Single endosteal dental implant-supported crown has long-term success rates as high as 90%–95%.¹ Although the literature shows an extensive success rate, Henry et al. reported 96.6% success associated with a 10% esthetic failure rate in a 5-year multicenter study for single tooth replacements in the anterior maxilla.²,³ Mechanical factors, especially the type of abutment material, affect the stability of the mucosa and crestal bone.⁴,⁵ Ingemar Abrahamsson et al. observed an increase in the amount of bone loss following the abutment connection.⁶ Although titanium abutments are the most widely considered standard treatment option for implant-supported restorations, they have an inherent esthetic disadvantage. Increased demands for highly esthetic restoration contribute to the development of nonmetallic abutment materials with improved material characteristics.⁷ However, ceramic abutments exhibited a complication of 2%, with a higher degree of fractures than metallic abutments that lead to implant failure.⁸,⁹,¹⁰

The mechanical properties of zirconia with an elastic modulus (210 Gpa) similar to those of metals were used as abutments to improve the esthetic outcome of an implant-supported prosthesis.¹¹,¹² However, an abutment material with an elastic modulus similar to bone was known to decrease the stress distribution to the supporting tissues.⁷ Poly-ether-ether ketone (PEEK), which is a dominant member of the poly-aryl-ether-ketone polymer family, exhibits an elastic modulus that varies from 3 to 4 Gpa. Moreover, the addition of fillers such as carbon fiber can modify the elastic modulus of PEEK from 3 GPa to 18 Gpa equivalent to cortical bone.⁷ PEEK, as a healing abutment, lowers the risk for marginal bone loss and soft-tissue recession during the initial healing period when compared to Titanium.¹³ Santing et al. suggested that the fracture strength of composite resin provisional crowns fabricated over PEEK and titanium abutments is comparable.¹⁴ However, the studies on PEEK abutments are limited to validate their application as a conventional implant abutment. Furthermore, the location and magnitude of occlusal forces can affect the quality and quantity of induced strain and stress in all components of the bone–implant prosthesis complex.¹⁵

The role of abutment material on the effective stress transmission around implants to reduce the marginal bone loss and facial marginal recession for an optimal esthetic outcome is still questionable. Hence, the purpose of the present study was to investigate the influence of different abutment materials such as titanium, zirconia, and PEEK on stress distribution around maxillary anterior implants under varied alveolar bone density and occlusal forces using a three-dimensional (3D) finite element analysis (FEA). The objective of the study was to analyze the stress distribution around the anterior maxillary implant with:

Different abutment materials: titanium, zirconia, and PEEK

Different bone densities: D2 and D3 in each abutment material

Different oblique and vertical loads under varying densities for each abutment material.

An in vitro study was conducted by FEA in the present study. A 3D FEA was done using the ANSYS (Analysis System Software, Ansys, Inc., Canonsburg, Pennsylvania, United States) Pro/Engineer Wildfire 2.0 software. The bone, implant, and abutment were modeled from the computed tomography scan obtained from a similar clinical situation. A 3D finite element model of a maxillary anterior section of bone with a single dental implant in the incisor region with two different bone qualities and three different abutment materials was constructed. Six FEA models were constructed with three different forces, and the loads were applied at the chosen coordinates.

Bone was constructed with a height of 24 mm and a width of 7 mm having a cortical bone thickness of 1 mm and 2 mm on the labial and palatal bone, respectively. The properties of the bone approximated the D2 and D3 as classified by Lekholm and Zarb.¹⁶ A 3.7 mm width, 11.5 mm length solid tapered screw-type implant that has a threaded helix and internal hex connection was modeled in the bone.

Three different abutment materials titanium, zirconia, and PEEK of the same dimensions (3 mm diameter and 6 mm length) were used. The abutments were prepared 1.5 mm on the incisal, buccal, and lingual aspects with a 0.5 mm width of chamfer margin. Porcelain fused to the metal crown was designed with a 10 mm length and a diameter of 8.5 and 7 mm in mesiodistal and labiolingual aspects, respectively. The crown had a 2 mm thickness of feldspathic porcelain laid over a 0.8 mm thickness of Co-Cr alloy that was cemented to the abutment with an intervening cement thickness of 50 μm.¹⁷

All the materials used in the models were isotropic, homogeneous, and linearly elastic considering 100% osseointegration of the implants. The Young's modulus and the Poisson's ratio for each material were taken from existing literature Table 1.¹,¹⁸,¹⁹ External loads of 100 N, 175 N, and 250 N were applied in the vertical and oblique directions. The vertical load was applied along the long axis of the implant prosthesis, and the oblique load was applied on the cingulum of the incisor crown at a 135° inclination.¹{Table 1}

The methodology was reviewed by an independent statistician, and the collected data were analyzed with the SPSS 28.0 version(IBM, SPSS Inc, Armonk, New York, United States). To describe the descriptive statistics, the mean and standard deviation were used. To find the significant difference between the bivariate samples in paired groups (vertical and oblique loads), the Friedman–Wilcoxon signed-rank test was used. Mann–Whitney U test was to find the significant difference between independent groups (cervical, medial, and apical regions of abutment), and Kruskal–Wallis test was used for the multivariate analysis (cervical, medial, and apical regions of abutment). In all the above statistical tools, the probability value < 0.05 is considered a significant level.

