Numerical simulation of the healing process in the tibia diaphysis fracture fixed with external fixation

The tibia diaphysis (shaft) fracture is one of the most common long bone fractures, and is usually treated with either the internal or the external fixations. How to choose a proper fixation type is still empirical and controversial. The objective of this study was to investigate whether the lateral external fixation (LEF) is suitable to treat the transverse and oblique tibia diaphysis fracture, from a mechanobiological perspective. The healing processes in the tibia fractures were simulated using the finite element method. The models of both the transverse and oblique (45°) tibia diaphysis fracture fixed with a LEF were built. A mechano-bioregulatory algorithm, which considered both the mechanobiological and biological environments, was developed to simulate the cell and tissue activities inside the callus. The results showed that both fractures healed in a typical secondary osteogenesis process. After 60 days, the regions of external callus and bone marrow were occupied with bone tissue. However, the mechanical stimulus in the inter-cortical region in the oblique fracture model with a less stiff LEF was greater than the stimulus in the transverse fracture model with the same LEF, indicating that the angled fracture was prone to generate greater instability. Moreover, increased osteogenic differentiation threshold only slightly affected the bone formation in the bridging areas, thus, had minor influences on the healing process. In conclusion, the lateral external fixation demonstrated satisfactory capacity in the treatment of the transverse and oblique tibia diaphysis fracture. The oblique fracture was more likely to be affected with a less stiff fixation.