Impact of Modeling Assumptions on Stability Predictions in Reverse Total Shoulder Arthroplasty

Abstract

Reverse total shoulder arthroplasty (rTSA) is commonly used in the shoulder replacement surgeries for the relief of pain and to restore function, in patients with grossly deficient rotator cuff. Primary instability due to glenoid loosening is one of the critical complications of rTSA; the implants are designed and implanted such that the motion between the glenoid baseplate and underlying bone is minimized to facilitate adequate primary fixation. Finite element analysis (FEA) is commonly used to simulate the test setup per ASTM F2028-14 for comparing micromotion between designs or configurations to study the pre-clinical indications for stability. The FEA results can be influenced by the underlying modeling assumptions. It is a common practice to simplify the screw shafts by modeling them as cylinders and modeling the screw-bone interface using bonded contact, to evaluate micromotion in rTSA components. The goal of this study was to evaluate the effect of three different assumptions for modeling the screw-bone interface on micromotion predictions. The credibility of these modeling assumptions was examined by comparing the micromotion rank order predicted among three different modular configurations with similar information from the literature. Eight configurations were modeled using different number of screws, glenosphere offset, and baseplate sizes. An axial compression and shear load was applied through the glenosphere and micromotion at the baseplate-bone interface was measured. Three modeling assumptions pertaining to modeling of the screw-bone interface were used and micromotion results were compared to study the effect of number of peripheral screws, eccentricities, and baseplate diameter. The relative comparison of micromotion between configurations using two versus four peripheral screws remained unchanged irrespective of the three modeling assumptions. However, the relative comparison between two inferior offsets and baseplate sizes changed depending on the modeling assumptions used for the screw-bone interface. The finding from this study challenges the generally believed hypothesis that FEA models can be used to make relative comparison of micromotion in rTSA designs as long as the same modeling assumptions are used across all models. The comparisons with previously published work matched the finding from this study in some cases, whereas the comparison was contradicting in other cases. It is essential to validate the computer modeling approach with an experiment using similar designs and methods to increase the confidence in the predictions to make design decisions.

Finite Element Analysis of Custom Shoulder Implants Provides Accurate Prediction of Initial Stability

“…To our knowledge, this is the first work to report a full in silico modeling of ASTM F2028-14 for a custom-made prosthesis. Although previous studies [13,14,22] reported FE analysis for a similar experimental set-up, the effect of different loading directions, which in case of a custom implant cannot be neglected due to the asymmetry of the design shape, was never systematically investigated. This approach resulted in a total of eight measurements that were used to support the FE predictions.…”

“…Major assumptions were made during the creation of the in-silico model, looking for a trade-off between accuracy and computational cost. The bone substitutes were modeled with homogeneous isotropic material properties, a simplification commonly accepted and implemented in the literature [13,14,16,22], although not fully representative of the behavior of the bone substitute. The porous structure of the implant was not explicitly modelled to reduce the complexity of the model.…”

“…All components were modeled with linear elastic material properties, which is an assumption commonly made under these experimental conditions [22]. The loading box and baseplate were assigned with a Young's modulus of 110,000 MPa and a Poisson's ratio, ν, of 0.3 (corresponding to Titanium Ti-6Al-4V, [23]).…”

“…Coulomb friction contact was implemented at the bone-implant interface. In the literature, values ranging from 0.5 to 0.7 are reported for the friction coefficient between bone and porous metal [13,14,22,27], thus an average friction coefficient of 0.6 was selected for the presented model. The 3D FE models were meshed with tetrahedral C3D4 elements.…”

“…In order to assess the impact of different screw types (compression and locking) on fixation, particular attention was paid to the screw modeling. A recent study showed that an excessive simplification of the screw shaft model has an impact on the micromotion in RSA implant design analysis [22]. Hence, the validity of the simplification assumptions has always to be evaluated against experimental measurements, aiming for a trade-off between acceptable computation times and prediction accuracy.…”