The glenohumeral (GH) joint is the most mobile joint in the human body, but its mobility inherently increases the risk of instability. The humeral head sits in a shallow glenoid in the scapula like a golf ball sitting on a tee. The stability in this joint is provided by the rotator cuff muscles and tendons that actively pull the humerus back into the socket to prevent dislocation, especially during overhead motions. However, the rotator cuff is prone to tears, resulting in pain, loss of mobility, and recreational limitations. Surgical reattachment of the tendon to the bone is challenging due to the mechanical disparity between the two tissues, resulting in stress concentrations and a high risk of retear. Notably, the specialized tissue at the tendon-to-bone attachment, which facilitates stress transfer between tendon and bone in healthy joints, does not regenerate after surgical reattachment and healing, making tendon-to-bone repairs prone to re-tears.

A comprehensive understanding of GH joint biomechanics is essential for developing early interventions to prevent rotator cuff injuries. Furthermore, improving tendon-to-bone fixation during rotator cuff repair is critical to improve post-surgery outcomes. In the last decade, bioinspired solutions have shown considerable promise for addressing several biomedical problems. This thesis draws bioinspiration from two animals that have evolved unique mechanical functions:​ (i) the bat shoulder joint, which facilitates repetitive overhead motions during flight and may offer insights into rotator cuff pathology and (ii) the curvature of python snake teeth, which enables secure grasping of prey without soft tissue tearing.

In the first part of the thesis, the bat shoulder was studied for its unique characteristics relative to mice. Overhead motions in humans often lead to shoulder injuries, partly because the bony anatomy of the unstable GH joint places greater stress on the joint’s surrounding soft tissues to stabilize these motions. Traditional animal models used to study shoulder pathology are quadrupeds, which lack the capacity for overhead motion. In contrast, bats consistently engage in overhead motion during flight, subjecting their shoulders to substantial loading throughout their relatively long lifespan. Remarkably, the biomechanical demands placed on a bat’s shoulder are estimated to exceed those of a competitive swimmer’s by 45-fold, despite sharing similar coracoacromial arch anatomy with humans. We were inspired to study functional adaptations in the shoulders of bats that enable this overhead motion. We performed comparative anatomy studies of the shoulders of bats and mice, similarly-sized quadrupeds. By quantifying the constraints imposed by the bony anatomy, we identified adaptations of the shoulder, including the rotator cuff tendons, that allow bats to sustain overhead motion in a high stress, repeated loading environment, without injury.

In the second part of the thesis, python teeth were used as inspiration to develop a repair device optimized to grasp the rotator cuff without tearing. Rotator cuff repair surgeries fail frequently, with 20-94% of the 600,000 repairs performed annually in the United States resulting in retearing of the rotator cuff. The most common cause of failure is sutures tearing through tendons at grasping points. To address this issue, we examined the specialized teeth of snakes of the Pythonoidea superfamily, which effectively grasp soft tissues without tearing. To apply this non-damaging and effective gripping approach to the surgical repair of tendons, we developed and optimized a python-tooth inspired array as an adjunct to current rotator cuff suture repair, and found that it nearly doubled repair strength. Integrated simulations, 3D printing, and ex vivo experiments revealed a relationship between tooth shape and grasping mechanics, and enabled optimization of a tooth array device to enhance rotator cuff repair to distribute stresses and increase tendon-bone contact. The efficacy of the approach was demonstrated via human cadaver tests, suggesting an alternative to traditional suturing paradigms that may reduce tendon re-tearing.

Collectively, these studies contribute to a better understanding of the biomechanics of the GH joint and offer novel, bioinspired approaches for rotator cuff repair. The functional adaptations of bats provide insight into developing new approaches to treat GH joint instability, and a clinically relevant python-tooth inspired device can ultimately reduce the high rates of re-rupture currently observed in rotator cuff repair.