Toe-out gait in patients with knee osteoarthritis partially transforms external knee adduction moment into flexion moment during early stance phase of gait: A tri-planar kinetic mechanism
Introduction
The high prevalence and burden of knee osteoarthritis (OA) have led to increased efforts to describe factors underlying disease initiation, progression and intervention (Badley et al., 1994; Badley and Wang, 1998; Felson, 2004). In vivo studies of ambulation suggest an important role for ambulatory mechanics, quantified using three-dimensional gait kinematic and kinetic analysis, in the study of knee OA (Andriacchi and Mundermann, 2006; Andriacchi et al., 2000; Astephen and Deluzio, 2005; Krebs et al., 2003). The external adduction moment about the knee, in particular, has been demonstrated to be a valid proxy for load on the knee medial compartment, a risk factor for disease progression (Amin et al., 2004; Hurwitz et al., 1998; Miyazaki et al., 2002; Schipplein and Andriacchi, 1991), and a useful outcome measure following interventions (Shimada et al., 2006).
During normal gait, the weight-bearing knee remains lateral to the dynamic line of action of the ground reaction force throughout stance phase, thereby resulting in an external adduction moment about the knee joint. The normal predominance of a knee adduction moment throughout the stance phase of gait, and subsequently larger medial knee compartment load relative to lateral, has been suggested to contribute to the markedly greater incidence of advanced knee OA in the knee medial compartment (Andriacchi and Mundermann, 2006; Andriacchi, 1994; Doherty, 2001; Hunt et al., 2006; Smith et al., 2004). Moreover, in the presence of medial compartment knee OA, articular cartilage degeneration leads to decreased medial joint space, creating a further shift in lower limb alignment toward varus (Dearborn et al., 1996), and an even greater adduction moment about the knee during gait (Johnson et al., 1980; Prodromos et al., 1985; Specogna et al., 2006). In an attempt to break this perpetuating cycle, several interventions are aimed at reducing knee medial compartment loading (Andriacchi, 1994; Hurwitz et al., 1998; Prodromos et al., 1985; Schipplein and Andriacchi, 1991; Shimada et al., 2006; Wang et al., 1990).
It has been suggested that some patients with knee OA adopt compensatory gait kinematics that serve to unload the medial compartment of the knee (McGibbon and Krebs, 2002). A frequently identified compensation is an increase in the toe-out angle of the weight-bearing foot during stance phase (Andrews et al., 1996; Guo et al., 2006; Hurwitz et al., 2002; Lin et al., 2001; Wang et al., 1990). The assumed mechanism is a more lateral placement of the center of pressure (CP) due to increased toe-out angle that moves the line of action of the ground reaction force closer to the weight-bearing knee, thus reducing its adduction lever arm. However, this mechanism has only been demonstrated in late stance, reducing the second peak of adduction moment. Although previous studies have reported a negative relationship between toe-out angle and knee adduction moment (Andrews et al., 1996; Baliunas et al., 2002; Guo et al., 2006; Hurwitz et al., 2002, Lin et al., 2001). We are unaware of any reports that have quantified the change in frontal plane lever arm acting about the knee, or changes in the components of the three-dimensional external knee moment. This is likely due to the difficulty in obtaining accurate estimates of kinematics and kinetics during toe-out and no-toe-out conditions within subjects during normal walking.
In the present study, we examine the mechanisms of reducing adduction moments and frontal plane lever arms about the knee with increased toe-out angle during stance phase. It is hypothesized that the lateral shifting of CP with toe-out acts only in late stance. However, we propose a complementary mechanism that acts in early to mid-stance phase that also tends to reduce adduction moment. By externally rotating the hip joint to create a toe-out angle, the anatomy of the knee joint is rotated in a way such that a portion of the external knee adduction moment is experienced by the knee instead as a flexion moment. Because a flexion moment is likely borne by the medial and lateral compartments together, the medial compartment would then be relatively off-loaded compared to the no-toe-out condition in early stance. We tested this hypothesis by using two frames of reference constructed on the same patient data. The first frame was fixed to the tibia (TFOR) and rotated with the knee anatomy during toe-out. It is in this frame that external adduction moment is most often reported in the literature. In this frame, adduction moment should be reduced with increasing toe-out angle. The second frame was fixed to the laboratory (LFOR). In this frame, the knee adduction moment was calculated always in the frontal plane of the laboratory frame of reference, resulting in an adduction moment that was unaffected by toe-out angle and that simulated subject gait without knee rotation (no-toe-out). The CP was mathematically ‘derotated’ along with the leg and foot by the negative of the foot progression angle. Using two frames of reference on the same data enabled within-trial repeated measures analysis.
Section snippets
Subjects
A total of 180 patients (141 males, 39 females) were recruited from an orthopedic injuries clinic. Patients had been assessed by an orthopedic surgeon for potential surgical treatment of one knee due to complaints of pain localized to the medial compartment. All subjects had varus alignment of the lower limb and were diagnosed with OA based on the Altman et al. (1986) criteria. Mean age was 48.1 years (range 21–76) and mean body-mass index (BMI) was 27.8 (range 18.0–48.8). Each subject signed
Results
The CP shifted by less than 1 cm for the first 30% of the gait cycle (approximately 50% of stance phase; Fig. 4B) and by less than 6 mm for the first 20% of gait (30% of stance). The ground reaction force lever arms acting about the knee joint in the frontal and sagittal planes (Fig. 5A and B) and the adduction and flexion components of the external knee moment (Fig. 6A and B) are plotted from a representative trial. The lever arm in the frontal plane (; causing an adduction moment) was
Discussion
This is the first study to directly examine multi-planar changes in knee joint biomechanics during gait as a function of toe-out angle in patients with knee OA. A novel analysis was used to investigate the affect of toe-out within the same data set.
The mathematical derotating of the CP suggests that the CP is shifted too little in early stance (Fig. 4B) to effectively reduce frontal plane lever arm and external adduction moment about the knee by itself. However, the present findings suggest
Conflict of interest
None of the authors has a conflict of interest in the presentation of this manuscript or any means of financial gain from this presentation.
Acknowledgments
This research was funded by a Canadian Institutes of Health Research grant, a Natural Science and Engineering Research postgraduate scholarship and by Arthrex, Inc.
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