Perfusion-limited recovery of evoked potential function after spinal cord injury.

STUDY DESIGN: The current study was designed to determine whether progressive spinal cord damage during residual compression is caused by low blood flow and ischemia. OBJECTIVES: The purpose of this experiment was to determine the effects of sustained spinal cord compression on regional blood flow and evoked potential recovery after time-dependent decompression. SUMMARY OF BACKGROUND DATA: Spinal cord injury after trauma is commonly associated with residual cord compression. Although decreased blood flow has been reported after spinal cord contusion, the effect of residual spinal cord displacement on reperfusion of blood flow or recovery of neurologic function remains unclear. METHODS: Eighteen beagles were anesthetized, and the spinal cord at T13 was loaded dorsally under precision loading conditions until evoked potential amplitudes were reduced by 50%. At this function endpoint, spinal cord displacement was maintained for 90 minutes. Somatosensory-evoked potentials were measured at regular intervals until 3 hours after decompression. Regional spinal cord blood flow was measured with a fluorescent microsphere technique at regular time points during and after spinal cord decompression. RESULTS: Within 5 minutes after dynamic cord compression was discontinued, evoked potential signals were absent in all dogs. Evoked potential recovery was observed after decompression in 7 of 18 dogs. Regional spinal cord blood flow at baseline, 21.8 +/- 1.9 mL/100 g. min (mean +/- SE), decreased to 3.9 +/- 0.9 mL/100 g. min after dynamic compression was discontinued. Although spinal cord-piston interface pressure dissipated by 87% of maximum interface pressure during sustained compression, mean blood flow recovered to only 34% of baseline flow. In the 7 dogs that recovered evoked potential function, blood flow increased to 11.3 +/- 2.7 mL/100g. min immediately before decompression (P < or = 0.05). In the 11 dogs that did not recover evoked potential function after decompression, regional blood flow did not improve during sustained compression. CONCLUSIONS: Recovery of evoked potential function after decompression corresponded with a greater return of blood flow during sustained displacement and greater reperfusion of blood flow associated with decompression.

Biomechanical evaluation of posterior cervical stabilization after a wide laminectomy.

STUDY DESIGN: In vitro biomechanical investigation with nondestructive and destructive testing in a human cadaveric model simulating a wide postlaminectomy condition. OBJECTIVES: To determine the relative stability conferred by a posterior cervical spinal rod system and posterior cervical plating. SUMMARY OF BACKGROUND DATA: Posterior cervical plate fixation has been shown to be biomechanically superior to wiring techniques, but lateral mass screws may injure neurovascular structures or facet joints if they are inserted improperly. A cervical rod system has been developed to enhance the safety of lateral mass instrumentation. METHODS: The cervical spines of 12 cadavers underwent biomechanical testing. After completion of the nondestructive intact testing, a wide laminectomy with subtotal facetectomies from C4 to C6 was performed. The specimens in two subgroups (group A, cervical spine rods with unicortical fixation, and group B, reconstruction plates with bicortical fixation) were tested in flexion, lateral bending, and torsion. Finally, destructive testing in flexion was performed. Stiffness, neutral zone, failure moment, energy to failure, and mechanism of failure were determined for each specimen. The data were analyzed using paired t tests and ANOVA. RESULTS: Group B had a greater mean screw torque value. The instrumented constructs had a greater stiffness ratio (instrumented/intact) than the intact specimens in flexion, lateral bending, and torsional testing. Group A had a significantly greater flexural stiffness than Group B. Neutral zone ratio values were significantly lower during flexural testing for the cervical rod construct. Destructive testing resulted in significantly greater failure moment and energy-to-failure values for group A. In the cervical rod construct, failure occurred primarily by superior screw loosening with pull-out from the lateral mass. Reconstruction plates consistently failed with fracture of the lateral mass and superior screw loosening. CONCLUSION: Significantly greater stability was noted in the cervical rod construct during nondestructive and destructive flexural testing.

Viscoelastic relaxation and regional blood flow response to spinal cord compression and decompression.

