1 28 Medical technology Implants made from PEEK ensure new quality in medicine Whether it is spinal disc problems or certain diseases of the gastrointestinal tract, dental technology or cardiac pacemakers: For many health concerns, implants are the method of choice for restoring a patient s quality of life. As an implant material, polyetheretherketone has established a firm position in a market defined not by large piece counts, but by custom-designed individual solutions. Evonik has met this challenge with VESTAKEEP PEEK. [ text Marc Knebel ] Cranial-Maxillo-Facial Dental Orthopedics Cardiovascular Spine Fields of application for implants made from PEEK polymers
2 Medical technology 29 Patients Who suffer from stomach or intestinal tumors can experience constriction of the digestive tract a condition called stenosis, which prevents food from passing through. In these cases, a stent an elastic, tube-shaped, self-expanding wire mesh that unfolds following placement in the esophagus or the affected section of the intestine reopens the pathway for food. The stent does not cure the patient s disease, but it does alleviate suffering and improves the patient s quality of life. Traditionally produced from polyamide or polyester, stents can now be made from Evonik s polyetheretherketone (PEEK) VESTAKEEP as well. The material boasts not only excellent biocompatibility, which Evonik has verified in numerous tests by external testing institutes, but also high biostability. The latter is a direct result of the aromatic, partially crystalline properties of the PEEK polymer, which not only imparts outstanding resistance to corrosion, hydrolysis, and chemicals but also renders it inert to bodily liquids the material retains its stability, even under aggressive conditions. The stents are one of the new medical applications Evonik has opened up with VESTAKEEP. The fact that EPT Evonik Polymer Technology GmbH, which manufactures PEEK semi-finished products is certified to EN ISO 13485:2003 simplifies marketing of the material. This certification makes it easier for the producers of medical products to qualify as suppliers, and ensures specific medical standards of care and traceability are met. This completes a key hurdle in advancing new applications. In addition to its biostability and biocompatibility (see also elements34, p. 30 ff), the advantages of VESTAKEEP include its outstanding mechanical properties, such as high dimensional stability based on low water intake, high stiffness at low weight, high resistance to heat distortion, and a long-term service temperature of 260 C. This high-performance plastic is also easy to work into versatile shapes an argument not only for the filigree construction of stents but also for use as an extremely thin-walled battery housing in cardiac pacemakers, in dental technology, or as an implant pump for patients who, for instance, suffer from excess fluid in the abdomen as a result of liver disease. 333 Orthopedics: A growing field of applications Synthesis of PEEK. The aromatic, partially crystalline properties of the PEEK polymer make it highly resistant to chemicals and bodily fluids F OH F O Na 2 CO 3 + NaF H O 2 O CO O 2 OH O n Evonik produces VESTAKEEP in a Class clean room, which commenced operation in April 2011
3 30 Medical technology 333 Historical areas of application for high-performance PEEK have been spinal implants, with a growing interest in orthopedic devices and recently accepted applications such as mouth, jaw, and skull implants. For spine surgery, in the case of herniated discs it can be advisable to remove the intervertebral disc and replace it with a prosthesis called a spine cage. Because the cage is supposed to create a bony connection between the neighboring vertebrae, it has a central cavity that can be filled with calcium phosphate or the patient s own bone material. Anatomically shaped, it also has a toothed surface for high primary stability the stability achieved solely through the clamping effect of the implant in the bones. While cage implants are machined from a higher volume, off-the-shelf design, cranial implants have moved to a more custom design for each patient. They are used in patients with extensive skull defects to protect the delicate brain whether it is a congenital malformation or the result of an accident or sickness. To this end, a 3D X-ray scan is used to reconstruct the exact three-dimensional anatomy of the surface of the skull on a computer. The implant is then adapted to this form and produced from a block of PEEK using CNC milling technology. Spine cage made of VESTAKEEP PEEK as a placeholder for the space between the vertebrae, which recreates the natural height of the intervertebral disc segment A protective shield for the skull Compared to the long-established implant materials titanium and cobalt-chromium, PEEK offers a variety of advantages in orthopedics. The plastic scores points in terms of ductility, elongation at break, and fatigue resistance blows or shocks to the implant are effectively absorbed. From a patient perspective they appreciate the extremely low thermal conductivity: Cranial plates made of titanium, which have excellent heat conductivity, can cause pain when temperatures fluctuate dramatically in the environment around them for example, when the patient leaves a wellheated house to go outdoors in winter. VESTAKEEP is also X-ray transparent and therefore invisible in computer tomography and magnetic resonance imaging, which allows easy monitoring of bone growth and structure during the healing process. Titanium and cobalt-chromium, on the other hand, are opaque to X-rays and generate artifacts that make it difficult to analyze the image. There are sometimes cases in which the physician wants to see the implant he may, for example, check As an added service, Evonik also offers customers CAE (computer-aided engineering). These simulations provide information on how much stress an implant receives and how it should be constructed X-ray image of 2-mm-thick VESTAKEEP platelets that were layered in stacks of varying heights. The absorption of the X-ray light increases as the thickness of the layers increases (from bottom to top). At the same time, it is possible to control the absorption by varying the content of the barium sulfate
