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Biomedical subjects

T A Zdeblick

Publications and source records attributed to T A Zdeblick.

At least 19 recordsLinked to original sources
PubMed1998

Biomechanical evaluation of translaminar facet joint fixation. A comparative study of poly-L-lactide pins, screws, and pedicle fixation.

STUDY DESIGN: Nine sheep cadaveric spines were used in this acute postoperative model. OBJECTIVES: To compare the biomechanical performance of translaminar facet joint fixation techique with that of cortical screws and bioabsorbable poly-L-lactide pins and with that of rigid pedicle screw fixation in the lumbar spine. SUMMARY OF BACKGROUND DATA: Among numerous posterior spine fixation techniques, pedicle screw fixation has been reported to be the most rigid construct and to provide high fusion rate. Translaminar facet joint screw fixation is an alternative to pedicle screw fixation and is the lowest profile construct that achieves stabilization. The authors have developed a new concept involving application of bioabsorbable poly-L-lactide pins to translaminar facet joint fixation. Degradation in the stiffness of the implants with time may be advantageous for fusion mass remodeling. METHODS: A total of nine sheep L2-L6 cadaveric spines were used. Each intact spine was nondestructively tested in flexion-extension bending (+/- 5-Nm peak bending moment with 100-N axial compression) on a modified testing machine. Loads were applied for 10-second periods using sinusoid waveforms. After testing the intact spine, bilateral fenestration was performed between L4 and L5 and the medial aspect of the facet capsule was resected. The L4-L5 functional spinal unit was than stabilized by five methods: translaminar facet joint fixation with smooth poly-L-lactide pins; translaminar facet joint fixation with cortical screws; pedicle screw fixation with the Texas Scottish Rite Hospital system; and without instrumentation, in that order. Linear displacement of L4 inferior and L5 superior articular processes in the sagittal plain (delta facet) and L4-L5 intervertebral rotation in the sagittal plain (theta sagittal) were measured by the extensometers mounted to the spine. Ranges of motion (delta facet and theta sagittal), neutral zones, linear elastic zone stiffness, and the total energy absorption during the load-unload cycle (hysteresis) were calculated. RESULTS: By resecting the facet joint capsules and ligamentum flavum, delta facet and theta sagittal were not increased significantly, whereas the increase of neutral zones and hysteresis were statistically significant. Compared with the intact spine, delta facet was significantly reduced to 41% of normal with translaminar facet joint fixation with poly-L-lactide pins, to 9% with translaminar facet joint fixation with screws, and to 11% with the Texas Scottish Rite Hospital system. Neutral zones of delta facet showed a similar pattern, and these differences were significant. Regarding linear elastic zone stiffness, translaminar facet joint fixation with screws provided a stiffer construct than did pedicle screw fixation in the flexion loading mode, whereas pedicle screw fixation yielded higher values for stiffness in extension loading. Translaminar facet joint fixation with poly-L-lactide pins increased linear elastic zone stiffness in extension loading, but the increase was less than was achieved with the other constructs. CONCLUSIONS: The facet joint is the only true articulation in the lumbosacral spine. It is logical to fix this part directly to achieve spine fixation. Translaminar facet joint fixation with screws show similar biomechanical performance to pedicle screw fixation. Translaminar facet joint fixation with poly-L-lactide pins is significantly less stiff than either type of screw fixation, but it also restricts the facet joint and intervertebral motions significantly when compared with the intact spine.

Analysis of Variance↗
PubMed1998

Cervical interbody fusion cages. An animal model with and without bone morphogenetic protein.

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

Animals↗
PubMed1998

Laparoscopic spinal fusion.

This article details the development of the laparoscopic technique of interbody spinal fusion. The rationale for this procedure is discussed as are indications, contraindications, and operative technique. The results of over 100 laparoscopic lumbar fusions are presented along with their complications and surgical recommendations.

Cervical Vertebrae↗
PubMed1997

Biomechanical comparison of posterior lumbar interbody fusion cages.

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

Animals↗
PubMed1997

Anterior instrumentation of the thoracolumbar spine. A biomechanical comparison.

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

Compressive Strength↗
PubMed1997

Mechanical evaluation of cross-link designs in rigid pedicle screw systems.

