MATERIALS
Time to revisit former designs
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Image: iStockphoto |
Medical device engineers are increasingly turning to implantable grade polyetheretherketone (PEEK) based materials to aid their development of innovative long and short term medical devices. This expansion in choice means that designers can revisit designs intended for traditional materials such as titanium and ceramics and also create new device platforms, surgical approaches and techniques. These can be realised completely with polymers or in combination with traditional biomaterials. The versatility of implantable grade PEEK enables its properties to be tailored through the addition of compounds to yield application specific benefits such as increased strength and load bearing, improved wear properties or specific radiopacity.
Advances for the spine
Spinal fusion was an early adopter of PEEK material. The polymer’s compatibility with radiograph and magnetic resonance imaging combined with a reduced modulus offers advantages over titanium, which creates imaging artefacts and is stiffer. In addition, limitations with autograft harvest and concerns with allograft are avoided with the assured and traceable supply of implantable grade PEEK. As suggested in the literature, PEEK may also offer lower complication rates compared with autograft (range of 6.24–12.26% for PEEK versus 16–64.14% for autograft).1
Evaluation of a retrospective review (a follow up average of 18 months) of the first 15 consecutive cases of single level anterior cervical interbody fusion using the Solis cage (Figure 1) (Stryker UK Ltd, Newbury, UK, www.stryker.co.uk) fabricated from PEEK for cervical spondylotic radiculopathy or myelopathy showed a 93.33% fusion rate at six months. The study authors concluded that the high fusion rate, low subsidence, stability provided by the cage and facilitation of radiological assessment are the result of the physical properties of PEEK as well as the design of the cage.2
Another study compared changes over time in cervical foraminal height after tricortical iliac graft or PEEK cage placement following anterior disectomy. Thirty patients underwent anterior cervical microdiscectomy and free bone graft insertion at 46 levels via the Smith–Robinson spinal fusion technique (the intervertebral disc is removed and a bone plug consisting of a slice of the iliac crest is inserted). Another 35 patients underwent the insertion of PEEK intervertebral cages at 41 levels. At the 18 month follow up examination, there were no differences between the groups with regard to clinical recovery, fusion status and Cobb angle. In both groups, the foraminal height increased sufficiently and the nerve root was decompressed postoperatively, thus supporting the authors’ conclusion that PEEK cages may provide sufficient preservation of foraminal height even 1.5 years after the procedure.3
A study compared PEEK cages with femoral cortical bone allograft as a single piece interbody space in transforaminal lumbar interbody fusion (TLIF). It found both to be highly effective in promoting interbody fusion, maintaining postoperative disc space height and achieving desirable clinical outcomes in patients who underwent TLIF with pedicle screw fixation. The advantages of PEEK cages included a lower incidence of subsidence and their radiolucency, which permits easier visualisation of bone growth. This retrospective study evaluated 39 patients following single level TLIF in whom either a PEEK cage (18 patients) or femoral cortical bone allograft (FCA) (21 patients) was placed as an interbody spacer. Radiographically documented fusion occurred in all patients in the PEEK group and 95.2% of those in the FCA group. Pseudoarthrosis developed in one patient in the FCA group and this patient underwent additional surgery. In both groups, the mean lordotic angle changed by less than 2.20 degrees during the postoperative period and the mean postoperative Oswestry disability index (ODI) score was more than 40 points lower than the mean preoperative score. There was no significant difference between the two groups in mean change in lordotic angle (p = 0.415) and mean change in ODI score.4
Advances in joint replacement
The potential for using implantable PEEK materials in bearing applications has been aggressively investigated recently through a number of multidirectional pin-on-plate screening studies. Scholes and Unsworth determined that PEEK and reinforced PEEK materials can provide lower wear in self mating wear couples and against hard counterfaces compared with ultra high molecular weight polyethylene against cobalt chromium molybdenum (UHMWPE/ CoCrMo) (Figure 1).5
Extensively evaluated under unidirectional testing, coupled motion and cross shear, the NUBAC nucleus arthroplasty device by Pioneer Surgical Technology (Marquette, Michigan, USA, www.pioneersurgical.com) has a PEEK-on-PEEK ball in socket design. These tests have shown that PEEK wear performance is largely insensitive to the direction of motion and more recent work has shown that wear performance is not effected by excessive gamma radiation treatment followed by prolonged ageing in oxygen. The use of PEEK-on-PEEK wear couples has also recently been extended to the cervical spine area where the radiolucent properties of PEEK provide substantial benefits in postimplantation imaging.6-8
A carbon fibre reinforced (CFR) PEEK-on-ceramic wear couple has been employed in a unique design of a flexible anatomically shaped acetabular cup, which utilises the mechanical strength of PEEK and may provide greater stress transfer to the bone. Preclinical testing has shown that the wear couple has a wear rate of approximately 1 mm3/million cycles when bearing against a 54 mm diameter alumina head and can be fixed to bone via porous titanium and hydroxyapatite coatings on the CFR PEEK surface.9
These and other studies offer encouragement that implantable grade PEEK can offer good tribological potential for the future.
