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27 RESTORATIVE Dental Tribune Middle East & Africa Edition | 5/2017 High performance polymers. Part one In the first of a series of articles, Professor Paul Tipton gives us an introduction to high performance polymers in dentistry. Here he discusses polyether ether ketone (PEEK), a new material for framework fabrication in prosthodontics By Prof Paul Tipton, UK The high-performance polymers (HPPs) are the uppermost class of plastics, possessing better tempera- ture and chemical stability and me- chanical properties than the com- modity plastics, but typically being manufactured in lower volumes and costing more. The family of HPPs that have entered dentistry are called the Polyarleth- erketones (‘PAEKs’), of which there are several members with varying chemical structures. Many of us in the dental industry are inadvertent- ly familiar with the family member called PEEK (Polyetheretherketone), through its use in healing caps, tem- porary abutments and scan bodies. However, the reason for the recent enthusiasm surrounding PAEKs has been their potential for use as a met- al alternative in broader indications such as removable dentures (Figures 1 and 2) and implant borne prosthet- ics (Fig. 3). It is here that the shock absorbing characteristics of the ma- terial could be extremely interesting for immediate loading or long term frameworks (Fig. 4). In this first article of a series of six, the background to these materials will be described. This will be fol- lowed by a series of case studies de- scribing their use in removable and implant-borne prosthetics. The PAEK family PEEK is the most well-known and most widely used PAEK family member. PEEK was invented in the UK in 1978 (ICI – now as Victrex plc) and was selected by aerospace, semi- conductor, automotive and medical industries as a standard material of use in all these sectors. It is typically used as a metal replacement, due to its strength to weight ratio and corro- sion resistance. Other family mem- bers also exist which are variations of the chemistry (eg PEK and PEKK), and these materials can also be filled with pigments or reinforcing agents. In their unaltered, unfilled state the materials are beige in colour. PAEKs in Medical Several of the properties of PEEK that were being exploited in industry (eg strength-to-weight ratio, chemical and wear resistance, radiolucency, and reduced stiffness versus met- als) were naturally intriguing for medical use. The first published PEEK medical research came in the 1980s (Williams et al., 1987) followed by the first implantable grade from Invibio Biomaterial Solutions in the 1990s (Victrex plc./Invibio Ltd, UK). Medical grades have a much tighter specification, and increased quality control than industrial grade materi- als, which is important in the wake of the silicone breast implant scandal (see http://www.nhs.uk/conditions/ breast-implants/pages/pip-intro- duction.aspx). PEEK remained the only medical PAEK for many years. Spine surgeons particularly adopted Invibio’s PEEK, liking the reduced Young’s Modulus (stiffness) of the material and the scatter-free CT and MRI imaging. PEEK has since become the standard alternative to titanium for load bearing spinal cage devices for the spine. Today, manufacturer Invibio claims PEEK has been used in around five million implantable de- vices, spanning some 500 separate US FDA 510k regulatory clearances. In more recent years, additional versions of PEEK and PEKK have ap- peared on the medical marketplace, but have been limited in use. PAEKs in dentistry Short term devices such as tempo- rary healing caps and abutments have been sold direct to dentists through the dental companies for many years. In these situations, ei- ther unfilled PEEK or PEEK with a 10% titanium dioxide pigment filler are typical and have been used in these temporary devices for over a decade. In the case of customised prosthe- ses, the upstream material or shape becomes the ‘device’ and is regulated and cleared for use for a defined set of indications. Here, the PAEKs have appeared as materials for use in in- jection press systems or as discs for computer aided design/manufac- ture (CAD/CAM). Types of PAEKs for prosthetics There are now many brands of PAEK dental devices becoming available for use in prosthetic frameworks. The most common formulations of the PAEKs are: • Unfilled, pure 100% PEEK (eg JU- VORA, Invibio/JUVORA Ltd). This is a beige material. • 80% PEEK with 20% nanoceramic filler (eg. BioHPP, Bredent GmbH). This is a white material. • 80% PEEK with 20% titanium diox- ide filler (eg Dentokeep disc, NT Trad- ing). This is a white material. • 80% PEKK with 20% filler including titanium dioxide (eg Pekkton Ivory, Cendres and Mettaux). This is an off- white material. Typically the particle size of these fillers (circa. 