Development of a musculoskeletal shoulder model considering anatomic joint structures and soft-tissue deformation for dynamic simulation.

The shoulder joint has a high degree of freedom and an extremely complex and unstable kinematic mechanism. Coordinated contraction of the rotator cuff muscles that stop around the humeral head and the deltoid muscles and the extensibility of soft tissues, such as the joint capsule, labrum, and ligaments, contribute to shoulder-joint stability. Understanding the mechanics of shoulder-joint movement, including soft-tissue characteristics, is important for disease prevention and the development of a device for disease treatment. This study aimed to create a musculoskeletal shoulder model to represent the realistic behavior of joint movement and soft-tissue deformation as a dynamic simulation using a rigid-body model for bones and a soft-body model for soft tissues via a spring-damper-mass system. To reproduce the muscle-contraction properties of organisms, we used a muscle-expansion representation and Hill's mechanical muscle model. Shoulder motion, including the movement of the center of rotation in joints, was reproduced, and the strain in the joint capsule during dynamic shoulder movement was quantified. Furthermore, we investigated narrowing of the acromiohumeral distance in several situations to induce tissue damage due to rotator cuff impingement at the anterior-subacromial border during shoulder abduction. Given that the model can analyze exercises under disease conditions, such as muscle and tendon injuries and impingement syndrome, the proposed model is expected to help elucidate disease mechanisms and develop treatment guidelines.

Evaluation of the pressure on the dorsal surface of the distal radius using a cadaveric and computational model: clinical considerations in intersection syndrome and Colles' fracture.

The fibers of the abductor pollicis longus (APL) and extensor pollicis brevis (EPB) muscles intersect the distal radius. This anatomical structure puts pressure on the dorsal surface of the distal radius when various wrist positions are adopted. An increase in this pressure is associated with the risk of intersection syndrome and with immobilization after Colles' fracture. However, the relationship between the pressure on the distal radius and various wrist positions remains unclear. This study was established to provide quantitative data on the mechanical effect of the pressure exerted by the APL and EPB. Ten cadaveric wrist models containing a force sensor were prepared and used to record pressure levels at various wrist positions, such as pronation, supination, flexion and dorsiflexion, and radial and ulnar deviation. A three-dimensional simulation model comprising four bones, one muscle, one tendon, and one tendon sheath was constructed and analyzed in detail using the finite element method. The contribution of the APL and EPB to the pressure exerted on the distal radius was quantified by dissecting muscles while measuring pressure. The position (pronation and ulnar deviation without flexion/dorsiflexion) associated with a strong force being exerted on the distal radius was determined by measuring and analyzing the mechanical effect. We concluded that this position increases the risk of intersection syndrome but provides effective immobilization after Colles' fracture. The cadaveric and computational method presented herein is the first to identify the anatomical relationship between the pressure on the distal radius and various wrist positions.