The distribution at the start of the study period of 84,759 patients in the 3 age groups was 22,885 (27%) in the 55- to 64-year group, 34,833 (41%) in the 65- to 74-year group, and 27,041 (32%) in the 75- to 84-year group. If patients received a second arthroplasty, it was usually a total hip prosthesis or a total knee prosthesis, but a few patients received a partial hip prosthesis or unicompartmental knee prosthesis.
In this description of the results, the term ‘events’ refers only to the type of events covered in this multi-state model, i.e. first primary arthroplasty (hip), second primary arthroplasty (either hip or knee), first revisions of these, and death.
shows the numbers and proportions of arthroplasty events that had occurred at the end of the study period. Following the first hip replacement (either left or right), 3 times as many patients had a contralateral primary hip replacement as a left or right knee replacement (12% vs. 4%), 2% had a revision, and 5% died (: from state 1 to 2, 3, or 4). Of the 1,929 patients who had a revision after the first arthroplasty, 6% subsequently received a hip and 4% received a knee (: from state 2 to 5 or 6). Of patients who had received a second arthroplasty, either hip (9,997) or knee (3,565), 1% went on to have a revision of the first arthroplasty and 2% went on to have a revision of the second arthroplasty (: from state 3 to 5 or 7 and from state 4 to 6 or 8).
Numbers and percentages of events in the 10-state model () at the end of the study period for patients whose first arthroplasty was either a left total hip arthroplasty for osteoarthritis or a right total hip arthroplasty for osteoarthritis
shows an example of the multi-state model with the SNAH code on a subsample of patients who received a left hip as first arthroplasty. (We arbitrarily chose to show the number of subsequent right-knee primary arthroplasties after the first hip, but one could instead have chosen left knees or both left and right knees). State 6, for example, consists of 20 patients with histories of HL10/HL21/KR30/ and 9 patients with HL10/KR20/HL31/.
Example of the multi-state model with SNAH code on a subsample of patients who received a left hip prosthesis as first arthroplasty, followed by another primary arthroplasty or a revision of the left hip. (Number of events is shown in parentheses).
The estimated state occupation probabilities at different time points since the first recorded hip arthroplasty for each age group are presented in . Because so few patients experienced events beyond the second event after the initial arthroplasty (, states 5–9), these events were combined. Among patients in the 2 youngest age groups, it appears that at each time point after having received the first hip arthroplasty the probability of occupying state 3 (having received a second hip arthroplasty and no further events) was higher than occupying any of the other event states. For the oldest age group, the probability of occupying state 3 was the highest until approximately 3 years after the first hip prosthesis; thereafter, the probability of being in the state ‘dead’ was the highest. The probability of occupying state 3 decreased with increasing age. For example, 5 years after having received a first total hip arthroplasty approximately 20% of patients aged 65–74 years were estimated to have received a contralateral hip (and had not experienced any other event in this model) as compared to 10% of patients in the oldest age group (aged 75–84 years). The estimated probability of occupying state 4 (having received a knee arthroplasty) rather than occupying state 3 was much lower throughout the study period. Thus, when a patient received a hip first, this was more likely to be followed by another hip than a knee.
State occupation probabilities for patients in 3 age groups after first hip arthroplasty, based on the model in (revision: state 2; hip: state 3; knee: state 4; dead: state 10; other: states 5–9).
shows the effect of gender on the transition hazards between the states pictured in for the 3 age groups. Because so few patients experienced events beyond a second event, these transitions are not included in the table. The transition rate from state 1 to 2 (from first arthroplasty to revision of the same arthroplasty) was higher for males than for females in the oldest (75- to 84-year) age group (HR = 1.3, 95%CI: 1.1–1.5; p = 0.004), but there was no evidence of a difference between the sexes for the youngest age groups for this transition. For the transition rates of receiving a second arthroplasty (hip or knee), these were lower for males than for females in all age groups (HR = 0.8–0.9; p < 0.05). The instantaneous risk of dying following the first arthroplasty was higher for males than for females in all 3 age groups (HR = 1.5, 1.6, and 1.7, respectively; all p < 0.01) during the study period. For patients in the oldest age group, the transition hazards to being dead from receiving second hip or receiving a knee were also higher for males than for females (HR = 2.1 and HR = 1.9, respectively; p < 0.01). In summary, the transition rates for revision after first arthroplasty were higher for males than for females in the oldest age group. In all age groups, males had lower transition rates for receiving a second primary arthroplasty of either hip or knee. Generally, males had a higher death rate than females.
Effect of sex, adjusted for age, on the transition hazards between states (see ) for patients whose first arthroplasty was a total hip arthroplasty for osteoarthritis