STUDY DESIGN: To better understand the relationships between primary mechanical factors of spinal cord trauma and secondary mechanisms of injury, this study evaluated regional blood flow and somatosensory evoked potential function in an in vivo canine model with controlled velocity spinal cord displacement and real-time piston-spinal cord interface pressure feedback. OBJECTIVES: To determine the effect of regional spinal cord blood flow and viscoelastic cord relaxation on recovery of neural conduction, with and without spinal cord decompression. SUMMARY OF BACKGROUND DATA: The relative contribution of mechanical and vascular factors on spinal cord injury remains undefined. METHODS: Twelve beagles were anesthetized and underwent T13 laminectomy. A constant velocity spinal cord compression was applied using a hydraulic loading piston with a subminiature pressure transducer rigidly attached to the spinal column. Spinal cord displacement was stopped when somatosensory evoked potential amplitudes decreased by 50% (maximum compression). Six animals were decompressed 5 minutes after maximum compression and were compared with six animals who had spinal cord displacement maintained for 3 hours and were not decompressed. Regional spinal cord blood flow was measured with a fluorescent microsphere technique. RESULTS: At maximum compression, regional spinal cord blood flow at the injury site fell from 19.0 +/- 1.3 mL/100 g/min to 12.6 +/- 1.0 mL/100 g/min, whereas piston-spinal cord interface pressure was 30.5 +/- 1.8 kPa, and cord displacement measured 2.1 +/- 0.1 mm (mean +/- SE). Five minutes after the piston translation was stopped, the spinal cord interface pressure had dissipated 51%, whereas the somatosensory evoked potential amplitudes continued to decrease to 16% of baseline. In the sustained compression group, cord interface pressure relaxed to 13% of maximum within 90 minutes; however, no recovery of somatosensory evoked potential function occurred, and regional spinal cord blood flow remained significantly lower than baseline at 30 and 180 minutes after maximum compression. In the six animals that underwent spinal cord decompression, somatosensory evoked potential function and regional spinal cord blood flow recovered to baseline 30 minutes after maximum compression. CONCLUSIONS: Despite rapid cord relaxation of more than 50% within 5 minutes after maximum compression, somatosensory evoked potential conduction recovered only with early decompression. Spinal cord decompression was associated with an early recovery of regional spinal cord blood flow and somatosensory evoked potential recovery. By 3 hours, spinal cord blood flow was similar in both the compressed and decompressed groups, despite that somatosensory evoked potential recovery occurred only in the decompressed group.

Early time-dependent decompression for spinal cord injury: vascular mechanisms of recovery.

