Implant-bone load transfer mechanisms in full-arch prostheses supported by four implants: a three-dimensional finite element approach

Statement of problem: complete-arch restorations supported by fewer than 5 dental implants can induce unbalanced load transfer and tissue overloading, leading to excessive bone resorption and possible clinical failure. This is primarily affected by the cantilever length, the implant design and positioning, and the morphology and properties of the bone. Purpose: The purpose of this study was to compare 2 different restorative techniques for complete-arch rehabilitations supported by 4 implants. The primary purpose was to highlight the possible risks of excessive stress and unbalanced load transfer mechanisms and to identify the main biomechanical factors affecting loading transmission. Material and methods: Three-dimensional (3D) numerical models of edentulous maxillae and mandibles restored with 2 techniques using 4 implants were generated from computed tomography (CT) images and analyzed with linear elastic finite-element simulations with 3 different static loads. The first technique used 2 vertical mesial implants and 2 tilted distal implants (at a 30 degree angle), and the second used vertical implants that fulfilled platform switching concepts. Bone-muscle interactions and temporomandibular joints were included in the mandibular model. Complete implant osseous integration was assumed and different posthealing crestal bone geometries were modeled. Stress measures (revealing risks of tissue overloading) and a performance index (highlighting the main features of the loading partition mechanisms) were introduced and computed to compare the 2 techniques. Results: Dissimilar load transfer mechanisms of the 2 restorative approaches when applied in mandibular and maxillary models were modeled. Prostheses supported by distally tilted implants exhibited a more effective and uniform loading partition than all vertical implants, except in the simulated maxilla under a frontal load. Tilted distal implants reduced compressive states at distal bone-implant interfaces but, depending on bone morphology and loading type, could induce high tensile stresses at distal crests. Overloading risks on mesial periimplant bone decreased when the efficient preservation of the crestal bone through platform switching strategies was modeled. Conclusions: Numerical simulations highlighted that the cantilever length, the implant design and positioning, and the bone's mechanical properties and morphology can affect both load transmission mechanisms and bone overloading risks in complete-arch restorations supported by 4 implants. Distally tilted implants induced better loading transmission than vertical implants, although the levels of computed stress were physiologically acceptable in both situations.