The primary principle ensuring the retention of an amalgam restoration is occlusal convergence. This refers to the slight taper of the cavity walls towards each other, creating a shape that resists dislodgement by occlusal forces. Convergent walls provide mechanical interlocking, stabilizing the restoration without relying solely on additional retention aids. While retentive pins and undercuts contribute to retention, the controlled convergence of cavity walls is fundamental for amalgam, which lacks inherent adhesive properties. Adhesion from dental cement is not a key factor since amalgam is a non-adhesive material. Thus, proper cavity design with occlusal convergence is critical for durability and retention.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.