A metal-on-metal pseudotumour, also known as aseptic lymphocyte-dominant vasculitis-associated lesion (ALVAL), is a mass-forming tissue reaction around a metal-on-metal hip or knee replacement.
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. There may be restricted range of movement with large pseudotumours.
The incidence of symptomatic pseudotumours following metal-on-metal hip arthroplasty is in the region of 5%.
Patients with bilateral resurfacing total hip replacement who develop a pseudotumour in one hip have a 1 in 3 chance of having a lesion on the contralateral side.
Pseudotumours are more common in females.
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 (nanometer sized). These are cytotoxic to macrophages once phagocytised, 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.
Sonographic features are non-specific but may show:
- mass of heterogenous echogenicity
- without internal power or colour Doppler signal
- fluid components
- 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
- less common
- typically involving the iliopsoas bursa
- solid components are more likely
- typical: contiguous with the joint capsule (representing distension of the iliopsoas bursa)
- 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)
Imaging differential diagnosis in MRI
There are two important diagnostic alternatives.
- less well defined than pseudotumours
- lack of a low signal intensity rim
Soft-tissue oedema 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
Pseudotumours can coexist with abductor tendon avulsion following hip arthroplasty.
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