Biomedical subjects

W C Hutton

Publications and source records attributed to W C Hutton.

At least 37 records · Page 2Linked to original sources

PubMed1999

Mechanical comparison of plates used in the treatment of unstable subtrochanteric femur fractures.

OBJECTIVES: To determine the stiffness and strength characteristics of certain plate-composite femur models designed to simulate unstable subtrochanteric femur fractures (OTA 31-A2.3). DESIGN: Fifteen identical composite femora were osteotomized to produce like models of an unstable subtrochanteric femur fracture. The femora were fixed with either the Synthes 95 degree angled condylar blade plate, a 95 degree dynamic condylar screw plate (DCS), or a 135 degree dynamic compression hip screw (DHS). MAIN OUTCOME MEASUREMENTS: A materials testing machine was used to apply compression to the femoral head through an adapter plate. Stiffness values were calculated from the load-deformation curves obtained. RESULTS: The DHS-femur model was the stiffest (586 newtons/ millimeter), followed by the 95 degree DCS (404 newtons/millimeter) and the 95 degree condylar blade plate (260 newtons/ millimeter). The DHS also had the highest ultimate load-to-failure (4,877 newtons), followed by the 95 degree DCS (3,107 newtons) and the 95 degree condylar blade plate (2,272 newtons). All of these differences were statistically significant (p < 0.00001 ). CONCLUSIONS: Our findings suggest that the Synthes 95 degree DCS has greater stiffness and strength than the Synthes 95 degree condylar blade plate when tested in this model of an unstable subtrochanteric femur fracture. This model may not be completely appropriate for testing the 135 degree DHS because the hard plastic "cortex" of the model prevented cut-out of the screw.

Bone Plates↗

PubMed1999

Correlation of medial/lateral rotation of the humerus with glenohumeral translation.

OBJECTIVES: To correlate glenohumeral translation in the anterior/posterior direction with medial and lateral rotation of the humerus. In addition, the length of the anterior and posterior component of the glenohumeral capsuloligamentous complex was varied in order to gain insight into the contribution of each component to limiting translation. All measurements were made with the humerus positioned at 90 degrees of abduction and 0 degrees of flexion/ extension. METHODS: Six fresh cadaveric shoulders were used. Each scapula was mounted in a cement pot to rest it in its correct anatomical position. Seven tests were carried out on each shoulder. A series of measurements of translation of the humerus in the anterior direction and posterior direction were taken at 20 degrees intervals of lateral rotation and then at 20 degrees intervals of medial rotation until the limit of lateral or medial rotation had clearly been reached (test 1). The capsuloligamentous complex was then incised and a beaded chain and catches were sutured across the joint to mimic the capsuloligamentous complex at different lengths (tests 2 to 7). RESULTS/CONCLUSIONS: (a) When the glenohumeral capsuloligamentous complex is intact, the humerus translates maximally in the glenoid (between 20 and 30 mm) when the humerus is between 40 degrees and 100 degrees of lateral rotation. (b) As the glenohumeral capsuloligamentous complex increases in length, so does the extent of translation. (c) In medial rotation, the length of the posterior capsule, rather than the length of the anterior capsule, has the greater effect on anterior/posterior translation. (d) In lateral rotation the length of the anterior capsule, rather than the length of the posterior capsule, has the greater effect on anterior/posterior translation. (e) The glenohumeral ligamentous complex acts more as a cuff, enclosing the joint, rather than as a sling, as is commonly thought.

Aged↗

PubMed1998

The effect of compressive force applied to the intervertebral disc in vivo. A study of proteoglycans and collagen.

STUDY DESIGN: Coil springs were stretched and attached to produce a compressive force across the lumbar intervertebral discs of dogs for up to 27 weeks. OBJECTIVE: To test the hypothesis that a high compressive force applied over a period of time affects the production of proteoglycans and collagen by the intervertebral disc cells. SUMMARY OF BACKGROUND DATA: It is a commonly held belief that high forces applied to the intervertebral disc, and to joints in general, play a role in causing degeneration. METHODS: Pairs of stainless steel coil springs were stretched and attached to produce a compressive force across the lumbar intervertebral discs (L1-L2 and L3-L4) of 16 dogs. Dogs were killed between 13 and 27 weeks after the springs were attached. The discs (L1-L2 and L3-L4) were excised and assessed using immunohistochemical analyses and enzyme-linked immunosorbent assay; T13-L1 and L4-L5 were used as controls. RESULTS: The main result relates to a group effect in the six dogs, assessed using enzyme-linked immunosorbent assay, that were generally at the highest values of force for the greatest number of weeks. For the nucleus, but not the anulus, Spearman rank correlations revealed a strong correlation between increases in force and force-weeks (force multiplied by number of weeks) and increases in collagen type I accompanied by decreases in proteoglycans, chondroitin sulfate, and collagen type II for both experimental discs (L1-L2 and L3-L4), as compared with corresponding values in the controls (T13-L1 and L4-L5). In other words, as either the force or the force-weeks increased, the effect on the nucleus became greater. CONCLUSION: A high compressive force applied to the disc over a period of time initiates changes in proteoglycans and collagen.

