Knee OA: The evidence for gait modification

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Gait retraining can potentially alter walking biomechanics such that knee adduction moment is reduced, an inexpensive off­loading option that does not require device wear. Gait modification studies to date have primarily focused on foot rotation, trunk lean, and knee medialization.

By Michael A. Hunt, PT, PhD

Retraining of gait and functional movement represents a common and important component of rehabilitation following an acute neurological or neuromuscular infarct (e.g., brain injury or stroke) or surgery (e.g., arthroplasty). The purpose of gait retraining in these instances is the recovery or optimization of global movement function within the context of neuromuscular or skeletal changes. This is generally achieved through nonspecific alterations in kinematic and muscle activation patterns at a number of lower limb joints, based on the unique clinical presentation of the patient. Optimizing movement efficiency and functionality are of primary importance when the patient presents with impaired muscle or joint function, and improvements may not be observed for weeks or months.

A number of recent published studies have focused on altering specific kinematic or kinetic variables to immediately redistribute loads passing through the knee joint during walking, with particular application to individuals with medial compartment knee osteoarthritis (OA). Gait modifications at the foot, hip, or trunk have the potential to alter ground reaction force lever arms at the knee such that load is reduced.

Excessive joint loading has been implicated in the breakdown of articular cartilage¹ and 3D motion analysis techniques can be used to measure movement and estimate intra-articular joint loading. Relevant to knee OA, the external knee adduction moment (KAM)—a valid and reliable proxy for medial compartment load during walking^2-4—has become an important outcome measure to minimize following surgical and nonsurgical treatment. Previous cross-sectional studies have shown significant relationships between the magnitude of the KAM during walking and clinically relevant outcomes such as alignment and disease severity⁵ as well as bone changes in the proximal tibia.⁶ Importantly, high KAM magnitudes have also been implicated in the progression of OA severity over a six-year period.⁷

Figure 1a. The KAM is predominantly composed of the magnitude of the ground reaction force (arrow and solid line) and its perpendicular distance from the knee joint center of rotation in the frontal plane—the lever arm (dotted line). Figure 1b. With external rotation of the foot, the position of the center of pressure (origin of the ground reaction force) moves laterally, bringing the GRF closer to the knee joint. Figure 1c. Leaning the trunk laterally over the stance limb shifts the center of mass laterally. Since the GRF is oriented toward the COM, the vector passes closer to the knee joint; again, a reduced lever arm magnitude and resultant KAM is expected. Figure 1d. Reduced lever arm and KAM magnitudes can also be accomplished by bringing the knee joint closer to the orientation of the GRF vector. For those with varus malalignment, walking with a more neutrally aligned knee would accomplish this.

The KAM is the rotational force tending to adduct the tibia with respect to the femur. Muscles generate an internal knee abduction moment to counteract the KAM, and this rotation is opposed primarily by compres­sion of the medial tibia and femur.⁴ A number of nonsurgical stra­tegies have been tested such as shoe inserts,⁸ knee bracing,⁹ and exercise,^10-13 with mixed results. In contrast, gait retraining approaches predicated on the known biomechanical properties of the KAM have consistently shown improvements in KAM magnitudes in those with and without knee OA.¹⁴

As a moment of force, KAM is predominantly a product of the frontal plane ground reaction force (GRF) and its perpendicular lever arm about the knee (Figure 1a).¹⁵ Therefore, reduc­tions in KAM can theoretically be achieved through reductions in the magnitude of the GRF, its lever arm, or both. Because GRF magnitude is influ­enced by factors that are difficult to modify (e.g., body mass) or highly linked to functionality (e.g., walking velocity), most gait retraining strategies aiming to reduce KAM focus on decreasing the moment arm by either bringing the GRF closer to the knee or the knee closer to the GRF.

A recent systematic review and meta-analysis¹⁴ found that gait retraining strategies designed to reduce lever arm magnitude generally result in favorable reductions in KAM. Gait retraining strategies relevant to the knee OA population include those focused on foot rotation, trunk lean, and knee medialization.

