Every full arch implant restoration designer must understand A-P spread before touching any CAD software. This article explains what it is, how to measure it, and how to use it for safe cantilever design.
What Is A-P Spread?
A-P spread (anterior-posterior spread) is the linear distance between the center of the most anterior implant and a line connecting the center of the two most distal implants. It represents the mechanical base of support for the prosthesis.
The Formula
Maximum cantilever length = A-P spread × 1.5. For example, with an A-P spread of 20mm, the maximum safe cantilever is 30mm total (15mm per side with bilateral symmetry). Exceeding this ratio exponentially increases force on distal implants.
Clinical Significance
Poor A-P spread (less than 15mm) in All-on-4 cases creates high-risk cantilever situations. We recommend at least 18mm A-P spread for standard full arch restorations. When A-P spread is limited, consider adding additional implants to convert to an All-on-5 or All-on-6 configuration.
Our design protocol measures A-P spread on every full arch case before beginning design. If the spread is inadequate, we flag it for the clinician before proceeding.