Optimal screw placement for base plate fixation in reverse total shoulder arthroplasty

Hypothesis: Scapular cortical thickness has not been fully characterized from the perspective of determining 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 35 mm, 9 degrees superior, 2 degrees posterior; (inferior screw-A) length 34 mm, 16 degrees inferior, 5 degrees anterior; (inferior screw-B) length = 31 mm, 31 inferior, 4 posterior; (posterior screw) length 19 mm, 29 degrees inferior, 3 degrees anterior. For scenario B: (superior screw) length 36 mm, 28 degrees superior, 10 degrees anterior; (inferior screw) length = 35 mm, 19 degrees inferior, 4 degrees anterior; (posterior screw) length 37 mm, 23 degrees superior, 3 degrees anterior. The anterior screw was consistent between scenarios A and B, averaged 29 mm in length and was directed 16 degrees inferior and 14 degrees 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. (C) 2011 Journal of Shoulder and Elbow Surgery Board of Trustees.