Anatomic considerations of anterior transarticular screw fixation for atlantoaxial instability

STUDY DESIGN: Anatomic parameters of C1 and C2 were measured in 30 dried human cervical spines. Anterior transarticular C1-C2 screws were placed in 15 cadaveric spines. OBJECTIVE: To provide anatomic data for anterior transarticular atlantoaxial screw or C1-C2 screw and plate fixation. SUMMARY OF BACKGROUND DATA: A posterior approach to fixation in the atlantoaxial joint has been well described. Damage to the vertebral artery is documented as a rare complication of posterior atlantoaxial transarticular screw fixation. An anterior surgical approach to exposing the upper cervical spine for internal fixation and bone graft recently has been developed. No anatomic information regarding the anterior transarticular atlantoaxial screw or screw and plate fixation between C1 and C2 is available in the literature. METHODS: Direct measurements using digital calipers and a goniometer were taken from 30 pairs of dried human C1 and C2 vertebrae. The anterior transarticular C1-C2 screw insertion point is at the junction of the lateral edge of the C2 vertebral body to 4 mm above the inferior edge of the C2 anterior arch. The parameters related to anterior transarticular atlantoaxial screw fixation or screw and plate fixation between the C1 lateral mass and the C2 vertebral body were measured. Fifteen embalmed cadavers were used for anterior C1-C2 transarticular screw placement. Longer screws (30-40 mm) were used to detect whether the screw tips violated the upper cervical canal or vertebral arteries. RESULTS: In the anterior transarticular atlantoaxial screw placement, lateral angulation of the screw placement relative to sagittal plane ranged from 4.8 +/- 1.8 degrees to 25.3 +/- 2.6 degrees. The posterior angulation of the screw placement relative to the coronal plane ranged from 12.8 +/- 3.1 degrees to 22.6 +/- 3.2 degrees. The length of the medial screw path ranged from 14.7 +/- 1.5 mm to 25.4 +/- 2.8 mm. In the anterior screw and plate fixation, the anteroposterior diameter of the inferior facet articular surface ranged from 16.2 +/- 1.6 mm to 17.1 +/- 1.8 mm. The anteroposterior diameter of the C2 vertebral body ranged from 9.3 +/- 1 mm to 16.2 +/- 1.8 mm. The anterior prevascular retropharyngeal approach appropriately exposed the atlantoaxial joint for anterior transarticular C1-C2 screw placement. No screws violated the vertebral artery and cervical canal. CONCLUSIONS: An anterior transarticular atlantoaxial screw 15-25 mm long can be inserted with a lateral angulation of 5-25 degrees relative to the sagittal plane and a posterior angulation of 10-25 degrees relative to the coronal plane. Additionally, in C1-C2 anterior plate fixation screws 15 mm long could be anchored in the inferior facet of the C1, and screws 9-15 mm long could be anchored in the C2 vertebral body.     

Preoperative oblique axial computed tomographic imaging for C1-C2 transarticular screw fixation: technical note

C1-C2 transarticular screw fixation is an increasingly popular surgical method of treating atlantoaxial instability. When properly performed, it can safely provide fusion rates near 100%. However, the technique of screw insertion into this region allows only a small margin for error. Preoperative radiological assessment is essential to analyze the morphology of the region, assess for vertebral bony and vascular anomalies, and define the tolerances for the transarticular screws along their planned trajectory. As an adjunct to the preoperative planning of C1-C2 transarticular screw fixation, a unique, easily obtainable method of computed tomographic imaging, using thin-section oblique axial computed tomographic images of the C1-C2 region, is described.     

