Biomechanical study of screws in the lateral masses: variables affecting pull-out resistance.

The purpose of this study was to investigate the effects of the design of the screw, the depth of insertion, the vertebral level, and the quality of the host bone on the pull-out resistance of screws used in the lateral masses. The study included twelve fresh cervical spines from human cadavera. Radiographs were made of each specimen to ensure the absence of defects, and then the cancellous-bone density of the vertebral bodies was measured at each level with quantitative computed tomography scanning. Six commercially available screws of various diameters and thread configurations (2.7, 3.2, 3.5, and 4.5-millimeter cortical-bone screws; a 3.5-millimeter cancellous-bone screw; and a 3.5-millimeter self-tapping screw) that are currently used for fixation of the cervical lateral masses were tested for axial load to failure. A twelve-by-twelve Latin square design was used to randomize the screws with regard to level (second through seventh cervical vertebrae), side (right and left), and depth of insertion (unicortical or bicortical purchase). Each screw was then subjected to uniaxial load to failure. The data were analyzed to determine if the diameter of the screw, the thread configuration, the number of cortices engaged, the cervical level, or the bone density was associated with the load to failure. Three major subgroups (greatest, intermediate, and lowest pull-out resistance) were identified. The subgroup with the greatest pull-out resistance included only screws with bicortical purchase; the 3.2, 3.5, and 4.5-millimeter cortical-bone screws and the 3.5-millimeter cancellous-bone screw were in this subgroup. Regardless of the thread configuration, no screw with unicortical purchase was in the group with the greatest pull-out resistance. Two of the three values in the subgroup with the lowest pull-out resistance were for the 3.5-millimeter self-tapping screw (with unicortical or bicortical purchase). The cancellous-bone density of the vertebral body was not associated with pull-out resistance and it did not vary significantly according to the cervical level, with the numbers available. However, the pull-out resistance of the screws varied significantly (p = 0.004) by level: it was the greatest at the fourth cervical level, decreasing cephalad and caudad to that level.

T-Saw laminoplasty for the management of cervical spondylotic myelopathy: clinical and radiographic outcome.

STUDY DESIGN: Independent evaluation of 18 patients with multilevel cervical spondylotic myelopathy who underwent threadwire T-saw laminoplasty. OBJECTIVES: Assess the efficacy of midline T-saw laminoplasty in non-Japanese patients based on clinical and radiographic criteria. SUMMARY OF BACKGROUND DATA: Spinous process-splitting laminoplasty has been well accepted in Japan. The results in non-Japanese patients are unknown. METHODS: A single physician performed independent clinical and radiographic evaluations at latest follow-up (mean, 24 months). In addition to a patient self-assessment questionnaire, objective measures included physical examination, Pavlov's ratio, sagittal canal diameter (by computed tomography), cord compression index, cervical lordosis, range of motion, and complications. RESULTS: Progression of myelopathy was arrested in all patients. Patients reported improvement in strength (78%), dexterity (67%), numbness (83%), pain (83%), and gait (67%). Bowel and bladder compromise resolved in five of six patients. The mean Nurick score improved from 2.7 to 0.9 (P < 0.001), and the mean Robinson pain score improved from 2.0 to 0.89 (P = 0.002). No patient required narcotic analgesics at latest follow-up compared with eight before laminoplasty. Objectively, 68% of patients with motor weakness regained normal strength (P = 0.001), whereas 50% regained normal sensation (P = 0.003). Radiographic canal expansion was verified by a statistically significant increase in the mean Pavlov ratio and osseous sagittal computed tomographic measurements. The mean cord compression index improved from 0.49 to 0.61 (P = 0.01). There was no significant change in mean cervical lordosis. Graft dislodgment or segmental instability did not occur. Complications included: infection (n = 1) and persistent postoperative motor root lesion at C5 (n = 1). CONCLUSIONS: T-saw laminoplasty appears to be a safe and effective method of arresting the progression of myelopathy and allowing marked functional improvement in most patients with multilevel cervical spondylotic myelopathy. [Key Words: cervical spine, decompression, laminoplasty, myelopathy, spondylosis]

Complications of posterior cervical plating.

