Discussion

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.