A methyl methacrylate cranial prosthesis is of heterogeneous density, similar to muscle, and contains locules of gas. The most common cause of a bony defect is cranioplasty for trauma. In the usual situation the excised bone can be replaced. In some cases the bone is no longer viable, and a synthetic prosthesis must be sought. These may be fashioned in theatre as required, cast to a direct mould of the patient’s skull fragment, or rebuilt using data from CT scans of both the patient’s bone defect and the bone fragment.
Gas within a methyl methacrylate cranioplasty is often due to the manufacture process and does not imply infection. Epidural gas or soft-tissue swelling on CT may be suggestive, but clinical assessment is the key to diagnosing infection. Infection occurs in around 4% of prostheses, and is more common when the frontal sinuses are involved.
Other factors which may lead to complications with methyl methacrylate cranioplasty include large defect, pre-existing infection, and local radiotherapy.
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