Materials Innovation: Fewer Surgeries with Metal-Ceramic Composite Degradable Implants
Researchers are working on a suture anchor made of metal and ceramic that completely degrades in the body.
No other joint in the human body is as highly mobile as the shoulder. However, it is also very sensitive and prone to injury, with athletes being particularly affected. The most common complaints include tendon rupture, which have to be treated surgically. The surgeon fastens the cracks using suture anchors. Such implants have traditionally been made of titanium or non-degradable polymers—with the disadvantages that either they remain in the body even after healing has occurred, or doctors have to remove them in a second procedure.
To avoid these issues, researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) in Bremen, Germany, have developed load-bearing, biodegradable implants that are completely degraded in the body. In the first step, they have used powder injection molding to manufacture a suture anchor, which is available as a demonstrator. The researchers recently presented this work at the COMPAMED trade fair in Düsseldorf.
Stimulating the Healing Process
“With the implant, severed tendons can be anchored to the bone until they have grown again,” said Dr. Philipp Imgrund, manager of the Medical Technology and Life Sciences business field at IFAM. “Since the function of the fixing element is satisfied after the healing process, it is no longer needed in the body. If implants or prostheses that are as wear-resistant as possible are required—such as in an artificial hip joint—metallic alloys such as titanium will certainly continue to be used. However, for plates, screws, pins and nails that should not remain in the body, there are other requirements.”
IFAM has worked jointly on the DegraLast project with the Fraunhofer Institutes for Laser Technology (ILT), for Biomedical Engineering (IBMT), and for Interfacial Engineering and Biotechnology (IGB) in establishing a materials and technology platform to produce degradable bone implants for use in trauma surgery and orthopedics. These materials are gradually absorbed by the body while new bone tissue is simultaneously formed. Ideally, the degree of degradation is adapted to the bone growth so that the degradation of the implant meshes with the bone formation. For this reason, the scientists are developing materials with specifically adjustable degradation. The challenge: the implants have to be mechanically stable enough to fix the bone in place during the entire healing process. At the same time, they cannot have any allergenic effects or cause inflammation.
The researchers at IFAM are relying on metal-ceramic composites. A metal component based on an iron alloy is being combined with beta-tricalcium phosphate (TCP) as the ceramic component. “Iron alloys corrode slowly and ensure high mechanical strength, while ceramic decomposes quickly, stimulates bone growth and aids the ingrowth of the implant,” said Imgrund in his explanation of the advantages of this material combination.
Manufacturing the Optimum Composition
In order to be able to manufacture the material composite, the researchers turned to powder injection molding. The process offers the ability to produce complex structures cost effectively and in large numbers. Properties such as density and porosity can be controlled selectively—an important factor, since high density and low porosity result in high mechanical strengths. Another advantage is that the materials are available as powders and can be mixed in any proportion prior to processing.
But what proportion is the right one? In laboratory experiments, the researchers found the optimum composition of the materials for the suture anchor. The demonstrator consists of 60% iron and 40% ceramic. “It is important to determine the right amount of ceramics as a function of the powder amount,” said Imgrund. “If the proportion is too high, the material will be brittle. On the other hand, the tricalcium phosphate accelerates the degradation of the implant.”
The researchers have succeeded in doubling the degradation rate from 120 to 240 micrometers per year in the laboratory model. The shoulder anchor would be absorbed by the body within one to two years.
While shaping processes such as powder injection molding are especially suited in large quantities as fixation elements for standard implants, additive manufacturing methods are used to produce individual implants—such as for bone replacement in the skull area—or implants with defined pore structure. The researchers from ILT who are also involved in the project are producing implants made of magnesium alloys through the use of selective laser melting (SLM).
To ensure the safety of the new composite materials from the outset, colleagues from IGB in the DegraLast project are establishing cell-based, in vitro test systems for analysis of the ingrowth behavior in the bone. The scientists at IBMT are in turn working on an in vivo monitoring system that can monitor and document the degradation behavior of the implants in the human body.
For more information, visit www.fraunhofer.de/en.