Total hip arthroplasty (THA) with ceramic-on-ceramic (CoC) bearings has become popular, especially in younger patients, largely because of the material’s durability. However, CoC bearings are susceptible to catastrophic failure through fracture. Although the definitive mechanistic pathway for ceramic fracture has not been elucidated, one of the proposed mechanisms is impingement between the ceramic acetabular liner and the metal neck of the femoral stem. In the January 20, 2021 issue of The Journal, Lee et al. take an illuminating radiographic dive into the patterns of impingement in CoC THA.
The authors analyzed 244 cases of CoC THAs that had ≥15 years of radiographic follow-up. They found impingement-related notches at 77 sites in 57 (23.4%) of the cases. The notches were seen either on the neck (28 cases) or on the shoulder (29 cases) of the stem. In 8 cases, notches were found in multiple locations.
All of the neck notches were found when either a medium-neck or long-neck head was used. Shoulder notches were found on the stem only when a short-neck head was used. Lee et al. observed that the use of medium-neck or long-neck heads prevents the ceramic liner from contacting the stem shoulder because the liner impinges on the neck first. The authors also noted that the mean cup inclination was significantly lower in the cases with notched stems compared to stems without notches (36.9° vs 39.8°), and that mean anteversion was higher in the cases with notches (19.9° vs 17.3°).
We have known that impingement can occur between the ceramic liner and metal stem in CoC THA, but this study suggests that it may happen in a significant proportion of patients, both along the neck and shoulder of the stem. Manufacturers should consider these findings when designing implants, and patients and surgeons considering CoC implants may want to avoid short-neck heads, if possible. Also, because impingement-related stem notching appears to occur more frequently with lower cup inclination and higher anteversion, surgical technique remains vitally important in these cases, independent of implant design and selection.
Finally, we should note that the patients in this study were young (mean age of 43 years) and Asian. Asian culture and lifestyle include frequent squatting and sitting cross-legged, which Lee et al. say “induces more impingement between the stem and liner.”
Matthew R. Schmitz, MD
JBJS Deputy Editor for Social Media