STUDY DESIGN: This study was designed to evaluate the biomechanical performance of 5 different cross-link brands to determine which design characteristics are biomechanically desirable. METHODS: The Cotrel-Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Texas Scottish Rite Hospital systems were assembled to vertebral models according to the manufacturer's specifications. Three constructs were tested for each brand of instrumentation: without cross-links, with one cross-link, and with two cross-links. Four modes of loading: axial, torsional, flexion-extension, and lateral-flexion were used. Load-displacement curves were plotted. The stiffness was calculated from the slope of these curves. OBJECTIVES: Five different rigid pedicle screw systems were tested to determine: 1) what are the characteristics of cross-link design that are most effective in limiting torsional motion; 2) whether two cross-links are more effective than one; and 3) whether cross-linkage increases the construct stiffness in lateral bending. SUMMARY OF BACKGROUND DATA: Cross-linkage has been shown to increase the torsional stiffness of rod and screw constructs. Increased construct stiffness has been correlated with higher fusion rates. RESULTS: Increases in axial, flexion-extension, or lateral-flexion stiffness, with the addition of one or two cross-links, were not statistically significant. In torsional loading, increases in stiffness within brands were statistically significant in every case. The average increase was 44% with one added cross-link and 26% with two. The magnitude of the increase in torsional stiffness was compared with the cross-sectional area of the respective cross-link. Greater stiffness correlated with larger cross-sectional area (r = 0.81 for one cross-link, and r = 0.60 for two). CONCLUSION: The use of cross-linkage in spinal fusion increases torsional stiffness in pedicle screw and hook constructs. This study 1) confirmed the effectiveness of cross-linkage in limiting torsional motion and showed the superiority of two cross-links to one cross-link in limiting torsional motion, 2) showed that increase of torsional stiffness of a cross-linked construct is proportional to the cross-sectional area of the cross-link, and 3) demonstrated that cross-links do not increases stiffness in the lateral flexion mode.

Biomechanical Phenomena↗
PubMed1997

Posterior lumbar interbody fusion. A biomechanical comparison, including a new threaded cage.

STUDY DESIGN: In vitro biomechanical testing was performed on eight lumbar calf spines. OBJECTIVES: To compare the initial stiffness of a standard method of posterior lumbar interbody fusion using structural autograft with the same procedure using additional posterior instrumentation. These constructs also were compared to a new titanium implant. SUMMARY OF BACKGROUND DATA: Posterior lumbar interbody fusion is gaining wide acceptance for the treatment of segmental spinal instability, spondylolisthesis, and discogenic pain. Many methods have been described, including use of autograft or allograft bone, in either structural or nonstructural form, with or without additional fixation. A new threaded titanium interbody implant has been designed to increase initial stability while allowing bony ingrowth for fusion. METHODS: Eight lumbar calf spines were subjected to axial compression, sagittal moments (flexion-extension), and axial torque while displacement was measured. Stiffness was calculated from the load displacement curves for each construct under each load pattern. RESULTS: The posterior lumbar interbody fusion by bone graft alone was the least stiff construct of all modes tested. In two of eight specimens the bone graft dislodged posteriorly into the canal during torsional testing. The titanium interbody implant was similar in stiffness to the bone graft posterior lumbar interbody fusion with posterior instrumentation group in all three modes. They were both significantly stiffer than the normal spine, the destabilized spine, and the posterior lumbar interbody fusion by bone graft alone (P < 0.05). CONCLUSIONS: In this model, the posterior lumbar interbody fusion with bone graft alone had less initial stiffness than that of the intact spine. The addition of posterior instrumentation or interbody implants can increase initial stiffness significantly.

Animals↗
PubMed1997

Anterior instrumentation in the management of thoracolumbar burst fractures.

Anterior instrumentation in the treatment of thoracolumbar fractures has progressed significantly during the past 2 decades. These fixation systems have evolved to meet the anatomic, biomechanical, and imaging challenges associated with internal fixation of the thoracolumbar spine. The evolution of these devices will be reviewed, and from this, the indications and surgical techniques necessary for the safe and effective use of the device will be discussed. This study also reports the authors' initial clinical experience using the Z plate anterior thoracolumbar plating system in the treatment of thoracolumbar burst fractures. The study consists of 12 consecutive adult patients who underwent a 1-stage anterolateral decompressive and stabilization procedure for burst fractures from T9-L3. The indications for surgery included neurologic deficits, deformity, progressive kyphosis, and late pain. Ten of the 12 patients maintained their postoperative sagittal alignment or a significant portion of their kyphosis reduction. Two patients with severe kyphotic deformities greater than 50 degrees lost 10 degrees and 20 degrees of their reduction at last followup. All 3 patients with neurologic deficits recovered. There were no neurologic or perioperative complications. Eleven of the 12 patients obtained a good or excellent functional outcome. Anterior arthrodesis using instrumentation stabilization after a 1-stage anterolateral decompression and reduction procedure can yield successful clinical results in the treatment of thoracolumbar burst fractures.