STUDY DESIGN: This retrospective clinical study evaluates complications occurring during or immediately after surgery of posterior cervical plating. OBJECTIVES: The present study quantifies risks associated with posterior cervical plating using lateral mass screw fixation. The observed clinical complications are compared with theoretical risks previously studied in cadavers. Unanticipated complications are identified. SUMMARY OF BACKGROUND DATA: There are many reports that describe posterior cervical plating and attempt to describe the indications for using this type of fixation, but few studies have discussed the clinical complications incurred by application of these plates and screws. METHODS: Seventy-eight consecutive patients whose treatment included posterior cervical lateral mass plating were independently reviewed to identify associated complications. The average patient age was 52.9 years, and the average follow-up period was 2 years (range, 10-47 months). Multiple indications for surgery were present, but complex reconstructive procedures were required in 70.5% of cases. Complication rates were calculated as either a percentage of the number of screws inserted or as a percentage of the number of cases performed or both. RESULTS: Six hundred fifty-four screws were inserted--an average of 8.4 screws per patient. Complication rates as a function of the number of screws inserted included nerve root injury, 0.6%; facet violations, 0.2%; vertebral artery injury, 0%; broken screw, 0.3%; screw avulsion, 0.2%; and screw loosening 1.1%. Complications as a percentage of the number of cases performed included spinal cord injury, 2.6%; iatrogenic foraminal stenosis, 2.6%; broken plate, 1.3%; lost reduction, 2.6%; adjacent segment degeneration, 3.8%; infection, 1.3%; and pseudoarthrosis, 1.4%. CONCLUSIONS: Cadaveric work has predicted certain anatomic complication rates associated with lateral mass screw insertion. This study finds the risk of lateral mass screw insertion to be considerably less than predicted in vitro. The present study reports other complications that were not predicted in laboratory studies.

Subarachnoid--pleural fistulae--management with a transdiaphragmatic pedicled greater omental flap: report of two cases.

STUDY DESIGN: This report describes the treatment of chronic subarachnoid--pleural fistulae using a pedicled greater omentum transfer flap. OBJECTIVE: To describe a new technique for the management of chronic subarachnoid--pleural fistulae resulting from thoracic dural tears. SUMMARY OF BACKGROUND DATA: Thoracic dural tears with leakage of cerebral spinal fluid into the pleural space can occur after thoracic spine surgery. The treatment of chronic subarachnoid--pleural fistulae using an omental flap, however, has not been reported. METHODS: The clinical, radiographic, and surgical details of two cases are described. RESULTS: Pedicled greater omentum transferred to the thoracic spine was an effective method in the treatment of intractable thoracic dural tears for two patients. CONCLUSION: These cases demonstrate that pedicled greater omentum transferred to the thoracic spine can be a safe and effective technique for the management of intractable thoracic dural tears and their complications.

Expansive midline T-saw laminoplasty (modified spinous process-splitting) for the management of cervical myelopathy.

STUDY DESIGN: The authors developed a method of spinous process-splitting laminoplasty using a threadwire saw in a prospective study of 25 patients with cervical myelopathy. This report describes the surgical technique and the results of the expansive midline laminoplasty performed with an threadwire saw. OBJECTIVES: To compare the efficacy of midline, threadwire-saw laminoplasty with that of the original spinous process-splitting laminoplasty. SUMMARY OF BACKGROUND DATA: The spinous process-splitting laminoplasty was described by Kurokawa in 1982. Although the procedure has a number of theoretical and practical advantages, it has not been widely used because of technical difficulties. METHODS: Twenty-five patients who underwent expansive, midline, threadwire-saw laminoplasty from C3 to C7 for cervical myelopathy were studied. The threadwire saw was used to split the spinous processes. The mean follow-up period was 34 months. Neurologic results were evaluated with pre- and postoperative scores, and recovery rates were evaluated by methods described in previous reports using the Japanese Orthopaedic Association scoring system. Radiographic data analyzed included plain radiographs and computed tomography scans. The duration of surgery and the amount of blood lost during this procedure using the threadwire saw were compared with the duration and blood loss that occurred during the original Kurokawa's procedure using a burr. RESULTS: In all cases, good enlargement of the cervical canal was achieved. The mean increase in cervical cross-sectional area was 36.1%, according to computed tomography scans. No dural tears occurred, and no patients experienced any decrease in neurologic function. The neurologic recovery rate was 72%, which was almost same as the neurologic recovery rate in the original procedure. Using the threadwire saw, the mean duration of surgery was 63 minutes shorter and the mean blood loss was 70 cc less than in procedures using burrs. CONCLUSIONS: The application of the threadwire saw to split the spinous processes made Kurokawa's procedure simpler, faster, and safer.

Pedicle and transverse process screws of the upper thoracic spine. Biomechanical comparison of loads to failure.

