Is time to joint replacement a valid outcome measure in clinical trials of drugs for osteoarthritis?

The rate of radiographic joint space narrowing is commonly used today as a structural outcome measure in clinical trials evaluating potential disease-modifying drugs in patients with hip osteoarthritis, but this results in a continuous variable. Among the methods proposed to circumvent this problem, it has been suggested that the incidence of total hip arthoplasty (THA) provides a "hard" outcome measure. It is a dichotomized variable, easy to measure and sensitive to change, with acceptable intrinsic validity. However, because this measure is limited by the variability of factors underlying the decision to perform surgery and the length of waiting lists, it has been suggested that "time to fulfill criteria for considering THA" may be a more appropriate endpoint.