Registries look at cancer, PJI and fracture
Cancer, PJI and periprosthetic fracture were the main subjects discussed at the Sixth International Congress of Arthroplasty Registries (ISAR) held May 20-22, 2017, in San Francisco, USA. According to the Australian registry, the overall incidence of cancer for THA patients was 6% higher than the population average. The authors did not find a significant difference in all-site cancer rates between metal-on-metal (MoM) and other bearings. Nevertheless, the incidence of Hodgkin's Lymphoma was 117% higher with MoM.
PJI remains one of the most frequent reasons for failure in THA worldwide, according to the authors of an international registry collaboration study. The British National Joint Registry (NJR) shows an increased risk of revision for PJI in males, smokers and patients with high BMI, diabetes, rheumatoid arthritis, chronic pulmonary disease or liver disease. The authors also identified an influence of dementia, fractured neck of femur, previous history of infection and lateral surgical approach. There was no evidence of association with the presence of a consultant (none, assisting, operating), grade of operating surgeon (consultant, other) and place of surgery (England, Wales). Ceramic bearings showed a protective effect against PJI.
In the discussion of periprosthetic fracture, there was a consensus that its incidence might be related to the difficulty of positioning a cementless stem. According to Steiger et al., in case of first revision for periprosthetic fracture the most common reason for a second revision was infection, followed by osteolysis, dislocation and re-fracture.
CoC results with patients under 30
In patients under 30 at the time of primary implantation, CoC bearings have a revision rate of 1.1% at 11.5 years compared to 7.9% at 7.3 years for hard-on-soft bearings. This is one of the results of a systematic review by Walker et al. The authors identified 14 articles from peer-reviewed journals published between 1981 and 2013. When only uncemented THA were assessed, CoC revision rate was only 0.78% at 12.2 years while the overall revision rate was 1.3% at 9.9 years. In this comparison, there was also a significant difference in the rate of radiographic loosening: 0.2% with CoC and 12.2% with hard-on-soft bearings.
The authors used the change in Harris Hip Score (HHS) as primary outcome measure. Secondary outcome measures were implant survivorship, effect of fixation (cemented/uncemented stem) and bearing surface (metal or ceramic hard-on-soft / CoC). The overall revision rate of 5% at a weighted mean follow-up of 8.4 years in this very young patient group is similar to the results in older patient groups. However, only a large randomized controlled trial comparing cemented and uncemented mode of fixation with CoC vs. metal or ceramic hard-on-soft bearings will be able to determine any significant difference.
The results suggest that CoC bearings outperform hard-on-soft couples in terms of improved HHS, revision rate and loosening. However, the differences in bearing surfaces are confounded by differences in fixation and the use of historical polyethylene varieties obsolete today, in the hard-on-soft group. Thus, no conclusions about modern bearing materials can be drawn from this study.
Cobalt toxicity can be fatal
Though very rare, cobalt toxicity is a severe and potentially fatal complication in THA. In a literature review, Zywiel et al. identified 18 cases, most of which reported systemic toxic reactions at cobalt levels above 100µg/L. Eight cases, one of which was fatal, were related to the use of metal-on-polyethylene (MoP) bearings in revision THA after the fracture of a ceramic component. The other ten cases were judged as possibly malfunctioning metal-on-metal (MoM) bearings. In all cases treated, cobalt levels in the serum decreased after revision. The authors recommended using cobalt-free materials such as titanium and ceramic for revision. They stress that early intervention with complete removal of cobalt components may help to prevent further complications. They also plead for increasing awareness for the broad variety of symptoms of cobalt toxicity.
Cobalt ion levels can be measured from whole blood, serum or erythrocytes. However, the values cannot be used interchangeably. Different units are used for cobalt levels, which are easily convertible: 1ppb = 1µg/L = 1ng/ml; which is approx. 16.97nmol/L. The natural level of Co in serum is believed to be about 0.1ppb. The authors were not able to define a pathological Co ion level since it is highly likely that the systemic risk from Co ion release is complicated by patient-specific factors that increase or decrease susceptibility.
36mm bearing diameter ranking on top
More ceramic heads in USA
In the USA, the use of ceramic femoral heads for THA has grown steadily according to registry data from the AJRR of the years 2012–2015. Ceramic heads are favourable for their friction and wear characteristics, and especially address concerns regarding trunnionosis and corrosion with metal heads, which probably has contributed to this growth. Ceramic heads are chosen more often for younger than for older patients, but there is also growth in the latter group.
When antioxidant or “enhanced” polyethylene acetabular liners are chosen, the combination with a ceramic head is favored in the majority of cases. Conventional polyethylene liners are typically chosen together with metal heads. Femoral head size has remained relatively constant between 2012 and 2015, with 36mm heads used in more than 50% of the procedures performed.