STUDY DESIGN: An In vitro biomechanical load-to-failure test. OBJECTIVES: To determine the comparative axial pullout strengths of pedicle screw versus transverse process screws in the upper thoracic spine (T1-T4), and to compare their failure loads with bone density as seen on computed tomography. SUMMARY OF THE BACKGROUND DATA: The morphology of the upper thoracic spine presents technical challenges for rigid segmental fixation. Though data are available for failure characteristics of cervical-lateral mass screws, analogous data are wanting in regard to screw fixation of the upper thoracic spine. METHODS: Ten fresh-frozen human spines (T1-T4) were quantitatively scanned using computed tomography to determine trabecular bone density at each level. The vertebrae were drilled and tapped for the insertion of a 3.5-mill meter-diameter cortical bone screw in either the pedicle or the transverse process position. A uniaxial load to failure was applied. RESULTS: The mean ultimate load to failure for the pedicle screws (658 N) was statistically greater than that of the transverse process screws (361 N; P < 0.001). The T1 pedicle screw sustained the highest load to failure (775 N). No significant difference was found between load to failure for the pedicle and transverse process screws at T1. A trend toward decreasing load to failure was seen for both screw positions with descending thoracic level. Neither pedicle dimensions nor screw working length correlated with load to failure. CONCLUSIONS: Upper thoracic pedicle screws have superior axial loading characteristics compared with bicortical transverse process screws, except at T1. Load behavior of either of these screws was not predictable based on anatomic parameters.

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

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

Morphology of the dens. A quantitative study.

Morphometric studies of the dens of the second cervical vertebra were performed on a sample of one hundred twenty bones from the Hamann-Todd Collection. Data were collected on the longitudinal and transverse dimensions of the dens, as well as the size of the dens relative to the centrum of C2. Relationships of dens dimensions to body size and sexual dimorphism were evaluated. In both longitudinal and transverse mean dimensions, the dens in males was slightly, but significantly larger (5-10%) than in females. The relative distributions of dens dimensions were consistently skewed toward the higher values in males and the lower values in females. Body height or weight were not significantly correlated with dens dimensions, and were therefore poor predictors of the size of the dens. Implications for screw fixation of fractures of the dens are discussed.

Quantitative internal dens morphology.

Recent work has demonstrated the highly variable and unpredictable external dimensions of the dens (odontoid process). No data have been available regarding internal dimensions. Quantitative computed tomography analysis of 120 axis vertebrae allowed nondestructive measurement of external and internal dens dimensions. The external computed tomography measurements correlated well with caliper-derived data. Minimum internal dens dimensions and cortical thicknesses in the sagittal and transverse planes are reported. Some axis vertebrae may not be amenable to internal fixation of type II dens fractures. Preoperative planning should include quantitative computed tomography analysis of the dens.

Anatomic comparison of the Roy-Camille and Magerl techniques for screw placement in the lower cervical spine.

The Roy-Camille and Magerl techniques for screw placement in the lower cervical spine were compared under simulated operating room conditions. Three surgeons with varying years of spine surgery experience participated. The anatomic morbidity risks of each screw insertion technique were evaluated by anatomic dissection and radiographic examination. The spinal cord and vertebral arteries were not threatened by either method. Analysis of the pooled data revealed that the Roy-Camille technique had less risk of nerve root injury, nerve roots placed "at risk" for injury, and errors in "zone" of placement. The Magerl technique had less risk of facet joint violation. A learning curve was observed. Once the surgeons gained experience with the two techniques, there were no longer any statistically significant differences among them. The number of years of spine surgery practice did not influence the morbidity risk of either technique. The surgeons' ability to aim the screws was identical in the axial plane, but consistently less accurate in the sagittal plane for the Magerl screws because of the normal prominence of the cervicothoracic junction. Thorough familiarity with the regional anatomy and practice in the anatomy laboratory are recommended before performing these techniques in patients.

Traumatic open anterior lumbosacral fracture dislocation. A report of two cases.

STUDY DESIGN: Case presentation. OBJECTIVES: To review the diagnosis and treatment of rare anterior lumbosacral fracture dislocations. SUMMARY OF BACKGROUND DATA: The severity of closed anterior and open and closed posterior lumbosacral dislocations has been documented; however, there have been no reports of open anterior lumbosacral dislocations in the literature. Two patients are reported who experienced acute open anterior lumbosacral fracture dislocations. METHODS: Review of the patient history and physical examination, radiologic review, operative techniques, and a review of the literature. RESULTS: Fractures healed in both patients, with no major infections. Both patients had persistent neurologic deficits at last follow-up. CONCLUSIONS: Open lumbosacral fracture dislocations are complex injuries that require diligence on the part of the surgeons involved the recognize the severity of the injury, to prevent or resolve any infectious process, to prevent further neurologic injury, and then to obtain and maintain alignment of the spine on the pelvis.

