumbar interbody fusion and twenty-three blocks of xenograft bone from goats and cows were tested in compression and compared with the clinical mechanical requirements of posterior lumbar interbody fusion. Variations in processing methods allowed evaluation of the effects of processing on mechanical strength. Fresh-frozen cancellous bone from Os Bone (Cleveland, OH) failed at an average load of 863 +/- 615 N. Fresh-frozen cancellous bone from the Mid American Tissue Center (Massilon, OH) failed at an average load of 3492 +/- 1720 N. Freeze-dried cancellous bone obtained from the American Red Cross failed at an average load of 1595 +/- 1031 N. Air-dried ethylene oxide sterilized cancellous bone from Os Bone failed at an average load of 1338 +/- 691 N. Air-dried ethylene oxide sterilized cancellous bone from Mid America failed at an average load of 1616 +/- 1157 N. Fresh-frozen tricortical bone from Mid America failed at an average load of 2257 +/- 1081 N. Air-dried ethylene oxide sterilized tricortical bone from Os Bone failed at an average load of 2474 +/- 1928 N. Air-dried ethylene oxide sterilized tricortical bone from Mid America failed at an average load of 2308 +/- 422 N. Bovine Surgibone from Unilab (Hillside, NJ) failed at an average load of 2967 +/- 399 N. Strength of bone in compression was not weakened by freeze drying, air drying, ethylene oxide sterilization, or by incubation at 37 degrees C for 1 week before testing.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

A biomechanical analysis of decompression and reconstruction methods in the cervical spine. Emphasis on a carbon-fiber-composite cage.

Biomechanical analysis of three different patterns of instability--that created by fifth and sixth cervical anterior discectomy, that created by one-level (fifth cervical) anterior corpectomy, and that created by two-level (fourth and fifth cervical) corpectomy--was performed in eighteen calf spines. Three types of anterior reconstruction--anterior iliac strut bone-grafting, use of an anterior carbon-fiber-composite cage packed with cancellous bone graft, as well as use of polymethylmethacrylate anteriorly--were cyclically tested in axial compression, torsion, and flexion-extension. Each of these types of reconstruction was also tested with supplemental posterior wire stabilization (the triple-wire technique of Bohlman). Regardless of the type of anterior instability, the carbon-fiber-reinforced cage packed with cancellous bone graft was more rigid than the iliac bone graft alone. The cage resulted in good stiffness in the axial compression and rotation tests and was the most rigid construct in the flexion-extension tests. The superior aspect of the polymethylmethacrylate constructs loosened at the bone-cement interface in eight of the twelve specimens during flexion-extension testing. The addition of the supplemental posterior wiring to the anterior constructs provided additional rigidity in flexion-extension testing.

Animals↗

PubMed1992

Quantitative histologic study of the influence of anterior spinal instrumentation and biodegradable polymer on lumbar interbody fusion after corpectomy. A canine model.

Histomorphometric and microradiographic studies were performed to investigate whether there are different rates of bone remodeling based on the intrinsic stability with anterior spinal instrumentation and to evaluate if biodegradable polymer could be used clinically as the material of choice for anterior spinal instrumentation. Twenty-one coon hounds underwent anterior and posterior spinal destabilizing procedures to produce a reproducible amount of spinal instability: corpectomy of L5, discectomies and partial facetectomies of L4-5 and L5-6, resections of L5 lamina, spinous process, supra- and interspinous ligament, and ligamentum flavum. Group 1 (N = 7) underwent anterior autogenous ulna strut graft alone at L4-6; group 2 (N = 7) underwent anterior biodegradable polymer strut alone at the same level; group 3 (N = 7) underwent same bone graft as in group 1, augmented by anterior Kaneda device. Six months after surgery quantitative histologic study showed that device-related osteopenia occurred in spines treated with Kaneda device. Within the L5 vertebral body the volumetric density of bone (mm3/cm3) was less for the group with Kaneda device (group 3) compared with that without instrumentation (group 1, P less than 0.05). In the spine treated with biodegradable polymer, no adverse host tissue responses were observed histologically. In addition, osteoconductive abilities of the polymer were suggested microscopically. Its mechanical property, however, was not rigid enough to stabilize the corpectomized spine.

Animals↗