Locations for screw fixation beyond the glenoid vault for fixation of glenoid implants into the scapula: An anatomic study

Abstract

Screw fixation is often required for glenoid implants and glenoid bone grafting, yet no study has evaluated the complex and irregular anatomy of the scapula to determine the best positions for the screws. Current implants rely solely on the bone in the glenoid vault for screw fixation, but in cases with glenoid bone loss, screw fixation may not confer adequate stability. The purpose of this study was to find the best positions for screws that are secured in bone beyond the glenoid vault. Twenty-seven scapulae from the Hamann-Todd osteologic collection underwent a computed tomography scan, and the images were transferred to a custom computer program that allowed 3-dimensional manipulation of the scapular images. Virtual screws were inserted into the scapula through the glenoid fossa, and the starting points and screw lengths were recorded. The coordinates of the starting points were scaled to allow comparisons among the different sizes of scapula. Once the ideal screw position was found, the angle of insertion was altered by 15 degrees in 4 directions, and the resulting screw length was recorded to show the effects of screw insertion error. Three screws could be inserted through the glenoid fossa into bone beyond the glenoid vault. The superior screw had a median length of 29 mm and exited at the junction of the coracoid and the suprascapular notch. The middle screw had a median length of 60 mm and exited at the junction of the spine and the scapular body. The inferior screw had a median length of 75 mm and exited posterior to the tip of the scapula. When the starting positions were constrained by a symmetric implant, the median superior screw length was 19 mm, the median inferior screw length was 61 mm, and the median middle screw length was 55 mm. Screw fixation in bone beyond the glenoid vault can be used for fixation of glenoid implants or glenoid bone grafting. Future studies are required to test the biomechanical stability of this new screw configuration, as well as methods and tools for precise implantation of the screws into the described locations. Once these studies are completed revision total shoulder arthroplasty with severe glenoid bone loss may be treated in 1 stage with screw fixation of a glenoid implant over bulk allograft.