title: Reports of joint instability in knee osteoarthritis: its prevalence and relationship to physical function.
abstract: OBJECTIVE
  To report the prevalence and relationship of self-reported knee instability to physical function in a sample of subjects with knee osteoarthritis (OA), and to discuss the implications of these observations for rehabilitation.
  
  
  METHODS
  Subjects were 105 individuals (80 females) with knee OA who rated their knee instability severity on a 6-point numeric scale in response to the query "To what degree does giving way, buckling, or shifting of the knee affect your level of daily activity?" A principal component analysis was used to combine The Western Ontario and McMaster Universities pain, stiffness, and physical function subscale scores, and the Timed Get Up and Go Test score into a principal component score for physical function (PCPF). Other variables that could affect the PCPF such as age, sex, years with knee OA, radiographic severity of knee OA, knee pain, knee motion, and quadriceps strength were also recorded. The prevalence of self-reported knee instability was determined by calculating the proportion of subjects who reported each severity level of knee instability. Hierarchical regression analysis was performed to determine if the level of self-reported knee instability could predict the PCPF, even after accounting for the effects of the other variables.
  
  
  RESULTS
  Sixty-three percent of the subjects reported knee instability during activities of daily living, and 44% reported that instability affects their ability to function. The severity of self-reported knee instability was associated with the PCPF (eta2=0.40, P <0.001), and after controlling for all other independent variables, significantly increased the prediction of the PCPF (r2=0.56, r2 change=0.05; P <0.001).
  
  
  CONCLUSIONS
  The results indicate that a substantial proportion of individuals with knee OA report episodes of knee instability during activities of daily living, and instability affects physical function beyond that which can be explained by contributions from other impairments such as knee pain, range of motion, and quadriceps strength. Knee instability is a problem that should be specifically addressed in rehabilitation programs and may require interventions beyond those that address pain, joint motion, and muscular strength, to maximize the effectiveness of rehabilitation for individuals with knee OA.
authors.affiliation: School of Health and Rehabilitation Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA. kfitzger@pitt.edu
authors.forename: G Kelley
authors.initials: GK
authors.lastname: Fitzgerald
authors.forename: Sara R
authors.initials: SR
authors.lastname: Piva
authors.forename: James J
authors.initials: JJ
authors.lastname: Irrgang
mesh_terms: D000203:Activities of Daily Living; D004185:Disability Evaluation; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000453:epidemiology / Q000209:etiology* / Q000503:physiopathology*; D007719:Knee Joint / Q000503:physiopathology*; D008297:Male; D020370:Osteoarthritis, Knee / Q000150:complications* / Q000503:physiopathology*; D015995:Prevalence; D025341:Principal Component Analysis / Q000379:methods; D012649:Self Concept; D012720:Severity of Illness Index