• Ara Nazarian, Beth Israel Deaconess Medical Center & Harvard Medical School
• Joseph DeAngelis, Beth Israel Deaconess Medical Center & Harvard Medical School

The human shoulder is a complicated system designed to balance mobility and stability. The dynamic nature of the shoulder joint, overuse, and age are major risk factors for injury, pain and disability – affecting 18-26% of adults at any point in time, making it one of the most common regional pain syndromes. Symptoms can be persistent and disabling in terms of an individual's ability to carry out daily activities both at home and in the workplace.

Shoulder mechanics is a complex topic that encompasses joint motion, muscle activation, rotator cuff tear and repair, tendon biology and mechanics, scapular positioning and capsular integrity. These elements work together in an intricate balance to safely achieve the joint's wide range of motion. If a shoulder injury is unattended, further ailments may follow. For example, instability can be associated with rotator cuff (RC) tear, and lead to persistent pain, dysfunction, and arthritis. Similarly, RC abnormalities can be asymptomatic, which makes it difficult for clinicians to determine when intervention is needed. These conditions and the uncertainty associated with when and how to treat shoulder injury motivate multifaceted studies of the shoulder.

Shoulder studies have been conducted using ex vivo (cadaveric), in vivo (patient) and in silico (simulation) modalities, with little cross-over between them. While cadaveric studies have been used to validate FE models, little has been done to leverage the strengths of each investigative modality to help conduct clinically relevant studies that can inform clinical decision making. These modalities have computational mechanics at their core, be it finite element modeling and simulation, or kinematics and kinetics in cadaveric- and patient-based studies.

In vivo studies cannot measure the pressure experienced within the joint nor can the forces produced by muscles be fully recorded. Ex vivo studies attempt to fill this gap by artificially loading muscles while measuring the forces on the joint. In this case, the muscles are not alive, so force is externally applied using estimations. In many cases, shoulder integrity is compromised during specimen preparation. Moreover, glenohumeral capsule integrity, scapular orientation and motion, and RC muscle-activation also limit transferring findings of cadaveric studies to clinic. In silico studies attempt to bridge the gap between the two approaches. Ideally, shoulder models that incorporate the optimal data and analysis from each setting would balance the drawbacks of using a single one to maximize clinical utility.

Understanding the strengths and limitations of a study's design and methodology can help to maximize the transfer of knowledge from the lab to clinic. A shift is occurring in shoulder research moving biomechanical studies from cadaveric models to computer-based simulations. This change can be attributed to the cost of lab-based studies and the lack of invasive procedures to measure different quantities. For this reason, dynamic shoulder studies will rely extensively on the balance between in vivo data and our ability to model it in simulations. At this stage, cadaveric and human motion studies are indispensable for model motivation and validation, and will play a major role in the study of dynamic motions of the shoulder.

Therefore, the aim of this mini-symposium is to offer a platform to present and discuss the latest findings in joint, rotator cuff, tendon, scapula and capsule computational mechanics and modeling, that comprise any one or a combination of in vivo, ex vivo and in silico modalities. The ultimate goal of this exercise is to further leverage evidence-based science towards informing clinical decision-making.