Theoretically, maximum retention is obtained if a tooth preparation has what type of walls?

Maximum retention in tooth preparation is achieved with parallel walls because this design maximizes the surface area in contact with the restorative material. When walls are parallel, they create a consistent resistance to dislodgement forces, which enhances stability and retention of the final prosthetic restoration.

In contrast, divergent walls can compromise retention as they allow for a wider opening toward the occlusal surface, which may lead to a lack of sufficient grip on the restoration. Ovate walls, while beneficial for some aesthetic considerations, do not provide the optimal retention, as their smooth, rounded nature does not create adequate resistance. Perpendicular walls can enhance certain aspects of retention, but they do not offer the same level of surface area and friction against the restoration as parallel walls do. Thus, for maximizing retention, parallel walls are the ideal choice in tooth preparation design.