What direction of forces do retention grooves on a crown prep primarily resist?

Retention grooves on a crown preparation are specifically designed to enhance the mechanical retention of the crown down into the tooth structure. Their primary function is to resist displacement forces that may occur as a result of occlusal loading and a patient's chewing forces.

The direction of forces that these grooves primarily resist is buccal-lingual. This is because, during mastication, the forces are often directed towards the sides of the preparation (buccal and lingual surfaces), which could potentially dislodge the crown. The grooves provide an area of increased surface area and a locking mechanism that helps prevent the crown from being easily displaced in this direction.

In contrast, mesial-distal forces, occluso-vertical forces, and diagonal forces do not primarily play as significant a role relative to the specific design and purpose of retention grooves. While these forces may occur and can affect the stability of the crown, the key role of retention grooves is to primarily combat buccal-lingual forces that threaten to dislodge the restoration.