Adolescent↗
PubMed1997

Failed anterior cervical discectomy and arthrodesis. Analysis and treatment of thirty-five patients.

Thirty-five patients were managed operatively after failure of an anterior cervical discectomy and arthrodesis. Failure was classified as the absence of fusion without deformity but with neck pain or radiculopathy, or both; the absence of fusion after anterior or posterior dislodgment of the graft; or kyphosis due to collapse of the graft or to an unrecognized posterior soft-tissue injury. Twenty-three patients had failure of the arthrodesis without deformity (with neck pain only, neck and arm pain, radiculopathy, or myelopathy). Four patients had dislodgment of the graft; in two of them the graft migrated anteriorly after a multilevel Robinson arthrodesis, and in two it migrated posteriorly after a Cloward arthrodesis. Eight patients had a failure because of a kyphotic deformity. Five of them had had a Cloward arthrodesis; one, a discectomy; and two, a Robinson arthrodesis. Six had received allograft bone. Operative treatment of the pseudarthrosis consisted of repeat resection of the disc space in the area of the failed arthrodesis followed by repeat anterior Robinson arthrodesis with decompression of the nerve root if the patient had radiculopathy. It consisted of anterior corpectomy or vertebral-body resection and strut-grafting with reduction of the deformity if the patient had migration of the graft and kyphosis. The reoperations were performed four months to fourteen years (average, thirty-two months) after the initial operation. The duration of follow-up after the second operation averaged forty-four months (range, twenty-four to 216 months). The result was excellent for twenty-nine patients, good for one, fair for four, and poor for one. We concluded that, in patients who have persistent symptoms after an anterior cervical arthrodesis, an excellent result can be achieved with repeat anterior decompression and autogenous bone-grafting.

Adult↗
PubMed1996

Lumbar disc disease with discogenic pain. What surgical treatment is most effective?

The diagnosis and treatment of painful degenerative disc disease remains one of the most controversial topics in the spine literature. The debate continues as to whether surgical treatment should be attempted, and if so, which surgical approach is optimal. Surgical options include the following general types of spinal fusions: posterolateral (instrumented or noninstrumented), anterior lumbar interbody, posterior lumbar interbody, and transarticular facet joint screws. Dr. Nachemson continues to take his long-standing position that the diagnosis is unreliable and that the outcomes are inadequate to justify the risks and costs. Dr. Zdeblick believes that instrumented posterolateral spinal fusions can offer a high degree of success with an acceptably low rate of complications. Finally, Dr. O'Brien believes that the pain generator is anterior and that the treatment should be anterior discectomy and interbody fusion combined with posterior transarticular facet screw fusion.

Bone Screws↗
PubMed1996

A prospective study of laparoscopic spinal fusion. Technique and operative complications.

OBJECTIVE: The authors hypothesized that anterior transperitoneal laparoscopic interbody fusion was feasible and safe. SUMMARY BACKGROUND DATA: Low back pain associated with degenerative disc disease is optimally treated with distraction of the disc space and permanent fusion of the adjacent vertebral bodies. Exposure usually is obtained by a posterior muscle-splitting incision. Although the posterior approach is effective, the procedure results in significant debility secondary to the muscle splitting incision, a 4- to 6-day inpatient hospital stay, and the loss of employment for 3 to 6 months after the procedure. METHODS: Twenty consecutive patients with discogenic back pain were prospectively treated with laparoscopic anterior lumbar instrumentation and fusion. The anterior L5-S1 or L4-L5 disc spaces were exposed laparoscopically. Distraction of the disc space and fixation were achieved by insertion of a threaded titanium cage packed with cancellous bone. RESULTS: Three technical complications occurred all in the first four patients and required two conversions to open transperitoneal fusion. Hospital stay for the patients treated by laparoscopic instrumentation and fusion averaged 1.7 days. Twelve of 20 patients reported excellent pain relief and returned to work in 3 to 8 weeks. No motion was identified in the 16 patients who obtained flexion extension radiographs at 6 months. CONCLUSIONS: Laparoscopic transperitoneal anterior lumbar instrumentation and fusion is safe, and the early results are encouraging.

