A search of the literature indicates several constrained total knee arthroplasty (TKA) systems are at risk for articular surface lockdown bolts backing out. The backing out of a lockdown bolt may lead to an unstable and/or painful knee and may necessitate revision. Upon backing out, the bolt may damage implant components and surrounding tissues. To date, studies in the literature have not simulated or replicated loosening of bolts in TKA. Therefore, the objectives of this study were to 1) develop a set of physiological loading parameters that challenge bolted articular surfaces; 2) evaluate whether significant bolt torque is lost during application of this loading to a CCK device with a bolt as a secondary locking mechanism. Physical test parameters to loosen lockdown bolts were developed based on loading experienced during activities of daily living. Sinusoidal waveforms and timing were used to simulate worst case walking gait conditions. Compared to data from everyday activities in instrumented TKR patients, anterior posterior loads and internal/external torques exceeding the absolute maximums observed were selected. To transfer more shear and torsion to the joint interface, compressive load lower than typically reported for walking gait was used. Frequency was representative of walking gait motion. The offset in torsional waveform enables a ratcheting motion to drive a loose bolt out of the joint: during external femoral rotation of a left knee, reduced compressive load and posterior directed femoral loading on a CCK spine creates a potential articular surface lift-off. The lift-off may grab the underside of the front bolt shoulder while external (CCW) rotation loosens the bolt. These loading conditions exist during toe-off of walking gait. Two CCK devices were evaluated to capture potential difference in performance: a medium articular surface combination and a smaller articular surface combination. Testing was performed on a load frame capable of rotation and vertical / horizontal translation.Introduction
Materials and Methods
Finite element analysis (FEA) is widely used to study micromotion between the glenoid baseplate and bone, as a pre-clinical indicator for clinical stability in reverse total shoulder arthroplasty (rTSA). Various key parameters such as the number, length, and angle of screws have been shown to influence micromotion [1]. This study explores the influence of screw preloads, an insufficiently studied parameter. Specifically, two rTSA configurations with 18mm and 48mm peripheral screws (PS) were analyzed without screw preloads, followed by analysis of the 48mm PS configuration with an experimentally measured screw preload. FEA models were created to simulate a fixation experiment inspired by ASTM F2028-14. The rTSA configurations used here have a superior and an inferior PS. The assemblies were virtually implanted into a synthetic bone block as per surgical technique. Sliding contacts were defined to model the interface between screw threads-bone, and between baseplate-bone. To determine the screw preload experimentally, the 48mm screw (n=5) was inserted through a hole in a metal plate, which rested on top of a Futek washer load cell, placed on top of the foam block with a predrilled pilot hole (Figure 1). The screw was inserted using a torque driver until the average human factors torque for the screw driver handle was reached. The resulting axial compressive load due to screw insertion was measured by the washer load cell. Two step analyses were performed using Ansys version 17.2 for 18mm and 48mm PS, where 756N axial and shear loads were applied sequentially. The model with the 48mm PS was then analyzed in a four step analysis; preload inferior and superior screws, followed by applying the axial and shear loads (Figure 2). Peak overall micromotion including tangential and normal components at the baseplate-bone interface was compared for all three models.INTRODUCTION
METHODS