Although surgical decompression is often advocated for acute spinal cord injury, the timing and efficacy of early treatment have not been clinically proven. Our objectives were to determine the importance of early spinal cord decompression on recovery of evoked potential conduction under precision loading conditions and to determine if regional vascular mechanisms could be linked to electrophysiologic recovery. Twenty-one mature beagles were anesthetized and mechanically ventilated to maintain normal respiratory and acid-base balance. Somatosensory-evoked potentials from the upper and lower extremities were measured at regular intervals. The spinal cord at T-13 was loaded dorsally under precision loading conditions until evoked potential amplitudes had been reduced by 50%. At this functional endpoint, spinal cord displacement was maintained for either 30 (n = 7), 60 (n = 8), or 180 min (n = 6). Spinal cord decompression was followed by a 3-h monitoring period. Regional spinal cord blood flow was measured with fluorescent microspheres at baseline (following laminectomy) immediately after stopping dynamic cord compression, 5, 15, and 180 min after decompression. Within 5 min after stopping dynamic compression, evoked potential signals were absent in all dogs. We observed somatosensory-evoked potential recovery in 6 of 7 dogs in the 30-min compression group, 5 of 8 dogs in the 60-min compression group, and 0 of 6 dogs in the 180-min compression group. Recovery in the 30- and 60-min groups varied significantly from the 180-min group (p < 0.05). Regional spinal cord blood flow at baseline, 21.4+/-2.2 ml/100/g/min (combined group mean +/- SE) decreased to 4.1+/-0.7 ml/100 g/min after stopping dynamic compression. Reperfusion flows after decompression were inversely related to duration of compression. Of the 7 dogs in the 30 min compression group, 5 min after decompression the blood flow was 49.1+/-3.1 ml/100 g/min, which was greater than two times baseline. In the 180-min compression group early post-decompression blood flow, 19.8+/-6.2 ml/100 g/min, was not significantly different than baseline. Of the 8 dogs in the 60-min compression group, 5 who recovered evoked potential conduction revealed a lower spinal cord blood flow sampled immediately after stopping dynamic compression, 2.1+/-0.4 ml/100 g/min, compared to the 3 who did not recover where blood flow was 8.4+/-2.1 ml/100 g/min (p < 0.05). Reperfusion flows measured as the interval change in blood flow between the time dynamic compression was stopped to 5, 15, or 180 min after decompression, were significantly greater in those dogs that recovered evoked potential function (p < 0.05). Three hours after decompression, spinal cord blood flow in the 3 dogs in the 60-min compression group with no recovery, 11.1+/-2.1 ml/100 g/min, was significantly less than the spinal cord blood flow of the recovered group (n = 5), 20.5+/-2.2 ml/100 g/min. These data illustrate the importance of early time-dependent events following precision dynamic spinal cord loading and sustained compression conditions. Spinal cord decompression performed within 1 h of evoked potential loss resulted in significant electrophysiologic recovery after 3 h of monitoring. This study showed that the degree of early reperfusion hyperemia after decompression was inversely proportional to the duration of spinal cord compression and proportional to electrophysiologic recovery. Residual blood flow during the sustained compression period was significantly higher in those dogs that did not recover evoked potential function after decompression suggesting a reperfusion injury. These results indicate that, after precise dynamic spinal cord loading to a point of functional conduction deficit (50% decline in evoked potential amplitude), a critical time period exists where intervention in the form of early spinal cord decompression can lead to effective recovery of electrophysiologic function in the 1- to 3-h post-decompression p

Altered ovarian sterol carrier protein expression in the pregnant streptozotocin-treated diabetic rat.

Reproductive dysfunction in the diabetic female rat is associated with impaired folliculogenesis, reduced corpus luteum progesterone output, and spontaneous abortion. The underlying mechanism for reduced steroid production remains unresolved. In this study we examined whether or not diabetes alters levels of P450 side-chain cleavage enzyme (P450scc), 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), or the cholesterol transport proteins, steroidogenic acute regulatory (StAR) protein and sterol carrier protein-2 (SCP2), leading to lower progesterone levels and pregnancy loss. Rats (Day 3 pregnant) received an injection of streptozotocin (STZ, 60 mg/kg; i.v.) to induce a diabetic state; P450scc, 3 beta-HSD, and SCP2 were examined by Western and Northern blot analysis in ovarian tissue 12 days after injection of STZ (diabetic rats, n = 12) or vehicle (nondiabetic rats, n = 12). Serum progesterone, triglyceride, and beta-hydroxybutyrate (beta-HBA) levels were also examined. Results indicate that diabetic rats that aborted (diabetic-fetus [Ft], n = 6) had significantly lower progesterone levels (7.04 +/- 2.6 ng/ml; p < 0.004) than nondiabetic animals (108.6 +/- 5.15 ng/ml) and diabetic +Ft animals (74.3 +/- 8.9 ng/ml, n = 6). Western blot analysis of ovarian P450scc and 3 beta-HSD in the nondiabetic rats and the diabetic rats with fetuses indicated no significant difference. In contrast, ovaries from diabetic animals without fetuses had significantly lower SCP2 levels (p < 0.017) compared to controls. Concomitant with the reduction in SCP2, a 58-kDa SCP2-immunoreactive protein, referred to as sterol carrier protein-X (SCPx), increased significantly (p < 0.001). The C-terminal sequence of SCPx is identical to SCP2, while its N-terminal region is homologous with 3-oxoacyl coenzyme A thiolase, an enzyme involved in fatty acid metabolism. Increased SCPx expression coincided with increased serum triglyceride and beta-HBA levels, suggesting that the enhanced SCPx level may coincide with an ovarian shift to fatty acid metabolism. When SCPx steady-state mRNA levels were measured using an SCPx-specific riboprobe (280-bp protected fragment) in a ribonuclease protection assay, ovarian SCPx mRNA levels in the diabetic animals were increased 4.2-fold compared to control SCPx mRNA levels. Ovarian StAR mRNA levels were increased slightly in the diabetic animals, and ovarian P450scc and 3 beta-HSD mRNA levels were increased 3-fold in the diabetic animals that aborted relative to the nondiabetic animals and the +Ft diabetic animals. Results of this study confirm that SCPx mRNA levels are elevated following diabetes onset and that StAR, P450scc, and 3 beta-HSD mRNA levels do not correspond with the reduced steroid hormone profile associated with diabetes. These results are concordant with the possibility that reduced steroid levels in the diabetic animals reflect a loss of SCP2-mediated cholesterol transport capacity as SCPx/3-oxoacyl coenzyme A thiolase expression is enhanced.

