Feasibility of using tapping torque during lumbar pedicle screw insertion to predict screw fixation strength.

Background: Rigid pedicle screw fixation is mandatory for achieving successful spinal fusion; however, there is no reliable method predicting screw fixation before screw insertion. The purpose of the present study was to investigate the efficacy of measurement of tapping torque to predict pedicle screw fixation.

Methods: First, different densities of polyurethane foam were used to measure tapping torque. The insertional torque during pedicle screw insertion and axial pullout strength were measured and compared between under-tapped and same-tapped groups. Next, for in vivo study, the tapping and insertional torque of lumbar pedicle screws using the cortical bone trajectory technique were measured intraoperatively in 45 consecutive patients. Then, correlations between tapping torque, the bone mineral density of the femoral neck and lumbar vertebrae, and insertional torque were investigated.

Results: Ex vivo tapping torque significantly correlated with the insertional torque and pullout strength regardless of tapping sizes (r = 0.98, p < 0.001). The mean in vivo tapping and insertional torque were 1.48 ± 0.73 and 2.48 ± 1.25 Nm, respectively (p < 0.001). Insertional torque significantly correlated with tapping torque and two BMD parameters, and the correlation coefficient of tapping torque (r = 0.83, p < 0.001) was higher than those of femoral neck BMD (r = 0.59, p < 0.001) and lumbar BMD (r = 0.39, p < 0.001).

Conclusions: Tapping torque is a reliable predictor of pedicle screw fixation and allows surgeons to improve the integrity of the bone-screw interface by making modification prior to actual screw insertion.