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Biomechanics

Biomechanics - Poster Session A

Poster E1 - Assessment of Long Bone Structure-Function Relationships in Mice

Thursday, October 24, 2024
10:00 AM – 11:00 AM EST
Location: Exhibit Hall E, F & G

Introduction:  Osteoarthritis (OA) is characterized as a degenerative joint disease that affects musculoskeletal joint structure and function. This can include changes within articular cartilage, subchondral bone, ligaments, capsule, synovial membrane, and periarticular membrane [1]. To better understand the effects of OA on skeletal structure, mouse models are frequently employed due to the wide availability of surgical and genetic tools available for inducing OA. High resolution 3D imaging, micro computed tomography (microCT) and MRI, as well as uni- and multi-axial mechanical testing are common tools used when studying the biomechanical implications of OA on the musculoskeletal system [2].

Previous research has highlighted changes in biomechanical function, including morphology, strength and integrity of bones associated with OA, but they have not identified relationships between structural and mechanical properties [3]. In this study, mechanical strength and morphological testing of healthy femora and tibiae will establish baseline bone structure-function relationships in a common mouse strain for genetic induction of OA, enabling future studies to quantify changes in bone mechanical and structural properties due to OA. It was hypothesized that femora would show greater resistance than tibiae to a force applied at the bones midshaft, and that femora would have larger trabecular and cortical bone properties than tibiae.

Materials and

Methods:  Left femora and tibiae (n = 10 per bone) were extracted post-mortem from 12-week-old, female C57BL/6 mice. All soft tissue was carefully dissected away from the joint using standard dissection techniques. The bones were wrapped in PBS-soaked gauze and stored at -20°C. Prior to micro-CT scanning bones were thawed to room temperature. Micro CT was conducted using a Brüker SkyScan 1276 Micro-CT with an 8 µm voxel size, an energy of 85,000 V, and an intensity of 47 µA. Trabecular and cortical bone properties typically described in the literature were measured from the microCT scans using Brüker microCT software (Nrecon, Dataviewer, CTan) [2]. Three-point bend testing was performed using an Instron 5943. Femora and tibiae were laid across a 7.6mm support span, the indenter rate was set at 0.1mm/s, and an initial contact force of 0.1N was applied to ensure contact between the bone and the indenter [2]. Custom MATLAB code was used to calculate, stiffness, yield point, fracture point, post-yield displacement (PYD), max force, and work-to-fracture (WTF) [3]. Statistical analysis was performed using custom code in RStudio and included two-sided Wilcoxon tests. Significance was set at alpha = 0.05.

Results, Conclusions, and Discussions: On average, the max force was 13.42N and 12.54N for the femora and tibiae respectively. Femora had a larger mean stiffness of 83.17 N/mm compared to tibiae which had a mean stiffness of 51.86 N/mm. The average work to fracture (WTF) of the femora was approximately 3 J higher than the tibiae, 9.84 J compared to 6.82 J. On average, femora had greater mechanical properties than tibiae.

Structural characteristics such as bone volume fraction, total cross-sectional area, minimum moment of inertia, and maximum moment of inertia were significantly larger in femoral samples compared to tibial samples (Figure 2). There were no significant differences between the two bone groups in bone surface volume fraction, trabecular thickness, cortical thickness, and tissue mineral density (Table 1).

The femora had both stronger mechanical properties and increased trabecular and cortical bone structural properties, which agreed with both our hypotheses and with prior studies that found increased mechanical properties in femoral compared to tibial bones in healthy mouse models in other wild type strains [4]. Our work supports previous research which has shown that bone is a mechanoresponsive tissue with a close structure-function relationship. Increased structural properties in tissues are directly related to greater mechanical demands. Limitations of this work include an all-female cohort of mice and experimental constraints that do not allow for longitudinal, in vivo micro-CT imaging of the mice. Future work will include using the mechanical and structural properties recorded to calculate the material properties of the two bones. As well as merging findings with MRI imaging and robotic mechanical testing of knee-joint soft tissues. Additionally, a healthy cohort of male and OA affected mice will be studied to characterize sex and injury differences in bone structure-function relationships.

Acknowledgements (Optional):  We would like to acknowledge Dr. Mary Boggs for assistance in this project and funding from the NIH NIGMS (P20GM103446), and NIH NIGMS (P20GM139760).

References:
[1] Hunter, Bierma-Zeinstra, The Lancelet, 2019.
[2] Tanguay et al, J Orthopaedic Research, 2023.
[3] Jepsen et al, JBMR, 2015.
[4] Schriefer et al, JBiomech, 2004.