The regenerative ability of bones is limited. If they are too badly damaged, e.g. by a tumor or an accident, they have to be replaced by implants. At present surgeons mainly use permanent titanium implants. To improve their ingrowth behavior and compatibility, research scientists at the Fraunhofer ILT have developed a method for the manufacture of porous titanium-alloy implants.
The implants are made by selective laser melting (SLM), an additive laser manufacturing process which was developed back in the early 1990s by the research scientists in Aachen and which has been tailored more specifically to the needs of the medical sector. At the St. Johannes Hospital in Duisburg, Germany, the first hip prosthesis made using SLM was successfully implanted in a patient in 2008. Because of an extreme deformation of the hip, the 35-year-old woman needed a customized implant, which it was possible to produce thanks to the innovative technique. The project partner is now producing 30-40 implants annually using the SLM process.
Often it is not necessary and can even be detrimental for implants to retain their original shape in the patient’s body. Particularly in pediatric surgery, permanent implants are a problem because they soon become too small in relation to the child’s growing skeleton and have to be removed or replaced in a further operation. A need therefore exists in medicine for biodegradable implants with osteo-inductive properties: implants that degrade at the same rate as bone growth which at the same time they specifically stimulate. After a time they should be completely replaced by native bone tissue and thus accelerate the healing process. What’s more, the implant should be individually adapted to the damaged bone structure and exhibit best-possible compatibility in the body. A great advantage of using degradable implants is that in the case of disease-associated bone damage the patient only has to undergo one surgery.
Ultra-fine channels enhance degradability
The research scientists at the Fraunhofer ILT took up this challenge in cooperation with Aachen University Hospital and RWTH Aachen University’s Institute of Mineral Engineering and Department of Dental Materials and Biomaterials Research. In the RESOBONE project they developed a process for the manufacture of biodegradable implants on the basis of their experience with permanent implants produced by means of the SLM method. This production process makes it possible to individually adapt the macrostructure of biodegradable implants to the patient’s specific bone damage prior to surgery. The precisely defined microstructure of the implant ultimately determines its porosity, which is crucial for the implant to be resorbed by the body over time. For this to happen the implant needs to have channels running through it so that blood and cells can completely penetrate. “Creating this sponge-like lattice structure was the biggest challenge of process development,” says Simon Höges, project manager at the Fraunhofer ILT. “Previous types of implant could only be penetrated by native cells to a limited extent. The new technique enables us to generate porous channels with a diameter of 500 to 1000 µm to an accuracy of 100 µm. The material used to manufacture the implants is also of decisive importance in this respect.” β-tricalciumphosphate (β-TCP) is a suitable material for making biodegradable implants because as a natural constituent of human bone it encourages optimal ingrowth behavior in the body. Owing to its chemical structure, however, β-TCP cannot be directly processed by melting. It is therefore only limitedly suitable for processing by means of the SLM method. “We therefore had to find an additive that can be mixed with the pulverized β-TCP to combine its advantages with better meltability. We ultimately found this material in polylactide (PLA), a degradable polymer,” explains Höges. PLA melts at a temperature below 200 °C and is excellent for use in SLM processing. This composite material made it possible to finally produce the biodegradable implant. It is based on β-TCP, but the PLA additive makes the material easier to shape.
Layer by layer to a personal implant
The first step in the manufacture of the degradable implant involves making a CT scan image of the existing bone structure. The contours of the implant are then defined using a virtual technique on the basis of the recorded image data. A software model is then used to integrate the desired pore structure in the virtual implant. The result is a precise template for the microstructure and macrostructure of the implant to be produced. The actual production process can now begin: A laser beam melts a thin layer of the powdered material by local application of heat, following the structural outlines and contours of the virtual model. Then a further layer of powder is added and fused to the first layer with micrometer accuracy. By repeating this procedure, the tailor-made implant is built up layer by layer from the powder material. None of the material is wasted, because any excess powder is recycled and reused. The result is a biodegradable implant with a porous structure produced in a single integrated process.
On the basis of the knowledge and experience gained in this project, SLM is now available as a reproducible shaping process with considerable potential as a means of manufacturing tailor-made biodegradable implants with a defined pore structure. The process is suitable both for the one-off production of personalized implants and for use in low-volume batch production. The range of applications is wide: SLM can be used to make non-load-bearing bone implants as well as biomedical products such as biodegradable stents, which after performing their task are completely resorbed by the body. The technique can also be used in oral surgery, to reconstruct defects in the jaw bone. In cooperation with the partners from the Department of Dental Materials and Biomaterials Research, Höges and his team are currently testing further materials for the production of implants in order to optimize their ingrowth behavior.
Simon Höges will be presenting the conclusive results from the RESOBONE project at a seminar to be held in Aachen University Hospital on July 14, 2010. Lectures on the subject by project partners are also planned. Further information about this event is available at www.ilt.fraunhofer.de in the Events / Trade Fairs section.
Axel Bauer | idw
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