Massive irreparable rotator cuff tears are a common cause of pain and disability. Several different treatment options are available for this pathology;​ however, these treatments have been associated with poor clinical outcomes when used to treat younger (<65 years), more active patients. The purpose of this thesis was to design and evaluate a subacromial implant in its ability to restore normal glenohumeral stability and range of motion. The implant was created as a modular device, which captured different implant thicknesses (5mm and 8mm) and constraints (high and low) within its design. In-vitro testing compared the ability of these implants to restore normal shoulder biomechanics. The results indicated the 5mm high constraint implant to be the most effective in restoring normal joint position. Furthermore, range of motion increased when the implant was paired with a tuberoplasty procedure. These results suggest this implant may be advantageous in treating younger patients.