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External foot rotation angle

Modifying the foot rotation angle in the transverse plane (known as external foot rotation angle, toe-out angle, or foot progression angle) during stance has received the most attention in the gait retraining literature on lowering the KAM. A number of studies have investigated the effects of increasing the amount of toeing-out during stance, based on the theory that rotating the foot externally will result in a concomitant lateralization of the GRF center of pressure and subsequent reduction of the GRF lever arm at the knee (Figure 1b).^5,16  Predetermined or self-selected increases in toe out have typically had little effect on KAM magnitude in early stance (first 50% of stance) in those with^17-18 and without knee OA.^19-20 In contrast, maintaining this increased toe-out angle has been shown to significantly reduce KAM in late stance (last 50% of stance).^{17, 19, 21-22}

This apparent discrepancy can be explained by the anticipated effects of lateralization of the center of pressure throughout the gait cycle. Given that the center of pressure is positioned under the heel in early stance, rotation of the foot would change the position of the center of pressure very little compared with natural gait. In contrast, during late stance, when the center of pressure is under the forefoot, external rotation of the entire foot would bring with it substantial changes in the position of the center of pressure compared with normal walking.²³ Taken together, these findings suggest that alterations in foot rotation angle may have a beneficial effect on frontal plane knee joint kinetics during walking.

Lateral trunk lean angle

Clinically, many individuals with knee OA already demonstrate an observable displacement of their trunk laterally over the stance limb during gait. The amount of self-selected lateral trunk lean during gait has been shown to be related to disease severity²⁴ as well as knee pain. ²⁵

While a direct cause and effect relationship cannot be determined from cross-sectional studies—thus limiting the ability to claim that lateral trunk lean is a true compensatory response—the biomechanical effect of such a movement is clear. Moving the center of mass laterally while maintaining the same foot placement changes the orientation of the GRF and brings the vector closer to the knee joint center to reduce the magnitude of the GRF lever arm (Figure 1c). Individuals with knee OA who exhibit greater amounts of lateral trunk lean also experience less loading in the medial compartment of the knee joint as quantified by KAM.²⁵ Two studies have shown that actively increasing the amount of lateral trunk lean toward the stance limb in young, healthy individuals significantly reduces KAM magnitudes in early stance.^26-27 In fact, Mundermann et al²⁶ found a mean peak KAM decrease of 65% across 19 healthy individuals walking with approximately 10° of additional lateral trunk lean. Almost all participants experienced KAM reductions in excess of 50%. In contrast to foot rotation angle modification, the effects of exaggerated lateral trunk lean walking are more pronounced during early stance.¹⁴ During normal gait, maximum ipsilateral lean occurs in early stance while lateral trunk lean angle is near neutral in late stance as the body prepares for contralateral foot contact. Thus the biomechanical benefits of lateral trunk lean on load distribution in the knee are typically not observed simply due to lack of observable ipsilateral trunk lean during this period of the gait cycle. However, if one were to maintain ipsilateral lean during late stance, reductions in KAM would be expected, but with the consequences of reduced balance and stability.

Knee medialization

Although foot rotation and trunk lean influence the KAM either by moving the GRF vector origin or changing the vector orientation, another approach would be to dynamically bring the knee closer to the GRF (Figure 1d). Though not discussed in the literature as much as other gait modifications, two studies employing this type of movement have shown significant reductions in KAM magnitude, in excess of 50%.^28-29 Participants are instructed to walk with their knees as close together as possible to reduce the dynamic varus malalignment that would be associated with large GRF lever arm values.

A number of strategies could be employed to achieve such a gait pattern, though a common approach would be to increase hip adduction and internal rotation in slight knee flexion. Indeed, a recent study that provided real-time biofeedback of frontal plane knee angle during treadmill walking showed that this type of gait retraining was associated with decreases in KAM and knee varus alignment over eight sessions, predominantly resulting from increased hip adduction and internal rotation.³⁰ Importantly, this study provided evidence of retention of the modified gait pattern in the absence of the real-time feedback, suggesting that longer term training may translate to more permanent benefits.

