Role of knee kinematics and kinetics on performance and disability in people with medial compartment knee osteoarthritis

Abstract

Background

Although gait characteristics have been well documented in people with knee osteoarthritis, little is known about the relationships between gait characteristics and performance or disability. Our purpose was to examine the role of knee kinematics and kinetics on walking performance and disability in people with knee osteoarthritis. We also examined whether pain mediated the relationship between the knee adduction moment and performance or disability.

Methods

Three-dimensional gait analysis was conducted on 54 people with medial compartment knee osteoarthritis. Performance was quantified with the Six Minute Walk test and disability was self-reported on the Short Form-36. The pain subscale of the Western Ontario McMaster Universities Osteoarthritis Index and the functional self-efficacy subscale of the Arthritis Self-Efficacy scale were completed.

Findings

A step-wise linear regression demonstrated that the variance in Six Minute Walk test scores was explained by functional self-efficacy (50%) and the range of knee motion (8%). The variance in Short Form-36 was explained by pain (36%), the peak extension angle (19%) and the range of knee motion (4%). Pain was unrelated to the knee adduction moment so analyses of pain as a mediator of the adduction moment on either performance or disability were halted.

Interpretation

Kinematic output from the motor control system is useful in understanding some variance in current performance and disability in people with knee osteoarthritis. The knee adduction moment was unrelated to these variables and pain did not mediate between the knee adduction moment and performance or disability. Therefore this moment does not explain current clinical status in people with knee osteoarthritis based on the measures of performance and disability used in this study.

Introduction

The prevalence of knee osteoarthritis (OA) makes this disease the single greatest cause of chronic disability in community-dwelling adults in the United States (Guccione et al., 1994). Although definitions vary, disability can be defined as an inability to fulfill a social role, such as work or family responsibilities (NIH, 1993). To quantify disability, researchers use self-report questionnaires that include items that reflect the impact of disease on social functioning. For example, Lingard and colleagues used the Short Form-36 (SF-36), which includes domains of emotional, physical and social functioning, as an outcome measure post-arthroplasty in a 2-year prospective study (Lingard et al., 2004). Although the SF-36 is frequently used as an outcome measure of disability, little research has examined the correlates of disability in people with knee OA. Our previous work showed that pain explained nearly 40% of the variance in SF-36 raw scores (Maly et al., 2006); but little is known about other factors that contribute to disability measures in people with knee OA.

Because disability in people with knee OA has been linked to limitations in lower extremity mobility performance (Guccione et al., 1994), some research has attempted to uncover the determinants of physical performance. Walking is the activity most commonly reported as difficult by those with knee OA (Guccione et al., 1994). Self-efficacy, a person’s beliefs in their capabilities to organize and execute actions required to achieve a goal (Bandura, 1998), is an important determinant of walking performance in people with knee OA (Harrison, 2004, Sharma et al., 2003). In one study, self-efficacy explained 50% of the variance in the distance walked in 6 min in people with knee OA; while mechanical variables like strength and body weight contributed an additional 12% (Maly et al., 2005). It may be surprising that a psychosocial variable such as self-efficacy would contribute more to physical performance than mechanical variables. However, strength and obesity may not be the only or the most influential of mechanical variables affecting performance. Gait characteristics could explain variance in walking performance in knee OA. For example, altered knee loading during weight-bearing may result in pain due to intraosseous pressure, effusion and ischemia (O’Reilly and Doherty, 1998), which would interfere with walking performance. Thus, it is possible that gait mechanics would demonstrate a stronger relationship to walking performance than other mechanical variables previously studied in people with knee OA.

Gait analysis has proven useful in differentiating between people with and without knee OA. Compared to healthy adults, people with knee OA walk more slowly due to a shorter stride length or decreased cadence (Stauffer et al., 1977, Brinkman and Perry, 1985, Gok et al., 2002, Kaufman et al., 2001) and with decreased sagittal plane knee motion (Stauffer et al., 1977, Brinkman and Perry, 1985, Kaufman et al., 2001, Messier et al., 1992). Most studies show that the external knee adduction moment is greater in people with knee OA, even when compared to age, sex-matched controls (Gok et al., 2002). The adduction moment correlated with bone density distribution at the proximal tibia (Hurwitz et al., 1998) suggesting that it is a reasonable proxy for medial loading. The adduction moment also relates to disease severity and progression (Miyazaki et al., 2002, Wada et al., 2001, Sharma et al., 1998) and change in pain intensity with medication (Hurwitz et al., 2000). In the presence of pain, the knee adduction moment is lower during stair-climbing or walking (Schnitzer et al., 1993, Hurwitz et al., 2000, Shrader et al., 2004). It is possible that this moment will relate strongly to performance and disability, but these relationships are unclear. Also unclear is whether an elevated knee adduction moment directly results in pain. It is possible that pain mediates a relationship between elevated medial loading and performance or disability (Fig. 1). A mediator is a variable (e.g., pain) that represents a mechanism through which an independent variable (adduction moment) influences the dependent variable of interest (performance) (Baron and Kenny, 1986).

Other gait mechanics may also be important to walking performance, including external rotation (Gok et al., 2002), flexion (Kaufman et al., 2001) and extension moments (Hurwitz et al., 2000). These kinetic variables have not been studied extensively, yet these may be useful in understanding performance. For example, people with knee OA have a higher peak knee flexor moment, which is thought to improve joint stability during level walking (Schipplein and Andriacchi, 1991). Because our current knowledge about the role of various gait characteristics on walking performance in people with knee OA is limited, we aimed to explore a variety of kinematic and kinetic variables as potential determinants of performance.

The purpose of this study was to examine the role of three-dimensional knee angles and moments on walking performance and self-reported disability. Because self-efficacy is a known determinant of performance and knee pain is a known determinant of self-reported disability, we controlled for these variables. Second, we investigated whether pain mediated the relationship between the adduction moment and both walking performance and disability. We hypothesized that (1) controlling for self-efficacy, knee kinematics and kinetics will be significantly related to performance; (2) controlling for pain, knee kinematics and kinetics will be significantly related to disability and (3) pain will mediate a relationship between the knee adduction moment and disability.