Monoaxial Versus Polyaxial Screw Insertion in Angular Stable Plate Fixation of Proximal Humeral Fractures: Radiographic Analysis of a Prospective Randomized Study

Background: Monoaxial and polyaxial screw insertion are used in angular stable plating of displaced proximal humeral fractures. Aim of the study was to compare both fixation techniques by radiographic evaluation. Methods: Prospective randomized treatment with monoaxial or polyaxial screw insertion in angular stable anatomic preshaped plates of displaced proximal humeral fractures. Analysis of standardized true anterior-posterior (true a.p.) and outlet-view radiographs at 1 day, 6 weeks, 3 months, and 6 months after surgery by two radiologists with respect to radiographic evidence of secondary varus displacement, cut out of screws, osteonecrosis, and hardware failure. Secondary varus displacement was defined as a varus decrease of the humeral head-shaft angle of >10 degree in true a. p. radiographs. Results: Sixty-six consecutive patients (48 women, [72.7%]; 18 men, [27.3%]; mean age 67.7 years [95% CI, 63.9-71.6]) with displaced proximal humeral fractures were evaluated in this study. Nineteen patients (29%) showed secondary varus displacement of >10-degree angle. In 6 cases (9%), intra-articular cut out of screws was found. Furthermore, 1 case (2%) of nonunion was observed. No relationship between monoaxial and polyaxial screw insertion was found regarding occurrence of secondary varus displacement (monoaxial, 11/polyaxial, 8; p = 0.91) or screw cut out (monoaxial, 4/polyaxial, 2; p = 0.64). Prevalence of secondary varus displacement and hardware cut out was related to patients age (p = 0.02) and fracture pattern, according to Neer- and AO/OTA-classification (p < 0.001). The average immediate postoperative head-shaft angle was 135.2 degrees (CI, 132.3-138.1) in the group without radiographic complication, compared with 126.7-degree angle (CI, 123.6-129.7) among those with secondary varus displacement of >10-degree angle and screw cut out (p < 0.001). Furthermore, in cases of an immediate postoperative head-shaft angle of <130 degrees, there was a 48% incidence of secondary varus dislocation (n = 13) versus 15% in cases with a head-shaft angle >130 degrees (n = 6, p = 0.004). Conclusion: Monoaxial and polyaxial screw insertion allow for mechanical stabilization in angular stable plating of unstable proximal humerus fractures. Radiographic evidence of secondary varus displacement of >10-degree angle and screw cut out was seen similarly often in both fixation techniques. To avoid secondary varus displacement and screw cut out, restoration of a humeral head-shaft angle of >130 degrees seems to be important in mono-axial and polyaxial fixation of proximal humeral fractures.