Sunday, April 28, 2013

Metal-on-metal pseudotumour

Radiopaedia

metal-on-metal pseudotumour is a mass-forming tissue reaction around a metal-on-metal hip (or knee) replacement.

Clinical presenation

Metal-on-metal pseudotumours are large focal solid or semiliquid masses around the hip (or knee) prostheses. The pseudotumours mimick 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.

Epidemiology

Incidence of symptomatic pseudotumours following metal-on-metal hip arthroplasty is in the region of 5%.
Patients with bilateral resurfacing THR 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.

Pathology

The pathophysiology is poorly understood. Metal-on-meta; pseudtumours are sterile inflammatory lesions, also termed ‘aseptic lymphocyte-dominated vascular-associated lesions’
Excessive wear is considered the initiating process, leading to the release of particles (nanometer sized). These are cytotoxic to macrophages once phagozytised, therefore leading to necrosis within the lesions.
Reactive masses are related to high serum and joint fluid ion levels, and a delayed typoe 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
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)

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 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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