Heterotopic Ossification (HO) is a potential long-term complication in orthopaedic surgery. It is commonly classified according to the Brooker classification, which is based on radiological findings. To our knowledge the correlation of histological features to the Brooker grade is unknown as is the association between HO and the indication for revision. The aim of this paper is to analyze the ossification grade of HO tissue in patients undergoing revision hip and knee arthroplasty and to propose a histologically based classification system for HO. We also assess the relationship between the grade of HO and the indication for revision (septic and aseptic revision). From January to May 2019 we collected 50 human HO samples from hip and knee revision arthroplasty cases. These tissue samples were double-blinded and sent for histopathological diagnostic. Based on these results, we developed a classification system for the progression of HO. The grade of ossification was based on three characteristics: Grade of heterotopic ossification (Grade 1-3), presence of necrosis (N0 or N1) and the presence of osteomyelitis (HOES-Score Type 1 to 5). Demographic data as well as surgical details and indication for surgery was prospectively collected from clinical records. Fifty tissue samples were harvested from 44 hips and 6 knee joints. Of these 33 exhibited Grade I ossifications (66%), followed by 11 Grade II (22%) and one Grade III (2%). Necrosis was noted in two tissue samples (4%) and 2 more had osteomyelitis findings according to HOES-Score. Six samples (12%) with radiologically suggestive of HO turned out to be wear-induced synovitis, SLIM Type 1. Of these cases 16 were septic (32%) and 34 aseptic (68%) revisions. Most of the HO tissue samples were classified as a low-grade. High-grade ossification-Score is rare. Higher grades of ossification seem to be associated with septic revision cases. Wear-induced synovitis potentially influences HO development. A histological scoring system for ossification grading can be derived from the data presented in this study.
