title: Occult spinous process fractures associated with interspinous process spacers.
abstract: METHODS
  Prospective observational study.
  
  
  OBJECTIVE
  To provide a more accurate estimate of the rate of acute spinous process fractures associated with IPS surgery.
  
  
  BACKGROUND
  Biomechanical cadaveric studies have suggested adequate spinous process strength to support placement of interspinous process spacers (IPS). Postoperative spinous process fractures have been reported in one%-to 5.8% of patients in previous series based on routine biplanar radiographic evaluation. However, most fractures occur between the base and midportion of the spinous process in an area that is typically difficult to visualize on plain radiographs due to device design.
  
  
  METHODS
  All patients underwent preoperative biplanar plain radiographs and computed tomography (CT) of the lumbar spine to confirm anatomy favorable for IPS placement and rule out fracture or spondylolysis. Postoperatively, all patients underwent repeat CT imaging within six months of surgery, biplanar radiographs at two weeks, six weeks, three months, six months, and one year. All studies were reviewed independently by a neuroradiologist and two orthopedic spine surgeons.
  
  
  RESULTS
  Fifty implants (38 L4-5, 12 L3-4) were placed in 38 patients who completed follow-up and were included in final analysis. Three IPS designs were included (34 Medtronic X-STOP titanium, 8 X-STOP PEEK, 8 Lanx Aspen). Postoperative CT revealed 11 nondisplaced spinous process fractures in 11 patients (28.9% of patients, 22% of levels). Five fractures were associated with mild to moderate lumbar back pain and six fractures were asymptomatic. No patient reported a traumatic incident. No fracture was identifiable on plain radiographs. One fracture displaced during follow-up evaluation. Three patients underwent IPS removal and laminectomy. Three fractures healed by CT in one year. Overall, patients with fractures tended toward poorer outcomes by Zurich Claudication Questionnaire (ZCQ) (28.5% vs. 34.8% improvement in symptom severity, P = 0.496; 21.4% vs. 30.7% improvement in physical function, P = 0.199) and tended toward lower satisfaction rates (50% vs. 73.7%, P = 0.24) at one year compared to patients without fracture.
  
  
  CONCLUSIONS
  Interspinous process spacer surgery appears associated with a higher rate of early postoperative spinous process fracture than previously reported. In all cases, in this series, plain radiographs were inadequate to identify fractures because all fractures were initially minimal or nondisplaced, many patients were osteopenic, and the metallic wings of the devices often obscured fractures. Moreover, in most patients, fractures were associated with mild or no acute localized pain. This study suggests that unrecognized spinous process fracture may be responsible for a significant number of patients who experience unsatisfactory outcome after IPS surgery. CT imaging is required to identify the vast majority of such fractures.
authors.affiliation: Tufts University School of Medicine, 125 Parker Hill Ave., Boston, MA 02120, USA. dhkim@caregroup.harvard.edu
authors.forename: David H
authors.initials: DH
authors.lastname: Kim
authors.forename: Mark
authors.initials: M
authors.lastname: Tantorski
authors.forename: Jeremy
authors.initials: J
authors.lastname: Shaw
authors.forename: Juli
authors.initials: J
authors.lastname: Martha
authors.forename: Ling
authors.initials: L
authors.lastname: Li
authors.forename: Nael
authors.initials: N
authors.lastname: Shanti
authors.forename: Tal
authors.initials: T
authors.lastname: Rencu
authors.forename: Stephen
authors.initials: S
authors.lastname: Parazin
authors.forename: Brian
authors.initials: B
authors.lastname: Kwon
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001416:Back Pain / Q000175:diagnosis / Q000209:etiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000293:injuries / Q000601:surgery*; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D011182:Postoperative Care; D011183:Postoperative Complications / Q000175:diagnosis* / Q000209:etiology; D011300:Preoperative Care; D011446:Prospective Studies; D019736:Prostheses and Implants; D016103:Spinal Fractures / Q000175:diagnosis* / Q000209:etiology; D013130:Spinal Stenosis / Q000150:complications / Q000601:surgery*; D011795:Surveys and Questionnaires; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome