Pedicle Screw Removal Torque Analysis

This was a prospective designed single-center study. Patients who underwent pedicle screw removal surgery were screened for eligibility. The indications for screw removal included the following: (1) pedicle screw fixation for thoracolumbar fracture without fusion and imaging confirmed solid fracture union, (2) patients required screw removal that presented persistent axial para-midline back pain to palpation or abnormal foreign body sensation due to pedicle fixation with imaging confirmed solid fusion, and no other cause found, e.g., infection. Those with significant pedicle malplacement or destructive spine disorders, such as bone metabolic disease, were excluded from our study. Informed consent was obtained from all individual participants included in the study.
The titanium alloy pedicle screws (Johnson&Johnson, USA; Medtronic, USA; Stryker, USA; Kanghui Med, China; FULE, China) with diameter of 4.0 to 6.5 mm and length of 30 to 55 mm were used in the primary surgeries. Conventional lateral and anteroposterior radiographs were taken before and at 3 to 6 months after primary surgery. The same X-ray and CT scan were scheduled before screw removal surgery to evaluate fracture union and spine fusion, as well as stability of instrumentation. Bone mineral density (BMD) was tested using dual energy X-ray absorptiometry. The maximum extraction torque was recorded while unscrewing the screw using torque gauge with a range of 0.06 to 6.00 Nm (Park Tool, China). The same surgeon (J.S. senior resident) with sufficient practice made all the extraction torque recordings. The radiographs were evaluated by a senior spinal surgeon (X.W. senior attending doctor), who was blinded to the extraction torque and patient information. X-ray criteria for screw loosening were a radiolucent zone surrounding the screw thicker than 1 mm and/or the “double halo” sign. CT scan criterion of loosening was a no signal zone surrounding the whole body of screw on the CT image. Because of metal artifact, no signal zone was usually seen around screw tail, which could not be interpreted as screw loosening (Fig. 1).Fig. 1

Typical CT image of metal artifact around screw tail. This picture shows metal artifact around screw tail in CT scan, which may confuse the interpretation of screw loosening

All values are given as the mean ± SD. Statistical analyses were performed using the IBM SPSS Statistics 13.0 (Chicago, USA). The Chi-square test was used for categorical variables. Mann-Whitney U tests or Kruskal-Wallis test was used for continuous variables. TwoStep Cluster was used to clustered screws by torque. We constructed a linear regression model with Pearson correlations analysis to assess whether clinical data, such as age and BMD, were correlated with the extraction torque of screw. The level of statistical significance was selected to be p = 0.05.