B Masson (Toulouse)
In some cases of a revision when the femoral stem is well fixed in the bone and the femoral head need to be changed, a metal femoral ball head was the only component that a surgeon could safely use. Ceramic femoral ball heads could not be recommended in these cases because of possible reliability issues resulting from a previously used stem taper. Specific ceramic revision ball heads now allow the surgeon to choose a femoral ball head made of ceramicwith a specially designed metal sleeve designed to reduce the effect of stem taper imperfections on the reliability of the ball head used with a firmly fixed femoral stem.
Ceramic revision ball headsystems consist of an alumina matrix composite femoral ball head and a specially designed titanium sleeve. The sleeve diminishes the effect of minor damage to the taper and ensures an even distribution of stresses on the inside of the femoral ball head.
Ceramic femoral ball heads are made of alumina matrix composite ceramics, BIOLOX® delta. They are available in diameters of 28mm, 32mm, 36mm and 40mm.The titanium sleeves are manufactured using a clinically proven alloy of titanium (TiAl6V4). They are available for neck sizes S, M, L and XL. Their skirt-free design does not affect the range of motion (ROM).
The addition of a new interface with the titanium sleeve can create new issues. Two interfaces need to be considered. Concerning the Titanium sleeve-Ceramic interface, recent investigation on retrievedceramic ball head compared with metal ball head shown that the corrosion on the taper is negligible with the ceramic component. Concerning the interface Stem – Titanium sleeve, depending on the material of the stem, the corrosion phenomenon will be different. In the case of a titanium stem, corrosion may be assisted by mechanical action, so called fretting corrosion. The passive layer is permanently damaged leading to accelerated corrosion of the unprotected surface. A badly fitted ball-head may lead to a corrosive attack of the stem taper, because the micro-motions destroy the passive layer. A well-fixed ball head will avoid this complication. In the case of a cobalt chromium stem, galvanic corrosion could appear.Galvanic corrosion needs 3 conditions, of which all have to be met: two different metals, physical contact and aqueous environment for ion transfer. The galvanic corrosion effect can be strongly reduced with an appropriate conical fixation, by reducing the electrolytic fluid present at the interface.
General or uniform corrosion cannot be avoided. However, for modern implant materials, the extremely slow process of general corrosion is such that the lifetime of an implant exceeds by far the lifetime of the patient. If corrosion cannot be avoided, care has to be taken to limit it as much as possible. All other forms of corrosion can be avoided by appropriate choice of materials, designs and handling. Specific ceramic revision ball heads provide safe alternativesand allow optimization of the wear couple for hip revision.