Changes Throughout Flexion

For Medical Students

The Principle

A joint whose geometry changes through its range cannot have one description. The honest answer to most questions about the knee begins with a flexion angle.

Three things change together: where the surfaces touch, which ligaments are tight, and how much rotation and translation are available.

They change together because they are the same fact described three ways. The surfaces set the ligament lengths, and the ligament lengths set the available motion.

At the Knee

Near full extension the surfaces are most congruent, the joint is locked, and rotation is close to zero. 3.3.5 covers the locking.

Through roughly 20 to 30 degrees the knee unlocks and the bony contribution to stability falls away, handing over to the ligaments.

Through mid flexion, roughly 30 to 90 degrees, rotation and translation are at their greatest and the ligaments are the limit. This is where most stability tests are performed.

Beyond about 120 degrees the contact moves to the posterior condyles, the surfaces are small and highly loaded, and the posterior soft tissues take a share again.

What Changes It

Which structures are intact, since an injury removes a limit at the angle that structure was the limit.

Load, which compresses the surfaces and reduces the available motion at every angle.

Muscle activity, which adds restraint through the whole arc.

Joint shape, which varies between people and sets where each handover occurs.

Clinical Relevance

Test at more than one angle, because a structure that is the limit at 30 degrees may contribute little at 90. Section 4 covers the tests.

Quote a flexion angle with every range, laxity or contact figure.

Expect symptoms to be angle specific. A patient with deep flexion pain and a patient with terminal extension pain are describing different parts of the joint.

The handover from bone to soft tissue around 30 degrees is why so many examination manoeuvres are performed there. 3.2.10 covers stability through range.

MRCS Level

The Principle

Divide the arc into zones and name what dominates in each, rather than quoting a value for the knee. The zones are approximate and the handovers are gradual.

The contact point migrates posteriorly on both condyles with flexion, further on the lateral than the medial, which is the kinematic account of coupled rotation. 3.2.9 covers coupling.

Ligament bundles are not uniformly tight. Within the anterior cruciate the anteromedial and posterolateral bundles reach maximal tension at different angles, which is why a single tunnel position is a compromise.

At the Knee

Zero to about 20 degrees: maximal congruence, terminal rotation, bony contribution greatest, posterior capsule and collateral structures taut.

About 20 to 30 degrees: unlocking, the bony contribution falling, and the angle at which collateral and rotational testing is most informative.

About 30 to 90 degrees: maximal axial rotation and anteroposterior translation, cruciates as primary restraints, and the range the clinical laxity tests sample.

Beyond about 120 degrees: posterior condylar contact, small contact area, high patellofemoral load, posterior meniscal and capsular involvement. 3.5 covers contact mechanics.

Patellofemoral contact migrates proximally on the patella and the contact area enlarges with flexion, which 3.12 covers.

What Changes It

Each structure's integrity, at the angle where it is the primary restraint rather than throughout.

Posterior tibial slope, which shifts the whole arc relative to the loading.

Meniscal status, particularly the posterior horns in the deeper part of the range.

Previous surgery, since a graft or an implant changes where each handover sits.

How It Is Measured

Incremental fluoroscopy or weight bearing imaging measures contact position at chosen angles and builds the picture from static points.

Cadaveric sectioning at several flexion angles is where the zone by zone apportionment comes from, with the sectioning order limitation.

Ligament length and strain measurement in cadavers gives the tension profile through the arc, in a specimen and under an applied load.

Clinical examination samples two or three angles, which is why the examination sequence is a sampling strategy rather than a ritual.

Clinical Relevance

Examine at 0, 30 and 90 degrees deliberately, because each samples a different restraint set.

Expect a finding to be present at one angle and absent at another, and record the angle rather than the finding alone.

Read a graft's behaviour as angle dependent, since the bundles tighten at different angles.

Consider deep flexion loading in a patient whose symptoms appear only at the end of range.

FRCS Level

At the Knee

Treat mid flexion as the zone where reconstructions and replacements fail quietly. Terminal extension and deep flexion are checked, and mid flexion instability is the recognised gap in both.

Mid flexion instability after replacement is described as a laxity at 30 to 60 degrees with a stable knee in extension and in flexion, and conventional teaching attributes it to femoral component position and to the radius of curvature.

The angle dependence of ligament bundle tension is the mechanical argument for anatomic tunnel placement and for the double bundle debate. 3.7 covers it.

What Changes It

Femoral component position and sagittal radius, which decide the gap through mid flexion rather than at the checked angles.

Joint line position, which shifts the whole arc and changes ligament tension through it.

Graft tunnel position and tensioning angle, which decide where a construct is tight.

Slope and alignment, which change the loading that the angle dependent restraints meet.

How It Is Measured

Intraoperative gap measurement at several flexion angles, with tensioning devices or navigation, is the practical measurement and it is made in an anaesthetised knee.

Cadaveric and simulator testing describes mid flexion laxity by design and by component position, under repeatable conditions.

Stress radiography at a chosen flexion angle quantifies a clinical suspicion and samples one angle.

Mid flexion instability has no agreed diagnostic threshold, which is why it is a clinical diagnosis supported by measurement rather than defined by it.

Clinical Relevance

Check gaps through mid flexion rather than at extension and 90 degrees alone.

Restore the joint line, since shifting it changes ligament tension through the whole arc.

Suspect mid flexion instability in a replaced knee that is stable at the checked angles and symptomatic in between.

Choose tunnel position and tensioning angle deliberately, since they fix where the graft is tight.