Benign fluid collection not uncommon
Fluid collection is not uncommon in asymptomatic patients with well-functioning CoXPE bearings. However, they are probably benign and without clinical consequence. Jennings et al. drew these conclusions after examining MRI scans of 44 patients (50 hips). 9 hips showed visible fluid collections, 5 hips manifested intracapsular synovitis with a mean synovial thickness of 5.1mm. Two of these were considered to have a thickened synovium (>6mm). 4 hips showed extra-articular fluid collection with intracapsular communication with a mean synovial thickness of 6mm. Two of these were considered to have a thickened synovium.
No tissue destructions or solid lesions were found. There were neither indications of implant loosening nor other signs of adverse local tissue reaction (ALTR). Nevertheless, the patients should be monitored closely in case the fluid collection progresses from benign to symptomatic or to even causing destructive lesions, the authors pointed out. 3 hips showed signs of potential early osteolysis. The osteolytic lesions may have been subclinical cystic lesions, which could not be identified on the preoperative radiographs. In addition, subtle osteolysis may be difficult to interpret on MRI.
The mean follow-up was 3.4 years, mean Harris Hip Score was 98.5. Only pain-free patients with at least 2-year follow up with appropriately positioned implant components (non-modular stem, no sleeves, BIOLOX®delta ball head and XPE insert without antioxidant) and serial radiographs available within two years prior to study enrollment were included. A musculoskeletal radiologist and an arthroplasty surgeon interpreted the MRI images.
Minor tribocorrosion with sleeves
Tribocorrosion can also occur with sleeved ceramic femoral heads. However, it is predominantly mild and lower than with metal heads. MacDonald et al. conducted a retrieval study of 37 BIOLOX®OPTION sleeved ceramic heads (titanium alloy, BIOLOX®delta ceramics). There was no difference in surface damage of the interface between heads used for primary and for revision surgery. According to the authors, sleeves are a reasonable solution for solving the dilemma with stems remaining in-situ during revision surgery.
Tribocorrosion of sleeve and stem surfaces was evaluated on a four-point qualitative scoring system. Mild to moderate tribocorrosion (score 2-3) was observed on 92% of internal sleeve surfaces in contact with the stems, on 19% of external surfaces in contact with ceramic heads, and on 78% of the stem tapers. A negligible score of 1 was observed on 8% of the internal, on 81% of the external sleeve surfaces, and on 17% of the stem tapers. Severe fretting corrosion (score 4) was observed only on one stem taper, which had remained in situ during a previous revision surgery.
The implants had mostly been revised for instability, infection, and loosening. Implantation time was between 0 and 3.3 years. 17 sleeved heads were implanted during revision and 19 during primary THA. There was no revision due to reactions to metal debris. All femoral stems were made of titanium alloy. They were only revised if not well fixed. The severity of tribocorrosion on the external sleeve taper was correlated with the implantation time, and sleeve length was correlated with the surface damage of the internal sleeve. No correlation between taper surface condition and patient or implant factors was observed.
Fretting corrosion: an issue for BIOLOX®OPTION?
Less biofilm on ceramics
There is significantly less biofilm mass on retrieved ceramic (34%) than on polyethylene (98%) and metal surfaces (66%). During the recent EFORT 2017 congress, Trampuz et al. updated their report on their large prospective multicenter explant study. Their findings potentially reflect special characteristics of ceramics, which seem to resist bacterial adhesion.
More PJI with metal
MoXP bearings are associated with a significantly higher risk of revision for PJI than CoC bearings (hazard ratio 1.42) in patients younger than 70 years when cementless femoral components are used. Madanat et al. presented their conclusion from Australian registry data of 177,237 primary THA procedures over a 14-year period during the recent EFORT 2017 congress.
Higher mortality with PJI
Varnum et al. analyzed the data of 68’504 primary THA from the Danish Hip Registry, of which 445 were revised for PJI and 1350 were exchanged for other causes within the first year after primary implantation. The authors found that revision for PJI is associated with a mortality risk 2.18 times higher than in the reference group of all THA patients in the first post-operative year and 1.87 times higher than in the aseptic-revision group in the first year following revision surgery.