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- SUBJECTS AND METHODS
- AUTHOR CONTRIBUTIONS
Footwear has traditionally been engineered to provide maximum foot support and comfort, and medical alterations to footwear have been largely directed at remediating disorders involving the feet, such as diabetic neuropathy. However, the lower extremity is known to be an interrelated functional and mechanical unit, and alterations at one aspect of the lower extremity (e.g., the foot) can have significant impact on distant areas of the extremity such as the knee (1–7). Therefore, footwear design alone may substantially affect the loading patterns of the entire lower body, and these biomechanic effects may have important implications for conditions in which mechanical factors are important to the pathogenesis and progression of the disease, such as osteoarthritis (OA) of the knee. Nonetheless, the potential impact of footwear on knee OA has not been widely recognized or critically evaluated.
OA is the most common arthritic condition, and symptomatic OA of the knee is a significant source of disability and impaired quality of life (8–10). Despite this, OA treatment remains largely palliative and generally focuses on oral analgesics rather than on the aberrant biomechanic loading that underlies much of its progression. OA of the lower extremity is mediated, at least in part, through aberrant dynamic loads transmitted across the joints. In OA of the medial compartment of the knee, radiographic severity (11), disease progression (12), and pain (13, 14) have all been associated with elevated medial joint loads, whereas reducing loading of the medial compartment may result in symptomatic benefits (15–17).
Recent evidence suggests that modern shoes have a substantial influence on joint loading in subjects with knee OA (2, 6). This is particularly relevant because shoes are worn during most of the day in Western societies, especially during ambulation, the most common daily activity, when the load on the lower extremity joints is significantly greater than it is at rest (18). These observations suggest that rational shoe design may provide a novel approach to reducing biomechanic loading of the knee during gait, and thereby may provide potential palliation for knee OA.
We recently reported that the use of modern, comfortable walking shoes results in an ∼14% increase in dynamic loading of the knees during ambulation compared with barefoot walking among individuals with knee OA (2). These data suggest a biomechanic advantage to the enhanced mobility of a bare foot compared with the constraints and stability provided by modern footwear. Taking into account the loading dynamics of the foot, we designed a “mobility” shoe, intended to affect the biomechanic milieu of the entire lower extremity and thereby to reduce joint loads at the knee during ambulation. Here, we report the biomechanic effects of this mobility shoe, relative to self-chosen conventional walking shoes and to a control shoe, in individuals with knee OA in order to test the hypothesis that the loads borne by osteoarthritic knees can be substantially reduced through innovative footwear design.
- Top of page
- SUBJECTS AND METHODS
- AUTHOR CONTRIBUTIONS
This study demonstrates that footwear designed to simulate the essential features of barefoot walking can effectively reduce both the peak knee loads (peak external knee adduction moment) and overall knee loading (adduction angular impulse) during normal ambulation among patients with knee OA, compared both with subjects' self-chosen, comfortable walking shoes and with commonly prescribed stability walking shoes. Because dynamic loading across the knee is pathophysiologically important to the onset and progression of knee OA, these findings suggest that altering the characteristics of footwear might have a substantial impact on the overall disease process of knee OA.
It should be noted that the difference in adduction angular impulse in group A (conventional shoes versus the mobility shoe) approached, but did not reach, statistical significance. However, this is probably a result of less power to detect this difference in this group (see Subjects and Methods). Although group A had a larger sample size, the effect sizes in this group for peak external knee adduction moment and adduction angular impulse were lower, presumably because of the greater variation in data with subjects wearing various conventional footwear versus the standardized control footwear worn in group B.
The mobility shoe in this study was designed to incorporate the potential biomechanic advantages of barefoot walking. For example, conventional walking shoes typically contain partial lifts at the heel, which previous studies have shown actually increase knee loads in healthy women (4); in contrast, the absence of such a heel during barefoot walking may effectively reduce peak torques at the knee. Another factor may be the stiffness imposed by shoe soles. The GRF varied between the types of footwear and, as mentioned previously, the GRF is a determinant of peak moments at the lower extremity joints. The GRF may be affected by the impact and position with which the foot contacts the ground. Thus, the flexible movement of a bare foot may be biomechanically advantageous by allowing for better force application of the foot with the ground. Some advantage may also be related to enhanced sensory input from skin contact with the ground, relative to an insulated foot contacting the ground. The increased sensory input would initiate protective neuromuscular reflexes to help minimize proximal joint impact and load (31–33).
