Predictive factors of distal pedicle screw loosening followed posterior corrective surgery for degenerative lumbar scoliosis

To explore incidence and predictive factors for distal pedicle screw loosening (DPSL) followed posterior corrective surgery for degenerative lumbar scoliosis (DLS). The diagnostic criteria for DPSL developed by X-ray including radiolucent area around screw and "double halo" sign. According to occurrence of DPSL at two-year follow-up, 153 patients were divided into two groups: study group (screw loosening) and control group (without screw loosening). To investigate predictive factors for DPSL, three categorized factors including general data, surgical data and radiological data were analyzed statistically. DPSL was detected in 72 patients at two-year follow up (study group). Hounsfield unit (HU) value was lower in study group than that in control group. Fusion level was longer in study group than that in control group. Lower instrumented vertebrae on L5 was less in study group than that in control group. Posterolateral fusion was less in study group than that in control group. Preoperative Cobb angle, postoperative Cobb angle, Cobb angle correction, preoperative lumbosacral coronal angle (LSCA), LSCA correction, preoperative thoracolumbar junction (TL), postoperative TL were larger in study group than those in control group. Logistic regression analysis revealed that low Bone mineral density (BMD) (< 169 HU), posterolateral fusion, Cobb angle correction (> 16 degrees), LSCA correction (> 9 degrees) were independently associated with DPSL. The incidence of DPSL following posterior decompression and instrumented fusion for DLS is 47.1%. Low BMD, large correction of both main curve and fractional curve are predictive factors for DPSL, posterolateral fusion is a protective factor.