Optimal retention of a complete denture primarily depends on the maximum extension of the denture base within the anatomic limits. This ensures a greater surface area for adhesion and cohesion between the denture base and the mucosa, leading to enhanced peripheral seal and stability. Proper extension also enables effective utilization of the natural anatomic undercuts and support areas. In contrast, factors like increased vertical dimension or adhesives help retention to a lesser extent and are adjuncts rather than primary determinants. Hence, achieving the correct and extended denture base coverage is crucial for optimal retention.

Reference: Prosthodontics, Sharry & Zarb, 13th Edition.