What is the angulation of a prep for a rest seat?

What is the angulation of a prep for a rest seat?

• A. Less than 90 degrees
• B. Exactly 90 degrees
• C. Greater than 90 degrees
• D. 120 degrees

Answer: (A)

The rest seat should be prepared with an angle that is less than 90 degrees. This creates a positive, slightly convergent internal surface that allows the rest to seat smoothly along the intended path of insertion and distribute the functional forces along the tooth without binding. If the angle were 90 degrees or more, the walls could form a corner or an outward-flaring shape that impedes seating, concentrates stress, or creates undercuts, making the rest difficult to place reliably. A 120-degree angle would be too obtuse and not provide the proper containment for the rest.

The rest seat should be prepared with an angle that is less than 90 degrees. This creates a positive, slightly convergent internal surface that allows the rest to seat smoothly along the intended path of insertion and distribute the functional forces along the tooth without binding. If the angle were 90 degrees or more, the walls could form a corner or an outward-flaring shape that impedes seating, concentrates stress, or creates undercuts, making the rest difficult to place reliably. A 120-degree angle would be too obtuse and not provide the proper containment for the rest.