Functions of the ACL

For Medical Students

The Principle

A ligament is a rope between two bones. It does nothing until it is tight, and then it stops the bones separating any further in that direction.

A rope running diagonally across a joint limits more than one motion, because moving in several directions can stretch the same rope.

So a ligament has a primary direction, the one it limits first, and secondary ones it contributes to.

At the Knee

The anterior cruciate runs from the back and outside of the femoral notch to the front and middle of the tibia. It therefore resists the tibia moving forward. 2.7.1 covers the anatomy.

That is its primary function and it is the one worth remembering: the primary restraint to anterior translation of the tibia. 3.7.2 covers it.

It contributes secondarily to rotational control, because rotation also stretches it. 3.7.7 covers that.

It carries nerve endings, so it contributes to position sense and to reflex muscle control. 2.7.5 covers the innervation.

What Changes It

Flexion angle, which changes how tight the ligament is and which part of it is tight. 3.7.4 covers the bundles.

Muscle activity, since the quadriceps pulls the tibia forward and the hamstrings pull it back.

The state of the other structures, which share the secondary restraint roles. 3.7.3 covers them.

Bony shape, particularly the backward slope of the tibia, which affects how much the tibia tends to slide.

Clinical Relevance

A torn anterior cruciate leaves a knee that can walk and gives way on turning, which follows from the primary and secondary functions.

The sensation patients describe is rotational rather than a forward slide, because the coupled rotation is what they feel. 3.2.9 covers coupling.

Reconstruction aims at the primary function and achieves the secondary ones less completely. 3.7.20 covers the construct.

Knowing the primary function makes the examination findings predictable. Section 4 covers the tests.

MRCS Level

The Principle

Primary restraint means the structure that takes the load first in a given direction at a given angle. Secondary means the structures that take it once the primary has gone. 3.2.10 covers the distinction.

A ligament's function is therefore directional and angle dependent, and both have to be stated for a claim about it to mean anything.

Function is measured by taking the structure away and remeasuring, which is why the apportionment literature is a sectioning literature.

At the Knee

Conventional teaching gives the anterior cruciate roughly 85 per cent of the restraining force to an anterior drawer in the intact knee, which is the figure the primary designation rests on.

It is a secondary restraint to internal rotation and to varus and valgus angulation, which is why a complete tear changes more than one examination finding.

It guides kinematics as well as limiting them. It contributes to the screw home mechanism and to the coupled rotation of flexion. 3.3.5 covers the mechanism.

Its mechanoreceptors contribute to reflex hamstring activation, so the functional loss after rupture is partly neuromuscular. 3.6.8 covers the comparable meniscal role.

What Changes It

Flexion angle, through bundle tension. 3.7.5 and 3.7.6 cover the bundles.

Quadriceps and hamstring activity, which load and unload it respectively.

Posterior tibial slope, which increases the anterior shear for a given compressive load.

Meniscal and anterolateral integrity, which share the secondary roles. 3.7.3 covers them.

How It Is Measured

Sequential sectioning in cadavers with a robotic or manual rig measures the apportionment, with the sectioning order affecting the figures.

In vivo strain measurement with an implanted transducer measures what the ligament actually experiences, in a small number of volunteer studies. 3.7.10 covers strain.

Instrumented laxity testing measures the clinical consequence of losing it rather than its function.

Histology demonstrates the mechanoreceptors, establishing the sensory role without quantifying it.

Clinical Relevance

State a direction and a flexion angle with any claim about what the ligament does.

Expect a complete tear to change several examination findings, through the secondary roles.

Treat the functional loss as partly neuromuscular rather than purely mechanical.

Read an apportionment figure with its sectioning order in mind.

FRCS Level

At the Knee

Treat the four functions as unequally restorable. A graft restores the anterior restraint well, the rotational contribution partly, the kinematic guidance less, and the sensory role hardly at all.

That ranking explains the commonest disappointment after a technically successful reconstruction: a stable knee that does not feel normal. 3.7.20 covers the construct.

It also explains why outcome depends on rehabilitation as much as on fixation. The functions a graft does not restore are the ones muscle has to cover. 3.4.7 covers internal moments.

What Changes It

Tunnel position, which decides how much of the native function the graft reproduces.

Graft choice and its material properties. 3.7.16 covers grafts.

Concurrent injury to the secondary restraints, which changes what the graft is being asked to do alone.

Rehabilitation and neuromuscular recovery, which cover the functions the graft cannot.

How It Is Measured

Robotic testing of reconstructed cadaveric knees compares a construct against the intact state in all six degrees of freedom.

In vivo kinematics measures whether motion was restored, in research cohorts performing prescribed tasks.

Arthrometry and pivot shift measure the clinical restraint, the first reproducibly and the second without an agreed threshold.

Patient reported outcome measures whether the knee feels normal, which is the function no instrument captures.

Clinical Relevance

Counsel that stability and normality are different outcomes, and name which the operation restores.

Address the secondary restraints where they are also injured, rather than relying on the graft.

Invest in neuromuscular rehabilitation, since it covers what the graft does not.

Read a construct study by which of the four functions it measured.