Combined Instability Patterns

For Medical Students

What It Tests

A combined instability pattern is a description of which movements a knee cannot resist, rather than a list of which structures are torn.

The distinction matters because the operation is planned from the movements. An imaging report names structures; the examination says which laxity the patient has. 4.1.6 covers that division.

The patterns worth knowing are few. Straight anterior, straight posterior, anterolateral rotatory, anteromedial rotatory, posterolateral rotatory, and the multiligament knee where several coexist. 4.11.7 covers the last.

How It Is Performed

Examine in a fixed order, so the pattern emerges rather than being assembled from remembered findings. 4.1.4 covers the structured approach.

Translation first: the Lachman test and the posterior drawer with the tibia reduced. 4.10.4 covers the Lachman test.

Then the collaterals: valgus and varus at thirty degrees and in full extension. 4.12.1 covers valgus stress testing.

Then rotation: the dial test at thirty and ninety degrees, the pivot shift, and the anterior drawer in external rotation. 4.13.5 covers the dial test.

Then the whole limb: alignment standing, gait, and the nerve and circulation in any knee with multiple findings. 4.11.5 covers dynamic assessment.

Record the pattern as a sentence rather than a list of positive tests, because that is what the next clinician needs. 4.1.8 covers the record.

What a Positive Means

Increased anterior translation with a large pivot shift and no collateral or rotational abnormality elsewhere is anterolateral rotatory instability. 4.14.3 covers it.

Increased anterior translation with valgus opening in extension and an increased anteromedial drawer is anteromedial rotatory instability. 4.14.4 covers it.

A posterior sag with an increased dial difference at thirty and ninety degrees is posterolateral rotatory instability with posterior cruciate deficiency. 4.13.8 covers it.

Abnormal findings in three or more planes, with a high energy mechanism, is a multiligament knee until proven otherwise, and the vascular and nerve examination comes first. 4.11.7 covers the sequence.

MRCS Level

How It Is Performed

In a viva this is the synthesis station, and the examiner is testing whether the findings become a diagnosis or stay a list.

The expected sequence is translation, collaterals at two angles, rotation at two angles, then the limb, with the nerve and circulation first in any knee that could have been dislocated.

Say the pattern aloud at the end, because that is the answer, and a candidate who recites positive tests has not answered.

Say that you would examine the other knee completely, since every one of these findings is a comparison. 4.1.2 covers the principle.

Say that examination under anaesthesia is where the pattern is established in an acute high energy knee. 4.14.6 covers it.

Say that stress radiographs convert the grades into numbers where a plan turns on one.

What a Positive Means

The reasoning expected is that each structure has a plane and a flexion angle in which it dominates, so a combination of deficiencies produces a reproducible pattern rather than an arbitrary set of findings. 3.9.9 covers the medial angles.

Secondary restraints matter as much as primary ones. Each structure is a secondary restraint for several movements, so a combined injury produces more laxity than the sum of the single injuries. 3.10.8 covers that argument.

That is the mechanical reason a cruciate reconstruction into an uncontrolled corner stretches, and the reason the pattern rather than the structure list determines the operation. 3.10.9 covers the combination.

A pattern can also be produced by a single structure plus a bony factor, such as a posterior tibial slope or a varus alignment, which is why the limb is part of the examination. 4.11.5 covers it.

How Good It Is

The examination establishes patterns well and grades components poorly, and every page in this section has reached the same conclusion.

Patterns are what the operation needs, so that is the right way round, and it is the argument for teaching the examination as a sequence rather than as a set of named tests.

The acute high energy knee is where the assessment fails, and it fails towards false reassurance because guarding reduces every finding. 4.13.4 covers the corner assessment that suffers most.

Imaging is the better description of the structures and the worse guide to the operation, so the two are read together. 4.1.6 covers that relationship.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the pattern is the operation. Which structures are reconstructed, in what order, whether the knee is staged, and whether a bony correction comes first all follow from it.

The patterns that change an operation most are the two that involve a corner, because a corner left untreated stretches whatever else is done. 4.13.8 covers the lateral combinations.

The pattern that most often goes unrecognised is anterolateral rotatory instability over a competent graft, because the Lachman test is normal and the examination looks reassuring. 4.14.3 covers it.

The pattern most often over treated is a borderline corner finding in a knee with a cruciate injury, where an unnecessary lateral reconstruction produces stiffness. 4.2.4 covers stiffness.

Alignment and slope are part of the pattern rather than a separate consideration, and in a revision they are often the main finding. 4.10.12 covers revision assessment.

Where It Misleads

The commonest error is recording tests rather than a pattern, which leaves the next clinician without the reasoning.

The second is examining in a different order each time, so findings are missed rather than contradicted.

The third is accepting a pattern established on a guarding patient.

The fourth is building the pattern from imaging, which names structures and cannot grade laxity.

The fifth is omitting the limb, where alignment and slope explain a pattern the soft tissues do not.

Fellowship Level

Where It Misleads

In referred practice the pattern is usually being reconstructed retrospectively from a failed operation, and the examination is better evidence than the operation note.

A stretched posterior cruciate graft with a dial difference at both angles is a posterolateral pattern that was present and unrecorded, and the revision addresses both. 4.13.5 covers the test.

A knee with two sound reconstructions and a persisting complaint usually has a bony factor: a posterior slope favouring posterior translation, or a varus limb loading a lateral reconstruction. Neither is a soft tissue problem and neither is fixed by revising soft tissue.

A pattern that does not fit any of the recognised combinations should raise the question of generalised hypermobility or a connective tissue disorder, where the knee is one joint of several and the operation is a different conversation. 4.23.3 covers those patients.

In a multiply operated knee the pattern is constrained by previous surgery, so the examination under reads some planes and over reads others, and the operation notes are needed to interpret it. 4.2.30 covers previous surgery.

The commonest reason a pattern cannot be established at all is an incompletely recovered common peroneal nerve palsy, which changes the gait and the dynamic examination permanently and is attributed to laxity. 4.23.6 covers complex injuries.