Ceramic components have been used for total hip arthroplasty in Europe since the early 1970's, with good results11,19. Such components afford a number of theoretical advantages compared with metal-alloy ones. They have been shown to have excellent biocompatibility both in animal studies and in clinical investigations in Europe6,7,10,20. Ceramic can be given a very high, scratch-resistant polish. This feature, combined with the wettability and corrosion resistance of the material, allows for low-friction articulations with excellent wear characteristics. A number of studies have demonstrated that the rates of wear for ceramic on ultra-high molecular weight polyethylene are two to twenty times lower than the rates for metal alloy on ultra-high molecular weight polyethylene8,9,13,17,18. Ceramic is brittle, which gives it the advantage, compared with ductile materials such as metal, of not being subject to cyclic or fatigue failure. The effective strength of ceramic is a function of the strain rate2. This makes it particularly suitable for the repetitive stress in the hip joint.
Historically, alumina has been the ceramic most commonly used for hip replacement. Zirconia was developed in an attempt to increase tensile strength. Zirconia has a much finer grain structure and a lower elastic modulus than alumina; these properties allow zirconia to be polished to a lower surface roughness and they provide the material with increased strength2,12. Zirconia may prove to be a better choice for hip replacement; however, there is some controversy with regard to its stability in vivo6,20,21.
The disadvantage of ceramic, which is brittle, is its susceptibility to fracture. Because ceramic has a high elastic modulus, it will not plastically deform as metal does. Instead, the formation and propagation of cracks may lead to fracture.
The reported prevalence of fracture of the femoral head is low, especially for hip replacements with ceramic-on-polyethylene articulations. We are aware of eleven instances of a fracture of a ceramic femoral head articulating with polyethylene that have been reported in the English-language literature3,12,15,16. The prevalence of this fracture has been higher for ceramic-on-ceramic articulations, especially those in which the ceramic head was manufactured before 1979, when the material was inferior because of its larger grain size and poorer surface finish7,14,22. The type of ceramic femoral head that was used in our patient has had a very good record in the United States; according to information supplied by the manufacturer, our findings represent only the second reported fracture of more than 7589 ceramic heads that have been implanted with cement to date.
Because the effective strength of ceramic is a function of the strain rate and is dependent on the purity of the material, several factors may increase the risk of failure of a ceramic femoral head. Increased weight and activity of the patient may increase the risk of fracture by increasing the load across the joint; however, in some studies, increased activity has not corresponded with an increased rate of fracture14. In fact, Nizard et al. found that the ceramic components in patients who were less than fifty years old (and who presumably were more active) had a better rate of survival than those in older patients. A history of trauma was associated with only three of the eleven instances of failure of a ceramic-polyethylene hip replacement that we found in our review of the literature3,12,15,16. The trauma involved a minor fall onto the contralateral hip in two patients and onto the ipsilateral hip in one patient. There was no causative event related to the fracture in the remaining eight patients.
Our patient was relatively heavy, which may have been a contributing factor. At the time of the revision, the femoral component was found to be well fixed. We chose to place a ceramic head onto the well fixed stem in light of the history of extensive polyethylene wear. Precise matching of the taper inside the femoral head and the taper of the trunnion of the femoral stem may be important in order to avoid areas of stress concentration. We believe that an unrecognized defect on the existing taper may have contributed to the fracture of the ceramic head. When a ceramic femoral head is used, careful insertion onto a taper with no imperfections may be important to the long-term performance of the ceramic.
We believe that a ceramic head may be a good choice for primary total hip arthroplasty, especially for a young patient. However, the use of a ceramic femoral head on an existing taper at the time of a revision arthroplasty probably should be avoided as minor, unrecognizable flaws on the taper may lead to the formation of cracks in the ceramic with subsequent fracture.