PEEK Dental Implants: A Review of the Literature

Dental implants increase the quality of life for many patients with tooth loss.¹  The material of choice for oral endosseous implants is pure titanium, introduced at the end of the 1960s by Branemark.²  Although implants based on titanium and titanium alloys, such as Ti-6Al-7Nb and Ti- 6Al-4V,^3,4  are well evidence-based, it was demonstrated that their use can be correlated with a range of problems. One problem is a potential hypersensitivity to titanium.^5–9  Another problem could occur due to the gradient difference in the elastic moduli of a titanium implant and its surrounding bone. This may cause stress in the implant-bone interface during load transfer,^10,11  probably resulting in peri-implant bone loss.^12,13  Also, titanium can cause esthetic problems due to its lack of light transmission.¹⁴  This can provoke a dark shimmer of the peri-implant soft tissue in cases of thin biotype mucosa and/or mucosa recession around a titanium implant. This can be problematic especially in the presence of a high smile line.^15,16  Additionally, an increasing number of patients are demanding dental reconstructions of completely metal-free materials.¹⁵  As an alternative to titanium, ceramic implants are proposed, which were first introduced about 40 years ago and were made from aluminum oxide.^17–22  Due to frequent fracture incidence, this material was substituted by titanium.²³  Nowadays, ceramic dental implants are made of zirconia, which seems to be a better suitable alternative to titanium because of its tooth-like color, mechanical properties, biocompatibility, and low plaque affinity.²⁴  But a systematic review of the literature by Andreiotelli et al¹⁵  from 2009 concludes that the scientific clinical data are not yet sufficient to recommend ceramic implants for routine clinical use. This corresponds to a grade C recommendation of the definitions of types of evidence originating from the US Agency for Health Care Policy and Research.¹⁵  Furthermore, the stress distribution of a zirconia implant to the surrounding bone could be associated with even higher stress peaks compared to titanium, due to the higher elastic modulus of zirconia of 210 GPa.²⁵ 

Another biocompatible material with an elastic modulus of 3.6 GPa, which is closer to that of bone, is polyetheretherketone (PEEK).²⁶  Its modulus can be modified by reinforcing it with carbon fibers, for example, to achieve a modulus of 18 GPa, similar to that of cortical bone.²⁷ 

Since PEEK showed resistance to degradation in vivo, it was offered commercially in April 1998 as a biomaterial for long-term implants (Invibio Ltd, Thornton-Cleveleys, UK).²⁸  Since then, PEEK has demonstrated to be a high-performance thermoplastic polymer able to replace metallic implant components in the field of orthopedics^29,30  and traumatology.^31,32  Also, calvarial reconstructions with PEEK implants were described.³³  These findings suggest that PEEK could substitute titanium as material for dental endosseous implants.

The current review pertains to literature published prior to December 1, 2010. The aim was to figure out whether there are existing studies about dental implants from PEEK, which probably underline the theory that PEEK could be considered as a viable alternative material for dental implants.

The articles for the current review were found using the PubMed search engine and searching for references cited within these articles. All articles published until December 2010 were reviewed. The following search terms were used together: “dental implant PEEK.”

Only articles about dental implants from PEEK or modified PEEK published until December 2010 were included, regardless of the kind of investigation (in vivo or in vitro), implant design, surface modifications, year of publication, and language.

The search yielded 5 articles, of which 3 were included in the review.^11,34,35  Two articles were excluded, because they did not report on dental implants of PEEK.^36,37 

Of the included articles, 2 reported on animal investigations^34,35  and 1 on an in vitro finite element study.¹¹ 

The aim of the 3-dimensional finite element study was to compare the stress distribution to the peri-implant bone in 4 distinct testing models consisting of either a titanium or a carbon fiber–reinforced PEEK (CFR-PEEK) implant, containing 30% carbon fibers to obtain an elastic modulus of 17.4 GPa similar to that of cortical bone,²⁷  each in combination with a titanium and a CFR-PEEK abutment, completed with a cemented artificial crown (Table 1).

The CFR-PEEK implants presented a higher load concentration in the cervical area and at the cortical bone than the titanium implants, whereas the titanium implants presented equivalent stress peaks in the cervical portion and a more homogenous load distribution throughout the whole implant body (Table 1).

The authors admit that the higher stress concentrations of the PEEK implant were not expected. A more homogenous stress distribution was intended to diminish stress peaks at the implant-bone interface. So, they conclude that the CFR-PEEK implant did not present any advantages in comparison to the titanium implant.

This material was also used for cylindrical implants in an animal experiment, which compared the bone-implant contact (BIC) and shear strength of 20 titanium-coated implants to 20 uncoated implants.³⁵  In this study 5 implants were inserted in each femur of 4 mongrel dogs, 5 uncoated on one side and 5 titanium-coated on the other side. After 4 and 8 weeks of healing, 2 dogs were sacrificed. Of each femur, 3 implants underwent pull-out testing to determine the mechanical integrity of the interface. The other 2 implants of each femur were reserved for intact histologic evaluation.

