Histopathological patterns seen around failed metal-on-metal hip replacements: Cluster and latent class analysis of patterns of failure

We aimed to establish latent subtypes of histopathological patterns in failed metal-on-metal hip replacements. Tissue samples of the synovia from the neocapsule were retrieved from 284 revised ASR (Articular Surface Replacement) hip replacements and analyzed histologically. Hierarchical cluster analysis and polytomous latent class analysis were performed to establish the underlying structure and relationships of the histological observations and to find similar cohorts of cases. Clustering analyses suggested four distinct subtypes that could be readily and reasonably labeled and mapped against a recent consensus statement. The first two subtypes showed synovial necrosis, lymphocyte sheets, and abundant or thin histiocyte sheets. In addition, the first subtype showed abundant germinal centers and no metal particles either extracellularly or intracellularly. Metal particles were, however, seen in the second subtype. Hence, the first subtype was labeled immunologic Type IV neosynovitis and the second subtype abrasion-induced inflammatory lymphocytic Type I neosynovitis. The third and fourth subtypes showed no perivascular and diffuse lymphocytes, but a higher number of metal particles intracellularly and extracellularly. The third subtype had synovial necrosis along with granulomas and was labeled abrasion-induced necrotic Type I neosynovitis, whereas the fourth subtype had readily intact synovial lining, and this subtype was labeled abrasion-induced foreign body Type I neosynovitis. Histopathological findings in failed MoM hips are not just one wide entity. These hips evince four different histological patterns that also differ at the macroscopic level. Moreover, the often stated ALVAL-type reaction seems to be dualistic in nature, which is a novel finding.