Spontaneous ambulatory activity as a quantifiable outcome measure for osteoarthritis of the knee.

Objective: Quantifiable outcome measures for disabling diseases such as osteoarthritis (OA) of the knee are necessary in order to compare the impact of different interventions competing for financial resources. Current subjective and questionnaire data are not satisfactory for such study. In this study, we examine the potential of the direct measurement of ambulatory activity as such a measure.

Population: Patients with X-ray evidence of OA of the knee recruited to studies of anti-inflammatory agents (n = 29). Patients with OA of the knee awaiting knee replacement surgery (n = 28).

Methods: Comparison of various standard measures with total energy output data from an activity monitor.

Results: Spearman rho correlations of ambulatory energy output (number of steps x average amplitude of steps) correlated with other measures. Correlation with physician's opinion was greater than with patient's opinion (r = 0.4 and 0.2, respectively). There was no correlation with visual analogue pain scale or OA severity index. Correlation with scales of the Nottingham Health Profile questionnaire were not significant either for mobility (r = - 0.15) or for pain (r = - 0.13). There was, however, a significant correlation between poor sleep and increased activity (r = 0.34, P < 0.05). Correlation with Kellgren X-ray grade was significant (r = - 0.45, P = 0.01). Patients recruited to anti-inflammatory studies were 69% more active than those awaiting replacement surgery.

Conclusion: The monitoring of ambulatory activity shows some construct and discriminant validity, and is worthy of further study.