Aggrecans↗

PubMed1998

Vascularization of the fusion mass in a posterolateral intertransverse process fusion.

STUDY DESIGN: A previously characterized rabbit model was used to study vascularization of the fusion mass in a posterolateral intertransverse process fusion. OBJECTIVES: To determine the interosseus origin of the new blood vessels in a posterolateral intertransverse process fusion mass and to test the hypothesis that bone incorporation and the extent of vascularization are closely related. SUMMARY OF BACKGROUND DATA: It has been reported that vascularization is essential for bone graft incorporation. There are, however, few reports dealing with vascularization of the spinal arthrodesis. METHODS: Thirty-one adult New Zealand White rabbits underwent bilateral intertransverse process fusion, using autogenous iliac crest bone graft. The rabbits were killed at 3 weeks (n = 6) and 6 weeks (n = 25) after surgery, and colored silicone was injected to fix the vasculature. A semiautomated image analysis system was used to assess the percentage of the area of vascularization in the fusion mass and the transverse processes. RESULTS: The major interosseus blood supply for vascularization of the autogenous bone graft came from upper and lower transverse processes. There were three types of fusion mass observed at 6 weeks after surgery: solid type, solid type with cartilaginous cleft, and nonunion type. There was significantly less vascularization of the fusion mass and of the transverse processes in the nonunion type compared with that in the solid type and with that in the cartilaginous cleft type. CONCLUSIONS: There is a close correlation between bone incorporation and the extent of vascularization in a posterolateral intertransverse process fusion.

Angiography↗

PubMed1998

Reversing the inhibitory effect of nicotine on spinal fusion using an osteoinductive protein extract.

STUDY DESIGN: The effect on spinal fusion of an osteoinductive bone protein extract in the presence of a known inhibitor of spinal fusion (systemic nicotine) was studied prospectively in an animal model of posterolateral lumbar fusion. OBJECTIVES: To evaluate the ability of a bovine-derived osteoinductive bone protein extract to overcome the inhibitory effect of nicotine in a rabbit spine fusion model. SUMMARY OF BACKGROUND DATA: Multiple studies have demonstrated the ability of a variety of osteoinductive growth factors to serve as a bone graft substitute for lumbar spinal fusion under "normal" healing conditions. METHODS: Forty-eight adult female New Zealand white rabbits underwent spine arthrodesis at L5-L6 while receiving systemic nicotine through a subcutaneous miniosmotic pump. Arthrodesis was performed using one of the following three graft materials: 1) autogenous iliac crest, 2) osteoinductive bone protein delivered in an allogeneic demineralized bone matrix/ collagen carrier, or 3) osteoinductive bone protein delivered with autogenous iliac crest. Fusions were assessed by blinded manual palpation, radiography, and biomechanical testing. RESULTS: Of the 44 rabbits manually tested by blinded observers, all 14 in the osteoinductive bone protein plus autogenous iliac crest bone group had solid fusions (14 of 14), whereas the fusion rate was less in the osteoinductive bone protein plus demineralized bone matrix group (nine of 14, 64%; P = 0.02), and there were no fusions in the autogenous iliac crest only group (0 of 16, 0%; P = 0.000001). The use of osteoinductive bone protein with autogenous bone produced stronger and stiffer fusions compared with those using autogenous bone alone or osteoinductive bone protein with allograft bone. CONCLUSIONS: Cigarette smoking and nicotine are inhibitory factors in the healing of fractures and spine fusions. This study shows that the inhibitory effect of nicotine can be overcome with an osteoinductive bone growth factor in an animal model.

Animals↗

PubMed1998

The effect of fluid loss on the viscoelastic behavior of the lumbar intervertebral disc in compression.

