Introduction and background:

Dental implants are crucial components in the restorative treatment of partially and completely edentulous patients. Following surgical placement, the supportive and protective tissues initiate a healing process that ultimately leads to the biological and functional integration of the implant into the host. In the case of peri-implant soft tissues, this process is guided by the use of healing abutments and temporary prostheses, which also play a significant role in establishing the microbiota in the new peri-implant environment.

Healing abutments can be fabricated from various materials, with titanium, polymethyl-methyl-methacrylate (PMMA), and polyetheretheretherketone (PEEK) being the most common. PEEK, in particular, has garnered attention in recent years due to its physicochemical and mechanical properties, which have been observed to promote healing and tissue evolution around the implant ^1-3. Moreover, it exhibits superior aesthetic performance compared to metal alloys, thereby encouraging its use in aesthetically challenging areas ⁽⁴. Titanium, on the other hand, exhibits excellent mechanical and physicochemical properties, as well as well-known biological compatibility, attributed to its high chemical stability and corrosion resistance, among other factors ⁵. However, some authors have noted its inferior antibacterial and anti-adherent capabilities compared to zirconia ⁶ or PEEK ⁽⁷.

Features influencing biofilm formation on abutments include the roughness of its surface, its surface energy, as well as intrinsic features of the materials used to manufacture it ^(8-10. However, the specific role of each feature, particularly the material's effect on biofilm formation and viability, remains unclear. Therefore, the objective of this descriptive study is to explore the effect of surface and material characteristics on the total volume and viability of bacterial biofilm developed on PEEK and titanium healing abutments.

Methods

Specimen Preparation

Four titanium healing abutments (Ø: 6.5mm; H: 6mm; RC; RC platform Ø: 4.1mm) and four PEEK healing abutments (Ø: 5mm; H: 7mm; NC; NC platform Ø: 3.3mm) from the Straumann/Neodent company (Straumann®, Basel, Switzerland) were prepared. The upper cylindrical portion of each abutment was separated from its base using a high-precision saw (IsoMet 1000, Buehler, Illinois, USA) with continuous irrigation, as depicted in Figure 1.

Figure 1. On the left, the titanium abutment, on the right, the PEEK abutment. The upper portion was used, which was sectioned from its base (dotted line) 

Surface Energy

Surface energy was determined by measuring the contact angle of each material using the sessile drop technique, with distilled water as the liquid. A 0.5 μL drop was poured onto the outer convex area of the abutments (Figure 2). Hydrophobicity was evaluated by measuring the angle formed by the drop with the abutment surface.

Figure 2. Sessile drop technique for contact angle measurement for selected titanium and PEEK abutments 

Statistical Analysis

Statistical analysis was conducted using Minitab 17 (Minitab Inc., USA). Due to the limited sample size, only descriptive statistics were performed. Effect size was estimated using Cohen's d, based on the differences in group means and weighted standard deviation, to determine if there is an effect, and whether it is small, moderate, or large ⁽¹².

Results

Surface Characteristics

Roughness and contact angle values are detailed in Table 1. Figure 3 illustrates characteristic images obtained with the confocal microscope of each surface.

Table 1. Average values, with their respective standard deviations in parentheses, for roughness and contact angle 

According to Cohen's criterion, the type of material had a significant effect (d < 0.2) of the material type on the contact angle was noted (Figure 2).

Figure 3. Roughness of PEEK (A) and Titanium (B) 

Total biofilm volume and bacterial viability

Confocal microscopy images revealed a higher volume of bacterial biofilm on titanium surfaces (696 µm³ ± 315 µm³) compared to PEEK (419 µm³ ± 279 µm³), as depicted in Figure 4. These values allow estimating a medium to large effect according to Cohen (d = -0.67). Additionally, titanium healing abutments exhibited a greater volume of live bacteria (384 µm³ ± 97 µm³) than the PEEK ones (234 µm³ ± 82.2 µm³). Nevertheless, the ratio of live bacteria to the total observed bacteria was similar (55% vs. 56% for titanium and PEEK, respectively), as illustrated in the CLSM images presented in Figure 5.