Posterior C1-C2 transarticular screw fixation for atlantoaxial arthrodesis

OBJECTIVE: To assess the outcomes associated with C1-C2 transarticular screw fixation. METHODS: The clinical outcomes of 121 patients treated with posterior C1-C2 transarticular screws and wired posterior C1-C2 autologous bone struts were evaluated prospectively. Atlantoaxial instability was caused by rheumatoid arthritis in 48 patients, C1 or C2 fractures in 45, transverse ligament disruption in 11, os odontoideum in 9, tumors in 6, and infection in 2. RESULTS: Altogether, 226 screws were placed under lateral fluoroscopic guidance. Bilateral C1-C2 screws were placed in 105 patients; each of 16 patients had only one screw placed because of an anomalous vertebral artery (n = 13) or other pathological abnormality. Postoperatively, each patient underwent radiography and computed tomography to assess the position of the screw and healing. Most screws (221 screws, 98%) were positioned satisfactorily. Five screws were malpositioned (2%), but none were associated with clinical sequelae. Four malpositioned screws were reoperated on (one was repositioned, and three were removed). No patients had neurological complications, strokes, or transient ischemic attacks. Long-term follow-up (mean, 22 mo) of 114 patients demonstrated a 98% fusion rate. Two nonunions (2%) required occipitocervical fixation. In comparison, our C1-C2 fixations with wires and autograft (n = 74) had an 86% union rate. CONCLUSION: Rigidly fixating C1-C2 instability with transarticular screws was associated with a significantly higher fusion rate than that achieved using wired grafts alone. The risk of screw malpositioning and catastrophic vascular or neural injury is small and can be minimized by assessing the position of the foramen transversaria on preoperative computed tomographic scans and by using intraoperative fluoroscopy and frameless stereotaxy to guide the screw trajectory.     

Biomechanical comparison of C1-C2 posterior fixations. Cable, graft, and screw combinations

STUDY DESIGN: Four combinations of cable-graft-screw fixation at C1-C2 were compared biomechanically in vitro using nondestructive flexibility testing. Each specimen was instrumented successively using each fixation combination. OBJECTIVES: To determine the relative amounts of movement at C1-C2 after instrumentation with various combinations of one or two transarticular screws and a posterior cable-secured graft. Also to determine the role of each component of the construct in resisting different types of loading. SUMMARY OF BACKGROUND DATA: Spinal stiffness increases after instrumentation with two transarticular screws plus a posterior wire-graft compared with a wire-graft alone. Other C1-C2 cable-graft-screw combinations have not been tested. METHODS: Eight human cadaveric occiput-C3 specimens were loaded nondestructively with pure moments, and nonconstrained motion at C1-C2 was measured. The instrumented states tested were a C1-C2 interposition graft attached with multistranded cable; a cable-graft plus one transarticular screw; two transarticular screws alone; and a cable-graft plus two transarticular screws. RESULTS: The transarticular screws prevented lateral bending and axial rotation better than the posterior cable-graft. The cable-graft prevented flexion and extension better than the screws. Increasing the number of fixation points often significantly decreased the rotation and translation (paired t test; P < 0.05). Axes of rotation shifted from their normal location toward the hardware. CONCLUSIONS: It is mechanically advantageous to include as many fixation points as possible when atlantoaxial instability is treated surgically.     

The reliability of the lateral radiograph in determination of the optimal transarticular C1-C2 screw length

STUDY DESIGN: This study assessed the value of using lateral radiographs in evaluating the optimal screw length in transarticular C1-C2 screw fixation. OBJECTIVES: To assess the reliability of the lateral radiograph in determining the optimal transarticular C1-C2 screw length. SUMMARY OF BACKGROUND DATA: Transarticular C1-C2 screw placement is usually performed using anatomic landmarks and fluoroscopy. A lateral fluoroscopic image is valuable when directing screws in the sagittal plane, but its exact role in determining screw length has not been investigated. METHODS: Eight cervical spine specimens were used in this study. Screw placements were performed in each specimen, fixed in the exact lateral position and under direct visualization. After each placement, a lateral radiograph was taken. The odontoid process was divided into three equal portions. Another portion anterior to the odontoid process was called the anterior tubercle region. The number of screw tips appearing in each portion on the radiograph was then recorded for each placement. In addition, 30 C1 specimens were measured to evaluate the anterior part of C1. RESULTS: The results showed that 12.5% of the screws placed 2 mm short of reaching the ventral cortex and 0 mm overpenetrating the ventral cortex of the lateral mass of C1 projected in the radiograph on the anterior tubercle region, 37.5% on the anterior region of the odontoid process, and 50% on the middle region of the odontoid process. Twenty-five percent of the screws that were placed to overpenetrate, by 2 or 4 mm, the ventral cortex of the lateral mass of C1 were projected on the anterior tubercle region in the radiograph, and 50% and 62.5% were projected on the anterior region of the odontoid process, respectively. The mean vertical distance between the anteriormost point of the anterior tubercle of the anterior ring and the middle of the ventral cortex of the lateral in all specimens was 5.6 +/- 1 mm, and the mean transverse angle of the anterior ring relative to the frontal plane was 21.1 +/- 3.5 degrees. CONCLUSIONS: This results in this study indicate that a lateral radiograph may not be reliable in determining the optimal screw length, although it is valuable in directing accurate screw angle in the sagittal plane. Preoperative computed tomographic evaluation of the C1-C2 region may be helpful in estimating the location of the screw tip on the lateral radiograph during surgery.     

