This is an interesting and topical case report. This report highlights that corrosion at the head-neck taper interface remains a serious concern and can have significant effects in well-positioned, low wearing hips. There have been many reports on tapers of hip replacements but the value of this report is that it concerns the taper of a large diameter MOM hip.
There are some important issues that should be raised and may become important as the field of hip tapers becomes a more widely reported subject in clinical orthopaedic journals. These can be divided into: unknown metallurgical issues; known implications of component mixing; and lastly, the terminology of “trunionosis”.
The use of the optical system called Redlux for the measurement of bearing surface wear is far from common practice and is in its infancy. Measurement of taper wear is not well described in general and should involve the use of a measurement probe small enough to reach between the grooves of the surface.
It is known that mixing components is not recommended in clinical practice. This study highlights this, even though the mechanism of failure is new and joins the list of reasons not to mix implant types.
Lastly, the terminology of “trunionosis” is misleading because it is the head taper (and not the femoral stem taper / trunion) that is usually worn, even though in most cases the stem taper is made of titanium (a softer material than cobalt-chromium). This is clinically important because metal-on-metal hip problems are likely to result in many bearing surface revisions without the need to change the stem.
Mr A Hart
Imperial College London, UK