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 femur on postnatal radiographs. We decided to determine the prevalence of growth lines in healthy infants.

Materials And Methods: Voiding cystourethrograms of 791 neonates and infants, 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 cohort of 633 healthy infants. Each study was coded for presence or absence of a growth line in the proximal femur, and the distance of the growth line from the metaphyseal edge was measured to the nearest 0.5 mm. Radiographs were obtained on a unit with fixed tube-film distance. Data on the mode of delivery 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 delivered 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 infants. The rate of longitudinal growth of the proximal femoral metaphysis, on radiographs, is approximately 1 mm per 11 days (1 mm per 13 days when corrected for magnification). Approximation of the timing of prenatal and postnatal stressful events that result in a growth disturbance line may be possible.