Optimal screw placement for base plate fixation in reverse total shoulder arthroplasty James Guido DiStefano, MD a, *, Andrew Y. Park, MD a , Thuc-Quyen D. Nguyen b,c , Gerd Diederichs, MD d , Jenni M. Buckley, PhD a,b,c , William H. Montgomery III, MD a a Department of Orthopaedic Surgery, St. Mary’s Medical Center, San Francisco, CA, USA b Biomechanical Testing Facility, UCSF/SFGH Orthopaedic Trauma Institute, San Francisco, CA, USA c Department of Orthopaedic Surgery, University of California San Francisco, San Francisco, CA, USA d Department of Radiology, University of California San Francisco, San Francisco, CA, USA Hypothesis: Scapular cortical thickness has not been fully characterized from the perspective of deter- mining optimal screw placement for securing the glenoid base plate in reverse shoulder arthroplasty. Materials and methods: Twelve fresh frozen cadaveric scapulae underwent high resolution CT scans with 3-dimensional reconstructions and wall thickness analysis. Digital base plates were positioned and virtual screws were placed according to 2 scenarios: A - intraosseous through the entire course and exits a ‘‘safe region’’ with no known neurovascular structures; B - may leave and re-enter the bone and penetrates the thickest cortical region accessible regardless of adjacent structures. Results: For scenario A, the optimal screw configurations were: (superior screw) length ¼ 35 mm, 9  superior, 2  posterior; (inferior screw-A) length ¼ 34 mm, 16  inferior, 5  anterior; (inferior screw-B) length ¼ 31 mm, 31 inferior, 4 posterior; (posterior screw) length 19 mm, 29  inferior, 3  anterior. For scenario B: (superior screw) length ¼ 36 mm, 28  superior, 10  anterior; (inferior screw) length ¼ 35 mm, 19  inferior, 4  anterior; (posterior screw) length 37 mm, 23  superior, 3  anterior. The anterior screw was consistent between scenarios A and B, averaged 29 mm in length and was directed 16  inferior and 14  posterior. Conclusion: Thicker cortical regions were present in the lateral aspect of the suprascapular notch, scapular spine base, anterior/superior aspect of inferior pillar and junction of glenoid neck and scapular spine. Regions with high cortical thickness were accessible for both scenarios except for the posterior screw in scenario A. Level of evidence: Basic Science Study, Radiologic Analysis. Ó 2011 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Reverse total shoulder arthroplasty; scapula; screw placement; base plate; cortical thickness; 3-dimensional The reverse design of the total shoulder arthroplasty (RTSA) has been utilized for elderly patients with severely degenerated glenohumoral joints that are rotator cuff defi- cient (rotator cuff arthropathy). Some of the most common causes of failure in reverse shoulder arthroplasty involve *Reprint requests: James Guido DiStefano, MD, Department of Orthopaedic Surgery, Saint Mary’s Medical Center, 450 Stanyan Street, San Francisco, CA 94117. E-mail address: distefanojg@gmail.com (J.G. DiStefano). J Shoulder Elbow Surg (2011) 20, 467-476 www.elsevier.com/locate/ymse 1058-2746/$ - see front matter Ó 2011 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2010.06.001