A viscoelastic finite element model of a L2-L3 motion segment was constructed and used to study: (1) the behavior of the intervertebral disc with different amounts of nucleus fluid loss; and (2) the effect of different rates of fluid loss on the viscoelastic behavior of the disc. The results indicate that: (1) The viscoelastic behavior of the intervertebral disc depends to a large extent on the rate of fluid loss from the disc; the intrinsic properties of disc tissue play a role only at the early stage of compressive loading; (2) the axial strain increases, whereas the intradiscal pressure and the posterior radial disc bulge decrease with increasing fluid loss; (3) a decreasing fluid loss rate with a total fluid loss of 10 to 20 percent (from the nucleus) during the first hour of compressive loading best predicts the overall viscoelastic behavior of a disc.

Biomechanical Phenomena↗

PubMed1998

Biomechanical analysis of hindfoot fixation using an intramedullary rod.

To measure the stiffness of the hindfoot when fixed with an intramedullary rod placed in a retrograde manner, two biomechanical experiments were carried out on five matched pairs of cadaveric below knee specimens. Experiment 1: In the right leg of each pair, an intramedullary rod was placed in a retrograde fashion through the calcaneus, talus, and into the tibial intramedullary canal. Biomechanical testing was done to determine hindfoot stiffness, with and without distal and proximal transverse interlocking screws. The uninstrumented left leg of each pair was tested as a control. Experiment 2: A series of similar biomechanical experiments were done on the same specimens to determine the effect on hindfoot stiffness of an intramedullary rod with one distal screw as compared with a method of tibiotalocalcaneal fixation using three cross-cannulated screws. The results show that an intramedullary rod placed in a retrograde manner stiffens the hindfoot and the placement of interlocking screws enhances that effect (Experiment 1). The intramedullary rod with one distal screw inserted provides more stiffness to the hindfoot than does three cross-cannulated screws (Experiment 2).

Aged↗

PubMed1998

Guidelines to decortication in posterolateral spine fusion.

Despite the development of innovative approaches and the general success that has been achieved with spinal fusion, the rate of nonunion in some studies has been reported as high as 35%. Decortication has been shown to promote the fusion process and provides not only a rich source of vascular supply from the underlying cancellous bone, but also access to pluripotent stem cells within the marrow. Although the blood supply to the lumbar spine has been described, little attention has been paid to relevant areas of the spine most affected by decortication during the posterolateral fusion process. To assess these areas of the spine and attribute some potential importance to spinal fusion outcome, a perfusion study was designed to delineate the vascular anatomy involved in a decortication procedure. Cadaver spines were perfused with a radiopaque contrast material, fixed, decalcified, and cleared en bloc by the method of Spalteholz. Transverse, sagittal, and coronal slabs were made and the vascular supply was documented. The dominant intraosseous architecture of the vertebra reflected a cancellous bone structure, characterized by marrow and a sinusoidal blood distribution within a trabecular matrix. A contrasting architecture could be differentiated in the pars interarticularis that was more consistent with dense, cortical bone. Matrix from this region typified haversian lamellar bone and exhibited parallel osteons that contained a central vascular component. The relevance of this variance could have multiple implications, given the differences between cortical and cancellous bone in function, formation, healing, and remodeling. In posterolateral intertransverse process arthrodesis, the transverse processes and lateral facets are good areas to be decorticated, whereas the pars interarticularis is less attractive.

Aged↗

PubMed1998

The Marshall R. Urist Young Investigator Award. Gene expression during autograft lumbar spine fusion and the effect of bone morphogenetic protein 2.

A prospective animal study of posterolateral lumbar spine arthrodesis was performed to determine the temporal and spatial pattern of gene expression and to determine the effect of recombinant human bone morphogenetic protein 2 on the gene expression pattern of a healing spine fusion mass. In Group 1, 20 adult New Zealand rabbits underwent L4-L5 posterolateral intertransverse process arthrodesis using autograft alone. Two rabbits were euthanized at each of the following points: 0, 2, and 4 days, and 1, 2, 3, 4, 5, 6, and 10 weeks after surgery. The same surgical technique was used for 16 rabbits in Group II, except that the autograft first was soaked in a solution of recombinant human bone morphogenetic protein 2 before implantation. Ribonucleic acid was extracted from different regions of the fusion mass at each point and analyzed for expression of bone and cartilage related genes using reverse transcription polymerase chain reaction. A reproducible temporal sequence and spatial pattern of gene expression was found in healing spine fusions. In the central portion of the fusion mass a temporal lag in gene expression was observed that parallels the lag in healing within the central zone previously observed in histologic studies. Treatment of bone graft with recombinant human bone morphogenetic protein 2 resulted in an increase in the early expression of bone morphogenetic protein 6 which was associated with expression of higher levels of Type I collagen, osteocalcin, and other bone related genes. These findings suggest that central nonunion may be associated with delayed expression of osteoblast related genes in the central region of the forming fusion mass. The growth factor, recombinant human bone morphogenetic protein 2, increased the level of bone related gene expression throughout the fusion mass, eliminated the delay in healing within the central zone, and may decrease the likelihood of a nonunion.

