title: A comparison of concentric and eccentric glenospheres in reverse shoulder arthroplasty: a randomized controlled trial.
abstract: BACKGROUND
  Inferior scapular notching following reverse shoulder arthroplasty is due to mechanical impingement and, in some studies, has been associated with poorer functional scores, lower patient satisfaction, and more limited shoulder motion. We aimed to test the hypothesis that inferior positioning of the center of rotation with eccentric glenosphere designs decreases the adduction deficit before impingement occurs and improves clinical outcome.
  
  
  METHODS
  A randomized, controlled, double-blinded trial was performed. According to the results of a power analysis, fifty patients undergoing reverse shoulder arthroplasty for the diagnosis of cuff tear arthropathy were randomized intraoperatively to receive either a concentric or eccentric glenosphere. The glenoid baseplate was positioned flush to the inferior border of the glenoid before the glenosphere was then attached. Notching was assessed using an anteroposterior radiograph, and clinical outcome was assessed using the visual analog pain scale score, shoulder function rating, American Shoulder and Elbow Surgeons score, and Oxford shoulder score. Active forward elevation and external rotation were assessed. The outcome assessor was blinded to the treatment group. The mean follow-up period for the groups was forty-three and forty-seven months.
  
  
  RESULTS
  Patient demographics and preoperative scores were similar between the groups. At the time of the final follow-up, four patients (14.8%) in the concentric group had developed inferior scapular notching (two with Nerot grade I and two with Nerot grade II), ranging in size from 1.1 to 7.4 mm, compared with one patient (4.3%; Nerot grade I) in the eccentric group (p = 0.36). No notching occurred in any patient with glenoid overhang of >3.5 mm. No significant difference between the groups was seen with respect to functional outcome scores, patient satisfaction, or shoulder motion.
  
  
  CONCLUSIONS
  There were no differences in notching rates or clinical outcomes between concentric and eccentric glenospheres following reverse shoulder arthroplasty. Inferior glenosphere overhang of >3.5 mm, however, prevented notching. This may be achieved with a modified surgical technique, but eccentric glenospheres provide an additional option.
  
  
  METHODS
  Therapeutic Level I. See Instructions for Authors for a complete description of levels of evidence.
authors.affiliation: Department of Orthopaedic Surgery, North Shore Hospital, Private Bag 93 503, Takapuna, North Shore City 0740, New Zealand. E-mail address for P.C. Poon: petercpoon@xtra.co.nz.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Poon
authors.affiliation: Department of Orthopaedic Surgery, North Shore Hospital, Private Bag 93 503, Takapuna, North Shore City 0740, New Zealand. E-mail address for P.C. Poon: petercpoon@xtra.co.nz.
authors.forename: Justin
authors.initials: J
authors.lastname: Chou
authors.affiliation: Department of Orthopaedic Surgery, North Shore Hospital, Private Bag 93 503, Takapuna, North Shore City 0740, New Zealand. E-mail address for P.C. Poon: petercpoon@xtra.co.nz.
authors.forename: Simon W
authors.initials: SW
authors.lastname: Young
authors.affiliation: Department of Orthopaedic Surgery, North Shore Hospital, Private Bag 93 503, Takapuna, North Shore City 0740, New Zealand. E-mail address for P.C. Poon: petercpoon@xtra.co.nz.
authors.forename: Tim
authors.initials: T
authors.lastname: Astley
mesh_terms: D000368:Aged; D019643:Arthroplasty, Replacement / Q000295:instrumentation*; D004311:Double-Blind Method; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007595:Joint Prosthesis; D008297:Male; D011183:Postoperative Complications / Q000517:prevention & control; D011474:Prosthesis Design; D000070636:Rotator Cuff Injuries; D012421:Rupture / Q000601:surgery; D012540:Scapula; D019534:Shoulder Impingement Syndrome / Q000517:prevention & control; D012785:Shoulder Joint / Q000601:surgery*; D016896:Treatment Outcome