Total shoulder arthroplasty (TSA) can fail for several reasons, such as component loosening, periprosthetic fracture, instability, infection, soft tissue failure, or joint overstuffing. Severe metallosis without loose glenoid components after TSA may result in the need for revision to reverse TSA./r/nFour years before the current presentation, an 86-year-old woman suffered from right shoulder pain and swelling. The initial diagnosis was osteoarthritis of the shoulder joint, for which she underwent TSA. Four years later, she complained of shoulder joint pain, swelling, and limited range of motion. On sonography, subscapularis and supraspinatus tendon tears were identified. Plain radiographs and computed tomography (CT) scans showed metallosis around the shoulder joint. Due to the rocking horse mechanism, wear of the upper portion of the glenoid component and bearing caused a foreign-body reaction and severe metallosis around the joint. Due to a massive rotator cuff tear combined with glenoid component wear, the patient eventually underwent reverse TSA (RTSA) and was satisfied with the final results./r/nSevere metallosis due to glenoid component wear combined with a massive rotator cuff tear in TSA may cause the need for revision to RTSA.