All CT scans were obtained with 0.6mm slices and allowed for coronal and sagittal reconstructions. Postoperative CT scans were primarily acquired for routine hardware evaluations (41.9%). CT scans were also ordered for persistent back or leg pain (28.0%), and the presence of postoperative complications such as new neurological deficit/symptoms, hematoma, or evaluation of surgical site infection (18.3%). The remaining scans (11.8%) were performed for non-neurosurgical indications including abdominal and pelvic pathology and pain. It is important to note that there were no significant differences in the indications for getting a CT scan between percutaneous and open cases. Also, there was no significant difference in facet violations when comparing across indications. The presence of facet violation on the postoperative CT at less than 3 months did not guide any acute management or clearly result in new acute symptoms. The grading criterion for facet joint violation was established prior to radiographic review (Table 3). A screw that was not in the facet and did not encroach upon the facet joint was labeled as a grade 0. If the screw was in the lateral facet but did not enter the articular facet, it was labeled as a grade 1 (Figure 1). Screws that penetrated the articular facet by ≤ 1 mm were assigned a grade 2 violation (Figure 2). Those screws travelling within the articular surface of the facet were given a grade 3 designation (Figure 3). All screws were graded by evaluating axial images together with the coronal and sagittal reconstructed ones.
The top-level screws were evaluated by 2 different observers for evidence and grade of facet violation. The inter- and intra-observer Pearson r coefficients were 0.98 and 1.0, respectively. Both sides were assessed independently. Patient parameters such as age, gender, and body mass index (BMI) in addition to surgical factors including the level or segment of spine at the top of the construct, the distance from the skin surface to the L4 lamina, and the number of operative levels were recorded. Operative reports and radiographic images were used to confirm whether the screws were placed through an open or percutaneous technique.
The primary outcomes for this study were the mean grade and incidence of facet violation due to open and percutaneous pedicle screw placement. Secondary outcomes included evaluating patient and surgery factors impacting primary outcomes. We also evaluated if facet violations impacted the development of clinically significant adjacent segment. The charts of patients with at least 3 years of follow up were reviewed to determine need for subsequent lumbar surgery due to adjacent level disease. We compared facet violation grade for patients requiring surgery for ASD compared to ones who remained asymptomatic.