DISTAL FEMORAL OSSIFICATION CENTER

PROXIMAL TIBIAL OSSIFICATION CENTER

The fetal distal femoral and proximal tibial epiphysis ossification centers have a characteristic predictable pattern of development that can be identified and assessed sonographically in the third trimester.

Both ossification centers are in the same plane and are therefore easily detected sonographically.

ULTRASOUND

• The distal femoral epiphysis is identified by visualizing the distal femur and locating the echogenic epiphysis.
• The proximal tibial epiphysis is also an echogenic structure adjacent to the tibial head.
• Measurements are obtained in an axial plane from outer to outer margins along the medial lateral surfaces of the epiphysis (1).
• Both centers increase in size and are more echo-dense in appearance with progressive gestational age.

DISTAL FEMORAL EPIPHYSIS (1)

• < 28 weeks gestation - Not identifiable.
• At 33 weeks gestation - Observed in 72% of fetuses.
• At 34 weeks gestation - Observed in 94% of fetuses.
  
• Size of 1-2mm = Gestational age > 33 weeks (87% of fetuses).
• Size of ³ 3mm = Gestational age > 37 weeks (85% of fetuses).

PROXIMAL TIBIAL EPHYSIS (1)

• < 34 weeks gestation - Not identifiable.
• At 36 weeks gestation - Observed in 56% of fetuses.
• At 37 weeks gestation - Observed in 80% of fetuses.
• At 39 weeks gestation - Observed in 100% of fetuses.
  
• Size of 1-2mm = Gestational age > 36 weeks (88% of fetuses).
• Size of ³ 3mm = Gestational age > 38 weeks (94% of fetuses).

OTHER OSSIFICATION CENTERS

Link to other Less Predictable Ossification Centers

REFERENCES

1. Goldstein I, Lockwood C, Belanger K et.al. Ultrasonographic assessment of gestational age with the distal femoral and proximal tibial ossification centers in the third trimester. Am J Obstet Gynecol 1988;158:127-130.