Regional modular responses in different bone compartments to the anabolic effect of PTH (1-34) and axial loading in mice

Reverse engineering Frost's mechanostat model in mouse tibia: Insights from combined PTH and mechanical loading

Osteoporosis is a widespread skeletal disease impacting billions, with treatments aimed at enhancing bone mass or preventing bone loss essential for reducing fracture risk and related health complications. Clinical evidence shows that intermittent parathyroid hormone (PTH) treatment increases cortical width at certain skeletal sites, with effects further amplified when combined with mechanical loading (ML), making this pharmacological and exercise approach promising for dual osteoporosis therapy. However, the mechanisms through which PTH enhances osteogenic response are not fully understood. This study uses μ CT endpoint imaging data from the mouse tibia loading model together with mechanical assessment of strain patterns in cortical bone to quantitatively compute parameters in Frost's mechanostat model. Particularly, we investigate the effects of PTH alone and in combination with ML on bone formation threshold and rate. Our analysis shows that PTH alone promotes periosteal bone formation independently of strain patterns induced by habitual loading in a dose-dependent manner. PTH lowers the bone formation modeling threshold (MESm) in bones undergoing ML in a dose-dependent and site-specific manner. The highest sensitivity is observed around 37 % of tibial height, where MESm decreases from 1060.6με in untreated bones to 212.1με at an 80μg/kg/day g PTH dose. This region also exhibits the highest adaptation response, with a maximum modeling velocity (MaxFL) of approximately 7με/day at 80μg/kg/day PTH. Although the formation velocity modulus (FVM) increases in PTH-treated bones compared to untreated ones across all regions, this change is not dose-dependent.

A novel framework for elucidating the effect of mechanical loading on the geometry of ovariectomized mouse tibiae using principal component analysis

Introduction

Murine models are used to test the effect of anti-osteoporosis treatments as they replicate some of the bone phenotypes observed in osteoporotic (OP) patients. The effect of disease and treatment is typically described as changes in bone geometry and microstructure over time. Conventional assessment of geometric changes relies on morphometric scalar parameters. However, being correlated with each other, these parameters do not describe separate fractions of variations and offer only a moderate insight into temporal changes.

Methods

The current study proposes a novel image-based framework that employs deformable image registration on in vivo longitudinal images of bones and Principal Component Analysis (PCA) for improved quantification of geometric effects of OP treatments. This PCA-based model and a novel post-processing of score changes provide orthogonal modes of shape variations temporally induced by a course of treatment (specifically in vivo mechanical loading).

Results and Discussion

Errors associated with the proposed framework are rigorously quantified and it is shown that the accuracy of deformable image registration in capturing the bone shapes (∼1 voxel = 10.4 μm) is of the same order of magnitude as the relevant state-of-the-art evaluation studies. Applying the framework to longitudinal image data from the midshaft section of ovariectomized mouse tibia, two mutually orthogonal mode shapes are reliably identified to be an effect of treatment. The mode shapes captured changes of the tibia geometry due to the treatment at the anterior crest (maximum of 0.103 mm) and across the tibia midshaft section and the posterior (0.030 mm) and medial (0.024 mm) aspects. These changes agree with those reported previously but are now described in a compact fashion, as a vector field of displacements on the bone surface. The proposed framework enables a more detailed investigation of the effect of disease and treatment on bones in preclinical studies and boosts the precision of such assessments.

Combining PTH(1-34) and mechanical loading has increased benefit to tibia bone mechanics in ovariectomised mice

Combined treatment with PTH(1-34) and mechanical loading confers increased structural benefits to bone than monotherapies. However, it remains unclear how this longitudinal adaptation affects the bone mechanics. This study quantified the individual and combined longitudinal effects of PTH(1-34) and mechanical loading on the bone stiffness and strength evaluated in vivo with validated micro-finite element (microFE) models. C57BL/6 mice were ovariectomised at 14-week-old and treated either with injections of PTH(1-34), compressive tibia loading or both interventions concurrently. Right tibiae were in vivo microCT-scanned every 2 weeks from 14 until 24-week-old. MicroCT images were rigidly registered to reference tibia and the cortical organ level (whole bone) and tissue level (midshaft) morphometric properties and bone mineral content were quantified. MicroCT images were converted into voxel-based homogeneous, linear elastic microFE models to estimate the bone stiffness and strength. This approach allowed us for the first time to quantify the longitudinal changes in mechanical properties induced by combined treatments in a model of accelerated bone resorption. Both changes of stiffness and strength were higher with co-treatment than with individual therapies, consistent with increased benefits with the tibia bone mineral content and cortical area, properties strongly associated with the tibia mechanics. The longitudinal data shows that the two bone anabolics, both individually and combined, had persistent benefit on estimated mechanical properties, and that benefits (increased stiffness and strength) remained after treatment was withdrawn.