Subchondral fracture begins from the bone resorption area in osteonecrosis of the femoral head: a micro-computerised tomography study

Purpose For successful joint preservation in osteonecrosis of the femoral head (ONFH), it is important to understand the mechanism of collapse. The purpose of this study was to investigate the initiation of subchondral fracture in ONFH by using micro-CT imaging of the whole femoral head, focusing on the three-dimensional relationship between the subchondral fracture and the bone resorption area. Methods A total of 40 femoral heads from 37 patients retrieved during total hip arthroplasty for stage 3A or 3B ONFH by Japanese Investigation Committee criteria were scanned using micro-CT with a 0.146-mm thickness cuts. We divided the cohort into early and late collapsed stages according to a threshold of 3 mm of collapse as measured by micro-CT. Results According to the analysis on multiple radial plane views in the whole femoral head, there were two interesting findings. First, the initial fracture cracks ran between separated bone resorption areas at the anterosuperior portions of all 18 femoral heads in the early collapsed stage. Second, fractures of the necrotic bone at the sclerotic boundary and a fibrous, granulation-like, low-density tissue along the necrotic side of the sclerotic boundary were seen in 19 of the 22 in the late collapsed stage. After bone resorption around the retinaculum and teres insertion initiates the subchondral fracture, bone resorption expanding at the anterosuperior portion of the femoral head may result in the spread of fracture and the potential for massive collapse. Conclusions Three-dimensional micro-CT showed bone resorption around the reparative zone initiates the subchondral fracture in ONFH.