Advances in trauma applications
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Figure 1: Solis cervical fusion cage used for spinal fusion applications (Image courtesy of Stryker). |
In 2007, medical device manufacturer icotec AG (Altstätten, Switzerland, www.icotec.ch) used continuous CFR PEEK to develop its heat moulded nonmetal osteosynthesis fixator plate to stabilise long bone fractures. Its aim was to develop a system with superior imaging properties that also offered stability for painless functioning, minimal elastic movement and minimal interference to blood flow supply. The resulting “Snakeplate” design used angular stable screws and was staggered to maximise stability while minimising contact between the plate and the bone, thus preserving the periostal blood flow. This type of innovation is an example of the flexibility afforded by the addition of PEEK into the armamentarium.
Advances in craniomaxillofacial applications
Titanium implants are used for the majority of approximately 80000 craniomaxillofacial (CMF) procedures performed annually in the United States. Limitations associated with implanting titanium during cosmetic and functional reconstruction of large and complex calvarial defects include material stiffness relative to bone, with possible refracture as a result of stress shielding, and uncomfortable levels of heat transfer; weight; feel and colour perception through the skin; interference with concurrent radiotherapy treatment and modern imaging technologies such as MRI; and concern over metal ion release.
Traditional CMF polymers such as polyethylene are somewhat pliable and do provide some shape memory, but can lack the high strengths that some CMF applications demand. Other alternative materials such as silicone rubber, polytetrafluoroethylene, tantalum mesh, vitallium mesh and poly(methyl methacrylate) have been considered and it would seem that the properties of PEEK merit its inclusion in this list. Indeed, the first studies are being published such as a complex orbito-fronto-temporal reconstruction using a computer designed patient specific PEEK implant. This study suggested that computer designed alloplastic implants may be a future alternative to titanium and may enhance cosmetic results, surgical success and patient outcomes.10
More recently, it was suggested that PEEK may replace bone during oral and maxillofacial reconstructive surgery and that the biocompatibility, individual shape and the possibility of compounding bioinert polymer powder with osteoconductive and bioactive materials may benefit bone formation in vivo.11
Extending the properties of PEEK
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Figure 2: Total wear factor for PEEK-OPTIMA reinforced compound against various counterfaces.
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Research into implantable PEEK has gained momentum and wider studies with impact across a range of applications have been reported such as cell interaction, coatings, surface modification and new forms, including porous and fibres. A 2007 evaluation of the in vitro response of human osteoblasts to PEEK substrates with that of commercially pure titanium found that, in general, human osteoblast responses to implantable grade PEEK were comparable with titanium. As with other biomaterials, the surface finish conferred by industrial processes such as injection moulding or machining subtly influenced cell behaviour.12
The versatility of PEEK to allow potential surface modifications and coatings associated with medical devices has been highlighted in several studies. One of these demonstrated that PEEK surfaces could be further influenced by chemical surface treatments such as low temperature plasma. It found that the effects on the osteoblast and fibroblast proliferation and differentiation were comparable with tissue culture plastic. In addition, the presence of cells could be controlled to micropatterned areas that had been exposed to the plasma treatment.13
Another study observed that PEEK implants placed into the femurs of four canines could have increased bone contact over an eight week period if they also had a titanium coating. This small study may suggest that the combined benefits of two totally different materials may be advantageous for some osseointegrative applications.14
Options beyond metal and ceramics
Interest in implantable grade PEEK has expanded across an array of device applications since its initial introduction in 1999 and this pipeline is reinforced by multiple supporting in vitro, in vivo and clinical studies. The adoption of implantable grade PEEK fills a natural vacancy in the biomaterial toolbox and widens the choice hitherto offered by pure metals and ceramics.
