Design of a rigid subacromial implant to treat massive irreparable rotator cuff tears

BACKGROUND: Massive rotator cuff tears often lead to superior migration of the humeral head and abnormal shoulder kinematics and function. In an attempt to restore normal joint kinematics, a modular subacromial implant was developed and evaluated in this biomechanical cadaveric study. METHODS: Eight cadaveric specimens were assessed using a muscle-loading shoulder simulator that applied loads at 0, 30, and 60 degrees of glenohumeral abduction. Testing states included the intact rotator cuff (native), the simulation of a massive rotator cuff tear, and multiple implant states. Two implant thicknesses (5 and 8 mm) and two implant articular surface constraints (low & high) were tested. Humeral head translation in both anterior-posterior and superior-inferior directions was measured using an optical tracking system. FINDINGS: The presence of a massive rotator cuff tear resulted in significant superior humeral head migration relative to the native state (P = 0.016). Both low and high constrained 5 mm thick implants restored superior-inferior humeral head position to within 1 ± 2 mm (P = 0.223) and 1 ± 2 mm (P = 0.928) respectively, of the intact position. Significant inferior translation relative to the native state was observed with both 8 mm implant designs (8 mm low constrained: -4 ± 2 mm, P = 0.060; 8 mm high constrained: -4 ± 2 mm, P = 0.060). No significant differences were observed for anterior-posterior translation. INTERPRETATION: These results demonstrate that an implant affixed to the acromion helps to restore the native position of the humeral head in the presence of a massive rotator cuff tear with superior migration. The 5 mm high constrained implant design was most effective at restoring native humeral head position. LEVEL OF EVIDENCE: Basic Science; Biomechanics.