Journal Article Acromioclavicular joint separations 107-73-3 9002-88-4 Coated Materials, Biocompatible Gels Phosphorylcholine Polyethylene Polymethacrylic Acids Reverse shoulder arthroplasty Rockwood classification conservative treatment indications for surgery initial treatment mid-term outcomes patient-reported outcome measures poly(2-methacryloyloxyethyl-phosphorylcholine) posterior displacement rheumatoid arthritis severity assessment surgical methods vertical displacement