Posterior Stability

For Medical Students

The Principle

Stability in a direction means the joint does not travel too far when pushed that way.

If one structure holds almost all of a direction's restraint, losing it produces a large and obvious change.

Where the loss also changes the resting position, the change becomes visible rather than only measurable.

At the Knee

Posterior stability is resistance to the tibia translating backwards relative to the femur. 3.2.8 covers the motion.

The posterior cruciate holds nearly all of it in flexion. Conventional teaching gives it roughly 95 per cent at 90 degrees. 3.8.1 covers the functions.

Losing it lets the tibia sag back under gravity when the knee is flexed, which produces a visible loss of the normal step off at the front of the knee.

Conventional teaching grades the loss by that step off: 0 to 5 millimetres, 5 to 10, and more than 10, which correspond to grades one to three.

What Changes It

Flexion angle, since the ligament's restraint rises with flexion.

Quadriceps activity, which pulls the tibia forward and reduces the sag.

Posterolateral corner integrity, which adds rotational laxity to the picture. 2.10.4 covers the corner.

Partial healing, which reduces the measured laxity over months.

Clinical Relevance

The step off is examined before any drawer, because the drawer is measured from it. 3.8.6 covers the drawer.

The quadriceps active test uses the muscle to pull the sagged tibia forward, which demonstrates the sag mechanically. Section 4 covers the tests.

A grade three injury suggests the corner is involved too, rather than a worse isolated injury.

Patients report difficulty with descending rather than with turning, which follows from the direction. 3.8.7 covers gait.

MRCS Level

The Principle

Grading by step off is grading a position, not an excursion, which makes it unusual among laxity grades. A grade is a statement about where the tibia sits.

That is why the measurement is more reproducible radiographically than by hand: a radiograph records a position and a hand records a judgement about one.

The distinction matters because the starting position is the quantity most affected by muscle activity and limb support.

At the Knee

Conventional teaching gives the normal anterior step off of the medial tibial plateau relative to the medial femoral condyle as around 10 millimetres at 90 degrees of flexion.

Grade one is a reduced but present step off, grade two is a flush plateau, and grade three is a plateau posterior to the condyle.

Posterior restraint is shared differently near extension, where the posterior capsule and the corner contribute more and the cruciate less.

The meniscofemoral ligaments contribute independently, which is why a divided posterior cruciate does not produce unlimited translation. 2.6.3 covers them.

What Changes It

Flexion angle, which changes both the restraint and the gravitational sag.

Muscle activity, which moves the starting position.

Corner integrity, which adds external rotation at 30 degrees.

Healing over time, which reduces the measured laxity.

How It Is Measured

Stress radiography with a standardised posterior load measures the translation against bony landmarks and is the reference clinical measurement.

Clinical grading by step off is reproducible between experienced examiners and depends on limb support and relaxation.

Instrumented arthrometry is less reliable posteriorly than anteriorly, because the start position is uncertain.

Examination under anaesthetic removes the quadriceps contribution and gives the structural figure.

Clinical Relevance

Grade by step off and record the flexion angle and the limb support used.

Use stress radiography where the grade decides the treatment.

Expect an unreliable arthrometer reading posteriorly and prefer radiography.

Suspect corner involvement in a grade three, and test rotation at 30 degrees.

FRCS Level

At the Knee

Treat the grade as the decision variable and the method of obtaining it as the weak point. Stress radiography is the measurement that settles it and is underused.

Conventional teaching holds that a side to side difference above roughly 12 millimetres on stress radiography suggests a combined rather than an isolated injury, which converts a grade into an anatomical inference.

The grade also drives the non operative decision, because partial healing is more likely in a lower grade injury. Section 6 covers management.

What Changes It

Corner and collateral integrity, which raise the grade beyond what the cruciate alone gives.

Alignment, which loads a deficient or reconstructed knee. 3.13 covers alignment.

Time and healing, which lower the measured grade.

Quadriceps capacity, which is modifiable and substitutes dynamically.

How It Is Measured

Stress radiography with a standardised technique is the reference and gives a side to side difference in millimetres.

Examination under anaesthetic is the cleanest structural assessment and is made in a knee that will swell.

Serial measurement captures healing and is more informative than a single reading.

No measurement separates a healed long ligament from a partly reconstructed one in a patient.

Clinical Relevance

Obtain stress radiography rather than grading by hand where the treatment turns on it.

Read a large side to side difference as suggesting a combined injury.

Reassess serially before committing to reconstruction.

Build quadriceps capacity, which substitutes for the restraint.