title: Factor analysis of the North American Spine Society outcome assessment instrument: a study based on a spine registry of patients treated with lumbar and cervical disc arthroplasty.
abstract: BACKGROUND
  Studies involving factor analysis (FA) of the items in the North American Spine Society (NASS) outcome assessment instrument have revealed inconsistent factor structures for the individual items.
  
  
  OBJECTIVE
  This study examined whether the factor structure of the NASS varied in relation to the severity of the back/neck problem and differed from that originally recommended by the developers of the questionnaire, by analyzing data before and after surgery in a large series of patients undergoing lumbar or cervical disc arthroplasty.
  
  
  METHODS
  Prospective multicenter observational case series.
  
  
  METHODS
  Three hundred ninety-one patients with low back pain and 553 patients with neck pain completed questionnaires preoperatively and again at 3 to 6 and 12 months follow-ups (FUs), in connection with the SWISSspine disc arthroplasty registry.
  
  
  METHODS
  North American Spine Society outcome assessment instrument.
  
  
  METHODS
  First, an exploratory FA without a priori assumptions and subsequently a confirmatory FA were performed on the 17 items of the NASS-lumbar and 19 items of the NASS-cervical collected at each assessment time point. The item-loading invariance was tested in the German version of the questionnaire for baseline and FU.
  
  
  RESULTS
  Both NASS-lumbar and NASS-cervical factor structures differed between baseline and postoperative data sets. The confirmatory analysis and item-loading invariance showed better fit for a three-factor (3F) structure for NASS-lumbar, containing items on "disability," "back pain," and "radiating pain, numbness, and weakness (leg/foot)" and for a 5F structure for NASS-cervical including disability, "neck pain," "radiating pain and numbness (arm/hand)," "weakness (arm/hand)," and "motor deficit (legs)."
  
  
  CONCLUSIONS
  The best-fitting factor structure at both baseline and FU was selected for both the lumbar- and cervical-NASS questionnaires. It differed from that proposed by the originators of the NASS instruments. Although the NASS questionnaire represents a valid outcome measure for degenerative spine diseases, it is able to distinguish among all major symptom domains (factors) in patients undergoing lumbar and cervical disc arthroplasty; overall, the item structure could be improved. Any potential revision of the NASS should consider its factorial structure; factorial invariance over time should be aimed for, to allow for more precise interpretations of treatment success.
authors.affiliation: Institute for Evaluative Research in Orthopaedic Surgery, University of Bern, Stauffacherstrasse 78, CH-3014 Bern, Switzerland. Electronic address: emin.aghayev@memcenter.unibe.ch.
authors.forename: Emin
authors.initials: E
authors.lastname: Aghayev
authors.affiliation: Department of Work and Organizational Psychology, Institute for Psychology, University of Bern, Uni Tobler, Muesmattstrasse 45, CH-3009 Bern, Switzerland.
authors.forename: Achim
authors.initials: A
authors.lastname: Elfering
authors.affiliation: Centre Hospitalier Universitaire Vaudois, Hôpital Orthopédique, University of Lausanne, Av P. Decker 4, 1011 Lausanne, Switzerland.
authors.forename: Constantin
authors.initials: C
authors.lastname: Schizas
authors.affiliation: Spine Center Division, Department of Research and Development, Schulthess Klinik, Lengghalde 2, CH-8008 Zurich, Switzerland.
authors.forename: Anne F
authors.initials: AF
authors.lastname: Mannion
mesh_terms: D000328:Adult; D000368:Aged; D001178:Arthroplasty / Q000379:methods*; D001416:Back Pain / Q000601:surgery*; D002574:Cervical Vertebrae / Q000601:surgery*; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000601:surgery*; D007405:Intervertebral Disc Displacement; D017116:Low Back Pain / Q000601:surgery; D008159:Lumbar Vertebrae / Q000601:surgery*; D008161:Lumbosacral Region / Q000601:surgery; D008297:Male; D008875:Middle Aged; D019547:Neck Pain / Q000601:surgery*; D017063:Outcome Assessment, Health Care; D010147:Pain Measurement; D011446:Prospective Studies; D012042:Registries; D012955:Societies, Medical; D011795:Surveys and Questionnaires; D016896:Treatment Outcome; D014481:United States
keywords: Factor analysis; NASS; Outcome; SWISSspine; Total disc arthroplasty