Computer-assisted analysis of functional internal rotation after reverse total shoulder arthroplasty: implications for component choice and orientation

PurposeFunctional internal rotation (IR) is a combination of extension and IR. It is clinically often limited after reverse total shoulder arthroplasty (RTSA) either due to loss of extension or IR in extension. It was the purpose of this study to determine the ideal in-vitro combination of glenoid and humeral components to achieve impingement-free functional IR.MethodsRTSA components were virtually implanted into a normal scapula (previously established with a statistical shape model) and into a corresponding humerus using a computer planning program (CASPA). Baseline glenoid configuration consisted of a 28 mm baseplate placed flush with the posteroinferior glenoid rim, a baseplate inclination angle of 96 degrees (relative to the supraspinatus fossa) and a 36 mm standard glenosphere. Baseline humeral configuration consisted of a 12 mm humeral stem, a metaphysis with a neck shaft angle (NSA) of 155 degrees (+ 6 mm medial offset), anatomic torsion of -20 degrees and a symmetric PE inlay (36mmx0mm). Additional configurations with different humeral torsion (-20 degrees, + 10 degrees), NSA (135 degrees, 145 degrees, 155 degrees), baseplate position, diameter, lateralization and inclination were tested. Glenohumeral extension of 5, 10, 20, and 40 degrees was performed first, followed by IR of 20, 40, and 60 degrees with the arm in extension of 40 degrees-the value previously identified as necessary for satisfactory clinical functional IR. The different component combinations were taken through simulated ROM and the impingement volume (mm(3)) was recorded. Furthermore, the occurrence of impingement was read out in 5 degrees motion increments.ResultsIn all cases where impingement occurred, it occurred between the PE inlay and the posterior glenoid rim. Only in 11 of 36 combinations full functional IR was possible without impingement. Anterosuperior baseplate positioning showed the highest impingement volume with every combination of NSA and torsion. A posteroinferiorly positioned 26 mm baseplate resulting in an additional 2 mm of inferior overhang as well as 6 mm baseplate lateralization offered the best impingement-free functional IR (5/6 combinations without impingement). Low impingement potential resulted from a combination of NSA 135 degrees and + 10 degrees torsion (4/6 combinations without impingement), followed by NSA 135 degrees and -20 degrees torsion (3/6 combinations without impingement) regardless of glenoid setup.ConclusionThe largest impingement-free functional IRs resulted from combining a posteroinferior baseplate position, a greater inferior glenosphere overhang, 90 degrees of baseplate inclination angle, 6 mm glenosphere lateralization with respect to baseline setup, a lower NSA and antetorsion of the humeral component. Surgeons can employ and combine these implant configurations to achieve and improve functional IR when planning and performing RTSA.