Abstract
Introduction:
Concerns remain regarding both the toughness of alumina, and stability of zirconia ceramics in total hip arthroplasty (THA). A zirconia-toughened alumina (ZTA) bearing has been introduced, in which yttria-stabilized, zirconia polycrystals are uniformly distributed in an alumina matrix. The goal is to combine the wear resistance of alumina with the toughness of zirconia. Zirconia's toughness is attributed to a tetragonal to monoclinic (t-m) phase transformation that occurs in response to a crack, hindering its propagation; however, it might decrease material stability. The purposes of this study were to investigate the degree and position of metal transfer, and the occurrence of t-m phase transformation using Raman spectroscopy, in a series of retrieved, ZTA femoral heads.
Materials and Methods:
Twenty-seven ZTA femoral heads were reviewed as part of an IRB-approved implant retrieval program. All acetabular liners were composed of highly cross-linked polyethylene. The length of implantation, age, body mass index (BMI), sex, and reason for revision were recorded.
Two independent graders assessed each femoral head for metal transfer over three regions (apex, equator, and below equator), using a previously validated grading system (Figure 1). The female trunnion of each head was graded in two regions: the deep and superficial 50% (Figure 2).
Raman spectra were collected with a confocal Raman imaging system (alpha300 R, WITec, Knoxville, TN) operating a 488 nm laser, using a microscope objective of 20X. Three scans were taken in each of the aforementioned regions of the femoral head surface. Scans were also performed in regions of visible wear or metal transfer.
Interobserver correlation coefficients for the measurement of metal transfer between the two graders were determined. One-way ANOVAs were used to compare differences of metal transfer between the 3 surface regions (p < 0.05 = significant).
Results:
The mean length of implantation was 2.2 + 3.4 years. The most common reasons for revision were osteolysis/aseptic loosening (9), device failure (6), and instability/dislocation (5). Of the femoral heads, 66.7% displayed metal transfer within the apex (mean score 1.7 + 0.5), 96.3% in the equator (mean score 2.2 + 0.6), and 100% below the equator (mean score 2.7 + 0.9), which was significantly greater than the apex (p = 0.002). All heads demonstrated metal transfer in both the superficial (mean score 3.1 + 0.3) and deep trunnion (mean score 3.6 + 0.5). Interobserver correlations were fair/moderate for all regions analyzed (r = 0.59 to 0.80).
Raman spectra indicated the occurrence of the t-m phase transformation with monoclinic bands observed at 180, 383, 477, 537, 570 and 622 cm−1. Similarly, tetragonal zirconia bands were noted at 146, 260, 641 cm−1 (Figure 3).
Discussion:
Current literature has limited information on the long–term results of ZTA femoral heads. Our study is the first large series of this kind to include visual assessment and Raman spectra analysis. While the metal transfer grade is less than reported for prior generations of alumina ceramic heads, the occurrence of t-m phase transformations may potentially compromise the long-term mechanical strength and stability of this material.