The coated implants showed significantly BIC values after a healing period of both 4 (P = .0014) and 8 weeks (P = .0261), whereas the BIC of both the uncoated and the coated implants generally decreased from 4 to 8 weeks. The shear strength for the uncoated implants was significantly higher after 4 (P = .0107) and lower after 8 weeks (P = .2496) (Table 2). The authors conclude that the addition of the titanium coating apparently increases the biocompatibility of the implant surface. Using the BIC ratio as a parameter for the grade of osseointegration, it can be stated that both CFR-PEEK implants (coated and uncoated) presented a desirable osseointegration in comparison to the BIC values of the titanium implants (40.91 ± 10.11%) of the second animal experiment³⁴  (Table 2). The aim of that study was to evaluate osseointegration of 1-piece zirconia vs titanium implants depending on their insertion depths after a healing period of 4 months due to a split-mouth design (submerged vs nonsubmerged healing). Therefore, the test implants made from zirconia and coated zirconia (covered by a calcium-liberating titanium oxide [TiO₂] sol-gel layer) were compared to a control implant made from titanium. Additionally, an experimental implant of PEEK was inserted. All implants had the same design, only differing in their biomaterials. In this study, the PEEK implants reached BIC rates of 26 ± 8.9%.

In neither of the 2 animal investigations were signs of inflammation or foreign body reactions observed.

Referring to a 3-dimensional finite element analysis of a CFR-PEEK and a titanium implant (Table 1), the authors concluded that due to its higher stress concentrations, the CFR-PEEK implant could not be recommended.¹¹ 

This deformation rate could probably be diminished by an inner stiffening of the implant, for example, by an abutment connection bolt which extends to the apical region of the implant, whereas the complete biomechanical behavior of a PEEK implant has to be tested experimentally to achieve accurate data.

Because CFR-PEEK is black due to the carbon fibers, its use could be unfavorable, especially in esthetic zones.

In an animal investigation from 1995, the BIC and shear strength of titanium-coated and uncoated CFR-PEEK implants were evaluated.³⁵  The shear strength of the uncoated implants was significantly higher after 4 and insignificantly lower after 8 weeks of healing, although the BIC rate of the coated implants was always significantly higher (Table 2).

The surface roughness as an important factor was not assessed, so this phenomenon is difficult to interpret.

Considering potential hypersensitivities to titanium, in such cases a titanium coating might provoke hypersensitive inflammatory reactions.

The aim of the second animal experiment was to evaluate osseointegration of 1-piece implants made from zirconia, coated zirconia (coated by a calcium-liberating TiO₂ sol-gel layer), titanium, and PEEK depending on their insertion depths after a healing period of 4 months due to a split-mouth design (submerged vs nonsubmerged healing).³⁴  Regrettably, the resulting BIC values of both the submerged and the nonsubmerged implant groups of this study were summarized to a mean value (Table 2). The different types of healing could have had an influence on the BIC values, due to different exposures of the implants to masticatory loads and oral flora.

For histomorphometric analysis, the bone level (BL) between the uppermost thread and the crestal bone level was subdivided into 2 sections. One section described the bone-related BL, ranging from the crestal bone level to the uppermost BIC. The other section was named implant-related BL, measured from the uppermost thread to the uppermost level of BIC. The level of the uppermost BIC was localized between the uppermost thread and the crestal bone level. All results, however, were expressed as negative values (Table 3). The authors argue that the bone levels in general presented higher values in the group of the submerged implants due to higher insertion depths, which were defined neither before nor while the implants were inserted. To get the mean insertion depths of the uppermost threads in relation to the crestal bone level, we took the values of the mean bone-related BL and the mean implant-related BL from the original article and added them together (Table 3). Then, we evaluated the differences in these calculated insertion depths for the nonsubmerged and the submerged implants to see how much deeper the submerged implants were inserted compared to the nonsubmerged (Table 4). The value of the difference was positive when the result revealed a greater insertion depth for the submerged implants and negative if the insertion depth of the submerged implants was less deep. In this way, we calculated the following values: for the submerged implants of zirconia +0.47 mm, of coated zirconia −0.31 mm, of titanium +0.6 mm, and of PEEK +0.81 mm (Table 4). These findings contradict the statement of the authors that the submerged implants in general were inserted deeper than the nonsubmerged implants.

Another finding in the article states that the PEEK nonsubmerged implants showed significantly lower bone-related BL than the nonsubmerged coated zirconia and titanium implants (P = .046, .028). There is no evidence mentioned, if the mean insertion depths of the coated zirconia (2.56 mm), the titanium (2.02 mm), and the PEEK implants (1.74 mm) could play an influencing role for the bone-related BL, as the PEEK implants of the nonsubmerged group presented the lowest mean insertion depth (Table 3).

Neither of the 2 animal investigations observed inflammation signs or foreign body reactions, which emphasizes the evidenced biocompatibility of PEEK. Another in vitro study demonstrated attachment of a collagen gel to PEEK by enzyme-induced mineral deposition.³⁸  If this kind of coating could be used to anchor a PEEK implant in the alveolar bone by collagen fibers like a natural tooth, this could represent another advantage of PEEK over titanium, giving back the physiologic tensile load to the bone.

Literature reporting on dental implants made from PEEK demonstrate that PEEK is basically osseointegrated as biocompatible material in vivo. Further investigations are necessary to find ways to improve the biomechanical behavior to achieve a more homogenous stress distribution to the surrounding bone, which has not yet been experimentally proven. Long-term investigations of loaded PEEK implants in vitro and in vivo are necessary. Experimental modulations of the surface are needed as well to achieve the highest possible grade of osseointegration. The design of a 2-piece implant made from PEEK, which allows the submerged healing method, has to be developed. PEEK used for a dental implant should have a light translucency similar to a natural tooth to achieve favorable esthetic results.

Abbreviations

BIC

bone-implant contact

BL

bone level

CFR-PEEK

carbon fiber–reinforced PEEK

PEEK

polyetheretherketone