Epiphyseal Ossification Centers

Distal Femoral Epiphysis (DFE)

Proximal Tibial Epiphysis + DFE

Proximal Humeral Epiphysis

Findings:

An echogenic focus is identified within the distal femoral epiphyseal cartilage. This is the developing ossification center for the distal femur. In a slightly older fetus, in addition to the distal femoral epiphysis, the proximal tibial epiphyseal ossification center is identified. In a still older fetus, the proximal humeral epiphyseal ossification center is seen.

Discussion:

In 1950's it was recognized that the identification of the fetal skeletal epiphyseal ossification centers was useful for estimating gestaional age, particularly at a time near fetal lung maturity. Fetal/maternal radiographs depicting these centers were often difficult to interpret due to fetal motion, overlapping structures etc. In the 1970's it was recognized that the distal femoral epiphyseal ossification center could be identified sonographically. In the 1980's ultrasound was first utilized to assess the appearance time for the epiphyseal centers of the lower extemities, speicifically the distal femoral epipyses (DFE) and the proximal tibial epiphyses (PTE). It was noted that the DFE was seen at 33 weeks in most fetuses and that when the PTE was identified, the fetus was at least 35 weeks gestation. Subsequent studies have: correlated the epiphyseal centers with fetal sex, identified other epiphyseal centers and have measured the size of these centers correlating them with both gestational age as well as fetal lung maturity as evidenced by a mature L/S ratio. These studies have shown that all fetuses older than 34 weeks had a visible DFE. Also, the DFE may appear as early as 29 weeks, often appearing 2-3 weeks earlier in females than males. Likewise, when the DFE was greater than or equal to 7 mm the gestational age was greater than or equal to 37 weeks.

Subsequent studies in 1986 demonstrated that identification of the proximal humeral epiphyseal ossification center correlated with a gestaional age of greater than or equal to 38 weeks and a mature fetal lung ratio. Likewise, when the combined diameters of the DFE and PTE were > 11 mm and of similar size (DFE greater than or equal to 1mm larger than the PTE) there was evidence of fetal lung maturity.

Several precautions should be taken as it relates to these centers: 1) While the presence of these centers may be helpful in determining fetal gestational age and/or lung maturity their absence does not indicate that a fetus is necessarily earlier in gestation or does not have fetal lung maturity 2) Adjacent sturctures to the cartilaginous centers ie synovium or capsule should not be misinterpreted as the epiphyseal centers and 3) the identification of a DFE of any size may not correlate with fetal lung maturity particularly in complicated pregnancies, such as the diabetic patient.

References:

Chinn DH, Bolding DB, Callen PW, Gross BH and Filly RA. Ultrasonographic identification of fetal lower extremity epiphyseal ossification centers. Radiology 147:815-818, 1983

Gentili P, Trasimeni A, Giorlandino C. Fetal ossification centers as predictors of gestational age in normal and abnormal pregnancies. J Ultrasound Med 3:193-197, 1984

Tabsh KMA. Correlation of ultrasonic epiphyseal centers and the lecithin:sphingomyelin ratio. Obstet Gynecol 64:92, 1984

Mahony BS, Bowie JD, Killam AP, Kay HH, Cooper C. Epiphyseal ossification centers in the assessment of fetal maturity: sonographic correlation with the amniocentesis lung profile. Radiology 159:521-524, 1986

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