Measurement of lateralization and distalization in Grammont and onlay reverse total shoulder arthroplasty

Abstract

BackgroundReverse total shoulder arthroplasty (RTSA) implants allow variation in lateral and distal offset (LO and DO). The aim of this study was to measure, compare, and classify LO and DO and to assess the effects of modular changes using 2 humeral RTSA implants (one Grammont and one onlay-type) paired with a common modular glenoid component. MethodsThree thousand eight hundred sixty virtual RTSA models were created. Measurements of LO and DO between reference points (B—center of the ‘baseplate;’ C—‘glenosphere’ center of rotation; P—pivot point of the ‘insert;’ M—midpoint of the distal surface of the ‘tray’ in the onlay humeral implant or midpoint of the proximal surface of the ‘spacer’ in the Grammont implant; J—junction of humeral stem and ‘tray,’ ‘liner’ or ‘spacer;’ H—distal end of stem) were taken. ‘Glenoid offset’ was distance BP (comprising BC and CP); ‘humeral offset’ was distance PH (comprising PJ, JM, and MH); their sum was the ‘global offset.’ ResultsGlobal LO and DO measurements were 14.9-30.9 mm and 13.1-36.3 mm, for the Grammont implant, and 23.0-52.6 mm and 18.0-42.2 mm for the onlay implant. Glenoid LO and DO measurements were 8.2-17.3 mm and 16.3-25.0 mm for the Grammont implant and 11.6-23.1 mm and 12.8-23.3 mm for the onlay implant. LO and DO did not vary proportionally in either implant type. Baseplate modularity affects distance BC; ‘glenosphere’ modularity affects distance BC, (‘glenosphere’ eccentricity and lateralization) and/or distance CP (‘glenosphere’ diameter). Humeral modularity affects distance PH (stem, ‘insert,’ ‘tray,’ and ‘spacer’ geometry, overall neck shaft angle [NSA]) and distance CP (overall NSA). Overall NSA also dictated the relative amount of humeral LO and DO resulting from other humeral modular changes. ConclusionBoth Grammont and onlay humeral RTSA implants paired with a common modular glenoid component can be configured to create a large range of LO and DO, but values are typically greater when using the onlay humeral component. This is despite the higher NSA of the Grammont implant which would be expected to result in more DO. The described classification of DO together with LO may be beneficial. Previous classifications of RTSA based on their ‘glenoid’ (distance BP) and ‘humeral’ (distance PH) offsets are misleading. Distance BC should be considered as the measure of ‘glenoid’ and distance CH as the measure of ‘humeral’ offset. The ‘glenosphere’ is best thought of as a ‘nonglenoid, nonhumeral spacer’ that can affect both BC and CH.