Total Knee Replacement

For Medical Students

What It Tests

After a total replacement the examination is checking the range, the alignment, the stability, the extensor mechanism and the wound, and each is read against the replaced knee rather than against a normal one. 4.18.7 covers the arthroplasty assessment.

The examination's main question in a symptomatic replacement is whether the problem is mechanical, infective or outside the knee. 4.8.9 covers synovitis.

It matters that a replaced knee's normal findings include a reduced range and a different end feel, so those are not abnormalities. 4.7.5 covers the end feel.

How It Is Performed

Establish when the replacement was done, what type, and what the recovery has been like. 4.2.30 covers previous surgery.

Look at the wound and the surrounding skin for healing, discharge and warmth. 4.3.6 covers skin changes.

Assess the temperature and the effusion, since both raise infection in a replaced knee. 4.6.2 covers temperature assessment.

Measure the range, recording the extension deficit and the flexion separately. 4.7.10 covers the post operative range assessment.

Assess the alignment standing, since the replacement should have corrected it. 4.3.3 covers standing alignment.

Assess stability in extension and in flexion, because a replaced knee can be stable in one and lax in the other. 4.12.1 covers valgus stress testing.

Assess the extensor mechanism, including the ability to hold the leg straight and the patellar tracking. 4.15.1 covers patellar tracking.

Assess function and the walking distance, since the operation was done for function. 4.7.9 covers the functional range.

What a Positive Means

A healed wound, a corrected alignment, a good range, a stable knee and improved function is the expected result.

Warmth, an effusion and pain raise infection, which is the finding that changes everything else. 4.6.2 covers temperature assessment.

Laxity in flexion with stability in extension is a recognised pattern and it produces instability on stairs. 4.12.1 covers valgus stress testing.

An extension deficit indicates a block or a contracture, and it is the deficit patients notice. 4.5.4 covers the flexion deformity.

Anterior pain with maltracking raises a patellofemoral cause, which is a common source of pain after replacement. 4.15.9 covers patellofemoral pain.

MRCS Level

How It Is Performed

In a viva this is the painful total knee replacement and the expected answer rules out infection before anything else.

The expected points are the history, the wound and skin, the temperature and effusion, the range with the deficit recorded, the alignment, stability in both positions, the extensor mechanism, the function and the hip and spine.

Say that you would rule out infection first and name the findings that raise it. 4.6.2 covers temperature assessment.

Say that you would assess stability in flexion as well as in extension, and give the reason. 4.12.1 covers valgus stress testing.

Say that you would examine the hip and the spine, since referred pain accounts for a proportion of painful replacements. 4.2.15 covers referred pain.

What a Positive Means

The reasoning expected is that a replacement substitutes implant geometry for the knee's own articular geometry, so the ligaments and the component positions together determine the stability. 3.13.10 covers alignment correction principles.

Flexion stability depends on the gap the components create in flexion, which is why a knee can be stable extended and lax flexed. 3.3.1 covers the introduction to kinematics.

A replaced knee has no cruciate contribution in many designs, so the examination does not test them. 3.7.1 covers the functions of the cruciate.

Infection changes the whole assessment, which is why it is excluded first and not last. 4.8.9 covers synovitis.

How Good It Is

The examination establishes the range, the alignment, the stability and the extensor function reliably in a replaced knee.

Its best contribution is the infection assessment, since the clinical findings start that pathway and a delay is costly. 4.6.2 covers temperature assessment.

It cannot assess loosening, wear, component rotation or the cause of pain in a well functioning replacement.

Radiographs, blood tests and aspiration carry those questions, so the examination triages rather than diagnoses here. 4.1.6 covers that relationship.

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 examination sorts a painful replacement into infection, instability, stiffness, extensor problems, and pain without a knee cause.

Warmth, effusion, rest pain and a poor recovery from the start point to infection, and the examination is the first filter. 4.6.2 covers temperature assessment.

Instability in flexion with a good range and a corrected alignment is a component or balance problem, and the examination defines it. 4.12.1 covers valgus stress testing.

Stiffness with a quiet knee and an early loss of range is a different problem again, and the end feel contributes. 4.7.5 covers the end feel.

Pain with a normal examination, a well aligned stable implant and a normal extensor mechanism is the group the examination identifies by exclusion, and it is the hardest group. 4.2.10 covers severity.

Where It Misleads

The commonest error is assessing a replaced knee against a normal knee's expected findings. 4.7.10 covers the post operative range assessment.

Infection is excluded on the basis of a normal temperature, which a low grade infection does not produce. 4.6.2 covers temperature assessment.

Flexion instability is missed because stability is tested only in extension. 4.12.1 covers valgus stress testing.

The extensor mechanism is not examined, so an extensor problem is called instability. 4.15.1 covers patellar tracking.

The hip and the spine are not examined, and both refer to the replaced knee. 4.2.15 covers referred pain.

Fellowship Level

Where It Misleads

At fellowship level the examination of a painful replacement is mostly a process of exclusion, and its limits define the referral pathway.

The examination cannot exclude low grade infection, and no combination of clinical findings does, which is why the pathway is laboratory and aspiration rather than clinical. 4.8.9 covers synovitis.

Component rotation produces pain and maltracking and is not clinically measurable, so the examination finds the consequence without the cause. 4.15.1 covers patellar tracking.

Flexion instability is a clinical diagnosis with no reliable test, so the history of instability on stairs carries as much weight as the examination. 4.2.2 covers instability.

Pain with a normal examination in a well functioning replacement is common enough to be its own category, and labelling it is a judgement rather than a finding. 4.2.10 covers severity.

Patient expectation shapes the reported result more than any clinical finding does, which puts the most predictive variable outside the examination. 4.22.8 covers the use of these scores.