Abstract: BACKGROUND: Distal radial fracture involving dorsal ulnar fragment type belongs to unstable fractures, AO classification of 23C type and the middle column fractures of three-column classification. If dorsal ulnar fragment cannot be effectively fixed, it will affect the stability of radial joints and ulnar radial joints. The fixation effect of dorsal volar plate alone on dorsal ulnar fragment is limited. Although the fixation effect of dorsal volar double plate is effective, it increases the trauma and treatment cost, which is not in line with the current Diagnosis Related Groups policy. The addition of dorsal Kirschner wire fixation may be a relatively feasible plan.  
OBJECTIVE: A finite element model of AO/ASIF 23-C1 distal radial fracture was established to compare the differences in biomechanical stability between volar locking plate and volar locking plate plus dorsal Kirschner wire, especially focusing on the fixation effect of dorsal ulnar fragment, providing a reference for clinical application.
METHODS: Using reverse modeling technology, the CT data of healthy femur of a 42-year-old male and internal fixation data were imported into relevant software to establish three-dimensional finite element models of AO/ASIF 23-C1 distal radial fractures with volar locking plate and volar locking plate with dorsal Kirschner wire. The models were subjected to 100 N axial stress, or 1 Nm bending or torsional stress, respectively. The maximum displacement, maximum Von Mises stress and stress distribution of dorsal ulnar fragment were compared between the two groups.  
RESULTS AND CONCLUSION: (1) The stress on the internal fixation was 2-10 times that on the bone under the two fixation methods, which met the requirements of fracture fixation. (2) Under the three different loads, there was no significant difference between the two groups in the stress peaks of both bone and internal fixation (< 30%). Under an axial load, the peak stress on the internal fixation was 3-4 times that on the bone. However, under bending load, the peak stress of internal fixation was 8-10 times that of bone, while under torsion load, the peak stress of internal fixation was only twice that of bone. (3) Under an axial load, the maximum displacements of the dorsal ulnar fragment between the two groups were very small and the difference was small, only 0.056 mm and 0.047 mm, respectively, indicating good stability of the fragment. Under bending load, the maximum displacement of single plate group was 2.4 times that of Kirschner wire group. Under torsion load, the maximum displacement of the single plate group was 2.7 times that of the Kirschner wire group. (4) It is indicated that the volar locking plate with Kirschner wire can more effectively fix the dorsal ulnar fracture fragment, with more uniform stress distribution, improve the fixation strength, and does not significantly increase the treatment cost. It is more suitable for the treatment of distal radial fractures involving the dorsal ulnar fracture fragment under the current Diagnosis Related Groups policy.

Key words: distal radius fracture, internal fixation, volar locking plate, Kirschner wire, displacement, stress, biomechanics, finite element analysis