Objective    To explore the effect of continuous dynamic pressure on cyclooxygenase mRNA expression

bone mass and mechanical properties during experimental fracture healing.      Methods    Unilateral osteotomy of left humeral diaphysis was performed on adult New Zealand rabbits. The humeral fracture was fixed by shape memory connector (SMC) in Study Group and by 4-hole dynamic compression plate (DCP) in Control Group.  Samples from the fracture gaps were obtained on day 3

7

14

21

28 and 56 and the temporal expression patterns of cyclooxygenase mRNA during the healing process were examined by real-time PCR.  The humeri were harvested and sent for biomechanical tests as well as dual-energy X-ray absorptiometry in 2

4

8 and 12 weeks postoperatively.      Results    The relative levels of COX-2 mRNA were elevated after fracture and were significantly higher in Study Group than in Control Group at 3 and 4 weeks postoperatively.  The BMD values of the Study Group were significantly higher than those in the Control Group from 4 to 12 weeks postoperatively. With regard to bone mechanical parameters

the Study Group showed significantly higher maximum torque

torsional stiffness

bending strength and bending rigidity than those in the Control Group at 4 and 8 weeks postoperatively.      Conclusion    The persistent dynamic longitudinal pressure produced by SMC may accelerate bone healing with increased bone mass and bone strength after fractures in this rabbit model by promoting the proliferation activity of the osteoblasts and callus mineralization

which may be closely related with a higher COX-2 mRNA expression.