title: Revision surgery is overestimated in hip replacement.
abstract: OBJECTIVE
  The Kaplan-Meier estimation is widely used in orthopedics to calculate the probability of revision surgery. Using data from a long-term follow-up study, we aimed to assess the amount of bias introduced by the Kaplan-Meier estimator in a competing risk setting.
  
  
  METHODS
  We describe both the Kaplan-Meier estimator and the competing risk model, and explain why the competing risk model is a more appropriate approach to estimate the probability of revision surgery when patients die in a hip revision surgery cohort. In our study, a total of 62 acetabular revisions were performed. After a mean of 25 years, no patients were lost to follow-up, 13 patients had undergone revision surgery and 33 patients died of causes unrelated to their hip.
  
  
  RESULTS
  The Kaplan-Meier estimator overestimates the probability of revision surgery in our example by 3%, 11%, 28%, 32% and 60% at five, ten, 15, 20 and 25 years, respectively. As the cumulative incidence of the competing event increases over time, as does the amount of bias.
  
  
  CONCLUSIONS
  Ignoring competing risks leads to biased estimations of the probability of revision surgery. In order to guide choosing the appropriate statistical analysis in future clinical studies, we propose a flowchart.
authors.affiliation: Leiden University Medical Center, Department of Orthopaedic Surgery, Albinusdreef 2, PO Box 9600, Postzone J-11-S, 2300 RC Leiden, the Netherlands.
authors.forename: J C
authors.initials: JC
authors.lastname: Keurentjes
authors.forename: M
authors.initials: M
authors.lastname: Fiocco
authors.forename: B W
authors.initials: BW
authors.lastname: Schreurs
authors.forename: B G
authors.initials: BG
authors.lastname: Pijls
authors.forename: K A
authors.initials: KA
authors.lastname: Nouta
authors.forename: R G H H
authors.initials: RG
authors.lastname: Nelissen
keywords: Bias; Competing risk; Hip replacement; Kaplan-Meier; Probability; Statistics