Biomechanical comparison of posterior cervical fixation.

STUDY DESIGN: Biomechanical stability using four different posterior cervical fixation techniques was evaluated in human cadaveric spine. OBJECTIVES: To introduce an alternative interspinous fixation technique using wavy-shaped rods, and to compare its in vitro biomechanical stability with that of other posterior cervical fixation techniques. SUMMARY OF BACKGROUND DATA: Fixation of the posterior cervical spine with interspinous wiring is well known as Rogers' or Bohlman's technique. Recently, several plate fixation techniques have been used for posterior cervical stabilization. Since 1983, the authors have developed the wavy-shaped rod system as an alternative to the interspinous fixation technique. This unique technique has been proven clinically useful in Japan. However, the authors are not aware of any prior biomechanical studies. METHODS: Seven fresh frozen cervical human spines were tested at the C5-C6 motion segment. Nondestructive static biomechanical testing was performed with flexion-extension, lateral bending, and axial rotation for the following stabilization techniques: intact spine, creation of a Stage 3 distractive-flexion injury followed by fixation with the wavy-shaped rods bounded by three multistrand cables, interspinous wiring with a multistrand cable, triple wiring technique using multistrand cables with a pair of unicortical grafts from the ilium, and lateral mass plate fixation with Magerl's screw technique. Testing was performed on a material testing machine (MTS 858 Bionix test system, MTS, Minneapolis, MN), and load displacement curves were obtained using four linear extensometers and one rotatory extensometer across the C5-C6 motion segment. RESULTS: In axial compression loading, the reconstructed specimens showed significant differences in range of motion measured at the anterior and posterior positions, and statistical analysis was performed using one-way analysis of variance. In a comparison of the four fixation techniques, the construct with the wavy-shaped rod indicated significantly less motion both anteriorly and posteriorly than with the other fixation techniques. Also in flexion-extension loading, all the techniques significantly limited the intervertebral motion below the level of the intact motion segment. Particularly, the construct with the wavy-shaped rod showed the smallest mobility, 49.9% anteriorly and 9.3% posteriorly, compared with that of the intact spine. In lateral bending, the lateral mass plate provided the greatest stability, which was superior to the intact segment, but the difference was not statistically significant. In axial rotation, all the reconstruction techniques limited the angular motion below the intact level (wavy rod, 68.0%; Rogers' wiring, 75.2%; Bohlman's triple wiring, 59.8%; lateral mass plate, 71.7%), but no significant differences were observed using one-way analysis of variance, as compared with the intact segment. CONCLUSIONS: All four reconstruction techniques restored the stability of the cervical motion segment to at least the level of the intact motion segment before destabilization. An alternative cervical posterior fixation technique, the Wavy Rod system, was considered the most effective technique in stabilizing a cervical motion segment, particularly in axial compression and flexion extension loading.

Biomechanical evaluation of occipitocervical fixation devices.

Human cadaveric occipitocervical specimens were implanted with three types of instrumentation. The devices were tested biomechanically under three modes of loading to determine the stiffness of spinal constructs and the failure mechanisms of the constructs under extreme flexion. The devices tested were the AXIS Fixation System (with custom plate), the Y-Plate, and the Luque rectangle. No significant differences in stiffness among the devices were found under compression and flexion. The stiffnesses of the plate systems were statistically higher than the Luque rectangle in extension and torsion. In extreme flexion, the plate systems failed by fracture of the C2 pedicles. Modern plate systems, for occipitocervical fixation, provide more stiffness and stability than traditional wiring techniques. This study provides surgeons with information on the relative merits of modern plate and screw systems compared with traditional rod and wire constructs.

A prospective comparison of surgical approach for anterior L4-L5 fusion: laparoscopic versus mini anterior lumbar interbody fusion.

