Optimal insertion site of glenoid baseplate in reverse total shoulder arthroplasty: anatomical simulation using three dimensional image processing software.

PURPOSE Conventionally, the central structure of the baseplate is inserted through the point where the vertical and horizontal axes of the glenoid intersect (conventional insertion site (CIS)). However, there is scanty theoretical evidence that CIS has the optimal bone stock. We evaluated the optimal insertion site for the glenoid baseplate through the three-dimensional volumetric measurement of the glenoid bone stock. METHODS Pre-operative computed tomography (CT) images of 30 consecutive reverse total shoulder arthroplasty procedures were analyzed. Three-dimensional image processing software was used to reconstruct CT and volumetrically measure the glenoid bone stock according to the simulated central peg. A simulated central peg was inserted to the medial pole of the scapula from 49 points determined along with the intersect point of the vertical and horizontal axes of the glenoid CIS at 2-mm intervals. The overlapped volume between the simulated central peg and glenoid vault, representing the amount of glenoid bone stock along the passage of the central peg, was then automatically calculated. RESULTS The depth of the glenoid vault was 25.5 ± 3.0 mm (range, 19.3-31.5), and the mean overlapped volume between the simulated central peg and the glenoid vault was 623.0 ± 185.8 ml. The optimal insertion site for the bony purchase of the central peg was 2 mm inferior and posterior from the CIS (765.3 ± 157.5). CONCLUSION The optimal insertion site of the baseplate is located slightly inferiorly and posteriorly to the CIS. This anatomical information may be used as a reference to determine the optimal insertion site of the baseplate according to an implant of a surgeon's choice.