The 2012 Frank Stinchfield Award: Decreasing patient activity with aging: implications for crosslinked polyethylene wear.

BACKGROUND: Patient activity influences polyethylene wear. However, it is unclear how individual activity changes with patient aging after THA. QUESTIONS/PURPOSES: We quantified changes in individual gait cycles and gait speed, assessed age-related differences in these parameters, and determined their relationship to polyethylene wear.
METHODS: A microprocessor was worn on the ankle to quantify the activity of 14 healthy patients with a well-functioning THA at two time periods: early (within 3.5 years of implantation) and late (10-13 postoperative years). Wear was measured on serial radiographs using edge detection-based software.
RESULTS: Mean activity decreased by 16% from the early to the late period: 2.04 million gait cycles/year to 1.71 million gait cycles/year. Mean gait speed decreased by 9%: 15.4 cycles/minute to 14.0 cycles/minute. The activity of the 10 patients who were younger than 65 years at surgery decreased by 14% (2.34 million gait cycles/year to 2.02 million gait cycles/year), while the four patients 65 years or older at surgery decreased by 28% (1.29 million gait cycles/year to 0.94 million gait cycles/year). Gait speed was 26% slower for patients 65 years or older than for patients younger than 65 years. The mean linear penetration rate decreased by 42% from the first 5 years (early wear rate) to the next 8 years (late wear rate, 5-13 years): 0.043 mm/year to 0.025 mm/year.
CONCLUSIONS: The greatest patient activity and wear occurred during the first 5 years. Walking speed and gait cycles both decreased with aging, resulting in deceasing wear over time.