• Myung Jin Shin, Hyojune Kim, Dong Min Kim, Dongjun Park, In-Ho Jeon, and Kyoung Hwan Koh.
• Department of Orthopedic Surgery, College of Medicine, Asan Medical Center, University of Ulsan, 88, Olympic-ro 43-gil, Songpa-gu, Seoul, 05505, Korea.
• Arch Orthop Trauma Surg. 2021 Sep 1; 141 (9): 1517-1523.

IntroductionThe purpose of this study was to evaluate the effect of a medial support screw through a proximal humerus fracture. For this purpose, we verified whether the biomechanics are different according to the position of the screw while using the same number of screws. In addition, we tried to verify whether the insertion of additional inferomedial screws would make a difference in stability.Materials And MethodsTwenty-four proximal humerus bones were included in the study. A two-part fracture was created and fixed using a locking plate. Cyclic loading and load-to-failure test were applied to three groups: group A (proximal 6 screws + calcar screws), group B (proximal 6 screws), and group C (proximal 4 screws + calcar screws). Interfragmentary gaps were measured following cyclic loading and compared. The failure was defined when the bone breakage or medial gap closing was observed during ultimate failure load applied. The load-to-failure, maximum displacement, stiffness, and yield load were recorded and compared.ResultsThe interfragmentary gap was differently reduced by 0.29 ± 0.14 mm, 0.73 ± 0.25 mm, and 0.53 ± 0.09 mm following 1000 cyclic loading for groups A, B, and C, respectively. The load-to-failure was 945.22 ± 101.02 N, 941.40 ± 148.90 N, and 940.58 ± 91.78 N in groups A, B, and C, respectively. The stiffness of group A (214.76 ± 34.0 N/mm) was superior when compared to that of group C (171.12 ± 23.0 N/mm; p = 0.025). The maximum displacement prior to failure, yield load, showed no significant difference between comparative groups.ConclusionOur study did not show any additional biomechanical effects with the use of inferomedial supporting screws in non-osteoporotic proximal humerus fracture, besides making the fracture-plate construct stiff. The role of the inferomedial supporting screw was also unclear. However, the groups that used increased screw fixation and inferomedial screw insertion seemed to be more resistant to cyclic loading.© 2020. Springer-Verlag GmbH Germany, part of Springer Nature.

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