Relatively lower dislocation risk with 36mm CoC
36mm metal-on-highly crosslinked polyethylene (MoXPE) bearings are associated with a dislocation risk 1.6 times greater than same size ceramic-on-ceramic bearings. Shah et al. speculate that this observation maybe an effect of the release of corrosion products from the trunnion-head junction. There was no significant difference in dislocation rate between the bearing types of the other sizes. For their study, the authors analyzed 101,915 MoXPE, 30,256 CoXPE and 60,104 CoC bearings of head sizes 28mm, 32mm and 36mm recorded in the Australian register. At 13 years, 1,219 THA had been revised for dislocation.
The cumulative revision rate calculated for the three groups (MoXPE, CoXPE and CoC) at two years was 0.6%, 0.4% and 0.5%, respectively. At 13 years, the rates were 1.2%, 1.0% and 0.9%. The authors included all primary THA for osteoarthritis using one of these bearings. The data were adjusted for age and gender.
Compared to CoXPE, MoXPE bearings showed an increased risk of revision for dislocation in all head sizes during the observation period of 15 years. The risk was also higher for MoXPE when compared to CoC after three months. Further analysis showed no effect of the bearing type for 28mm heads, except during the first three months when CoC bearings had a higher revision rate for dislocation. No differences were found for 32mm bearings.
However, the authors also point to the limitations of their study. Dislocation risk depends, among many other things, on approach, positioning, laxity of the joint or capsule retention. None of these confounding factors was assessed. There may also be a selection bias as MoXPE bearings are more likely to be used in public hospitals.
ORS on cobalt issues in hip and knee
Different forms of corrosion and the release of cobalt were among the hot topics discussed at the 63rd Annual Meeting of the Orthopedic Research Society (ORS), March 19-22, in San Diego. Several studies confirmed that material loss from CoCr heads in THA increases with time in situ. Several presentations showed that cobalt release is also an issue in knee arthroplasty. In abrasive wear situations, it can reach the same level as in metal-on-metal THA. In patients with painful TKA, the studies found increased cobalt levels.
Efforts to predict corrosion in laboratory settings were presented, aiming at parametric testing and optimized development of design and material. A research team from Chicago’s Rush University confirmed the observation made on cell-induced corrosion. The team also found that the amount of cobalt in Co-alloy particles from a failed THA trunnion was considerably reduced, showing the solubility of this potentially cancerogenic* metal.
More lysis than suspected
The prevalence of osteolysis in THA patients with a metal-on-highly crosslinked polyethylene (MoXPE) bearing might be higher than previously reported. Howie et al. reviewed 100 hips using CT. They identified radiolucent lines in 40.8% of the patients, even in the absence of significant XPE wear. The prevalence was higher with 36mm (47%) than with 28mm heads (34%, p=0.08). Overall, the mean wear rate was 0.04mm/year with no significant difference between 28mm and 36mm diameters (p=0.48). The authors stated that previous reports may have underestimated the prevalence of osteolysis due to poor imaging and small patient cohorts. They also identified an increased total volume of osteolysis at longer-term follow-up. The authors consider the relatively high prevalence of osteolysis in the absence of XPE wear to be of significant concern regarding the long-term survival of these MoXPE implants.
Zirconia toughening works
Phase transformation of the zirconia particles in BIOLOX®delta ceramics toughens the component’s surface; cracks affect only the first micrometers in depth. These findings by Perrichon et al. confirm that the crack resistance of the zirconia-toughened alumina works as intended. BIOLOX®delta specimen also showed an excellent resistance against hydrothermal ageing which did not affect wear or mechanical performance.
The authors tested the components under three different conditions: friction, hydrothermal ageing and severe shock. They state that shocks are the main origin of wear damage as represented by the formation of wear stripes in which phase transformation occurs. High contact stresses during shocks cause the formation of micro-cracks within the wear stripes, which locally trigger a zirconia phase transformation, reducing their propagation in response. The remaining limited crack paths cause a weakening of the mechanical properties, but only in a limited damage area. The phase transformation by shocks is therefore not detrimental for the zirconia-toughened alumina, but rather a positive effect that limits the extension of the wear stripes.
Microstructure of BIOLOX®delta ZTA ceramics
86% ceramic in Germany
According to German registry data from 2016, 86% of the THA bearings had at least one ceramic component. 58% of the bearings had a 32mm, 33% had 36mm diameter bearing. The most common cause for revision of a primary THA was aseptic loosening of one or more components (41%). While the percentage of this indication has been descending over the past three years, the incidence of PJI-induced revisions has increased to 15.6%. The numbers of dislocation and periprosthetic fracture have also increased. They were the cause of revision in around 10% of cases, while implant wear was the cause in 7%.