Prostaglandin F2 alpha mediates ovarian sterol carrier protein-2 expression during luteolysis.

In the corpus luteum, prostaglandin F2 alpha (PGF2 alpha) appears to be a physiological agent with both antisteroidogenic and luteolytic actions. It is hypothesized that the antisteroidogenic action of PGF2 alpha acts through altered transport of cholesterol to the mitochondrial cytochrome P450 side-chain cleavage enzyme (P450scc). However, the effect of PGF2 alpha on the expression of the putative cholesterol transport protein, sterol carrier protein-2 (SCP2; 13.2 kilodaltons), has not been examined. In this study, the decline in serum progesterone after PGF2 alpha injection was examined in parallel with altered ovarian SCP2, P450scc, and 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) protein and messenger RNA (mRNA) levels. Rats (28 days old) were treated with 8 IU PMSG to induce follicular development and ovulation. Ten days after ovulation, animals were treated with PGF2 alpha (single or multiple injections; 100-250 micrograms each) or left untreated. Ovarian SCP2, P450scc, and 3 beta HSD protein and mRNA levels were examined 0 (time zero), 4, and 8 h post-PGF2 alpha treatment using Western and Northern blot analysis. SCP2 mRNA levels were also examined using a highly sensitive ribonuclease protection assay that detects a 429-base pair SCP2-mRNA specific sequence. The results indicate that serum progesterone was significantly reduced 4 and 8 h after PGF2 alpha injections (P < 0.001; n = 6/time point). The decline in progesterone paralleled a 50-60% reduction in 3 beta HSD protein and mRNA levels by 4 h post-PGF2 alpha. Protein and mRNA levels for 3 beta HSD returned to control values by 8 h post-PGF2 alpha treatment. P450scc expression was also reduced at 4 h (44-54%), but by 8 h, both protein and mRNA levels had increased above the normal control levels (P < 0.02). In contrast, the 0.8-kilobase SCP2-specific mRNA transcript was reduced to 50% and 80% of the pre-PGF2 alpha treatment level at 4 and 8 h, respectively (P < 0.01). SCP2 ribonuclease protection assay analysis also indicated that SCP2 mRNA levels were reduced 65% (P < 0.03) and 85% (P < 0.01) by 4 and 8 h post-PGF2 alpha treatment compared to those in time zero ovarian tissue. Consistent with the loss of SCP2 mRNA expression, Western blot analysis indicated that a 15-kilodalton SCP2-immunoreactive protein (presumably the pro-SCP2 form) was significantly reduced or absent in the PGF2 alpha treated animals (P < 0.04).(ABSTRACT TRUNCATED AT 400 WORDS)

Differential expression of hepatic sterol carrier proteins in the streptozotocin-treated diabetic rat.