Safe lateral-mass screw lengths in the Roy-Camille and Magerl techniques. An anatomic study

STUDY DESIGN: Investigation of the mean safe lateral-mass screw lengths in the Roy-Camille and Magerl screw techniques in cadaveric cervical specimens. OBJECTIVES: To report the mean screw path length and to evaluate the relation of the screw trajectory to the nerve root in the Roy-Camille and Magerl screw techniques. SUMMARY OF BACKGROUND DATA: Potential injury to the cervical nerve root caused by too long a screw remains a major concern. Few studies regarding proper screw length and its relation to the adjacent nerve root are available. METHODS: Fourteen cervical spines were used for this study. Each lateral mass from C3 to C7 was drilled according to the techniques described by Roy-Camille (right side) and Magerl (left side). The cervical spines were harvested from the cadavers, and the anterior aspect of the lateral mass and spinal nerve were exposed. The screw path length between the dorsal and ventral cortices of the lateral mass were measured. An additional measurement was taken from the ventral aspect of the lateral mass to the nerve root along the screw path. RESULTS: The mean screw path length in the Roy-Camille technique decreased consistently from C3 (15.7 +/- 1.7 mm) to C7 (11.3 +/- 0.8 mm). The mean distance from the ventral cortex to the nerve root ranged from 1.2 to 2.3 mm, and the smallest value was at C7. The mean screw path length in the Magerl technique also decreased from cephelad to caudal, with a range of 15-16 mm at C3-C6 and a mean value of 13.8 mm at C7. CONCLUSIONS: A safe screw length is 14-15 mm in the Roy-Camille technique and 15-16 mm in the Magerl technique at C3-C6. A short screw may be used at C7 if desired.     

Anatomic study for ideal and safe posterior C1-C2 transarticular screw fixation

STUDY DESIGN: Directions of the C1-C2 posterior transarticular screw trajectories making the longest path or violating the transverse foramen were measured by using an objective measuring method. OBJECTIVES: To clarify the directions of the screw trajectory marking the longest paths without violating the transverse foramen. To achieve this, diverse directions of the screw trajectories were objectified by measuring the locations of the points of screw intersection on the superior articular surface of C2. SUMMARY OF BACKGROUND DATA: The principal limitation of posterior C1-C2 transarticular screw fixation is the location of the vertebral artery. Because of the lack of an objective measuring method, surgical unsuitability has been decided on the basis of individual experiences as reported in 18% to 23% of cases. METHODS: Sagittal reconstructed computed tomographic images were made at 3.5 mm and 6 mm from the spinal canal. C1-C2 transarticular screw trajectories making the longest path or violating the transverse foramen (dangerous trajectory) were drawn, and their points of screw intersection on the superior articular surface of C2 were measured from the posterior rim of the superior articular surface of C2. When the space available for the screw behind the points of screw intersection by the dangerous trajectory was equal to or less than 3.5 mm, the case was defined as "unacceptable"; when the space available for the screw was more than 3.5 mm but equal to or less than 4.5 mm, it was defined as "risky" for the placement of the screw. RESULTS: Trajectories make the longest paths when they pass an average of 3.6 mm and 2.8 mm anterior to the posterior rim of the posterior articular surface of C2 at 3.5-mm lateral images and 6-mm lateral images, respectively. Four of 64 cases were unacceptable or risky unilaterally on 3.5-mm lateral images, and 21 cases were unacceptable or risky on 6-mm lateral images. A sigmoid-shaped increment curve of the risk was noted as the increasing forward inclination of the screw trajectories increased. CONCLUSIONS: The areas on the superior articular surface of C2 intersected by the trajectories making the longest paths without violating the transverse foramen are clarified as a guide to the ideal and safe trajectories. The theoretical minimal risk and usual risk of the posterior C1-C2 transarticular screw fixation are presented as well.     

