Time evolution of the mechanical properties of the callus of fresh fractures

The temporal evolution of mechanical properties was investigated up to 10 weeks in 240 rat radial osteotomies. After discarding the displaced fractures, the remaining 111 undisplaced fractures were mechanically tested for tensile failure. Analysis of the recorded data versus healing time (tensile failure load, stiffness, energy to failure, displacement to failure) shows that callus tissues behavior proceeds essentially in two steps. A rubbery-like behavior (low strength, low stiffness, large elongation) is observed up to the time of 6 weeks, which is followed by a hard tissue like behavior (high stiffness, small elongation) up to the end of complete recovery. The mechanical parameters change at different rates and reach the values of intact sham bone between 9 and 10 weeks. The stiffness vs time curve has a sigmoid shape with an abrupt increase between the 6th and 7th weeks, whereas the tensile failure load rises steadily. Strength and stiffness are correlated only at the earlier stage of healing.