4 Medical technology 31 In permanent bending tests, VESTAKEEP holds up three times as long as a comparable competing PEEK product VESTAKEEP Ultimate Standard PEEK polymer Bending force [N] , , , ,000 Cycles On the computer, 3D X-ray scans are used to reconstruct the exact three-dimensional anatomy of the skull. This is done for the purpose of manufacturing skull implants, where the implant is adapted to this form. CNC milling technology is used to make the implant from one block of PEEK Visible thanks to barium sulfate the primary stability of a spine cage to make sure it is properly seated, and whether the bones are actually growing through it. This is why tantalum pins are used in PEEK cages to make the implant visible. However, incorporating the pins is a pretty tedious, laborintensive post-processing step because they have to be seated perfectly flush the cavities around the pins could cause problems during cleaning and sterilization. For such cases, Evonik has developed a new VESTAKEEP PEEK variant that contains barium sulfate. The idea comes from diagnostic radiology: Physicians give patients a suspension made of barium sulfate to display the esophagus, stomach and small and large intestines, as well as mobility disorders of these organs, in a CT image. Decades of experience have proven the safety of using barium sulfate. Studies by Evonik have shown that a content of less than ten percent barium sulfate is enough to make the implant visible on an X-ray, without generating the kinds of disruptive artifacts caused by metals. While the quality of the X-ray depends on a number of factors such as the strength of the X-ray radiation and geometry, and the wall thickness of the implant a filling level of six percent barium sulfate is wellsuited to a majority of applications. For example, a spine cage can be portrayed clearly in an X-ray image, and at the same time allows the monitoring of bone growth in the cavity. Osteointegration the healing process in the bones in which the bone cells graft to the implant and implant surface, which normally takes about six to twelve months is essential for a stable fit for an implant in the spinal column. The factors that influence osteointegration are the individual properties of the bone, the construction of the implant, and its surface condition. With regard to the latter, titanium is superior to PEEK, because the metal surface shows improved grafting behavior, and the bone substance anchors faster to the metal. Consequently, the 333 Coating should accelerate bone growth
5 32 Medical technology Interview Material artifacts diminish the image quality Dr. Markus Braun has two jobs: He is attending physician at the Klinikum Westfalen in Dortmund and team doctor at Borussia Dortmund football club. A specialist in sports medicine, Braun knows a great deal about premature signs of exhaustion, fast healing processes, and the question of the right implant material. You ll be standing on the sidelines again next weekend, Dr. Braun: As a fan or a concerned physician? Braun: All of the above! My first responsibility is as a physician, but you automatically become a fan. When you spend that much time with the players, you develop a personal relationship beyond the normal doctor-patient relationship. How much time do you actually spend with the players? Braun: In any given week I spend a couple of hours on the training ground almost every evening. And I go to every game on the weekend. This time we go to Berlin: We ll be traveling from Friday afternoon to Saturday evening. Then, it s right back to training Sunday morning. Editor s note: Dr. Braun also had his work cut out for him this weekend. In the win over Hertha BSC, Lukasz Piszczek suffered a concussion and contusion of the skull when he collided with another player. What materials do you work with? Braun: There is a trend in medicine toward minimally invasive surgery and away from large-scale operations. Minimally invasive surgery makes only small incisions and is controlled by imaging methods such as CT or MRT. In the case of CT, small steel needles are used, and in MRT, amagnetic materials such as carbon fibers or titanium. Many materials are well known to generate artifacts on the screen. How do you handle that? Braun: Artifacts on the image are a disaster! For me, they are a real problem, particularly with the equipment. Braun shows a CT scan from a spinal column cementing procedure. The area around the bone punch cannula is obscured by the white of the artifacts. Without this buildup of artifacts, I could work with greater precision. The material PEEK doesn t leave artifacts. Braun: That is, in fact, an interesting feature of my work. To get that kind of information, I usually have to do my own research or trust the claims of the implant manufacturers. Right now, there is no direct exchange between doctors and material manufacturers. And these are the experts who ultimately have to convert my requirements into concrete technical data. Everyone knows that professional athletes are especially prone to injury. What long-term impact does this kind of permanent stress have on the body? Braun: Signs of exhaustion are often the first to appear. By age 40 or 45, osteoarthritis is not unusual. Top athletes are also frequent candidates for implants. What criteria are crucial to the optimal implant? Braun: Naturally, it would be best to completely forego the use of foreign material. This is not always possible, however. When foreign material has to be used, the material properties are crucial: Biocompatibility is the first consideration. Of course, the material must be as durable as possible to prevent the need for a second operation. Wear resistance and the right amount of elasticity are crucial to stability. If these properties are not what they should be, the implant can cause problems later.
