Hip joint: sleeve versus endoprosthesis

The project partners develop a sleeve, which is meant to act as a cushion in the hip joint. In this interview with MEDICA-tradefair.com, Dr. Josef Jansen details the advantages of this sleeve, talks about the role of the right synthetic material and how this intervention differs from a prosthetic implant.

Dr. Jansen, what is the objective of the "MioHIP" project?

Dr. Josef Jansen: Our goal is to delay or ultimately maybe even entirely avoid endoprosthesis implantation in patients suffering from hip osteoarthritis. Based on the current state of orthopedic medicine, this pertains to patients with osteoarthritis in the moderate or late stage. We want to achieve this goal by developing a sleeve that can be slipped over the femoral head to balance the abraded, damaged cartilage.

This sleeve is intended to enable an intervention where all structures of the natural hip joint are preserved. In the case of an endoprosthesis implantation, bones and cartilage are recessed and the ligaments that connect the hip socket with the ball are severed. This would benefit patients because quite often not all parts of the joint are damaged by arthritis and need replacement. Severe pain is the actual reason for the replacement surgery. This is a completely new approach, which has yet to be developed and tested.

How did you come up with this idea?

Jansen: We are also developing a similar product intended for use in the knee joint. In this case, it is an elastic cushion designed to delay the prosthesis implant and quickly restore patient mobility again and eliminate pain.

This topic subsequently gave way to the idea of a femoral head sleeve. This is similar to the idea of a synthetic cup inlay, which in the case of an artificial hip joint is made of high molecular weight polyethylene. However, polyethylene is very stiff. Polyurethane is better suited for this type of sleeve.

Why is that?

It is very resistant to abrasion and fatigue and can be easily produced and tested in varying degrees of hardness and properties. Polyurethanes have been used in the medical technology field for many years in a wide range of applications. At this point, we know which type is suited best for what purpose as it pertains to biostability and biocompatibility.

We determined the load magnitude at our test rig, where an industrial robot simulates the walking motion. Materials that exhibit similar elasticity to natural articular cartilage, are very quickly damaged and destroyed by the load force from walking. This is why we had to run tests with many different types of polyurethane to develop the right formulation. Not only does the material have to withstand extremely high loads on the joints, it also needs to maintain its properties in continuous suspension in synovial fluid. In the case of the MioHIP project, the Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT modifies the polymer to create a biomimetic surface and increase the compatibility of the material with the human body.

You mentioned that the intervention intends to keep the hip joint intact. What would this process look like?

Jansen: We are aiming for a minimally invasive procedure to position the sleeve. We want to reach the joint via a small access point while bypassing all ligaments and then uncouple it. That means you extend it a bit to enlarge the space between the acetabulum and femoral head. The sleeve could then be wrapped around the head and closed like a zipper before you rejoin the joint again. This will minimize the risks for patients and reduce the recovery time.

Alongside the development of the sleeve, our medical cooperation partner, the Department of Orthopedic Surgery at the RWTH Aachen University, will develop and test the procedure. We are entering uncharted territory.