Session: 05-09-02: Computational Modeling in Biomedical Applications - II

Paper Number: 95176

95176 - A Computational Parametric Design Approach for Orthopedic Implants Under Highly Nonlinear Conditions 

Employing a computational modeling approach is advantageous during the development and evaluation of orthopedic devices. The ability to perform parametric studies can reduce design loops and thus the design development timeline. Typically, the models are simplified which leads to more efficient simulations. However, the models also need to be thoroughly verified and validated as per standards such as ASME VVUQ 40 (Verification, Validation, And Uncertainty Quantification in Computational Modeling of Medical Devices) to ensure their accuracy relative to the device design, its configuration and application.

In some scenarios the complexity of loading conditions, geometry or material response can lead to challenges in adopting a computational approach during the design development cycle. As a case study, this paper presents the development of a computational approach to simulate off axis screw insertion followed by cantilever loading on a predicate/on-market trauma locking plate implant. 

During a surgical procedure the screw insertion angle can vary depending on the bone geometry and properties. It is governed by the conditions the surgeon faces at the time of the procedure where his focus is on obtaining a good bone purchase. Due to off-axis insertion a high probability of cross threading between the threads of the screw head and the plate exists. This can introduce significant damage to the threads on both the screw as well as the plate which then potentially impacts the load bearing capacity of the screw-plate interface.

Due to the complexity of thread interactions, the joint performance is typically characterized through laboratory testing. Once the screw is inserted into the plate, the joint is subjected to different loading conditions. Two key tests are a) push through wherein the load acts along the screw axis and b) cantilever wherein a lateral (off axis) load is applied to the screw tip.

The number of parameters of interest are large, including, screw and plate material, insertion angles, thread geometries, and different loading scenarios. Additionally, random effects such as screw clocking relative to the plate threads are difficult to quantify. In such a scenario, relying solely on a testing approach can extend the design development timelines. Hence, even though its application presents difficulties due to cross threading and the corresponding thread damage a computational approach presents significant advantages.

This paper compares two simulation approaches i) Lagrangian approach with element deletion and ii) Coupled Eulerian Lagrangian (CEL). Simulations are performed using a general-purpose finite element code ABAQUS® along with an explicit time integration approach. To account for the high expected local thread deformations due to cross-threading a continuum damage mechanics (CDM) based material model is implemented.  The modeling approach is validated by comparing the predicted response of the screw-plate interface with cantilever loading results from laboratory tests. The thread deformation profiles are compared to the observed thread damage from testing. Effect of simulation parameters such as mesh density and mass scaling for an explicit quasi-static simulation are studied for both these approaches.

The results show potential in continuing further investigations to enable employing the models as tools to perform parametric studies. For the insertion configurations simulated, the models are able to simulate the desired screw head prominence. The deformation profiles and the regions with thread damage as predicted by the analysis compare well with the observed thread damage during the tests. When the simulations diverge from the observed test data, the reasons for this variability are investigated and potential solutions or additional tests/simulations are recommended.

The paper highlights the challenges and potential evaluation approaches while attempting to develop a robust computational methodology which can be employed for design development when specifically applied to highly nonlinear complex loading scenarios.

Presenting Author: Mandar Kulkarni Stress Engineering Services Inc.

Presenting Author Biography: Dr. Mandar Kulkarni has over 10 years of engineering experience and has been working with Stress Engineering Services since 2012. He mainly practices in the areas of solid mechanics, fatigue and fracture mechanics covering a range of materials and applications straddling different industries ranging from oil &amp; gas to the medical devices and automotive. He also holds a professional engineering license in the state of Texas.<br/><br/>As a finite element analysis expert he has performed a number of static, dynamic, steady-state and transient heat transfer and coupled thermo-mechanical evaluations with geometric and material nonlinearities on systems such as pressure vessels and piping. <br/>Dr. Kulkarni has also developed computational models of the complex device-bone interactions to help drive device development. He has experience in establishing credibility of the simulations per industry standards and help with regulatory submissions. His efforts are supported through extensive material characterization testing as well.

Authors:

Mandar Kulkarni Stress Engineering Services Inc.
Akshay Dandekar Stress Engineering Services, Inc
Mark Burchnall Stress Engineering Services
Ryan Dewall DePuy Synthes Trauma
Joel Oberli DePuy Synthes Trauma