Clinical implications and limitations

Gait modification is a feasible and inexpensive treatment that can potentially offload a painful compartment of an arthritic knee. With minimal equipment and time—most reported studies employ a single testing session with training lasting no more than a few minutes—almost instantaneous changes in knee joint biomechanics can be achieved. Of course, given that gait modification is not unlike other skills, optimal results are likely to be expected when training approaches adhere to motor learning principles. Various attempts have been made to improve the learning and accuracy of gait modification, such as the use of real-time biofeedback regarding modifications. Though able to provide very precise data pertaining to the movement, biofeedback systems relying on 3D motion analysis are expensive and not available to most clinicians. As a result, the utility of these systems may be limited to obtaining evidence of the effectiveness of certain gait modifications, while the actual implementation clinically may continue to rely on therapist interactions. Investigation into optimization of gait modification training in the clinical setting is needed, including the effectiveness of visual feedback via mirrors, which is a commonly used clinical technique and has been used for research.³¹

The gait modifications described above are designed to decrease loading across the medial compartment of the knee by reducing the external KAM. In essence, the actual load is not reduced; rather, it is simply redistributed away from the painful and arthritic medial compartment—the compartment most commonly affected in tibiofemoral OA. As a result, these strategies are only applicable to those with medial compartment OA and may, in fact, be detrimental to those with lateral compartment OA. Further, when gait biomechanics change, adverse effects at other joints are possible due to altered joint function and muscle requirements. Thus, it is possible that by “fixing” one joint, damage will be created at another joint. That said, a patient with painful and debilitating knee OA may be willing to take that risk if knee pain can be reduced.

Finally, though the KAM is a reliable and valid proxy for medial compartment knee joint load, this measure has a number of limitations. The actual distribution of load at the tissue level cannot be accurately determined at a given time from external measures, let alone changes in the distribution following an intervention. Instead, the load passing through the medial compartment—and ultimately, the bone and cartilage—is estimated based on biomechanical principles and previous research showing high correlations between the KAM and direct internal measures of load.² However, it is an assumption that reductions in the externally measured joint moments will correlate with reductions in the total internal moments required to maintain equilibrium. Indeed, while some studies have shown a significant relationship between the KAM and directly measured joint contact forces using an instrumented prosthesis,^{2, 32} others have found the KAM does not necessarily correlate with medial contact force.³³ Thus, caution must be exercised when implying internal loading based on external measures.

Future directions

Though results of studies to date are promising, more clinically relevant questions need to be answered before these gait modification strategies should be implemented on a wider scale. First, though reductions in KAM have been documented, the effects of these modifications on clinically relevant outcomes such as knee pain are unknown. Very little previous research has been conducted in this area with people with a diagnosis of knee OA. Therefore, more research is needed to examine if changes in knee joint biomechanics translate to clinical improvement.

On a similar note, as indicated above, the majority of research in this area has been limited to immediate changes in knee biomechanics after a single training session. Conducting longer-term interventions will provide valuable information regarding the feasibility (especially in the older population), adherence (given the potential for aesthetic changes in walking style, especially with a lateral trunk lean or knee medialization gait pattern), and effectiveness of gait modification interventions. Longer-term interventions will also provide the ability to monitor changes in biomechanics and symptoms at other joints to assess the risk of negative consequences of these gait modifications. Finally, though studied as a stand-alone treatment to test efficacy, clinical implementation of these gait modifications would be as part of an overall treatment strategy. How gait modification would optimally fit into clinical management and how it could be combined with other interventions have yet to be determined.

Michael A. Hunt, PT, PhD, is an assistant professor in the Department of Physical Therapy at the University of British Columbia in Vancouver, BC.

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