Combined ACL and MCL Injury

For Medical Students

The Principle

Two injuries together are worse than the sum of two injuries apart, when each structure was also a secondary restraint for the other's direction.

Shared restraint is the reason. Losing both removes the primary and the backup at once.

The two can also differ in healing. One structure can recover while the other does not, which changes the management over time.

At the Knee

The anterior cruciate and the medial structures share rotational restraint, so losing both produces more rotatory laxity than either alone. 3.9.7 covers the pattern.

The valgus and rotation mechanism that injures the cruciate is the same mechanism that injures the medial side, which is why the combination is common. 3.7.14 covers the mechanism.

The medial side heals reliably and the cruciate does not, so the combined injury usually becomes an isolated cruciate problem over weeks. 3.9.3 covers the medial ligament.

Conventional teaching therefore allows the medial side to heal first and reconstructs the cruciate afterwards in most cases. Section 6 covers management.

What Changes It

Whether the medial injury involves the corner, since corner injuries heal less reliably. 3.9.6 covers the corner.

The grade of medial laxity.

Time, which allows the medial side to heal.

Alignment, which loads whichever side remains lax. 3.13 covers alignment.

Clinical Relevance

The usual sequence is to let the medial side heal and then reconstruct the cruciate, which follows from the different healing capacities. Section 6 covers management.

A persisting rotatory laxity after the medial side has healed means the corner was involved. 3.9.7 covers the pattern.

Reconstructing the cruciate in a knee with residual medial laxity loads the graft more, which is a reason to address both.

6.20 covers the clinical condition.

MRCS Level

The Principle

The mechanical statement is that each structure is a secondary restraint for the other's direction, so the combined deficit exceeds the sum of the two isolated ones. 3.7.3 covers secondary restraints.

The clinical statement is that the two structures have different healing capacities, so the injury changes shape over weeks.

Managing it means acting on the second statement while measuring the first.

At the Knee

Sectioning studies show that dividing the medial structures in a cruciate deficient knee produces a larger increase in anterior translation and rotation than dividing them in an intact knee. 3.9.7 covers the pattern.

Conventional teaching holds that most medial injuries in this combination heal with bracing and early motion, and that a grade three injury or a corner injury may not. 3.9.6 covers the corner.

Reconstructing the cruciate in a knee with residual valgus or rotatory laxity places higher loads on the graft, which is the mechanical argument for addressing both.

Stiffness is the other risk. Operating on both at once, acutely, is conventionally associated with a higher rate of arthrofibrosis. 6.138 covers arthrofibrosis.

What Changes It

Medial injury grade and whether the corner is involved.

Timing, since early motion favours medial healing and early surgery favours stiffness.

Alignment, which loads the lax side.

Whether the medial laxity persists after a period of healing.

How It Is Measured

Serial valgus stress radiography measures the medial healing over weeks.

Examination under anaesthetic at the time of cruciate reconstruction assesses residual medial laxity.

Robotic testing apportions the restraint in the combined state.

Range of motion at follow up measures the stiffness risk that early combined surgery carries.

Clinical Relevance

Brace and mobilise the medial injury and reassess, rather than operating acutely on both.

Reassess medial laxity under anaesthetic at the time of cruciate reconstruction.

Address the medial side where laxity persists, since it loads the graft.

Weigh the stiffness risk of early combined surgery against the laxity risk of delay.

FRCS Level

At the Knee

Treat the sequencing as the decision and the mechanics as what informs it. Medial healing favours delay and graft protection favours addressing residual laxity, and the two pull against each other.

Conventional teaching resolves it by bracing and mobilising the medial side, reassessing, and reconstructing the cruciate once motion is restored, addressing the medial side at that sitting only if laxity persists. 3.9.8 covers the medial construct.

The exception is the knee that is grossly unstable or has a displaced medial avulsion, where acute repair of the medial side is conventionally accepted. 3.9.3 covers that pattern.

What Changes It

Medial grade, corner involvement and displacement.

Restoration of motion before cruciate surgery.

Alignment, particularly valgus. 3.13 covers alignment.

Patient demand and the timeline that allows.

How It Is Measured

Serial examination and valgus stress radiography measure the medial healing.

Range of motion determines readiness for cruciate reconstruction.

Examination under anaesthetic at surgery determines whether the medial side needs addressing.

Outcome series for this combination are heterogeneous in grade and in timing, which limits comparison.

Clinical Relevance

Brace, mobilise and reassess rather than operating acutely on both.

Restore motion before reconstructing the cruciate.

Address the medial side at that sitting only if laxity persists.

Repair acutely where there is a displaced medial avulsion or gross instability.