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
Good outcomes in reverse shoulder arthroplasty (RSA) rely in part on stability of the humeral component. Traditionally humeral components have been cemented, however there has been recent interest in press-fit fixation of humeral components in RSA. Lateralization of the head center in RSA can impart larger moments on the humeral component than for anatomic reconstructions, increasing the importance of distal humeral canal preparation for implant stability. To date, the primary stability of any type of press-fit humeral prosthesis has been largely unexplored. The goal of this study is to evaluate the effect of over-reaming the distal humeral canal in a press-fit humeral component in RSA. Computed tomography (CT) data of the shoulder were obtained from 55 shoulders. Images were segmented to produce digital models of the humerus. Humeral components for RSA (2mm diameter size increments) were sized and placed per the surgical technique, including preparation of the humerus with the appropriate reamers (1mm increments). Finite element models for each specimen were created with heterogeneous bone properties derived from the CT scan. Pressfit between the bone and stem was resolved to quantify the initial contact pressure on the stem; each stem was then loaded at 566N oriented 20° lateral and 45° anterior. Overall motion of the stem was measured, as well as interfacial micromotion in the porous coating region (Fig. 1). The effect of line-to-line (L2L) reaming and over-reaming by 1 mm was evaluated using an unpaired Student's t-test, with significance defined at p<0.05.Introduction
Methods
Comprehensive research and retrieval analyses of metal on metal / metal on polyethylene hip fretting and corrosion have been reported. Design choices such as modularity, material couples, geometry and offsets, as well as surgical variability and patient sensitivity have been cited as factors contributing to revision. Findings are informing new designs, surgical techniques and patient testing. However, similar efforts have not been performed on the shoulder. Do reduced joint reaction forces imply lower risk of fretting and corrosion? In this study we designed an accelerated corrosion fatigue (ACF) test specific for the shoulder to allow for evaluation of varying designs, and compared results to a reported shoulder retrieval study [Day ORS 2015]. Anatomic configuration and reverse shoulder ACF tests were developed with loads and orientations determined from instrumented shoulder data and reported literature. Scaled loads of 1480 N and 962 N were applied to anatomic (Fig 1.A) and reverse (Fig 1.B) prostheses, respectively (n=5 each, with additional assembly control), in potential worse case loading directions (α=25°, β=20°: anatomic; α=0°, β=0°: reverse), at 5 Hz for 3.0 Mc with R=0.1. Test environment included 0.9% NaCl solution at elevated temperature (50° C) and a decreased pH (3.5). Mass, roughness (Ra) and taper damage (modified Goldberg scoring system) measures were taken before and after testing. Taper connections were assembled at impact loads of 3600 N +/− 20% based on cadaveric studies. Goldberg scores for 79 humeral heads and 61 stems from an IRB approved collection served as the comparator.Introduction
Methods
Porous scaffolds for bone ingrowth have numerous applications, including correcting deformities in the foot and ankle. Various materials and shapes may be selected for bridging an osteotomy in a corrective procedure. This research explores the performance of commercially pure Titanium (CPTi) and Tantalum (Ta) porous scaffold materials for use in foot and ankle applications under simplified compression loading. Finite element analysis was performed to evaluate von Mises stress in 3 porous implant designs: 1) a CPTi foot and ankle implant (Fig 1) 2) a similar Ta implant (wedge angle = 5°) and 3) a similar Ta implant with an increased wedge angle of 20°. Properties were assigned per reported material and density specifications. Clinically relevant axial compressive load of 2.5X BW (2154 N) was applied through fixtures which conform to ASTM F2077–11. Compressive yield and fatigue strength was evaluated per ASTM F2077–11 to compare CPTi performance in design 1 to the Ta performance of design 3.Introduction
Methods
