Pathogenetic relevance of the pregnancy hormone relaxin to inborn hip instability.

The etiology of inborn hip dysplasia is unknown. In general, a multifactorial genesis is assumed. The influence of hormones on the development of the fetal hip joint and its stability is discussed as well as mechanical influences. This study was carried out with the intention to examine the correlation between the concentration of the pregnancy hormone relaxin and the stability of the hip joint in newborns. Both hips of 90 newborn children were examined clinically and sonographically. In 25 hips (13.9%), pathological sonograms according to the classification of Graf were found. The relaxin concentration was measured in cord blood using a heterologous radioimmunoassay. Statistical evaluation revealed an insignificant decrease of relaxin concentration with increasing sonographic hip instability. The results indicate that hip instability frequently occurs with decreasing relaxin concentration. These facts contradict the earlier assumption that hip instability coincides with increased relaxin concentrations in newborns. We assume that there is a worse preparation of the pelvis and the birth canal during pregnancy due to the lower relaxin concentration and thus that there could be a higher pressure on the fetus in the perinatal phase. A decreased relaxin concentration seems to have no direct effect on the hip joint tissue, but indirectly there is consequent rigidity of the tissue in mother and child, which can further promote the development of hip instability.