Animals↗

PubMed1998

Biomechanical study of lumbar pedicle screws: does convergence affect axial pullout strength?

We tested the hypothesis that two pedicle screws placed in convergence offer more resistance to axial pullout than do two pedicle screws placed in parallel. Eight fresh cadaveric lumbar spines, L2-L5, were harvested. Individual vertebra were then paired for testing. Into each L2 and L3 vertebra, a pair of pedicle screws were inserted. The screws were placed parallel (i.e., at 0 degrees convergence) in L2 and at 30 degrees convergence in L3, in the first pair. In the second pair of L2 and L3, this order was reversed. Alternating the convergence angle was carried out on each successive pair of L2 and L3. A section of standard longitudinal rod was attached to each pedicle screw by using top-loading three-point shear clamp fixation with tangential lock screws. Transverse connectors were attached superiorly and inferiorly to the longitudinal rods. As in L2 and L3, each L4 and L5 received a pair of screws placed at either 0 or 30 degrees convergence, with the angle of convergence alternated on each successive pair. Each pair of screws in each vertebra was tested in axial pullout at 1 mm/s. Fourteen pairs of vertebra were tested and two findings emerged: paired pedicle screws at 30 degrees of convergence offered more resistance to axial pullout (28.6% on average) and sustained higher loads at the clinical threshold of loosening (101% on average) than paired pedicle screws placed in parallel. Thus there seems to be an advantage to screws placed in 30 degrees of convergence as compared with screws in parallel.

Bone Screws↗

PubMed1998

A biomechanical study of the ankle syndesmosis after fibular graft harvest.

Harvesting the fibula as corticocancellous bone graft is not without complications. Multiple clinical studies chronicled the morbidity at the donor site, both immediately after surgery and at long-term follow-up. To our knowledge, no cadaveric or biomechanical studies addressed the effect that fibular osteotomy has on the ankle syndesmosis. We carried out two experiments by using cadaveric tibias/fibulas to determine the ability of the ankle syndesmosis to resist both lateral and vertical displacement of the intact fibula, as well as lateral and vertical displacement of the distal fibular stump after serial fibular osteotomies had been made at distances of 14, 10, 6, and 4 cm from the tip of the lateral malleolus. The results of this study show that, once the fibula is cut, the syndesmosis provides significantly less resistance to lateral and vertical displacement. Further, although this resistance continues to decrease with each decrease in fibular stump length, the length of the fibular stump does not significantly affect the stability of the syndesmosis.

Aged↗

PubMed1997

Cervical pedicle screws versus lateral mass screws. Anatomic feasibility and biomechanical comparison.

STUDY DESIGN: Biomechanical comparison of the pull-out strengths of lateral mass and pedicle screws in the human cervical spine. Measurements of pedicle dimensions and orientation were compiled. OBJECTIVES: To determine if transpedicular screws provide greater pull-out resistance than lateral mass screws and to investigate the anatomic feasibility of pedicle screw insertion. SUMMARY OF BACKGROUND DATA: Cervical pedicle screws have been reported in limited clinical and biomechanical studies, and some quantitative cervical pedicle anatomy has been reported. No direct biomechanical comparisons have been made between lateral mass and pedicle screws. METHODS: Fifty-six fresh disarticulated human vertebrae (C2-C7) were evaluated with computed tomography to determine morphometry and vertebral body bone density. Lateral mass and pedicle screws were randomized to left versus right. A 3.5-mm cortical screw was used for both techniques, unless a pedicle was narrower than 5.0 mm; then a 2.7-mm cortical screw was used instead. Pedicle wall violations were recorded. Screws were subjected to a uniaxial load to failure. Mean pedicle height, width, and angle with respect to the vertebral midline were tabulated for each level. RESULTS: The mean load-to-failure was 677 N for the cervical pedicle screws and 355 N for the lateral mass screws. No significant correlations for either screw type were found between pull-out strength and bone density, screw length, or vertebral level. Pedicle and lateral mass dimensions were highly variable and not predictive of pull-out strength. Seven (13%) minor pedicle wall violations were observed. CONCLUSIONS: Cervical pedicle screws demonstrated a significantly higher resistance to pull-out forces than did lateral mass screws. The variability in pedicle morphometry and orientation requires careful preoperative assessment to determine the suitability of pedicle screw insertion.

