Biomechanical evaluation of rescue configurations following posterior implant loss in all-on-4 rehabilitation under different occlusal loads: a finite element analysis

This study aimed to evaluate stress distribution and prosthetic displacement associated with different implant configurations used as biomechanical rescue strategies following posterior implant loss in All-on-4 rehabilitation under functional and parafunctional occlusal loads. Three-dimensional finite element models were developed using computed tomography data of an edentulous maxilla to simulate three rehabilitation scenarios following posterior implant loss in a maxillary All-on-4 case. Model 1 (M1) restored the original All-on-4 configuration using a new tilted posterior implant. Model 2 (M2) combined three conventional implants with a subperiosteal implant featuring two abutment exits, while Model 3 (M3) simulated replacement of the lost implant with a zygomatic implant. All models incorporated screw-retained full-arch polymethyl methacrylate prostheses supported by titanium frameworks. Functional unilateral loading scenarios included: Load 1, a 450 N vertical posterior load distributed over the first premolar, second premolar, and first molar; Load 2, a 93 N oblique canine-guided load applied at 45° from the palatal-to-buccal direction; and Load 3, a 200 N oblique group-function load applied at 30° to the premolar and molar buccal cusps. Parafunctional loading scenarios included: Load 4, a 1000 N vertical posterior load; Load 5, a 250 N oblique canine-guided load; and Load 6, a 500 N oblique group-function load applied to the same contact points. The highest cortical bone stress was observed in M1 under Load 4 (95.944 MPa), whereas the highest implant-body stress was recorded at the posterior abutment exit of M2 under Load 6 (630.449 MPa). The highest framework stress was observed in M1 under Load 4 (448.413 MPa), whereas the greatest prosthetic displacement occurred in M1 under Load 6 (0.442 mm). The lowest framework stress and prosthetic displacement were recorded in M2 under Load 1 (58.971 MPa; 0.050 mm, respectively). The subperiosteal rescue configuration generally showed lower bone and framework stresses and prosthetic displacement, although high implant-body stress was observed under parafunctional loading. The conventional All-on-4 rescue configuration showed greater mechanical demand within the superstructure. The zygomatic rescue configuration exhibited intermediate performance. Parafunctional loading and group-function loading generated less favorable biomechanical responses than functional loading and canine-guided loading. Posterior implant loss in All-on-4 rehabilitation presents a clinical challenge, particularly in patients with parafunctional habits. This study evaluates the biomechanical performance of subperiosteal and zygomatic rescue configurations following posterior implant loss in All-on-4 rehabilitation under different occlusal loading conditions. The findings may provide preliminary biomechanical information to support treatment planning in patients with elevated occlusal risk.