(B-63) The Effect of Central Screw or Post Vault Containment on Reverse Total Shoulder Arthroplasty Baseplate Stability

Introduction:: Reverse total shoulder arthroplasty (RTSA) is a surgical procedure that replaces irreversible rotator cuff damage and degenerative glenohumeral joint diseases that hinder arm abduction. Rotator cuff injuries affect ~2 million people every year in the US and tears occur in 62% of people over the age of 80 [1][2]. Glenohumeral osteoarthritis, a disease that causes progressive degeneration of the glenohumeral joint articular cartilage, occurs in 27.5% of people over 80 [3]. To address these problems, RTSA can be used to repair joint functionality; however, RTSA implant complications occur 19-68% of the time which often require implant revision [4]. One major complication is glenoid baseplate failure resulting from increased movement at the bone-baseplate interface. The stability of the baseplate’s initial fixation plays a large role in how much micromotion the implant will experience overtime. There were two main goals of this study. The first was to determine if glenoid baseplate micromotion was affected by glenoid vault perforation by the central post. The second was to evaluate if micromotion was affected by an increased lateralization construct, if the amount of micromotion differed between central bicortical screw and post fixation, and to find the amount of shear load required to cause failure in all test groups. By testing a variability of baseplate components, the optimal configuration for baseplate fixation could be established.

Materials and Methods:: This study examines 38 composite scapula specimens split into six study groups with varying Arthrex glenohumeral implant components. All 36 specimens received 24 mm modular glenoid baseplates with 4 mm of lateralization. 18 specimens received standard 36 mm modular glenospheres and 18 received 36 mm modular glenospheres with 4 mm of lateralization. Split equally between the two different glenosphere groups, the baseplates either received a central post contained within the vault, a central bicortical post, or a central bicortical screw. The Arthrex Virtual Implant Positioning (VIP™) glenoid guide was used to aid the surgeons during prosthesis implantation to ensure reproducible implantation angles. After implantation, each specimen was secured in a metal pot and then placed in a custom-made Instron 8874 fixture. A linear variable differential transformer (LVDT), used to measure implant micromotion, was attached underneath the scapula to allow light glenosphere contact. Each specimen was subjected to cyclic shear loading beginning at 50 microns at 1 Hz and increased until the baseplate experienced 150 microns of micromotion. Each specimen was then subjected to failure testing; for this study, failure was classified as 1000 microns of baseplate movement or scapula fracture. After data collection, statistical analysis was done between the six study groups to determine differences in baseplate micromotion and fixation strength.

Results, Conclusions, and Discussions:: Thus far in our study we have validated our method development and experimental setup. To begin, the LVDT was calibrated and a sensitivity analysis was performed to determine measurement resolution. To create a calibration curve, metal gauge blocks with displacements of 1 mm to 1.4 mm in increments of 0.01 mm were placed under the LVDT actuator to record voltage. As expected, the actuator measurements produced linear results with an R2 value of 0.9892. To find the LVDT sensitivity a T-test was performed between voltages for two different displacements. It was found to be statistically significant at 10 microns which validated that the LVDT could perform measurements with a 10-micron resolution. Next, the method of analysis used to determine the implantation angles for each study group was established. To ensure accurate data collection of the implantation angles, we created a repeatable process of measuring the angle from the midpoint of the central post/screw relative to the horizon. Lastly, the Instron experimental setup was finalized to ensure that all modular glenospheres experienced perpendicular shear loading. The results of validating the experimental setup ensured that the data collected during the study would be accurate, reproducible, and beneficial to the orthopaedic industry.
There were two hypotheses for the study results. First, due to baseplate post placement being an anatomical fixation that only interfaces with the bone, we expected that bicortical post placement in the glenoid vault would not compromise baseplate stability compared to contained post placement. Second, because increased construct lateralization will offset the center of rotation and therefore increase shear stresses, we expected that the test specimens with 4 mm lateralized glenospheres would have increased micromotion and decreased failure strength. From the data we will collect in the following months, we will be able to draw statistical conclusions for each goal and hypothesis.
This study has potential to inform surgeons on implantation techniques that will increase implant lifespan, resulting in reduced revision rates and increased patient satisfaction. On a broader scale, reduced revisions would decrease the use of expensive materials, relieve patients of unwanted complications, and improve the reputation of RTSA procedures.

Acknowledgements (Optional): : I would like to acknowledge the Hawkins Foundation and Arthrex for funding, and Arthrex for the supply of surgical materials and implants.

References (Optional): : [1] Athwal & Armstrong. (2022). OrthoInfo; [2] May & Garmel. (2022). National Library of Medicine; [3] Ibounig et al. (2020). Sage Journals 110(3); [4] Familiari et al. (2018). Efort Open Reviews 3(2).