Annals of Biomedical Engineering, Vol. 9, pp. 383-391, 1981 0090-6964181/040383-09 $02.00/0 Printed in the USA. All rights reserved. Copyright 9 1982 Pergamon Press Ltd. TIME EVOLUTION OF THE MECHANICAL PROPERTIES OF THE CALLUS OF FRESH FRACTURES P. Christel G. Cerf and A. Pilla Bioelectrochemistry and Orthopaedic Research Laboratories Columbia University New York, New York The temporal evolution of mechanical properties was investigated up to 10 weeks in 240 rat radial osteotomies. After disearding the displaced fractures, the remaining 111 undisplaeed 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 proeeeds 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 reaeh the values of intact sham bone between 9 and 10 weeks. The stiffness vs time curve has a sigmoid shape with an abrupt inerease 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. INTRODUCTION The systematic study of experimental animal fractures is a means by which a better understanding of fracture healing and the factors which could speed its course may be obtained. The most common difficulty arising from Address correspondence to P. Christel, Laboratoire de Recherches Orthoprdiques, 10, avenue de Verdun, 75010 Paris, France. Acknowledgements-P. Christel received partial grants from the French Society of Orthopaedic Surgery (S.O.F.C.O.T.), the Claude Bernard Association and the GETROA while on leave overseas. The authors wish to express their sincerest thanks to the staff of Electro Biology Inc. and the Ortho- paedic Research Laboratories of Columbia University, to C.A.L. Bassett for providing facilities for this work, to A. Meunier (Laboratoire de Recherches Orthop~diques) for help with data analysis and to J.L. Leray (INSERM, U. 18) for his helpful criticism and preparation of this paper. This study also received f'mancial support from the Electrobiology Foundation. 383