Contact Mechanics After ACL Injury

For Medical Students

The Principle

A ligament holds a joint in position as well as limiting its travel. Lose the ligament and the resting position changes.

A changed resting position means a changed contact point. The same load now falls on cartilage that was not built for it.

The change is small and constant. A few millimetres, on every step, for years, which is how a mechanical change becomes a biological one.

At the Knee

Losing the anterior cruciate allows the tibia to sit further forward relative to the femur under load, which moves the contact posteriorly on the tibia.

Rotation changes too, because the cruciate restrains the coupled rotation as well as the translation. 3.2.9 covers coupling.

Conventional teaching places the altered contact chiefly in the medial compartment in chronically deficient knees, which matches where degeneration is seen.

The contact area is not necessarily smaller. The problem is that the load falls somewhere different, on cartilage that has not adapted to it.

What Changes It

Whether the knee is loaded, since the shift appears under load rather than at rest.

Associated meniscal injury, which is common and compounds the change. 3.5.5 covers meniscal loss.

Posterior tibial slope, which affects how far the tibia translates.

Activity, since each pivoting episode is an extreme version of the same shift.

Clinical Relevance

Cruciate deficiency leads to degeneration through a contact mechanics route as well as through repeated injury. 6.5 covers chronic deficiency.

Meniscal injury is common alongside it, and the two mechanical problems add. Section 6 covers management.

Reconstruction aims to restore the position as well as the limit, which is why tunnel position matters. Section 7 covers technique.

A patient with a stable feeling but deficient knee still has a mechanical problem, which is worth saying plainly.

MRCS Level

The Principle

Distinguish the acute mechanical change from the chronic one. Acutely, the knee translates further under the same load. Chronically, the secondary restraints stretch and the resting position shifts as well.

Contact mechanics after injury is therefore a moving target, and a figure measured at six weeks does not describe the knee at five years.

The relevant quantity is where the load falls rather than how much there is, which is why this is a contact mechanics problem rather than a load one. 3.5.2 covers pressure.

At the Knee

Cadaveric and in vivo imaging work describes a posterior shift of the tibiofemoral contact point after cruciate sectioning, larger in the medial compartment in most reports.

The medial meniscus posterior horn becomes a secondary restraint to anterior translation, so it is loaded more and a ramp lesion becomes likelier. 6.35 covers ramp lesions.

Conventional teaching holds that reconstruction improves but does not normalise contact position, which is consistent with the persistent degeneration risk after reconstruction.

Increased posterior tibial slope increases the anterior translation for the same load, so it compounds the contact shift. 3.13 covers alignment.

What Changes It

Time since injury, through secondary restraint stretching.

Meniscal and ramp lesion status, which changes the secondary restraint.

Slope, which changes the translation for a given load.

Reconstruction and its tunnel position, which partly restores the position.

How It Is Measured

Cadaveric sectioning with contact sensors measures the acute change under an applied load, without muscle.

In vivo imaging with magnetic resonance or biplanar fluoroscopy during loading measures the position in patients, over limited activities.

Cartilage imaging measures the consequence over years rather than the mechanics. 5.11 covers it.

No clinical measurement of contact position exists, so it is inferred from laxity and from the imaging consequence.

Clinical Relevance

Treat a chronically deficient knee as mechanically different from an acutely injured one, not simply as a later stage.

Look for a ramp lesion in cruciate deficiency, since the posterior horn becomes a secondary restraint.

Measure slope where translation is larger than the injury explains.

Explain that reconstruction improves the contact position without normalising it.

FRCS Level

At the Knee

Treat the contact shift as the mechanical account of post-traumatic degeneration after cruciate injury, and treat it as only partly addressable. 6.77 covers the natural history.

That framing changes what reconstruction is for. It restores stability reliably and contact position partially, so counselling about long term joint health should not rest on the first achievement alone.

Slope correction is the mechanical response where the shift is driven by bony morphology rather than by the graft, and it is the reason a failed graft without a new injury prompts a slope measurement.

What Changes It

Graft position and tension, which set how much of the position is restored.

Meniscal and ramp lesion repair, which restores the secondary restraint.

Slope, correctable by anterior closing wedge osteotomy in selected cases.

Time to reconstruction, since secondary restraints stretch while the knee waits.

How It Is Measured

In vivo kinematic and contact imaging measures whether position was restored, in research cohorts performing prescribed tasks.

Lateral radiography and magnetic resonance imaging measure slope, which is the actionable bony variable.

Long term cohorts measure the degeneration outcome and cannot attribute it to contact position specifically.

No clinical measurement confirms restored contact mechanics in a given patient.

Clinical Relevance

Repair a ramp lesion alongside reconstruction, on secondary restraint grounds.

Measure slope in revision and in a graft that failed without a new injury.

Counsel that stability and long term joint health are different outcomes.

Reconstruct without avoidable delay, since secondary restraints stretch meanwhile.