The minimum acceptable crown-to-root ratio for a fixed prosthesis is 1:1.5, meaning the root embedded in bone should be at least one and a half times the length of the clinical crown. This ratio ensures adequate periodontal support to withstand occlusal forces and maintain prosthesis stability. Ratios less than 1:1 can result in increased mobility and risk of failure due to insufficient root anchorage. Therefore, a 1:1.5 ratio provides an optimal balance between biological support and mechanical leverage necessary for long-term success of fixed dental prostheses.

Reference: Weiss, Herbert, Essentials of Fixed Prosthodontics, 4th Edition.