Alignment and OA

For Medical Students

The Principle

Two things can each cause the other. A knee that is bow legged wears on the inside, and a knee worn on the inside becomes more bow legged.

Once that happens, the problem accelerates, because each step makes the other worse.

Breaking the loop early is easier than breaking it late, which is why timing matters more than it first appears.

At the Knee

Varus loads the inner compartment, and the inner compartment is where wear usually starts. 3.13.3 covers varus.

As cartilage and then bone are lost on that side, the leg becomes more bow legged. 1.2.2 covers cartilage.

The increased deformity increases the load, and the loop closes. 6.73 covers medial compartment disease.

The same applies on the other side, with valgus and the outer compartment, and it is less common. 3.13.4 covers valgus.

What Changes It

The alignment the leg started with. 3.13.5 covers constitutional varus.

Body weight, which scales the load. 3.5.7 covers obesity.

Previous meniscal loss or injury, which removes a load spreader. 3.5.5 covers it.

Activity, which decides how many loading cycles the joint sees.

Clinical Relevance

It explains why osteoarthritis in the knee is usually one sided rather than even. 6.71 covers the introduction.

It is the argument for intervening while one compartment is still intact.

It is also why a severe deformity is less correctable: much of it is now inside the joint. 3.13.10 covers the principles.

5.3.4 covers how osteoarthritis looks on imaging.

MRCS Level

The Principle

Alignment is both a risk factor for progression and an outcome of it, which makes the epidemiology harder than it looks.

A cross sectional association between varus and medial disease cannot separate the two directions. Only longitudinal data can.

Where longitudinal data exists, it supports alignment as a predictor of progression rather than of incidence, which is a meaningful distinction.

At the Knee

Baseline varus predicts medial compartment progression in longitudinal cohorts, with the effect size rising with the degree of deformity.

The adduction moment predicts more strongly than the static angle, which points at load as the mechanism. 3.13.7 covers it.

Evidence for alignment predicting the first appearance of disease is weaker than for predicting its progression.

Deformity acquired through joint space loss adds an intra-articular component that no extra-articular surgery addresses. 3.13.6 covers it.

Meniscal extrusion and subchondral change accompany the process and contribute to the measured deformity. 3.5.5 covers meniscal loss.

What Changes It

Baseline alignment, which is the exposure in question. 3.13.5 covers its origin.

Body mass, which is the strongest modifiable risk factor and acts through load. 3.5.7 covers obesity.

Meniscal status, which changes load distribution independently of alignment. 3.5.5 covers it.

Previous injury, which starts the process at a site load alone would not have chosen. 6.77 covers post-traumatic disease.

Muscle strength and gait, which change the moment without changing the angle. 3.13.7 covers them.

How It Is Measured

Standing long leg radiography at baseline and follow up, which is how cohort studies measure it. 5.3.2 covers the technique.

Radiographic grading of joint space, which is the usual progression endpoint and is insensitive early. 5.3.4 covers it.

Magnetic resonance imaging of cartilage and bone marrow, which detects change earlier and has no agreed threshold. 5.11 covers it.

Gait analysis for the adduction moment, which gives the mechanism and is confined to research. 3.16.7 covers it.

Separating cause from consequence requires a baseline measurement taken before disease, which most cohorts do not have.

Clinical Relevance

Distinguish alignment as a progression risk from alignment as a cause of incident disease, since the evidence differs.

Apportion the measured deformity between bone and joint, because only the first part is correctable. 3.13.6 covers it.

Address body mass alongside alignment, since it is the stronger modifiable factor. 3.5.7 covers it.

Avoid reading a cross sectional association as a causal direction. 3.13.7 covers the mechanism.

FRCS Level

At the Knee

The careful statement is that alignment predicts progression of established disease, that its role in incident disease is weaker, and that the loop makes both hard to measure.

The practical consequence is that alignment correction has its best rationale in the knee with a deformity and a preserved opposite compartment, which is a narrow window. 3.13.10 covers the targets.

A severe varus knee has most of its deformity inside the joint, so an extra-articular correction can restore the axis without restoring the joint. 3.13.6 covers obliquity.

Correcting alignment after that point redistributes load onto a compartment that may also be worn, which is the reason for assessing the opposite side before correcting. 6.74 covers lateral disease.

Conventional teaching has kept alignment as a central factor while accepting that the mechanical and the biological accounts of progression are both incomplete. 6.154 covers the controversies.

What Changes It

How much of the deformity is bony against intra-articular, which decides what correction achieves.

The state of the opposite compartment, which decides whether load can be moved there. 5.11 covers cartilage imaging.

Body mass and gait, which act on the moment rather than the angle. 3.13.7 covers them.

Meniscal and ligament status, which change the compartment's tolerance. 3.5.5 covers meniscal loss.

The patient's age and expected remaining demand, which decide whether a joint preserving strategy makes sense at all. 6.76 covers the young patient.

How It Is Measured

Longitudinal cohort data with baseline alignment, which is the only design that can address causation and is scarce.

Joint space measurement, which is the regulatory endpoint and a poor proxy for cartilage health. 5.3.4 covers it.

Compositional magnetic resonance imaging, which detects earlier change and lacks a threshold. 5.11 covers it.

Gait analysis, which measures the mechanism and does not appear in the large cohorts.

The mechanical and the biological literatures use different endpoints, which is part of why they have not converged.

Clinical Relevance

Assess the opposite compartment before correcting alignment, because the correction moves load onto it. 5.11 covers the imaging.

Apportion the deformity and state the apportionment, since an extra-articular correction cannot address an intra-articular loss.

Offer weight reduction and gait modification as part of the mechanical plan rather than instead of it. 3.13.7 covers them.

Describe alignment as a predictor of progression rather than as a cause of the disease, which is what the evidence supports.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into the decision, because the window in which correction is rational is narrow and the operation is large.