title: The Effects of Spinopelvic Parameters and Paraspinal Muscle Degeneration on S1 Screw Loosening.
abstract: 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).
  
  
  CONCLUSIONS
  In addition to well known risk factors, spinopelvic parameters and the degeneration of paraspinal muscles also showed significant effects on the S1 screw loosening.
authors.affiliation: Department of Neurosurgery, Chung-Ang University Hospital, Seoul, Korea.
authors.forename: Jin-Bum
authors.initials: JB
authors.lastname: Kim
authors.affiliation: Department of Neurosurgery, Chung-Ang University Hospital, Seoul, Korea.
authors.forename: Seung-Won
authors.initials: SW
authors.lastname: Park
authors.affiliation: Department of Neurosurgery, Gangneung Asan Hospital, University of Ulsan College of Medicine, Gangneung, Korea.
authors.forename: Young-Seok
authors.initials: YS
authors.lastname: Lee
authors.affiliation: Department of Neurosurgery, Chung-Ang University Hospital, Seoul, Korea.
authors.forename: Taek-Kyun
authors.initials: TK
authors.lastname: Nam
authors.affiliation: Department of Neurosurgery, Chung-Ang University Hospital, Seoul, Korea.
authors.forename: Yong-Sook
authors.initials: YS
authors.lastname: Park
authors.affiliation: Department of Neurosurgery, Chung-Ang University Hospital, Seoul, Korea.
authors.forename: Young-Baeg
authors.initials: YB
authors.lastname: Kim
keywords: Instrumentation; Lumbosacral region; Muscle; Pedicle screws; Risk factors; Spinal fusion