LAUSR

We are seeing more orthopaedic surgeons choosing to use the largest femoral head possible when performing THAs, undoubtedly to prevent against dislocation. Ironically, the founder of the operation, John Charnley, decreased the size of the head from 40 mm to 22 mm to reduce the frictional torque and thicken the cemented acetabulum polyethylene. Thanks to the development of highly crosslinked polyethylene (HXLPE), orthopaedic surgeons are far less concerned about friction [1] than they seem to be about dislocation. But is our increased use of large femoral heads (> 32 mm) as a way of preventing a dislocation an overreaction? That is the question Tsikandylakis and colleagues [7] addressed in the current study comparing 36 mm heads to 32 and 28 mm heads with data from the Combined Scandinavian Registries. The authors found that 36 mm heads underwent more revisions compared to 32 and 28 mm heads because of loosening, not dislocation [7]. There are two reports of increased loosening of stems with the direct anterior approach in the first 5 years, and the data of this study would be consistent with those previously published [4, 6]. In the current study, 93% of hips with a 36 mm head had a posterior approach, yet that approach was not correlated to increased loosening, which means the increase in loosening was principally driven by the 7% of the hips with 36 mm heads that had anterior approaches [7]. I am curious whether the surgeons used too large a cup for the acetabulum, and as such, increased the risk of an unrecognized acetabular fracture or impingement. Future studies should address this question. A recently published registry study [1] confirms the finding of increased revision with femoral heads larger than 32 mm at 14 years postoperative in hips with HXLPE. The Australian Registry did not find any difference until after 10 years, but at 14 years, the revision rate was 8.5% (66,973 primary THA) with 36 mm heads and 4.6% (84,157 primary THA) with 32 mm heads. The mean followup of hips with HXLPE was 4.2 years with a cumulative revision rate of 6.2% at 16 years. The leading causes of revision, in order, were dislocation, infection, fracture, and loosening. Noticeably missing as a cause of failure of the HXLPE articulations was osteolysis and wear, which were common with conventional polyethylene [1].