【Abstract】 Objective To investigate corresponding relation between structure change of the femoral head with“crescent sign” and stress exerted on the avascular necrosis of femoral head, to explore the mechanism of the “crescent sign” formation. Methods From March 1998 to April 2003, the femoral heads of 18 hips in 16 cases having osteonecrosis and “crescent sign” in X-ray film before total hi p arthroplasty, were collected. General and coronal section plane morphology of the femoral heads were observed. The princi ple of effective stress and stress concentration theory were used to explain the phenomena and structure changes in osteonecrosis of the femoral head. Results Cancellous bone existed as a threedimensional,interconnected network of trabeculae rods and plates, with 50%-90% of porosity and 20-30 mmHg bone marrow pressure. According to the definition of porous media, bones especially cancellous bone was a kind of sol id and l iquid two phases porous media. Cross-sectional structure changes in the junction between subchondral plate and cancellous were the place where stress concentrated. The principle of effective stress and stress concentration theory could explain the phenomena and their relationship that occurred in avascular necrosis of the femoral head. Conclusion The “crescent sign” starts in an area of very focal resorption in the subchondral plate laterally and peripherally. The focal resorption in the subchondral plate breaks the continuity of subchondral plate and causes stress concentration in the resorption region. The concentrated stress accumulates in the junction between subchondral plate and unrepaired necrotic cancellous bone brings on the fracture right below the subchondral plate. The focal resorption of the subchondral plate also provides a pathway for the pore water in the unrepaired necrotic bone skeleton to outflow, therefore cause effective stress increase and unrepaired necrotic bone skeleton be compacted by increased effective stress appl ied on unrepaired necrotic cancellous bone skeleton, and results in the volume decrease of unrepaired necrotic cancellous bone and the formation of cavum below the subchondral plate. The cavum shows “crescent sign” in the X-ray film.
Objective To explore the differences between transient osteoporosis of the hi p (TOH) and bone marrow edema (BME) associated with osteonecrosis of the femoral head (ONFH) in terms of cl inical practice and imaging. Methods From January 2006 to February 2008, 5 hips of TOH in 5 cases (1 male and 4 females, aged 29-42 years) and 67 hips of BME associated with ONFH in 63 cases (53 males and 10 females, aged 18-70 years) were analyzed. According to ARCO classification, there were 23 hi ps of stage II, 43 hi ps of stage III, and 1 hi p of stage IV. The induced factors, the degree of pain, the duration of pain and commemorative symptoms were compared. The X-ray, MRI and ECT were also compared. Results There were no differences in induced factors, the degree of pain, joint effusion and ECT between TOH and BME associated with ONFH. TOH had no risk factors, antecedent symptoms and commemorative pain. There were 2 hips with TOH which showed osteopenia on X-ray films. The location of edema was in the superior part or the whole femoral head.A total of 65 hips with BME associated with ONFH had related causes of disease, 10 had antecedent pain and 59 had change of hardening on X-ray films. The X-ray films showed sclerotic area and the edema changes on MRI surrounded the necrotic lesion. The pain just amel iorated when BME disappeared. Conclusion There are differences between TOH and BME associated with ONFH on symptoms, X-ray films and MRI.