Adult↗
PubMed1996

Cervical spine infections.

Cervical spine infections arise from a variety of etiologies including postsurgical, iatrogenic, and hematogenous routes. Clinical history, physical examinations, and diagnostic studies all play an integral role in the diagnosis and treatment of these infections. Successful treatment depends on a proper and timely diagnosis, understanding the etiology, and defining the extent to which the infectious process involves the spinal supporting and neurologic elements. Surgical treatment is required when there is abscess formation, instability, progressive kyphosis secondary to vertebral body collapse, or canal compromise with neurologic deficits.

Adult↗
PubMed1995

The treatment of degenerative lumbar disorders. A critical review of the literature.

STUDY DESIGN: A literature review. OBJECTIVES: To clarify the surgical indications for spinal fusion in the various lumbar conditions. SUMMARY OF BACKGROUND DATA: This article summarizes the published literature regarding the treatment of lumbar degenerative conditions. Retrospective and prospective studies regarding the treatment of nonspecific low back pain, disc herniation, spinal stenosis, isthmic spondylolisthesis, facet syndrome, discogenic pain, and lumbar spondylosis are reviewed. METHODS: A review of the English Medical Literature regarding the treatment of low back pain, disc herniation, spinal stenosis, spondylolisthesis, discogenic pain, and lumbar spondylosis was performed. RESULTS AND CONCLUSIONS: Past research does not support the use of spinal fusion for nonspecific low back pain. Fusion is rarely indicated for primary disc herniation but may have a role in recurrent herniations. Specific guidelines regarding fusion in spinal stenosis and spondylolisthesis are described based on the literature. Less rigorous data are presented for indications for fusion in facet joint syndrome, discogenic pain, and lumbar spondylosis. Spinal fusion plays an important role in the treatment of degenerative disorders of the lumbar spine, and the indications for this procedure can be refined by reviewing the existing literature.

Humans↗
PubMed1995

Instrumented laparoscopic spinal fusion. Preliminary Results.

STUDY DESIGN: Seventeen consecutive patients underwent laparoscopic instrumented interbody fusions using custom-designed delivery instrumentation and "BAK" fusion cages; both are manufactured by Spinetech and the former was developed by the authors. The cases were performed at two spine centers under Food and Drug Administration investigational device evaluation clinical trials. OBJECTIVES: We expect this approach will maintain a high fusion rate with diminished hospitalization time, recovery time, patient discomfort, and expense. The rehabilitative aspects of the procedure are a great improvement over traditional fusion approaches. SUMMARY AND BACKGROUND DATA: Extraordinary advances in many endoscopic surgical fields have resulted in many endoscopic surgical fields have resulted in lowered morbidity, expense, and suffering associated with their open surgery counterparts. The authors have developed prototype of delivery instruments for the current laparoscopic fusion cage delivery system. METHODS: The procedure is performed transperitoneally with carbon dioxide insufflation to enable video-assisted visualization through a 10-mm endoscope. Three 10-mm incisions and one 13- to 20-mm incision are required for one-level procedures. Two hollow titanium-threaded interbody implants are packed with autologous bone and inserted into the diseased interspace. RESULTS: Seventeen patients, with an average follow-up period of 8 months and a range of 6-12 months, underwent the procedure. There were 14 single-level fusions and three two-level fusions, all involving L4-S1 levels. There were two cases that required conversion to open procedures without sequelae; two patients had remote donor site wound infections eradicated with incision and drainage and antibiotics, and one patient required subsequent posterior spinal decompression because of a displaced endplate fracture. Average hospital stay was an average of 2 days, excluding two patients with complications and very prolonged stay. CONCLUSIONS: Although this procedure is associated with a long learning curve, the technique, once mastered, is effective and advantageous over current approaches to lumbar fusion. Operative time and hospital stay are expected to decrease with future instrumentation development and surgeon experience.

