Theoretical cantilever lengths versus clinical variables in fifty-five clinical cases

Cantilever loading increases loads distributed to implants, potentially causing biomechanical complications. The implemented length is often less than what is considered to be optimal.

This study investigated the effects of clinical variables on predicted cantilever lengths. Theoretically, calculated maximum cantilever was defined as the length that would not cause gold screw loosening or fatigue failure. The variables investigated included number and distribution of implants, arches placed, and the clinician's "optimal" cantilevers.

Implant and prosthesis location coordinates of 55 clinical cases were determined from casts. The distribution of an applied 143 N vertical load to implants was calculated through the Skalak model for more than 500 loading sites. Gold screw joint overload was assumed to occur at 200 and 250 N in compression and tension. Calculated lengths were compared with clinical variables.

For a set number of implants, the relationship between calculated cantilever length and anterior-posterior spread was linear. The sum of length on both sides versus prosthesis length between the most distal implants was linear, regardless of the number of implants. Predicted satisfaction was defined as calculated length greater than the clinicians' optimal length. Satisfaction rates were 100%, 56%, 33%, 8%, and 0% for cases supported by 8 and 7, 6, 5, 4, and 3 implants (44% overall), respectively. Ninety-eight percent of cases with anterior-posterior spreads greater than 11.1 mm were satisfied.

Within the limitations of the model, predicted complications of the gold screw joint may be reduced if: (1) cantilever length is less than calculated from linear equations, and (2) anterior-posterior spread is greater than 11.1 mm.