Sterol carrier protein-2 (SCP2) is a 13.2-kilodalton protein that has been implicated in intracellular cholesterol transport, whereas a related sterol carrier protein, sterol carrier protein-X (SCPx; 58 kilodaltons) has been suggested to function also in the beta-oxidation of fatty acids. Although diabetes-related hyperlipidemia and altered cholesterol metabolism have been extensively studied, the intracellular cholesterol transport capacity during hyperglycemic states has not been examined. The fact that beta-oxidation is increased in diabetes whereas hepatic cholesterol metabolism is reduced suggests that differential expression of these sterol carrier proteins may accompany diabetic dyslipidemia. In this study, SCP2 protein levels were reduced by 60% in mildly hypercholesterolemic (cholesterol, > 130 and < 150 mg/dl; P < 0.01) diabetic rats and by 90% in severely hypercholesterolemic (cholesterol, > 150 mg/dl; P < 0.002) diabetic animals. In contrast, hepatic SCPx protein expression increased (3.5-fold) after diabetes induction with streptozotocin (STZ). The decline in SCP2 was inversely related to serum cholesterol levels. Hepatic SCP messenger RNA levels examined by ribonuclease protection assay demonstrated that hepatic SCP messenger RNA was increased 2-fold in diabetic animals. Northern blot analysis indicated that both the 0.8-kilobase SCP2-specific and the 2.1-kilobase SCPx-specific transcripts increased after STZ injection. SCPx protein induction preceded the decline in SCP2 by 4-5 days. Insulin treatment reversed the increase in SCPx and prevented the decline in SCP2. We conclude that SCP2 and SCPx are differentially expressed in the STZ-diabetic rat and suggest that this change in SCP expression should be considered a potential contributing mechanism through which cholesterol metabolism may be altered in diabetes.

Reduced hepatic sterol carrier protein-2 expression in the streptozotocin treated diabetic rat.

While a strong relationship between the hypercholesterolemia of diabetes and premature atherosclerosis is established, the etiology for the elevation in serum cholesterol in this disease is unknown. To determine whether diabetic hypercholesterolemia may be related to alterations in hepatic cholesterol transport capacity, sterol carrier protein-2 (SCP2) expression was examined in rats treated with streptozotocin (SZT). Furthermore, this study examined whether 17ß-estradiol and insulin confer a protective effect on liver cholesterol homeostasis by maintaining hepatic SCP2 levels. SCP2 protein and mRNA expression were examined 13 days following SZT-induced diabetes onset and in diabetic rats treated with estradiol (1 cm silastic implant) or insulin (12 units/day). Data indicate that SCP2 protein levels were significantly reduced in the diabetic animals and that SCP2 protein expression in the liver was inversely related to the level of serum cholesterol in the diabetic animals. In contrast, SCP2 mRNA levels examined by slot blot, ribonuclease protection assay, and Northern blot analysis were significantly elevated. Both insulin and estradiol were able to enhance the expression of SCP2 protein in the liver following SZT treatment. The results of this investigation clearly indicate that hepatic SCP2 protein levels are significantly altered in the diabetic state suggesting that cholesterol transport capacity is reduced in the SZT-treated diabetic rat. The inverse relationship between serum cholesterol and hepatic SCP2 protein content suggests that the reduction in this protein may be a contributing factor in diabetic hypercholesterolemia.

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)

Anterior spinal fixators. A biomechanical in vitro study.

In vitro calf spine testing was performed in flexion, rotation, and axial load, using a vertebral body corpectomy and anterior iliac crest bone grafting model. Anterior spinal fixation devices then were sequentially tested, and axial stiffness, torsional stiffness, and flexural strain determined. The constructs tested were the Contoured Anterior Spinal Plate (CASP), the Kaneda device, the Kostuik-Harrington device (KH), and the Texas Scottish Rite Hospital (TSRH) vertebral body screw construct. In torsion, the Kaneda device returned spinal stability to that of the intact spine. The Kostuik-Harrington device was unstable in torsion. In axial loading and flexion, the Kaneda device and the TSRH construct proved the most stiff, with the KH and CASP systems significantly lower in stiffness. The authors believe that the Kaneda device and the TSRH vertebral body screw construct are effective in restoring acute stability to the lumbar spine after corpectomy.

Biomechanical study of sternal closure techniques.

