Experimental study of the time effect of controlled micromovement on the influence of the fracture healing

Objective To explore the influence and mechanism of time effect of the controlled micromovement on fracture healing. Methods Forty-eight rabbit models of femoral fracture were prepared and fixed with unilateral two-bar external fixator. They were randomly divided into four groups: continuing immobilization group, instant micromovement group, 1-week micromovement group and 2-week micromovement group. Postoperative radiographs were taken at 1, 2, 3 and 5 weeks to observe callus growth. The maximum load, deflection and rigidity of callus at fracture end were measured 5 weeks after operation. At 1, 2 and 3 weeks after operation, the histological morphology of callus was observed, and the expression and distribution of osteocalcin (oc) in callus were detected. Results At 5 weeks after operation, the X-ray scores of fracture line in 1-week micromovement group and 2-week micromovement group were 10.384±0.744 mm, 10.412±0.482 mm, significantly higher than those in continuing immobilization group (7.518±0.536). The anteroposterior diameter and the exterior and interior diameter of the external callus in 1-week micromovement group and 2-week micromovement group were 14.3±3.2 mm, 14.0±2.8 mm and 14.6±2.1 mm, 15.2±3.1 mm, which were smaller than those in the continuing immobilization group 15.3±2.3 mm and 16.7±1.9 mm, but there was no significant difference. The bone mineral density value and proportion rate in the fracture site were 0.446±0.020 g/cm2, 0.416±0.021 g/cm2 and 1.171%±0.056%, 1.143%±0.040% in 1-week micromovement group and 2-week micromovement group, which were significantly higher than those in continuing immobilization group which were 0.376±0.022 g/cm2 and 0.912%±0.051%. The maximum load of callus in 1-week micromovement group and 2-week micromovement group was 415.6±27.2 N, 400.3±28.5 N, which was significantly higher than that in continuing immobilization group 329.2±18.4 N and instant micromovement group 272.8±22.7 N. There was no difference of the deflection of callus between groups. The rigidity of callus in 1-week micromovement group was 590.4±24.2 N/mm, which was significantly higher than that in other groups; the rigidity of callus in the 2-week micromovement group was 540.6±22.8 N/mm, which was significantly higher than those in the instant micromovement group and the continuing immobilization group (152.4±21.7 N/mm, 174.8±20.6 N/mm). Conclusion Micromovement begins from one or two weeks can significantly raise external callus formation and vagueness level of fracture line, accelerating bridging callus formation, and can significantly raise bone mineral density and rigidity of callus. It also accelerates the maturity, hypertrophy and mineralization of chondrocyte, resulting in the stimulation of the fracture healing through endochondral ossification; it seemingly can improve the amount and density of osteoclasts in callus to stimulate the maturity and mineralization of chondrocyte. The strengthening coupling of osteoblasts and osteoclasts can promote the transformation from soft callus to hard callus and the remolding of hard callus. Key words: Fractures, bone; Fracture healing; External fixators