Flexion Assessment

For Medical Students

What It Tests

Flexion assessment measures how far the knee bends, actively and passively, and establishes what stops it.

Unlike extension, flexion is frequently limited by something other than the joint: an effusion filling the capsule, thigh and calf bulk meeting, or pain. 4.8.1 covers the capsular volume.

Deep flexion matters functionally because several daily tasks need it, and the threshold for each is worth knowing when a patient describes what they cannot do. 4.7.9 covers functional range.

How It Is Performed

Lay the patient supine and ask them to slide the heel towards the buttock as far as they can, and measure the angle. 4.7.1 covers active motion.

Then flex the knee passively to its limit, supporting the limb, and measure that angle. 4.7.2 covers passive motion.

Note what stops the flexion: tissue contact between calf and thigh, a firm resistance, or pain.

Measure the heel to buttock distance as well, which is a simple comparative measure and easier to reproduce than an angle in deep flexion.

Compare both sides and record both, since normal flexion varies widely. 4.1.2 covers the principle.

What a Positive Means

Flexion limited by tissue contact in a muscular or obese patient is a normal finding and not a restriction. 4.23.4 covers those patients.

Flexion limited by a firm resistance indicates capsular shortening or an intra-articular cause, and the end feel distinguishes them. 4.7.5 covers end feel.

Flexion limited by pain with a full passive range indicates an irritable joint rather than a restricted one, usually from an effusion or inflammation. 4.8.2 covers it.

A reduction in flexion after surgery indicates swelling, pain or developing stiffness, and the time since surgery decides which is likely. 4.7.10 covers post-operative range.

Loss of deep flexion with preserved extension is the pattern in a knee with an effusion, since the capsule's volume limits flexion first. 4.6.3 covers the effusion assessment.

MRCS Level

How It Is Performed

In a viva the expected answer says what stopped the flexion as well as the angle, and the three possible answers are the content of the question.

The expected points are the active then passive measurement, what limits it, the heel to buttock distance, and the comparison.

Say that you would measure the heel to buttock distance where the angle is hard to measure, since deep flexion angles are poorly reproducible.

Say that you would assess the effusion first, since an effusion limits flexion mechanically. 4.8.3 covers the bulge sign.

Say that you would ask what the patient cannot do, because the functional consequence of lost flexion is specific and the angle alone does not convey it. 4.7.9 covers functional range.

What a Positive Means

The reasoning expected is that flexion requires the femur to roll back on the tibia and the posterior structures to accommodate it, so a limit can be bony, capsular, muscular or fluid. 3.3.2 covers roll and glide.

The capsule's volume is finite, so an effusion reduces the available flexion before it reduces extension, which is the mechanism behind the characteristic pattern. 4.8.1 covers normal fluid physiology.

Deep flexion loads the patellofemoral joint heavily, which is why flexion is painful early in patellofemoral disease and why a painful arc in deep flexion points there. 4.7.6 covers the painful arc.

Posterior structures and hamstring length also limit flexion, so a restriction can be extra-articular, and the passive assessment with the hip flexed and extended distinguishes it. 4.7.2 covers passive motion.

How Good It Is

Flexion angles are less reproducible than extension angles, because the limit is softer and the landmarks move, which is why the heel to buttock distance is useful.

The distinction between a tissue limit and a joint limit is reliable and is the part that matters most for interpretation.

It is a poor measure of a knee's problem in isolation, since many knees with significant pathology have full flexion. 4.10.3 covers one such group.

It is the most useful range to follow after an operation that risks stiffness, because it is the range lost first in the post-operative knee. 4.7.10 covers it.

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

FRCS Level

How Good It Is

At decision level flexion contributes to feasibility, to the assessment of post-operative stiffness and to the functional conversation.

A knee that cannot flex adequately cannot be operated on arthroscopically in the usual way, and that is a practical constraint the examination establishes.

A loss of flexion after a reconstruction or an arthroplasty is the earliest sign that the knee is stiffening, and acting early is what prevents an established problem. 4.7.7 covers contractures.

The functional thresholds matter for the conversation. A patient who cannot achieve the flexion their occupation or their religious practice requires has a specific problem that an angle alone does not convey. 4.7.9 covers functional range.

Flexion contributes little to ligament decisions, and a knee with a full range and gross laxity is a common and unremarkable combination. 4.11.2 covers one such grade.

Where It Misleads

The commonest error is recording an angle without saying what limited it.

The second is reading tissue contact as a restriction.

The third is measuring flexion in an effused knee and treating the figure as structural. 4.8.2 covers it.

The fourth is comparing with a textbook normal rather than the other knee.

The fifth is relying on an angle in deep flexion, where reproducibility is poorest.

Fellowship Level

Where It Misleads

In referred practice lost flexion after surgery is a common and under investigated complaint, and the examination separates its causes.

A knee with a firm block in flexion after a reconstruction may have a mechanical cause, and the end feel is what raises it. 4.7.5 covers it.

A knee with a global leathery restriction in both directions has arthrofibrosis, and flexion and extension are lost together. 4.7.7 covers contractures.

A patient whose flexion is limited by anterior pain rather than resistance has a patellofemoral problem, and the sitting examination is where it is localised. 4.3.4 covers it.

A patient whose flexion was never measured before the operation cannot be assessed for loss, which is the commonest reason this complaint cannot be resolved. 4.1.8 covers the record.

In a knee with an arthroplasty the achievable flexion is partly determined by the implant and the soft tissue balance, so the target is not the other knee and that has to be said. 4.7.9 covers functional range.