The Effects of Spinopelvic Parameters and Paraspinal Muscle Degeneration on S1 Screw Loosening

Objective : To investigate risk factors for S1 screw loosening after lumbosacral fusion, including spinopelvic parameters and paraspinal muscles. Methods : We studied with 156 patients with degenerative lumbar disease who underwent lumbosacral interbody fusion and pedicle screw fixation including the level of L5-S1 between 2005 and 2012. The patients were divided into loosening and non-loosening groups. Screw loosening was defined as a halo sign larger than 1 mm around a screw. We checked cross sectional area of paraspinal muscles, mean signal intensity of the muscles on T2 weight MRI as a degree of fatty degeneration, spinopelvic parameters, bone mineral density, number of fusion level, and the characteristic of S1 screw. Results : Twenty seven patients showed S1 screw loosening, which is 24.4% of total. The mean duration for S1 screw loosening was 7.3 +/- 4.1 months after surgery. Statistically significant risk factors were increased age, poor BMD, 3 or more fusion levels (p < 0.05). Among spinopelvic parameters, a high pelvic incidence (p < 0.01), a greater difference between pelvic incidence and lumbar lordotic angle preoperatively (p < 0.01) and postoperatively (p < 0.05). Smaller cross-sectional area and high T2 signal intensity in both multifidus and erector spinae muscles were also significant muscular risk factors (p < 0.05). Small converging angle (p < 0.001) and short intraosseous length (p < 0.05) of S1 screw were significant screw related risk factors (p < 0.05). Conclusion : In addition to well known risk factors, spinopelvic parameters and the degeneration of paraspinal muscles also showed significant effects on the S1 screw loosening.