Intracortical remodeling in adult rat long bones after fatigue loading

Abstract

Intracortical remodeling in the adult skeleton removes and replaces areas of compact bone that have sustained microdamage. Although studies have been performed in animal species in which there is an existing baseline of remodeling activity, laboratory rodents have been considered to have limited suitability as models for cortical bone turnover processes because of a lack of haversian remodeling activity. Supraphysiological cyclic axial loading of the ulna in vivo was used to induce bending with consequent fatigue and microdamage. Right ulnae of adult Sprague-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 were fatigued in the same way. Ulnae were harvested immediately to allow comparison of the immediate response of the left 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 intracortical remodeling activity, microdamage, and osteocyte integrity. Bone microdamage (linear microcracks, as well as patches of diffuse basic fuchsin staining within the cortex) occurred in fatigue-loaded ulnar diaphyses. Ten days after fatigue loading, intracortical resorption was activated in ulnar cortices. Intracortical resorption occurred in preferential association with linear-type microcracks, with microcrack number density reduced almost 40% by 10 days after fatigue. Resorption spaces 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 resorption is associated both with bone microdamage and with regions of altered osteocyte integrity. From these studies, we conclude that rats can initiate haversian remodeling in long bones in response to fatigue, and that osteocyte death or damage may provide one of the stimuli for this process.

Introduction

Intracortical remodeling in the adult skeleton removes and replaces areas of compact bone that have sustained bone microdamage as a consequence of fatigue.7, 8, 17, 18, 24, 26 Left undetected and unrepaired, the accumulation of microdamage in bone leads to compromised mechanical properties and bone fragility. Microdamage accumulation due to an imbalance between damage-causing processes and intrinsic repair processes underlies the development of stress fractures7, 30 and may play a significant role in the increased bone fragility associated with aging and osteoporosis.39, 40 Bone microdamage and fragility are also implicated in bone implant failure and fractures associated with long-term usage of drugs that suppress bone remodeling physiology.7 Recently, with the introduction of wide clinical usage of drugs that reduce bone remodeling globally in the skeleton, concerns have been raised about long-term pharmacological inhibition of bone remodeling leading to accumulation of unremodeled matrix damage, resulting in increased bone fragility in the population.31 Accordingly, examination of factors that influence microdamage accumulation in the skeleton, and those factors that influence its detection and repair, are fundamental to understanding skeletal health and disease.

There have been few experimental studies of the relationship between bone microdamage and intrinsic repair processes, owing in part to an incomplete understanding of bone fatigue and resulting bone matrix damage processes, and in part to the inherent difficulty and expense of performing these studies in vivo. Numerous studies over the last several years have shown that bone fatigues quite readily with normal mechanical usage, with fatigue microdamage occurring at multiple levels of the bone microarchitecture.4, 5, 7, 16, 39, 40 The few previous experimental studies examining whether intracortical remodeling in the adult skeleton removes and replaces microdamaged areas of compact bone were performed in dogs, because canine bone, like human bone, is osteonal and has an existing baseline of intracortical remodeling activity.8, 26 However, it is unknown whether repair of microdamage through remodeling is characteristic only of haversian bone, or whether bone fatigue will similarly activate osteonal remodeling processes to effect matrix repair in a species within which intracortical turnover is characteristically absent. In the current study, we tested the hypothesis that bone fatigue loading in vivo can activate the remodeling process in bones in which haversian remodeling characteristically does not occur. Accordingly, the objectives of the current studies were to develop a system by which adult rat long bones could be fatigued in vivo, and to use that system to determine whether fatigue loading in vivo would activate intracortical remodeling in adult rat long bones.