AB1137 CLASSIFICATION OF THE EARLY STAGE OF RAPIDLY DESTRUCTIVE COXOPATHY ACCORDING TO THE FEMORAL HEAD DESTRUCTION

Background: Rapidly destructive coxopathy (RDC) is an unusual subset of osteoarthritis of the hip characterized by rapid chondrolysis with progressive loss of the joint space as the first manifestation of the disease. Because rapid progression of RDC makes it difficult to obtain sequential radiographs in its early stage, the process of disease progression in the early stage remains unclear. Although the pathogenesis of RDC is still unclarified, the potential causes of RDC include subchondral insufficiency fracture of the femoral head resulting from osteoporosis, pelvic posterior inclination in RDC as a mechanical factor, and increased serum levels of matrix metalloproteinase (MMP)-3 as a biological factor. Objectives: This study aimed to differentiate the process of disease progression in the early stage of RDC and provide its new classification system. Methods: This monocentric retrospective study included 42 female patients who met the criteria of RPOH, chondrolysis >2 mm during 12 months from the onset of hip pain based on a series of radiographs and computed tomography (CT). This study also included 9 female patients with osteoarthritis secondary to developmental dysplasia of the hip (DDH), who demonstrated chondrolysis >2 mm during 12 months from the onset of hip pain. Cortical thickness index (CTI) correlated with bone mineral density of the hip, pelvic tilt, and serum concentrations of matrix metalloproteinase (MMP)-3 were analyzed. Results: RDC were classified into two types based on the absence (type 1, n=17) and presence (type 2, n=25) of subsequent femoral head destruction shown by CT within 12 months after the onset of hip pain. MMP-3 significantly increased in RDC type 2 compared with type 1 and DDH. Increased posterior pelvic tilt was found in RDC type 2 compared with DDH. Logistic regression and receiver operating characteristic curve analyses indicated that MMP-3 may be associated with differentiation between RDC types 1 and 2. No difference was found in CTI between RDC types and DDH. RDC type 2 hips developed partial (type 2A) and massive (type 2B) femoral head destruction within the first 12 months. Whereas partial destruction showed 40% collapse ratio. Increased posterior pelvic tilt was found in massive destruction. Femoral head destruction started earlier within the first 6 months in massive destruction compared with that in partial destruction. From receiver operating characteristic curve analysis, pelvic tilt differentiated the femoral head destruction types using the initial radiograph at the onset before first demonstration of femoral head destruction. No difference was found in CTI or MMP-3 between the two subtypes. Conclusion: Disease progression of RDC during 12 months after the onset of hip pain could be classified into two distinct types based on the absence (type 1) and presence (type 2) of femoral head destruction in association with MMP-3 and pelvic tilt as biological and mechanical factors, respectively. MMP-3 may be helpful to differentiate those two types in the early stage of RPOH. The extent of femoral head destruction could further differentiate RDC type 2 into two subtypes based on pelvic tilt. Acknowledgments : This study was supported by the Japan Hip Joint Foundation. Disclosure of Interests: None declared