Abstract: BACKGROUND: The intramedullary fixation treatment for the middle clavicle fracture is effective and safe. However, issues such as nail loosening and displacement, as well as soft tissue irritability of the nail tail, are prevalent. Some interlocking intramedullary nails are demanding to operate; thus, their widespread utilization is confined. Treatment of intramedullary clavicle fracture can be further refined. Accordingly, to alleviate these issues, a novel intramedullary fixation instrument needs to be developed.  
OBJECTIVE: To explore the biomechanical stability of ortho-bridge system intramedullary fixation in the treatment of middle clavicle fracture by means of finite element analysis so as to offer a scientific foundation for clinical application.
METHODS: The finite element model of transverse middle clavicle fracture was established by relevant software utilizing the CT data of the clavicle from a young volunteer. The locking plate, ortho-bridge system, and titanium elastic nail finite element models were assembled in accordance with the principle of internal fracture fixation. Finally, meshing, applying loads, and data processing were accomplished with the ANSYS 15.0 program. In each model, the stress distribution, maximum Von Mises stress and maximum displacement of clavicle and internal fixation were compared.  
RESULTS AND CONCLUSION: (1) Under different loads, the stress distribution of the ortho-bridge system intramedullary fixation of the middle clavicle fracture was comparable to that of the titanium elastic nail fixation. Near the fractured end of the intramedullary connecting rod, the phenomenon of stress concentration was observed. The secondary stress concentration occurred in the extramedullary locking area of the ortho-bridge system, causing the overall stress of the ortho-bridge system group to be more dispersed than that of the titanium elastic nail group. (2) Under the axial compression force, the maximum Von Mises stress of the clavicle in the ortho-bridge system group 251.96 MPa was greater than that of the other groups, whereas the maximum Von Mises stress of the fractured surface 33.79 MPa was lower than the two control groups. Under cantilever bending and clockwise/counterclockwise torsion loads, the maximum Von Mises stresses of the clavicle and the fractured surface of the ortho-bridge system group were less than those of the titanium elastic nail group, but greater than those of the locking plate group. (3) Under varying loads, the maximum Von Mises stress of the implant in the three groups of internal fixation appeared near the fractured end. Among them, the titanium elastic nail exhibited the highest peak implant stress, followed by the ortho-bridge system, and the locking plate exhibited the least. (4) Under different loads, the maximum displacement of the clavicle in each group occurred primarily at its distal clavicle. Among them, the titanium elastic nail group displayed the greatest maximum displacement of the distal clavicle and fractured end, followed by the ortho-bridge system group, the locking plate group, and the normal clavicle. (5) The findings indicate that the biomechanical stability of ortho-bridge system intramedullary fixation is superior to that of titanium elastic nail in the treatment of middle clavicle fracture. The risk of loosening and displacement of the internal fixation is lower than the titanium elastic nail; however, the fixation strength is not as strong as the locking plate.

Key words: clavicle fracture, intramedullary fixation, ortho-bridge system, intramedullary nail, locking plate, finite element analysis, biomechanics, complications