PURPOSE OF THE STUDY To investigate the pressure levels on the axillary nerve at different plate positions after plate fixation of a proximal humerus fracture. MATERIAL AND METHODS Eight fresh-frozen cadaveric specimens were used. The plates were applied on the lateral side of the humerus. A FlexiForce pressure sensor was placed between the nerve and the plate, and the pressure on the nerve was measured (Group 1). The plates were then placed in two different positions such that distal portion of the plates would have 30° anterior and 30° posterior angles to the anatomical axis of the humerus (Group 2 and 3). The anterior of the distal attachment of the deltoid muscle was then elevated and the plates were placed in the middle of the anatomical axis of the humerus (Group 4). The position of the plates were controlled by fluoroscopy and the pressure was measured for each configuration sequentially. RESULTS The mean age of the cadavers was 70.5 ± 6.8 years (range: 61-80 years). Mean pressure values of the groups were 2.65 ± 0.8, 2.52 ± 0.8, 5.65 ± 1.4, and 2.56 ± 0.9 N, respectively. Group 3 had statistically highest-pressure values than the other groups, while no difference was found among groups 1, 2, and 4. DISCUSSION Numbness and weakness of the shoulder muscles are other clinical findings. If numbness on the deltoid muscle is reported, then atrophy is noted in the deltoid muscle in later stages. Persistent pain may be seen even if fracture union occurs after PHF surgery. Axillary nerve entrapment may be considered after the removal of common complications such as avascular necrosis due to fracture, screw migration, infection, and biceps tendon and rotator cuff problems. The sensory branch of the axillary nerve provides the sensation of the anterior joint capsule and lateral part of the deltoid muscle. CONCLUSIONS Proximal humerus plates, which are angled posteriorly along, lead to an increased pressure on the axillary nerve. Anterior orientation of the plate or elevation of deltoid insertion may be used to prevent the possible complications related to axillary nerve. Level of evidence Level II. Key words: proximal humerus fracture, Philos, axillary nerve, nerve injury.

Istanbul Medeniyet University Department of Orthopaedic Surgery and Traumatology Istanbul Turkey

Medilife Private Hospital Department of Orthopaedic Surgery and Traumatology Istanbul Turkey

Ministry of Health Silopi State Hospital Department of Orthopaedic Surgery and Traumatology Sirnak Turkey

University of Health Sciences Istanbul Fatih Sultan Mehmet Training and Research Hospital Department of Orthopaedic Surgery and Traumatology Istanbul Turkey

University of Health Sciences Istanbul Umraniye Training and Research Hospital Department of Orthopaedic Surgery and Traumatology Umraniye Istanbul Turkey