Subchondral osteopenia and accelerated bone remodelling post‐ovariectomy – a possible mechanism for subchondral microfractures in the aetiology of spontaneous osteonecrosis of the knee?

Osteopenia and subchondral microfractures are implicated in the aetiology of spontaneous osteonecrosis of the knee (SPONK). The ovine tibia shows significant alterations of the trabecular architecture within the subchondral bone of the medial tibial plateau post‐ovariectomy (OVX), including reduced trabecular bone volume fraction. We hypothesise that accelerated subchondral bone resorption may also play a role in increasing microfracture risk at this site. Twenty‐two sheep were examined in this study; 10 of the sheep underwent OVX , while the remainder ( n  = 13) were kept as controls ( CON ). Five fluorochrome dyes were administered intravenously at 12‐week intervals via the jugular vein to both groups, to label sites of bone turnover. These animals were then killed at 12 months post‐operatively. Bone turnover was significantly increased in the OVX group in both trabecular bone (2.024 vs. 1.047 no. mm −2 ; P  = 0.05) and within the subchondral bone plate (4.68 vs. 0.69 no. mm −2 ; P  < 0.001). In addition to the classically described turnover visible along trabecular surfaces, we also found visual evidence of intra‐trabecular osteonal remodelling. In conclusion, this study shows significant alterations in bone turnover in both trabecular bone and within the subchondral bone plate at 1 year post‐OVX. Remodelling of trabecular bone was due to both classically described hemi‐osteonal and intra‐trabecular osteonal remodelling. The presence of both localised osteopenia and accelerated bone remodelling within the medial tibial plateau provide a possible mechanism for subchondral microfractures in the aetiology of SPONK. Further utilisation of the OVX ewe may be useful for further study in this field.