The ideal bone screw: naturally "organic", but nevertheless strong

There is hardly a medical field that does not use an implant of one kind or another, from dental to ENT medicine to orthopedics and, of course, accident surgery. Easier said than done since the challenge in bone surgery is to make an implant strong enough to stabilize fractures, for instance, but which will dissolve over time, eliminating the need for its surgical removal. Magnesium implants do just that. The latest innovative implant developments will be showcased at the trade fair MEDICA, world forum for medicine (to take place from November 16 - 19, 2011 in Düsseldorf, Germany) and at the parallel held trade fair for the medical supplier industry COMPAMED - High tech solutions for medical technology (November 16 - 18, 2011).

Implants such as those successfully used for decades in orthopedic and accident surgery should, in general, meet multiple criteria such as:

• must have a surface that will readily grow into one with the bone,

• must be biocompatible and not release any toxic substances,

• must be bioresorbable, being replaced with the body's own tissue,

• must be able to withstand sterilization without losing any positive properties

• must be porous enough that cells and blood vessels can grow into it,

• and must be mechanically stable so that patients can use their limbs as soon as possible after the surgery.

Still a great challenge

"Despite all efforts, there is still no implant or bone replacement material that completely fulfils all these criteria," says Dr. Sascha Heinemann of the Max Bergmann Center of Biomaterials at the Technical University of Dresden.

Decisive for implant quality is not only the correct choice of material but also an optimal surface structure. For instance, the biocompatibility is clearly determined by the roughness of the surface. This also applies for dental implants, whose roughness in the nanometre range (billionths of a meter) determines its protein binding capacity and consequently how fast it will grow into the jaw bone. In light of this, Alicona Imaging GmbH (exhibitor at COMPAMED) has developed an innovative 3-D surface measuring technology that is excellently suited for the surface characterization of implants. "Cost-efficient measuring means analyzing all the relevant parameters with a single system, and that's exactly what our InfiniteFocus does," explains Dr. Stefan Scherer, CEO of Alicona Imaging GmbH. It combines the options of a roughness and a shape-measuring device, thus providing all the functionalities of an optical profilometer and a micro-coordinate measuring machine. Even with complex shapes and different material properties, the user obtains a resolution of up to 10 nanometres and that even over large vertical and lateral scanning areas. For complete measurement of the shape, it has an optional rotation unit that turns the sample 360°. "We know of no comparable optical measuring system that provides such substantiated information on roughness, even over wide measuring ranges," confirms Dr. Frank Rupp, Head of the "Interface analysis of medical materials" team at the Tübingen Polyclinic.

Implants that dissolve in the body within a defined period of time represent a particular challenge. For a long time now, implants such as these have been developed and used to fix small bone fragments in place, for instance. The self-dissolving polymers used for this include: polylactide (PLA), polyglycolid (PGA), polydioxanon (PDS), poly hydroxy butyric acid (PHB) and polyorthoester (POE). Pins made of PDS (Ethipin) and PGA (Biofix), for instance, have been used clinically since 1985. However, both materials degrade very quickly and therefore rapidly lose their strength. The degradation times were increased significantly by using the polymer PLA.

Cardiovascular implants - a market worth billions

The market for cardiovascular implants, worth around $ 15 billion worldwide, is considered extremely attractive. In addition, the importance of coating technologies for catheters and stents is increasing because, amongst other reasons, they make innovative and individualized medical therapies possible. "Coatings increase functionality, lifetime and cost efficiency, improve the guidance and positioning of parts, and they facilitate handling and reduce the patient's risk of injury," explains Prof. Hans-Wilhelm Engels, Head of the Innovation Community Council and Head of Research in the area of paints, adhesives and specialties at Bayer MaterialScience (COMPAMED exhibitor). The coating materials that can be used include hydrophile polymers and polymers with good gliding properties such as polyvinyl pyrrolidone (PVP), polyethylene glycol / poly ethylene oxide (PEG/PEO), polyvinyl alcohol (PVA) as well as hyaluronic acid. As these compounds are often water soluble they must be stabilized through cross-linking. Just recently Bayer introduced Baymedix CD 500, a new coating into which the agents can be integrated. These stable coatings allow the programmable release of a number of different agents, from small molecules all the way to protein therapeutics. The biocompatibility of these systems was confirmed in in-vitro and in-vivo tests. "We are also already working on optimal systems for biodegradable stents," says Prof. Engels.

