The influence of foot progression angle on the knee adduction moment during walking and stair climbing in pain free individuals with knee osteoarthritis

Abstract

The external knee adduction moment during walking and stair climbing has a characteristic double hump pattern. The magnitude of the adduction moment is associated with the development and progression of medial compartment knee osteoarthritis (OA). There is an inverse relationship between the magnitude of the second peak adduction moment and foot progression angle (FPA). Increasing FPA beyond a self-selected degree of toe-out may further reduce the magnitude of this moment for persons with knee OA. In this study, subjects with medial compartment knee OA walked and climbed stairs using their natural (i.e. self-selected) and an increased FPA (i.e. self-selected + 15° of additional toe-out). Increasing FPA did not change the magnitude of the first peak adduction moment but it did significantly decrease the second peak during walking. The first peak moment during stair ascent was significantly greater for the increased FPA condition, and a significant reduction was noted for the second peak. No significant differences were noted during stair descent. These results suggest that walking with a toe-out strategy may benefit persons with early stages of medial knee OA.

Introduction

Osteoarthritis is the most prevalent form of arthritis affecting more than twenty million adults [1]. It is the leading reason for total knee and hip replacement and a primary cause of lower limb disability in the elderly [2], [3]. Larger than normal joint forces may lead to cartilage damage [2]; worsening the severity and debilitating symptoms such as joint stiffness and pain during activities of daily living. The knee adduction moment is positively correlated with the severity of medial compartment OA [4], and it is an often-used predictor of knee joint loading [5]. Repetitive joint loading related to activity and profession has been associated with the onset and progression of OA [6], [7], [8]. Thus, treatments to reduce the magnitude of the knee adduction moment are indicated for individuals with medial compartment knee OA.

The external knee adduction moment during walking has a characteristic double-hump pattern with two distinct peaks. The first peak is generally larger than the second, and it is greater for individuals with medial compartment OA than healthy, age-matched controls [9]. From a mechanical perspective, altering the kinematics and kinetics at one joint can influence the mechanics of all other joints in a linkage [10], [11]. Thus, changing the position of the foot at contact may have a modifying effect on the knee adduction moment. This potential was recognized by Wang et al. who noted that the ankle inversion moment was correlated with the peak knee adduction moment [12]. They speculated that walking with a toe-out gait reduced the ankle inversion moment, and by association, the adduction moment at the knee. Andrews et al. examined the relationship between foot progression angle and the knee adduction moment in healthy subjects during walking [13]. They found a negative relationship between the second peak adduction moment and the foot progression angle. That is, subjects that walked with a greater degree of toe-out tended to have a smaller knee adduction moment. A similar finding was reported by Hurwitz et al. for subjects with knee OA [9]. Subjects that walked with a larger degree of toe-out tended to have a smaller knee adduction moment during late stance.

The relationship between FPA and the first peak adduction moment is less clear because of limited studies and conflicting findings. For example, Andrews et al. [13] and Hurwitz et al. [9] reported no relationship between FPA and the first peak adduction moment, while Wada et al. [14] found a relationship between FPA and the peak adduction moment during walking for subjects with end-stage knee OA. The peak adduction moment was the first peak in “most cases” as stated by the authors, but it was the second peak for others. It is important to note that these studies used linear regression to determine if self-selected FPA and the magnitude of the knee adduction moment were related. That is, these studies did not investigate the effect of increasing FPA beyond a self-selected degree of toe-out might have on the knee adduction moment.

The only study to compare the effect of walking with a FPA greater than a self-selected degree of toe-out was conducted by Lin et al. [15]. They found that the first peak adduction moment increased when healthy teenagers walked with a 30° increase in their FPA. The effect that increasing FPA might have on subjects with medial compartment knee OA may not be well represented by healthy teenagers because of differences in skeletal structure, joint flexibility and lower limb alignment. Thus, the effect of increasing FPA on the knee adduction moment in persons with medial compartment knee OA is not yet known. Stair climbing is a more difficult task than walking for individuals with knee OA, and the effect of increasing FPA might be more clearly revealed during these more demanding tasks. Thus, the purpose of this study was to examine the effect of increasing FPA on the knee adduction moment during walking, stair ascent and descent for a group of pain free individuals with medial knee OA. Based on our current knowledge about the relationship between FPA and the knee adduction moment, it was hypothesized that increasing FPA would further decrease the magnitude of the second peak moment during walking and stair climbing. The literature regarding the effect of increasing FPA on the magnitude of the first peak is less clear, and therefore the role of increasing FPA on the first peak moment was tested using the null hypothesis.