Abstract
Dissociation of the polyethylene liner of modular acetabular components is a rare occurrence, and previous reports have commented on the difficulty in diagnosis from plain radiographs. The radiograph is often incorrectly reported by radiologists as showing advanced polyethylene wear, causing delay in referral and increasing the complexity of treatment required.
We report 9 cases of late polyethylene liner dissociation of the cementless Harris-Galante II porous-coated acetabular component (Zimmer Inc, Warsaw, IN) which occurred without trauma or injury. This is the largest reported series to date.
In all cases, there was a common pattern of clinical symptoms and signs which is described.
Radiographs showed a distinct appearance with a radiolucency medial to the femoral neck in association with an eccentrically placed femoral head lying in contact with the acetabular metal shell. We have termed this the ‘crescent sign’.
We believe that the diagnosis can be made from a single antero-posterior pelvic radiograph without the need for previous films for comparison, or the need for arthrography. Clinicians should look specifically for the crescent sign when an eccentrically placed femoral head has been noted, in order to differentiate the more unusual diagnosis of dissociation from that of polyethylene wear. Early diagnosis and prompt referral prevents further damage to the femoral head and metal acetabular shell, thus reducing the complexity of revision surgery.
Editoral Secretary Mr Peter Howard. Correspondence should be addressed to BHS at the Royal College of Surgeons, 35 - 43 Lincoln’s Inn Fields, London WC2A 3PN.
