Strength of the baseplate fixation of a reversed shoulder prosthesis

A reversed shoulder prosthesis is the general treatment of glenohumeral arthritis and concomitant rotator cuff arthropathy. However, failure due to loosening of the components remains a concern. The purpose of this study was to examine the forces that occur at the screws of the glenoid implant during the fixation process. An experimental setup was developed allowing for measuring both force and torque depending on turn angle for each screw separately. Experiments were performed on artificial bone of two different densities (0.16 g/cm3 and 0.24 g/cm3) and on six human glenoid bones of unknown age and gender. Characteristic graphs could be obtained for every material and each screw position. On average maximal force values were 180 N in lower density foam and 341 N in higher density foam. Corresponding mean peak values for torque were 0.65 Nm and 1.33 Nm, respectively. In the human bone experiments force peak values for the anterior compression screw ranged from 179 N to 317 N, torque values from 0.26 Nm to 2.36 Nm. For the posterior compression screw force peak values ranged from 320 N to 545 N, torque values from 1.35 Nm to 4.66 Nm. Peak values were consistently higher in higher density foam and at the posterior screw position. These results show a high dependency of the occurring screw forces on material density and position within the glenoid and contribute to further research in glenoid implant design optimization.