STUDY DESIGN: A prospective comparison of 50 consecutive patients who underwent L4-L5 anterior lumbar interbody fusion (ALIF). OBJECTIVES: To compare surgical time, blood loss, time in hospital, complications and adequacy of exposure between laparoscopic and mini-ALIF surgical approaches for L4-L5 anterior spinal fusion. SUMMARY OF BACKGROUND DATA: Advances in minimally invasive laparoscopic techniques have resulted in many centers adopting the endoscopic approach to L5-S1 as routine. However, the endoscopic approach to L4-L5 can be much more difficult. A direct comparison of open and laparoscopic techniques of exposure has not been reported. METHODS: From 1995 through 1998, data were prospectively collected on a series of 50 consecutive patients who underwent L4-L5 anterior interbody fusion with a threaded device, by either a laparoscopic or an open mini-ALIF approach. RESULTS: Twenty-five patients underwent a laparoscopic procedure and 25 an open mini-ALIF approach. For single-level L4-L5 fusions, there was no statistical difference in operating time, blood loss, or length of hospital stay between laparoscopic or mini-ALIF groups. For two-level procedures, only the operative time differed, with laparoscopic procedures taking 25 minutes longer (P = 0.035). The rate of complications was significantly higher in the laparoscopic group (20% vs. 4%). In the laparoscopic group, 16% of patients had inadequate exposure, with the result that only a single cage was placed. In the open mini-ALIF group, two cages were placed in all cases. CONCLUSIONS: There does not appear to be a significant advantage at the L4-L5 level of the transperitoneal laparoscopic surgical approach when compared with an open mini-ALIF retroperitoneal technique.

The use of rhBMP-2 in interbody fusion cages. Definitive evidence of osteoinduction in humans: a preliminary report.

STUDY DESIGN: A prospective randomized controlled human clinical pilot trial. OBJECTIVES: To determine the feasibility of using rhBMP-2/collagen as a substitute for autogenous bone graft inside interbody fusion cages to achieve arthrodesis in humans. SUMMARY OF BACKGROUND DATA: Preclinical studies have shown rhBMP-2 to be an effective substitute for autogenous bone graft, but there are no studies to date documenting such efficacy for human spine fusion. METHODS: Fourteen patients with single-level lumbar degenerative disc disease refractory to nonoperative management were randomized to receive lumbar interbody arthrodesis with a tapered cylindrical threaded fusion cage filled with rhBMP-2/collagen sponge or autogenous iliac crest bone. Patients were evaluated with radiographs, sagittally reformatted computed tomography scans, and Short Form-36 and Oswestry outcome questionnaires. RESULTS: All 11 patients who received rhBMP-2 were judged by three independent radiologists to have solid fusions (at 6, 12, and 24 months postimplantation), whereas only 2 of the 3 control patients, who received the standard treatment of autogenous iliac crest bone, were deemed to be fused. The Oswestry Disability Questionnaire scores of the rhBMP-2 group improved sooner (after 3 months) than those of the autograft group, with both groups demonstrating similar improvement at 6 months. Short Form 36 scores continued to improve up to 24 months. CONCLUSION: The arthrodesis was found to occur more reliably in patients treated with rhBMP-2-filled fusion cages than in controls treated with autogenous bone graft, although the sample size was limited. There were no adverse events related to the rhBMP-2 treatment. This study is one of the first to show consistent and unequivocal osteoinduction by a recombinant growth factor in-humans.

Laparoscopic lumbar spinal fusion: the role of the general surgeon.

The refinement of minimally invasive surgical techniques has impacted all areas of surgical practice. Laparoscopic approaches to lumbar spine fusion via the transperitoneal and retroperitoneal routes have similarly evolved with the development of new techniques and instruments unique to this procedure. The benefits of laparoscopic fusion techniques include shorter hospital stay, improved postoperative relief of pain, and preservation of critical spinal musculature. A general surgical laparoscopist is a critical member of the operative team. Although the technical details of the procedure are becoming standardized, patient selection is critical to maximize benefit and minimize risk.

The efficacy of anterior cervical plating in the management of symptomatic pseudoarthrosis of the cervical spine.

STUDY DESIGN: A retrospective review of 16 consecutive patients treated with anterior resection of the pseudoarthrosis, autogenous iliac crest bone grafting, and stabilization with an anterior cervical plate. OBJECTIVES: To determine the efficacy of anterior cervical plating used to manage symptomatic pseudoarthrosis of the cervical spine and obtain safe radiographic fusion and improved clinical results. SUMMARY OF BACKGROUND DATA: It is generally recognized that the clinical outcome of anterior cervical discectomy and fusion correlates with rates of fusion. There is debate in the literature as to how the patient with symptomatic cervical pseudoarthrosis should be addressed. Recent reports would support a posterior approach rather than a revision anterior approach. METHODS: Sixteen consecutive patients with symptomatic pseudoarthrosis of the cervical spine were treated with anterior resection of the pseudoarthrosis, autogenous iliac crest bone grafting, and stabilization with an anterior cervical plate. The average follow-up period was 51 months, and patients were assessed using physical examinations, questionnaires, and flexion-extension lateral radiographs. RESULTS: In all, 75% of the patients reported improvement of their symptoms, and 69% of patients returned to work. Fusions were graded I or II in 81% of the patients. No patient demonstrated radiographic instability, and none required revision surgery. Involvement with workers' compensation litigation negatively affected the clinical outcome. CONCLUSIONS: Patients in whom symptomatic cervical pseudoarthrosis develops after cervical anterior discectomy and fusion may be managed successfully with anterior resection of the pseudoarthrosis, autogenous bone grafting, and an anterior cervical plate. Successful clinical results regarding return to work status and general satisfaction with the surgical procedure depend not only on obtaining a successful radiographic fusion, but also on patient selection.

