Measurement of Fastening Torque of Rod Securing Spinal Implant Screws

Background/Objectives: The mechanical performance of spinal instrumentation depends on appropriate fixation of rod-securing screws. Although manufacturer-recommended torque values and torque-limiting instruments are widely used in spinal surgery, the contribution of surgeon-related factors to manually applied tightening torque remains insufficiently characterized. This study aimed to quantify the magnitude and variability of tightening torque applied by spine surgeons and to investigate the influence of handling technique and surgeon orientation on torque generation. Methods: A thoracolumbar spine model instrumented with pedicle screw–rod constructs was used in an experimental setup. Twenty-two spine surgeons from six institutions performed a total of 220 screw-tightening procedures using a standardized manual screwdriver setup. Tightening torque was estimated from measured loosening torque using a previously established calibration relationship. Hand usage, hand position, and surgeon orientation relative to the construct were recorded. Statistical analyses were performed to compare torque values between different handling conditions. Results: The mean estimated tightening torque was 4.52 Nm, with considerable inter-individual variability among surgeons. Two-handed grip produced significantly (p = 0.006) higher torque (5.76 ± 1.46 Nm) compared to thumb-downward (3.47 ± 1.19 Nm). Among the univariate linear regression analyses, only the two-handed grip technique showed a significant association with higher fastening torque (B [95% CI]: 1.88 [0.56; 3.21], p = 0.008). The other factors (age, height, weight, experience) did not show a significant relationship. Conclusions: Manual tightening torque during spinal instrumentation shows substantial variability among surgeons and is influenced by handling technique and surgeon position. These findings demonstrate the importance of considering human factors in biomechanical evaluations of spinal fixation procedures. Further studies are required to determine the clinical significance of this variability.