Friction of Laser-Textured Zirconia Coated with a Platelet-Rich Fibrin

Abstract

In the present in vitro study, we evaluated the adhesion of an injectable platelet-rich fibrin (i-PRF) to laser-textured zirconia surfaces and their resultant friction behavior against bone tissue. Three types of zirconia surfaces were compared regarding the i-PRF coating effects: 1) grit blasted with 250-μm spherical alumina particles and acid etched with 20% hydrofluoric acid (ZLA), 2) laser textured with a random (RD) surface pattern, or 3) laser textured with a designed pattern based on 16 lines and 8 passages (L16N8). The coefficient of friction (COF) of the specimens was assessed on a reciprocating sliding pin-on-plate tribometer at 1-N normal load, 1 Hz, and a 2-mm stroke length. Sliding wear tests were carried out against bovine femoral bone tissue in 0.9% sodium chloride solution at room temperature. Surfaces were then assessed by scanning electron microscopy. COF mean values for test groups (0.35, ZLA; 0.45, L16N8) were lower when compared with the control groups (0.52, ZLA; 0.60, L16N8), with the exception of the RD group (0.47, test; 0.43, control). Results did not show significant differences in COF mean values between RD and L16N8 surfaces after coating with i-PRF. The 3-dimensional fibrin network embedded with leukocytes, platelets, and red blood cells was responsible for decreasing COF mean values over the zirconia surfaces, thus providing a lubricant effect. Also, the morphologic aspects of the laser-treated zirconia surfaces increased the adhesion of the platelet-rich fibrin, which could speed up the osseointegration process of zirconia implants.

Keywords

coefficient of friction dental materials laser surface texturing osseointegration dental implants lasers friction;bone

Introduction

The early osteointegration process of dental implants depends on several factors such as: patients’ health conditions, materials, implant-to-bone interface integrity, materials, and implant design (Davies 2003; Howe et al. 2019; Monje et al. 2019; Schünemann et al. 2019; Souza et al. 2019). Dental implants composed of yttria-stabilized tetragonal zirconia polycrystals, known as zirconia, have recently gathered attention since their optical properties and high chemical stability providing enhanced aesthetic and clinical success (Kohal et al. 2016; Dantas et al. 2019; Schünemann et al. 2019; Rohr et al. 2021). However, such high chemical stability decreases the interactions between zirconia surfaces and blood products or osteogenic cells during the first stage of the osseointegration (Schünemann et al. 2019; Rohr et al. 2021). The migration and differentiation of osteogenic cells are also dependent on the roughness and morphologic aspects of the zirconia implants (Schünemann et al. 2019; Rohr et al. 2021; da Cruz et al. 2022). The formation of a stable fibrin clot in contact with the moderately rough implant surfaces is a key factor in the healing process, as it provides an autogenous scaffold for the migration and differentiation osteogenic cells toward the bone-to-implant interface (Davies 2003; Varela et al. 2019; Andrade et al. 2021).

Implant surfaces can be functionalized with inorganic or organic bioactive materials, although many alternative approaches are not clinically utilized, considering the lack of in vitro and in vivo studies to validate the findings (Rao et al. 2019; Shah et al. 2021; Noronha Oliveira et al. 2023). Injectable platelet-rich fibrin (i-PRF) is an autologous product produced from human blood centrifugation at specific spin centrifugation and equipment (Miron et al. 2017). Several guidelines for platelet-rich fibrin (PRF) are currently reported in literature, providing products with varied content of blood cells, proteins, and growth factors (Dohan et al. 2006; Castro, Cortelini et al. 2019). The flowable i-PRF is rich in leukocytes, fibrinogen, platelets, fibrin, and growth factors, as reported by previous studies (Miron et al. 2017; Castro et al. 2019; Varela et al. 2019). Regarding its low viscosity, i-PRF can be handled by syringes and mixed with other bone substitutes, while a few studies reported the coating potential of dental implants with i-PRF (Cortellini et al. 2018; Strauss et al. 2018; Andrade et al. 2021; Sampaio et al. 2023). The flowable i-PRF used as implant coating can provide a dense fibrin-based clot on micro- and nanoscale features on implant surfaces (Lollobrigida et al. 2018; Strauss et al. 2018). However, the i-PRF coating or some components (leukocytes, platelets) can be detached from the implant-to-bone interface on implant placement (Andrade et al. 2021; Noronha Oliveira et al. 2023). The removal of the coating and its components can occur upon friction between rough implant surfaces and bone tissues, so placement should be performed on a torque monitor to achieve primary stability. In fact, zirconia implant surfaces should be modified by increasing the roughness and enhancing the morphologic aspects for mechanical interlocking of i-PRF coatings, as well as for maintaining the primary stability of the endosseous implant. Yet, the i-PRF coating can decrease the frictional forces on the zirconia implant surface during implant placement against bone tissues toward the positioning into the surgical site.

