Femoral growth lines: Bony birthmarks in infants

OBJECTIVE. During a study of infants who were being examined for antenatal renal dilatation, we noted that many had growth lines in their proximal fem ur on postnatal radiographs. We decided to determine the prevalence of grow th lines in healthy infants. MATERIALS AND METHODS, Voiding cystourethrograms of 791 neonates and in fan ts, 0-6 months old, were reviewed. All who had documented severe illness or no reasonable view of the proximal femur were excluded, resulting in a coh ort of 633 healthy infants. Each study was coded for presence or absence of a gowth line in the proximal femur, and the distance of the growth line fr om the metaphyseal edge was measured to the nearest 0.5 mm. Radiograpbs wer e obtained on a unit with fixed tube-film distance. Data on the mode of del ivery were collected for 136 infants. RESULTS. Of 633 eligible infants, 247 (39%) of 633 had a discernible growth line. The distance of the growth line from the metaphysis, in millimeters, correlated significantly with age in days (r = .81, p < .01). Infants deli vered vaginally were more likely to have a growth line than were those born by cesarean delivery (p = .049). CONCLUSION. A growth line in the proximal femur is common in healthy infant s. The rate of longitudinal growth of the proximal femoral metaphysis, on r adiographs, is approximately 1 mm per 11 days (1 mm per 13 days when correc ted for magnification). Approximation of the timing of prenatal and postnat al stressful events that result in a growth disturbance line may be possibl e.