Early mortality after elective hip surgery

In this issue of Acta Orthopaedica, a study by Blom and colleagues on postoperative mortality following primary total hip arthroplasty appears in print (pages 347–50). The authors nicely present their data and different aspects of early postoperative mortality. Furthermore, none of the patients received chemical thrombosis prophylaxis routinely. The study is therefore an important contribution to the discussion on early mortality and causes of death in individuals without chemical thrombosis prophylaxis.

In Figure 1, Blom et al. show survival curves for categories of age and in Table 2 they quantify differences in 30- and 90-day accumulated mortality for the age categories. Furthermore, Table 1 is essential for discussion of what day deaths occur, and especially the causes of death. Tables 1 versus 2 demonstrate that there are basically two ways to present mortality, either as accumulated or instant mortality.

Accumulated mortality is counting the number of deaths within a given time interval (i.e. within 30, 60, or 90 days), or preferably using survival probabilities (e.g. Kaplan-Meier curves). Instant mortality, on the other hand, is the rate of deaths at a given time point (for example, the 20th postoperative day). Consequently, the word mortality alone is unspecific! (There is a strict formulation for the two measures and the relationship between them, which I omit here). Also, another important issue is that an increase in the instant mortality for just one day (e.g. the first postoperative day) would increase the accumulated mortality for infinity. Thus, accumulated numbers can not be used when it comes to discussing changes in mortality. Based on the curves in Figure 1 of Blom et al., showing differences in survival for age, one can however argue that there must be an age difference in the instant mortality, but not necessarily for the whole period.