Defining high activity in arthroplasty patients.

There is evidence that high levels of physical activity following arthroplasty of the hip or knee can lead to early revision. However, the term 'highly active' is not well defined. A validated ankle accelerometer was used to quantify activity in 13 patients, who had undergone a total of 20 arthroplasties of the lower limbs and who had active lifestyles. The assessments were taken at a mean of 8.7 years post-operatively (1.8 to 15.8). The mean gait cycles per day was 8273 (5964 to 12,557), which extrapolates to 3.0 million cycles per year (cpy) (2.2 to 4.6). The mean percentage of time spent in high activity mode was 4.3%, or about one hour per day. The mean percentage of cycles in high activity was 40%. Based on these data, we propose the following definitions of high activity: > 3 million cpy; one hour per day in high activity mode; 40% of cycles in high activity mode. Extrapolating the sample of activity over the time since operation, the mean cycles per arthroplasty was 25.2 million, with a maximum of 44.1 million. No joint has been revised, or shows evidence of impending failure. ©2016 The British Editorial Society of Bone & Joint Surgery.