Skeletal Biomechanics and Tibial Bone Adaptive Response to Mechanical Stimuli in the Green Iguana (Iguana iguana)

Mechanical loading models are used to study adaptive skeletal mechanobiology mechanisms. However, most studies have used mammal models, leaving a knowledge gap regarding how these mechanisms differ among vertebrate groups. To address this gap, we evaluated the in vivo bone strain environment of the left tibia in green iguanas during locomotion, axial compressive loading, and with finite element analysis (FEA). Our study involved examining subadult green iguanas (n=7) over a range of speeds (0.4 - 1.3 m/s) and axial load magnitudes (-25 to -100 N) to determine peak strains. Bone strains were measured using single-element strain gauges (n=18) and rosette strain gauges (n=3), surgically attached to the tibial anterior, posterior, and medial surfaces. At a speed of 1.3 m/s, peak strains ± standard deviation observed were 645 ± 699 µε, -448 ± 464 µε, and 206 ± 168 µε at the anterior, posterior, and medial surfaces, respectively. Peak principal tensile and compressive strains on the medial surface were 199 ± 113 µε and -153 ± 98 µε at 1.3 m/s. During -100 N compressive loading, peak strains were 403 ± 277 µε, -506 ± 460 µε, and -52 ± 177 µε at the anterior, posterior, and medial surfaces, respectively. Our FEA model demonstrated a close correlation with experimentally measured strain values at the gauge sites (slope = 1.07, R2=0.7). Using these foundational in vivo strain results and a daily strain stimulus formula, our objective was to develop a novel noninvasive axial compressive tibial loading model to induce a cortical bone adaptive response in the green iguana tibia. However, following three weeks of daily applied compressive loading, no significant difference was detected in critical bone parameters at 37% and 50% (midshaft) volumes of interests from the proximal tibia (P<0.05).  While this study did not yield significant differences in critical bone parameters following the application of daily compressive loading, it provided new knowledge regarding the bone strain environment and the potential for inducing adaptive responses in the green iguana tibia. Further research may refine our understanding of skeletal mechanobiology mechanisms across vertebrate groups and develop more effective loading models for studying bone adaptation. Overall, the findings of this study contribute to the broader field of musculoskeletal mechanobiology, giving insights that may inform bone health and adaptation in diverse species, including humans.