Can insertional torque predict screw loosening and related failures? An invivo study of pedicle screw fixation augmenting posterior lumbar interbodyfusion

Study Design. An investigation of the relation between intraoperative inser tional torque of pedicle screws, bone mineral density of the vertebra, and development of screw loosening in vivo. Objectives. To determine the usefulness of intraoperative measurement of th e insertional torque of pedicle screws. Summary of Background Data. Some biomechanical studies have demonstrated th at the insertional torque is highly correlated with bone mineral density an d the stability of pedicle screws in vitro. Methods. Pedicle screw fixation was performed with posterior lumbar interbo dy fusion in 62 consecutive patients. The mean age of the patients at the t ime of surgery was 58 years. The insertional torque of pedicle screws was m easured intraoperatively in all patients. The mean follow-up period was 2.7 years. Results. The mean insertional torque was 1.28 +/- 0.37 Nm in patients with screw loosening and 1.50 +/- 0.40 Nm In patients without the problem. The m ean insertional torque in patients with compression fractures in the upper vertebra adjacent to the fixed segment was 0.83 +/- 0.23 Nm. There was no s ignificant difference between the mean insertional torque in patients with screw loosening and those without the condition. The mean insertional torqu e in patients without screw loosening was significantly greater than in pat ients with compression fractures (P < 0.01). A high correlation was found b etween insertional torque and bone mineral density (P < 0.01). Conclusions. Although a high correlation was found between the insertional torque of pedicle screws andbone mineral density in viva the insertional to rque could not objectively predict screw loosening.