Naoto Yamamura

Patient-Specific Skeletal Muscle Fiber Modeling from Structure Tensor Field of Clinical CT Images

We propose an optimization method for estimating patient-specific muscle fiber arrangement from clinical CT. Our approach first computes the structure tensor field to estimate local orientation, then a geometric template representing fiber arrangement is fitted using a B-spline deformation by maximizing fitness of the local orientation using a smoothness penalty. The initialization is computed with a previously proposed algorithm that takes account of only the muscle’s surface shape. Evaluation was performed using a CT volume (1.0 mm\(^\text {3}\)/voxel) and high resolution optical images of a serial cryo-section (0.1 mm\(^\text {3}\)/voxel). The mean fiber distance error at the initialization of 6.00 mm was decreased to 2.78 mm after the proposed optimization for the gluteus maximus muscle, and from 5.28 mm to 3.09 mm for the gluteus medius muscle. The result from 20 patient CT images suggested that the proposed algorithm reconstructed an anatomically more plausible fiber arrangement than the previous method.

Influence of intramuscular fiber orientation on the Achilles tendon curvature using three-dimensional finite element modeling of contracting skeletal muscle

Tendon curvature plays a key role in mechanical gain (amplifying the joint excursion relative to fiber length change) during joint motion, but the mechanism remains unresolved. A three-dimensional finite element (FE) model was used to investigate the influence of intramuscular fiber orientation upon the curvature pattern of the Achilles tendon during active muscular contraction. Two simulation models, with fiber pennation angles of θ = 25° and 47° were tested for the gastrocnemius and soleus muscles. A smaller pennation angle (25°) of the soleus muscle fibers was accompanied by a large change in curvature whereas a larger pennation angle (47°) of the soleus muscle was accompanied by small effects. These results suggest that the fiber pennation angle determines the curvature of the tendon, and the magnitude of the curvature varies along the length of the aponeurosis. Such FE modeling has the potential of determining changes in force output consequent to changes in intramuscular fiber orientation arising from resistance training or unloading, and provides mechanism for predicting the risk of Achilles tendon ruptures.

Registration-Based Patient-Specific Musculoskeletal Modeling Using High Fidelity Cadaveric Template Model

We propose a method to construct patient-specific musculoskeletal model using a template obtained from a high fidelity cadaver images. Musculoskeletal simulation has been traditionally performed using a string-type muscle model that represent the line-of-forces of a muscle with strings, while recent studies found that a more detailed model that represents muscle’s 3D shape and internal fiber arrangement would provide better simulation accuracy when sufficient computational resources are available. Thus, we aim at reconstructing patient-specific muscle fiber arrangement from clinically available modalities such as CT or (non-diffusion) MRI. Our approach follows a conventional biomedical modeling approach which first constructs a highly accurate generic template model which is then registered using the patient-specific measurement. Our template is created from a high-resolution cryosectioned volume and newly proposed registration method aligns the surface of bones and muscles as well as the local orientation inside the muscle (i.e., muscle fiber direction). The evaluation was performed using cryosectioned volumes of two cadavers, one of which accompanies images obtained from clinical CT and MRI. Quantitative evaluation demonstrated that the mean fiber distance error between the one estimated from CT and the ground truth was 4.16, 3.76, and 2.45 mm for the gluteus maximus, medius, and minimus muscles, respectively. The qualitative visual assessment on 20 clinical CT images suggested plausible fiber arrangements that would be able to be translated to biomechanical simulation.

A Multi-modality Approach Towards Elucidation of the Mechanism for Human Achilles Tendon Bending During Passive Ankle Rotation

The in vitro unconstrained Achilles tendon is nearly straight, while in vivo experiments reveal that the proximal region of the Achilles tendon, adjacent to Kager’s fat pad, bends ventrally during plantarflexion but remains nearly straight during dorsiflexion. Tendon bending is an important factor in determining the displacement of the foot compared to the shortening of the muscle fibers. The objective of this study was to elucidate the various mechanisms that could cause tendon bending, which currently remain unknown. Examination of Thiel-embalmed cadavers, with preservation of native articular joint mobility, revealed that the Achilles tendon still bent ventrally even when its surrounding tissues, including the skin surface, Kager’s fat pad, and distal portions of the soleus muscle were removed. Shear modulus and collagen fiber orientation were distributed homogeneously with respect to the longitudinal line of the tendon, minimizing their causative contributions to the bending. Given that tendon bending is not caused by either the nature of the deformations of the tissues surrounding the Achilles tendon or its physical properties, we conclude that it results from the geometric architecture of the Achilles tendon and its configuration with respect to the surrounding tissues.

Development of Multi-scale Musculo-Skeletal Simulator

This paper presents preliminary status of our project for developing a multi-scale simulation of musculo-skeletal system for providing a useful tool in the field of musculo-skeletal physiology and related healthcare issues. Here, a mechanically and physiologically detailed model of skeletal muscles, as a key building block of the system, is considered, in which hierarchical structure of skeletal muscles is modeled by multiple one-dimensional muscle fiber models embedded in a three-dimensional finite element continuum-mechanics model. For modeling the excitation-contraction coupling, we combined models at multiple scales including electrophysiology for action potential generation in muscle fibers, Ca2+ dynamics of sarcoplasmic reticulum describing calcium release and uptake, and cross-bridge dynamics that can reproduce stochastic cross-bridge kinetics with a state transition model of the myosin molecules via Monte Carlo simulation. We then validated each of these models for the excitation-contraction coupling by comparing dynamics of the models with physiological data available in literatures on the muscle function.