granulomatous pseudotumour related to joint replacements

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Metal-on-metal pseudotumors represent mass-forming inflammation around a metal-on-metal hip or knee replacement. The term describes one presentation on the spectrum of adverse reaction to metal debris.

Clinical presentation

Metal-on-metal pseudotumors are large focal solid or semiliquid masses around the hip (or knee) prostheses. The pseudotumors mimic local effects of neoplasia or infection in the absence of either of these. The principal symptom is pain. There may be restricted range of movement with large pseudotumors.

Epidemiology

In patients with metal-on-metal hip arthroplasty, diagnostic imaging studies (ultrasound and MRI) show the incidence of asymptomatic pseudotumors at the level of 27-32% . The incidence of symptomatic pseudotumors following metal-on-metal hip arthroplasty is approximately 5%.

Patients with bilateral resurfacing total hip replacement who develop a pseudotumor in one hip have a one in three chance of having a lesion on the contralateral side.

Pseudotumors are more common in females.

Pathology

The pathophysiology is poorly understood. Metal-on-metal pseudotumors are sterile inflammatory lesions.

Excessive wear is considered the initiating process, leading to the release of particles (nanometer-sized). These are cytotoxic to macrophages once phagocytosed, therefore leading to necrosis within the lesions.

Reactive masses are related to high serum and joint fluid ion levels, and a delayed type IV hypersensitivity reaction has been implied.

Radiographic features

Ultrasound

Sonographic features are non-specific but may show:

mass of heterogenous echogenicity
without internal power or color Doppler signal
fluid components

MRI

Metal artifact reduction sequences (MARS) may be helpful for evaluation,

Posterolateral lesions

located at posterolateral aspect of the joint, often in continuity with the greater trochanter
typically cystic in nature
frequently with layering of contents, and a low signal intensity wall
foci of susceptibility artefact due to metal content
representing extension through posterior capsular defects (typical surgical approach in hip arthroplasty

Anterior lesions

less common
typically involving the iliopsoas bursa
solid components are more likely
typical: contiguous with the joint capsule (representing distension of the iliopsoas bursa)

Arthrography

may more definitively reveal a connection between the periprosthetic collection and the joint space
aspiration of the collection will reveal elevated cobalt and chromium ion levels (may be elevated in serum as well)

Signal characteristics

T1 C+ (Gd): generally show no enhancement

Differential diagnosis

Imaging differential diagnosis in MRI

There are two important diagnostic alternatives:

Infection

less well defined than pseudotumors
lack of a low signal intensity rim

Soft-tissue edema can be seen with both infection or pseudotumor. Extensive perifascial fluid is more suggestive of infection.

Abductor tendon avulsion-associated fluid collections

pure fluid signal
lack of a low signal intensity rim
typical: location at the site of abductor avulsion

Pseudotumors can coexist with abductor tendon avulsion following hip arthroplasty.