Dynamic Valgus

For Medical Students

The Principle

Alignment measured standing still is one thing. Alignment during a movement is another, and they can disagree completely.

A joint in the middle of a limb takes the position the joints above and below allow it to take.

So a knee that collapses inwards is often reporting a problem elsewhere rather than having one of its own.

At the Knee

Dynamic valgus is the knee moving inwards relative to the hip and the foot during a loaded movement.

It happens most on landing from a jump, on changing direction and on stepping down. 3.3.13 covers landing kinematics.

When the knee falls inwards, the line of the quadriceps pull moves outwards relative to the knee, so the patella is pushed laterally harder.

The hip is usually where it starts, through weakness or poor control of the muscles that hold the thigh out. Section 8 covers the training.

What Changes It

Hip abductor and external rotator strength and control.

Fatigue, which makes the pattern worse within a single session.

Foot position and how much the arch collapses.

Trunk position over the planted foot, which moves the whole load path. 3.15 covers sports biomechanics.

Clinical Relevance

It is a trainable pattern, unlike bone shape, which is why it gets attention. 3.12.4 covers the groove.

It raises the lateral push on the patella, so it matters in both pain and instability. 3.12.8 covers instability.

It is also a recognised risk pattern for cruciate injury, so the same training serves two purposes. 3.7.14 covers ACL injury mechanisms.

Watching a single leg squat or a step down shows it. A standing photograph does not. 4.5.7 covers dynamic alignment.

MRCS Level

The Principle

A lateral vector on the patella has a static component from skeletal geometry and a dynamic component from limb control. The two add.

The static component is fixed and the dynamic component is modifiable, which is why the same anatomy produces symptoms in one person and not another.

Treating the modifiable part first is therefore reasonable even when the fixed part is abnormal.

At the Knee

Dynamic valgus combines hip adduction, hip internal rotation, knee abduction and often foot pronation in one pattern.

During the pattern the femur rotates internally beneath the patella, which moves the trochlear groove medially relative to the extensor mechanism.

That is mechanically equivalent to moving the tubercle laterally, and it is the reason rotation and tubercle position appear together in this chapter. 3.12.12 covers the tubercle.

Patellofemoral joint reaction force also rises during the movement, so pressure and displacement increase together. 3.12.3 covers pressure.

Fatigue amplifies the pattern measurably within a training session, which matters for when injuries happen. 3.15 covers sports biomechanics.

What Changes It

Hip abductor and external rotator strength, and more importantly their timing.

Trunk control over the stance limb, which sets the whole load path.

Fatigue, which degrades control before it degrades strength.

Footwear, surface and foot mechanics, which change the base of the pattern.

Fixed skeletal factors, which set the baseline the dynamic component adds to. 3.12.11 covers them.

How It Is Measured

Two dimensional video of a single leg squat, drop jump or step down, scored by observation or by a frontal plane projection angle.

Three dimensional motion capture, which measures the hip and knee angles properly and is confined to laboratories. 3.16.7 covers motion capture.

Markerless and wearable systems, which are closing that gap with less precision. 3.16.8 covers wearables.

Clinical observation, which identifies the gross pattern reliably and grades it poorly. 4.5.7 covers the assessment.

No measurement predicts an individual's patellofemoral symptoms, and the pattern's link to cruciate injury is better established than its link to this joint.

Clinical Relevance

Assess movement as well as posture, because the two disagree often enough to matter. 4.5.7 covers the examination.

Address the modifiable component first in a knee with normal morphology. Section 8 covers the programmes.

Expect fatigue to reveal the pattern, so test late in a session rather than fresh.

Treat the link with patellofemoral symptoms as weaker than the link with cruciate injury, and say which is which. 3.7.14 covers the latter.

FRCS Level

At the Knee

The mechanically precise statement is that dynamic valgus produces relative femoral internal rotation under the patella, and the patella does not follow the femur.

That decouples the extensor mechanism from the groove in the transverse plane, which is the same geometric error as a lateralised tubercle or an anteverted femur. 3.12.11 covers the fixed versions.

Because the error is geometric, its effect is additive with the fixed factors rather than an alternative explanation to them.

Simultaneous rise in joint reaction force means displacement and pressure increase together, which distinguishes this from a purely static malalignment. 3.12.3 covers pressure.

Conventional teaching treats the pattern as modifiable and the evidence for modifying patellofemoral outcomes with it is weaker than the evidence for modifying cruciate risk.

What Changes It

Neuromuscular control of the hip, which is the trainable target.

Trunk and pelvic control, which many programmes under address.

Fatigue state, which decides whether a programme's gains hold when they are needed.

Fixed femoral and tibial rotation, which the pattern adds to. 3.12.11 covers them.

Sport and position, which set how often the pattern is loaded. 3.15 covers them.

How It Is Measured

Three dimensional motion capture is the reference and measures a laboratory task rather than the sport. 3.16.7 covers it.

Two dimensional projection angles correlate moderately with three dimensional measurement and are feasible in a clinic.

Instrumented insoles and inertial sensors measure in the field with less accuracy and far more repetitions. 3.16.8 covers them.

Patellofemoral consequences are inferred from modelling rather than measured, because the joint cannot be instrumented in a living athlete. 3.16.4 covers modelling.

The chain from an observed pattern to a patellofemoral pressure figure therefore runs through several assumptions, and the figure at the end should be read as a direction.

Clinical Relevance

Separate the fixed and the modifiable contributions before planning, because only one of them responds to training. 3.12.11 covers the fixed ones.

Expect training to change the pattern reliably and the patellofemoral symptoms less reliably, and counsel on that basis.

Test under fatigue, since an unfatigued assessment overestimates control.

Avoid presenting a modelled pressure change as a measured one.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because adherence decides whether a training programme is the right choice at all.