INTRACORTICAL REMODELING IN ADULT-RAT LONG BONES AFTER FATIGUE LOADING

Intracortical remodeling in the adult skeleton removes and replaces ar eas of compact bone that have sustained microdamage. Although studies have been performed in animal species in which there is an existing ba seline of remodeling activity, laboratory rodents have been considered to have limited suitability as models for cortical bone turnover proc esses because of a lack of haversian remodeling activity, Supraphysiol ogical cyclic axial loading of the ulna in vivo was used to induce ben ding with consequent fatigue and microdamage, Right ulnae of adult Spr ague-Dawley rats were fatigue-loaded to a prefailure stopping point of 30% decrease in ulnae whole bone stiffness, Ten days after the first loading, Left ulnae mere fatigued in the same may. Ulnae mere harveste d immediately to allow comparison of the immediate response of the lef t ulna to the fatigue loads, and the biological response of the right leg to the fatigue challenge. Histomorphometry and confocal microscopy of basic fuchsin-stained bone sections were used to assess intracorti cal remodeling activity, microdamage, and osteocyte integrity. Bone mi crodamage (linear microcracks, as well as patches of diffuse basic fuc hsin staining within the cortex) occurred in fatigue-loaded ulnar diap hyses, Ten days after fatigue loading, intracortical resorption was ac tivated in ulnar cortices, Intracortical resorption occurred in prefer ential association with linear-type microcracks, with microcrack numbe r density reduced almost 40% by 10 days after fatigue. Resorption spac es were also consistently observed within areas of the cortex in which no bone matrix damage could be detected. Confocal microscopy studies showed alterations of osteocyte and canalicular integrity around these resorption spaces. These studies reveal that: (1) rat bone undergoes intracortical remodeling in response to high levels of cyclic strain, which induce microdamage in the cortex; and (2) intracortical resorpti on is associated both with bone microdamage and with regions of altere d osteocyte integrity. From these studies, we conclude that rats can i nitiate haversian remodeling in long bones in response to fatigue, and that osteocyte death or damage may provide one of the stimuli for thi s process, (Bone 23:275-281; 1998) (C) 1998 by Elsevier Science Inc, A ll rights reserved.