Abstract: BACKGROUND: The treatment of distal tibial comminuted fractures with soft tissue injury has always been challenging. The new retrograde tibial nailing and supercutaneous locking plate are important treatment methods, but their strain and stress shielding at the fracture end during different periods of fracture healing and different load conditions have not been reported. 
OBJECTIVE: To explore the biomechanical stability of retrograde tibial nailing and supercutaneous locking plate in different periods of fracture healing by finite element analysis to offer a scientific foundation for clinical application and rehabilitation exercise.
METHODS: The finite element model of distal tibial comminuted fracture was established by utilizing the CT data of the tibia from a 40-year-old healthy male. Retrograde tibial nailing, supercutaneous locking plate, and callus models were assembled in accordance with the principle of fracture fixation. The finite element analysis was performed using ANSYS software to compare the displacement of the fracture end, the stress shielding of the tibia, the stress of the callus, and the stress distribution of the tibia and the fixation device during different periods of fracture healing.
RESULTS AND CONCLUSION: (1) The relative displacement of the tibial fracture decreased gradually with the healing of the fracture, and the displacement decreased significantly after 3 months. At 0 and 1 months after operation, the vertical displacement and total displacement of the supercutaneous locking plate group were higher than those in the retrograde intramedullary nail group. The Z-axis displacement (horizontal medial and lateral displacement) of the two fixation methods was more obvious than the X-axis and Y-axis, and the Z-axis displacement of supercutaneous locking plate group was the most obvious. The maximum Z-axis displacement of the two fixation methods was located on the outside of the tibia, and the minimum displacement was located on the inside of the tibia. (2) The stress shielding rate at different periods of fracture healing gradually decreased with time. The stress shielding rate of the retrograde intramedullary nail was higher than that of the supercutaneous locking plate at different stages of fracture healing. After 3 months, the stress shielding rate of the supercutaneous locking plate was reduced to about 4%, and the stress shielding rate of the retrograde intramedullary nail was reduced to about 40%. (3) The stress of the stress concentration site of the callus in the two fixation methods increased with the increase of the load, and the stress of the callus in the supercutaneous locking plate group was always greater than that in the retrograde intramedullary nail group. The maximum stress distribution of the callus was approximately equally distributed among the two modes of fixation, both in the lateral portion of the tibia. (4) As the fracture healed, the maximum stress of the tibia in the two groups decreased gradually, and the stress in the supercutaneous locking plate group was always greater than that in the retrograde intramedullary nail group. The average stress of the maximum stress area of the tibia in the supercutaneous locking plate group under 1 500 N load was 285 MPa, while that in the retrograde intramedullary nail group was 26 MPa. (5) As the fracture healed, the stress of the fixation device in the two groups decreased gradually, and the stress in the supercutaneous locking plate group was significantly higher than that in the retrograde intramedullary nail group. After 3 months, the stress of the two fixation devices decreased significantly. (6) It is indicated that in the early stage of fracture healing, the strain on the fracture end in the retrograde intramedullary nail group is small, and the maximum stress of the tibia is moderate, allowing early loaded. The fractured ends in the supercutaneous locking plate group had too large strain and too large maximum stress of the tibia, which needed to be partially loaded under protection and could not be fully loaded. In the middle and late stages of fracture healing, the tibial retrograde intramedullary nail and the supercutaneous locking plate could be completely loaded, and the stress shielding rate of the supercutaneous locking plate was significantly lower than the tibial retrograde intramedullary nail.

Key words: distal tibia fracture, callus, fracture healing, retrograde nailing, supercutaneous locking plate, finite element analysis, biomechanics, stress shielding