Do new materials improve acetabular fixation and reduce wear? Commentary on an article by David C. Ayers, MD, et al.: "radiostereometric analysis study of tantalum compared with titanium acetabular cups and highly cross-linked compared with conventional liners in young patients undergoing total hip replacement".

Abstract

Commentary The bearing articulation remains the nemesis of total hip arthroplasty. Obtaining and maintaining component stability and minimizing wear of the articulation represent the two most important factors for the longevity of any hip replacement. This article by Ayers et al. evaluates the stability of two different types of cementless fixation, a titanium fiber mesh compared with ultra-porous tantalum. The authors further investigate wear of an ultra-high molecular weight conventional polyethylene liner compared with a highly cross-linked polyethylene liner. This article represents a sequel to a previously published two-year follow-up evaluation of noncemented total hip arthroplasty acetabular migration and polyethylene wear as determined by radiostereometric analysis (RSA)1. A total of forty-six young, active individuals undergoing primary total hip arthroplasty were randomized into four cohorts: ultra-high molecular weight conventional polyethylene liner and titanium cup, highly cross-linked polyethylene liner and titanium cup, ultra-high molecular weight conventional polyethylene liner and tantalum cup, and highly cross-linked polyethylene liner and tantalum cup. There were no clinical or non-RSA radiographic differences among the groups. RSA demonstrated significantly less femoral head penetration, a surrogate for polyethylene wear, in the highly cross-linked polyethylene liners compared with ultra-high molecular weight conventional polyethylene liners by an order of magnitude. There was no significant difference in proximal migration between the titanium and tantalum acetabular components through five years of follow-up. The type of polyethylene liner or acetabular component had no effect on patient activity, pain, and well-being, which were maximized by six months following surgical intervention. The strength of this article is that it is a prospective, randomized, blinded study utilizing a very sophisticated technology, namely RSA, to obtain precise measurements of component micromotion relative to the bone that is otherwise undetectable by routine radiographs and to measure femoral head penetration2. The radiodense tantalum cups obscure the identification of the center of the femoral head with use of edge detection. Therefore, Ayers et al. developed a novel method known as the “fictive femoral head” to assess wear. This method is based on the identification of two radiopaque tantalum markers attached to the stem proximally and a priori knowledge of the femoral head and stem configuration of the inserted total hip replacement. From a series of radiographs, a stem configuration library was created, which provided geometrical information used to place a fictive femoral head center in three-dimensional space. This fictive femoral head center will be used by the RSA software. Although others have reported a substantial decrease in the wear of highly cross-linked polyethylene3, this article clearly shows the superiority of highly cross-linked polyethylene over conventional polyethylene, with a tenfold reduction in wear. Wear rates from one to five years were 0.04 mm/yr for ultra-high molecular weight conventional polyethylene liners and 0.004 mm/yr for highly cross-linked polyethylene liners. With respect to fixation, Ayers et al. demonstrated excellent fixation of both titanium mesh and tantalum acetabular components, as has been previously documented4. At the five-year follow-up, the median migration (and standard error) was 0.05 ± 0.20 mm proximally for titanium mesh cups and 0.21 ± 0.05 mm for tantalum cups. No implants were shown to be loose on radiographs. There was no evidence of osteolysis and no revisions were performed through five years of follow-up. The take-home points from this article are as follows. With use of femoral head penetration according to precise RSA measurements as a surrogate for wear, highly cross-linked polyethylene demonstrates ten times lower wear than ultra-high molecular weight conventional polyethylene at up to five years of follow-up3. Titanium mesh and tantalum (Trabecular Metal; Zimmer) materials provide excellent cementless fixation for acetabular components with little evidence of clinically relevant migration as measured by sophisticated RSA technology at up to five years of follow-up4. These results cannot and should not be extrapolated to other types of porous and ultra-porous cementless fixation.