Dynamic PCL Examination

For Medical Students

What It Tests

Dynamic examination looks at the posterior cruciate deficient knee while it is loaded and moving, rather than while it is lying still on a couch.

The reason is the same reason the history matters. A graded laxity is a property of a relaxed joint, and the patient's problem is a property of a working one. 4.2.2 covers instability as a symptom.

The posterior cruciate is unusual here. Its deficiency is compensated well by muscle in many patients and badly in a few, and nothing in the resting examination separates the two. 4.11.6 covers the chronic knee.

How It Is Performed

Watch the patient walk, before touching the knee and before they are undressed if possible.

Look for a flexed knee pattern, where the knee avoids the last part of extension, and for a varus thrust, which suggests the lateral structures rather than the cruciate alone. 4.13.4 covers the corner.

Ask the patient to squat, and watch whether the knee is willing to go to full flexion and whether they shift weight away from it.

Ask them to step down from a low step, leading with the affected leg. Descent loads the extensor mechanism against gravity and is the task most patients name when they describe the problem. 4.2.6 covers giving way.

Watch the quadriceps while they do it, because visible wasting and a reluctance to load are the two findings that predict how the knee will behave. 4.11.6 covers that.

What a Positive Means

A knee that avoids terminal extension in gait, shifts weight in descent and has a wasted quadriceps is a knee whose compensation is failing, whatever the drawer grade says.

A knee with a third degree drawer that walks normally, squats fully and descends a step without hesitation is a knee that has compensated, and the examination has just told you something the grade could not. 4.11.2 covers the drawer.

A varus thrust in gait moves the problem beyond the cruciate, because the cruciate does not resist varus. 4.11.7 covers the combined knee.

Pain at the front of the knee during descent, rather than instability, points at the patellofemoral joint, which is where this deficiency sends its load. 4.11.6 covers that consequence.

MRCS Level

How It Is Performed

In a viva the dynamic assessment is what separates a candidate who has examined these knees from one who has learned the tests, and it is usually asked as what else would you do.

The expected points are gait before couch, the flexed knee pattern, the varus thrust, the squat, the step down, and the quadriceps.

Say that you would examine the hip and the ankle in a patient with an abnormal gait, because a thrust can come from above or below the knee. 4.1.4 covers the structured approach.

Say that you would ask the patient to show you the movement that causes the problem, which is the single most useful instruction in this part of the examination.

Say that you would record what the patient could do rather than only what the joint allowed, since that is what the next clinician needs. 4.1.8 covers the record.

What a Positive Means

The reasoning expected is that the posterior cruciate resists posterior translation and that the quadriceps, pulling through the patellar tendon, produces an anterior force that substitutes for it through part of the flexion range. 3.8.2 covers posterior stability.

That substitution is why quadriceps strength predicts function better than laxity grade does, and why rehabilitation is the first treatment for most isolated injuries.

The substitution is weakest in deep flexion and in loaded descent, where the tendon angle is least favourable, which is why the step down is the task that exposes the knee. 4.11.3 covers the same geometry used as a test.

Gait following injury changes with time rather than immediately, so a recently injured knee walks better than a chronically deficient one with the same grade. 3.8.7 covers gait after injury.

How Good It Is

Dynamic examination is the part of this chapter that correlates best with what the patient complains about and worst with what the ligament is doing.

That is its value rather than its weakness. The static tests grade the ligament accurately and the dynamic assessment grades the problem, and the chapter needs both.

It is the least standardised part of the examination, so it is the least comparable between clinicians and the least suited to a research protocol.

Patient reported outcome scores capture some of the same ground more reproducibly, and they capture it at a visit rather than in the clinic room. 4.22.8 covers their use in practice.

No figures appear for any of these comparisons, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the dynamic assessment is close to being the decision, which is unusual and is specific to this ligament.

An isolated posterior cruciate injury is managed non operatively in most patients, and what selects the minority for reconstruction is function rather than grade. The dynamic examination is the assessment of function.

A patient with a wasted quadriceps who has not completed rehabilitation is not a patient whose knee has failed non operative treatment, and the distinction decides whether the next step is a physiotherapist or an operation.

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. 4.11.7 covers the assessment.

Where the dynamic findings and the grade disagree, the dynamic findings decide the plan and the grade decides the technique.

Where It Misleads

The commonest error is examining the knee on the couch and never watching the patient move, which is also the commonest reason a clinic letter describes a knee the patient does not recognise.

The second is attributing every abnormal gait to the knee in front of you. A flexed knee pattern can be hip flexion contracture or an ankle problem.

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

The fourth is reading an abnormal descent as cruciate instability when it is patellofemoral pain, which is common in these knees and is treated differently.

Fellowship Level

Where It Misleads

In referred practice the dynamic examination is often the only part of the assessment nobody has done, and it is frequently where the answer is.

The commonest referral in this chapter is a patient with a reconstructed posterior cruciate, an acceptable examination and a knee they do not trust. Watching them descend a step usually separates a mechanical problem from a confidence problem, and the two need opposite treatments. 4.22.6 covers the score built for that question.

A patient whose gait has deteriorated since reconstruction, with an unchanged drawer, is pointing at the corner or at early medial compartment change rather than at the graft. 4.11.6 covers the progression.

Quadriceps wasting that has not recovered after a technically sound reconstruction is the finding most predictive of persistent complaint, and it is also the finding most often recorded as present and then not acted on.

A knee that looks worse walking than it measures on the couch, with no corner signs and no wasting, should raise the question of whether the complaint is the knee at all. 4.2.15 covers referred pain.

In the elite athlete the dynamic examination has to be done at the intensity the sport demands, because a knee that passes a step down can still fail a deceleration, and the clinic cannot show that. 4.23.2 covers those patients.