Feature Article

Open-cell metal foams can be fabricated in various porosities.

Stress shielding
Bone defects are a major problem in medicine. Lesions have to heal in a stable manner until the bone produced naturally by the body is able to regain its mechanical function. To achieve rapid mobilisation of the patient, medical treatment requires the use of implants that can carry the full mechanical load postoperatively. Defects can be treated either with autogenic bone or with solid bone-replacement material. Autogenic bone typically is taken from the pelvic area and, thus, requires additional invasive procedures. These operations always carry a risk and are painful for patients. Current implants, on the other hand, are very stiff. Implant stiffness greatly exceeds that of the surrounding bone, and it assumes the load, which has an effect on that part of the body. The bone-healing process itself is stimulated by a local biomechanical stress field. Thus, repeated stress results in bone thickening and in the formation of a new bone matrix. Massive implants with less flexibility interfere with the natural stress field, since they carry nearly the full load, resulting in so-called stress shielding. The consequent absence of biomechanical stimulation may lead to bone degradation and premature loosening.

To overcome stress-shielding related problems, cellular metals with low stiffness have been developed. The use of open-cell foam as a bone implant material has its roots in the 1970s. Researchers thought a material of this type would promote integration of the bone and blood vessels and overcome the stiffness mismatch between the bone and implant. Because of their porous structure, cellular metallic materials achieve a stiffness that is within the range of cancellous bone. Situated in the vicinity of joints, cancellous bone has a highly porous structure at its end and is frequently subject to fractures, especially among people suffering from osteoporosis. Open-cell metals enable integration of bone cells and blood vessels, which are absolutely necessary for the metabolism and, thus, for bone growth. Moreover, the strength of these materials is comparable to bone. Because of these desirable and complementary properties, cellular metallic materials are attracting significant interest among medical researchers.

Open-cell metal foams
Open-cell porous metals are under development at the Fraunhofer Institute Center IZD in Dresden, Germany, in a co-
operative effort between the Fraunhofer IFAM and IKTS institutes. Research is focused on implants made of steel or titanium. These materials are produced by means of a powder metallurgical replication technology that involves impregnating reticulated polyurethane foams with a metal powder-binder suspension. In the next step, the organic material is removed thermally, and the powder skeleton is sintered. Thus, on the one hand, structures with high open-cell porosity can be manufactured. On the other hand, the structure itself may be influenced by the selection of a suitable template. In principle, this enables all sinterable materials to be structured in an open-cell construction.

The process allows the production of homogeneous foam-like structures with approximate porosities between 75 and 95%. The mechanical properties of the bone-replacement material may be specially adapted to the corresponding values of the bone through a targeted manipulation of the material’s density and structure. This may be done, for instance, by setting up the desired coating thickness or by selecting a particular basic material. Specifically, this technology makes possible an individual adaptation to the environment of the surrounding bone material. The material can be adjusted to fit into either an adolescent bone or an older, osteoporotic bone.

This innovative material with its fascinating properties has been analysed in interdisciplinary projects; the Fraunhofer institutes co-operate with their industrial partners and clinics to explore materials as well as biological and medical aspects of engineering up to their application in an animal model.

Permanent implants
Initially, the new implant material was conceived as a permanent implant. In other words, after the operation, the implant remains in the human body forever. Titanium is the material of choice for these applications. This is mainly because of its low density and extraordinarily strong biocompatibility, coupled with excellent corrosion resistance. For nonabsorbable bone replacement materials, this means contact with the bone that is free of connective tissue and inflammation. Titanium’s osteoconductivity also makes the material ideal for endoprosthestic devices. Among metallic replacement materials, titanium and its alloy Ti6Al4V enjoy the highest level of acceptance on the market. Consequently, it seems reasonable to design a bone replacement material based on titanium. This is the subject of the joint project TiFoam funded by the German Federal Ministry for Science (BMWi).

Figure 1: The implant material’s compression strength (left) and stiffness (right) are adjustable and can be matched to the bone properties.

The fundamental properties of this material are mechanical strength and stiffness. These properties are mainly governed by two aspects: structural properties, such as porosity and pore size, and the alloy’s impurities, which influence the mechanical properties to a large extent. Titanium presents a challenge in this regard, because of its extraordinary affinity with oxygen, carbon and nitrogen. When these elements appear as oxides, carbides or nitrides, even at low volumes, they render the material brittle. Through process optimisation, however, open-cell porous titanium foam has been produced with impurity levels approaching a titanium Grade 5 classification. Thus, the compression strength of open-cell titanium can be adjusted to the 10 to 55 MPa range by altering the porosity of the implants (Figure 1). These values match the strength of cancellous vertebral bone, which is typically 5 to 10 MPa. Similarly, the stiffness of the implants matches bone stiffness.

