Abstract: BACKGROUND: For many years, surgical treatment for intra-articular fractures of the distal radius has been widely recognized, but there is still controversy over which surgical approach can achieve better efficacy and maximize joint function recovery. Regardless of the chosen treatment method, the goal is anatomical reduction and stable internal fixation. Although palmar locking plate fixation has recently become the most commonly used surgical method, one of the main limitations of this fixation method is the inability to achieve sufficient stability of the ulnar dorsal fracture fragment.
OBJECTIVE: To analyze the biomechanical characteristics of treating distal radius fractures with dorsal ulnar fragment with only volar locking plate fixation and combined dorsal mini-plate fixation, and to explore the optimal internal fixation methods for different fracture patterns of distal radius fractures with dorsal ulnar fragment.
METHODS: Thirty radial bone specimens were produced using 3D printing technology to establish distal radius fracture models with dorsal ulnar fragment and divided into three groups. Group 1 established distal radius fracture models with a dorsal ulnar fragment not exceeding the Lister tubercle. Group 2 established distal radius fracture models with a dorsal ulnar fragment exceeding the Lister tubercle. Group 3 established distal radius fracture models with comminuted fractures with dorsal ulnar fragment. Each group of models was randomly fixed with either a simple palmar locking steel plate or a palmar locking steel plate combined with a dorsal mini steel plate. The prepared model was installed into a customized biomechanical fixture. Cyclic loading and failure loading experiments were conducted on each model in sequence. Biomechanical stability was observed. 
RESULTS AND CONCLUSION: (1) In the cyclic loading experiment, all test models did not fail. Comparison of stiffness among different models: There was no significant difference in fixing method at the initial stage of group 1 (P > 0.05), but there was a significant difference in fixing method values at the 1 000th, 2 000th, and 3 000th cycles (P < 0.05). There was no significant difference in fixing method between each stage of group 2 (P > 0.05). There were significant differences in each stage of group 3 (P < 0.05). (2) In the failure load experiment, the elastic limits of each group of models were compared: there was a significant difference between group 1 and group 3 (P < 0.05). There was no significant difference in group 2 (P > 0.05). Comparison of failure loads among different models: There was a significant difference between group 1 and group 3 (P < 0.05). There was no significant difference in various fixing methods in group 2 (P > 0.05). (3) It is concluded that for comminuted distal radius fractures or distal radius fractures with a relatively small dorsal ulnar fragment not exceeding the Lister tubercle, the stiffness and axial compressive strength of volar locking plate combined with dorsal mini-plate fixation are superior to only volar locking plate fixation, which can provide stronger fixation in the early stages of fracture healing. For distal radius fractures with a larger dorsal ulnar fragment exceeding the Lister tubercle, the biomechanical stability of the two fixation methods is basically the same.

Key words: distal radius fracture, dorsal ulnar fracture fragment, volar locking plate, dorsal mini-plate, biomechanics, orthopedic implants