Median sternotomy is the most commonly used incision in cardiothoracic surgery. Closure of this incision is usually performed with parasternal wires, but alternate techniques have been proposed to improve closure stability. This study compares biomechanical stability of standard wire (No. 5 stainless steel) with that of three types of band closure: 5-mm Mersilene ribbon, 5-mm stainless steel band, and 5-mm plastic band. Eight bisected cadaver sterna were reapproximated using each method of sternal fixation and tested for biomechanical stability using an MTS Bionix 858 Biomechanical Tester. Loads of 50, 100, 150, and 200 Newtons (1 Newton = 1 kg.m/s2) were applied as a distracting force across the closure. A linear regression line of displacement as a function of increasing load was determined for each closure method; the slope of this line is inversely proportional to fixation stability. Displacement and load correlated linearly for each closure (r = 0.99). Mean slopes were 0.012 mm/Newton (95% confidence limits, 0.0098 to 0.0142 mm/Newton) for No. 5 stainless steel wire, 0.014 mm/Newton (95% confidence limits, 0.0118 to 0.0162 mm/Newton) for plastic band, 0.017 mm/Newton (95% confidence limits, 0.0148 to 0.0192 mm/Newton) for Mersilene ribbon, and 0.017 mm/Newton (95% confidence limits, 0.0148 to 0.0192 mm/Newton) for 5-mm steel band. No. 5 stainless steel wire provided the most stable closure, although statistical significance was achieved only in comparison with Mersilene ribbon and stainless steel band (p < 0.05). The superior stability of stainless steel wire closure may be due to tightening of the wires by twisting, which results in more tension across the reapproximated sternal halves than with other methods.

A biomechanical analysis of solvent-dehydrated and freeze-dried human fascia lata allografts. A preliminary report.

UNLABELLED: This study compares the basic mechanical properties of two groups of commercially available fascia lata allografts processed by different means (solvent-dehydrated and sterilized via gamma radiation, and freeze-dried without secondary sterilization). The results reveal significantly (P less than 0.05) higher stiffness, higher maximum load to failure, and higher maximum load per unit width of graft with the solvent-dried as opposed to the freeze-dried fascia lata. Subsections of individual solvent-dried specimens were also more uniform in their mechanical properties than those of the freeze-dried allografts. CLINICAL RELEVANCE: Fascia lata is used as a graft material in a variety of orthopaedic procedures. Allograft fascia lata offers an increased cross-sectional area of material and eliminates the morbidity associated with the harvesting of autologous tissues. However, the structural uniformity of such large grafts has been questioned. Processing techniques used in the sterilization and storage of such grafts is varied and represents a potential source of variation in the mechanical properties of allograft specimens. The results of this study suggest that a commercially available solvent-dehydrated form of fascia lata provides a more suitable grafting material than freeze-dried specimens obtained from tissue banks.

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.

Influence of disc degeneration on mechanism of thoracolumbar burst fractures.

In order to clarify the pathomechanism of thoracolumbar burst fractures and to evaluate the influence of disc degeneration and bone mineral density, a biomechanical study was performed using cadaveric spines. Eleven motion segments of thoracolumbar spines from human cadavers were compressed vertically until a fracture occurred. In addition, bone mineral density and degree of disc degeneration were determined for each specimen. Compression of 7 of 11 specimens resulted in the typical burst fracture characterized by retropulsion of a bony fragment into the spinal canal and an increase of the interpedicular distance. All seven specimens showed disruptions of the middle end plate and disc materials in the vertebral body. The fracture line was located between the middle of the end plate and the middle of the posterior wall cortex. No burst fractures were seen in the specimens with severely degenerated discs and osteoporosis. In order to confirm the stress state in a vertebra that induces the burst fracture, finite element analysis of one motion segment was also carried out under the same mechanical conditions as the experiments in this study. As a result of calculation for the healthy disc, the highest stresses under axial compression were concentrated in the following areas: the middle of the end plate, the cancellous bone under the nucleus pulposus, and the middle of the posterior wall cortex. This implies that the above regions are more vulnerable to vertical compressive load. In the analysis of specimens with severely degenerated discs, stresses were very low at the end plate and cancellous bone under the nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Cervical stability after foraminotomy. A biomechanical in vitro analysis.

