Primary Restraints

For Medical Students

The Principle

When several structures could resist a movement, one of them usually takes most of the load. That one is the primary restraint.

Being primary is a statement about order as well as amount. The primary restraint is loaded first, and the others only once it has stretched or failed.

A door held by one strong bolt and three weak hooks is the picture. The bolt is the primary restraint, and the hooks matter only once it is gone.

At the Knee

The anterior cruciate is the primary restraint to the tibia translating forward on the femur. 3.2.7 covers anterior translation.

Conventional teaching gives it roughly 85 per cent of the resisting force in the intact knee, with the remainder shared by the other structures.

That share is why a complete tear produces a large increase in translation, and why the increase is detectable on examination. Section 4 covers the tests.

Other directions have other primaries. The posterior cruciate is primary for posterior translation and the collaterals for coronal angulation. 3.8 and 3.9 cover them.

What Changes It

Flexion angle, which changes how much of the ligament is tight.

Whether the secondary restraints are intact, since they decide what happens after the primary goes. 3.7.3 covers them.

Muscle activity, which can substitute for a lost primary restraint partly.

Load, since compression holds the joint together and reduces translation.

Clinical Relevance

A large increase in anterior translation means the primary restraint has gone, which is why the finding is reliable.

A smaller increase means the ligament is partly torn or that guarding is reducing the measurement. Section 4 covers the tests.

Side to side comparison matters more than the absolute figure, because individual variation is large.

Reconstruction is an attempt to replace a primary restraint, which is a different task from augmenting a secondary one.

MRCS Level

The Principle

Apportionment is measured by subtraction. A structure's share is the drop in resisting force when it is cut, in a specimen loaded in a chosen direction at a chosen angle.

The method has a known weakness: the order of sectioning changes the figures, because cutting one structure changes the load the next one sees. 3.2.9 covers that limitation.

So a primary designation is robust and a percentage is not. The anterior cruciate is clearly primary for anterior translation; the exact share depends on the protocol.

At the Knee

Conventional teaching gives the anterior cruciate roughly 85 per cent of the restraint to an anterior drawer at 90 degrees, and a comparable or greater share at 30 degrees.

Its share is greatest in the mid range, where the bony contribution to stability has fallen away. 3.3.8 covers the zones.

Near full extension the geometry and the other structures contribute more, which is part of why a test at 30 degrees is more sensitive than one at 0.

For internal rotation it is a secondary rather than a primary restraint, with the anterolateral structures and the menisci sharing the role. 3.11 covers the anterolateral complex.

What Changes It

Flexion angle, which is the largest factor in the share.

Sectioning order in the study being quoted.

Applied load direction, since a combined load distributes differently from a pure one.

Specimen age and condition, which affect the tissue properties being measured.

How It Is Measured

Robotic universal force moment sensor testing applies a known load and records the force in each structure by subtraction, which is the current standard.

Manual sectioning with a laxity measurement is the older method and gives displacement rather than force.

In situ force measurement with a buckle or transducer on the ligament measures it directly and disturbs the mechanics by being there.

No in vivo measurement of a ligament's share exists in a patient.

Clinical Relevance

Rely on the primary designation and treat the percentage as protocol dependent.

Test at 30 degrees where sensitivity matters, since the share is higher in mid range.

Expect a combined injury to distribute load differently from the sectioning studies.

Describe the anterior cruciate as a secondary restraint to rotation, not a primary one.

FRCS Level

At the Knee

Treat the primary and secondary language as the framework for deciding what a reconstruction has to include. Replacing a primary restraint is one operation; replacing a primary and augmenting a secondary is another.

The anterolateral augmentation debate is exactly this question. If rotational control is shared, a single intra-articular graft restores part of it, and whether to add to that is a question about the share. 3.11 covers it.

Conventional teaching holds that a well placed single bundle graft restores anterior translation to near normal and rotational control less completely, which is the apportionment argument applied to a construct.

What Changes It

Tunnel position, which determines how much of the native restraint the graft reproduces.

Whether secondary restraints are injured, which decides what the graft carries alone.

Bony morphology, particularly slope, which sets the load the restraint meets. 3.13 covers alignment.

Graft tension at fixation, which sets the angle at which the construct becomes the limit.

How It Is Measured

Robotic testing of reconstructed knees apportions the restored restraint and compares constructs under identical loads.

Instrumented pivot shift measures the rotational result clinically, without an agreed threshold.

Registry and cohort reinjury rates measure the clinical consequence of an incompletely restored share.

The sectioning order limitation applies to every apportionment figure a construct argument rests on.

Clinical Relevance

Decide whether to augment by asking which share is not being restored.

Assess the secondary restraints before relying on a single graft to carry rotation.

Measure slope where the restraint is meeting an excessive load.

Read construct comparisons with the sectioning order limitation in mind.