Assessment of maximal insertional torque of cervical and thoracic screws during posterior spinal surgery

Study Design : Multicenter intraoperative biomechanical analysis. Purpose : This study aimed to assess the maximal insertional torque (MIT) of lateral mass screw (LMS) and pedicle screw (PS) in the cervical or thoracic vertebrae during surgery. Methods : During posterior spinal fusion, cervical or thoracic multi-axial screws were placed at different cervical or thoracic levels and the MIT was recorded for each screw　revolution using an analogue torque wrench. Screw number was as follows : 11 PS at C7, 134 LMS (C3-6) (Magerl technique, 70 ; Roy-Camille technique, 64), and 33 PS in the thoracic region. Results : Average MIT　values (cNm) were as follows : for PS (diameter : 3.5 mm) at C7, 45.3±21.9 in men and 60.0±20.1 in women (p=0.28) ; for Magerl screws, 69.0±20.5 in men and 58.6±15.7 in women (p=0.13) ; and for Roy-Camille screws, 51.0±17.9 in men and 42.4±15.9 in women (p=0.52). The average MIT for Magerl screws was significantly higher than that for Roy-Camille screws in men and women (both p < 0.01). Conclusions : Intraoperative insertional torque could be a good indicator to evaluate the purchase and help guide decisions on screw type and insertion technique. Further postoperative assessments with sequential X-rays are needed to reveal the significance of MIT during posterior spinal fusion.