Background: The role of the presence of the femoral head ossific nucleus as a risk factor for the development of osteonecrosis of the femoral head in infants with developmental dysplasia of the hip has been investigated in several small studies, but the results have been inconsistent. The purpose of the present study was to determine the effect of the presence of the ossific nucleus on the development of osteonecrosis.
Methods: A systematic review of the medical literature from 1966 to 2007 was performed. Two independent reviewers evaluated all articles. Interrater agreement was determined, and the quality of evidence was evaluated. A meta-analysis was then performed with the main outcome defined as the development of osteonecrosis of the femoral head two years after reduction.
Results: Six observational studies (five retrospective and one prospective) met the inclusion criteria. Inconsistency was found in that half of the studies demonstrated a protective effect of the ossific nucleus on the development of osteonecrosis whereas half of the studies did not. A meta-analysis (including 358 patients) showed no significant effect of the presence of the ossific nucleus on the development of grades-I through IV osteonecrosis, with forty-one cases of osteonecrosis (19%) found in infants in whom the ossific nucleus had been present at the time of hip reduction compared with thirty cases (22%) in the group without an ossific nucleus (relative risk = 0.75, 95% confidence interval = 0.46 to 1.21). When only radiographic changes of grade II or worse were considered to represent osteonecrosis, a significant difference in the prevalence of osteonecrosis was found, with fourteen cases of osteonecrosis (7%) in infants with an ossific nucleus compared with eighteen cases (16%) in those without an ossific nucleus (relative risk = 0.43, 95% confidence interval = 0.20 to 0.90). A subgroup analysis showed that the presence of the ossific nucleus reduced the probability of osteonecrosis by 60% (relative risk = 0.41, 95% confidence interval = 0.18 to 0.91) after closed reduction, but no significant effect was found in patients treated with open reduction (relative risk = 1.14, 95% confidence interval = 0.62 to 2.07). All studies demonstrated methodological weaknesses compromising the quality of evidence.
Conclusions: We did not find that the presence of the ossific nucleus had a significant effect on the development of osteonecrosis of any grade after hip reduction in infants with developmental dysplasia of the hip. The meta-analysis suggested that the presence of the ossific nucleus has a protective effect against the development of the more severe forms of femoral head osteonecrosis. However, the quality of evidence is moderate, and additional research is likely to have an important impact on the confidence in the estimate of the effect and may change this estimate.
Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.