The correlation between insertion torque of pedicle screws with bone mineral density values in posterior lumbar pedicle screw fixation

INTRODUCTION: In a pedicle screw fixation, the degree of osseointegration between a screw and the bone is an essential factor for the stability and fusion. In patients with osteoporosis, however, there is a high possibility for a pedicle screw loosening. Owing to this, there is a difficulty in determining the surgical indications. Previous biomechanical studies have reported that a pedicle screw fixation has a higher degree of relationship with the bone mineral density (BMD). Some experiments confirmed findings that insertion torque was found to be a strong correlating factor in terms of screw pull-out strength. Nevertheless, there are also some reports that there is no consistent relationship between the torsion force during a pedicle screw fixation and that during the removal of a screw. It has been known, however, that the initial stability of a pedicle screw is a very crucial factor for the successful treatment outcomes in the spinal fusion. Therefore, the prediction of initial fixation force of a pedicle screw prior to the surgery would be essential in making a treatment plan for the insertion of a screw and reducing the rate of surgical failure. Given the above background, we attempted to identify the correlation between the BMD and the torque during a pedicle screw fixation in patients who are indicated in posterior lumbar pedicle screw instrumentation. Then, we also attempted to clarify the predictable torque depending on the BMD during a pedicle screw fixation.