THE JAW INDEX

There appears to be a linear relationship between mandibular growth and gestational age or BPD (1).

The sonographic diagnosis of micrognathia is subjective resulting in some error in the diagnosis. Workers (1) have attempted to develop an accurate and more objective method to diagnose fetal micrognathia (The Jaw Index).

MEASUREMENT OF JAW INDEX

• This technique was derived, as lateral growth of the mandible, even in micrognathia appears to be relatively normal. The growth process is primarily impaired in an antero-posterior plane.
• Axial plane at the base of the cranium just caudal to the lower dental arch where the mandible has a horseshoe shape.
• Laterolateral diameter was obtained by joining the bases of the two rami.
• AP diameter from symphysis mentis to the middle of the laterolateral diameter.
• Jaw index = AP mandibular diameter / BPD × 100.

Jaw index	Sensitivity (%)	Specificity (%)	PPV (%)	NPV (%)
<24 <23 <22 <21	100 100 90.1 72.7	98.1 98.1 99.2 100	68.7 68.7 83.3 100	100 100 99.6 98.9

REFERENCES

1. Paladini D, Morra T, Teodoro A et.al. Objective diagnosis of micrognathia in the fetus: The Jaw Index. Obstetrics and Gynecology 1999;93:382-386.