Background: Humeral head inferior subluxation often occurs immediately after osteosynthesis for proximal humeral fracture; however, the underlying cause remains largely unknown. In addition, the prognosis of postoperative inferior subluxation has not been fully investigated. This study aimed to clarify the predictive factors that affected the onset of postoperative inferior subluxation using multivariate analysis and examine the postoperative course of inferior subluxation and its influence on postoperative outcomes. Methods: We retrospectively reviewed 212 patients who underwent osteosynthesis for Neer 2- or 3-part proximal humeral fractures. In the multivariate analysis, the dependent variable was set as the inferior subluxation observed 1 week after the surgery. The explanatory variables included age, sex, affected side, body mass index, smoking, local osteoporosis, preoperative axillary nerve injury, time from injury to surgery, fracture dislocation, fracture pattern, preoperative inferior subluxation, surgical procedure, surgical approaches, blood loss, operative time, and postoperative drainage. Baseline variables, which were observed to be significant in the univariate analysis, were included in multivariate models. Furthermore, based on the presence of inferior subluxation at 1 week after the surgery, we divided the patients into two groups: with inferior subluxation (+IS group) and without inferior subluxation (−IS group). We compared the postoperative outcomes (incidence of postoperative complications and range of motion) between these two groups. Results: Of 212 patients, 64 (30.7%) experienced inferior subluxation at 1 week after the surgery. On multivariate analyses, preoperative inferior subluxation (odds ratio = 4.69; 95% confidence interval = 2.45-9.76; P < .001) and longer operative time (odds ratio = 1.01; 95% confidence interval = 1.00-1.02; P = .049) were the risk factors for postoperative inferior subluxation. In the +IS group, inferior subluxation resolved at 1 year after the surgery in 89.5% of patients. There was no significant difference in the postoperative outcome between the +IS and −IS groups. However, four of six patients with persistent inferior subluxation, more than 6 months after the surgery, experienced complications, such as varus angulation of the humeral head or screw joint perforation. Conclusions: This study provides new information on the risk factors for and prognosis of postosteosynthesis inferior subluxation in patients with proximal humeral fracture. Longer operative time and presence of preoperative inferior subluxation was associated with an increased risk of postoperative inferior subluxation, although it was temporary in most cases, and had no significant influence on the postoperative outcomes. However, in patients with persistent inferior subluxation of more than 6 months duration, inferior subluxation may be related to postoperative complications.
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