In the present in vitro study, we reported the adhesion of laser-textured zirconia surfaces to a flowable and injectable autologous PRF on reciprocating sliding testing against bone in a saline solution. We hypothesized that the laser-treated zirconia can provide well-designed topographic surfaces to enhance their adhesion to the i-PRF. A second hypothesis is based on a probable low coefficient of friction (COF) on zirconia surfaces coated with the PRF over implant placement.

Materials and Methods

Zirconia surfaces were treated by 3 approaches intending to increase the retention of the i-PRF. A first group of sintered zirconia cylindrical specimens (TZ-3YSB-E; Tosoh Corporation) with a dimension of 8 mm in diameter and a thickness of 5 mm (Fig. 1A) were grit blasted with 250-μm spherical Al₂O₃ particles at 6 bar and 50 mm away from the surface for 30 s (Appendix; Faria et al. 2020). Grit-blasted zirconia (ZLA) disks were then immersed in 20% hydrofluoric acid for 1 min and then ultrasonically cleaned in an isopropyl alcohol for 5 min.

The other 2 groups of presintered zirconia discs were laser texturized on Nd:YAG laser (OEM Plus; SISMA) at 6 W through a 3-μm spot size and at a pulse width of approximately 35 ns. A focusing unit containing a fused quartz lens was used with a nominal focal length of 160 mm, a wavelength of 1.064 μm, and a maximum pulse energy of 0.3 mJ/pulse. Laser texturing was carried out on zirconia surfaces as an alternative approach to standard grit basting (ZLA). No surface pattern was designed in one of the groups (random [RD]); therefore, the laser texturing was randomly performed for comparison with ZLA and with well-designed laser-textured surfaces. Another group of surfaces was subjected to well-designed laser texturizing with a 16-line and 8-passage (L16N8) pattern, which was previously planned by computer-aided design. The planning of the L16N8 surfaces considered a probable increased mechanical interlocking of i-PRF coatings and stable primary stability of the implant to the bone.

Surface texturing was carried out at room environment. An air braided jet was carried out to remove debris produced during laser processing. The sintering of laser-textured zirconia was carried out at 1,500 °C and at 8 °C/min with a high-temperature furnace (Zirkonofen 700; Zirkonzahn) for 2 h of holding time.

Then, we coated zirconia specimens with a flowable i-PRF, as seen in Figure 1K. On i-PRF preparation, blood was harvested from 3 human volunteers (Appendix) and immediately centrifuged at room temperature with a high-quality table centrifuge (IntraSpin; IntraLock) at 2,700 rpm (408g) for 3 min (Fig. 1E). After centrifugation, the flowable PRF (upper yellow liquid phase) was immediately harvested from the plastic tubes with a plastic disposable transfer pipette (avoiding red blood cells). Thereafter, we performed reciprocating wear sliding tests on the zirconia specimens to evaluate the dynamic COF evolution and the structural integrity of the surface patterns. Wear tests were performed against bovine bone tissue (1-mm diameter, 5-mm length) mimicking the placement of a dental implant (Fig. 1K, L). The tests were performed with a reciprocating sliding pin-on-plate tribometer (UMT-2; Bruker) on 1-N axial loading, at 1 Hz, and over a 2-mm stroke length at room temperature (20 ± 2 °C) for 5 min (Souza et al. 2010a; Souza et al. 2012). We applied a low-magnitude and clinically relevant loading at 1 N to evaluate the gradual effects of the i-PRF coating on the evolution of the COF on zirconia surfaces following previous studies. COF values were statistically analyzed by 2-way analysis of variance and Tukey test (P < 0.001) with Origin Lab statistical software program.

Surfaces were inspected by scanning electron microscopy (SEM; JSM-6010 LV [JEOL]) equipped with an energy-dispersive spectrometer. SEM preparation (Appendix) was adapted from our previous studies on i-PRF (Souza et al. 2010; Souza et al. 2012; Varela et al. 2019; Oliveira et al. 2024). SEM images were acquired at magnification ranging from ×30 to ×8,000 at 3 areas for each specimen (n = 9).