Frameless stereotactic guidance for surgery of the upper cervical spine

OBJECTIVE: The goal was to evaluate and describe the use of a frameless, computed tomography-guided, stereotactic technique in complex procedures involving the craniocervical junction. METHODS: Eleven procedures, including transoral odontoid resection, posterior atlantoaxial fusion with transarticular C1-C2 screw fixation, and spinal tumor resection, were performed in the preceding 26 months. In each case, frameless stereotaxy was used to plan the incision, to define resection margins, and to determine the appropriate orientation of instrumentation. RESULTS: There were no intraoperative complications noted. Each patient underwent adequate resection of the pathological lesion and satisfactory placement of instrumentation. The stereotactic system provided detailed anatomic visualization, which increased the confidence of the surgeon during the procedure. The system limited the need for extensive surgical exposure, reduced fluoroscopy time, and decreased the risk of neurovascular injury. CONCLUSION: Frameless stereotaxy provided the surgeon with intraoperative information regarding the extent of bone and soft tissue resection. It provided a multidimensional view of anatomic relationships in the operative field, which significantly increased surgical accuracy and safety.     

Anterior transpedicular fixation of the lower thoracic and lumbar spine. Experimental verification using a new direction finder

Currently, no anterior spinal implant provides a strong bone-screw interface because of the cancellous characteristics of the vertebral body. A more secure anchorage could be obtained by anterior transpedicular screw fixation. Four hundred transpedicular screws located between T7 and L5 were placed using the newly developed direction finder. Measurements were obtained directly from radiographs of the cadaveric specimens. In 10 cases (2.5%), the screws crossed the medial pedicle border, but never by more than 1.4 mm. A lateral protrusion was noted in another 41 screws (10%), with no protrusion greater than 2.2 mm. Encroachments beyond the superior or inferior border were not observed. The mean angle of the screws at each level measured between 7 and 19 in the transverse plane and between 2 and 4.5 in the sagittal plane. This technique should be reserved for vertebrae without significant arthritic changes. The rare screw with minimal infraction through the medial or lateral pedicle wall should not cause any vascular or neural compromise. The anterior transpedicular screw technique appeared relatively safe (88%) and encouraged the development of the new plate system for anterior spinal stabilization.     

Computer assisted spine surgery

When inserting screws into a vertebral pedicle, the surgeon usually exposes the back part of the vertebra and uses his or her anatomic knowledge to align the drill in the proper direction. A slight error in direction may result in an important error in the position of the tip of the screw. This is done with no direct visibility of crucial structures (spinal cord, pleura, vessels). Statistical analysis of a series of surgical procedures has shown that 10% to 40% of the screws are not installed correctly. To reduce the risk of complication, a computer assisted method is proposed that enables the surgeon to place a screw at a position preoperatively defined in 3 dimensions using computed tomography images. This allows the surgeon to align a standard surgical drill with the optimal position and direction. The depth of the pilot hole during drilling also is monitored by the system to prevent penetration of the anterior cortex of the vertebral body. Using this procedure, in vitro tests were performed and showed that an accuracy of less than 1 mm can be obtained. Clinical trials were done in 10 patients who suffered severe scoliosis or spondylolisthesis. The trajectory of the holes drilled in L2, L3, L4, and L5 vertebrae were checked for all clinical tests. Postoperative radiographs and computed tomography scans showed that the screws were well inserted in each plane for each pedicle. This technique also can be used to perform osteosynthesis at the thoracic and cervical levels.     