Maximum stability through "bone welding"

A procedure in bone surgery has been creating a furor for several years now as the idea originated in the watch manufacturing industry and was developed further in the furniture industry. The procedure in question is the WoodWelding technology of the Swiss company WoodWelding. In this procedure, also known as bone welding, synthetic implants (lactic acid polymers) are fixed in the bone by means of ultrasound.

The first company licensed to use the method was Germany's KLS-Martin SA (exhibitor at MEDICA). The first application of the patented technology has been approved in the U.S. and Europe for face and skull surgery since the end of 2005. The technology can also be used in spinal column surgery as well as in dental and sports medicine. With the SonicWeld Rx procedure of KLS Martin Group, the pin is made to vibrate by precisely defined ultrasound frequencies. When it is placed on a pre-drilled hole, the pin liquefies and penetrates hollow spaces that conventional screws cannot reach. The three-dimensional anchoring ensures maximum stability. Bone welding works in cancellous and cortical bone. The connection is stable within seconds. Nevertheless, the bone pin self-degrades completely. A second surgical intervention is not necessary. And: The implantation time is significantly shorter than for conventional resorbable implants. Moreover, there is no risk of implant fractures.

Resorbable metal implants: Magnesium and iron make it possible

So-called magnesium-based implants are almost the "latest rage" in bioresorbable implants. What's so special? Over time they dissolve through corrosion, but they are metal implants and, being different from other bioresorbable implants made of polymers, are very stable and therefore suitable for supporting functions. As implants made of Mg alloys have similar elasticity modules as bone tissue, there is no bone resorption at the implantation location. Attempts were made to use magnesium as implant material as early as the 19th century, for instance for blood vessel ligatures. However, no commercial magnesium-based or magnesium alloy products are available to date. Nevertheless, the positive developments raise hopes that the special properties of the light metal would benefit the clinical field, says Hanover implant researcher Professor Frank Witte, whose research won the Ministry of Education and Research prize for innovation in medical technology in 2009.

Currently, the research in metal-based bone replacement materials is primarily focused on magnesium-based materials, which are considered promising. But to date they do not possess the necessary mechanical properties and degrade too quickly. Researchers at LMU Munich, the Fraunhofer Institute for Manufacturing Technology (exhibitor at COMPAMED) and InnoTERE GmbH Dresden have therefore developed a degradable iron-based alloy which in toxicological studies proved to be well-tolerated. However, it will be several years before such iron-based implants will be commercially available. What Sirris, the Belgian center of competence for the technology industry, has planned is also still a thing of the future: resorbable implants from a 3-D printer in conjunction with the quick-change artists amongst the cells: stem cells. It appears that the old adage "nature is the best engineer" still holds true.

The current developments in implant medicine are also the topic at the MEDICA Congress held in the Congress Center Düsseldorf (CCD South) and will be discussed in connection with various disease patterns, for example at the event "Aortic Valvular Disease - Surgery or Intervention" (November 18, 2:30 pm, no. 323) headed by Prof. Dr. Jan Gummert or in connection with aspects of tissue engineering at the event "Stem Cell and Tissue Therapy between Basic Research and Clinical Application" (November 19, 10:00 am, no. 416) headed by Prof. Dr. Hans M. Klein.

Information about exhibitors presenting implant technologies at MEDICA and COMPAMED 2011 is available online at http://www.medica-tradefair.com and http://www.compamed-tradefair.com

Authors: Dr. Thomas Kron and Klaus Jopp