Background Functional outcome in surgical treatment of Neer three- and four-part proximal humerus fractures (PHF) varies greatly and depends on multiple parameters. Important parameters are the amount and direction of displacement and the necessary reduction of fragments during surgery. These are patient-specific parameters and are difficult to determine using traditional modalities such as radiographs and computed tomography (CT). Methods A 58-year-old female patient was reported in the emergency department with a three-part PHF. CT scan images showed that the humeral shaft was medialised and internally rotated, but with a marginally displaced greater tuberosity fragment. Using a bone-determined range of motion (ROM) simulation system, we analysed the CT scan and calculated the required correction needed to prevent post-operative impingement. The fracture was reduced and stabilised by a locking plate, realigning the medialised and internally rotated humeral shaft. The post-operative bone-determined ROM was determined using a post-operative CT scan and the motion simulation system. Results ROM limitations due to bony impingement visible in the simulations of the pre-operative CT scan had mostly disappeared in the simulations of the post-operative CT scan. Twelve weeks post-surgery the patient has regained close to 80% of her ROM. Conclusions This case demonstrates the applicability of a new diagnostic tool that can be used to identify bony impingement and helps in making the decision for conservative or surgical treatment of a PHF. The simulation of post-trauma function was indicative of functional outcome. This supports our claim that the system may be used to facilitate the treatment decision regarding PHF.