Computer-aided pedicle screw placement using frameless stereotaxis

STUDY DESIGN: In vitro assessment of accuracy and reliability of frameless stereotaxis for insertion of pedicle screws in human cadaveric lumbar spine. OBJECTIVES: To assess a new method of targeting and placing pedicle screws in a human cadaver study. SUMMARY OF BACKGROUND DATA: Pedicle screw instrumentation is common. Complications may occur from improper placement of screws. Even when performed by experienced spinal surgeons, improper placement can occur in 5.2% of pedicles instrumented. Development of computer-guided methods of pedicle screw insertion may decrease this complication rate. METHODS: The technique used preoperative computed tomography scans together with a commercial neurosurgical navigational computer system to assist in placing guidewires in the pedicles. A section of human cadaver spine was first scanned and the data transferred to the workstation. The image data set and physical specimen were then registered by using an instrumented articulated arm to identify selected points on the specimen and randomly sample surface points. Eight highly repeatable locations on each vertebral body were found to be suitable for registration, but better overall accuracy was obtained when surface matching was used in combination with these points. Under guidance of image on the computer, Kirschner wires were inserted into the pedicles of four vertebral bodies. The spine was rescanned, and the planned and resulting positions of the wires compared. RESULTS: The average distance between the planned and resulting wire entry point was 1.2 mm, with an average difference in planned and resulting trajectories of 6.0 degrees. CONCLUSIONS: Computer-aided pedicle screw instrumentation is feasible. Further technical points require clarification before widespread use is possible.     

The role of anteromedial foraminotomy and the uncovertebral joints in the stability of the cervical spine. A biomechanical study

STUDY DESIGN: The biomechanical role of the cervical uncovertebral joint was investigated using human cadaveric spines. Sequential resection of cervical uncovertebral joints, including clinical anteromedial foraminotomy, was conducted, followed by biomechanical testing after each stage of resection. OBJECTIVES: To clarify the biomechanical role of uncovertebral joints and clinical anteromedial foraminotomy in the cervical spine and their effects on interbody bone graft stability. SUMMARY OF BACKGROUND DATA: Although the biomechanical role of the cervical uncovertebral joints has been considered to be that of a guiding mechanism in flexion and extension and a limiting mechanism in posterior translation and lateral bending, there have been no studies quantifying this role. According to results in quantitative anatomic studies, anatomic variations exist in uncovertebral joints, depending on the vertebral level, articular angulation, and relative height of the joints. METHODS: Fourteen human functional spinal units at C3-C4 and C6-C7 underwent sequential uncovertebral joint resection, with each stage of resection followed by biomechanical testing. The uncovertebral joint was divided anatomically into three parts on each side: the posterior foraminal part, the posterior half, and the anterior half. The loading modes included torsion, flexion, extension, and lateral bending. A simulated anterior bone graft construct was also tested after each uncovertebral joint resection procedure. RESULTS: Significant changes in stability were observed after sequential uncovertebral joint resection in all loading modes (P < 0.05). The biomechanical contribution of uncovertebral joints decreased in the following order: the posterior foraminal part, the posterior half, and the anterior half. Unilateral and bilateral foraminotomy most affected the stability of the functional spinal unit during extension, causing a 30% and 36% decrease in stiffness of the functional spinal unit, respectively. The effect was less in torsion and lateral bending. After sequential resection, there was a statistically significant difference between decreases in torsional stiffness at C3-C4 and C6-C7 (P < 0.05). The stiffness of the simulated bone graft construct decreased progressively during flexion and lateral bending after each foraminotomy (P < 0.05). Increased bone graft height of 79% returned stability to the preforaminotomy level. CONCLUSIONS: This is the first study to quantitate the biomechanical role of uncovertebral joints in cervical segmental stability and the effect at each intervertebral level. The effect differs because of anatomic variations in uncovertebral joints. The major biomechanical function of uncovertebral joints includes the regulation of extension and lateral bending motion, followed by torsion, which is mainly provided by the posterior uncovertebral joints. This study highlights the clinical assessment of additional segmental instability attributed to destruction of the uncovertebral joints during surgical procedures or by neoplastic lesions.     

