Dynamic Alignment

For Medical Students

The Principle

A measurement taken standing still describes one instant. Most of what a joint does happens in movement.

Standing on two legs halves the load on each. Standing on one puts all of it through that leg, and more than all of it when moving.

So the number that matters is taken during the activity, and the number usually available is taken in a radiology department.

At the Knee

During walking the body passes over one leg at a time, and the load on that knee rises above body weight. 3.14.2 covers stance phase.

How far the body's weight passes to one side of that knee changes through the step.

Some knees also shift suddenly outwards as weight comes onto them, which is called a varus thrust. 3.13.3 covers varus.

A thrust means the knee is moving into deformity under load, which a standing film cannot show.

What Changes It

Walking speed, which changes the forces.

Trunk position, because leaning over the loaded leg reduces the sideways lever. 3.14.1 covers normal gait.

Muscle strength and control around the hip and knee.

Ligament laxity, which decides whether the knee shifts under load.

Clinical Relevance

Watching a patient walk shows things a radiograph cannot. 4.3.2 covers gait observation.

A thrust is a bad sign and it is free to look for.

Teaching someone to lean slightly over the loaded leg reduces the load on that side, which is a real intervention.

Section 8 covers the rehabilitation.

MRCS Level

The Principle

The mechanically correct quantity is the external knee adduction moment during stance, which is the product of the ground reaction force and its moment arm. 3.4.6 covers external moments.

Static alignment contributes the moment arm and nothing else. Force magnitude, direction and timing all come from the movement.

That is why a static angle and a dynamic moment correlate moderately rather than closely.

At the Knee

The adduction moment peaks early in stance and again late, and its first peak is the value usually reported.

It predicts medial compartment progression more strongly than the static hip knee ankle angle does. 3.13.8 covers progression.

Varus thrust is a visible dynamic increase in varus during loading, and it is an independent predictor of progression.

Trunk lean towards the stance limb reduces the moment arm and measurably reduces the moment, which is the basis of gait retraining.

Toe out walking and a lateral wedge also alter the moment, with smaller and less consistent effects.

What Changes It

Static alignment, which sets the baseline moment arm. 3.13.1 covers it.

Walking speed, which scales the ground reaction force. 3.4.2 covers it.

Trunk and pelvic kinematics, which move the force's line of action.

Soft tissue laxity, which permits thrust. 3.10.1 covers the lateral restraints.

Body mass, which scales the force directly. 3.5.7 covers obesity.

How It Is Measured

Three dimensional gait analysis with force plates, which is the reference and is confined to laboratories. 3.16.7 covers motion capture.

Clinical observation of thrust during walking, which is free and has moderate reproducibility. 4.3.2 covers it.

Wearable sensors and instrumented insoles, which estimate the moment in the field with less accuracy. 3.16.8 covers them.

Musculoskeletal modelling, which converts measured kinematics into estimated moments and inherits its assumptions. 3.16.4 covers it.

No routine clinical pathway measures the adduction moment, which is the chapter's central gap.

Clinical Relevance

Observe gait in every patient in whom alignment is being considered, because thrust predicts independently of the film. 4.3.2 covers it.

Explain gait retraining and trunk lean as a genuine mechanical intervention rather than as advice. Section 8 covers the programmes.

Expect walking speed and body mass to change the moment without changing the angle.

State that the predictive quantity is not the one being measured when quoting an alignment angle. 3.13.1 covers the angle.

FRCS Level

At the Knee

This page is where the chapter's argument lands. The variable that predicts is not the variable that is measured, and it is not the variable that surgery corrects.

Correcting the static angle changes the moment arm and leaves force magnitude, direction and timing untouched, so the moment changes by less than the geometry suggests.

That partly explains why osteotomy outcomes vary between patients with identical radiographic corrections. 3.13.9 covers the operation.

It also explains why gait retraining, lateral wedges and weight loss have measurable mechanical effects without changing a single degree on a film.

Conventional teaching plans from the static angle because it is reproducible and correctable, and the honest position is that it is the available surrogate rather than the right variable. 6.154 covers the controversies.

What Changes It

Geometry, which surgery changes and which is only the moment arm.

Force magnitude, which body mass and walking speed change. 3.5.7 covers obesity.

Force direction and timing, which gait retraining changes.

Laxity and thrust, which decide whether the loaded alignment matches the standing one.

Neuromuscular control, which is modifiable and is the least measured of these in clinic.

How It Is Measured

Laboratory gait analysis, which is the reference and measures a laboratory walk rather than a life. 3.16.7 covers it.

Instrumented implants, which give direct in vivo load and only in replaced knees.

Wearables, which trade accuracy for the number of steps measured, and which are the most likely route to a clinical measure. 3.16.8 covers them.

Clinical thrust assessment, which is the only part of this available today at no cost. 4.3.2 covers it.

The absence of a routine dynamic measurement is a limitation of practice rather than of knowledge: the quantity is known, it is simply not measured.

Clinical Relevance

Assess thrust clinically in every alignment decision, since it is the only dynamic information routinely obtainable. 4.3.2 covers it.

Expect a radiographic correction to change the moment by less than the geometry implies, and counsel on that basis.

Offer gait retraining and weight reduction as mechanical interventions in their own right rather than as adjuncts. Section 8 covers them.

State clearly that the static angle is a surrogate for the adduction moment when explaining the rationale for surgery.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, since the non operative levers here depend entirely on what the patient will do.