Results

Grit-blasted and acid-etched zirconia surfaces (ZLA; Fig. 2A–C) revealed a smoother morphologic aspect when compared with the laser-textured (L16N8) zirconia surfaces (Fig. 2D–K). Differences in the fibrin network coatings were noticed between the ZLA and L16N8 groups. Even though the ZLA surfaces revealed a lower roughness (Fig. 2A–C), we detected a remaining flowable PRF onto some surface regions, which is a promising outcome for a potential coating of commercial implants. Laser-textured surfaces showed a higher area covered by a dense fibrin network when compared with the ZLA surfaces. Thus, the morphologic aspects of the laser-structured surfaces provide a high retention of the fibrin network enriched with platelets, leukocytes, and growth factors (Fig. 2K, L). The flowable i-PRF covered peaks and valleys of the laser-textured zirconia surfaces (Fig. 2J–L). Blood cells can also be seen trapped into the fibrin network on the laser-textured surfaces (Fig. 3K).

In the present study, we noticed the retention of bone fragments and cells entrapped in the fibrin network over laser-textured zirconia surfaces after the wear tests (Fig. 3). The reciprocating sliding direction is indicated on the SEM images. The existence of cracks and adherent thin tribolayers was detected only at the edges of the wear track on the ZLA surfaces (Fig. 3C). The absence of cracks over the laser-textured surfaces (RD and L16N8) occurred due to the high PRF retention throughout the surface. A high amount of bone tissue (dark regions) was transferred to the laser-textured zirconia surfaces (RD and L16N8; Figs. 3 and 4).

The evolution of the COF recorded for grit-blasted/etched or laser-textured zirconia-based specimens against bone in a 0.9% NaCl solution is shown in Figures 3 and 4 (right side). Initial COF values on zirconia free of PRF (control group) ranged from 0.42 to 0.90, while the initial COF of zirconia coated with flowable PRF (test groups) ranged from 0.53 to 0.94. Regarding the COF evolution for the ZLA surfaces free of i-PRF (control), we noticed an initial running-down period that was followed by a progressive increase in the COF values (Fig. 3). On the randomly laser-textured zirconia surfaces (RD group), the COF evolution was not linear over the surfaces free of PRF (Fig. 3G). The COF mean values ranged from 0.3 to 0.7 at a few spots.

On the COF evolution for the L16N8 surfaces free of i-PRF, different plots were noted, although a relative steady-state regime with some oscillations in COF was representative for all specimens (Fig. 4). A significant decrease in COF values from around 0.6 to 0.4 validate the lubricant effect of the i-PRF coating over the L16N8 group. Also, oscillations in the COF evolution showed lower values at around 0.1 and 0.2 that indicate an ultra-low friction on some zirconia spots due to the fibrin network coating effect. The lubricating effect of PRF was maintained over the test, probably because fibrin covered the zirconia, providing a smoother surface as seen by SEM (Figs. 2 –4).

Mean COF values for ZLA and L16N8 coated with flowable PRF were significantly lower (0.35, ZLA; 0.45, L16N8) when compared with the control groups free of flowable PRF (0.52, ZLA; 0.60, L16N8; P < 0.0001). A decrease in the COF values suggests a lubricant effect of the flowable PRF. The COF mean values for the RD surfaces free of flowable PRF coating was lower (0.43) than those recorded for RD surfaces coated with flowable PRF (0.47) but was not significant (P = 0.143). On the i-PRF coated surfaces, results did not show statistically significant differences in COF mean values between RD and L16N8 surfaces (P = 0.265). The lowest COF mean values were recorded for the surfaces coated with flowable PRF, ranging from 0.35 to 0.47 (Fig. 4), while an extensive running-down period was characterized by a progressive decrease in the COF values up to 300 s. Statistical analysis of the COF values for the groups is shown in the Table.

Table.

Descriptive Statistics and ANOVA on the Coefficient of Friction Values at the Steady-State Regime Recorded for ZLA, RD, and L16N8.

Group^a	Mean	SD	Min	Max	PE	95% CI
ZLA
Control	0.52	0.028	0.48	0.56	0.009	0.022 to 0.53
Coated with i-PRF	0.35	0.020	0.32	0.38	0.006	0.015 to 0.36
RD
Control	0.43	0.036	0.39	0.47	0.011	0.027 to 0.45
Coated with i-PRF	0.47	0.027	0.43	0.50	0.008	0.019 to 0.48
L16N8
Control	0.60	0.035	0.56	0.64	0.011	0.027 to 0.62
Coated with i-PRF	0.45	0.026	0.40	0.47	0.008	0.020 to 0.45
ANOVA	Square Sum	df	Mean Square	F	P Value	F crit
Variation
Surface	0.07166	2	0.03582	42.22	2.63E-11	3.19
i-PRF coating	0.13093	1	0.13093	154.29	1.32E-16	4.04
Interaction	0.11928	2	0.05964	70.28	5.47E-15	3.19
Within	0.04073	48	0.00084
Total	0.36260	53

Abbreviations: ANOVA, analysis of variance; F crit, F critical value; i-PRF, injectable platelet-rich fibrin; L16N8, designed pattern (16 lines and 8 passages); PE, ; RD, random; ZLA, grit blasted and acid etched.

n = 9 per group.

