What orientation do extended occlusal rests have relative to the long axis?

What orientation do extended occlusal rests have relative to the long axis?

• A. Perpendicular to the long axis
• B. Parallel to the long axis
• C. Angled buccolingually
• D. Oblique to the occlusal plane

Answer: (A)

Extended occlusal rests are meant to carry masticatory forces vertically and transfer them along the tooth’s long axis. To do this effectively, the rest seat is prepared so that its floor lies perpendicular to the tooth’s long axis. This orientation creates a uniform, adequately thick rest that seats flush and transmits forces in a straight path down the root, reducing tipping moments on the abutment. If the rest were parallel to the long axis, loading could introduce unwanted lever effects and uneven seating, while an oblique or buccolingual angle would introduce shear forces and off-axis loading, compromising stability and tooth integrity. So, the resting surface is perpendicular to the long axis.

Extended occlusal rests are meant to carry masticatory forces vertically and transfer them along the tooth’s long axis. To do this effectively, the rest seat is prepared so that its floor lies perpendicular to the tooth’s long axis. This orientation creates a uniform, adequately thick rest that seats flush and transmits forces in a straight path down the root, reducing tipping moments on the abutment.

If the rest were parallel to the long axis, loading could introduce unwanted lever effects and uneven seating, while an oblique or buccolingual angle would introduce shear forces and off-axis loading, compromising stability and tooth integrity. So, the resting surface is perpendicular to the long axis.