Research

Purpose: To evaluate the effectiveness of using orthogonal plain radiographs and a systematic method of interpretation in assessing the placement of lumbar and lumbosacral pedicle screws

Methods: Five adult cadaveric spines were instrumented with titanium pedicle screws from L1 to S1. Screws were placed outside the confines of the pedicle in all four quadrants or within the pedicle using a Latin-Square design. Each cadaver was imaged with orthogonal radiographs and high – resolution CT scans. The spines were then re-imaged after the instrumentation was replaced with stainless steal screws placed in the identical position. Finally, each spine was dissected to assess the exact position of the screws. Images were read in a blinded fashion and the results compared to known screw positions at dissection.

Results: In total, 120 pedicle screws were placed, 44 (38%) outside the confines of the pedicle. The positive predictive value (PPV) of correctly identifying screw position was 83.3% using orthogonal plain radiographs, 82.4% using 1mm axial CT scans, and 85.2% using coronal CT reconstructions. Sensitivity, defined as the percent of the misplaced screws that were correctly identified, was similar across the three diagnostic tests. Similarly, specificity, defined as the percent of screws read as correctly placed that were actually placed within the pedicle, was similar.
Plain X-Ray Axial CT Coronal CT
Sensitivity 70.0 77.5 75.0
Specificity 83.8 87.0 86.3
There was an observed association with anatomic level, with a consistently less accuracy in detecting screw position at L5 in all three assessments (p=0.001). Additionally, correct positions of stainless steel screws were more difficult to detect as compared to the titanium (p=0.033). Other variables examined, such as screw length and screw diameter, did not have an effect on the ability to read the positioning.

Conclusions: CT scans; often considered the “gold standard” for clinical assessment of pedicle screw placement, have limitations when validated with gross anatomical dissection. This systematic method for evaluating pedicle screw placement utilizing orthogonal plain radiographs attained accuracy comparable to high – resolution CT scans.