Discussion

The present study shows a novelty on the friction and wear testing of a flowable i-PRF onto laser-textured zirconia surfaces for dental implants. In the present study, we utilized the reciprocating sliding tests to simulate friction forces as found in an implant placement procedure. Then, we assessed the COF and wear of the laser-textured zirconia surfaces coated or not with an i-PRF. Results validate the first hypothesis, considering that the morphologic improvement of zirconia surfaces can increase the retention of the flowable PRF. We validate the retaining of i-PRF over grit-blasted and laser-textured zirconia surfaces due to the mechanical interlocking of the i-PRF throughout the rough surfaces. In fact, the fibrin fibers from PRF do offer proper mechanical behavior and elasticity thanks to the formation of a 3-dimensional fibrin network. The absence of aligned abrasion groves onto the laser-textured surfaces can be explained by the low-level loading and the presence of i-PRF during the wear testing. The use of flowable PRF may bring benefits since the low frictional forces decrease the implant surface degradation by wear against bone tissues. Thus, we assessed a low-level loading in this study to avoid the complete destruction of the bone counterparty and therefore to allow the gradual monitoring of the COF evolution. Also, the loading magnitude mimics the one occurring over implant placement. In previous wear studies, the normal and frictional loading varied since such a parameter is quite variable in the oral cavity or over the implant placement (Souza et al. 2010; Souza et al. 2012; Macedo et al. 2017; Udomsawat et al. 2018; Demirbas et al. 2022; Fabris et al. 2022).

We noticed mean values of COF for zirconia surfaces free of flowable PRF similar to those reported in previous studies (Moura et al. 2017; Faria et al. 2020). In our study, the transference of bone tissue to zirconia surfaces indicates a stronger and effective contact between implant and bone that can also explain the highest COF values recorded within the reciprocating sliding wear tests. Laser-textured zirconia surface with a well-designed pattern (i.e., L16N8) could be a good choice for increasing the primary stability of zirconia implants to bone and for retaining the flowable PRF layer in the microscale valleys throughout the surface, as seen by microscopic images and COF values. Still, fibrin itself and the debris resulting from test materials can lead to a third viscous-elastic body effect able to distribute loads, decreasing the contact pressure at the surface (Dantas et al. 2019). Additionally, textured surfaces play a role on mechanical interlocking into bone, leading to a primary stability that could accelerate osseointegration at an early stage (Öncü et al. 2016; Torkzaban et al. 2018). Also, a reliable bone-to-implant contact is essential to prevent implant microscale motion and maximize the long-term implant success rate (Schünemann et al. 2019; Souza et al. 2019). That could be particularly significant in patients with compromised tissue healing, as in severe periodontal disease and radiotherapies or bisphosphonate-based therapies (Olmedo-Gaya et al. 2016; Apaza-Bedoya et al. 2017).

Although 5-y clinical data exist for different zirconia implants, no analysis has yet been performed focusing on how the surface morphologic aspects of implants affect clinical parameters (Rohr et al. 2021). Also, studies on i-PRF coating onto zirconia surfaces could not be found in the literature. Previous studies reported only the morphologic aspects of PRF onto titanium implant surfaces. Thus, the in vitro biological characterization of PRF leads to a better understanding of its influence on the friction and wear of implants into a bone defect, which can support the development of further effective coatings for implants and guidelines for surgical procedures. A detailed cell, morphologic, and protein characterization of an i-PRF was reported in a previous study (Varela et al. 2019). This study showed a higher density of the 3-dimensional fibrin network for i-PRF formed at 700 rpm for 3 min than that for the peripheral blood clot. However, the morphologic aspects and mechanical behavior of the fibrin network depend on the proportion of harvested fibrinogen and thrombin from the initial blood sample.

