Ceramic-on-ceramic alumina bearings (ALX) have demonstrated low wear with minimal biological consequences for almost four decades. An alumina-zirconia composite (BIOLOX-DELTATM) was introduced in 2000 as an alternative ceramic. This contains well-distributed zirconia grains that can undergo some surface phase transformations from tetragonal to monoclinic. We analyzed 5 cases revised at 1–7 years to compare to our simulator wear studies. For the retrieved DELTA bearings, two important questions were

how much tetragonal to monoclinic transformation was there in the zirconia phase and

The retrieval cases were photographed and logged with respect to clinical and revision details. The DELTA balls varied from 22mm to 36mm diameters. These had been mated with liner inserts varying by UHMWPE, BIOLOX-FORTE and BIOLOX-DELTA materials. Bearing features were analyzed for roughness by white-light interferometry, for wear by SEM, for dimensions by CMM and for transfer layers by EDS technique. Surface transformations on DELTA retrievals were mapped by XRD. The four combinations of 36mm diameter BIOLOX-FORTE and BIOLOX-DELTA were studied in a hip simulator, which was run in ‘severe’ micro-separation test mode to 5 million cycles. Wear rates, wear stripes, bearing roughness and wear debris were compared to the retrieval data.

In two DELTA ball cases, there were conspicuous impingement signs, stripe wear and black metallic smears. It is to be noted that the metal transfer sites (EDS) appeared to be from the revision procedures. The retrieved balls run with alumina liners showed monoclinic phase peaking at 32% on the particular surface and internal bore. On the fracture surface of case 1, the monoclinic content had increased to 40%. Various surface roughness indices were assessed on the bearings. The polished articular surfaces averaged roughness (Sa) of the order 3 nm, representing extremely smooth surfaces. The main wear zone was only marginally rougher (5 nm). In contrast the stripe wear zones had roughness of the order 55–140 nm.

In the laboratory, the DELTA bearings provided a 3–6 fold wear reduction compared to FORTE controls. Roughness of stripes increased to maximum 113nm on controls. Roughness of wear stripes showed FORTE with the highest and DELTA with the lowest values. DELTA bearings also revealed much milder wear by SEM imaging. Phase transformations showed peaks at < 30% for both main wear zone and stripe wear sites. It is hypothesized that the concentration of monoclinic phase reached a certain level due to compression contraint imposed by the alumina matrix. With implant wear, additional tetragonal grains of zirconia are exposed and these will also transform to tetragonal. This consistency between laboratory and retrieval studies confirmed the stable nature of the bearings. The BIOLOX-DELTA combination provides optimal potential for a clinically relevant reduction in stripe wear.

Wear in polyethylene liners appears to be exacerbated by 3rd-body abrasion effects with the CoCr ball combinations used for total hip replacements. This has implications for various wear modes encountered in patients. Yet clinical and laboratory studies have offered weak and sometimes contradictory wear relationships with respect to crosslinking, ball diameter and roughness, and 3rd-body wear effects. Our hip simulator model investigated the effect of severe wear challenges by 3rd-body cement particles, using large diameter CoCr and alumina balls, with highly-crosslinked polyethylene liners (HXPE) irradiated to 75kGy compared to contemporary controls (CXPE 35kGy).

The polyethylene liners were gamma-irradiated to 35/75kGy under N2 (CXPE/HXPE). We used 32 and 44mm CoCr balls (ENCORE, Austin, TX) and 44mm alumina-ceramic (Biolox-forte, CeramTecAG) as ‘scratch-resistant’ standard of comparison. We compared 5 bearings pairs with different roughness characteristics using both new and pre-worn polyethylene liners. A 12-station orbital hip simulator with a physiological load profile (0.2kN–3kN load, frequency 1Hz) with cups mounted in “Inverted- position”. Diluted bovine serum (Hyclone Inc., Logan, UT) was used as lubricant (20mg/ml protein, 400ml volume). In phase I, all cups were run in standard (‘clean’) lubricant for 1.5 million cycles (1.5Mc). In phase II, the liners were run in a PMMA slurry of serum (5mg/ml) for 2Mc. In phase III, implants were run ‘clean’ for 1.5Mc. Wear-rate was measured each 0.25Mc event, and surface roughness measured by SEM (XL-30FEG) and white light interferometry (Newview600, Zygo) every 0.5Mc.

In phase I, Wear withnew CXPE and HXPE liners averaged 182mm3/Mc and 30mm3/Mc. Thus the HXPE liners averaged a 6.0-fold wear reduction compared to controls. Compared to new liners, the pre-worn CXPE and HXPE liners showed 10% and 25%, greater wear respectively. Here it was noted that CoCr balls maintained similar roughness (Sa:8–12nm). And alumina balls showed small, gradual increase (Sa: 2 to 2.5nm). The HXPE maintained a superior finish to CXPE controls. Roughness revealed a gradual decrease with time, pre-worn CXPE from 0.28 to 0.15um and pre-worn HXPE from 0.18 to 0.04um (Sa). In contrast, new HXPE showed a dramatic smoothing (0.8 to 0.1um) 92.8% decreased in first 0.5Mc. These effects have not been previously quantified. In phase II with abrasive mode, the liner wear-rates increased dramatically by 6 and 80-fold for CXPE and HXPE, respectively. These data confirmed that HXPE was sensitive to ‘severe’ wear against CoCr and alumina balls. In phase III, the polyethylene roughness dropped by > 90% and wear decreased to phase-I values. The wear-ratio was now 2:1 for CXPE:HXPE as predicted by the ‘diameter’ and ‘crosslinking’ algorithms.

It was clear that surface roughness was not a confounding factorfor either the CoCr or alumina balls. It was the polyethylene surface roughness that appeared to influence wear rates. Our analysis showed that there was a transient due to patches of abrasive cement transferring onto CoCr ball surfaces. Overall the actual roughness of the CoCr balls did not change and was therefore not a factor in increased polyethylene wear.