Revision hip surgery is increasing each year. We describe a case of revision hip surgery of a fully ingrown modular stem in which the supplied manufactures replacement modular head did not fit the original taper. This resulted from a change in the manufacturing process and taper dimensions between the primary index hip replacement and the revision operation. The manufacturing company was not aware of this change in their manufacturing process. This caused problems during surgery.

A thirty-nine year old male patient had a Mittel-meier uncemented hip prosthesis performed for avascular necrosis of the femoral head in 1988. Twelve years following the index primary hip replacement the cup was loose and migrating so revision of the acetabular component was planned.

A selection of Mittelmeier heads was obtained from the company. The ceramic head was removed and the acetabular component revised for a cemented cup. It was then discovered, during the operation, that the supplied heads did not fit the taper on the stem. The manufacturers had changed the dimensions of the taper since the original prosthesis had been implanted in 1988. Several companies had marketed this prosthesis since 1988 and the current supplier was unaware of a change in the taper. With the cup already cemented in place there were limited options. By chance the Exeter heads fitted the taper on the femoral component and were used. The patient recovered uneventfully.

The Mittlemier is not the only prosthesis to change its taper during its lifetime. If as in this case the manufacturers are unaware of a change in the manufacturing process the surgeon has no way of confirming the information provided prior to the surgery. This case highlights the potential problems when trying to partially revise a modular component.