title: The Natural History of Osteoarthritis: What Happens to the Other Hip?
abstract: BACKGROUND
  Idiopathic osteoarthritis (OA) is a common diagnosis leading to hip arthroplasty. Patients undergoing unilateral hip arthroplasty often wonder whether their other hip will follow the same path as the one that was operated on, and if so, when? There also are limited data available to predict from AP radiographs which contralateral hips will have OA develop and which will not.
  
  
  OBJECTIVE
  We sought (1) to determine the incidence of contralateral osteoarthritic degeneration in a group of patients who were treated with unilateral hip arthroplasty; and (2) to identify clinical and radiographic features associated with the development of contralateral OA.
  
  
  METHODS
  Between 1998 and 2010, we performed 398 hip arthroplasties on patients with unilateral primary hip OA, who at the time of surgery did not have any symptoms in the contralateral hip. Of those, 367 (92%) had a minimum 2-year radiographic followup (mean, 11 years; range, 2-17 years). The 31 patients dropped from the study for lack of radiographic followup had comparable preoperative features as the study group. We performed a radiographic analysis on the baseline AP radiographs to see what factors were associated with arthritis progression, and we performed Kaplan-Meier survivorship analysis with contralateral hip pain and contralateral THA as the endpoints.
  
  
  RESULTS
  Kaplan-Meier survival estimates indicated that 10 years after the baseline evaluation, 59% (95% CI, 53%-65%) of the patients remained free of symptoms on the contralateral hip and 81% (95% CI, 75%-85%) remained free of an arthroplasty on the contralateral hip. Sex, age, weight, or BMI were not associated with the development of OA on the contralateral hip with the numbers available. Reduced minimum joint space width (hazard ratio, 0.299; 95% CI, 0.237-0.378), low center-edge angle (hazard ratio, 0.941; 95% CI, 0.915-0.968), low head-to-neck ratio (hazard ratio, 1.555; 95% CI, 1.088-2.223), and the presence of osteophytes (hazard ratio, 1.453; 95% CI, 1.001-2.110) were associated with the development of contralateral OA. In hips with a center-edge angle greater than 25°, a head-to-neck ratio of 1.3 or less increased the chances of development of OA by 86% (hazard ratio, 1.857; 95% CI, 1.235-2.793).
  
  
  CONCLUSIONS
  The variables we studied can easily be assessed from an AP pelvis radiograph so physicians can predict the occurrence of contralateral OA and the need for future hip arthroplasty in their patients needing unilateral arthroplasty. However, the data available might have led us to underestimate the need for contralateral arthroplasty. Future studies with a prospective design should aim at completing the list of radiographic features associated with the development of OA by adding a review of lateral radiographs.
  
  
  METHODS
  Level IV, prognostic study.
authors.affiliation: Joint Replacement Institute, 2200 West Third Street, Suite 400, Los Angeles, CA, 90057, USA. harlanamstutz@verity.org.
authors.forename: Harlan C
authors.initials: HC
authors.lastname: Amstutz
authors.affiliation: Joint Replacement Institute, 2200 West Third Street, Suite 400, Los Angeles, CA, 90057, USA.
authors.forename: Michel J
authors.initials: MJ
authors.lastname: Le Duff
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects*; D001696:Biomechanical Phenomena; D018450:Disease Progression; D005260:Female; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery*; D006801:Humans; D015994:Incidence; D053208:Kaplan-Meier Estimate; D015141:Los Angeles / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D016017:Odds Ratio; D015207:Osteoarthritis, Hip / Q000175:diagnosis / Q000453:epidemiology / Q000503:physiopathology* / Q000601:surgery*; D011183:Postoperative Complications / Q000000981:diagnostic imaging / Q000453:epidemiology / Q000503:physiopathology* / Q000601:surgery; D011237:Predictive Value of Tests; D016016:Proportional Hazards Models; D012086:Reoperation; D012307:Risk Factors; D013997:Time Factors; D016896:Treatment Outcome