Knee osteoarthritis (OA) is a degenerative joint disease characterized by pain and stiffness.1 These symptoms are related to disruption of the articular surfaces and are associated with significant impairment in functional ability.2 The disease prevalence increases with age, and currently 12% to 16% of people older than 65 years of age in the United States have been diagnosed with knee OA.3,4 It has been estimated that almost 45% of all people in the United States will develop knee OA during their lifetime.5 With the growing prevalence of the disease, a concomitant growth in the cost of treating the disease also has been observed. The cost for end-stage treatment of the disease has been estimated to be $38,000.6 Projections show that the cost will continue to rise as the baby boomer generation enters the age range for typical symptomatic disease presentation.
Tibiofemoral knee OA can develop in either the medial or lateral compartment.7 However, it is 9 times more common in the medial compartment.7 Static lower-extremity alignment has been shown to influence which compartment is involved, with genu valgus being associated with lateral OA and genu varus being associated with medial OA.8–11 Genu valgus alignment often is associated with hip adduction proximally and rear-foot eversion distally.12 Genu varus alignment, however, is associated with decreased hip adduction proximally and increased rear-foot inversion distally.
To date, only one study has compared the gait mechanics of people who have medial knee OA with those of people who have lateral knee OA.13 These researchers reported that, relative to people in a control group, people with medial knee OA exhibited more knee adduction but less hip adduction during gait than people with lateral knee OA.13 Alterations in hip and knee alignment are likely to lead to differences in respective joint loading. Weidow et al13 noted that people with medial knee OA had 52% higher internal knee abduction moments than people who were healthy (control group). People with lateral knee OA had 63% lower internal knee abduction moments than the control group.13 Surprisingly, lower internal peak hip abduction moments were reported in both people with medial knee OA and people with lateral knee OA than in the control group. Mundermann et al14 observed similar reductions in hip abduction moments in people with severe medial knee OA and in the control group. However, they found no difference between people with less severe medial knee OA and the control group. Alterations at the hip and knee are likely to influence mechanics distally and thus can significantly alter gait mechanics. Unfortunately, previous research studies did not include an evaluation of the differences in these distal mechanics, such as peak rear-foot eversion, eversion excursion, and the peak inversion moment.