Flexion Deformity

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For Medical Students

What It Tests

A flexion deformity is a knee that cannot be brought into full extension, whether by the patient or by the examiner.

Assessing it separates three causes: something inside the joint blocking extension, something outside it holding the knee flexed, and a knee choosing flexion because extension hurts. 4.7.5 covers the end feel that distinguishes them.

The distinction matters because the three have different treatments and only one of them is a contracture. 4.7.7 covers contractures.

How It Is Performed

Lay the patient supine with both legs straight and nothing under the knees. 4.3.5 covers the supine assessment.

Look at the two knees in profile and note whether either rests in flexion.

Ask the patient to straighten the knee actively and note how far it goes. 4.7.1 covers active motion.

Then straighten it passively and note how much further it goes and what stops it: a firm block, a soft stretch, or pain. 4.7.2 covers passive motion.

Measure the deficit in degrees with the knee at its passive limit, and record both the active and the passive figures. 4.1.8 covers the record.

Lift the leg by the heel and see whether the knee falls into extension under its own weight, which separates a true block from a held position.

What a Positive Means

A firm abrupt block to passive extension suggests something mechanical inside the joint, which in a reconstructed knee raises a graft impingement lesion. 4.7.10 covers post-operative range.

A soft spongy resistance that gives slightly suggests a soft tissue contracture, which is the posterior capsule shortening. 4.7.7 covers it.

Pain limiting extension with a knee that can be extended further under distraction suggests an effusion or an inflamed joint rather than a deformity. 4.8.2 covers the effusion.

A deficit that is larger actively than passively is an extension lag rather than a deformity, and it is a quadriceps problem. 4.7.8 covers it.

A knee that falls into extension when held by the heel has no fixed deformity at all. 4.3.10 covers deformity.

Key Learning Points

Three causes: a mechanical block, a soft tissue contracture, and a knee avoiding a painful position.
The end feel at the passive limit is what separates them.
Record the active and the passive figures separately.
A deficit larger actively than passively is an extension lag, not a deformity.
A knee that falls into extension held by the heel has no fixed deformity.

MRCS Level

How It Is Performed

In a viva the active and passive distinction is the answer, and a candidate who gives one figure has not performed the assessment.

The expected points are the supine position with nothing under the knees, the active then the passive attempt, the end feel, the measurement of both, and the heel lift.

Say that you would measure with a goniometer and state the reference you used, since an unqualified figure is not comparable at the next visit. 4.1.8 covers the record.

Say that you would look for an effusion, because an effused knee rests flexed and resolves. 4.8.3 covers the bulge sign.

Say that you would assess the hip, since a hip flexion contracture produces an apparent knee flexion deformity in standing. 4.4.5 covers a related pattern.

What a Positive Means

The reasoning expected is that full extension requires the posterior structures to be at full length and nothing to be interposed, so a deficit localises to one or the other. 3.3.5 covers the screw home mechanism.

The posterior capsule shortens when a knee is held flexed, which is why a deformity becomes fixed over weeks and why early extension work prevents it. 4.7.7 covers contractures.

An effusion holds the knee in slight flexion because that is the position of greatest capsular volume, which is a held position rather than a shortened structure. 4.8.1 covers normal fluid physiology.

The screw home mechanism requires rotation in terminal extension, so a knee that cannot rotate cannot fully extend, which is a less obvious cause worth knowing. 3.3.6 covers the reverse mechanism.

How Good It Is

The measurement is reproducible when the method is stated and unreliable when it is not, which makes the record rather than the skill the limiting factor.

The end feel is the most informative part and the least reproducible, because it is a judgement about resistance. 4.7.5 covers it.

The active and passive comparison is reliable and is the part most often collapsed into one figure. 4.7.8 covers the finding it reveals.

Imaging answers what a firm block suggests, and in a reconstructed knee it is the right next step. 4.1.6 covers that relationship.

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

Key Learning Points

Full extension needs the posterior structures at full length and nothing interposed.
The posterior capsule shortens when a knee is held flexed, so early extension work prevents a fixed deformity.
An effusion produces a held position rather than a shortened structure.
The screw home mechanism means a knee that cannot rotate cannot fully extend.
The end feel is the most informative part and the least reproducible.

FRCS Level

How Good It Is

At decision level a flexion deformity changes feasibility rather than indication, and feasibility is the harder constraint.

A knee that cannot be extended cannot be reconstructed to a normal ligament length, so a deformity is corrected before a reconstruction rather than alongside it. 4.7.7 covers contractures.

A fixed deformity also changes the alignment's loading, so a knee with a deformity and a varus limb has two problems that interact. 4.5.2 covers varus alignment.

A firm block in a reconstructed knee raises a mechanical lesion, and that is a treatable finding with a clear operation. 4.7.10 covers post-operative range.

A deformity that is painful and new raises infection or an inflammatory process, and that has to be excluded before it is treated as a stiffness problem. 4.8.9 covers synovitis.

Where It Misleads

The commonest error is recording one figure rather than the active and the passive.

The second is measuring with a pillow under the knee.

The third is calling an effusion a contracture, which resolves and does not need treating as a deformity.

The fourth is attributing a standing flexion deformity to the knee when the hip is contracted.

The fifth is reconstructing a ligament in a knee with an uncorrected deformity, which produces a tight and stiff result.

Key Learning Points

A flexion deformity changes feasibility rather than indication.
A knee that cannot be extended cannot be reconstructed to a normal ligament length.
A firm block in a reconstructed knee raises a treatable mechanical lesion.
A new painful deformity raises infection or an inflammatory process.
A standing flexion deformity may be a hip contracture.

Fellowship Level

Where It Misleads

In referred practice a persisting flexion deformity after a reconstruction is one of the commonest reasons for a dissatisfied patient, and it is usually recorded once and not followed.

A knee with a firm block and a reconstruction has a mechanical lesion whose treatment is straightforward, and the referral is often late because the end feel was never recorded. 4.7.5 covers it.

A knee with a soft contracture and a reconstruction has arthrofibrosis, and the treatment is different and the result less certain. 4.7.7 covers contractures.

A knee with a deformity and a chronically deficient posterior cruciate has an abnormal resting position that makes every laxity measurement unreliable, and that limitation has to be stated. 4.11.6 covers that knee.

In a multiply operated knee a deformity may be the best achievable result, and recognising that prevents an operation offered against an unachievable target. 4.10.12 covers revision assessment.

In medicolegal work the absence of a recorded extension figure at any stage is the commonest reason a stiffness claim cannot be assessed, which is an argument for the record rather than for a better examination. 4.1.8 covers it.

Key Learning Points

A persisting deformity after reconstruction is a common cause of dissatisfaction and is rarely followed.
A firm block with a reconstruction has a straightforward treatment and is often referred late.
A soft contracture after reconstruction is arthrofibrosis, with a less certain result.
A deformity with posterior cruciate deficiency makes every laxity measurement unreliable.
The absence of any recorded extension figure is the commonest reason a stiffness claim cannot be assessed.

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.