Deformity

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For Medical Students

What It Tests

Deformity is an abnormal resting shape of the knee, and the examination's first job is to establish whether it corrects.

A deformity that corrects passively is a position the knee is being held in. One that does not is a structural change, and the two have different causes and different treatments. 4.7.7 covers contractures.

The alignment chapter covers measurement of the named deformities. This page covers recognising a deformity and deciding whether it is fixed. 4.5.1 covers the measurement.

How It Is Performed

Look at the standing alignment from the front and the side, with the feet together. 4.3.3 covers standing alignment.

Look at the knee supine with both legs straight and nothing under them, and compare the two resting positions. 4.3.5 covers the supine assessment.

Attempt to correct the deformity gently by hand, and note whether it corrects fully, partly, or not at all.

Note whether correction is limited by a firm bony stop, a soft tissue stretch, or pain, since those are three different findings. 4.7.5 covers end feel.

Measure the deformity you cannot correct, in degrees, and record the limb it is in, because a tibial deformity and a femoral deformity are corrected in different places. 4.5.1 covers the mechanical axis.

What a Positive Means

A varus or valgus deformity that corrects passively indicates ligament laxity rather than bony change, and the ligament examination is what localises it. 4.12.1 covers valgus stress testing.

A varus or valgus deformity that does not correct indicates bony or cartilage change, and the radiograph rather than the examination grades it. 4.5.2 covers varus alignment.

A fixed flexion deformity indicates a contracture, an effusion holding the knee flexed, or a mechanical block, and the end feel distinguishes them. 4.5.4 covers it.

Hyperextension, particularly asymmetrical, indicates posterolateral corner deficiency or generalised hypermobility. 4.5.5 covers it.

A rotational deformity points at the tibia, the femur or the hip rather than at the knee joint itself. 4.5.6 covers rotational alignment.

Key Learning Points

The first job is establishing whether the deformity corrects.
A correctable deformity is a position; a fixed one is a structural change.
Correction limited by a bony stop, a soft tissue stretch or pain are three different findings.
A fixed flexion deformity is a contracture, an effusion, or a mechanical block.
A rotational deformity points away from the knee joint itself.

MRCS Level

How It Is Performed

In a viva the question behind the deformity is always whether it is fixed, and answering it without being asked is the mark.

The expected points are the standing and supine inspection, the attempt at passive correction, the end feel at the limit, the measurement of the uncorrectable part, and where in the limb it sits.

Say that you would examine standing and lying, because a deformity that appears only standing is a dynamic one and belongs to the alignment assessment. 4.5.7 covers dynamic alignment.

Say that you would assess the hip and the ankle, since a knee deformity can be compensating for one above or below it. 4.1.4 covers the structured approach.

Say that you would obtain long leg standing radiographs for any deformity you intend to quantify. 4.1.6 covers the relationship with imaging.

What a Positive Means

The reasoning expected is that the knee's resting shape reflects the balance between the bony geometry, the ligaments and the muscles crossing it, so a deformity localises to one of those three.

A correctable deformity means the bony geometry is intact and the restraint is not, which is the mechanical definition of laxity. 3.13.3 covers varus alignment.

A fixed deformity means the geometry has changed, by cartilage loss, by bone loss or by a malunion, and no soft tissue procedure corrects it. 3.13.8 covers alignment and osteoarthritis.

A flexion deformity has a separate mechanism again, because the posterior capsule shortens when a knee is held flexed, and the shortening becomes the limit. 4.7.7 covers contractures.

How Good It Is

The clinical assessment of deformity is a reliable screen for a fixed deformity and a poor measurement of any deformity.

The fixed or correctable distinction is the part it does well, and it is also the part that changes the plan, which is a convenient arrangement.

It is unreliable for small deformities and in obese patients, where the contours are hidden and the radiograph carries the assessment. 4.23.4 covers those patients.

It cannot distinguish a femoral from a tibial deformity reliably, which matters because the correction is performed where the deformity is. 4.5.1 covers the mechanical axis.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

Key Learning Points

A deformity localises to the bony geometry, the ligaments, or the muscles.
A correctable deformity means the geometry is intact and the restraint is not.
A fixed deformity means the geometry has changed, and no soft tissue procedure corrects it.
A flexion deformity has its own mechanism in the shortening posterior capsule.
The clinical assessment cannot reliably separate a femoral from a tibial deformity.

FRCS Level

How Good It Is

At decision level the fixed or correctable distinction decides whether the operation is bony or soft tissue, which is the largest single fork in this section.

A fixed varus deformity with lateral ligament laxity is an osteotomy problem first, and a lateral reconstruction into it stretches. 4.13.8 covers combined injuries.

A correctable valgus deformity with medial laxity is a ligament problem, and the reconstruction has a limb that will protect it. 4.12.8 covers that assessment.

A fixed flexion deformity changes feasibility. A knee that cannot reach extension cannot be reconstructed to a normal length, and correcting the deformity precedes the reconstruction. 4.7.7 covers contractures.

Where the deformity is in the bone, the examination's job is to establish that and hand the measurement to the radiograph, rather than to quantify it. 4.5.1 covers the measurement.

Where It Misleads

The commonest error is not attempting correction, which leaves the central question unanswered.

The second is assessing deformity lying only, where a loaded deformity disappears.

The third is attributing a deformity to the knee when it is in the tibia, the femur, the hip or the foot.

The fourth is reading a flexion deformity in an acutely effused knee as a contracture, when it resolves with the effusion. 4.8.2 covers the mechanism.

The fifth is recording deformity present without saying whether it corrected, which is the only part of the finding that changes anything.

Key Learning Points

The fixed or correctable distinction decides whether the operation is bony or soft tissue.
A fixed varus deformity with lateral laxity is an osteotomy problem first.
A fixed flexion deformity changes feasibility rather than indication.
A flexion deformity in an acutely effused knee is not a contracture.
Recording deformity without saying whether it corrected records nothing useful.

Fellowship Level

Where It Misleads

In referred practice the deformity is often the finding that explains a failed operation, and it is often absent from the first assessment.

A patient with a stretched lateral reconstruction and a fixed varus deformity has a sequencing error, and the revision is an osteotomy rather than another graft. 4.13.4 covers the corner.

A patient with a fixed flexion deformity after a reconstruction has either a cyclops lesion, an arthrofibrotic knee, or a graft placed with the knee in flexion, and separating them needs the operation note and the end feel together. 4.7.5 covers end feel.

A deformity that has developed since the injury is a different finding from one that preceded it, and in a chronically deficient knee the deformity is often a consequence of the laxity rather than an independent problem. 4.11.6 covers that progression.

In a knee with a previous osteotomy the deformity has been deliberately altered, so the question is whether the correction achieved what was planned, and the pre-operative films are required. 4.5.1 covers the measurement.

In medicolegal work the pre-injury alignment is almost never recorded, which makes attributing a deformity to an injury difficult in either direction, and saying so is the honest position. 4.23.7 covers that setting.

Key Learning Points

A stretched lateral reconstruction with a fixed varus deformity needs an osteotomy, not another graft.
A fixed flexion deformity after reconstruction has three causes separated by the end feel and the notes.
In a chronically deficient knee the deformity is often a consequence of the laxity.
After an osteotomy the question is whether the correction achieved what was planned.
Pre-injury alignment is almost never recorded, which complicates attribution.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 023 rather than an omission. The comparative standing of these tests is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page, and chapter 3.17 lists the studies the figures would come from. Any measurement here, including a flexion angle, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.