title: Screw loosening in the Dynesys stabilization system: radiographic evidence and effect on outcomes.
abstract: OBJECTIVE
  Dynamic stabilization systems are used to stabilize degenerative lumbar spondylosis. Loosening of the pedicle screws in such nonfusion implants is predictable. This retrospective study evaluated the incidence of screw loosening and its effect on clinical outcomes.
  
  
  METHODS
  Charts, radiographic films, and medical records of 71 consecutive patients who underwent decompression using Dynesys dynamic stabilization for 1- or 2-level lumbar spondylosis were reviewed. Radiographic films were evaluated and compared to detect screw loosening. A visual analog scale (VAS) for back pain and the Oswestry Disability Index (ODI) were used for measuring clinical outcome. Statistical analysis was conducted using the chi-square test and Student t-test.
  
  
  RESULTS
  The 71 patients in the study sample had a mean age of 59.2 +/- 11.65 years (range 23-80 years), with slight female predominance (39 women, 32 men). The mean follow-up duration was 16.6 months (range 8-29 months). There were loose screws in 14 of 71 patients (19.7%), for a rate of 4.6% per screw (17 of 368 screws). Most screw loosening occurred in patients >/= 55 years old (13 of 14 patients) although age and sex had no effect on screw loosening (p = 0.233 and 0.109, respectively). Both the loose screw and solid screw groups experienced significant improvement after the surgery in VAS and ODI scores. On the VAS, scores improved from 5.9 +/- 2.99 to 2.1 +/- 2.14 in the loose screw group (p = 0.003), and from 5.7 +/- 3.45 to 2.9 +/- 2.68 in the solid screw group (p < 0.001). For the ODI scale, scores improved from 43.5 +/- 16.78% to 28.0 +/- 18.18% (p = 0.006) in the loose screw group, and from 52.1 +/- 20.92% to 24.6 +/- 19.78% (p < 0.001) in the solid screw group. There were no significant differences between the 2 groups (p = 0.334 for VAS, p = 0.567 for ODI).
  
  
  CONCLUSIONS
  The preliminary study of this pedicle-based dynamic stabilization device for 1- and 2-level lumbar spondylosis shows radiographic evidence of screw loosening in 19.7% of patients and 4.6% of screws. Nonetheless, the loosening of screws has no adverse effect on clinical improvement.
authors.affiliation: Department of Neurosurgery, Neurological Institute, Taipei Veterans General Hospital, Taipei, Taiwan.
authors.forename: Chin-Chu
authors.initials: CC
authors.lastname: Ko
authors.forename: Hsiao-Wen
authors.initials: HW
authors.lastname: Tsai
authors.forename: Wen-Cheng
authors.initials: WC
authors.lastname: Huang
authors.forename: Jau-Ching
authors.initials: JC
authors.lastname: Wu
authors.forename: Yu-Chun
authors.initials: YC
authors.lastname: Chen
authors.forename: Yang-Hsin
authors.initials: YH
authors.lastname: Shih
authors.forename: Hung-Chieh
authors.initials: HC
authors.lastname: Chen
authors.forename: Ching-Lan
authors.initials: CL
authors.lastname: Wu
authors.forename: Henrich
authors.initials: H
authors.lastname: Cheng
mesh_terms: D000328:Adult; D000368:Aged; D001863:Bone Screws / Q000009:adverse effects* / Q000592:standards; D019544:Equipment Failure Analysis / Q000379:methods; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D019635:Neurosurgical Procedures / Q000009:adverse effects* / Q000295:instrumentation / Q000379:methods; D017063:Outcome Assessment, Health Care / Q000379:methods; D011183:Postoperative Complications / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000517:prevention & control; D011237:Predictive Value of Tests; D019736:Prostheses and Implants / Q000009:adverse effects* / Q000592:standards; D019919:Prosthesis Implantation / Q000009:adverse effects* / Q000295:instrumentation / Q000379:methods; D012189:Retrospective Studies; D055009:Spondylosis / Q000000981:diagnostic imaging* / Q000601:surgery*; D014057:Tomography, X-Ray Computed; D055815:Young Adult