300-500 nanometers for the nanoceramic) is not likely to give significant reinforcing proper- Elastic modulus comparison of different dental materials and natural bone ties to the material, since they are not fibres. Instead the fillers act more as a pigment and alter surface topog- raphy. These levels of 20% filler will make the material stiffness slightly higher, but consequently also slight- ly increase brittleness. It should also be noted that the inclusion of titani- um dioxide mean that these brands - BioHPP; Dentokeep and Pekkton should not be pitched as ‘metal free’ since this could be in breach of Ad- vertising Standards and/or Govern- ing Bodies. The reader should also take note as to the cleared indica- tions for use as the different mate- rials and forms may have varying clearances. To date, PAEKs with these specific 20% fillers only have a limited his- tory of use in dental and actually no prior medical history in any other medical applications. Therefore it is fair to say that the jury is still out as to the effects of adding these levels of these specific fillers to the PAEKs and the author advises the use only of the pure material where there are long-term studies. Methods of framework manufacture There are two methods for labora- tories to manufacture substructure frameworks from PAEKs. These are: (i) injection moulding or (ii) CAD/ CAM. (i) Industrial injection moulding ma- chines process the polymer under very high speed and pressure (eg. 1000’s bar), which are typically two orders of magnitude higher than the typical bench top pressing machines available to the dental laboratory (eg. 10’s bar). This means that small scale injection moulding of PAEK is no mean feat, due to tight process- ing windows and design limitations. Also these re-melting of PAEKs can also increase the risk of unpredicta- ble mechanical and physical proper- ties (eg brittleness, flexibility, colour, warping) if the framework has not cooled and recrystallised correctly. Finally, re-melting of PAEK materi- als can also cause degradation of the polymer (eg generation of phenol) unless very closely controlled using the correct equipment. This polymer degradation can be accentuated by the inclusion of fillers in the mate- rials (such as reinforcing agents or pigments). Therefore, melt process- ing of these materials should only be done by a competent laboratory and using the equipment recommended by the supplier. (ii) The alternative manufacture route uses CAD/CAM technology. This manufacturing route avoids all of the risks mentioned previously for re-melting the polymer. The ma- terial properties remain consistent and the framework manufacture can also benefit from the increase preci- sion and reproducibility of a digital workflow. Although it does require a more significant capital investment by the laboratory, many laboratories are seeing that it is necessary to align with other industries and adopt dig- itisation to increase efficiencies. PAEK materials further extend these CAD/CAM efficiencies when com- Fig 1: PAEK removeable denture pared to milling metal substruc- tures, since there is typically less tool wear and faster milling times and the capital equipment necessary to mill them does not need to be as expensive as machines for milling metal frameworks. It is the author’s view that the op- timum use of these materials only comes from the CAD/CAM milling process as opposed to the injection moulding process. Polymer properties When handling a prosthetic frame- work made from a PAEK, a striking thing is the difference in weight. When identical full arch implant prosthetic substructure frameworks were made from four different mate- rials, the results from weighing were: PAEK 4.9g, titanium 17g, zirconia 23g and cobalt chrome 33g. However, it is the possibility to intro- duce shock absorption to a prosthe- sis that is perhaps the most exciting. This could have positive implications for patient comfort and for damage limitation. In my view, the most rel- evant mechanical property related to the aspect of shock absorption is not ultimately compressive strength (as is sometimes promoted), but ac- tually flexural strength and elastic modulus. Obtaining an increasingly stronger material becomes academic since clearly it would be simplistic to pre- fer the highest value. Metals have very high compressive strengths relative to PAEKs but are not shock absorbing. Naturally, a design must also consider the influ- ence of thickness and shape as well, but values for flexural strength and elastic modulus are more indicative of the stiffness of a material and how much it will deflect the load. Stiffer materials, like metals, have a high elastic modulus (see Table 1) mean- ing that metals require high loads to elastically deform them. Therefore, one can look at natural materials like bone for clues as to an ideal for stiffness. Common denture materi- als like PMMA have an elastic modu- lus range of 1.8-3.1 GPa, but limited strength. The PAEKs have an elastic modulus closer to bone (4-5GPa) al- lowing the framework to be stiffer, yet still shock absorbing. Fig 2: PAEK removeable denture Fig 3: PAEK implant bourne prosthesis Fig 4: Long-term framework in PAEK However, PAEKs also additionally possess sufficient strength to be con- sidered as a metal alternative. Conclusions The high-performance polymers called PEEK and PEKK have excit- ing potential in dentistry as a metal alternative for removable and im- plant prosthetic frameworks. Their stiffness properties confer promise as a substructure that could add an element of shock absorption. This may have benefits for patient com- fort, addressing parafunction and damage limitation. In the following series of case studies, I shall describe the use of a PEEK high performance polymer as a framework for remov- able and implant prosthetics. References 1. Williams, D.F., McNamara, A., and Tutner, R.M. (1987) Potential of poly- etheretherketone (PEEK) and car- bonfibre-reinforced PEEK in medical applications. Journal of Material Sci- ence 2. Letters. February 1987, volume 6, issue 2, pp 188-190 Professor Paul Tipton BDS, MSc, DGDP RCS (UK) DENTAL SURGEON Visiting Professor of Restorative and Cosmetic Dentistry, City of London Dental School | www.colds.co.uk SPECIALIST IN PROSTHODONTICS | www.drpaultipton.co.uk T Clinic @ Manchester , London | www.tclinic.co.uk TIPTON TRAINING Ltd | www.tiptontraining.co.uk www.bard.uk.com President of the British Academy of Restorative Dentistry (BARD)