Laminectomy or facetectomy of the cervical spine, or both, may be needed for decompression of the spinal cord or of the nerve-roots. Acute stability of the cervical spine was tested after laminectomy and progressive staged foraminotomies in an in vitro model. Twelve cervical spines from human cadavera were used in the experiment. Biomechanical testing included the application of an axial load, the application of a flexion and extension moment, and the application of a torsional moment. Each specimen was tested intact, after laminectomy of the fifth cervical vertebra, and after progressive foraminotomy of the sixth cervical root. Foraminotomy was performed by resection of 25, 50, 75, and 100 per cent of the facet joint and capsule. Torsional stiffness decreased dramatically when more than 50 per cent of the facet had been resected. Statistically equivalent subsets were the intact specimen, laminectomy, 25 per cent facetectomy, and 50 per cent facetectomy in one subset, and 75 and 100 per cent facetectomy in the least-stiff subset. Flexion-moment testing showed that the posterior strain did not differ among three groups: the intact specimens, those that had been treated with laminectomy, and those that had been treated with a 25 per cent facetectomy. The 50 per cent facetectomy resulted in a 2.5 per cent increase in posterior strain, and the 75 or 100 per cent facetectomy, in a 25 per cent increase in posterior strain compared with the intact specimen. Segmental hypermobility of the cervical spine results if a foraminotomy involves resection of more than 50 per cent of the facet.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of acetabular stability in uncemented prostheses.

The stability of two commonly used uncemented acetabular prostheses fixed with either pegs or screws has been established by evaluating the torque strengths in ten cadaveric acetabulae. The initial stability of implants to simulated frictional loads was satisfactory compared to previous estimations of the frictional torques in polyethylene-on-metal and metal-on-metal articulations. Testing of the torque required to produce 2 degrees of rotation at the bone-implant interface showed that the peg fixation required significantly greater loads than did the screw fixation (P less than .03). Testing the prostheses to failure gave good results, although lower than previous cemented arthroplasty estimations. The margin of stability over the frictional torques that can be generated is such that it may affect the bony ingrowth into these prostheses. This may account for the common finding of fibrous ingrowth in these prostheses.

The effect of spinal implant rigidity on vertebral bone density. A canine model.

An animal model of anterior and posterior column instability was developed to allow in vivo observation of bone remodeling and arthrodesis following spinal instrumentation. After an initial anterior and posterior destabilizing lesion was created at the L5-L6 vertebral levels in 63 adult beagles, various spinal reconstructive surgical procedures were performed--with or without bilateral posterolateral bone grafting, with or without bilateral oophorectomies, and with or without spinal instrumentation (Harrington distraction, Luque rectangular, Cotrel-Dubousset pedicular, or Steffee pedicular implants). Observation 6 months after surgery revealed a significantly improved probability of achieving a spinal fusion if spinal instrumentation had been used (X2 = 5.84, P = .016). Nondestructive mechanical testing after removal of all metal instrumentation in torsion, axial compression, and flexion revealed that the fusions performed in conjunction with spinal instrumentation were more rigid (P less than .05). Quantitative histomorphometry showed that the volumetric density of bone was significantly lower (ie, device-related osteoporosis occurred) for fused versus unfused spines. In addition, a linear correlation occurred between decreasing volumetric density of bone and increasing rigidity of the spinal implant (r = .778); ie, device-related osteoporosis occurred secondary to Harrington, Cotrel-Dubousset, and Steffee pedicular instrumentation. Oophorectomized dogs became more osteoporotic than their surgically matched controls (posterolateral bone grafting alone, Cotrel-Dubousset pedicular instrumentation, and Steffee pedicular instrumentation); device-related osteoporosis added to the degree of hormonally induced osteoporosis (t = 5.0, P less than .0001). This is the first study to date documenting the occurrence of stress shielding in the spine secondary to spinal instrumentation.(ABSTRACT TRUNCATED AT 250 WORDS)

Triangulation of pedicular instrumentation. A biomechanical analysis.

