Framework stiffness and superstructure material govern stress distribution in graftless full-arch rehabilitation of the severely atrophic maxilla: a three-dimensional finite element analysis

Rehabilitation of the severely atrophic edentulous maxilla increasingly relies on graftless strategies, but the way framework stiffness and superstructure material interact with implant design to govern load transfer remains incompletely understood. This study used three-dimensional finite element analysis (FEA) to evaluate how implant design (All-on-Four [BLX/BLT], zygomatic [ZYGi], and patient-specific subperiosteal [SPi]), framework stiffness (titanium versus an idealized high-stiffness graphene reference), and superstructure material (monolithic zirconia versus polymethyl methacrylate [PMMA]) influence stress distribution in the severely atrophic maxilla. A severely atrophic maxilla (Cawood-Howell IV-V) was reconstructed from cone-beam computed tomography data and modeled in 16 implant-prosthetic configurations. Two static loading conditions were applied: 50 N oblique at 30 degrees (Scenario 1) and 150 N vertical (Scenario 2). First (tensile) and third (compressive) principal stresses were recorded in cortical and trabecular bone, and von Mises stresses in implants, abutments, frameworks, and superstructures. Graphene was assigned its intrinsic (bulk) in-plane elastic modulus (approximately 1000 GPa) as a theoretical upper-bound rigidity reference rather than a clinically manufacturable material; titanium alloy (110 GPa) served as the conventional metallic baseline. Because FEA is deterministic, configurations were compared descriptively by relative differences in peak stress; no inferential statistical analysis was applied. Zygomatic implants showed the lowest cortical and trabecular bone stresses overall, and subperiosteal implants showed similarly low trabecular and compressive stresses, although subperiosteal designs produced the highest cortical tensile stress under oblique loading; BLT showed the highest cortical stress under vertical loading. Increasing framework stiffness (titanium to the graphene reference) reduced compressive bone stress and the stress transmitted to the implants while concentrating stress within the framework; effects on abutment stress were small and inconsistent in direction, and effects on tensile bone stress were smaller and more variable. Monolithic zirconia retained more stress within the superstructure and thereby reduced the stress transmitted to the bar, abutments, and implants, whereas the low-modulus PMMA superstructure carried little stress itself but transmitted more load to the bar, abutments, and implants, particularly under vertical loading. The lowest bone-level stresses were observed for zygomatic implants combined with the high-stiffness framework reference and a zirconia superstructure. Within the limitations of this in silico, static, linear-elastic model, framework stiffness and implant design jointly governed load transfer in the atrophic maxilla. The findings characterize the limiting biomechanical behavior of framework stiffness and are hypothesis-generating; the response of clinically realizable graphene-reinforced composites, whose effective modulus is substantially lower, would be expected to lie between the PMMA and titanium cases.