The time sequence and anatomical locailsation of bone loss in the mid-diaphysis of the forelimb inresponse to long-term non-traumatic immoblllsation were studied in Beagle dogs aged seven to eight years.The changes in the right forelimb encased in plaster relative to the left one serving as a control were evaluated by means of radiomorphometry and histomorphometry and compared with the responses observed previously in young adult dogs.

At the beginning ofthe experiment the older dogs had a bone volume reduced by some eight per centdue to the age-related expansion of the cavity of the bone marrow. After immoblllsation, the bone lossoccurred in three stages, as in the young adult dogs althoughless clearly. During the first two months bonevolume fell rapklly to a level of 90 to 95 per cent of the volume in the control limbs;​ a rapid reversalfollowed to which both periosteal and endosteal surfaces and intracortical porosity contributed. A second slower but longer lasting bone loss began 12 weeks after the hnmoblllsation;​ it tended to level off towardsthe end of the experiment. A third stage was characterised by maintenance of the loss which equalled 40per cent of the original volume.

In contradistinction to young adult dogs, in the third metacarpal of the older dogs the endostealenvelope rather than the periosteal was the main contributor to the permanent bone loss, while in theremaining bones the periosteal envelope also contributed substantially. In both age groups the magnitude of bone loss was greatest in the distal bones. The extent of the bone resorption surface on the endosteal envelope paralleled the rate of bone loss.

In the older dogs the appositional rate was reduced by 30 per cent in both the control and in theexperimental limb relative to young adult dogs, suggesting that the bone loss after long-term immobilisation is mediated by the changes in bone-cell kinetics rather than in the cell function ofdifferentiated existing cells.