Occipitocervical fusion with C1 laminectomy in children

STUDY DESIGN: Eight children in whom atlantoaxial dislocation had developed underwent occipitocervical fusion using a rectangular rod. The postoperative results are presented, and the postoperative growth and deformation of the cervical spine were determined radiographically. OBJECTIVES: To investigate in a relatively long-term follow-up study whether occipitocervical fusion affects the growth of the cervical spine and induces spinal deformation. SUMMARY OF BACKGROUND DATA: It has been reported that children who have undergone C1-C2 posterior fusion are likely to develop abnormal curvature or deformation of the cervical spine as a result of a disturbance of growth of the fused vertebrae. There have been no studies, however, to confirm that these changes occur after occipitocervical fusion in children. METHODS: The subjects were one boy and seven girls who had undergone occipitocervical posterior fusion during childhood. The average age at the time of surgery was 8.3 years, and the average follow-up period was 5.9 years. The following were assessed radiographically: redislocation of the atlas, bone union, changes in the curvature of the cervical spine, the height and width of the vertebral bodies, and the anteroposterior diameter of the spinal canal. RESULTS: Solid bone union was achieved in all patients with maintenance of the reduced position at the time of surgery. None of the patients exhibited abnormal curvature of the cervical spine. The rate of increase in height of the C2 vertebral body was significantly less than that of vertebral bodies below C3. The rate of increase in width of the vertebral body and the anteroposterior diameter of the spinal canal of the C2 vertebral body and vertebral bodies below C3 did not differ significantly. CONCLUSIONS: Occipitocervical fusion with a rectangular rod is useful for treating atlantoaxial dislocation in children and yields excellent results because of the firm internal fixation it achieves. This surgery induced no apparent postoperative spinal deformations.     

Carbonated apatite cement augmentation of pedicle screw fixation in the lumbar spine

STUDY DESIGN: Biomechanical testing with human cadaveric lumbar vertebral bodies was used to determine the utility of an injectable carbonated apatite cancellous bone cement for improving the structural performance of pedicle screws subjected to axial pull-out or transverse cyclic loading. OBJECTIVES: To ascertain whether augmentation with a carbonated apatite cement can enhance pedicle screw fixation in the lumbar spine. SUMMARY OF BACKGROUND DATA: The beneficial effects of polymethylmethacrylate augmentation on pedicle screw pull-out strength have been demonstrated. Cancellous bone cement, however, may provide an attractive alternative in this application, as it is remodelable, biocompatible, and nonexothermic. METHODS: Forty-three cadaveric lumbar vertebral bodies were instrumented with pedicle screws. In 20 of these specimens, axial pull-out strength was compared between the control screws and those augmented with cancellous bone cement. In the remaining 23 specimens, the screws were loaded in the superior-inferior direction with a peak displacement of +/- 1 mm at a frequency of 3 Hz for 5000 cycles. Three parameters were calculated from the force-versus-time data: 1) the energy dissipated, 2) the peak force at the start of the test, and 3) the peak force at the end of 5000 cycles. RESULTS: The pull-out strength of the augmented pedicles averaged 68% greater than that of the control side. In response to cyclic loading, all measures of bio-mechanical performance improved 30-63%. CONCLUSIONS: The data suggest that augmentation with this carbonated apatite cancellous bone cement can enhance immediate screw fixation.     

In vitro simulation. Early results of stereotaxy for pedicle screw placement

STUDY DESIGN: Frameless stereotaxy with doppler ultrasound and three dimensional computer model registration is assessed in vitro for pedicle screw placement. OBJECTIVE: To identify feasibility of pedicle screw navigation and placement using this technology. SUMMARY OF BACKGROUND DATA: Inaccurate pedicle screw placement can lead to neurovascular injury or suboptimal fixation. Present techniques in pedicle screw placement involve only confirmation of hole orientation. METHOD: Forty-four pedicle screws were placed in lumbosacral models and cadaver specimens. Accuracy was assessed with a computed tomography scan and vertebral cross sectioning. RESULTS: All screws were intrapedicular. Accuracy of anterior cortical fixation was 1.5 mm, with a range of 2.5 mm. CONCLUSION: In vitro frameless stereotaxy is accurate for pedicle screw placement. This technology adds a component of navigation to pedicle screw placement.     

