Hip Replacement Prototype FEA

An Experimental new design for a hip replacement needs to be analyzed under realistic loading conditions. Before beginning physical stress testing of the device, we need to ensure that we are moving forward with the best design possible. The design team sent over a late-stage iteration to be signed off on before manufacturing. I am running it thru simulations to give feedback on the design. I will submit a report with my findings and recommendations on design improvements and materials.

My first task is to find the forces that a human hip must endure within relatively common tasks. The site orthoload.com has the raw data on the forces a hip will see under conditions like standing, walking, traversing stairs, and getting in and out of a car. My first step is to transform this raw data into useful loading conditions for Solidworks FEA. I take their vector mapping of forces on the ball of the hip joint and translate them into the loading conditions for Solidworks. 

It's important to think of the loads on the hip as dynamic. Since the forces are relative to the fixed restraint position, I decide to run simulations of the load from the lower leg to a fixed point at the ball of the hip, and from the hip to a fixed point on the prosthetic femur. The prosthetic has multiple parts and materials, including a bone cup fused to the hip joint, the accepital cup that meets the bone cup, and the ball and femur of the prosthetic. Choosing the correct bonding criteria in SolidWorks, displacement, and force type are crucial to accurate results, ensuring joints will move realistically and displace naturally.

Using three different mesh types and the forces derived from orthoload.com, I find the area of the prosthetic with the highest stress concentration. Upon finding this I refine the mesh in the area and choose the mesh that gave the maximum stress. 

Under the loading conditions related to walking, climbing stairs, and getting in and out of the car, I found the maximum force to be 311% the patients body weight. With this force, the maximum Von Mises stress concentration was found to be in the loft of neck between the ball and the femur. The Von Mises stress from the simulation was  8.65 x 108 𝑁 /𝑚^2, which exceeded the yield strength of the titanium alloy proposed of  3.7 x 108 𝑁 /𝑚^2. With this finding my recommendation was that failure of the current design was imminent under standard conditions and that a redesign the prosthetic was needed before moving onto manufacturing a prototype for physical testing. Strengthening the neck loft was needed by adding additional material, changing the material, or redistribution of the force. 

This project taught me a lot about accurate FEA analysis. Mainly, that there is more to it than just running a study in SolidWorks. Research needs to be conducted about the type of forces that a design will encounter. The biggest surprise during this project was the difference that fractions of a mm in displacement would cause to the Von Mises Stresses. Anticipating natural displacement and deformation was critical to an accurate assessment. Additionally, accurate assessment of the degrees of freedom each piece will experience, the degrees of freedom of any restraint, and the bonding of surfaces made critical differences in the analysis.

If we had the time and resources, manufacturing the product and going thru physical testing would have been very informative as to the accuracy of the FEA modeling and analysis.