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.