Cervical spine pedicle screws: a biomechanical comparison of two insertion techniques.

STUDY DESIGN: Biomechanical testing of the pullout strengths of pedicle screws placed by two different techniques in adult human cadaveric cervical spines. OBJECTIVES: To determine whether there is a significant difference in screw purchase of two commonly proposed methods of cervical pedicle screw insertion. SUMMARY OF BACKGROUND DATA: Wiring techniques remain the gold standard for posterior cervical fixation. However, absent or deficient posterior elements may dictate the use of alternative fixation techniques. Cervical pedicle screws have been shown to have significantly higher pullout strength than lateral mass screws. METHODS: Fifty fresh disarticulated human vertebrae (C3-C7) were evaluated with computed tomography for anatomic disease and pedicle morphometry. The right and left pedicles were randomly assigned to either a standard method or the Abumi insertion method. In the latter technique the cortex and cancellous bone of lateral mass are removed with a high-speed burr, which provides a direct view of the pedicle introitus. The pedicle is then probed and tapped and a 3.5-mm cortical screw inserted. Each screw was subjected to a uniaxial load to failure. RESULTS: There was no significant difference in the mean pullout resistance between the Abumi (696 N) and standard (636.5 N) insertion techniques (P = 0.41). There was no difference in pullout resistance between vertebral levels or within vertebral levels. Two (4%) minor pedicle wall violations were observed. CONCLUSION: In selected circumstances pedicle screw instrumentation of the cervical spine may be used to manage complex deformities and patterns of instability. Surgeons need not be concerned about reduced screw purchase when deciding between the Abumi method and its alternatives.

Cervical pedicle screws: comparative accuracy of two insertion techniques.

STUDY DESIGN: Independently assessed radiographic and anatomic comparison of device implantation methods. OBJECTIVES: To compare the relative accuracy of two techniques of inserting cervical pedicle screws. SUMMARY OF BACKGROUND DATA: In an attempt to define the anatomic risks of cervical pedicle screw insertion, image-guided stereotactic technology was shown to be superior to some other methods in vitro.- Meanwhile, in vivo experience with Abumi's technique of screw insertion has had few clinically relevant instances of screw malposition. There has been no direct comparison between current image-guided technology and Abumi's fluoroscopically assisted technique. METHODS: The pedicles (C3-C7) of human cadaveric cervical spines were instrumented with 3.5-mm screws with either of two techniques. Cortical integrity and potential neurovascular injury were independently assessed by computed tomographic (CT) scans and anatomic dissection. A cortical breach was considered "critical" if the screw encroached on any vital structure. If any part of the screw violated the cortex of the pedicle but no vital structure was at risk for injury, the breach was classified as "noncritical." RESULTS: In Group I (StealthStation; Sofamor-Danek, Memphis, TN), 82% of screws were placed in the pedicle, and 18% had a critical breach. In Group II (Abumi technique), 88% of screws were placed in the pedicle, and 12% had a critical breach. No statistically significant differences were demonstrated between each group (P = 0.59). Regarding pedicle dimensions and safety of insertion, a critical pedicle diameter of 4.5 mm was determined to be the size below which a critical breach was likely, but above which there was a significantly greater likelihood for safe screw placement. The most common structure injured in each group was the vertebral artery. CONCLUSIONS: The use of a computer-assisted image guidance system did not enhance safety or accuracy in placing pedicle screws compared with Abumi's technique. Both techniques have a noteworthy risk of injuring a critical structure if inserted into the pedicles with a diameter of less than 4.5 mm. Under laboratory conditions, pedicles with a diameter of more than 4.5 mm have a significantly greater likelihood of being safely instrumented by either technique. These data indicate that cervical pedicle screw placement is feasible, but it should be reserved for selected circumstances with clear indications and in the presence of suitable pedicle morphology.

Cervical myelopathy and congenital stenosis from hypoplasia of the atlas: report of three cases and literature review.

STUDY DESIGN: Case reports of patients with cervical myelopathy to hypoplasia of the atlas. OBJECTIVES: To report cases of cervical myelopathy due to congenital hypoplasia of the atlas and to review the literature. SUMMARY OF BACKGROUND DATA: Six previously documented cases of congenital hypoplasia of the atlas as a cause of cervical myelopathy are reported in the literature. METHODS: Three patient's clinical record and radiologic imaging studies as well as a thorough literature search are reported. Plain radiographs, computed tomography scans, magnetic resonance images, as well as somatosensory-evoked potential changes are displayed. RESULTS: Cervical myelopathy developed in three patients who were found to have congenital hypoplasia of the atlas. Laminectomy of C1 provided neurologic improvement in all three patients presented. CONCLUSION: Congenital hypoplasia of the atlas is a rare cause of cervical myelopathy. This report should broaden the radiographic differential diagnosis when seeking an explanation for the signs and symptoms of cervical myelopathy.

