Knee Stability During Motion

For Medical Students

The Principle

Stability means that the joint stays on its intended path under load. It does not mean stiffness, and a knee can be both mobile and stable.

Four contributors produce it. Bone gives shape, ligaments give passive limits, muscles give active control, and the menisci deepen the surfaces.

The proportions are not fixed. A contributor that dominates near full extension may contribute little in deep flexion, so stability has to be described at an angle.

At the Knee

Near full extension, bone contributes most. The condyles sit in the deepest part of the tibial surface, the joint is compressed, and available rotation is close to zero.

Through mid flexion the bony contribution falls and the ligaments take over. This is the part of the arc where most ligament tests are performed, because it is where the ligaments are the limit being tested.

In deep flexion the soft tissues behind the joint and the compressed posterior surfaces take a larger share again.

Muscle contributes throughout. The quadriceps resists posterior translation and the hamstrings resist anterior translation, which is why a strong limb tests tighter than a weak one.

What Changes It

The flexion angle, which changes which contributor dominates.

Load, because compression holds the surfaces together and makes the bony contribution larger.

Injury to any one contributor, since the others then carry a larger share and may be loaded beyond what they were doing before.

Fatigue and pain, both of which reduce the muscular contribution without changing any structure.

Clinical Relevance

Give a flexion angle with every stability finding, because the contributors change through the arc.

Expect a patient with a ligament injury to improve with rehabilitation without the ligament healing, since the muscular share can be increased. Section 6 covers management.

Treat giving way as a report of the path being lost rather than as a report of pain, and ask what the knee was doing at the time.

Remember that a knee can be unstable in one direction and normal in the others, which is why examination is a set of tests.

MRCS Level

The Principle

Separate primary from secondary restraint. The primary restraint is the structure that takes the load first in a given direction at a given angle, and the secondary ones take it once the primary has failed or elongated.

Secondary restraint is why an injured knee can examine as nearly normal. The finding depends on whether the secondary structures have been recruited, which depends on load and on time.

Static and dynamic stability are separate accounts of the same joint. Static stability is what an anaesthetised knee shows, and dynamic stability is what the patient achieves in function.

At the Knee

Anterior translation has the anterior cruciate as its primary restraint, with the menisci, the collateral structures and the anterolateral capsule as secondary. 3.7 covers the apportionment.

Posterior translation has the posterior cruciate as primary and the posterolateral corner and meniscofemoral ligaments as secondary.

Valgus has the medial collateral as primary, with the posterior oblique fibres taking a larger share as the knee extends.

Varus has the lateral collateral as primary near extension, with the popliteus complex and the iliotibial band contributing as flexion increases.

The handover from bone to soft tissue occurs over roughly the first 30 degrees of flexion, which is why the same test gives different answers at 0 and 30 degrees.

What Changes It

Flexion angle, which reassigns the primary restraint in several directions.

Chronicity, because secondary restraints stretch under repeated load and a knee that compensated early can decompensate later.

Alignment, since a varus limb loads the lateral restraints differently from a neutral one in the same activity.

Neuromuscular control, which decides whether the muscular contribution arrives before the joint has left its path or after.

How It Is Measured

Sequential sectioning in cadavers apportions the passive restraint and gives the primary and secondary language its content. The specimens have no muscle activity.

Instrumented laxity testing measures the passive limit in one direction in a clinic, which is the static account.

Functional testing, hop and landing tasks among them, measures the dynamic account indirectly through performance rather than through joint motion.

Examination under anaesthetic removes the muscular contribution deliberately, which is how a static finding is separated from a dynamic one.

Clinical Relevance

Test at more than one flexion angle, since the primary restraint changes across the arc.

Suspect a combined injury when a finding is larger than the primary restraint alone should allow, because that means a secondary restraint has gone too.

Expect examination under anaesthetic to show more laxity than the clinic, and plan around the larger figure.

Treat the muscular contribution as modifiable and the ligamentous one as not, which is the basis of non operative management in selected injuries.

FRCS Level

At the Knee

Treat stability as a system with redundancy rather than as a set of independent structures. Redundancy is what allows function after injury and what hides the injury on examination.

The cost of redundancy is transferred load. A deficient cruciate loads the menisci and the cartilage of one compartment, which is the mechanical account of why instability has a degenerative endpoint. 6.77 covers that natural history.

Conventional teaching separates the restraints by direction and angle, and the clinically important cases are the ones where more than one direction is involved at once. A multiligament knee is a failure of the system rather than a sum of single injuries.

What Changes It

Bony morphology, slope and condylar shape in particular, which change the demand placed on the soft tissue restraints for the same activity.

Alignment, which can be addressed surgically and therefore changes the stability problem rather than only describing it.

The state of the menisci, which are both restraint and load distributor and therefore appear in both halves of the account.

Previous surgery, since a graft, an osteotomy or a meniscectomy each reassign shares within the system.

How It Is Measured

Robotic and sectioning work gives the passive apportionment, and its order dependence is a stated limitation. 3.2.9 covers the same issue for coupled motion.

In vivo kinematics during function measures the system as it operates, over a narrow range of activities.

Patient reported outcome measures record whether the system is achieving stability in the patient's terms, which is the endpoint the other methods are proxies for.

No single measurement captures stability, so a description combines a passive figure, a functional task and a patient account.

Clinical Relevance

Plan multiligament reconstruction around the system rather than structure by structure, since restoring one restraint changes the load on the others.

Correct alignment where it is loading a reconstruction, on mechanical grounds rather than on appearance.

Preserve meniscal tissue in an unstable knee, because it carries both restraint and load distribution.

Combine a passive measurement, a functional test and the patient's account before judging stability restored.