Spontaneous tibial insert dislodgement in unicompartmental knee prosthesis

Case contributed by Dr Chris O'Donnell

Presentation

Sudden severe left knee pain. Previous medial unicompartmental knee prosthesis insertion.

Patient Data

Age: 70 years
Gender: Female

Two linear radio-opaque markers outline the polyethylene insert that normally sits on the tibial component of the unicompartmental (medial) knee prosthesis. 

The markers are anteromedial to the articulation with loss of the lucent articular gap, indicating insert dislodgement.

In a previous X-ray exam, the markers were symmetrically positioned between the femoral and tibial components with an associated lucent gap - this is normal.

Case Discussion

Polyethylene spacers or inserts are inserted between the 2 components of a prosthesis for articulation, much like articular cartilage. They are attached to the tibial component in a knee prosthesis and are prone to wear over time and can lead to foreign body reaction (particle disease or aggressive granulomatosis). Rarely they can dislodge. 

Polyethylene is radiolucent; thus, radio-opaque markers are inserted by the manufacturer so that the position of the spacer could be monitored.

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