Adult↗
PubMed1994

Anterior cervical discectomy and fusion using a porous hydroxyapatite bone graft substitute.

OBJECTIVES: This study analyzed the use of a coral hydroxyapatite bone substitute for use in ACDF both with and without an anterior cervical plate. STUDY DESIGN: The healing of multilevel anterior cervical fusions was tested using a goat model. Comparisons were drawn with histologic, radiographic, and biomechanical test data. METHODS: Forty-nine mature alpine goats had three-level anterior discectomies performed. Seven treatment groups of seven goats each were used; Group I with no fusion, Group IIa having tricortical iliac crest autograft, Group IIb having autograft plus an anterior plate, Group IIIa having tricortical iliac crest fresh-frozen allograft, Group IIIb having allograft plus an anterior plate, Group IVa having rectangular-shaped implants of porous hydroxyapatite, and Group IVb having ProOsteon 500 implants with an anterior cervical plate. RESULTS: Histologically, at 12 weeks 48% of the ProOsteon (Interpore, Irvine, CA) implants were rated as incorporated, 10% as possessing a fibrous gap, 29% as collapsed, and 14% as extruded. Anterior cervical plating improved the results with 71% of the implants showing good incorporation, 24% with collapse, and 5% with a fibrous gap. These histologic results compare favorably with autogenous bone and are improved over allograft bone. Fluorochrome analysis showed that none of the implants had complete turnover with host bone, but that all possessed peripheral creeping substitution with cutting cones of new bone formation at 12 weeks. Biomechanically, the spines using the ProOsteon implant were less stiff in torsion than autograft, but equal in stiffness to allograft. Flexion-extension neutral zone stiffness was lower in the ProOsteon implant group than either allograft or autograft. CONCLUSIONS: The use of a coral-based hydroxyapatite bone graft substitute for anterior cervical fusions led to significant rates of implant collapse at 12 weeks but showed excellent biologic compatibility with good early creeping substitution of the implant by host bone. The concomitant use of an anterior cervical plate with the implant prevents extrusion.

Animals↗
PubMed1994

A multi-degree of freedom system for biomechanical testing.

A system is described that allows axial, torsional, and bending testing of biomechanical specimens. The system uses electric motors under closed loop control in its grips allowing application of pure bending moments. These grips attach to an axial/torsional testing system. Thus, it provides simultaneous closed loop control of all three degrees of freedom (D.O.F), so that under any given test condition either the loads or the displacements for each D.O.F. can be maintained at zero, selected constant values, or simultaneously controlled. This enables the expedient evaluation of the mechanical behavior of biological structures under complex loadings or simple loadings (one D.O.F.) with no artificially induced constraints in the other two D.O.F.'s due to specimen mounting.

Biomechanical Phenomena↗
PubMed1994

A biomechanical comparison evaluating the use of intermediate screws and cross-linkage in lumbar pedicle fixation.

In lumbar fusion, controversy remains regarding the effectiveness of cross-linking and the necessity of placing pedicle screws at the intermediate levels of the segment to be fused. The purpose of this study is to evaluate the stiffness of various rod/screw constructs used to instrument a three-level fusion with specific emphasis on the effect of cross-linking and the intermediate pedicle screws. Nine lumbar calf spines were mounted at L1 and L5. Pedicle screws (TSRH, Danek, Memphis, TN) were then placed bilaterally in the L2, L3, and L4 pedicles. Random sequence testing of the following constructs was then conducted: TSRH rods connected bilaterally to the L2 and L4 pedicles with and without a cross-link, and rods connected bilaterally at the L2, L3, and L4 levels with and without a cross-link. The tests were conducted on a modified MTS testing machine (MTS, Minneapolis, MN) and consisted of cyclic application of axial load, torsion, and flexion and extension. The tests yielded axial, sagittal, and torsional stiffness values. Statistical analysis was performed using log transformation and Fischer's test of least significant difference. In axial testing the use of additional screws in the intermediate pedicles increased stiffness an average of 160% (p = .007). The addition of a cross-link did not increase stiffness with axial loading. In flexion testing the six-screw construct was 84% stiffer when compared with the four-screw construct (p = 0.0001). There was no significant change in flexion stiffness with addition of cross-links. In torsional testing the six-screw construct was 38% stiffer than the four-screw construct (p = 0.042).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