153 - Structural Properties of Bone Under Habitual Physiological Tension – a Study of Popliteal Sesamoid Bones (cyamella) from New Zealand White Rabbit

Poster Board Number: 153
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Abstract Body : The principle of bone functional adaptation predicts that bone tissue, and specifically trabecular bone, will respond to habitual physiological mechanical stimuli by modifying (modeling) its structure accordingly. This concept, if validated, could have important implications in our understanding of bone development and healing, as well as the use of internal and external bone architecture to infer locomotion behavior in extinct vertebrates, and specifically hominin species. So far, most studies that supported this paradigm were investigating bone functional adaptation in response to compressive stresses. Much less is known, however, whether bone functional adaptation takes place in a similar manner in response to tensile stresses. Sesamoid bones are ossified structures that are embedded in muscle tendons and are subjected primarily to tensile forces; hence, they are ideal for studying bone functional adaptation under tensile stresses. The objectives of this preliminary study is to investigate whether sesamoid bone architecture holds any evidence regarding the direction of (tensile) loading, and to analyze for the first time, the cyamella 3D trabecular architecture. The right knee joint region of 5 female New Zealand white rabbits was micro-CT scanned at an isotropic voxel-size of 17 µm. The popliteal sesamoid bone (cyamella) was visualized and segmented within the proximal tendon of the popliteus muscle using Dragonfly v2022.2 software (ORS inc.), and cortical and trabecular bone parameters were measured. Visualization showed that the cyamella is an irregularly-bean shaped bone with its long axis running roughly proximal to distal. Cyamella dimensions were 4.94 ± 0.38 mm, 4.11 ± 0.39 mm, and 2.35 ± 0.07 mm for length, width and thickness, respectively. The caudal face, which is embedded in the popliteal tendon, was coarse and irregular compared to the smooth anterior face, which articulates with the lateral condyle of the tibia. Total bone volume (cortical and trabecular bone) was 77.1% ±1.5%, where trabecular bone accounted for about 40% of total bone tissue. Trabecular bone analysis revealed a relatively high BV/TV of around 60% with an average trabecular thickness (Tb.Th) exceeding 0.2 mm. Degree of anisotropy (DA) were above 2.0 with the principal trabecular orientation aligned closely to the proximal-distal axis (corresponding to the direction of tensile stresses). 3D distance maps demonstrated that the distance from the proximal to the distal cortical shell was very close to the actual length of the bone, supporting the principal trabecular orientation results and indicating a predominantly trabecular network in the proximal-distal orientation. In conclusion, these preliminary results support the principle of bone functional adaptation and suggest that both tensile and compressive stresses may regulate bone modeling. Future studies should look at sesamoid bones in immobilized animals to confirm a less organized trabecular network in response to the absence of tensile forces. Demonstrating the impact of tensile forces on bone modeling will support the use of bone entheses (sites where muscle tendons are attached to bone, and thus are subjected to tensile forces) in studies that try to infer locomotion behavior from remnants of extinct species.