PCL Deficient Gait

For Medical Students

What It Tests

A posterior cruciate deficient knee walks with an increased reliance on the quadriceps, because the quadriceps pulls the tibia forwards and the deficient ligament is what was holding it there. 3.14.10 covers the biomechanics.

This is the opposite of the anterior cruciate deficient gait, where the quadriceps pull is the problem and the knee avoids it. 4.4.3 covers that pattern.

The consequence is that a posterior cruciate deficient knee walks well while the quadriceps is strong and badly when it is not, which is why the muscle rather than the laxity predicts function. 4.11.5 covers the assessment.

How It Is Performed

Watch the patient walk from the side at knee level over a repeating distance.

Look for a knee that avoids the last part of extension and walks in slight flexion, which is the pattern in the deficient knee with a weak quadriceps.

Look for a varus thrust, since a posterior cruciate injury with a posterolateral corner injury produces one and the corner changes the diagnosis. 4.13.4 covers the corner.

Watch the patient descend a step, because descent is where the quadriceps works hardest against gravity and where this knee fails first. 4.11.5 covers it.

Assess the quadriceps bulk and strength, since the gait follows the muscle. 4.3.9 covers wasting.

What a Positive Means

A knee avoiding terminal extension in a patient with a posterior sag indicates a knee whose compensation is failing. 4.11.1 covers the sag.

A normal gait in a patient with a grade three posterior drawer indicates compensation, and it is common. 4.11.2 covers the drawer.

A varus thrust changes the diagnosis from an isolated cruciate injury to a combined one, whatever the drawer grade says. 4.11.7 covers that assessment.

Difficulty descending a step with a normal walking gait is the characteristic functional finding, and it is often the patient's only complaint. 4.2.11 covers aggravating factors.

Anterior knee pain in a patient with this gait is a consequence of the altered contact mechanics rather than a separate problem. 4.11.6 covers the chronic knee.

MRCS Level

How It Is Performed

In a viva this is the gait to name when asked why posterior cruciate injuries are treated non operatively, and the mechanism is the answer.

The expected points are the side view, the avoidance of terminal extension, the search for a varus thrust, the step descent, and the quadriceps assessment.

Say that the quadriceps is the compensating mechanism and that rehabilitation is therefore the first treatment, since that is the clinical consequence of the mechanism. 4.11.5 covers it.

Say that you would look for a thrust deliberately, because a corner injury changes the management and the gait is where it is first visible. 4.13.1 covers varus stress testing.

Say that gait changes with time rather than immediately, so a recent injury walks better than a chronic one of the same grade. 3.14.10 covers the gait.

What a Positive Means

The reasoning expected is that the patellar tendon's line of pull is anterior to the tibia through much of flexion, so quadriceps contraction reduces a posteriorly subluxed tibia. 4.11.3 covers the test built on that.

The substitution is weakest in deep flexion and in loaded descent, where the tendon angle is least favourable, which is why those are the tasks that fail. 3.3.12 covers kinematics during stair climbing.

A knee walking in slight flexion is avoiding the position where the posterior cruciate would normally be tightest, which is the inverse of the anterior cruciate deficient knee's strategy. 3.14.9 covers that gait.

The altered contact position loads the patellofemoral joint and the medial compartment, which is the mechanism behind the anterior pain and the late degenerative change. 4.11.6 covers that progression.

How Good It Is

The gait assessment correlates with function in this knee better than any laxity measurement, which is the strongest version of a claim this section makes repeatedly.

It is not specific. A knee walking in slight flexion does so for pain, for effusion, for a flexion deformity and for quadriceps weakness as well. 4.5.4 covers flexion deformity.

The varus thrust is the most specific gait finding here, because the cruciate alone does not produce it.

It is the part of this chapter's assessment that changes with treatment, which makes it the most useful thing to follow. 4.4.7 covers post-operative gait.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the gait is close to being the decision in an isolated posterior cruciate injury, which is unusual and specific to this ligament.

Most of these injuries are managed non operatively, and what selects the minority for reconstruction is function rather than grade. The gait and the step descent are the functional assessment. 4.11.6 covers the argument.

A varus thrust changes the operation rather than the decision to operate, since a corner that is not addressed will stretch whatever is done to the cruciate. 4.13.8 covers combined injuries.

A patient with a wasted quadriceps who has not completed rehabilitation has not failed non operative treatment, and the gait is the evidence of that. 4.3.9 covers wasting.

Where the gait and the grade disagree, the gait decides the plan and the grade decides the technique, which is this section's recurring division. 4.11.2 covers the grade.

Where It Misleads

The commonest error is not watching the gait at all, which leaves the decision resting on a grade that does not predict function.

The second is attributing the flexed knee pattern to the cruciate when it is a flexion deformity or an effusion.

The third is missing a varus thrust, which is the finding that changes the operation.

The fourth is reading a normal gait as a normal knee, when the demand that fails may be above walking. 4.4.9 covers single leg landing.

The fifth is calling rehabilitation failed in a patient who has not had it, which is the error that generates the avoidable reconstructions in this chapter.

Fellowship Level

Where It Misleads

In referred practice this gait is usually being assessed in a knee that has already been reconstructed, and the question is what changed.

A patient whose gait has deteriorated with an unchanged drawer is developing compartment change or has a corner that has stretched, and neither is a graft problem. 4.11.6 covers the progression.

A patient with a varus thrust after a posterior cruciate reconstruction has an unaddressed corner or an uncorrected alignment, and the revision is decided by which. 4.13.4 covers the corner.

A knee with a fixed posterior subluxation walks in a characteristic way and measures deceptively well, because the drawer has nowhere to go, and the gait is the more honest assessment. 4.11.4 covers that failure.

An incompletely recovered common peroneal nerve palsy produces a thrust that is not ligamentous, and in a knee with corner surgery that distinction has to be made before any revision. 4.13.4 covers it.

In the athlete the walking gait is normal and the failing task is deceleration or a loaded descent at speed, which the clinic cannot reproduce. 4.23.2 covers those patients.