A variable-stiffness shoe lowers the knee adduction moment in subjects with symptoms of medial compartment knee osteoarthritis

Abstract

The purpose of this study was to evaluate the effectiveness of variable-stiffness shoes in lowering the peak external knee adduction moment during walking in subjects with symptomatic medial compartment knee osteoarthritis. The influence on other lower extremity joints was also investigated. The following hypotheses were tested: (1) variable-stiffness shoes will lower the knee adduction moment in the symptomatic knee compared to control shoes; (2) reductions in knee adduction moment will be greater at faster speeds; (3) subjects with higher initial knee adduction moments in control shoes will have greater reductions in knee adduction moment with the intervention shoes; and (4) variable-stiffness shoes will cause secondary changes in the hip and ankle frontal plane moments. Seventy-nine individuals were tested at self-selected slow, normal, and fast speeds with a constant-stiffness control shoe and a variable-stiffness intervention shoe. Peak moments for each condition were assessed using a motion capture system and force plate. The intervention shoes reduced the peak knee adduction moment compared to control at all walking speeds, and reductions increased with increasing walking speed. The magnitude of the knee adduction moment prior to intervention explained only 11.9% of the variance in the absolute change in maximum knee adduction moment. Secondary changes in frontal plane moments showed primarily reductions in other lower extremity joints. This study showed that the variable-stiffness shoe reduced the knee adduction moment in subjects with medial compartment knee osteoarthritis without the discomfort of a fixed wedge or overloading other joints, and thus can potentially slow the progression of knee osteoarthritis.

Introduction

Knee osteoarthritis affects 20–40% of individuals over age 60 (Dillon et al., 2006; Felson, 1990), and the medial compartment of the knee is involved approximately 10 times more frequently than the lateral compartment (Ahlback, 1968). The increase in involvement of the medial compartment is theorized to be a consequence of greater loads on the medial compartment articular cartilage (Andriacchi, 1994; Johnson et al., 1980; Morrison, 1970; Noyes et al., 1992; Schipplein and Andriacchi, 1991). Analytical methods have demonstrated that an increase in medial compartment joint loading is related to an increase in knee adduction moment (Crenshaw et al., 2000; Schipplein and Andriacchi, 1991). A high peak knee adduction moment during walking has been associated with the severity, rate of progression, and treatment outcome of medial compartment knee osteoarthritis (Baliunas et al., 2002; Miyazaki et al., 2002; Mündermann et al., 2004; Prodromos et al., 1985; Sharma et al., 1998). Consequently, many surgical and mechanical interventions for knee osteoarthritis are aimed at reducing the maximum knee adduction moment.

Surgical interventions such as high tibial osteotomy can reduce the adduction moment at the knee (Prodromos et al., 1985; Wang et al., 1990), but this reduction lessens post-operatively with time (Wang et al., 1990). Furthermore, such procedures are highly invasive, have associated complications (Magyar et al., 1999), and require a long recovery period (Amendola and Panarella, 2005).

Non-surgical mechanical interventions include non-invasive methods to lower the adduction moment and improve joint biomechanics. Numerous studies have examined footwear interventions including laterally wedged insoles and shoes in both healthy (Crenshaw et al., 2000; Fisher et al., 2007; Schmalz et al., 2006) and osteoarthritic (Kakihana et al., 2005, Kakihana et al., 2007; Kerrigan et al., 2002; Sasaki and Yasuda, 1987; Shimada et al., 2006) populations, and have found varying results in adduction moment reduction. In several studies, lateral wedges have been associated with discomfort in subjects (Kerrigan et al., 2002; Toda et al., 2004).

Thus, while wedged insoles have been shown to reduce the knee adduction moment, subjects may find such interventions uncomfortable. Variable-stiffness shoes with a greater lateral sole stiffness have also been shown to reduce the knee adduction moment in healthy individuals (Fisher et al., 2007), and were not associated with subject discomfort. It is not known, however, if this reduction in adduction moment also occurs in an osteoarthritic population. It is also not known if the variable-stiffness shoes create secondary gait changes in frontal plane kinetics, such as increased knee and hip abduction moments adopted in the natural gait pattern of subjects with medial compartment knee osteoarthritis (Mündermann et al., 2005).

The overall goal of this study was to evaluate the effectiveness of variable-stiffness shoes in lowering the knee adduction moment in subjects with symptoms of medial compartment knee osteoarthritis. Specifically, the following hypotheses were tested: (1) variable-stiffness shoes with a greater lateral sole stiffness will lower the knee adduction moment in the more affected leg of individuals with symptomatic medial compartment knee osteoarthritis compared to control shoes with a constant-stiffness sole; (2) reductions in knee adduction moment will be greater at faster speeds; (3) subjects with higher initial knee adduction moments in control shoes will have greater reductions in adduction moment with the intervention shoes; and (4) gait changes related to the use of variable-stiffness shoes will cause secondary changes in hip and knee abduction moments, hip adduction moment, and ankle inversion and eversion moments, compared to control.