Contact Mechanics After Osteotomy

For Medical Students

The Principle

If load divides according to where the force line falls, moving the force line moves the load. That is what an osteotomy does.

Cutting and realigning a bone changes the geometry of the limb, so the same body weight passes through the knee in a different place.

It is the only operation here that changes the load rather than the area carrying it. Everything else in this chapter tries to spread load; this moves it.

At the Knee

Correcting a varus knee towards valgus moves the force line laterally and reduces the medial compartment's share. 3.5.3 covers the distribution.

The pressure in the medial compartment falls accordingly, which is the intended effect and is measurable in cadaveric studies.

The lateral compartment's share rises by the same amount. That is not a side effect; it is the mechanism.

Correcting too far overloads the lateral compartment, which is why the target matters more than the direction. 3.5.6 covers malalignment.

What Changes It

The amount of correction, which is the controllable variable.

Where the correction is made, since a deformity in the femur is not corrected in the tibia without consequences.

The state of the compartment receiving the load, which has to be able to take it.

Body mass, which scales what is being moved. 3.5.7 covers it.

Clinical Relevance

Osteotomy is a mechanical operation with a mechanical target, and the target is a load line rather than an appearance. Section 7 covers technique.

The receiving compartment has to be healthy, which is part of why the indication is specific. Section 6 covers management.

Undercorrection leaves the problem and overcorrection creates a new one, so the acceptable range is narrow.

It buys time rather than curing anything, and the mechanics explain why.

MRCS Level

The Principle

The quantity being changed is the external adduction moment, and the operation changes it by altering the moment arm rather than the force. 3.4.6 covers external moments.

That is why correction is planned to a mechanical axis target: the axis position determines the moment arm, and the moment arm determines the share.

The relationship is non linear, so the last few degrees of correction change the moment more than the first few. Conventional teaching reflects this in the slight overcorrection usually specified.

At the Knee

Conventional teaching specifies correction of a varus knee to slight valgus rather than to neutral, on the ground that neutral mechanical alignment still leaves the medial compartment carrying more. 3.5.3 covers the distribution.

Cadaveric studies demonstrate the medial pressure fall and the lateral rise against the degree of correction, which is the dose response the target is chosen from.

Joint line obliquity changes with correction and affects how the load is distributed within the receiving compartment as well as between compartments.

Slope is altered by some tibial osteotomies as a consequence rather than an aim, which matters in a cruciate deficient knee. 3.5.8 covers that.

What Changes It

Correction magnitude and the resulting mechanical axis.

Level and plane of the osteotomy, which decide the obliquity and the slope change.

Soft tissue laxity, which determines how much of the measured deformity the bony correction addresses.

The receiving compartment's cartilage and meniscus, which set what it can carry.

How It Is Measured

Long leg standing radiography before and after measures the achieved mechanical axis and is the clinical measurement.

Cadaveric contact studies against imposed correction give the dose response.

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

No clinical measurement confirms the pressure change in the patient, so the axis stands in for it.

Clinical Relevance

Plan to a mechanical axis target rather than a correction angle alone.

Assess the receiving compartment before correcting load into it.

Account for the slope change a tibial osteotomy produces, particularly in a cruciate deficient knee.

Confirm the achieved axis radiographically, since the plan and the result can differ.

FRCS Level

At the Knee

Treat osteotomy as the mechanical foundation that other procedures are built on, rather than as an alternative to them. A cartilage or meniscal procedure in an uncorrected limb is being loaded in the same place that destroyed the original.

The correction is also a commitment. It changes the mechanics of any later arthroplasty, including the approach, the soft tissue balance and the joint line, which is part of the counselling rather than a technical footnote.

Slope correction by anterior closing wedge osteotomy is a separate operation with a separate mechanical target, acting on anterior translation rather than compartment share. 3.3.14 covers cutting.

What Changes It

The correction achieved against the correction planned, which is the technical variable.

Whether the deformity was localised correctly before correction.

The receiving compartment's state, which can progress and change the division later.

Body mass and activity, which set the loads the correction has to manage.

How It Is Measured

Deformity analysis on long leg standing films plans it and measures the result.

Stress radiography separates bony from soft tissue deformity before planning.

Registry and cohort survivorship measures how long the correction serves, and conversion to arthroplasty is the usual endpoint.

Cadaveric and modelling work gives the dose response the plan is drawn from.

Clinical Relevance

Correct alignment as the foundation for cartilage and meniscal work rather than as an alternative.

Localise the deformity before correcting, and confirm the achieved axis afterwards.

Counsel that the correction changes any later arthroplasty.

Treat slope correction as a separate operation with its own indication.