Insertion torque is not a good predictor of pedicle screw loosening after spinal instrumentation: a prospective study in 8 patients

BACKGROUND Pedicle screw loosening is a major safety concern in instrumented spinal surgery due to loosening with potential pseudarthrosis and possible loss of correction requiring revision surgery. Several cadaver studies have compared insertion torque of pedicle screws with resistance to pullout or cyclic loading. In most of these studies, a correlation has been found between these variables. Clinical studies have been made, comparing insertion torque to bone mineral density or radiological signs of screw loosening. There are no clinical studies comparing insertion torque to extraction torque or other biomechanical parameters in vivo. This study was designed to investigate whether the insertion torque of pedicle screws can be used to predict the purchase of the screws. METHODS The insertion torque of stainless steel pedicle screws was recorded in eight patients undergoing lumbar fusion surgery with four-screw constructs. Torque gauge manometers were used for the recordings. The implants were removed after one year, and the extraction torque of the screws was recorded. RESULTS The mean insertion torque was 76 ± 41 Ncm and the mean extraction torque 29 ± 36 Ncm. The r value was 0.591, suggesting that there was a correlation between the insertion and extraction torque. However, the scattergram revealed that the screws could be divided into two groups, six screws with a high correlation between insertion and extraction torque, and 26 screws where no correlation could be demonstrated. CONCLUSIONS In this unique human in-vivo study, the insertion torque could not be used to predict the purchase of lumbar pedicle screws one year after implantation. It could be demonstrated that in vivo insertion torque alone is of minor value to estimate pullout strength, and should be combined with or replaced by more accurate measures.