The age distribution of Legg-Perthes disease. An analysis using Sartwell's incubation period model.

The etiology of Legg-Perthes disease of the hip is unknown. One current hypothesis suggests that an environmental factor affects the growth mechanism at a critical prenatal stage. The time interval between the critical exposure and the onset of clinical disease can be regarded as the incubation period, and the distribution of ages at onset of disease will reflect the distribution of incubation periods. Published series of cases were examined for fit to Sartwell's logarithmic normal model of incubation periods. All fit except two. The study results are consistent with a single cause acting at a critical stage of life, i.e., before two years of age. The findings are compatible with either a prenatal or postnatal influence.