Factors affecting fixation of the glenoid component of a reverse total shoulder prothesis Caroline Chebli, MD, Philippe Huber, Jonathan Watling, Alexander Bertelsen, PA-C, Ryan T. Bicknell, MD, and Frederick Matsen III, MD, Seattle, WA The semiconstrained design of the reverse arthroplasty allows loads from the humerus to challenge the fixation of the glenoid component to the scapula. We examined some of the factors affecting the quality of glenoid screw fixation, including the density of the material into which the screws are placed, the purchase of individual screws, and the direction of loading in relation to screw placement. Loads were applied by the humeral component to glenoid components with different conditions of fixation. The load to failure for each set of conditions was measured and compared statistically. Load to failure was less when the glenoid component was fixed to material of lesser density. Each screw contributed to the quality of fixation; the screw nearest the point of load application made the largest contribution. Load to failure was less when the load was colinear with a line through the nonlocking holes in the base plate compared to colinear with a line through the locking holes. In performing a reverse total shoulder, surgeons should emphasize secure intraosseous placement of the fixation screws in the best quality bone available. The placement of the inferior screw appears to be the most critical. (J Shoulder Elbow Surg 2008;17:323-327.) Reverse total shoulder prostheses are now often used when the shoulder is seriously compromised by various combinations of irreparable cuff deficiency, arthropa- thy, failed prior reconstructions, severe bone loss or de- formity, and instability. 5,8-10,13,14,16-19,27,28,30,31 With these semiconstrained prostheses, loads applied to the humerus are transferred directly to the fixation of the glenoid component. As a result, failure of glenoid component fixation (Figure 1) is one of the most com- mon complications of the reverse total shoul- der. 4,9,11,14,18,19,27,30 While previous reports have concerned the anatomy of scapular bone and its rele- vance to glenoid component fixation, 2,3,7,12, 20-24,26,29 few have considered optimization of screw fixation of the glenoid component. 1,19 A review of ar- ticles regarding reverse total shoulder prostheses, es- pecially those showing glenoid component fixation failure, reveals a wide variability in the placement of glenoid fixation screws in the limited bone available in the scapula. 4,9,11,14,18,19,27,30 No study has been published regarding the effect of screw placement on the load to failure of glenoid fixation of the most com- monly used type of reverse total shoulder arthroplasty, one with a glenoid hemisphere applied to the surface of glenoid bone using 4 screws, the superior and infe- rior of which are fixed-angle locking screws. To understand better factors affecting the security of screw fixation of the glenoid component, we have tested the hypotheses that (1) the load to failure of fix- ation of the glenoid component is significantly af- fected by the density of the material into which the fixation screws are placed; (2) the load to failure of glenoid component fixation is significantly reduced if any of the 4 screws fails to achieve purchase in the ma- terial; and (3) the fixation is strongest when the 2 lock- ing screws are in line with the applied load. MATERIAL AND METHODS We tested a commonly used and commercially available reverse total shoulder prosthesis, in which a glenoid hemi- sphere is applied directly to the glenoid bone with 4 equally spaced screws (Delta, DePuy, Warsaw, IN) (Figure 2). Two of the fixation screws, opposite each other, lock into the base plate of the prosthesis, and 2 of the screws, also oppo- site each other, are nonlocking. We selected the 36 mm gle- nosphere, the standard metaglene, a standard humeral component with a 3 mm polyethylene cup, and 36 mm long locking and nonlocking screws. Consistent with previ- ous publications in The Journal, 19 such as Harman et al, we used commercially prepared blocks of rigid, unicellular polyurethane foams (Last-A-Foam FR-3700, General Plas- tics, Tacoma, WA) to ensure uniformity of the material into which the fixation screws of the metaglene were inserted. In contrast with the study of Harman et al, which used foam resembling cortical bone of ‘‘excellent’ quality’’, 19 From the Department of Orthopaedics and Sports Medicine, Univer- sity of Washington Medical Center. Reprint requests: Frederick A. Matsen III, MD, Department of Ortho- paedics and Sports Medicine, University of Washington Medical Center, 1959 NE Pacific Street, Box 356500, Seattle, WA 98195 (E-mail: matsen@u.washington.edu). Copyright ª 2008 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2008/$34.00 doi:10.1016/j.jse.2007.07.015 323