What is the relationship between knee adduction moment and ankle joint function?

Knee Adduction Moment and Its Relation to the Ankle Joint

The knee adduction moment (KAM) is a biomechanical force that loads the medial knee compartment during gait, and the ankle joint directly influences this moment through its control of the center of pressure and ground reaction force lever arm—specifically, lateral shifts in foot center of pressure and ankle positioning can reduce the medial-to-lateral distance between the knee and ankle joint centers, thereby decreasing the KAM. 1, 2

Understanding Knee Adduction Moment

The external knee adduction moment represents the primary determinant of medial-to-lateral load distribution across the tibiofemoral joint during walking 3. This moment:

• Correlates strongly with medial compartment osteoarthritis severity, with every 1.0-unit increase in adduction moment associated with a 0.63-mm decrease in medial joint space width 3
• Predicts medial contact force best during early stance phase (R² = 0.76), though correlation is only moderate during late stance (R² = 0.51) 4
• Consists of four distinct components generated by ground reaction forces and their associated lever arms, which can be targeted individually for intervention 1

The Ankle-Knee Biomechanical Connection

Direct Mechanical Relationship

The ankle joint controls KAM through two primary mechanisms:

• Lateral shifts in the center of pressure (COP) at the foot level are negatively correlated with knee flexion angle, the second peak of knee extensor moment, and knee abductor moment in patients with medial knee OA 2
• The mediolateral distance between knee and ankle joint centers (controlled by ankle positioning) represents the second component of KAM and is the main cause of increased KAM in patients with medial knee OA 1

Clinical Implications of Ankle Control

In patients with severe medial knee OA, controlling lateral shifts in the COP may be an effective intervention for reducing mechanical loads on the knee during stance phase 2. However, individual responses vary significantly—the same ankle-based intervention (such as laterally wedged insoles) can produce different KAM reductions in different subjects because the four components of KAM contribute differently across individuals 1.

Ankle-Based Interventions for KAM Reduction

Laterally Wedged Insoles

Full-length laterally wedged insoles can reduce KAM in patients with mild to moderate medial compartment OA, but their effectiveness is inconsistent and depends on ankle joint mechanics. 5

• The mechanism works by increasing foot pronation and shifting the center of pressure laterally, thereby shortening the moment arm between ground reaction force and knee joint center 5
• Full-length wedged insoles have demonstrated KAM reduction immediately and at 1-3 months in some studies, while heel wedges and two-thirds length insoles have not shown consistent effects 5
• A critical limitation exists: some of the wedge effect is lost at the ankle joint, with significant between-subject differences in response 5

Patient-Specific Factors at the Ankle

Patients with excessive subtalar joint valgus angle receive no analgesic or functional benefit from laterally wedged insoles, and in some patients with knee OA, lateral wedges paradoxically increase rather than decrease KAM 5. This occurs because:

• The ankle joint can compensate for or negate the intended biomechanical effect of the insole 5
• Static measurements of ankle alignment do not correlate well with dynamic alignment during gait 5

Proximal Control and the Kinetic Chain

While the ankle influences KAM, proximal muscle control is equally critical:

• Excessive hip adduction during running gait is a predictor of tibial bone stress injuries in female runners and indicates increased adduction moment 5, 6
• Strengthening hip and core musculature optimizes lower extremity biomechanics and may reduce KAM through improved control of the entire kinetic chain 5, 6
• The adductor muscle group (attaching to the pelvis and femur) plays a significant role in controlling knee alignment and moments during dynamic activities 5, 7

Clinical Decision-Making Algorithm

When addressing elevated KAM through ankle interventions:

1. Assess static ankle alignment first, particularly subtalar joint position—excessive valgus predicts poor response to lateral wedging 5
2. Perform dynamic gait analysis to identify which of the four KAM components is elevated, as the ankle primarily affects the second component (vertical GRF × mediolateral knee-ankle distance) 1
3. Trial full-length laterally wedged insoles only if the second KAM component is elevated and subtalar alignment is neutral or varus 5, 1
4. Monitor for paradoxical effects at 1-3 months, as some patients experience increased rather than decreased KAM due to ankle compensation 5
5. Consider alternative interventions (proximal strengthening, gait retraining) if ankle-based approaches fail, as moderate correlations (R² = 0.51-0.56) during late stance indicate ankle control alone is insufficient 4

Common Pitfalls

• Assuming all patients respond similarly to ankle interventions—individual ankle mechanics create high inter-subject variability in KAM response 5, 1
• Using static alignment alone to predict dynamic KAM—static measurements correlate poorly with dynamic moments during gait 5
• Ignoring the late stance phase, where ankle-KAM correlation is weakest and other factors (muscle activation, hip control) become more important 4
• Focusing solely on ankle interventions without addressing proximal control, particularly in female athletes where hip adduction is a strong predictor of injury 5, 6