Alignment Correction Principles

For Medical Students

The Principle

Deciding how far to correct something needs a target, and a target needs a reason.

A reason can be a measurement, an argument from how the thing works, or what has been done before. Those are not equally good, and it is worth knowing which one a target rests on.

At the Knee

Correcting a bow legged knee moves load off the worn inner side and onto the outer side. 3.13.9 covers the operation.

Stopping at neutral leaves the inner side still carrying most of the load, because it does that even in a straight leg. 3.5.3 covers load distribution.

So the usual practice is to go slightly past neutral into mild knock knee, which is called overcorrection.

The amount usually quoted comes from one surgical series from the 1980s, and no trial has compared it with any other amount.

What Changes It

The state of the compartment receiving the load. 3.13.8 covers that.

How much of the deformity is inside the joint rather than in the bone. 3.13.6 covers the joint line.

What the patient needs the knee for.

The patient's own original alignment. 3.13.5 covers constitutional varus.

Clinical Relevance

A target that comes from practice rather than from a trial should be described that way.

Correcting too far loads a compartment that may not want it. Correcting too little leaves the problem that prompted the operation.

7.68 covers the surgical principles.

MRCS Level

The Principle

A correction target is a position on a trade-off, between unloading a worn compartment and overloading a healthy one.

The trade-off has no measured optimum, so the target is a convention supported by argument and by survivorship data rather than by comparison.

That changes how firmly it should be stated.

At the Knee

For medial compartment disease the classical target is slight valgus overcorrection, usually quoted as a mechanical axis passing through the lateral compartment at around 62 per cent of the plateau width.

That figure comes from the Fujisawa series and was an observation about where cartilage appeared to recover, rather than the result of a comparison.

The rationale is that neutral leaves the medial compartment with its usual majority share, so unloading means passing neutral. 3.5.3 covers the distribution.

For valgus correction at the femur the target is nearer neutral, because the lateral compartment tolerates load less well. 3.13.4 covers the asymmetry.

Joint line obliquity limits how much can be taken at one level, which often binds before the target does. 3.13.6 covers it.

What Changes It

The receiving compartment's cartilage status, which sets the cost of overcorrection. 5.11 covers the imaging.

The intra-articular proportion of the deformity, which limits what is achievable. 3.13.6 covers it.

Patient age and demand, which change how long the correction has to last. 6.76 covers the young patient.

Constitutional alignment, which is what the knee developed at. 3.13.5 covers it.

Ligament competence, since a lax knee does not hold the correction under load. 3.13.7 covers dynamic alignment.

How It Is Measured

Mechanical axis deviation expressed as a percentage of plateau width, which is the planning expression. 5.16.5 covers the planning.

Hip knee ankle angle, which is the reporting expression and converts approximately.

Postoperative long leg radiography, which confirms what was achieved.

Survivorship series stratified by achieved correction, which is the evidence actually available.

No randomised comparison of correction targets exists, which is the honest statement about the evidence base.

Clinical Relevance

State the target as a convention supported by survivorship rather than as a measured optimum.

Assess the receiving compartment before choosing how far to correct. 5.11 covers the imaging.

Expect joint line obliquity to limit the correction before the target does in a large deformity. 3.13.6 covers it.

Aim nearer neutral for valgus correction than for varus, because the compartments differ. 3.13.4 covers them.

FRCS Level

At the Knee

The classical overcorrection target is the clearest example in this chapter of a number that has survived on plausibility and survivorship rather than on comparison.

The argument for it is sound as far as it goes: the medial compartment carries a majority share at neutral, so unloading it means passing neutral. 3.5.3 covers the share.

The argument against is that overcorrection loads a compartment that is not guaranteed healthy, and that constitutional variation unsettles neutral itself. 3.13.5 covers that.

Survivorship series support correction into slight valgus and do not discriminate finely between amounts within that range.

Conventional teaching has moved towards a range rather than a point, and towards accepting less correction where the lateral compartment is suspect. 6.154 covers the controversies.

What Changes It

Lateral compartment and meniscal status, which decide whether overcorrection is affordable. 3.5.5 covers meniscal loss.

The intra-articular component, which sets a ceiling on achievable correction. 3.13.6 covers it.

Constitutional alignment, which sets what the soft tissue envelope developed at.

Expected remaining demand, which decides how long the correction must serve. 6.76 covers the young patient.

Dynamic factors, which decide whether the achieved static correction changes the moment. 3.13.7 covers them.

How It Is Measured

Achieved mechanical axis deviation on a standardised postoperative film, which is what series report.

Survivorship stratified by achieved correction, which is observational and subject to selection.

Gait analysis before and after, which would measure the mechanical objective and is almost never reported. 3.16.7 covers it.

Compositional cartilage imaging of the receiving compartment, which would measure the cost. 5.11 covers it.

The evidence therefore measures neither the benefit nor the cost directly, only survivorship, which is a composite of both.

Clinical Relevance

Describe the target as a range supported by survivorship, not as an optimum.

Reduce the intended correction where the lateral compartment is suspect, and record the reasoning. 5.11 covers the imaging.

Name the Fujisawa figure as a single series observation when quoting it.

Expect the achieved correction to change the adduction moment by less than the geometry implies. 3.13.7 covers it.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into the target decision, because the cost of overcorrection falls on a compartment the patient will use for decades.

Fellowship Level

At the Knee

At fellowship level the target question becomes which plane's target leads, and whether the patient's own alignment should be the reference instead of a population figure.

In arthroplasty it produced kinematic and restricted kinematic alignment, which take the patient's own alignment as the target within limits. 7.81 and 7.83 cover them.

In osteotomy the same logic argues for a smaller correction in a constitutionally varus knee, and the literature does not support a specific adjustment. 3.13.5 covers the finding.

A sagittal target has entered practice separately, in the revision cruciate knee with a steep slope, where the coronal target becomes the constraint. 3.7.20 covers it.

Conventional teaching now sets a target per plane with a range, and the ranges are conventions rather than findings.

What Changes It

Which plane the indication leads with, which decides the primary target.

The patient's constitutional alignment, proxied by the contralateral limb.

The receiving compartment and the ligament envelope, which set the cost.

Concurrent procedures, which may impose their own targets. 7.74 covers combined procedures.

Previous correction, which makes the current alignment partly surgical. 7.75 covers revision.

How It Is Measured

Three dimensional planning, which can express a target in each plane simultaneously. 3.16.5 covers it.

The contralateral limb, which is the only practical reference for constitutional alignment and is unavailable in bilateral disease.

Achieved correction in all three planes postoperatively, which is rarely reported together.

Registry and series data, which report coronal correction and survivorship and little else.

No study sets a target in one plane and measures the consequences in the other two, which is what the multiplanar argument requires.

Clinical Relevance

Set a target in each plane and state which leads, rather than planning one angle.

Use the contralateral limb as the constitutional reference where it is available. 3.13.5 covers the reasoning.

Reduce the coronal target where the sagittal correction is the point of the operation. 3.7.20 covers that case.

Record achieved correction in all three planes, since that is the data the field lacks.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because a target chosen from a population figure is a decision about a person.