NORMAL MANDIBLE AND MAXILLA

• Scan plane parallel to mandible including (fetal hypopharynx is visible).
• Echogenic tip of the mandible should always be seen in the same plane as the nose and lips on the coronal view.

• Transverse diameters (inner-inner mandible measurements so that the line between these touched the anterior fetal hypopharynx)
• AP diameters (from inner surface of the jaw to intersect with the transverse line).
• Both AP and transverse diameters increased with advancing gestation. The transverse diameter is always larger than the AP distance.
• Normal maxilla – note integrity of alveolar ridge (note defect in alveolar ridge in cleft involving hard palate).

• Jaw Index

THE INFERIOR FACIAL ANGLE (IFA)

1. The IFA was derived from the Z-angle established by Merrifield to study facial harmony (2). The Z-angle was defined as the inferior angle formed by the crossing of two lines. The reference line was the Frankfort line that joins the cephalic portion to the inferior border of the orbit. It is easily visualized, both in the patients external appearance and in the lateral radiograph.
2. The IFA was defined on a sagittal view by the crossing of two lines:
• A line orthogonal to the vertical part of the forehead, drawn at the level of the synostosis of the nasal bones (reference line
• A line joining the tip of the mentum and the anterior border of the more protrusive lip (profile line) (3).
3. The mean (standard deviation) value of the inferior facial angle was 65.5 (8.13)°. Consequently, an inferior facial angle value below 49.2° (mean - 2 standard deviations) defined retrognathism.

MANDIBULAR (MD) AND MAXILLARY WIDTH (MX) (3)

• The MD and MX were measured on an axial plane caudal to the base of the cranium, at the level of the alveolus (dental arch).
• A line orthogonal to the sagittal axis was drawn 10 mm posteriorly to the anterior osteous border (approximately at the level of the canines).
• Measurements were obtained from one external bone table to the other.
• The MD/MX ratio was derived from these two measurements.
• In normal fetuses, the mandible width/maxilla width ratio was constant over the time interval studied.
• The mean (standard deviation) value was 1.017 (SD = 0.116).
• A mandible width/maxilla width ratio < 0.785 defined micrognathism

Axial scan at level of Mandible	Axial scan at level of tongue (just above mandible)	Axial scan at level of Maxilla

· Various investigators have assessed mandible size. Fetal mandible length (FML) measurements have been reported by numerous authors ( 4,5). The FML measurements were obtained in a plane that imaged the main portion of one ramus of the jaw, between the temporomandibular joint and the symphysis mentis. Both groups found similar results. The increase in FML was almost linear with increasing gestational age, and varied from a mean value of approximately 2.0 cm at 18 gestational weeks to 3.7 cm at 28 weeks. Only one pathological case, a fetus with trisomy 18, was available for analysis during the course of the first study (4). In spite of a visually normal profile, mandible length was below the 95 prediction limit for gestational age (late second trimester).

Mandible length -Chervenak et al. 1991 (Tables) (Graphs)

• Other authors (6,7) assessed the size of the mandible body through the measurement of its transverse diameter (TD) and anteroposterior diameter (APD). The measurements were obtained on an axial plane at the base of the cranium, just caudal to the lower dental arch, where the full extent of the horseshoe-shaped mandible is imaged. The TD was the distance between the bases of the two rami. The APD was the distance between the symphysis mentis and the middle of the TD.

Normal AP And Transverse Measurements

REFERENCES

1. Watson WJ, Katz VL. Sonographic measurement of the fetal mandible: Standards for normal normal pregnancy. Am J Perinatol 1993;10:226-228.
2. Merrifield LL. The profile line as an aid in critically evaluating facial esthetics. Am J Orthodontics 1966; 52: 804-22
3. Rotten D, Levaillant JM, Martinez H,H et.al. The fetal mandible: a 2D and 3D sonographic approach to the diagnosis of retrognathia and micrognathia. Ultrasound Obstet Gynecol 2002;19:122- 129.
4. Otto C, Platt LD. The fetal mandible measurement: an objective determination of fetal jaw size. Ultrasound Obstet Gynecol 1991; 1: 12-7
5. Chitty LS, Campbell S, Altman DG. Measurement of the fetal mandible feasability and construction of a centile chart. Prenat Diagn 1993; 13: 749-56
6. Watson WJ, Katz VL. Sonographic measurement of the fetal mandible: standards for normal pregnancy. Am J Perinatol 1993; 10: 226-8
7. Paladini D, Morra T, Teodoro A, Lamberti A, Tremolaterra F, Martinelli P. Objective diagnosis of micrognathia in the fetus: the jaw index. Obstet Gynecol 1999; 93: 382-6