Laterally wedged insoles are advocated for the management of medial tibiofemoral knee OA [5
] and biomechanical data have shown that such an intervention can immediately reduce the knee adduction moment by 5-10% on average [7
]. No study to date has evaluated whether the biomechanical effects of lateral wedges decline over time. This study evaluated whether the effect of laterally wedged insoles on the adduction moment declined over one month of wear. Results revealed that the immediate beneficial effects of the insole on the adduction moment remained even after the insoles had been worn by participants on a daily basis for one month. Furthermore, high users of the insoles demonstrated similar reductions in adduction moment parameters at one month to participants who were low users of the insoles over the preceding four weeks.
This is the first study to evaluate change in the biomechanical effects of laterally wedged insoles over any timeframe. Previous studies in knee OA have evaluated the immediate effects of insoles only, thus there are no similar studies with which to compare our results. Contrary to expectations, effects of the insoles on the adduction moment did not decline over one month. A limitation of this study is the sample size of 20 participants, and although we detected significant main effects, it is possible that there were insufficient participants to detect a significant interaction. It is also quite possible that a decline in the biomechanical benefits of wedges could be seen after a longer timeframe. As we did not objectively measure the degree to which our participant's wedges had compressed or worn down, it is impossible to know whether the insoles in this study did in fact degrade over such a relatively short period. Nonetheless, participants reported an average total insole use of 222 hours over four weeks, indicating use of insoles for approximately 8 hours each day. These data suggest that participants were wearing the insoles as prescribed and it is reasonable to assume that some compression/degradation would occur over a one-month period.
From a clinical perspective, findings from this study suggest that the biomechanical effectiveness of laterally wedged insoles does not decline after one month of wear. However the sample size of this study was limited to 20 participants and further studies of larger samples are needed to validate the current findings. While it is not presently known how frequently insoles need to be changed in order to optimise outcome, this study suggests that patients can continue to wear their insoles for periods greater than one month. Future research should serially evaluate the effects of insoles over longer periods of time, to more precisely ascertain exactly when biomechanical effects begin to decline and to guide health practitioners as to how frequently new insoles should be provided. It is also important to recognise that laterally wedged insoles are not always inserted into shoes, but may also be worn without shoes via strapping to the foot or within a sock-type ankle supporter [21
]. It is possible that results of the current study may differ if other types of lateral wedges are used and further research is needed to determine if this is the case.
Although consistent with other studies, the reductions in the adduction moment observed with lateral wedged insoles in this study (4.2-5.1%) are marginally smaller than the reductions reported by us [10
] and others [7
] and somewhat smaller than the 8.7% reduction reported by Butler et al [9
]. It is not clear why this is the case but differences in the design of wedges and the footwear utilised across studies may explain variations in results. For example, in comparison to our study, Butler et al [9
] customised the degree of lateral wedging for each participant in order to achieve maximal pain relief during a step-down task. A mean of 9.6° (+/- 3.2) of wedging was used, which may explain the greater reductions in the adduction moment observed by these authors compared to us.
Symptomatic response to insoles was not evaluated in this study, thus it should not be assumed that clinical benefits of insoles also remain after one month of use. While it is believed that alleviation of knee pain occurs in response to the reduction in medial compartment load observed with laterally wedged insoles [23
], results from clinical trials remain inconclusive about the benefits of insoles on symptoms [24
]. Future research should also evaluate whether any symptomatic benefit with insoles declines over time as insoles undergo continuous wear, as it is possible that decline in symptomatic benefit (if present) occurs at a rate different to decline in biomechanical benefits.