THE RESPONSE OF BONE TO EXTERNAL LOADING REGIMENS

The results of Hert et al.(1) have been re-examined using an external pin fixation technique involving New Zealand White rabbits. New bone f ormation. was assessed with tetracycline epifluorescence in 49 animals . The animals were operated upon using inhalational anaesthesia, and p ins were placed at opposite ends of the tibia of both hind limbs. load ing and on the fourteenth day of the loading regimen. The bones were l oaded for 45 min per 24 h. Undemineralized specimens were sectioned an d standardized photographs produced. The area of new bone was determin ed by a planimeter and by digitization. In dynamic loading the shape o f the force/time curve is of little significance; the response is freq uency dependent and for higher frequencies the response is greater tha n that from continuous loading; there was no statistical significance between the rapid relief loading regimens. For continuous loading the response is a function of the load value. In all cases more bone is la id down in immature subjects than in adults. With regard to osteoclast ic activity, all loading increased the areas of resorption irrespectiv e of character, in all adult animals intermittent loads produced a gre ater area of ostoeclastic resorption than static load; and produced a greater change as the number of load cycles per minute increased. The adult control animals had more resorptive activity than the immature a nimals although there was no statistical significance. Loading, irresp ective of its character, led to a net increase of bone, the osteoblast ic effects thus outweighing the osteoclastic effects.