Abstract: BACKGROUND: Clinical studies suggest that both bone quality and excessive crown-to-implant ratio are factors that affect the success rate of implant surgery, but there is no consensus on how large the crown-to-implant ratio under each bone quality will affect the prognosis of implant repair.
OBJECTIVE: To analyze the stress and strain of bone tissue around implant restorations with different crown-to-implant ratios under different bone types after stress loading using three-dimensional finite element method. Combined with the Frost bone mechanics regulation system theory, the physiological limits of bone strain were observed for each bone type within a certain range of crown to implant ratios. 
METHODS: Cone beam CT data were selected from a patient to establish solid bone block models with four different bone types (class I bone: cortical bone thickness 3 mm + dense cancellous bone, class II bone: cortical bone thickness 2 mm + relatively dense cancellous bone, class III bone: cortical bone thickness 1 mm + relatively low-density cancellous bone, class IV bone: cortical bone thickness 1 mm + low-density cancellous bone). Implant restorations with five different crown-to-implant ratios (1, 1.5, 2, 2.5, and 3) were created on each bone block model, for a total of 20 models. Vertical and oblique forces were applied to the dental crown to observe the von Mises stress values, strains, and displacement of the cortical and cancellous bones, as well as the degree of implant displacement.
RESULTS AND CONCLUSION: (1) In the class I bone model, when the crown-to-implant ratio reached 3 under oblique loading, the cortical bone strain exceeded the physiological limit. In the class II bone model, under oblique loading, when the crown-to-implant ratio reached 2.5, the cortical bone strain exceeded the physiological limit. In the class III bone model, under oblique loading, cortical bone exceeded the physiological limit when the crown-to-implant ratio reached 2.5, while cancellous bone reached the critical physiological limit at 1. In the class IV bone model, under oblique loading, when the crown-to-implant ratio reached 1.5, 2, 2.5, and 3, the cortical bone strain exceeded the physiological limit. In the class IV bone model, all five crown-to-implant ratios of cancellous bone exceeded physiological limits. (2) Under vertical load, the cancellous bone strain exceeded the physiological limit when the crown-root ratio reached 1, 2, 2.5, and 3 in the class III bone model, and when the cancellous bone strain of the five crown-root ratios in the class IV bone model exceeded the physiological limit. (3) Under oblique or vertical load, the implant displacement of the 20 groups of models did not exceed 100 μm. (4) From the perspective of biomechanics, when patients with insufficient bone height choose implant restoration, the crown-root ratio that class I bone can tolerate is up to 2.5 times, the crown-root ratio that class II bone can tolerate is up to 2 times, and the crown-root ratio that class III and class IV bones can tolerate is up to 1 times due to excessive strain of cancellous bone; but the crown-root ratio that cortical bone in class III bone can tolerate is up to 2 times. Whether class III bone can tolerate implant restoration with a high crown-root ratio and whether cancellous bone can tolerate higher strains needs further study.

Key words: short implant, crown-root ratio, bone, finite element, biomechanics, strain, engineered oral material