Resection of a vertebral hemangioma after preoperative embolization. Case report

Preoperative arterial embolization of a vertebral hemangioma allowed surgical excision of the vertebral body, restoration of normal anatomic continuity of the spinal canal, and improvement in myelopathy.     

The vertebral body depths of the cervical spine and its relation to anterior plate-screw fixation

STUDY DESIGN: A study was performed to measure the vertebral body depths in different locations from C2 to C7. OBJECTIVES: To measure the vertebral body depths in 10 linear dimension from C2 to C7. SUMMARY OF BACKGROUND DATA: Anterior plate-screw fixation of the cervical spine has been the common surgical procedure for management of multilevel degenerative disc disease and fracture dislocation. However, injury to the spinal cord during drill or screw placement is the most feared complication of this procedure. It is beneficial for one to have a knowledge of the vertebral body depths in different locations of the vertebral body before anterior cervical plating. METHODS: Twenty-seven cervical spines from C2 to C7 were evaluated directly for this study. Anatomic evaluation of the vertebral body included the anteroposterior midline sagittal depth and the anteroposterior parasagittal depth 5 mm lateral to midline on the superior and inferior endplates, as well as on the middle body. Measurements also were made of anteroposterior parasagittal vertebral depth with both medial and lateral inclination of 10 degrees, with respect to the parasagittal plane of the vertebral body. RESULTS: In general, the measurements of male specimens were larger than those of female specimens. Significant differences were noted at 21 measurements over C3 through C7. The mean depths of the superior endplate for all male and female specimens increased consistently from C3 to C7. The mean depths of the inferior endplate varied but generally increased from C2 to C6, then decreased to C7. The mean sagittal and parasagittal middle vertebral body depths were both 14 mm. CONCLUSIONS: This information, in conjunction with preoperative computed tomographic evaluation, may be helpful in determining proper screw length during anterior plating of the cervical spine.     

Anterior vertebral body screw pullout testing. A comparison of Zeilke, Kaneda, Universal Spine System, and Universal Spine System with pullout-resistant nut

STUDY DESIGN: A biomechanical study of pullout of anteriorly implanted screws in cadaveric vertebral bodies. OBJECTIVES: To investigate and compare the pullout strength of the Zielke, Kaneda, Universal Spine System (USS) pedicle screw, and USS pedicle screw with a new pullout-resistant nut. SUMMARY OF BACKGROUND DATA: A common problem with anterior purchase regardless of the implant system is screw pullout at the proximal and distal ends of multilevel constructs. There is limited information on a solution to this problem. METHODS: The L1 to L4 vertebral bodies from four cadavers had one each of Zielke and Kaneda pedicle screws (Acromed Corp., Cleveland, OH), USS pedicle screw (Synthes Spine, Paoli, PA), and USS pedicle screw with pullout-resistant nut implanted transversely across the center of the vertebral body with bicortical purchase in a similar fashion as would be used clinically. The screws were extracted using a servohydraulic material testing system. The maximum axial forces were recorded. RESULTS: The Zielke and Kaneda screws had no significant difference in mean pullout strength (P = 0.542). The USS screw alone was less strong (P = 0.009). The USS screw and pullout-resistant nut increased the pullout strength by twofold (P = 0.00006). In the screw pullout tests, the mode of failure was at the screw thread's interface. The USS screw and pullout-resistant nut failed by imploding the body around the nut. With the USS screw and pullout-resistant nut, the pullout strength was determined by the compressive strength of the bone. CONCLUSIONS: The addition of a pullout-resistant nut to an anterior vertebral body screw improves the pullout strength by twofold and changes the mode of failure to rely ultimately on the inherent vertebral body strength rather than the screw's characteristics. The addition of a pullout-resistant nut may be applicable to multilevel implant constructs to prevent screw pullout at the top and bottom.     

Revision pedicle screws. Bigger, longer shims--what is best?

