Flexion and Extension

For Medical Students

The Principle

Flexion and extension is rotation in the sagittal plane, and it is the knee's dominant motion by an order of magnitude.

It is not rotation about a fixed axis. The axis moves as the knee bends, because the femoral condyles roll as well as glide on the tibia. 3.3.2 covers that.

The practical consequence is that a description of knee flexion as hinge motion is close enough for a drawing and wrong for a measurement.

At the Knee

Conventional teaching gives the range as about 0 to 140 degrees, with 5 to 10 degrees of hyperextension present in many normal knees.

Functional ranges are smaller. Level walking uses about 0 to 60 degrees, stairs about 80 to 90, sitting about 90, and squatting beyond 130.

Extension is limited by the posterior capsule, the cruciate ligaments and the shape of the condyles together. Deep flexion is limited by soft tissue contact between calf and thigh before any ligament is reached.

The last degrees of extension involve a small rotation as well, which 3.3.5 covers as the screw home mechanism.

What Changes It

Effusion limits flexion before anything else does, because the capsule has a volume at which it is most comfortable and that volume corresponds to about 30 degrees.

Pain and guarding limit measured range without any structural cause.

Soft tissue bulk limits deep flexion, so the same knee allows different ranges in different bodies.

Scarring, arthritis and previous surgery limit it structurally, and the pattern of loss differs between them.

Clinical Relevance

Losing the last 10 degrees of extension is more disabling than losing the last 30 of flexion. A knee that cannot straighten is loaded by the quadriceps throughout stance, which is tiring and painful.

Fixed flexion is measured as the angle at which extension stops, and it is recorded separately from the flexion range for that reason.

Range of motion is recorded as two numbers rather than one, because the same arc at different positions means different things functionally.

MRCS Level

The Principle

Describe the motion as rotation about a migrating axis. The instant centre of rotation traces a curve through flexion rather than sitting at a point, which 3.3.7 covers.

Rolling without gliding would roll the femur off the back of the tibia within about 20 degrees. The cruciates convert most of the roll into a glide, which is the classical four bar linkage account.

That conversion is why cruciate injury changes the path of the joint as well as its laxity.

At the Knee

Posterior femoral rollback in the lateral compartment exceeds that in the medial, which produces the axial rotation that accompanies flexion. 3.3.3 carries it.

The extension limit is a combination: the posterior capsule tightens, the anterior cruciate tightens, and the condylar geometry brings the surfaces into their most congruent position.

Conventional teaching puts the tibiofemoral contact point moving posteriorly by of the order of 15 mm from extension to deep flexion in the lateral compartment and considerably less medially.

Patellofemoral contact migrates proximally on the patella with flexion, which 3.12.2 covers.

What Changes It

Effusion volume, pain, soft tissue bulk and structural change, as at student level, with posterior capsular contracture added as the usual structural cause of fixed flexion.

Hamstring tightness limits extension with the hip flexed and not with it extended, which separates a two joint muscle cause from a capsular one.

Implant design changes the flexion arc achievable, because the surfaces that permit rollback are replaced.

How It Is Measured

Goniometry is the clinical standard and its reliability is better for flexion than for small extension deficits, which is where the clinically important measurement lies.

Fluoroscopic analysis gives contact point migration in vivo and is the source of the rollback figures.

Recording convention matters: a knee described as having 10 to 120 degrees has a fixed flexion deformity, and the same arc described as 110 degrees of movement hides it.

Clinical Relevance

Record two numbers and name the deficit, because an arc alone conceals fixed flexion.

Distinguish capsular from muscular limitation by testing extension with the hip flexed and extended.

Expect effusion to limit flexion early, and reassess range once it has settled rather than recording a limited range as fixed.

Prioritise restoring extension over gaining the last degrees of flexion in rehabilitation, since the functional cost is asymmetrical.

FRCS Level

At the Knee

Treat the flexion arc as the product of surface geometry and ligament length, and note that surgery can alter either independently.

Rollback is asymmetrical between compartments and that asymmetry is the knee's rotation. An implant that reproduces one without the other reproduces neither.

The extension limit is multifactorial, which is why fixed flexion after arthroplasty has several candidate causes and the anatomy does not point to one.

What Changes It

Tibial slope changes the relationship between flexion and anteroposterior position, and is now part of both the arthroplasty and the cruciate revision conversation.

Posterior condylar offset changes the flexion achievable before impingement.

Soft tissue balance decides whether a reconstructed or replaced knee reaches its geometric range, so the number a design permits and the number a patient achieves are different quantities.

How It Is Measured

In vivo fluoroscopy during weight bearing activity is the method that matters for judging a design, and it is expensive and uncommon.

Passive range under anaesthetic, passive range awake and active range are three different measurements, and the gap between them carries information.

Published flexion figures for implants are usually passive and usually best case, which is worth stating when comparing them.

Clinical Relevance

Measure extension deliberately rather than as part of a sweep, since small deficits are both the hardest to measure and the most disabling.

Record passive and active range separately when assessing a stiff knee, because the difference localises the cause.

Consider slope and offset when a knee does not achieve its expected arc after surgery.

Avoid quoting a design's published flexion as a patient's expected flexion.