N. Zhang
Center of Osteonecrosis and Joint Preserving & Reconstruction, Orthopedic Surgery Department of China—Japan Friendship Hospital
Yinghua East Road, Chaoyang District, Beijing P.R. China
Introduction: The formation mechanism of the “crescent sign” in the ONFH is not quite clear. We sought to find the formation mechanism of the “crescent sign”.
Methods: We observed the pathologic structure changes of femoral heads with “crescent signs” in general, using coronal sections and X-ray films. We used the principle of effective stress and stress concentration theory to analyze and explain the reasons of pathologic changes occurring in the ONFH with the “crescent sign”.
Results: The concentrated stress accumulated in the junction between the subchondral plate and the unrepaired necrotic cancellous bone brings on the fracture line right beneath the subchondral plate. Increased effective stress causes the stress-to-strength ratio to increase progressively until reaching the critical level of collapse onset. The volume decrease of unrepaired necrotic cancellous bone leads to the formation of the crescent cavum.
Discussion: Bones are a kind of living porous media. The principle of effective stress is one of the important fundamental concepts in porous media mechanics. The principle of effective stress and stress concentration theory could clearly explain phenomena occurring in ONFH. This is the first time an explanation of the formation mechanism of the “crescent sign” has been done with the principle of effective stress. The results are useful for prediction and prevention collapse of ONFH.