Radiological Outcomes, Complications, and the Influence of Risk Factors in PHILOS Repair of Three- and Four-Part Proximal Humerus Fractures with and Without Femoral Head Allograft: Insights from a Cohort of 116 Patients

<b>Background</b>: Complications after proximal humerus osteosynthesis are not uncommon. The aim of this study was to compare the outcomes of osteosynthesis using PHILOS with fresh-frozen femoral head allograft augmentation and without it, and to assess the influence of risk factors and their impact on the occurrence of postoperative complications. <b>Methods</b>: This retrospective study evaluates the radiological outcomes and complications of treating proximal humerus fractures (Neer III-IV) in 116 patients over 50 years of age treated between 2017 and 2021. <b>Results</b>: Osteosynthesis without allograft was performed in 84 patients and with allograft in 32 patients. In total, 42 patients (36%) had a three-part fracture and 74 (64%) had a four-part fracture. The Deltoid Tuberosity Index was comparable between the groups (1.59 ± 0.25 vs. 1.50 ± 0.26; <i>p</i> = 0.802). The average duration of surgery was 101.3 ± 21.3 min with allograft and 86.0 ± 31.9 min without allograft (<i>p</i> = 0.004). AVN was verified in four patients (3.5%), head collapse in nine (8%), cut-out in six (5%), reoperation in eight (7%), infection in three (2.5%), and pseudoarthrosis in one (1%) case. <b>Conclusions</b>: An allograft augmentation improves construct stability, but cannot compensate for inadequate surgical technique. None of the risk factors significantly influenced the development of AVN and pseudoarthrosis. The greater tubercle comminution (<i>p</i> = 0.005), calcar loss (<i>p</i> = 0.020, <i>p</i> = 0.112), allograft augmentation (<i>p</i> < 0.001), and medial hinge restoration (<i>p</i> = 0.012, <i>p</i> = 0.002) were significant risk factors associated with HC and screw cut-out, respectively. The greater tubercle redislocation was influenced by its comminution, calcar loss, and the use of allograft augmentation. HFZ and DTI had no significant impact on surgery results or complications.