Cancellous bone adaptation to in vivo loading in a rabbit model

Abstract

Biophysical stimuli are important to the development and maintenance of cancellous bone, but the regulatory mechanisms need to be understood. We investigated the effects of mechanical loading applied in vivo to native cancellous bone in the rabbit on bone formation and trabecular realignment. A novel device was developed to apply controlled compressive loads to cancellous bone in situ. The effect of loading on cancellous bone volume fraction and architecture was quantified. A 4-week experiment was performed in rabbits with devices implanted bilaterally. Cyclic 1 MPa pressures were applied daily to the right limb for 10, 25, or 50 cycles at 0.5 Hz, and the left limb served as the control without any applied loading. Microcomputed tomography and histomorphometry were used to characterize the cancellous tissue within a 4-mm spherical volume located below the loading core. In vivo cyclic loading significantly increased the bone volume fraction, direct trabecular thickness, mean intercept length, and mineral apposition rate in the loaded limbs compared with contralateral limbs. Insufficient evidence was found to demonstrate an effect of number of cycles on the cancellous adaptation between loaded and control limbs. Using a rabbit model, we demonstrated that mechanical loading applied to cancellous bone in situ increased bone formation and altered trabecular morphology. This in vivo model will allow further investigation of cancellous functional adaptation to controlled mechanical stimuli and the influence of mechanical loading parameters, metabolic status, and therapeutic agents.

Trabecular bone adaptation to loading in a rabbit model is not magnitude‐dependent

“…A custom implantable loading device was previously designed to apply controlled compressive loads to trabecular tissue of the distal lateral femur of the rabbit in situ 15,20 (Fig. 1).…”

“…A limitation to our initial model was the manual application of the loading and high variability of the load magnitude, 19% coefficient of variability of loading. 15 To address concerns regarding potential inaccuracies with manual loading, we developed an automated version to apply controlled loads using an electromagnetic actuator to deliver direct, compressive loads to the top of the loading device 21 ( Fig. 1).…”

“…The profile of the automated version was lower and allowed load magnitude and frequency to be controlled consistently, while maintaining the identical contact area with the trabecular bone as in earlier experiments. 15,20 Animal Model Under an institutional animal care and use committee approved protocol, loading devices were implanted bilaterally in twenty skeletally mature adult (31 weeks old) New Zealand white rabbits. The surgical procedure was similar as described previously.…”

“…In this model cyclic compressive loading increased trabecular bone volume fraction and altered trabecular architecture. 15,20 Specimen specific finite element models demonstrated that the mean strains in the trabecular tissue were lower in the animals that underwent 1 MPa compressive loading daily for 4 weeks compared to the contralateral control tissue values. 20 The volume of tissue experiencing high strains decreased and was accompanied by an increase in the tissue volume experiencing low strains.…”

“…14 While clearly showing adaptive changes in bone to loading, the morbidity of this model limited its broader application. More recently, less invasive 15,16 and noninvasive 10,17,18 loading models have been introduced that apply controlled loads to corticotrabecular sites in vivo. Noninvasive models avoid the surgical insult that may augment the healing response, which may in turn confound the adaptive response.…”