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^bc, Gerd Diederichs, MD^d, Jenni M. Buckley, PhD^abc, 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}
 
Journal of Shoulder and Elbow Surgery, Volume 20, issue 3, Pages 467-476 (March 2011)

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° superior, 2° posterior; (inferior screw-A) length = 34 mm, 16° inferior, 5° anterior; (inferior screw-B) length = 31 mm, 31 inferior, 4 posterior; (posterior screw) length 19 mm, 29° inferior, 3° anterior. For scenario B: (superior screw) length = 36 mm, 28° superior, 10° anterior; (inferior screw) length = 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.