Central Fixation Element Type and Size Affect Glenoid Baseplate Micromotion in Reverse Shoulder Arthroplasty

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Torkan, Lawrence
Shoulder , Arthroplasty , Reverse shoulder arthroplasty , Shoulder biomechanics , Micromotion , Glenoid micromotion , Orthopaedic surgery , Baseplate micromotion
Reverse shoulder arthroplasty (RSA) is typically performed in patients with injured or deficient rotator cuff muscles. Loosening remains to be one of the principal modes of implant failure and the main complication leading to revision. The objective of this study was to mechanically evaluate factors affecting baseplate fixation to inform clinicians of possible implant configurations that may decrease micromotion and reduce the risk of loosening. Delta XTEND™ baseplates (DePuy Synthes) were implanted into Sawbones™ polyurethane foam blocks (N = 40) and varied in terms of their method of central fixation (screw vs. peg), central length (13.5 vs. 23.5 mm), anterior-posterior peripheral screw type (nonlocking vs. locking), and bone density (10 vs. 25 pcf). Samples were cyclically loaded under compression and shear with a 500 N force at 60° to simulate peak loads during shoulder abduction. Implant-bone motion was measured using four linear variable differential transformers. A theoretical plane was used to interpolate micromotion at the four peripheral screw positions for each sample. Micromotion measurements yielded a mean absolute percentage error (MAPE) of (9.5 ± 2.6)%; average displacements at the implant-bone interface were analyzed with a Kruskal-Wallis H test and nonparametric univariate analysis (α = 0.05). Central peg fixation generated greater micromotion at all four screw positions when compared to central screw fixation (p < 0.001). Furthermore, 13.5 mm central elements generated greater micromotion at all four measurement positions than 23.5 mm samples (p = 0.001). A significant interaction existed between the method of central fixation and central element length at all four screw positions (p < 0.001), as well as between peripheral screw type and bone density at all four screw positions (p = 0.015). Use of a central compression screw and a greater central element length that engages with the cortex of the scapula may provide improved baseplate fixation and reduce interface micromotion. Use of locking screws in the anterior and posterior positions may also offset loss of fixation due to lower bone quality. Future work that considers other baseplate factors may reveal additional surgical considerations and factor interactions.
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