Shoe inserts or lateral wedge orthotics have been examined as potential devices to reduce medial compartment joint loads in knee OA with varying success. In comparison with an 8–13% reduction in peak joint loads with the mobility shoes, most studies of lateral wedge orthotics have demonstrated a 5–8% reduction in load with the inserts compared with the same shoes without inserts. Kuroyanagi et al found larger reduction of 8–13% with lateral wedge insoles, without and with subtalar strapping, respectively (5). However, their results were compared with barefoot walking, and the practical, long-term use of such interventions in traditional footwear is not clear.
The concept of engineering shoes specifically to promote joint load reduction in arthritis is relatively novel. Recently, presumably based on previous lateral wedge orthotic data, Fisher et al reported on a shoe that incorporates a lateral wedge into its design (6); in healthy subjects without OA, reductions of up to 16% were observed in peak knee loads with specific shoe modifications (lateral wedge and lateral stiffness variations) compared with the subjects' conventional walking shoes. To our knowledge, this is the only other report of a shoe designed specifically to lower knee loads, although the mechanisms underlying load reduction and the corresponding design of this shoe are quite different from the mobility shoe described in the present study, and the advantages and long-term effects of each will need to be evaluated in controlled clinical trials.
Interestingly, in addition to significant variations in knee loads among the various footwear conditions, there were also notable differences observed in other gait parameters (stride, cadence, and joint ROM) while wearing shoes compared with walking barefoot. This was also observed in our previous study, in which conventional shoes were compared with barefoot walking in a larger group of subjects with OA of the knee (2). When evaluating whether variations in loads could be attributable to these loads, significant but small associations were seen between knee ROM and both peak external knee adduction moment and adduction angular impulse. Interestingly, these associations were only observed in group A, and not in group B. This may be due to insufficient power to detect these relationships with the smaller sample size in group B. However, in our previous study with >60 subjects with OA, this association was not observed. Furthermore, although variations in ROM were observed, they do not appear to be out of the range of what would be considered normal ROM at each of the lower extremity joints (34).
The other significant association observed in both groups was that between cadence and adduction angular impulse. As mentioned previously, this relationship is expected considering that adduction angular impulse is the integral of the adduction moment during the stance phase of the gait cycle. Because increased cadence would imply a shorter duration of stance phase, the adduction angular impulse would be expected to decrease accordingly. Nevertheless, the peak external knee adduction moment is not related to cadence, and reflects that loads were indeed decreasing while wearing the mobility shoe and walking barefoot compared with walking with conventional shoes and stability shoes.
Several limitations of the current study deserve consideration. First, this study evaluates the short-term variations in joint loads with different types of footwear; it is not yet clear that the observed loading advantages will be maintained after prolonged use of the footwear. Second, although load reduction presumably has clinical benefits in OA, the magnitude of any long-term symptomatic palliation or delay in structural disease progression provided by these shoes will need to be determined in a prospective study, and the long-term safety and comfort of the shoes will need to be established. Finally, this was a relatively small study and may be insufficiently powered to detect subtle relationships and differences in secondary outcomes. Therefore, it should be emphasized that this study was not intended to be a clinical trial or to establish the safety and utility of footwear in clinical practice. The main purpose of the study was to suggest the importance of footwear in terms of lower extremity joint loading, and to examine properties of footwear that may be responsible for knee loading variations. In certain populations, flat, flexible footwear such as the mobility shoe may be an inappropriate choice. Future decisions about footwear should weigh individual patient characteristics, and more detailed information about the clinical effects of the mobility shoe should be examined in a prospective clinical trial.
In summary, we report innovative footwear, designed to simulate barefoot walking, that induces significant reductions in dynamic knee loads during ambulation compared with conventional walking shoes and control shoes. In light of the pathophysiological role of mechanical loading in the progression of knee OA, these findings suggest that footwear may represent a novel therapeutic target for the treatment of knee OA. Moreover, the types of shoes worn by subjects with knee OA should be evaluated more closely in terms of their contribution to the disease, and long-term intervention trials to evaluate the clinical effects of shoe design on pain and disease progression in OA should be considered.