Structure and Healing of Bone as a Response to Strain and Stress 



95 



this means the screws were exposed to the constant growing pressure of the epi- 

 physeal cartilage. As a result we see in the corresponding bone thread on the weight- 

 bearing side an appositional ossification, as an adaptation of bone tissue to mechani- 

 cal pressure (Fig. 1). This adaptation does not occur in cancellous bone only, but also 

 in compact bone, where the osteons change their direction according to the stress-lines 

 of the screws. 



The primary stability of metal implantation into bone tissue and a continuous 

 mechanical pressure between the screw and the bone are the basic conditions for this 

 biomechanic adaptation. A primary instability leads to fibrotic bone resorption. 



In another experiment a screw was inserted between radius and ulna of a dog 

 (Fig. 2). The slight movement between both bones produced a discontinuous pressure 

 and a movement between bone and metal surface and led to an osteolytic reaction. 



F.g. 



Screw inserted across radi 

 Because of the lesser d 



IS and ulna in the dog. The slight movement produced tibrotic and osteolytic 

 the instability was relatively greater in the ulna, as well as the bone 

 resorption (hlstolog. section below) 



Under the influence of rigid fixation fractures and osteotomies heal by primary 

 ossification without chondro-fibrotic callus. At the ends of the fragments the Haver- 

 sian cavities are reorganized by the so-called "creeping substitution", and the Haver- 



