The limitations of this study include the following. First, we had only 14 patients with SAM activity data at two time periods over a minimum of 10 years. The challenges in acquiring quantitative activity data are patient compliance in the daily use of the SAM and having dedicated research personnel to work directly with patients during the activity assessments. This explains, at least in part, the paucity of reports that include quantitative activity data. Nevertheless, these 14 patients are representative of the patient with modern joint arthroplasty who can challenge the wear resistance of a bearing. They all were healthy, Charnley Class A, with a relatively low mean age of 55 years and a broad range of ages (26-78 years). Because of the small number of patients and the heterogeneous nature of this sample, a multivariate analysis to control for potentially confounding variables and identify independent predictors was not possible. Second, the sensitivity of the digital radiographic analysis method limits comparisons between the early and late wear periods for individual patients. Third, there was only one type of crosslinked polyethylene analyzed (Marathon®), and our results may not translate to other crosslinked polyethylenes. Another limitation is that the total number of gait cycles is an extrapolation from the two activity sampling periods, which may be either more or less than the actual number of accumulated cycles. These patients were all healthy, and we are unaware of anything that would contradict the activity extrapolations. The issue of activity sampling has been previously investigated. In a study of 131 patients with total joint arthroplasty, the total number of sampling days for each subject ranged from 7 to 123 days. Random samples of four consecutive measurements were compared to the total days of observation/subject (7-123 days). The gait activity measured over 4 days was correlated to that of the longer sampling period (r2 = 0.945), with no difference in the activity assessment for a sampling of 4 versus 7 days or more . On this basis, our minimum sampling of 5 days is likely to be representative of patient activity over a longer time. Further, we found individual activity in the early and late sampling periods to be correlated.
Individual patient activity generally decreases with aging. In a cross-sectional cohort study, age was associated (p = 0.048) with activity . On average, patients who were younger than 60 years walked 30% more (p = 0.023) than those who were 60 years or older. There was a high degree of variability, with a 45-fold difference between the least and most active patients studied. The linear regression analysis from that study predicts a 9.8% decrease in activity over the 11.4 years of observation in the current study. The actual decrease in mean patient activity we observed was 16.2%, suggesting the decrease in patient activity with aging is nonlinear.
Gait speed generally decreases with time. Early gait speed averaged 15.4 cycles/minute and late gait speed averaged 14.0 cycles/minute, an 8.8% decrease. With the numbers available, this difference is not statistically different (p = 0.19). We are unaware of any other such studies of gait speed over time in patients with THA. Based on our observations, as patients age, gait speed tends to slow down first, and then the number of gait cycles decreases.
The combination of an 8.8% decrease in walking speed and a 16% reduction in the number of gait cycles over the decade of observation was associated with a 40% decrease in polyethylene wear rate. The highest activity and the highest wear both occurred during the first 5 years, and both decreased over the next 5 to 8 years. These data have implications for the longer-term performance of hips with crosslinked polyethylene bearings. The first 5 years were predictive of subsequent wear. There was a correlation between individual early (first 5 years) and late (5-13 years) linear penetration rates and between individual early and late volumetric wear rates.
A unique aspect of our analysis was the reporting of an adjusted wear rate: the wear/million gait cycles of a 70-kg patient weight. More commonly used variables, such as sex, age, diagnosis, BMI, and Charnley class, are actually surrogates for the fundamental variable: the amount of use of the implant. Our method more directly assesses that fundamental variable. Further, the adjusted wear rate allows for comparison to the results of hip simulator studies . In a preclinical wear simulator test of this 5-Mrad crosslinked polyethylene, the mean wear rate was 5 mm3/million gait cycles . Our average adjusted wear rate was 6.5 mm3/million gait cycles.
In a systematic review of intermediate-term reports, the mean linear penetration of all types of highly crosslinked polyethylenes was 0.042 mm/year, and this wear rate was associated with a low risk of radiographically apparent osteolysis . This pooled average wear rate is nearly identical to our average 5-year linear penetration rate (0.043 mm/year). At a minimum of 10 years, the mean linear penetration rate was 0.037 mm/year, which was consistent with the reduction in patient activity with aging.
In summary, quantitative activity data indicate the number of gait cycles and gait speed both decrease with age. The number of gait cycles decreased an average of 16% over the decade of observation. The average gait speed of patients 65 years or older was 26% slower than that of patients younger than 65 years. The highest activity and highest wear rates occurred in the first 5 years postoperatively. The decrease in activity with aging was associated with a 40% decrease in late polyethylene wear rate. The wear rates observed are associated with a low risk of developing osteolysis.
The authors acknowledge and thank Dr. John Martell for his assistance with the radiographic wear analysis.
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