Rotator cuff injuries often occur at the site of tendons-to-bone attachment, also called the insertion site or enthesis. The sensitivity of the musculoskeletal system to its loading environment may augment the risk of injury at insertion sites due to extended periods of microgravity or unloading. Long-term changes in mechanical loading on joints, such as may be experienced during extended space travel, will lead to modifications in the tissues’ structural and therefore mechanical properties. Alix-Deymier Black will be examining the multi-scale structural and mechanical changes in the rotator cuff enthesis with unloading. A better understanding of structural and mechanical changes will help elucidate techniques for minimizing the risk of injury.
Effect of Unloading on the Structure and Mechanics of Rotator Cuff Tendon-to-Bone Insertion (First Award Fellowship)
Alix Deymier-Black, Ph.D.
Washington University School of Medicine
Rotator cuff injuries often occur at the site of tendons-to-bone attachment, also called the insertion site or enthesis. The sensitivity of the musculoskeletal system to its loading environment may augment the risk of injury at insertion sites due to extended periods of microgravity or unloading. Long-term changes in mechanical loading on joints, such as may be experienced during extended space travel, will lead to modifications in the tissues' structural and therefore mechanical properties. Joint deterioration during space flight would magnify the injury risk to the tendons. Even in the best conditions on earth, these injuries do not heal well (e.g., the failure rate for repaired rotator cuffs may be as high as 94% and will severely debilitate an injured astronaut. The negative effect of unloading has most clearly been demonstrated in bone. Unloading of bone leads to rapid bone resorption, loss of bone mass, and decreased mechanical properties. However, much less is known about the results of extended weightlessness or unloading on the interfaces between hard and soft joint tissues.
The insertion site achieves an effective attachment between tendon and bone through a multi-scale structural organization. On the nanometer scale, mineralized collagen fibrils are composed of regularly arrayed collagen molecules which act as templates for incorporation of nanometer-sized crystals of mineral intrafibrillarly within the gap zones or extrafibrillarly on the surface of the fibrils. The deposited mineral acts as a mechanical reinforcement within the collagen matrix and depending on its size, location, and organization provides tunable levels of strength and stiffness to the insertion. On the micrometer scale, these mineralized fibrils are arranged such that the enthesis contains gradients in both mineral content and collagen fibril orientation that spans ~20 µm in mice. Gradients such as these are responsible for dissipating stress concentrations, which are prevalent at the interfaces of dissimilar materials, thus limiting the risk of failure. Variations in enthesis structure caused by joint unloading at both the nanometer and micrometer scale can result in significant changes in the structure of the insertion and therefore in insertion mechanics. The overall objective of this project is to determine the effect of unloading on the structural, and in turn mechanical, properties of the enthesis. To mimic spaceflight conditions, the bony attachments of mouse rotator cuffs will be unloaded using a muscle paralysis model resulting in modified enthesis organization. This unloading is expected to lead to changes in collagen organization, collagen structure, mineral content, mineral organization, and mineral arrangement relative to collagen.
Rotator cuff tears are extremely prevalent, especially in the elderly population (~50% prevalence in individuals over 80 years). Even in the best of situations these tears are difficult to repair with a failure rate for repaired rotator cuffs as high as 94%. Rotator cuff tears tend to occur at the interface between tendon and bone. Such interfaces between dissimilar materials are prone to stress concentrations and increased failure risk. In healthy tissue, a number of structural mechanisms such as gradients in mineral content, collagen orientation, and matrix composition serve to dissipate these stress concentrations. The increased occurrence of rotator cuff injuries in the elderly population, suggests that there may be changes in the interfacial structure due to unloading as a result of disuse or decreased use of the shoulder. Understanding how changes in the enthesis structure affect the mechanics of the insertion in loaded and unloaded systems will help us to develop enhanced techniques for treatment and repair. Therefore, the research performed in this project will not only help the astronaut population, but will also provide essential information in regards to the mechanics of rotator cuff tissues and how they respond to use and disuse.