The i-PRF is slowly transformed in a viscous fibrin network depending on the centrifugation method and equipment. Fibrin contains binding sites for integrins, growth factors, and other extracellular matrix components, including fibronectin, which provides molecular signals to direct cell function. Thus, the use of a flowable injectable fibrin-based gel could be advantageous since the fibrin cross-linking occurs in direct contact with the implant surface. Also, a higher density of blood platelets was reported for i-PRF (~4,000 × 10² µL⁻¹) than for the blood clot (3,500 × 10² µL⁻¹) as well as on leukocytes (~8,000 µL^-1 in i-PRF and 6,000 µL⁻¹ in peripheral blood). Another study reported the capability of i-PRF to release higher content of various growth factors (e.g., PDGF, TGF-β) and type I collagen and therefore to stimulate higher fibroblast migration when compared with platelet-rich plasma (Miron et al. 2017).

In a previous study, an improvement on osteoblast proliferation and mineralization was noticed after application of i-PRF onto machined commercially pure titanium (Shah et al. 2021). Traini et al. (2014) found significantly lower blood clot extension on grit-blasted and acid-etched zirconia surfaces when compared with titanium surfaces modified by grit-blasting and acid-etching procedures. Previous studies compared the behavior of different implant titanium surfaces after contact with 3 flowable PRF products: a fibrinogen concentrate, a fluid harvested from L-PRF clots, and a combined approach with fibrinogen and L-PRF fluid (Lollobrigida et al. 2018; Andrade et al. 2021). Such studies revealed that some textured implant surfaces are more suitable for a biomimetic functionalization with platelet concentrates (Andrade et al. 2021). Thicker fibrin fibers were noted in contact with smooth and textured implant surfaces. A significant increase in bone formation has been achieved on the micro- and nanoscale structured surfaces. However, the high resorption rate of i-PRF may not sustain long-term volume stability in an in vivo condition. Considering that i-PRF is completely resorbed over the tissue-healing process, a combined application with enhanced implant surfaces or porous structures is highly recommended to repair large bone defects. Yet, further studies are required to validate the use of i-PRF for implant coating, considering the types of surfaces, material, loading, and design for dental implants. Clinical attention has been taken into account when choosing the type of implant surfaces for functionalization with i-PRF, since the adhesion of the i-PRF layer can become compromised over smooth implant surfaces. Also, randomized controlled clinical trials and histologic analysis are required to validate those findings on machined- or laser-textured surfaces. Histologic assessment could bring additional information regarding the resorption of the flowable PRF and bone-healing process.

Conclusions

The morphologic aspects and roughness of laser-textured zirconia increased the adhesion of an i-PRF. The PRF coating performed as a lubricant layer decreasing the COF between the zirconia and bone surfaces. After wear testing, laser-textured zirconia surfaces revealed a higher number of regions covered by a dense fibrin network layer, including platelets and leukocytes, when compared with zirconia surfaces modified by grit-blasting and acid-etching procedures. On potential clinical applications, the PRF layer can become adhered onto the rough zirconia surfaces after implant placement into the surgical site in bone tissues. That can enhance the events of the bone-healing processes, including the adsorption of proteins as well as the migration, adhesion, and differentiation of osteogenic cells.

Author Contributions

M. Noronha Oliveira, contributed to conception, data acquisition, drafted and critically revised the manuscript; N. Sahoo, contributed to design, data acquisition, drafted and critically revised the manuscript; O. Carvalho, contributed to design, data analysis, drafted and critically revised the manuscript; F.S. Silva, M. Özcan, B. Henriques, contributed to conception, design, data analysis and interpretation, drafted and critically revised the manuscript; J. Gomes, contributed to conception, design, data interpretation, drafted and critically revised the manuscript; J.C.M. Souza, contributed to conception, design, data acquisition, analysis, and interpretation, drafted and critically revised the manuscript. All authors gave final approval and agree to be accountable for all aspects of the work.

Supplemental Material

sj-docx-1-jdr-10.1177_00220345241305318 – Supplemental material for Friction of Laser-Textured Zirconia Coated with a Platelet-Rich Fibrin

Supplemental material, sj-docx-1-jdr-10.1177_00220345241305318 for Friction of Laser-Textured Zirconia Coated with a Platelet-Rich Fibrin by M. Noronha Oliveira, N. Sahoo, O. Carvalho, F.S. Silva, J. Gomes, M. Özcan, B. Henriques and J.C.M. Souza in Journal of Dental Research

Footnotes

Acknowledgements

The authors acknowledge the Portuguese Foundation for Science and Technology and the University of Zurich for the financial support.

A supplemental appendix to this article is available online.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the Portuguese Foundation for Science and Technology (UIDB/04436/2020, UIDP/04436/2020, PTDC/EMEEME/4197/2021) and the University of Zurich.

ORCID iD

J.C.M. Souza

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