Humeral head size predicts baseplate lateralization in reverse shoulder arthroplasty: a comparative computer model study

Abstract

BackgroundIn reverse shoulder arthroplasty (RSA), the ideal combination of baseplate lateralization (BL), glenosphere size (GS) and overhang (GOH) with a commonly used 145° neck shaft angle (NSA) is unclear. This is the first study evaluating correlations of body height (BH), humeral head size (HS), glenoid height (GH) and association of gender with best glenoid configurations for range of motion (ROM) maintaining anatomic lateralization (aLAT) for optimized muscle length in 145° and less distalized 135° RSA. MethodsIn this computer model study, 22 CTs without joint narrowing were analyzed (11 male/female). A standardized semi-inlay 145° platform stem was combined with 20 glenoid configurations (baseplate (B) 25, 25+3/+6 lateralized (l), 29, 29+3/6l combined with glenosphere36, 36+2eccentric (e), 36+3l, 39, 39+3e, 39+3l , 42, 42+4e). Abduction-adduction, flexion-extension, external rotation-internal rotation, total ROM (TROM) and total notching relevant (TNR) ROM were computed, best TROM models respecting aLAT (-1mm to +1mm) and HS/GH recorded. Secondly, the 145° models (Ascend Flex stem; Stryker, Kalamazoo, MI, USA) were converted and compared to a 135° inlay RSA (New Perform stem; Stryker, Kalamazoo, MI, USA) maintaining GOH (6.5-7mm) and aLAT. ResultsBest 145° models had eccentric glenospheres (mean BL: 3.5mm, GOH 8.8mm, GS 38.1mm, distalization (DIS) 23mm). The 135° models had concentric glenospheres, mean BL 3.8mm, GOH 6.9mm, GS 39.7mm, DIS 14.1mm. Head size showed the strongest positive correlation with BL in 145° and 135° models (0.65/0.79). Despite reduced GOH in smaller females with a 135° NSA, adduction, external rotation, extension, TNR ROM and TROM were significantly increased (p=0.02, p=0.005, p=0.005, p=0.004, p=0.003), abduction however reduced (p=0.02). The same trends were seen for males. ConclusionsHead size is a practical measure in surgery or preoperatively, and the strong positive correlation with BL is a useful planning aid. Despite reduction of GOH, conversion to a less distalized 135° NSA is powerful to maintain and even significantly increase all components of TNR ROM (extension/external rotation/adduction) in small females with the drawback of reduced abduction which may however be compensated by scapula motion. Lateralization with a less distalized 135° RSA optimizes muscle length, may facilitate subscapularis repair and maintains highest rigid body motion.