Biomechanical Phenomena↗

PubMed1997

Nuclear magnetic resonance timecourse studies of glyphosate metabolism by microbial soil isolates.

1. Triple Resonance Isotope EDited nmr spectroscopy (TRIED) has been developed to detect and examine minute levels of glyphosate metabolites in microbial soil isolates. Using stable isotopic labelling (13C and 15N), TRIED allows the simultaneous detection of multiple metabolites in crude matrices at submicrogram levels. An improvement over earlier techniques where milligrams are needed, TRIED can detect 500 ng of triply labelled compound in a crude sample (1:14,000 mass ratio) in just hours. 2. TRIED is used here to compare the kinetics and metabolic pathways of glyphosate metabolism by two strains of Ochrobactrum anthropi, LBAA and S5. Both LBAA and S5 appear to metabolize glyphosate primarily via the aminomethylphosphonate (AMPA) pathway, since no detectable levels of glycine or sarcosine are observed in the media or lysates of either microbe. The formation of N-methylAMPA is common to the metabolism of both microorganisms, but N-acetylAMPA is observed only in LBAA. N-methylacetamide is detected predominantly in media and lysates of S5, although some evidence also points to the formation of this metabolite in LBAA. 3. Results are consistent with conventional radioactive tracer studies. TRIED nmr provides more specific structural information complementary to radiolabel methods. Both nmr and radioactivity studies show S5 glyphosate metabolism to be much slower than that of LBAA.

Animals↗

PubMed1997

Intraarticular calcaneus fractures: a biomechanical comparison or two fixation methods.

OBJECTIVE: To compare the strength of fixation obtained using two unicortical 4.0 mm diameter cancellous screws versus two bicortical 3.5 mm diameter cortical screws in the fixation of intra-articular calcaneus fractures. DESIGN: In eight paired fresh frozen cadaver feet, the subtalar joint complex, consisting of the talus, interosseous ligaments, and calcaneus was removed as a unit. An oblique osteotomy of the calcaneus was then created in the sagittal plane. The two fragments were reduced and stabilized using either two unicortical 4.0 mm diameter cancellous screws or two bicortical 3.5 mm diameter cortical screws. A force was then applied to the medial calcaneal tuberosity until failure occurred. In all specimens testing was discontinued due to ligamentous failure, without causing bony disruption. RESULTS: In all specimens the forced applied to the calcaneus created a disruption of the interosseous ligaments across the subtalar joint and rotation of the calcaneus on the talus. Thus in each specimen, testing was discontinued due to interosseous ligamentous failure, without causing bony disruption. There was no statistically significant difference in force to failure between the specimens fixed with 3.5 mm diameter cortical screws as compared to those fixed with 4.0 mm diameter cancellous screws. CONCLUSIONS: The lack of a statistically significant difference in force to failure of the intra-articular calcaneus fractures fixed using 3.5 mm diameter cortical screws as compared to those fixed using 4.0 mm diameter cancellous screws and, the fact that the mode of failure was a disruption of ligaments rather than bony disruption suggest that either method of screw fixation is biomechanically adequate.

Aged↗

PubMed1997

In vivo evaluation of a resorbable osteoinductive composite as a graft substitute for lumbar spinal fusion.

The purpose of this prospective animal study was to evaluate the efficacy of a resorbable coral particulate to serve as a carrier with several doses of a bovine-derived osteoinductive bone protein mixture. A previously validated rabbit model for posterolateral intertransverse process lumbar spinal fusion was used. Posterolateral intertransverse process arthrodeses were performed at L5-L6 in 64 adult New Zealand white rabbits. The bone graft substitute evaluated consisted of a Biocoral/collagen composite with one of four doses (0, 100, 300, or 1,000 micrograms) of a bovine-derived osteoinductive bone protein extract (BP). Fusions were assessed at 5 weeks by manual palpation, radiography, histology, and biomechanical testing. Use of the Biocoral/collagen carrier without BP resulted in no solid fusions. Biocoral/collagen with 100 micrograms BP resulted in solid fusions in 31% (4 of 13) of the rabbits. Biocoral with 300 micrograms or 1,000 micrograms BP resulted in solid spinal fusion in all rabbits (27 of 27). There were no neurologic complications related to the growth factor or carrier. Small subcutaneous collections of serous fluids were noted in 26% of the animals in the 1st postoperative week, but resolved without problems by the 5th week. Such seromas may limit the clinical utility of this growth factor-carrier combination. Biocoral/collagen combined with the appropriate dose of bovine-derived osteoinductive bone protein was efficacious as a bone graft substitute for achieving posterolateral lumbar spinal fusion in the rabbit model. A dose of 300 micrograms BP was determined as the threshold to reliability produce solid spinal fusions.