Tensile load-to-failure pullout tests were performed on 54 cadaveric spinal segments. The vertebrae were grouped by adjacent levels and matched for bone mineral density (g/cm2), which was measured by dual-photon absorptiometry. Triangulation of Steffee screws and CD pedicle screws was accomplished by transverse plates specifically designed to increase fixation within the same vertebra before the longitudinal Steffee plate or CD rod was applied. A transverse plate with adjustable length was also tested to accommodate variable interpedicular distances. Comparative pullout load-to-failures were as follows: laminar hook, 809 SE 99.4 N; single CD pedicular screws, 863 SE 108 N; single Steffee pedicular screw, 1245 SE 75.3 N; adjustable transverse plate, 1341 SE 114; triangulated Steffee pedicle screws with a transverse plate, 1701 SE 151 N; and triangulated CD pedicle screws with a transverse plate, 2096 SE 115 N. Three triangulated constructs with pedicle screws and a transverse plate (CD, Steffee, and Kirschner) provided significantly greater fixation than conventional pedicular or laminar hook based instrumentation systems (P less than 0.05). Improved treatment of spinal deformities in the elderly and osteoporotic population is dependent on improving the fixation at the metal-bone interface of spinal implants Particularly in osteoporotic vertebrae, the strength of fixation of two triangulated pedicle screws is better than either laminar hooks or single pedicle screws. The strength of fixation of triangulated pedicle screws connected by a transverse plate is superior to a single pedicle screw because it is dependent on the mass of bone between the screws rather than simply the amount of bone within the screw thread.(ABSTRACT TRUNCATED AT 250 WORDS)

Two-point fixation of the lumbar spine. Differential stability in rotation.

Single vertebral motion segments were tested in torsion. The adjacent vertebrae were transfixed by two 3-mm Steinman pins placed vertically. These were applied in five different positions: between the anterior vertebral bodies, posterior vertebral bodies, pedicles, transverse processes, and lamina. Rotational displacement was limited the most by transfixation between the vertebral bodies (position one or two). Disrupting the anulus fibrosus significantly increased rotation in all positions except those in the vertebral body. These findings may imply that spinal fixation devices that engage the vertebral bodies may offer inherent advantages over purely posterior devices in stabilizing a vertebral motion segment. In addition, an interbody arthrodesis may prevent intervertebral motion better than a posterior or posterolateral fusion.

Biomechanical evaluation of methods of posterior stabilization of the spine and posterior lumbar interbody arthrodesis for lumbosacral isthmic spondylolisthesis. A calf-spine model.

In order to evaluate biomechanically the efficacy of four types of posterior instrumentation for the stabilization of isthmic spondylolisthesis of the lumbosacral spine, mechanical non-destructive cyclic testing in axial compression, flexion, extension, and rotation was performed on six fresh lumbosacral spines from calves. Each segment contained four motion segments, including the lumbosacral junction. Isthmic spondylolisthesis was created by sectioning the pars interarticularis of the sixth lumbar vertebra and all posterior ligaments between the fifth and sixth lumbar levels. Eight constructs were tested sequentially: (1) the intact spine, (2) the destabilized spine, (3) the spine fixed with Harrington double-distraction rods, (4) the spine treated with transpedicular Cotrel-Dubousset instrumentation with a transverse approximating device, (5) the spine treated with Steffee transpedicular screws and plates, (6) the spine treated with posterior lumbar interbody arthrodesis, (7) the spine treated with Cotrel-Dubousset instrumentation and posterior lumbar interbody arthrodesis, and (8) the spine treated with Steffee instrumentation and posterior lumbar interbody arthrodesis. One motion segment was involved in each construct, except for the spine that was fixed with Harrington instrumentation, which involved three segments. Strain across the supraspinous and anterior longitudinal ligaments was measured with two extensometers that were attached at the spondylolisthetic level and at the intact motion segments adjacent to the fixed level. Harrington instrumentation was the least rigid construct under any type of loading except axial compression (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)