AMRI Biomechanics

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Student MRCS FRCS

For Medical Students

The Principle

An instability pattern is named by what moves and in which direction, rather than by what is torn.

Naming patterns is useful because one pattern can arise from several injury combinations, and the examination finds the pattern rather than the tear.

A pattern involving two directions at once is missed by a test that loads only one.

At the Knee

Anteromedial rotatory instability is forward translation of the medial tibial plateau combined with external rotation of the tibia.

It requires loss of the medial rotational restraints, which are the deep layer and the corner rather than the long superficial ligament. 3.9.2 covers rotational stability.

It is usually accompanied by anterior cruciate injury, because the cruciate also restrains the anterior component. 3.7.7 covers rotational stability anteriorly.

A valgus test at 30 degrees can be nearly normal in a knee with this pattern, which is why the pattern is missed. Section 4 covers the tests.

What Changes It

Which medial structures are injured, since the rotational ones are the relevant ones.

Cruciate integrity, which shares the anterior restraint.

Flexion angle, which changes which structures are taut.

Chronicity, since the pattern becomes more marked as structures stretch.

Clinical Relevance

Testing rotation on the medial side is how the pattern is found, and it is a separate manoeuvre from a valgus test. Section 4 covers the tests.

A patient with a medial injury who continues to give way may have this pattern rather than a failed valgus restraint.

The pattern implies the corner is involved, which changes the management. 3.9.6 covers the corner.

6.18 covers the clinical condition.

Key Learning Points

A pattern is named by what moves and in which direction.
Anteromedial rotatory instability is medial plateau translation with external rotation.
It requires loss of the deep layer and the corner rather than the superficial ligament.
It usually accompanies anterior cruciate injury.
A valgus test at 30 degrees can be nearly normal in a knee with this pattern.

MRCS Level

The Principle

The pattern is a coupled motion, so it cannot be produced or measured by loading one degree of freedom. 3.2.9 covers coupling.

That is the general reason rotatory instabilities are harder to assess than translational ones, and it applies on both sides of the knee. 3.7.7 covers the anterolateral equivalent.

Conventional teaching pairs anteromedial rotatory instability with anterolateral rotatory instability as the two rotatory patterns, each involving a cruciate and a corner.

At the Knee

The injured structures are conventionally the deep medial collateral ligament, the posteromedial corner and the anterior cruciate, in varying combinations. 3.9.4 and 3.9.6 cover the medial structures.

The anteromedial drawer test applies an anterior force with the tibia externally rotated, which loads the coupled pair. Section 4 covers the tests.

Sectioning studies show that dividing the medial structures after the cruciate produces a larger anteromedial excursion than either alone, which is the apportionment behind the pattern.

External rotation laxity at 30 degrees distinguishes a medial rotatory injury from an isolated valgus one, and the dial test at 30 and 90 degrees separates medial from lateral involvement.

What Changes It

Which combination of structures is injured.

Flexion angle at which the rotation is assessed.

Chronicity, through progressive stretching.

Cruciate reconstruction, which removes the anterior component.

How It Is Measured

Sectioning studies with combined anterior and rotational loads apportion the restraint.

The anteromedial drawer and the dial test assess it clinically, with modest reproducibility.

Examination under anaesthetic is where the pattern is most reliably demonstrated.

No instrument quantifies anteromedial rotatory laxity in routine use.

Clinical Relevance

Perform an anteromedial drawer, since a straight anterior drawer and a valgus test both miss the pattern.

Use the dial test at 30 and 90 degrees to separate medial from lateral rotatory involvement.

Expect the pattern to be clearest under anaesthetic.

Treat the pattern as implying corner involvement.

Key Learning Points

The pattern is a coupled motion and cannot be loaded one degree of freedom at a time.
It pairs with anterolateral rotatory instability as the two rotatory patterns.
Deep ligament, corner and anterior cruciate are the structures involved.
External rotation laxity at 30 degrees distinguishes it from an isolated valgus injury.
No instrument quantifies anteromedial rotatory laxity in routine use.

FRCS Level

At the Knee

Treat this pattern as the reason a medial injury alongside a cruciate tear sometimes needs surgery. The cruciate reconstruction addresses the anterior component and leaves the rotational one. 3.9.10 covers combined injury.

Conventional teaching holds that a residual anteromedial rotatory laxity after cruciate reconstruction predicts a poorer result and is an indication to address the medial side at the same sitting.

That is an apportionment argument: the cruciate graft restores its share and the medial structures' share is not restored by it. 3.7.2 covers primary restraint.

What Changes It

Whether the medial side is addressed at the same sitting.

Chronicity, which stretches the remaining restraints.

Alignment, particularly valgus, which loads the medial side. 3.13 covers alignment.

Which medial structures are reconstructable.

How It Is Measured

Examination under anaesthetic before and after cruciate reconstruction is the practical assessment.

Valgus stress radiography measures the coronal component only.

Robotic testing apportions the restraint in combined injury states.

No instrument quantifies the rotatory component clinically, which limits outcome measurement.

Clinical Relevance

Assess anteromedial rotation under anaesthetic after cruciate fixation.

Address the medial side at the same sitting where rotatory laxity persists.

Correct valgus alignment where it loads the medial reconstruction.

Accept that the rotatory component cannot be quantified clinically.

Key Learning Points

The pattern is why a medial injury alongside a cruciate tear sometimes needs surgery.
The graft restores the anterior component and leaves the rotational one.
Residual anteromedial rotatory laxity predicts a poorer result.
It is an indication to address the medial side at the same sitting.
The rotatory component cannot be quantified clinically.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

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