Effects of Malalignment

For Medical Students

The Principle

A limb is a column, and where the load passes through that column decides which part of it carries the weight.

Alignment is the geometry of that column. A limb bowed outwards carries load through its inner side, and a limb angled inwards through its outer side.

Moving the load line a small distance changes the share a lot, because the distance it moves is multiplied by the force. 3.4.4 covers lever arms.

At the Knee

Varus alignment, a bow legged limb, moves the force line further medial and raises the medial compartment's share. Valgus does the reverse.

The medial compartment already carries more in a normal limb, so varus makes a heavy side heavier. 3.5.3 covers the distribution.

The contact area does not grow to match, so the extra load is carried at a higher pressure. 3.5.2 covers pressure.

Malalignment and compartment disease reinforce each other. Cartilage loss on one side increases the deformity, which increases the load on that side.

What Changes It

The degree of deformity, and whether it sits in the femur, the tibia or the joint itself.

Body weight, which multiplies the effect of any given deformity. 3.5.7 covers it.

Activity, since the deformity acts on every loading cycle.

Previous surgery, including a meniscectomy that allowed the compartment to collapse.

Clinical Relevance

Varus alignment and medial compartment osteoarthritis travel together, and each makes the other worse. 6.73 covers the condition.

Correcting alignment moves the load line and is therefore a mechanical treatment with a mechanical target. 3.5.10 covers the effect.

A meniscal or cartilage procedure in a malaligned limb is being asked to work against the mechanics, which is why alignment is assessed first. Section 6 covers management.

A patient asking why only one side of the knee is worn is asking an alignment question.

MRCS Level

The Principle

Alignment acts through the external adduction moment, not directly. The deformity lengthens the moment arm of the ground reaction force about the knee, and the moment is what the joint has to resist. 3.4.6 covers external moments.

That is why alignment is measured as a mechanical axis rather than as an appearance, and why the measurement is made standing.

Deformity in the frontal plane is the one usually discussed, and sagittal slope and torsional alignment act on contact mechanics too. 3.13 covers alignment in full.

At the Knee

Conventional teaching holds that medial compartment load rises steeply with varus, and that the relationship is non linear: successive degrees of deformity add more than the ones before them.

A normally aligned knee already places the mechanical axis slightly medial to the joint centre, so neutral mechanical alignment is not equal loading.

Lateral compartment overload from valgus is less common and behaves differently, partly because the lateral compartment carries its load over a smaller area. 3.5.1 covers area.

Joint line obliquity and the location of the deformity change what a correction has to do, which is why the deformity is localised before it is corrected.

What Changes It

The magnitude and the location of the deformity.

Lateral or medial soft tissue laxity, which adds to the measured deformity under load without a bony cause.

Body mass, which scales the force the lengthened arm acts on.

Gait, since trunk lean and foot progression angle alter the moment for the same alignment. 3.4.6 covers it.

How It Is Measured

Long leg standing radiography measures the mechanical axis and the hip knee ankle angle and is the clinical measurement.

Gait analysis measures the adduction moment, which is the quantity alignment acts through, and is a research measurement.

Cadaveric rigs measure contact pressure against imposed alignment and provide the dose response.

Supine imaging underestimates deformity, because the deformity has a load dependent soft tissue component.

Clinical Relevance

Measure alignment standing and on a long leg film, since supine imaging underestimates it.

Localise the deformity to the femur, the tibia or the joint before planning a correction.

Account for soft tissue laxity, which adds to the loaded deformity.

Treat the adduction moment as the quantity alignment is a proxy for.

FRCS Level

At the Knee

Treat alignment as the only mechanical variable in this chapter that can be changed by a large amount. Area can be preserved and partly restored; load share can be moved wholesale.

That is the mechanical case for osteotomy and the mechanical reason it precedes or accompanies cartilage and meniscal work rather than following it.

Conventional teaching identifies increased posterior tibial slope as a separate mechanical risk, acting on anterior translation and on graft failure rather than on compartment share. 3.3.14 covers cutting.

What Changes It

Correction magnitude and level, which decide the new load line and whether it overshoots.

Joint line obliquity, which a correction can improve or worsen.

Slope, which is a separate correction with a separate indication.

Soft tissue laxity, which determines how much of the measured deformity a bony correction will fix.

How It Is Measured

Long leg standing radiography with deformity analysis is the planning measurement.

Stress radiography separates bony deformity from soft tissue laxity, which changes the correction required.

Gait analysis quantifies the moment change achieved, in research settings.

Postoperative long leg imaging confirms the correction achieved, which is the available measure of whether the mechanical aim was met.

Clinical Relevance

Correct alignment before or with any cartilage or meniscal procedure in a malaligned limb.

Separate bony deformity from soft tissue laxity with stress radiography before planning.

Assess slope separately, since it acts on a different mechanism from compartment share.

Confirm the correction radiographically rather than assuming the plan was achieved.