Objective:To explore which configuration schemes of proximal humerus internal locking system (PHILOS) fixation with endosteal augmentation can provide the optimal biomechanical stability for treatment of osteoporotic proximal humeral fractures by means of finite element analysis.Methods:Based on the CT data of the humerus of an old female volunteer (78 years old, with a bone density T-value of -3.0), a three-dimensional finite element model of the humerus was established by digital medical software such as Mimics 19.0, Geomagic Studio 12, and Creo 2.0 ANSYS Workbench2019. Next, a model of unstable proximal humerus fracture was established and subjected respectively to 5 different fixations: simple PHILOS fixation (PHILOS group), PHILOS plus 6-cm fibula fixation with calcar screws (PHILOS-F-C-6 group), PHILOS plus 6-cm fibula fixation without calcar screws (PHILOS-F-6 group), PHILOS plus 9-cm fibula fixation with calcar screws (PHILOS-F-C-9), and PHILOS plus 9-cm fibula fixation without calcar screws (PHILOS-F-9 group). After a stress mode of shoulder joint abduction at 25° was simulated, a compressive load of 200N was applied to the 5 fixation models. The stress distribution and displacement of fracture ends in different fixation models were tested, and the biomechanical stability was compared among the 5 different internal fixations.Results:Under a shoulder joint abduction at 25° and a load of 200 N, the maximum stress and the displacement of the fracture ends in PHILOS-F-C-9 group (38.678 Mpa and 0.012 mm) decreased by 30.08% and 45.45%, respectively, compared with PHILOS-F-C-6 group (55.321 Mpa and 0.022 mm), and decreased by 12.48% and 15.38%, respectively, in PHILOS-F-9 group (77.012 Mpa and 0.033 mm) compared with PHILOS-F-6 group (88.106 Mpa and 0.039 mm). The maximum stress and the displacement of the fracture ends in PHILOS-F-C-6 group decreased by 37.21% and 43.59%, respectively, compared with PHILOS-F-6 group while decreased by 49.83% and 63.63% in PHILOS-F-C-9 group compared with PHILOS-F-9 group, respectively.Conclusion:For treatment of osteoporotic proximal humeral fractures with medial instability, PHILOS fixation with longer fibula endosteal augmentation plus insertion of calcar screws is a more appropriate choice which can reduce the stress of internal fixation and reduce the displacement of the fracture ends.