Biomechanical comparison of plates and rods in the unstable thoracic spine.

Thoracic spine stabilization after trauma or in tumor reconstruction cases frequently is performed with hook and rod internal fixation systems, the use of which is not always possible. Pelvic reconstruction plates with pedicle screw fixation offer an alternative to hooks and rods. In this study, we biomechanically compared a plate construct with a hook and rod system in an acute postoperative, unstable thoracic spine model. We found that the hook and rod system offered more resistance to flexion and extension bending than the plate construct; the opposite was true for lateral bending and axial torsion. We further determined that the addition of pars interarticularis screws to the plate construct provided increased resistance to all loading modes. Our study indicates that plate constructs can effectively stabilize the thoracic spine.

Anterior cervical graft and plate load sharing.

Anterior discectomy and fusion with an interbody bone graft and anterior plate is a common procedure in cervical spine surgical management. However, the graft may be shielded from some mechanical loading by the plate. Mechanical testing was performed on six cadaveric calf spines that were subjected to a simulated anterior cervical discectomy and fusion with an interbody bone graft alone and with an anterior plate to determine the amount of load sharing between the graft and plate. The load-displacement data were used to compute the amount of load sharing between the graft and the plate as a continuous function of the applied axial compression load. Although the percent load transmitted through the graft decreased (53 to 41%) as the axial load increased (45 to 90 N), the magnitude of load transmitted through the graft increased (24 to 37 N), with corresponding intervertebral strains <6%. In a single-level procedure, an anterior cervical plate serves as a load-sharing device rather than a load-shielding device, enabling graft consolidation as observed in clinical studies.

Biomechanical comparison of spondylolysis fixation techniques.

STUDY DESIGN: A load-controlled biomechanical analysis of flexion, extension, and torsional stiffness in instrumented calf spines. OBJECTIVES: To compare biomechanically the performance of various fixation techniques for the repair of spondylolytic defects in the pars interarticularis. SUMMARY OF BACKGROUND DATA: Several techniques have been developed to stabilize a spondylolytic defect in the lumbar spine. There are, however, no comprehensive biomechanical studies in which these techniques are compared. METHODS: Nine fresh-frozen and thawed calf cadaveric lumbar L2-L6 spines were used for mechanical testing. Scott's technique, Buck's technique (screw fixation in the lamina across the defects), modified Scott's technique (wire loops around cortical screws placed into both pedicles and tightened under the spinous process), and screw-rod-hook fixation were applied on the calf lumbar spines in which bilateral spondylolytic defects were created in the L4 vertebra. Motion across the defects for each direction of loading in flexion, extension, and rotation was measured using extensometers. The intervertebral rotations and the strain at the site of the spondylolytic defect were computed from the acquired load-displacement data. RESULTS: Each fixation technique significantly increased stiffness and returned the intervertebral rotational stiffness to nearly intact levels. Displacement across the defect under flexion loading was significantly suppressed by each instrumentation technique, but the least motion (P < 0.05) was allowed with the screw-rod-hook fixation or Buck's technique. CONCLUSIONS: All four fixation techniques restored the intervertebral rotational displacements under flexion and torsional loading to the intact condition. The screw-rod-hook fixation allowed the least amount of motion across the defect during flexion.

Cervical spondylotic myelopathy. Approaches to surgical treatment.

Cervical spondylotic myelopathy is the leading cause of spinal cord dysfunction in older patients. This review article looks at the natural history of the condition and examines the role of different surgical treatments for it. Anterior and posterior surgical approaches have a role in the treatment of cervical spondylotic myelopathy dependent on the number of levels involved and the alignment of the spine. Anterior decompression and fusion is useful in patients who have disease at three or fewer levels or in patients with kyphotic alignment. In more extensive disease, a posterior decompression and fusion is usually best. Canal expansive laminoplasty is useful in the treatment of myelopathy without radiculopathy in a patient with lordotic alignment. With the exception of laminoplasty, nonfusion procedures have little role in the treatment of cervical spondylotic myelopathy.

