A simple formula for predicting diameter of safely inserted cortical bone trajectory screws for fixation of the lower lumbar spine

Cortical bone trajectory (CBT) screws are popular for spinal fixation, but their ideal diameter has not been determined. Studies using postoperative computed tomography (CT) have revealed ample bone marrow space around 5.5-mm screws, which are commonly used. However, evidence indicates that a larger screw diameter provides a greater fixation strength. This study aimed to develop a generalizable formula for computing the diameter of CBT screws that could be inserted safely for fixation of the lower lumbar spine. Records of 44 consecutive patients who had undergone posterior fusion with CBT screws for single-level degenerative lumbar spondylolisthesis were retrospectively reviewed. We estimated the maximum diameter for conventional pedicle screws by the minimum diameter of the pedicle using preoperative CT (PSD). We measured the minimum endosteal diameter of the pedicle on the reconstructed plane of the postoperative CT which passed through the cannula used for the screw and estimated the maximum diameter for the CBT screws that could be inserted within the bone marrow space of the pedicle (CBTD). Among the 176 pedicles measured, there were 151 (85.8%) with a PSD of 8.5 mm and 13, 7.5 mm. Because of a slight pedicle wall breach, 13 screws were excluded from the sample. There were 64 (39.3%) screws with a CBTD of 8.5 mm; 45, 7.5 mm; and 40, 6.5 mm. Of 163 screws, 156 (95.7%) had PSD minus CBTD ≤2 mm for each pedicle. PSD minus the minimum outer cortical diameter was ≤1 mm for each pedicle in 155 (95.1%) screws. Our results show that CBT screws with a diameter 1 mm smaller than the endosteal diameter of the pedicle were inserted safely. Design: Clinical study.