KEYWORDSproximal humeral fracture, locking plate, internal fixation, biomechanics, finite element analysis 2 Abstract Background. We investigated the therapeutic effects associated with the combined use of lateral and posterior plates for treating complex proximal humerus fractures. Methods. We used in vitro biomechanical experiments and computer three-dimensional finite element analyses to investigate the biomechanical properties of combined lateral and posterior plates when treating proximal humerus fractures. Eighteen left SAWBONE (Pacific Research Labs) humerus bones were randomly divided among three groups. We established a medial column defect model for surgical neck fractures in each bone specimen and achieved fixation using a proximal humerus locking plate. Each of the three groups used a different fixation method. Group A used a Proximal Humerus Internal Locking System (PHILOS) plate support only. Group B used the PHILOS plate with a posterior locking plate (but without medial column support screws). Group C used the PHILOS plate with both the posterior locking plate and medial column support screws. We subjected three sets of specimens to axial compression, torsion, shear compression, model failure, fatigue testing, and micro-strain analyses.Results and conclusions. In vitro biomechanical analysis and three-dimensional finite element analyses showed that the PHILOS plate, in combination with the posterior locking plate and medial column support screws (Group C), had significantly enhanced biomechanical properties when compared with traditional single plate supports.