6 Medical technology 33 Dr. Markus Braun, 45, is a trained medical specialist in orthopedics, and has worked as the team doctor for first-division German soccer team Borussia Dortmund since Previously, he spent five years as the team doctor for Rot-Weiß Essen. In his full-time career, Braun has held various positions at clinics in Duisburg, Essen, and Bochum. Since mid-2011, he has worked as attending physician at the newly established sports medicine department at Klinikum Westfalen (Dortmund). His specialty is minimally invasive surgery using imaging methods. You are an expert in healing processes. It s incredible sometimes how fast professional soccer players recover even from serious injuries: What s the magic behind that? Braun: (laughs) No magic! Essentially, there are three simple reasons for that: First, the patients are young and in top condition. This is why the body responds well to treatments and therapies. Second, the players have access to their own physical therapists, who create an optimal environment that allows the natural healing processes to run their course without interference. And third: The players go to rehab far more for the optimal stimulation therapy under the direction of skilled rehab trainers. 333 spinal segment that received the implant stabilizes more quickly and is also able to take stress faster. On the other hand, with an E-modulus of 3.3 Gigapascal the elasticity of PEEK roughly corresponds to that of a bone. This allows an optimal transmission of force between the implant material and the natural bone, which can in turn have a positive effect on bone healing. Both advantages can be combined by coating the PEEK implant, which improves the grafting behavior of bone cells on PEEK. Common coating materials are hydroxylapatite, calcium phosphate, titanium, or a special nano-coating. Depending on the material, they are applied by vacuum plasma spraying, chemical treatment, or physical vapor deposition. Evonik is currently working on a number of formulas in cooperation with various companies that specialize in coating medical products. The current results are quite promising, as tests by an independent accredited testing laboratory show: VESTA- KEEP PEEK substrates, for example, demonstrate exceptionally high tensile and shear strength the coatings adhere extremely well and meet the requirements of the FDA on these points. The potential of PEEK polymers, however, is far from being exhausted. To increase the rigidity of the material even further, Evonik is working on a variant strengthened with carbon fibers (CF). Potential applications are plates and screws in the trauma segment used by surgeons to stabilize broken or fractured bones. The advantage lies in the fact that patients receive a metal-free solution, which helps those with allergic reactions to metal ions. Patients can also undergo MRI examinations later on, which is not possible with metal implants. Beyond this, Evonik has even more development plans for promoting the use of VESTAKEEP PEEK as a metal substitute in the body and more reports will follow as VESTAKEEP accomplishes these technologies. 777 Who decides whether a player is fit enough to play again? Braun: That s an interdisciplinary decision between the physical therapists, rehab trainers, and the doctor. To some extent, the player is responsible. Everyone tries for the fastest possible recovery time. Do you ever just have to say no? Braun: That happens sometimes. The players always want to get back on the field by the next game. At that moment, they re just highly motivated employees. We doctors have to act as a brake if the risk to their health is too great. Marc Knebel is Business Manager of VESTAKEEP Medical in Evonik s High Performance Polymers Business Line ,