Internal Rotation

For Medical Students

The Principle

Internal rotation is rotation of the tibia about its long axis so that the foot turns inwards relative to the thigh.

It is a small motion that the knee permits rather than performs. No muscle group exists to produce it as a primary action in the way the quadriceps produces extension.

Its range depends almost entirely on flexion angle, which makes it the clearest example of a degree of freedom whose availability is positional.

At the Knee

In full extension there is almost none. The condyles are at their most congruent and the collaterals are tight, so the joint is locked against rotation.

As the knee flexes the collaterals slacken and the contact areas become less congruent, and rotation becomes available. Conventional teaching gives internal rotation of about 10 to 15 degrees at 90 degrees of flexion.

The structures that resist it are the anterior cruciate ligament, the anterolateral structures and the distal part of the superficial medial collateral ligament. 3.7 and 3.11 carry the apportionment.

Popliteus produces it actively at the start of flexion, which is the unlocking movement 3.3.6 covers.

What Changes It

Flexion angle is the dominant variable, and rotation is greatest in mid flexion rather than at either end.

Load reduces it. A compressed joint resists rotation through its surfaces as well as its ligaments.

Injury increases it. An anterior cruciate deficient knee permits more internal rotation, and more again if the anterolateral structures have gone with it.

Clinical Relevance

Internal rotation is half of what a pivot shift test produces. The other half is anterior translation, and the two occur together rather than in sequence. 3.2.9 covers the coupling.

Because the motion is small and there is no instrument for it, rotational instability is graded by hand rather than measured.

A knee that gives way on turning rather than on walking straight is describing a rotational problem, which is why the history is worth taking carefully.

MRCS Level

The Principle

Separate the motion from the position. Internal rotation is a movement; internal tibial torsion is a fixed anatomical position. The two are confused often enough to be worth stating.

Rotation is measured relative to the femur, so femoral rotation at the hip changes the apparent rotation at the knee without the knee having moved.

That is why rotational assessment is done with the hip controlled.

At the Knee

Total axial rotation, internal and external together, is conventionally given as about 25 to 30 degrees at 90 degrees of flexion, with external rotation taking the larger share.

The medial compartment acts closer to a pivot and the lateral translates more, so the axis of rotation sits nearer the medial side rather than at the centre of the joint.

The anterolateral complex restrains internal rotation and its contribution appears when the anterior cruciate has failed, which 2.11.5 covers.

The iliotibial band's deep layers restrain it through their femoral attachment, which is why dividing those fibres increases it.

What Changes It

Flexion angle, applied load and muscle tone, as for every degree of freedom here.

Injury to the anterior cruciate, the anterolateral complex or both increases it, and the increase from a combined injury exceeds the sum of the isolated ones.

Previous lateral surgery can reduce it, and over-constraint in this direction is a described complication of lateral extra-articular procedures. 3.11.7 covers them.

How It Is Measured

There is no routine clinical instrument. The pivot shift is graded 0 to 3 by feel, and interobserver agreement is moderate at best.

Instrumented pivot shift measurement using accelerometers or image analysis exists in research settings and has not entered routine use.

Cadaveric robotic testing gives the restraint apportionment, with the order of sectioning affecting the result.

The absence of a clinical number is the reason rotational outcomes after reconstruction are reported as grades rather than millimetres.

Clinical Relevance

Grade the pivot shift and record the grade, because it is the only routine measure of this motion.

Control the hip when assessing rotation, since femoral rotation masquerades as knee rotation.

Expect a combined anterior cruciate and anterolateral injury to produce more rotation than either alone.

Distinguish rotational giving way in the history from the straight line instability of a different problem.

FRCS Level

At the Knee

Treat internal rotation as the degree of freedom that exposes the limits of current practice: it is the motion patients complain about, the one surgery most often fails to restore, and the one with no number.

Its restraints are distributed rather than concentrated, which is why no single structure's reconstruction reliably normalises it.

The medial pivot pattern means rotation and anteroposterior translation are geometrically linked, so a construct restraining one affects the other.

What Changes It

Tibial slope, notch geometry and generalised laxity are associated with greater rotational laxity and none is modifiable without an operation.

A lateral extra-articular procedure reduces it, and the margin between restoring and over-constraining is small because the structures involved have little compliance. 2.11.2 covers them.

Graft stiffness changes the rotational result as well as the translational one, which is under-reported relative to its effect.

How It Is Measured

Grading by hand is the practical method and its reliability limits what the literature can show. A trial comparing constructs on a three point scale assessed by unblinded examiners is measuring less than it appears to.

Quantitative pivot shift methods are the live research area and their adoption would change which questions can be answered.

Patient reported instability correlates with rotational findings better than with translational ones, which is an argument for measuring it properly.

Clinical Relevance

Grade and document rotation at every assessment, since the absence of an instrument makes the serial record the only comparison available.

Consider an anterolateral procedure where rotational laxity is high, and understand it as restoring a distributed restraint rather than replacing a structure.

Set expectations in terms of rotation rather than translation, because that is what patients describe.

Avoid reading a normal arthrometer reading as a stable knee.