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.
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.
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% 6. 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.
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.
Sonographic features are non-specific but may show:
- mass of heterogenous echogenicity
- without internal power or color Doppler signal
- fluid components
Metal artifact reduction sequences (MARS) may be helpful for evaluation,
- 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)
- T1 C+ (Gd): generally show no enhancement 5
Imaging differential diagnosis in MRI
There are two important diagnostic alternatives:
- 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.
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