Figura 4. Volume of viable (green) and total (red + green) biomass for both materials 

Figure 5. Confocal microscopy image (CLSM) of the abutment surfaces colonized by the biofilm. While a higher volume of total bacteria is visible in the titanium abutments, the proportion of live bacteria (in green) versus dead bacteria (in red) is similar in both materials. 

Discussion

This descriptive study took an exploratory approach to investigate the impact of material and surface characteristics on the development and viability of bacterial biofilm. While the extrapolation of the findings is constrained by the small number of evaluated samples, the observed trend of lower total bacteria volume on PEEK healing abutments’ surfaces is noteworthy for their indication and use as an alternative to titanium ones.

The development and growth of bacterial biofilm on a surface are influenced by several factors, including surface topography, hydrophobicity, surface energy, and charge ¹³. Studies have shown that both, an increase in roughness (S_a) exceeding 0.2 µm ¹⁴, and a higher surface energy promote biofilm formation in restorative materials, although the effect of roughness is more decisive ¹⁵. This relationship has been proven across different materials, such as composite resin ¹⁶, vitreous ceramics ¹⁷, and titanium ¹⁸. The presence of deeper and wider depressions increases the contact area, enhancing bacteria protection against external removal forces. This creates a more favorable surface for biofilm colonization and growth ¹⁹. In this study, despite observing larger roughness on the PEEK abutments compared to the titanium ones (Table 1), it was the latter that exhibited a larger total volume of biofilm on their surfaces (Figure 4). However, both materials showed biofilms with similar bacterial viability (Figure 5). In vitro studies examining these same materials have yielded disparate results. While Barkamo et al. also measured higher roughness (S_a) in PEEK than in titanium, they observed larger biofilm formation on PEEK, especially for strains like S. sanguis and S. oralis ⁽⁹. Interestingly, in the same study, measurements after 120 hours of culture showed no significant differences in the biofilms of the different materials ⁹.

Similarly, Hahnel et al. found no significant differences in the viable biomass of PEEK, titanium, or zirconia surfaces, despite PEEK exhibiting the lowest roughness values among the materials studied ⁽²⁰.

On the other hand, D'Ercole et al. noted significantly higher nanoroughness on PEEK compared to both machined and etched titanium. However, the total volume of bacteria and viable bacteria (S. oralis) was notably lower on PEEK surfaces ⁽⁷. Lastly, despite the absence of differences in the surface roughness of titanium and PEEK, Peng et al. identified a substantially higher total biomass (S. mutans, A. actinomycetemcomitans) on titanium than on PEEK- although, similar to the outcomes of this study, both materials exhibited similar cell viability in their biofilms ²¹.

Regarding surface energy, the abutments of both materials examined here were predominantly hydrophobic (contact angles exceeding 90°), albeit with very similar values between them (Figure 2). Literature reports on this property vary considerably, with some authors noting higher hydrophobicity for titanium ⁷, while others for PEEK ^{(9, 20, 21)}. However, there is no consensus among them regarding their impact on the quantity and viability of formed biomass. It appears that the ability to promote or inhibit biofilm formation is not solely or predominantly linked to a surface characteristic but rather to a combination thereof, also highly contingent on the bacterial composition of the colonizing biofilm ¹⁹.

While the results of this study are not directly applicable to clinical scenarios, they do provide insight into how both the nature of the material and surface characteristics influence the formation and development of bacterial biofilms. When considering healing abutments, designed for relatively short-term use, their impact on the longevity of final restorations may not appear significant at first glance. However, the establishment of a potentially pathogenic microbiota in the peri-implant niche, facilitated by the characteristics of the healing abutment utilized, could influence the subsequent development of peri-implant diseases and treatment success. Hence, studies of greater complexity and scope are needed to elucidate the role of this early microbiota in such diseases.

Conclusions

Bacterial viability of the biofilms developed on healing abutments exhibited no differences between PEEK and titanium, and there was no direct correlation between the total biofilm volume and the roughness and hydrophobicity of both materials.