The effect of operative position and short-segment fusion on maintenance of sagittal alignment of the lumbar spine.

STUDY DESIGN: A retrospective radiographic assessment of the maintenance of sagittal alignment in patients undergoing short-segment instrumented fusions in a knee-chest position. OBJECTIVE: To determine whether the use of the knee-chest position adversely effects the maintenance of lumbar sagittal alignment in patients undergoing short-segment instrumented fusions of the lumbar spine. SUMMARY OF BACKGROUND DATA: Previous authors have demonstrated that lumbar lordosis is reduced in the knee-chest position, but it is unknown whether the kneeling position adversely affects the maintenance of lumbar lordosis in short-segment instrumented fusions during the follow-up period. METHODS: Twenty-eight patients (17 men and 11 women, 20-72 years of age) were reviewed retrospectively, having undergone short-segment fusions in the kneeling position. Seven patients underwent posterior spinal fusion from L4 to L5, 13 patients from L5 to S1, and 8 patients from L4 to S1. Transpedicular instrumentation and autogenous iliac crest bone grafting was used in all cases. Radiographs were assessed for sacral tilt, lumbar lordosis, and intervertebral angulation. Data were analyzed with repeated measures analysis of variance. RESULTS: The minimum follow-up period was 33 months. For all 28 patients, lumbar lordosis measured 51 degrees before surgery, 37 degrees during surgery (P = 0.0001), and 50 degrees after surgery (P = 0.6135). In patients undergoing L4-S1 posterior spinal fusion, sacral tilt measured 49 degrees before surgery and 45 degrees after surgery (P = 0.039). Although overall lumbar lordosis was maintained, lordosis was shifted proximally in the lumbar spine, increasing at L1-L4 from 19 degrees before surgery to 24 degrees after surgery, while decreasing at L4-S1 from 32 degrees before surgery to 26 degrees after surgery. CONCLUSION: Overall lumbar lordosis is well maintained in patients undergoing short-segment instrumented fusion in the kneeling position. With compensatory lordosis being shifted proximally and sacral tilt not returning to the preoperative status in L4 to S1 fusions, caution should be exercised in using the kneeling position for longer instrumented lumbar fusions.

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.

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.

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.

Posterolateral fusion for isthmic spondylolisthesis in adults: analysis of fusion rate and clinical results.

This is a retrospective study of 83 consecutive adult patients with isthmic spondylolisthesis who underwent identical decompressive surgery combined with posterolateral spine fusion. We sought to determine factors that affect the fusion rate and clinical outcomes for adult patients with isthmic spondylolisthesis. The outcome of operative treatment for isthmic spondylolisthesis in adults has been poorly documented, as opposed to the treatment of children and adolescents. From 1989 to 1994, 83 consecutive adult patients (age 19-66 years; average, 38 years) underwent surgical treatment consisting of the Gill procedure and posterolateral fusion for isthmic lumbosacral spondylolisthesis. Seventy-three patients (46 men and 27 women) were available for an average of 3.8 years' follow-up (1.0-7.4 years). Thirty-eight underwent one-level fusion, and 35 underwent two-level fusions. Pedicle screw instrumentation was performed in 69 patients. A postoperative questionnaire including the Roland index, clinical charts, and radiographs were reviewed by an independent observer to assess the postoperative course, clinical results, and fusion status. Twenty-five variables were evaluated to determine which affected the fusion and success rates. Primary radiologic fusion and clinical success rates were 78 and 71%, respectively. There was a strong positive correlation between radiologic fusion and clinical success. Overall, single-level fusions showed an 82% fusion rate, and two-level fusions, a 74% rate. For two-level fusions, a significantly higher fusion rate was achieved with a rigid pedicle screw-fixation system than a semirigid system (79 vs. 57%). For smokers, cessation from smoking postoperatively did not increase the fusion rate, and patients who continued to smoke after surgery showed a significantly higher rate of pseudarthrosis. Worker's compensation status did not affect clinical results significantly. Patients who continued to take nonsteroidal antiinflammatory drugs (NSAIDs) >3 months postoperatively showed significantly lower fusion and success rates (44 and 37%). Single-level lumbar fusion for isthmic spondylolisthesis was equally effective with either rigid or semirigid pedicle screw instrumentation. For multilevel spine fusion in isthmic spondylolisthesis, rigid pedicle screw-fixation systems resulted in a high fusion rate. A smoking history or NSAIDs use postoperatively had strong negative influences on the fusion and clinical success rates.

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.

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.

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.