Hip Replacement: "Custom-made cups are currently only used in the case of major deformations"

Interview with graduate engineer Stefanie Betancur Escobar, Institute of Forming Technologies and Machines at the Leibniz University Hannover

Patients who receive a hip replacement look back on a long case history: it begins with abrasion in the old joint and via implantation and rehabilitation ideally leads to restoration of life’s quality: the replacement increases mobility and reduces the pain felt during walking. All the worse it becomes for the patient if the implant makes revision surgery necessary.


Stefanie Betancur Escobar; © private

Dipl.-Ing. Stefanie Betancur Escobar; © private

A reason for a second surgery can be migration of the replacement into the bone that is caused through natural bone remodeling processes after implantation. COMPAMED.de talked to graduate engineer Stefanie Betancur Escobar, Institute of Forming Technologies and Machines at the Leibniz University Hannover, about a cost-efficient production process for patient-individual cups used in hip replacements. These cups are supposed to alleviate bone remodeling and its consequences. The process is currently under development in the Collaborative Research Centre (Sonderforschungsbereich, SFB) 599 “Sustainable Bioresorbable and Permanent Implants of Metallic and Ceramic Materials”.

COMPAMED.de: Miss Betancur Escobar, how does the migration of an artificial hip cup after an implantation occur?

Stefanie Betancur Escobar: Generally, migration can occur with any implant. A material, whose properties differ from those of the surrounding bone, is being implanted. These cause an unphysiological load transmission to the bone – in a different way than it occurs in the original skeleton. The bone adjusts to the new, changed stress, which is a natural process.

Due to unphysiological load transmission, bone can also be resorbed where it normally does not happen. The prosthesis can then migrate into the pelvis, sometimes even by several millimeters and then eventually also become loose. The load transmission does not just depend on the material, but also on the geometry of the prosthesis.

COMPAMED.de: Prosthetic cups that are individually customized to the bone of the patient are meant to cushion or prevent this process. How common are they?

Betancur Escobar: Custom-made cups are currently only used in the case of major deformations. This is conceivable with tumors that develop in the hip and need to be removed by wide excision. Individual models are also being considered in case of revision, when an already existing prosthesis needs to be replaced. However, they are still not common in primary care.

COMPAMED.de: What processes are being used so far to make the individual hip cups?

Betancur Escobar: The most commonly used process is selective laser melting, where material is being melted in a powder bed until it achieves the desired shape.
Photo: x-ray hip

After the implantation of a hip replacement, the load transmission to the pelvic bone changes and natural bone remodeling with it. This can cause migration or even loosening of the replacement; © panthermedia.net/Sally Williams

COMPAMED.de: In the SFB 599 you are working on a manufacturing process with sheet metal forming, which represents a cost-effective and less elaborate alternative. What role does the so-called universal acetabulum geometry that you developed play?

Betancur Escobar: During the first part of our process, we want to achieve large-scale production of a standardized component with a high-pressure sheet metal forming process. The second part involves the individualization step. Universal geometry is essential for serial production, because it is intended to make the process more economical.

COMPAMED.de: Which data did you use to draw up this geometry?

Betancur Escobar: So far, we have established the principle based on CT data of canine hips, but we want to still expand this database. Then we deduct this principle onto the human being to develop the universal acetabulum geometry of the human hip based on CT data.

COMPAMED.de: How is the crucial step from universal to individual geometry performed?

Betancur Escobar: First a virtual adjustment is being done: the universal acetabulum geometry is placed over the individual acetabulum geometry of the patient and adjusted at the required locations. During production, the universal component is meant to be fitted using a double-acting rubber die forming process.

We are planning the forming process with two rubber matrices that can be varied with an adjustable device. The variation of the matrices is then meant to enable small expansions of the universal component at the required locations.

COMPAMED.de: What does the testing for this process look like?

Betancur Escobar: We are planning biomechanical testing. This includes implant tests and endurance testing of the manufactured component.

At the moment, everything is still simulated. We analyze the geometries we want to produce in finite element simulations (FE) and are hopefully already able to make a statement on the bone formation after implantation. Bone geometry and density are stored in the FE simulations. When we virtually implant a prosthesis and simulate the load transmission, the model provides information on the area and extent of bone formation. This way, we are able to predict in which areas the bone changes are becoming critical and how the geometry needs to be changed.
Photo: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated by Elena O'Meara.