Laminoplasty versus laminectomy and fusion for multilevel cervical myelopathy: an independent matched cohort analysis.

STUDY DESIGN: A matched cohort clinical and radiographic retrospective analysis of laminoplasty and laminectomy with fusion for the treatment of multilevel cervical myelopathy. OBJECTIVES: To compare the clinical and radiographic outcomes of two procedures increasingly used to treat multilevel cervical myelopathy. SUMMARY OF BACKGROUND DATA: Traditional methods of treating multilevel cervical myelopathy (laminectomy and corpectomy) are reported to have a notable frequency of complications. Laminoplasty and laminectomy with fusion have been advocated as superior procedures. A comparative study of these two techniques has not been reported. METHODS: Medical records of all patients treated for multilevel cervical myelopathy with either laminoplasty or laminectomy with fusion between 1994 and 1999 at our institution were reviewed. Thirteen patients that underwent laminectomy with fusion were matched with 13 patients that underwent laminoplasty. All patients and radiographs were independently evaluated at latest follow-up by a single physician. RESULTS: Cohorts were well matched based on patient age, duration of symptoms, and severity of myelopathy (Nurick grade) before surgery. Mean independent follow-up was similar (25.5 and 26.2 months). Both objective improvement in patient function (Nurick score) and the number of patients reporting subjective improvement in strength, dexterity, sensation, pain, and gait tended to be greater in the laminoplasty cohort. Whereas no complications occurred in the laminoplasty cohort, there were 14 complications in 9 patients that underwent laminectomy with fusion patients. Complications included progression of myelopathy, nonunion, instrumentation failure, development of a significant kyphotic alignment, persistent bone graft harvest site pain, subjacent degeneration requiring reoperation, and deep infection. CONCLUSIONS: The marked difference in complications and functional improvement between these matched cohorts suggests that laminoplasty may be preferable to laminectomy with fusion as a posterior procedure for multilevel cervical myelopathy.

Nicotine on the revascularization of bone graft. An experimental study in rabbits.

STUDY DESIGN: In 24 rabbits, the authors transplanted autologous cancellous bone to the anterior chamber of the eye. Half of the rabbits received nicotine and half received placebo (albumin) from mini-osmotic pumps that were implanted subcutaneously. Revascularization of the bone graft was evaluated postoperatively using ophthalmology slit-lamp and fluorescein angiography, and after sacrifice using microvascular silicone injection and histology. OBJECTIVES: The hypothesis that nicotine inhibits the revascularization of bone graft because of its pharmacologic action on the microvasculature was tested. SUMMARY OF BACKGROUND DATA: Pseudoarthrosis after spinal fusion occurs more frequently in smokers as compared with nonsmokers. METHODS: Observations of the bone graft were made regarding the time after implantation when vessels within the graft were noted and the pattern of these vessels. Revascularization of the graft was graded based on the observed percent area of fluorescence after injection of fluorescein. Serum levels of nicotine were measured weekly. Colored silicone was injected at sacrifice to fix the vasculature of the bone graft. Histologic analysis of undecalcified sections was performed. RESULTS: Nicotine, as compared with placebo, was associated with delayed revascularization within the graft, a smaller percent area of revascularization, and a larger number of grafts showing necrosis. CONCLUSIONS: Nicotine inhibits, but does not prevent, the revascularization of cancellous bone grafts. Inhibition of early revascularization by nicotine is proposed as the pathophysiologic mechanism by which smoking may adversely affect the healing of spinal fusions.

The syndromes of degenerative cervical disease.

Senescence of the cervical spinal motion segment is manifest by changes in each anatomic component. The pathoanatomical change that predominates varies from one person to the next. Sufficient compromise of local neural structures will precipitate the onset of symptoms. The pattern of symptoms and any associated physical signs will vary according to which structures are stimulated or compressed. Radicular and myelopathic syndromes are produced by nerve root and spinal cord compression, respectively. These patterns can occur individually or in combination. Other less clearly understood patterns of local and referred symptoms may be mediated by the sinu-vertebral nerves or the medial branches of the posterior ramus. Techniques for the precise diagnosis of these syndromes are needed. Separating patterns of symptomatic degenerative cervical disease from other causes of neck, shoulder, and arm symptoms rests on an awareness of the broad spectrum of subjective complaints, a thorough physical examination, and confirmatory diagnostic studies. Clear delineation of the etiology will increase the likelihood of successful treatment.