Poster AA2 - Modeling Tendon Strains Experienced During Chick Embryo Development

Introduction: Abnormal fetal movements are implicated in common musculoskeletal birth defects, including arthrogryposis and clubfoot. Using the chick embryo (equivalent of mammalian fetus), we recently discovered that fetal paralysis inhibits tendon mechanical development, whereas hypermotility improves mechanical properties[1]. The study revealed the importance of mechanical loading for tendon development but tendon strains experienced during movements could not be empirically measured. We have developed a computer simulation model (forward dynamics) to examine relationships between movement and tendon development in the chick embryo. Limb movement begins after embryonic day (D)6-D8 and peaks during D12-D17[2]. These movements involve not only kicks but also impacts with the eggshell[3]. While chick embryos grow rapidly and likely only impact the eggshell for a limited period, the role of these eggshell impacts in tendon development are unknown. Our objective is to create a computational model that predicts the strain induced by muscle contraction (active strain) and the strain induced by ankle flexion (resting strain) experienced by the calcaneal (Achilles) tendon during development. This computational model can be used to predict strain contributions of the calcaneal tendon during normal and perturbed development, which is needed to better understand the role of mechanics in normal development and in tendon-related birth disorders. Furthermore, this model can be expanded to evaluate strains experienced by cartilage, bone, and other tissues during fetal movements under normal as well as aberrant conditions, providing a powerful tool to enable new insights into the mechanobiology of musculoskeletal tissue development.

Materials and

Methods: Fertilized White Leghorn chicken eggs were incubated until D10-D19, sacrificed, and staged. The leg was dissected into thigh, shank, and foot, and each segment weighed and measured for length. MotionGenesis was used to create a sagittal plane musculoskeletal model of the chick embryo. Forward dynamic simulations of the chick embryo kicking at each stage were performed using MATLAB software. The model consisted of four rigid segments (thigh, shank, foot, toes) connected at pin joints (Fig 1 left). Each joint was actuated by a three-component Hill-based muscle model representing the extensor muscles of the leg[4]. Muscle geometry and masses were based on previous studies and were scaled to size for the model[5]. Calcaneal tendon stiffness and dampening were based on previous studies[6,7]. Contact points between the toes and the eggshell were defined by Hunt-Crossley contact springs[8]. Viscous forces applied to the segment mass centers modeled embryonic fluid drag. Egg dimensions were defined from measurements at the widest section in the sagittal plane. The hip was fixed to the eggshell and the joints, when possible, were initially posed in their neutral resting angles with the muscles deactivated and all forces in static equilibrium. Kicks were simulated by inputting a 50-ms twitch excitation to each extensor muscle. The outcome variables were active and resting strains of the ankle extensor’s series elastic component, representing the calcaneal tendon. Ten simulations were performed, each representing an embryonic day, with the model’s dimensions and masses set to the average leg segment measurements on each day from D10-19.

Results, Conclusions, and Discussions: Leg segment masses increased nonlinearly whereas leg segment lengths increased linearly during development (Fig. 1). Before D14, the modeled chick leg was too short to impact the eggshell (Fig. 2). After D17, the leg was too long to set the neutral initial pose, and a more flexed pose with the toes already resting against the eggshell was used (Fig 2). Based on the model, the leg is able to impact the eggshell during kicking from D14-D17 (Fig. 2). The model showed that active strain is greatest when the embryo is able to kick the eggshell (D14-D17) (Fig 3), whereas resting strain is greatest at D18-D19 due to dorsiflexion of the foot as the embryo becomes constrained (Fig 3). Thus, active strain is the greatest contribution to total tendon strain throughout development (Fig 3). Mechanobiology studies of tendon development have been limited by the inability to directly measure tendon strains during embryonic movement. We have developed a computer simulation model that predicts the strains experienced in the calcaneal tendon during normal development. In particular, the model is able to predict the contributions of muscle contraction and ankle flexion to tendon active and resting strains. Future iterations of the model will examine the effects of the embryo free-floating, orientation change that occurs at D14, different kicking motions, and modeling other limb tendons. This model is an exciting significant step toward understanding the roles of dynamic and static tensile loading in normal tendon development, and how disturbances to active and static loading may lead to tendon-related musculoskeletal disorders. This powerful tool can be expanded to evaluate strains experienced by other fetal musculoskeletal tissues to gain new insights into the mechanobiology of their development, which has the capacity to inform the development of fetal interventions and therapeutics for musculoskeletal birth disorders.

Acknowledgements (Optional):

References: [1] Pan et al., Phil. Trans. R. Soc., 2018; [2] Wu et al., J. Exp. Zool., 2001;[3] Bradley et al., J. Neurophysiol., 2005; [4] Miller et al., Handbook of Human Motion, 2018; [5] Hutchinson et al., J. Morphol., 2004; [6] Navarro et al., J. Biomech., 2022; [7] Alexander et al., Comp. Biochem. Physiol. Part A, 2002; [8] Sherman et al., Symposium on Human Body Dynamics, 2011