Absorption↗

PubMed1997

Analysis of chondroitin sulfate in lumbar intervertebral discs at two different stages of degeneration as assessed by discogram.

Previous studies have presented evidence that an underlying cause of intervertebral disc degeneration is related to changes in the sulfation of the proteoglycans. The sulfation of the chondroitin in cadaveric lumbar intervertebral discs, at two different stages of degeneration as assessed by discogram, were analyzed. Fourteen of 28 lumbar discs were graded 2 and the other 14 were graded 4 (i.e., more degenerated). From each disc, six regional segments were carefully isolated. Proteoglycans were solubilized from the disc tissue with 4 M GuHCl. Chondroitin sulfate chains were analyzed by diethylaminoethyl (DEAE)-Sephacel and high-performance liquid chromatography (HPLC) anion exchange chromatography. The major differences in sulfation of the chondroitin between grade 2 and grade 4 discs only occurred in the posterior central annulus and nucleus segments. The chondroitin in the posterior central nucleus segments of the grade 2 and grade 4 intervertebral discs were undersulfated as compared with the other segments, and the differences between these segments and the others were more accentuated in the grade 4 discs than in grade 2 discs.

Cadaver↗

PubMed1997

Impingement after total hip arthroplasty related to prosthetic component selection and range of motion.

This study was designed to define the relationship between different combinations of prosthetic head diameters, neck lengths, and acetabular containment angles, and range of motion before component impingement. Three cadaveric pelves with attached lower limbs were mounted in their correct anatomic position. Acetabular and modular femoral stems were inserted into each of the six hips. For each combination of femoral head diameter, neck length, and acetabular liner overhang, the range of motion to impingement was measured in flexion, extension, abduction, adduction, external rotation, internal rotation, and internal rotation with 90 degrees hip flexion. These experiments suggest that the maximum range of motion before impingement can be attained by increasing the prosthetic head diameter and avoiding longer neck lengths with skirts. Acetabular liners with greater overhang decrease motion in all planes except flexion when the overhang is positioned posteriorly.

Arthroplasty, Replacement, Hip↗

PubMed1996

Do bending, twisting, and diurnal fluid changes in the disc affect the propensity to prolapse? A viscoelastic finite element model.

STUDY DESIGN: A finite element model of a lumbar motion segment was constructed. OBJECTIVES: The model was directed toward understanding the effect of compression, bending and twisting, and diurnal fluid changes in the disc on the propensity to disc prolapse. Tensile stresses in the anulus fibers were computed and used to determine the successive steps required to create a fissure in the disc. SUMMARY OF BACKGROUND DATA: Disc prolapse is more likely under combined loading involving compression and bending and twisting. Changes in fluid content in the disc also affect the mechanical behavior of the disc. METHODS: The three-dimensional model accounted for the viscoelastic material properties of the anulus fibers and ligaments. Diurnal fluid exchange was simulated by changing the fluid content in the nucleus of the disc. Combined with bending and twisting, a compressive load was applied at different loading rates. RESULTS: The maximum tensile stress in the anulus fibers always occurred in the fibers at the inner posterior anulus at the junction of the disc and the endplate. Of the three models tested, the "weakest" (or the first to fail) was the saturated disc subjected to compression and bending and twisting. As the loading rate increased, anulus fiber failure was initiated at a lower value of compressive load. An increasing compressive load applied to a flexed, twisted, and saturated disc resulted in progressive failure, or fissure propagation, starting at the posterior inner anulus at the junction of the disc and the endplate. CONCLUSIONS: The results from this study suggest that there are several key factors involved in the initiation and propagation of anulus failure: axial compressive load, bending and twisting, and disc saturation. If one of these is lacking, anulus failure is harder to achieve.

Elasticity↗