Mechanical behavior of screw versus Endobutton for coracoid bone-block fixation

Abstract

IntroductionArthroscopic coracoid bone-block fixation by Endobutton was developed to avoid the complications associated with screwing. However, few studies have assessed the mechanical characteristics of the two. The aim of the present study was to assess and compare fixation rigidity by screw versus Endobutton. The study hypothesis was that rigidity is lower with Endobutton than with screws. Material and method3D print-outs of a glenoid and a coracoid process were obtained from CT scans of a patient showing anterior shoulder instability with significant bone defect. Four types of coracoid fixation were implemented: 1 or 2 4.5mm malleolar screws, and 1 or 2 Endobuttons. Three specimens per assembly were placed on a specific test bench. Lateromedial bone-block compression was exerted at 0.1mm/sec at 3 points: superior, central, inferior. The resultant force and bone-block displacement were recorded. ResultsMean fixation rigidity with 1 screw, 2 screws, 1 Endobutton and 2 Endobuttons was respectively 158N/mm (range, 133–179), 249N/mm (241–259), 10N/mm (5–13) and 14N/mm (13–15), with significant difference between the screw and Endobutton groups (p<0.001). Displacement was greater with 1 than 2 Endobuttons under superior or inferior force, while the difference was non-significant under central force (7.45 vs 6.93mm; p=0.53) ConclusionsScrew fixation showed greater rigidity, while the Endobutton assembly showed less tension, leading to greater bone-block mobilization. The interest of using two Endobuttons is to reduce displacement under polar pressure. the present biomechanical study confirmed the mechanical vulnerability of bone-blocks fixed by endobutton until consolidation is achieved. Level of evidenceBiomechanical study.