UC San Diego

Orthopaedic surgery procedures can provide relief for patients with impaired hand function, restoring lost capabilities. These procedures are often quite complex and understanding biomechanics is critical for proper surgical planning and execution. Surgeons must simultaneously consider many biomechanical factors, especially during procedures focused on modifying muscles or tendons, such as tendon transfers. Thus, the purpose of this work was to investigate aspects of human biomechanics that are relevant to tendon transfers. An approach combining studies of macroscopic and microscopic parameters was implemented to better understand clinically relevant aspects of biomechanics. In Chapters 2-5, several studies are summarized which illustrate the importance of understanding macroscopic biomechanical principles such as moment arms, force transmission, and passive load bearing properties. These properties are investigated in a number of muscle-tendon systems throughout the forearm and hand. Skeletal muscles also exhibit incredible diversity in form, shape and structural organization. Thus, lessons learned in the forearm and hand are not necessarily broadly applicable to other areas of the body. Likewise, animal muscles may not well represent their human structural analogues. In an effort to better understand the diversity of human muscle, an extensive biochemical study was conducted. In this study, summarized in Chapter 6, a comprehensive analysis of several biochemical parameters was carried out in 100 human muscles to search for common themes and trends in the muscular organization of the human body. Anatomic specialization occurred in collagen content, titin molecular mass and myosin heavy chain distributions, and human muscles did not correlate well with analogous muscles in mouse, rat or rabbit. In Chapter 7 we investigated the way in which biochemical parameters correlate with mechanical parameters in several relevant systems. In muscles commonly used in tendon transfers, we investigated passive load-bearing properties at several different size scales. This is specifically relevant to tendon transfers because these load bearing properties are commonly utilized by surgeons to assist in intraoperative and perioperative decisionmaking. At the single fiber and fiber bundle levels, passive stiffness was similar between the three muscles studied. These results were only poorly predicted by titin molecular mass and collagen content, factors that have often been hypothesized as primary load bearing structures. In Chapter 8 we describe our experience developing a method of measuring the passive length-tension relationship in whole muscles in humans. Utilizing a new device in conjunction with unique access afforded by tendon transfers, we can reliably and safely measure the length-tension relationship intraoperatively and we propose extension of this work to a full study in order to better understand clinically relevant mechanical differences between muscles used in tendon transfer