Pivot Shift Mechanics

For Medical Students

The Principle

A joint that has lost a restraint can sit out of place in one position and snap back into place in another.

The snap back happens when a structure that was slack becomes tight enough to pull the joint home.

The event has two halves: a subluxation and a reduction. Both are necessary for it to be felt.

At the Knee

Near extension, a cruciate deficient knee allows the lateral part of the tibia to sit forward. That is the subluxation. 3.2.7 covers anterior translation.

As the knee flexes, the iliotibial band passes behind the flexion axis and its tension pulls the tibia back into place. That is the reduction. 2.11.1 covers the band.

The reduction is sudden, which is why it is felt as a shift rather than as a gradual movement.

The whole event is a coupled motion, involving rotation as well as translation. 3.2.9 covers coupling.

What Changes It

Whether the iliotibial band is intact, since it produces the reduction.

The secondary restraints, which determine how far the knee subluxates. 3.7.3 covers them.

Muscle activity, which prevents the subluxation and is why the event is hard to reproduce in an awake patient.

Bony shape, since a convex lateral plateau and a steep slope both allow more subluxation.

Clinical Relevance

Patients describe this event rather than a forward slide, which is why their account is about turning rather than about laxity.

The test that reproduces it is the most informative one for function, and the hardest to perform in a guarding patient. Section 4 covers the tests.

Each episode loads the secondary restraints and the cartilage, which is part of how a deficient knee degenerates. 3.5.8 covers contact mechanics.

A reconstruction that abolishes this event has restored what the patient complained of.

MRCS Level

The Principle

The mechanics are a changing moment arm. The iliotibial band lies anterior to the flexion axis in extension and posterior to it in flexion, so its line of action reverses as the knee bends. 3.3.7 covers the instant centre.

Before the reversal it permits or contributes to the subluxation; after it, it reduces the joint. The reduction angle is where the reversal occurs.

Conventional teaching places the reduction at roughly 20 to 30 degrees of flexion, which is where the line of action crosses.

At the Knee

The subluxation is chiefly of the lateral compartment, because the lateral side translates further. 3.3.2 covers the asymmetry.

Grading is by the examiner's perception: a glide, a clunk, or a locked knee that will not reduce, which conventional teaching numbers one to three.

A high grade shift suggests involvement of the anterolateral structures or the lateral meniscus in addition to the cruciate. 3.7.3 covers secondary restraints.

The test's sensitivity is low in an awake patient and high under anaesthetic, which is a statement about muscle rather than about the knee.

What Changes It

Anterolateral complex and lateral meniscal integrity, which set the grade.

Slope and lateral plateau convexity, which permit more subluxation.

Muscle guarding, which suppresses the event.

Examiner technique and experience, which is a recognised source of variation.

How It Is Measured

Graded clinical examination is the standard and has modest inter observer reliability.

Instrumented measurement uses skin mounted accelerometers, image analysis or electromagnetic tracking to quantify the reduction, with no agreed threshold.

Robotic simulation applies a combined load and measures the coupled motion in cadavers.

Examination under anaesthetic is where the grade is most reliably obtained.

Clinical Relevance

Grade under anaesthetic where the grade will change the operation.

Read a high grade as suggesting injury beyond the cruciate alone.

Treat instrumented measures as research tools without a clinical threshold.

Account for examiner variation when comparing grades between assessments.

FRCS Level

At the Knee

Treat the pivot shift as the clinical sign that corresponds to what the patient reports, and treat its measurement as the weakest link in the whole chapter.

That weakness has consequences for the evidence. Construct comparisons that rest on rotational control rest on a sign with modest reliability and no threshold, which is why reinjury rates carry the argument instead. 3.7.7 covers rotation.

Conventional teaching holds that a residual pivot shift after reconstruction is associated with poorer outcome and with graft failure, which is the reason it remains the sign to abolish.

What Changes It

Femoral tunnel position, which determines the rotational control restored. 3.7.6 covers it.

Addition of a lateral extra-articular procedure. 3.11 covers the constructs.

Slope and plateau morphology, and whether they were addressed.

Lateral meniscal posterior horn repair.

How It Is Measured

Examination under anaesthetic before and after reconstruction is the practical intraoperative measurement.

Instrumented quantification is used in research cohorts and has no clinical threshold.

Robotic testing compares constructs under a simulated pivot shift load.

Reinjury rates are the endpoint the sign is a proxy for.

Clinical Relevance

Check the pivot shift under anaesthetic after fixation, before leaving theatre.

Add a lateral procedure where a residual shift is anticipated or demonstrated.

Address slope and lateral meniscal pathology as part of abolishing the sign.

State the sign's reliability limits when it is used to compare treatments.