References
1. S.G. Capps, “PEEK Cages and Spacers in Cervical Spine Fusion Applications,” Spinal News International, 4, (September 2007).
2. A. Kulkarni, H. Hee and H. Wong, “Solis Cage (PEEK) for Anterior Cervical Fusion: Preliminary Radiological Results with Emphasis on Fusion and Subsidence,” The Spine Journal, 7, 2, 205–209 (2007).
3. S.E. Celik, A. Kara and S. Celik, “A Comparison of Changes Over Time in Cervical Foraminal Height After Tricortical Iliac Graft or Polyetheretherketone Cage Placement Following Anterior Discectomy,” J. of Neurosurgery, 6, 1, 10–16 (2007).
4. A.R. Cutler et al., “Comparison of Polyetheretherketone Cages with Femoral Cortical Bone Allograft As a Single Piece Interbody Spacer in Transforaminal Lumbar Interbody Fusion,” J. of Neurosurgery, 5, 6, 534–539 (2006).
5. S.C. Scholes and A. Unsworth, “The Wear Properties of CFR-PEEK-OPTIMA Articulating Against Ceramic Assessed On A Multidirecitonal Pin-On-Plate Machine,” Proc. Inst. Mech. Eng., 221, 3, 281–289 (2007).
6. Q.B. Bao et al., “NUBAC Intradiscal Arthroplasty: Preclinical Studies and Preliminary Safety and Efficacy Evaluations,” Spinal Arthroplasty Society Journal, 1, 1, 36–45 (2007).
7. M.A. Wimmer et al., “The Effect of Accelerated Ageing on the Wear of PEEK for Use in Disc Arthroplasty,” Orthopaedic Research Society (2008).
8. T. Brown et al., “A Comprehensive Wear Assessment of Self-Mating PEEK-OPTIMA for Disc Arthroplasty Applications,” World Biomaterials Congress, Amsterdam, The Netherlands, 28 May to 1 June 2008.
9. A.M.H Latif et al., “Pre-Clinical Studies to Validate the MITCH PCR Cup: A Flexible and Anatomically Shaped Acetabular Component with Novel Bearing Characteristics,” J. of Materials Science, Materials in Medicine, 19, 1729–1736 (2008).
10. P. Scolozzi et al., “Complex Orbito-Fronto-Temporal Reconstruction Using Computer-Designed PEEK Implant,” J. of Craniofacial Surgury, 18, 1, 224–228 (2007).
11. C. von Wilmowsky et al., “Effects of Bioactive Glass and Beta-TCP Containing Three Dimensional Laser Sintered PEEK Composites on Osteoblasts In Vitro,” J. Biomed Mater Res A. in electronic copy (2008).
12. K.B. Sagomonyants et al., “In Vitro Response of Human Osteoblasts to PEEK Substrates Compared to CP Ti,” Biomaterials, 29, 11, 1563–1572 (2007).
13. D. Briem et al., “Response of Primary Fibroblasts and Osteoblasts to Plasma Treated Polyetheretherketone (PEEK) Surfaces,” J. of Materials Science, Materials in Medicine, 16, 7, 671–677 (2005).
14. S.D Cook and A.M. Rust-Davicki, “Preliminary Evaluation of Titanium Coated PEEK Dental Implants,” J. of Oral Implantology, 21, 3, 176–181 (1995).
Marcus Jarman-Smith, PhD, is a Senior Project Manager at Invibio Ltd, Technology Centre, Hillhouse International, Thornton Cleveleys, FY5 4QD, UK, tel. +44 1253 898 000, e-mail: mjarman-smith@invibio.com, www.invibio.com
