Metal-on-metal pseudotumours 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.
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Clinical presentation
Metal-on-metal pseudotumours are large focal solid or semiliquid masses around the hip (or knee) prostheses. The pseudotumours mimic local effects of neoplasia or infection in the absence of either of these. The principal symptom is pain, with a restricted range of motion also likely with larger pseudotumours.
Epidemiology
In patients with metal-on-metal hip arthroplasty, diagnostic imaging studies (ultrasound and MRI) show the incidence of asymptomatic pseudotumours at the level of 27-32% 6. The incidence of symptomatic pseudotumours following metal-on-metal hip arthroplasty is approximately 5%.
Patients with bilateral resurfacing total hip replacement who develop a pseudotumour in one hip have a one in three chance of having a lesion on the contralateral side.
Pseudotumours are more common in females.
Pathology
The pathophysiology is poorly understood. Metal-on-metal pseudotumours are sterile inflammatory lesions.
Excessive wear is considered the initiating process, leading to the release of particles (nanometre-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 colour 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 artifact 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 5
Differential diagnosis
Imaging differential diagnosis in MRI
There are two important diagnostic alternatives:
Infection
less well defined than pseudotumours
lack of a low signal intensity rim
Soft-tissue oedema can be seen with both infection or pseudotumour. 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
Pseudotumours can coexist with abductor tendon avulsion following hip arthroplasty.