From a medical perspective, the focus of researchers is on the applicability of the designed material. Tests have been conducted at the university hospital of the Technical University of Dresden. The new implant material was incorporated as a replacement for vertebral bodies in sheep. The first tests with permanent implants demonstrated outstanding in-growth of bone cells into the material. Even larger implants were fully integrated after six months.

Absorbable implants
The development of biodegradable materials is also an area of great interest. At the start of the healing process, the implant ideally would assume the full stabilisation load. As the bone regenerates, the implant materials would resorb and transfer the load-bearing function to the bone. In an ideal scenario, progressive osteointegration (in-growth of the bone) would be coupled with resorption of the implant to achieve optimal adaptation to the corresponding strength state at any time (Figure 2). The implant should carry the load for a period of six months and, ideally, the healing process should be completed after 18 to 24 months.

Figure 2: Degradable implants carry the full load right after implantation. As the healing process progresses and the bone recovers, the implant gradually disappears.

Magnesium has been discussed as an alternative bone material for some time. This material is highly biocompatible and has been successfully tested in clinical studies of degradable cardiovascular stents. However, its rapid corrosion rate makes its use as a bone implant material problematic, because it degrades before the newly established bone is able to carry the load. An alternative is to use iron as a biomaterial. Experiments have been conducted using degradable iron implants in cardiovascular surgery. The implanted stents were completely absorbed and did not produce inflammatory reactions. Implants made of degradable iron have received far less consideration in bone surgery. Nevertheless, the concept is interesting, since iron corrodes considerably slower than magnesium.

Because of the lower degradation rates in physiological media, the biological performance of this material is more comparable to a permanent implant. These properties have been demonstrated by in vivo studies. Furthermore, iron-based alloys used as a matrix material offer high mechanical strength. At Fraunhofer IFAM Dresden, open-cell porous metal foams are developed on a base of unalloyed and low-alloyed steel. Even the influence of low-volume alloy additives is monitored. Generally, only alloy elements that achieve a rate of biocompatibility similar to what is naturally produced in the body are considered. For instance, even a slight addition of phosphor—on the order of <1%—leads to a significant increase in the strength of metal foams. The degradation rate of the foam is the first point of interest. In vitro investigations of this material carried out by partner company InnoTERE GmbH showed that degradation occurs within about two years. At present, the absorption of this implant material is being verified in an in vivo model at the University Hospital of Ludwig-Maximilian-University of Munich. Here, the implant study revealed residual implant material even after an implantation time of 12 months, showing that the in vitro tests only give indications for the degradation time in vivo. Therefore, further work has to be done to accelerate the degradation rate of iron alloys. On a positive note, no inflammatory response was detected in the surrounding bone tissue, giving iron the seal of good biocompatibility.

Prospects
Previous projects have proved that open-cell porous metal foams are suitable for use as synthetic bone replacements, both in permanent and degradable implants. The next important step is the commercialisation of this technology. The development of degradable iron-based alloys and their approval for medical use, however, would appear to be several years away. The first commercially available cellular implant materials are likely to be permanent implants. InnoTERE has announced plans to manufacture such products in the near future.

Peter Quadbeck
is Team Manager at Fraunhofer Institute Manufacturing and Advanced Materials (IFAM), Dresden Branch Lab, Winterbergstraße 28, 01277 Dresden, Germany
tel. +49 3512 537 372
e-mail: peter.quadbeck@ifam-dd.fraunhofer.de
www.ifam-dd.fraunhofer.de
Ralf Hauser
is Scientist at the Dresden Branch Lab of Fraunhofer IFAM
Gisele Standke
is Scientist at Fraunhofer IKTS, Dresden
Jan Heineck
is Senior Physician at the University Hospital of Dresden
Bernd Wegener
is Senior Physician at the University Hospital of Ludwig-Maximilian-University, Munich
Günther Stephani
is Head of Department at Fraunhofer IFAM and
Bernd Kieback
is Director of Institute at the Dresden Branch Lab of Fraunhofer IFAM