Risk Factors for Clinically Relevant Loosening of Percutaneous Pedicle Screws

INTRODUCTION: (1) To evaluate the influence of pedicle screw loosening on clinical outcomes; (2) to clarify the association between the pull-out length and screw loosening 1 year after surgery; and (3) to determine radiographically which screw parameters predominantly influence the pull-out resistance of screws. METHODS: We analyzed 32 consecutive patients who underwent minimally invasive lumbar or thoracic spinal stabilization by intraoperative three-dimensional computed tomography (CT)-guided navigation without anterior reconstruction and were followed up for 1 year. The screw pull-out length was measured on axial CT images obtained both immediately after screw insertion and postoperatively. Loosening of screws and clinical outcomes were evaluated radiographically, clinically, and by CT 1 year after surgery. RESULTS: There were no significant differences in the mean age, sex, bone mineral density, mean stabilized length, and smoking habits of patients with (+) or without (−) loosening. The Oswestry Disability Index and the lumbar visual analog scale 1 year after surgery were significantly higher in patients with loosening (+) than in those without (−). The overall pedicle screw pull-out rate was 16.2% (47/290) of screws and the overall screw loosening rate was 15.2% (44/290) of screws. Screws with loosening (+) had significantly lower (axial) trajectory angles and higher screw pull-out lengths than those without (−). Approximately 82% of loosened screws had been pulled out during rod connection. CONCLUSIONS: A lower axial trajectory and an increased screw pull-out length after rod reduction are crucial risk factors for screw loosening.