Evaluation of strain at the terminal abutment site of a fixed mandibular implant prosthesis during cantilever loading

Cantilever lengths from 10 mm to 20 mm have been empirically recommended for Brånemark fixed mandibular implant prostheses. However, functional stresses generated within the framework and at the crestal bone associated with various cantilever lengths have not been well researched. The purpose of this investigation was to evaluate the strain generated within an implant-supported prosthesis and on a simulated bone surface during functional cantilever loading.

A symmetrical mandibular fixed-implant framework supported by six Nobelpharma 7.0 x 4.0-mm abutments and 15.0 x 4.0-mm fixtures was fabricated. The fixtures were embedded in a simulated bone matrix of polymethyl methacrylate resin. Fourteen different arrangements of active supporting abutments were tested during 15 lb unilateral static cantilever loading 7 mm, 14 mm and 20 mm distal to the terminal abutments. T-rosette strain gauges were placed immediately distal to the terminal abutment site on the right side of the framework and on the corresponding simulated bone surface.

There was no difference in framework microstrain as abutment number and arrangement were varied. Microstrain distal to the terminal abutment increased significantly with increasing cantilever length. Distal abutment microstrain increased 213% (63 mu epsilon to 197 mu epsilon) when cantilever length was increased from 7 mm to 14 mm and an additional 55% (197 mu epsilon to 306 mu epsilon) when cantilever length was increased from 14 mm to 20 mm. Overall, microstrain increased 306% when cantilever length was increased from 7 mm to 20 mm. Microstrain on the framework was always tensile (positive). Microstrain at the simulated bone reached higher maximum levels than on the framework (-588 mu epsilon versus 314 mu epsilon) and was compressive in nature (negative). In contrast to framework microstrain, microstrain at the simulated bone site varied dramatically with changes in abutment arrangement. Strains observed at the simulated bone surface increased dramatically as the distance to the adjacent active abutment increased or as the anterior-posterior span of abutments decreased. Distal abutment microstrain also increased significantly at the bone site as cantilever length increased, however, percent increases were less (7 mm to 14 mm, 55%; 14 mm to 20 mm, 30%; 7 mm to 20 mm, 101%).

The results of this study indicate that an optimum biomechanical environment should exist when cantilever spans exceeding 7 mm are planned regardless of the number of supporting abutments. Strain transmitted to the crestal bone can be decreased by maximizing the number and anterior-posterior spread of supporting fixtures while minimizing the distance between the distal abutment and its adjacent abutment.