Anatomic considerations for anterior instrumentation of the lumbar spine

Computed tomography morphometric analysis of anterior instrumentation in the pediatric thoracic spine

OBJECT The authors' goal in this paper was to quantify reference data on the dimensions and relationships of the maximum posterior screw angle and the thoracic spinal canal in different pediatric age groups. METHODS One hundred twelve pediatric patients were divided into 4 age groups, and their thoracic vertebrae were studied on CT scans. The width, depth, and maximum posterior screw angles with different screw entrance points were measured on a Philips Brilliance 16 CT. The statistical analysis was performed using the Student t-test and Pearson's correlation analysis. RESULTS The width and depth of the thoracic vertebrae increased from T-5 to T-12. The width ranged from 18.5 to 37.1 mm, while the depth ranged from 16.1 to 28.2 mm. The maximum posterior screw angle decreased from T-5 to T-12 in all groups. The ranges and mean angles at the entrance points were as follows: initial entrance point, 6.9° to 12.3° with a mean angle of 9.1°; second entrance point, 20.6° to 27.0° with a mean angle of 24.2°; and third entrance point, 29.2° to 37.5° with a mean angle of 33.7°. There were no significant age-related differences noted for the maximum posterior screw angles. CONCLUSIONS The angle decreased from T-5 to T-12. No significant age-related differences were noted in the maximum posterior screw angles. Screws should be placed between the initial and second points and parallel to the coronal section or at a slight anterior orientation.

Preoperative radiographic factors and surgeon experience are associated with cortical breach of C2 pedicle screws

STUDY DESIGN

A retrospective review study.

OBJECTIVE

In this study, we attempt to identify radiographic variables associated with likelihood of intraoperative breach during C2 pedicle screw placement. In addition, we attempt to correlate surgeon experience with breach rate.

SUMMARY OF BACKGROUND DATA

Pedicle screws have emerged as an effective approach for obtaining fixation of the axis, yet placement of C2 pedicle screws is technically demanding and poses the risk of injury to the vertebral artery. Given the evidence for substantial variation in C2 anatomy, preoperative assessment of computed tomography (CT) scans may indicate, which patients are at increased risk for cortical breach during the pedicle screw placement.

MATERIALS AND METHODS

A retrospective review of all patients undergoing C2 pedicle screw fixation at a single institution over the last 6 years was conducted. Radiographic cortical breaches were defined on postoperative CT scans as visualization of the screw beyond the cortical edge. Radiographic measurements were determined from preoperative CT scans and were then correlated with breaches via Student t test. The association of breach rate with surgeon experience was evaluated using univariate linear regression.

RESULTS

Ninety-three patients underwent placement of 170 screws. Cortical breach was detected on postoperative CT scans in 43 screws (25.3%). One clinically significant breach occurred with damage to the left vertebral artery intraoperatively. On axial CT sections, mean pedicle isthmus diameter was significantly smaller in patients with breach than in patients without breach for both left and right sides, P=0.006 and P=0.010, respectively. Specifically, a diameter of less than 6 mm was associated with a nearly 2-fold increase in risk of cortical breach (37% vs. 21%). Surgeons with greater experience in placing C2 pedicle screws were noted to have a lower breach incidence (P=0.004).

CONCLUSIONS

During placement of C2 pedicle screws, likelihood of cortical breach may be associated with size of pedicle and surgeon experience. Extensive preoperative evaluation of CT scans and consideration of technical demands of procedure may help avoid complications with such internal fixation.