Which angle decreases in deep occlusion?

Which angle decreases in deep occlusion?

• A. FMA
• B. FMIA
• C. SNA
• D. SNB

Answer: (A)

In deep occlusion, the vertical dimension is reduced, affecting angles that describe vertical relationships rather than sagittal jaw positions. The FMA is the angle between the Frankfort horizontal plane and the mandibular plane. With a reduced vertical dimension, the mandible tends to rotate in a way that makes the mandibular plane align more closely with the Frankfort horizontal, so this angle becomes smaller. The other angles reflect different aspects: SNA and SNB measure sagittal positions of the maxilla and mandible relative to the cranial base, which don’t necessarily decrease just from a deep bite. FMIA assesses the tilt of the lower incisors to the Frankfort plane; in deep bite, the lower incisors often retrocline to increase overbite, which would tend to increase FMIA rather than decrease. So the angle that decreases in deep occlusion is the FMA.

In deep occlusion, the vertical dimension is reduced, affecting angles that describe vertical relationships rather than sagittal jaw positions. The FMA is the angle between the Frankfort horizontal plane and the mandibular plane. With a reduced vertical dimension, the mandible tends to rotate in a way that makes the mandibular plane align more closely with the Frankfort horizontal, so this angle becomes smaller.

The other angles reflect different aspects: SNA and SNB measure sagittal positions of the maxilla and mandible relative to the cranial base, which don’t necessarily decrease just from a deep bite. FMIA assesses the tilt of the lower incisors to the Frankfort plane; in deep bite, the lower incisors often retrocline to increase overbite, which would tend to increase FMIA rather than decrease. So the angle that decreases in deep occlusion is the FMA.