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19. June 2023

Induction heating facilitates revision of implants

IW | How can hip and knee implants be removed if they become loose or an infection occurs– without damaging the surrounding bone? This question is being investigated by the Institute of Materials Science (IW) as part of the collaborative research center “SIIRI”. In search of possible solutions, a process for inductive heating of metallic implants is being simulated and tested.

Hip and knee arthroplasties – i.e., artificial joint replacements, as they are used in therapy for osteoarthritis – have been performed for decades and represent proven procedures. Nevertheless, the demanding conditions under which hip and knee arthroplasties are used repeatedly lead to complications that can cause the implants to fail. These include stress shielding – a loss of bone density in the immediate vicinity of the implant – and bacterial infections of the surrounding tissue.

In these cases, the load-bearing capacity of the implants is often significantly reduced. This can cause patients to experience high levels of pain. As a result, prostheses often have to be removed to prepare for the insertion of a replacement prosthesis.

Mechanical implant revision can damage surrounding tissue

These revision surgeries account for roughly10 percent of the total hip and knee replacement surgeries performed in Germany each year. Nevertheless, revision surgeries still play a minor role in the development of prostheses, and explantations are usually performed purely mechanically. In this procedure, the surgeon pulls the partially exposed implant out of its seat using mechanical force, such as a draw hammer, and also uses chisels and burs to chip and grind away the surrounding bone cement.

This procedure carries a considerable risk of damaging the surrounding bone  by erosion or fracture. Increased bone loss is associated with larger revision prostheses and more costly surgeries as well as longer reconvalescence times. The goal is therefore to preserve as much healthy bone tissue as possible.

Research goal: gentler procedure for removing implants

The aim of project A08 in the collaborative research center SIIRI (Safety-Integrated and Infection-Reactive Implants) is to develop a gentler explantation procedure using inductive heating processes. To this end, the IW is transferring its experience in the field of inductive heat treatment of metals to hip and knee endoprostheses. IW researchers are supported by the biomechanical and medical expertise of researchers from the Laboratory of Biomechanics and Biomaterials (LBB) at Hannover Medical School.

Induction heating involves passing a high-frequency alternating current through a coil. This creates eddy currents that heat metallic materials without contact. Induction stoves work according to this principle as well. In the same way, eddy currents can be generated in the implant surface that heat metallic implants withoutdirectly interacting with the surrounding tissue.

Implants detach more easily after targeted induction heating

The implant conducts the generated heat into the bone cement and softens it, which in turn makes it easier to loosen the bond between implant, bone cement and bone. In preliminary tests on cylinders made of Ti-6Al-4V, a titanium alloy frequently used in implant technology, IW researchers have achieved a reduction in the necessary pull-out force of about 50 percent. This would almost completely eliminate the need to chisel out the bone cement in revision operations.

However, since the thermal energy introduced can also cause damage to the surrounding bone, muscle and nerve tissue, it is essential that the heat output is precisely controlled. To ensure this, the IW is developing simulations that can be used to predict the temperature distributions achieved in the implant and bone cement. This simulation model enables a definition of the heating parameters and a design of the required coils for different implant types and installation situations.

In addition to the development of the simulation, IW and LBB are performing validation in several surrogate models. Various heating and pull-out tests are planned for this purpose, the results of which will be used to improve the simulations.

Perspective: Avoid bone damage, accelerate recovery

In the future, simulation should make it possible to predict an adapted heating strategy for endoprostheses in the perioperative environment of revisions. By tailoring the use of this technology to individual patients, it will be possible to avoid bone damage during revision surgery and thus significantly shorten recovery periods in the future.

by Patrick Evers

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Benefits of the research results

  • Novel method for inductive heating of hip and knee implants
  • Easier explantation, reducing tissue damage
  • Necessary pull-out forces were reduced by about 50 percent in preliminary tests
  • IW simulation model enables adapted heating strategies for different implant types
Thermographic image of a sample of Ti-6Al-4V (glowing yellow) in an internal field inductor. (Photo: Evers)
Thermographic image of a sample of Ti-6Al-4V (glowing yellow) in an internal field inductor. (Photo: Evers)
Schematic representation of inductive heating of endoprostheses with bone cement. (Graphic: Evers)
Schematic representation of inductive heating of endoprostheses with bone cement. (Graphic: Evers)
Experimental setup for induction heating of implant materials. (Photo: Evers)
Experimental setup for induction heating of implant materials. (Photo: Evers)
3D-scanned hip implant in the internal field inductor (simulation model). (Graphic: Evers)
3D-scanned hip implant in the internal field inductor (simulation model). (Graphic: Evers)

Contact

Patrick Evers, M.Sc.

+49 (0)511 762-18171
evers@iw.uni-hannover.de
www.iw.uni-hannover.de/en
Funded by the German Research Foundation (DFG) – SFB/TRR-298-SIIRI – project number 426335750.

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