Partial Knee Replacement

For Medical Students

What It Tests

A partial replacement resurfaces one compartment and leaves the others and the ligaments intact, so the examination covers the implant, the other compartments and the ligaments. 4.18.2 covers established disease.

The examination is checking the range, the alignment, the stability and whether pain is coming from the replaced compartment or from somewhere else. 4.3.3 covers standing alignment.

It matters that the commonest reason for a symptomatic partial replacement is progression elsewhere rather than a problem with the implant. 4.18.5 covers the patellofemoral worn knee.

How It Is Performed

Establish which compartment was replaced, when, and what was done about the alignment. 4.2.30 covers previous surgery.

Look at the scars, which indicate the approach. 4.3.7 covers surgical scars.

Assess the effusion, since a recurrent effusion after a partial replacement is a finding rather than a stage. 4.8.3 covers the bulge sign.

Measure the range, recording any extension deficit, because a partial replacement should restore a good range. 4.7.10 covers the post operative range assessment.

Assess the alignment standing, since a partial replacement does not usually correct a large deformity. 4.3.3 covers standing alignment.

Palpate the other compartments' joint lines and the patellofemoral joint, which is where progression shows. 4.6.4 covers joint line tenderness.

Assess the collateral ligaments, since a partial replacement relies on them. 4.12.1 covers valgus stress testing.

Assess the cruciate function, because an anterior cruciate deficient knee behaves differently with a partial replacement. 4.10.4 covers the Lachman test.

What a Positive Means

A good range, a settled knee, a maintained alignment and quiet unreplaced compartments is the expected result.

Pain and tenderness over an unreplaced compartment or the patellofemoral joint indicates progression, which is the commonest cause of a symptomatic partial replacement. 4.18.5 covers the patellofemoral worn knee.

A changing alignment indicates subsidence, loosening or progression, and the examination is often where the change is noticed first. 4.3.10 covers the deformity.

Collateral laxity after a partial replacement suggests the balance has changed, which affects the implant's loading. 4.12.5 covers the posterior oblique ligament.

A persistent effusion with a quiet examination raises loosening or an inflammatory cause. 4.8.9 covers synovitis.

MRCS Level

How It Is Performed

In a viva this is the symptomatic partial replacement and the expected answer examines the whole knee rather than the implant.

The expected points are the compartment replaced and the date, the scars, the effusion, the range, the standing alignment, all the joint lines, the patellofemoral joint, the collaterals, the cruciate and the function.

Say that you would examine the unreplaced compartments, because progression is the commonest problem. 4.6.4 covers joint line tenderness.

Say that you would assess cruciate function, since the implant depends on it. 4.10.4 covers the Lachman test.

Say that you would examine the hip, because referred pain explains some of these. 4.2.15 covers referred pain.

What a Positive Means

The reasoning expected is that a partial replacement resurfaces one compartment and leaves the knee's own kinematics in place, which is why the ligaments and the other compartments govern the result. 3.5.5 covers the effects of meniscal loss.

The implant is loaded by the alignment, so an uncorrected or a changing deformity loads it abnormally. 3.13.8 covers alignment and arthritis.

A cruciate deficient knee translates abnormally, which loads a medial implant's posterior edge, and that is why the cruciate is assessed. 3.7.8 covers anterior stability.

Progression in an unreplaced compartment produces the original symptoms again, which is why the examination returns to the compartment findings. 4.18.2 covers established disease.

How Good It Is

The examination localises the symptomatic compartment reliably and says nothing about fixation.

Its best contribution is separating progression from an implant problem by localisation, which is the first question. 4.6.4 covers joint line tenderness.

It cannot assess loosening, wear or component position.

Radiographs assess the implant and the compartments, so the examination localises and the imaging explains. 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 separates progression, loosening, instability and a cause outside the knee, and each leads somewhere different.

Pain localised to an unreplaced compartment with tenderness and an effusion is progression, and the examination establishes it. 4.6.4 covers joint line tenderness.

Diffuse pain with a normal localising examination raises loosening or infection, and the examination's negative result is what raises them. 4.8.9 covers synovitis.

Instability with a well aligned implant raises ligament imbalance or cruciate deficiency, and both are clinical findings. 4.12.1 covers valgus stress testing.

A changing deformity on serial examination is a structural finding, and the serial record is the clinical evidence. 4.1.8 covers examination documentation.

Where It Misleads

The commonest error is examining the implant and not the other compartments. 4.6.4 covers joint line tenderness.

The patellofemoral joint is omitted, and it is a common site of progression. 4.18.5 covers the patellofemoral worn knee.

Diffuse pain is attributed to progression when the localising examination does not support it. 4.2.8 covers the location of pain.

Hip disease is not excluded, and it reproduces the pattern. 4.2.15 covers referred pain.

An effusion is read as mechanical when it is the only sign of a low grade infection. 4.8.9 covers synovitis.

Fellowship Level

Where It Misleads

At fellowship level the examination is deciding whether a symptomatic partial replacement needs revising, and the clinical findings answer only part of that.

Progression and loosening both produce pain on loading, and the localisation separates them only when the progression is in a different compartment. 4.6.4 covers joint line tenderness.

An implant can be well fixed and malpositioned, which produces pain without any clinical sign the examination can name. 4.3.3 covers standing alignment.

Low grade infection presents with pain, stiffness and an effusion, which is the same presentation as every mechanical cause, so the examination cannot exclude it. 4.8.9 covers synovitis.

Ligament balance after a partial replacement is assessed against the operated knee's own baseline, and that baseline is often not recorded. 4.1.8 covers examination documentation.

Pain with a normal examination after a partial replacement is a recognised and unresolved problem, and the examination's negative result is the finding rather than a failure of technique. 4.2.10 covers severity.