A retrospective study analyzed 236 patients with osteoporosis and traumatic fracture of the proximal humerus treated in 2024, divided into a group with shoulder stiffness (119 cases) and without (117 cases). The incidence of shoulder joint stiffness 6 months after surgery was 50.4%. The stiffness group had a higher mean age (52.83 ± 6.65 years vs. 43.31 ± 6.48 years), a higher proportion of patients with BMI ≥24 kg/m² (73.1% vs. 50.4%, p < 0.001), more smokers (42.9% vs. 10.9%, p = 0.003), and less preoperative physical therapy. Multivariate analysis confirmed advanced age (OR = 1.297), overweight (OR = 5.599), smoking (OR = 3.270) and prolonged disease course (OR = 2.409) as independent risk factors, while preoperative standardized physical therapy was a protective factor (OR = 0.187). There was no difference in the incidence of stiffness between the plate and nail fixation methods. ROC analysis showed a high predictive value of age (AUC = 0.857) and disease course (AUC = 0.770). A three-step prevention is recommended: smoking cessation, weight control, and early physical therapy with personalized measures.