Proximal humerus fractures account for about 4%-5% of systemic fractures in clinical practice, ranking third in human incidence after hip and distal radial fractures. The classification and treatment of proximal humeral fractures are varied, and the optimal treatment is still widely debated. In recent years, internal fixators have been developed due to the continuous improvement of surgical technology. Various internal fixation methods, such as percutaneous internal fixation technology, locking plates, intramedullary nails, and shoulder joint replacement, are available. Studies have confirmed that using a lateral locking plate (LLP) for proximal humeral fractures has good biomechanical properties and become a commonly used fixation method in clinical practice for proximal humeral fractures with significant efficacy. It has the advantages of a stable structure and mature technology. However, LLP has shown a high reoperation rate with the increasing use of locking plates in proximal humeral fractures. Many associated complications have begun to trouble most orthopedic doctors, such as postoperative fracture varus deformity, malunion, screw release, bone nonunion, subacromial impingement, humeral head ischemic necrosis, infection, etc. Among them, the incidence of postoperative fracture varus deformity is high. Among the factors leading to this series of postoperative complications, inadequate medial column support is widespread in proximal humerus fractures. The lack of adequate medial column support for treating proximal humeral fractures with a LLP alone has not been identified. Some scholars have proposed adding humerus talus screws to the lateral plate. In addition, they also used the bone cement filling, the combined application of anterolateral and lateral plates, and the combined application of posterior and lateral plates to address the absence of the proximal humeral medial column. However, none of these surgical methods directly support the medial humerus to improve biomechanical stability. Yu He et al. used both the LLP and the medial locking plate (MLP) to provide strong support for the medial column to enhance the stability of the fracture site and reduce varus deformity. However, this kind of double-incision surgery will inevitably cause excessive damage to the soft tissue of the shoulder joint. In addition, due to the limited space at the proximal end of the humerus, the screws inevitably interact with each other. Therefore, to reduce the damage to the soft tissue of the shoulder joint and avoid the interaction between screws, and provide direct support to the medial column, we wondered if a MLP could be used alone to treat proximal humeral fractures.

To compare the biomechanical stability of medial column support for proximal humeral fractures using LLP and MLP with three-dimensional modeling technology and finite element analysis to put forward relevant suggestions for the clinical treatment of such fractures.

The CT DICOM data of the proximal humerus were obtained and processed by Mimics 21.0 and Geomagic Wrap 2017 software. The proximal humerus fracture defect model was designed by SolidWorks 2017 software, and the internal fixation model containing the locking plate and screw was established. Subsequently, according to the principles of AO fracture treatment, the implants were assembled with the designed proximal humeral fracture model and divided into group A (LLP group) and group B (MLP group). The above two groups of models were imported into the ANSYS Workbench 2019 finite element analysis software in STEP format for mesh assignment and calculation. The distal humerus was defined as complete fixation. The axial compression, shear, and torsional forces were tested in groups A and B, respectively. The humerus and implants' stress magnitude, distribution, and displacement were compared between the two internal fixation models under three different loading conditions.

Under axial compression, shear, and torsion, the maximum stress and displacement of the humerus, the maximum stress and displacement of the implant, and the relative displacement of the fracture end in group B were less than in group A.

In proximal humeral fractures with intact lateral cortex but absent medial column, the MLP fixation provides better structural stability than that of the LLP under axial, shear, and torsional loads, which is an effective method for the treatment of medial column instability in proximal humeral fractures. This study provides evidence for the biomechanical advantages of this new medial support technique. It thus offers additional clinical options for the surgical treatment of proximal humeral fractures with medial column loss.