BACKGROUND: There is increasing demand for knee and hip arthroplasty with considerable health system cost implications. Despite the high surgical costs relating to the prosthesis used, little is known about which factors are most influential in prosthesis choice, nor is it clear what level of variation may be warranted for clinical reasons. OBJECTIVES: This study had dual objectives: (1) identify factors influencing prosthesis selection for total hip arthroplasty (THA) and total knee arthroplasty (TKA) and (2) develop a working definition for unwarranted variation in THA and TKA implant selection. DESIGN: A three-stage prospective study: (1) develop and pilot a questionnaire with input from orthopaedic surgeons
(2) a cross-sectional survey with orthopaedic surgeons across Australia and (3) an expert panel to finalise the working definition of unwarranted variation. SETTING AND PARTICIPANTS: Study activities were conducted both virtually and in person, primarily with hospital-based orthopaedic surgeons and orthopaedic registrars across Australia, but also included health economists and health administrators in the expert panel stage. RESULTS: Revision rates, familiarity with an implant and implant quality were prioritised when choosing a prosthesis, while other factors, including cost, were reported to have limited influence. Technological advancement and revision rates were influential for past changes in prostheses, and strong clinical evidence is expected to precede future changes. The consensus was reached on a working definition of unwarranted variation that focused on implants with revision rates of 20% or higher compared with published benchmarks. The use of multiple cost thresholds was recommended for defining narrow and broad definitions of unwarranted variation. CONCLUSION: Study findings provide valuable insights into the decision-making process for prosthesis selection, as well as what surgeons believe might constitute unwarranted variation. This information can advance our understanding of the magnitude and impact of unwarranted variation in prosthesis selection, as well as inform strategies to address it.