STUDY DESIGN: To evaluate the effect of change in screw dimensions and hole augmentation in pedicle screw revisions, the insertional torque was determined, and results were compared with those in control specimens in an in vitro study using cadaveric thoracolumbar spines. OBJECTIVES: To determine the best method of salvage for failed pedicle screws, by evaluating the insertional torque after placing a larger diameter or longer screw into a stripped hole. Use of a shim and use of larger and longer screws were also investigated. Finally, the effect on insertional torque of simply removing and replacing a pedicle screw in its original hole was investigated. SUMMARY OF BACKGROUND DATA: The effects of using bigger or longer screws and shims to salvage failed pedicles have been studied. The interaction between how much larger, how much longer, and inserting with or without shims, has not been well studied. Optimizing reinsertional torque through the use of bigger screws risks exceeding the pedicle capacity. Using longer screws risks violation of the anterior vertebral body, thereby placing the great vessels and viscera at risk. By knowing the relative contribution of increase in length and diameter, the surgeon can optimize the risk-benefit ratio. METHODS: Eight cadaveric spines from T10 to S1 were harvested. The specimens underwent radiographic screening and bone densitometry. A modified Latin square randomization was designed to evaluate the screw diameters and lengths. Each pedicle was its own control. A 35- x 6.5-mm screw was used as a control. Test screws were placed after pedicle screw hole failure was achieved and documented by stripping. For the test screws, the diameters were increased by 1 mm and 2 mm, the lengths were increased by 5 mm and 10 mm. Shims were added randomly. The peak insertional torque was measured for each control screw and test screw placement. In addition, during each screw placement, the screw was removed and replaced to determine the effect. RESULTS: Insertional torque, after the pedicle screw is removed and replaced in the same hole, was decreased by 34% (P < 0.000005). Increasing the diameter of the salvage screw by 2 mm caused the insertional torque to be increased by 8.4% of the original. Increasing the length of the screw did not improve the salvage screw insertional torque. There was an interaction effect for the 1-mm increase in diameter and the increase in length. At this diameter, increasing the length had a significant effect (P = 0.009) on the salvage torque. Using a shim created no improvement in salvage insertional torque (P = 0.77). There was a poor linear correlation between torque and bone mineral density (r = 0.18) in these osteoporotic specimens. CONCLUSIONS: Removing and replacing a pedicle screw in its original hole substantially decreases its mechanical fixation. For pedicle salvage, increasing the diameter causes the greatest restoration of strength. Shims had no effect in pedicle salvage in osteoporotic specimens.     

Prediction of fatigue screw loosening in anterior spinal fixation using dual energy x-ray absorptiometry

STUDY DESIGN: A biomechanical study was performed to investigate a relation between the bone mineral density of the vertebral body and the number of loading cycles to induce fatigue loosening of an anterior vertebral screw. OBJECTIVES: The objective of this study was to investigate the potential usefulness of dual energy x-ray absorptiometry of measuring bone mineral density of the vertebral body in predicting the fatigue loosening of th anterior vertebral screw. SUMMARY OF BACKGROUND DATA: Loosening of the vertebral body screw is a well know failure in spinal instrumentation, and more commonly observed than pullout failure. The relation between bone mineral density and pullout strength of the screw has been investigated previously, but no studies are available on the fatigue loosening in anterior spinal fixation. METHODS: Bone mineral density was measured using dual energy x-ray absorptiometry and the screw loosening was produce by a cyclic loading in the cephalad-caudal direction. Screw loosening was defined as 1 mm displacement of the screw relative to bone, and the number of loading cycles to induce the screw loosening was obtained and statistically correlated with bone mineral density. RESULTS: There was a positive correlation between the number of loading cycles to induce screw loosening and bone mineral density (R = 0.8, P < 0.01). The average number of loading cycles to induce screw loosening was significantly less for specimens with bone mineral density < 0.45 g/cm2 compared to those with bone mineral density > or = g/cm2. CONCLUSIONS: These findings suggest that bone mineral density may be a good predictor of anterior vertebral screw loosening. Bone mineral density < 0.45 g/cm2 may be critical value of loosening of the anterior vertebral body screw. However, further biomechanical and clinical studies are required before using threshold value clinically.