The excellent study by de Steiger and colleagues can be seen as a culmination and confirmation of the efforts of researchers to use radiation crosslinking to improve the wear resistance of ultra-high molecular weight polyethylene in hip replacements. As the authors (and Dr. Namba, who previously provided a commentary), stated, the study “completes the loop” on the progressive process of laboratory testing, closely monitored early clinical studies, widespread surveillance and mid-term clinical evaluation. The results show an early and consistent substantial reduction in the rate of revision of hips with highly XLPE. It is satisfying and reassuring that this confirms the trends that were reported in the great majority of shorter-term studies involving fewer patients and fewer types of hips — as well as the pre-clinical hip simulator testing.
It also is valuable that, due to the large number of patients in the study, the authors were able to control for potentially confounding variables, including ball diameter and patient age. Another import factor was the separation of revisions due to loosening from those due to wear/lysis. Especially in the early clinical follow-up, aseptic loosening can be primarily due to problems with fixation or subluxation, when it is too soon to detect a reduction in loosening subsequent to wear-induced lysis.
In this regard, it is important to keep in mind the complex and interactive nature of wear, lysis and loosening. For example, loosening of a cemented stem can generate third-body particles of bone cement, which then migrate to the ball-socket interface, which increases the wear (of either type of PE), which increases the production of debris, which induces osteolysis, which further undermines the fixation, which increases the loosening — ie, a classic feedback loop. Despite the potential for such complex wear scenarios, the large number of patients and the 16-year follow-up of the study by de Steiger and his colleagues provide the ability to “see through the clinical fog” and accurately discern the substantial reduction in wear and subsequent clinical complications that have been provided by the highly XLPEs.
Namba RS. J Bone Joint Surg Am. 2018; doi:10.2106/JBJS.18.00416.
McKellop HA. Biomaterial. 2007; doi:10.1016/j.biomaterials.2007.07.040.
Harry A. McKellop, PhD
University of California, Los Angeles
Disclosures: McKellop reports he is a co-inventor of one of the highly-crosslinked polyethylenes evaluated in the study by de Steiger and colleagues, used in the Duraloc hip (DePuy Synthes) and he received royalties from the license of that technology.