Adjustable Acetabular Cage

Overview

Our sponsor, The Penn State Hershey Medical Center, has tasked us with designing an adjustable acetabular cage for use in hip revision surgery with large bone defects. Currently, surgeons have to screw a cage into the pelvis and cement a shell at a certain position into this cage to test the femur’s mobility within the implant. If the femur seems likely to dislocate, then the surgeons will have to scrape out the cement manually and start the process over in a different position which is difficult and takes up time. Our design will solve this problem by making the shell adjustable within this cage, so if the femur seems likely to dislocate, the surgeon can change the position without having to re-cement.

Objectives

– Develop a way to lock the position of the acetabular cup in total hip revision surgery to ensure adequate hip mobility trials prior to cementing.

– The final design is a grid-cup design that includes multiple holes lined along the cage. A screw is placed through the cup and screwed into any hole in the cage based on the orientation desired

Approach

– We have gathered our needs through talking to our sponsors which include both an engineering and surgeons perspective allowing us to properly define our needs using both perspectives and then coming up with and refining with our sponsor.

– Once we all made a large quantity of designs we decided that each person would choose their favorite 2 ideas and talk in more detail to the rest of the group on how it would work. We then ended up using a weighting matrix to decide on our 2 favorite designs to which we created CAD Models. Through prototyping and further discussion, we finally decided on our final design.

– Our analysis in our research was looking at existing products that are commonly used today and diving deep into the functionality of those designs to further develop our understanding of the Acetabular cages that exist. As well as find what aspects are common flaws in the designs that exist today and/or how we can combine these designs to make a design that is an improved version of our previous standard.

– We gathered data on a biweekly basis with our sponsors through remote meetings where we gained feedback from our sponsors consistently through giving them updates on what we have done for the past 2 weeks whether that be design generation, defining our needs, simulations, or presentations. We would then take that feedback and apply it as need be to whatever we are working on at that time to show them how we applied that feedback.

– We used Solidworks to simulate the screw of our design that connected our shell and cage because we considered it to be the point of highest stress concentration. We looked through different loading distributions and discussed those with our sponsor to come up with a more accurate simulation as to how and where that screw would break.

– For the final design, three separate models of the cage, shell, and threaded pin were made. These models went through numerous changes to get to a more ideal spot after receiving feedback from the sponsors about what could be improved. There were also a few models of earlier concepts made to better show ideas.

– There were three prototypes printed. The first prototype was 3D-printed and had dimensioning issues that did not allow for all the parts to fit together properly. It followed a pronged cup design that was scrapped after receiving feedback from the sponsors. The second prototype was 3-D printed once again, but the cup was a full semispherical shape with one hole at the bottom and the cage had holes that allowed for the pin to fit in. These holes were not extruded radially and needed to be fixed for the final prototype. After fixing that and making a few minor changes to the dimensions, the final prototype model was made. The cage and shell were metal printed to give a better idea of how the model should work. The pin was excluded due to size and cost limitations so the group chose to order a premade screw from McMaster-Carr. These screws were almost the exact size of the model and, with a little bit of tape, fit in the cage and shell perfectly.

– Physical testing of the prototype was limited. It was found, however, that all the parts can easily be put together as is and should function properly in the operating room should it be made out of proper material and any changes that need to be made should be minor. The biggest worry with this design is that the pin may be too small and could get lost during surgery. Fortunately, the purchased pin came with a drive that could be used to more easily fasten it into the cage with tools, so that should continue to be used in the future.

– One of the only concerns with how the design may fail was through the shear forces acting on the pin. To attempt to calculate the expected forces the pin would undergo, a free body diagram (FBD) and finite element analysis (FEA) were performed. The FBD assumed the pin would act as a cantilever beam as the threaded portion would be fixed into the cage and the head would feel a force pushing on it from the shell. The FEA was performed in SolidWorks to attempt to show how the pin may deform under this force as well. In both cases, it was assumed that this model would be made out of Ti-5Al-2.5Sn and would undergo a point load of 116N.

– The FBD showed that the maximum stress found would be about 127 MPa, which is well below the tensile strength of the material (517 MPa). It also showed that the maximum deflection of the screw would be 0.004 mm or 4 μm, a very small amount that has no cause for concern. The FEA on the other hand shows more qualitatively how the pin would deform and where it would feel the stress the most. What both of these show is that the pin, under the estimated conditions of a mobility trial, would not be likely to fail.

Outcomes

– Hip revision surgeries will take less time and resources

– Surgeons will have an easier time finding the perfect alignment for the shell