[Dynamic investigation of tibial biomechanical property endured persistent intensive stress]

For investigating the changes in continuous biomechanical nature of bone bearing intensive stress in vivo, we adopted the animal model of persistent intensive stress. Thirty-seven rabbits were involved in the experiment; they were randomly divided into control group (5 rabbits) and trained group (32 rabbits). All animals were forced to jump and run about 300 times everyday in the electric stimulation cage so as to simulate the persistent intensive stress on tibias for different periods. Subsequently, all animals were sacrificed at different times (1-11 weeks), and all tibias of them were collected for biochemical investigation. By torsion destroy test, the experiment showed that the changes of biomechanical nature appeared obviously in a period of 6 weeks or so in the experiment. Furthermore, during the 2nd-3rd week of each period, the biomechanical indexes decreased strikingly; the abilities of deformation resistant dropped, the twisting rigidity decreased (69.7% lower than control, P< 0.01), the flexible index inereased (203.2% higher than control, P<0.05), the angle of twisting destroy increased (102.9% greater than control, P< 0.05). And the tibial mechanical strength declined too; the destroy torsion, energy absorption and energy absorb density decreased (most of them being 50% lower than control, even accounting for 34.5% of control; P< 0.05 or P< 0.01). We noticed that the biomechanical properties of bone endured persistent intensive stress presented periodicity and the period of change in bone biomechanical nature covered about 6 weeks in the experiment. Therefore, we have ground to conclude that a weakness period did exist when rabbit tibias endured 2-3 weeks persistent intensive stress, and such weakness did exist at the 2nd-3rd week of each training period.