Center of Rotation Medialization in Reverse Total Shoulder Arthroplasty
Medialization of the reverse total shoulder arthroplasty (rTSA) center of rotation (COR) is used to increase the deltoid lever arm and optimize joint loading mechanics.[1] A commonly cited “Grammont-style” design principle places the COR approximately 10 mm medialized relative to the native anatomic COR.[1]
Biomechanical Effects on Deltoid Lever Arm
Medializing the COR increases the deltoid abductor moment arm.[2] Grammont-style medialization is associated with a ~20–42% increase in deltoid moment arm at the shoulder with the arm at the side across reported biomechanical evaluations.[3] Cadaveric work supports increased deltoid moment arms when COR medialization is applied, with reported anterior and middle deltoid moment-arm increases of ~10 mm and ~15 mm, respectively.[4]
Effects on Stability
Medialization of the COR is designed to position the reverse joint center at or near the glenoid bone–implant region, which reduces deleterious shear forces across the glenoid interface.[5] Medialization also increases compressive forces at the bone–glenoid interface, which contributes to a favorable resultant force vector at the joint.[5]
Effects on Scapular Notching
Medialized COR positioning is associated with higher rates of scapular notching in clinical series.[6] In reviews of medialized COR designs, scapular notching incidence has ranged from ~44% to 96% across studies reporting that design strategy.[6]
Effects on External Rotation
Medialized COR designs are associated with reduced external rotation compared with lateralized COR designs.[6] Lateralized COR designs are specifically developed to mitigate the external-rotation limitation seen with traditional medialized COR designs.[6]
Practical Implication for “Recommended” Medialization
The medialization magnitude is best framed as a design target from the original Grammont concept, rather than a patient-specific isolated “mm” instruction.[1] A frequently cited target is ~10 mm medialization of the COR in Grammont-style constructs.[1] Traditional medialized COR positioning increases deltoid efficiency and joint stability through altered loading mechanics.[5] Traditional medialized COR positioning increases risk of scapular notching and is associated with worse external-rotation outcomes versus lateralized constructs.[6]