The stress distribution at the peri-implant area was compared between nine groups (Ti 100 N, Ti 175 N, Ti 250 N, Zi 100 N, Zi 175 N, Zi 250 N, PEEK 100 N, PEEK 175 N, and PEEK 250 N) in both D2 and D3 bone quality Table 2, Table 3 and Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6. There was no statistical difference between the three materials in any of the six types of load that was assessed. However, higher stress values were observed with PEEK abutment (P > 0.05) Table 4. The results showed a generalized increase in the stress values on the D3 bone when compared to the D2 bone in all the groups with statistical insignificance (P > 0.05) Table 5. The stress on the peri-implant area significantly increased from 100 N to 175 N and 100–250 N (P < 0.05) which was more predominant with an oblique load Table 6.{Table 2}{Table 3}Figure 1

Stress distribution in titanium abutment with D2 bone: (a) 100 N oblique load, (b) 175 N oblique load, (c) 250 N oblique load, (d) 100 N vertical load, (e) 175 N vertical load, (f) 250 N vertical load.

Figure 1Figure 2

Stress distribution in titanium abutment with D3 bone: (a) 100 N oblique load, (b) 175 N oblique load, (c) 250 N oblique load, (d) 100 N vertical load, (e) 175 N vertical load, (f) 250 N vertical load.

Figure 2Figure 3

Stress distribution in zirconia abutment with D2 bone: (a) 100 N oblique load, (b) 175 N oblique load, (c) 250 N oblique load, (d) 100 N vertical load, (e) 175 N vertical load, (f) 250 N vertical load.

Figure 3Figure 4

Stress distribution in zirconia abutment with D3 bone: (a) 100 N oblique load, (b) 175 N oblique load, (c) 250 N oblique load, (d) 100 N vertical load, (e) 175 N vertical load, (f) 250 N vertical load.

Figure 4Figure 5

Stress distribution in PEEK abutment with D2 bone: (a) 100 N oblique load, (b) 175 N oblique load, (c) 250 N oblique load, (d) 100 N vertical load, (e) 175 N vertical load, (f) 250 N vertical load.

Figure 5Figure 6

Stress distribution in PEEK abutment with D3 bone: (a) 100 N oblique load, (b) 175 N oblique load, (c) 250 N oblique load, (d) 100 N vertical load, (e) 175 N vertical load, (f) 250 N vertical load.

Figure 6{Table 4}{Table 5}{Table 6}

Among the various abutment materials used as an implant superstructure, PEEK had an elastic modulus closer to the bone. Hence, the study was aimed at comparison of PEEK abutment material with titanium and zirconia abutment to evaluate the stress in the peri-implant bone around maxillary anterior implants for an optimal esthetic outcome. Clinically, it is impossible to assess the stress and strain distribution of implant-supported prostheses, although strain gauges may be used to measure strains at the abutment level. The use of simulation models such as the photoelastic method had disadvantage of being non-transparent, and stress-strain analysis had the disadvantage of having poor stress output. However, FEA has the remarkable advantage of analyzing areas that are difficult to access without risks to a living subject during the investigation and allows researchers to predict the biomechanical performance of dental implant designs.²⁰

We modeled the implants as threaded design with fine meshing to ensure greater accuracy and to prevent the underestimation of the stress generated at the bone-to-implant interface.²¹ Furthermore, porcelain fused to metal was used as the crown material, as there was no difference in the stress distribution between the porcelain fused to metal and zirconia crown.²² The fracture resistance of zirconia is very high, which could deviate the stress developed due to the abutment, and hence, porcelain fused to metal was used as the crown material.

PEEK (18 Gpa) though being closer to the elastic modulus of bone when compared to titanium (110 Gpa) and zirconia (210 Gpa),⁷,¹¹,²³,²⁴ we observed insignificant higher stress values around PEEK abutment material. The outcome of the study was in concurrence with Sarot et al., indicating that PEEK had higher stress concentration in the implant neck and the adjacent bone, due to decreased stiffness and higher deformation.⁷ However, our comparative study revealed that the increased stress observed with PEEK was comparable with titanium and zirconia abutments.

Further, stress observed in both vertical and oblique loads in D2 and D3 bones with titanium and zirconia abutment was indifferent from the PEEK abutment. The titanium abutment distributed the stresses in a more homogeneous manner in our study due to lesser deformation of the material. However, we observed that the zirconia abutment showed the least stress compared to the other two abutment materials. The literature claims that titanium abutments exhibit the highest bending than zirconia,²⁵ and the stress distribution in zirconia was less than titanium with the least bacterial contamination.²⁶,²⁷

We observed that both on D2 and D3 bones, stress on the peri-implant area increased up to 175 N with higher stress observed for D3 bone, but at a load above 175 N, the changes were insignificant. The absence of stiffer cortical bone to support the implant neck in a low-density bone and the implant being stiffer than the surrounding cancellous bone could have increased stress concentration in D3 bone.¹⁸ However, D2 and D3 bones did not have any difference at a load of 250N. There was also a generalized insignificant increase in stress on the labial bone compared to the palatal bone, and the maximum stress values were observed in the cortical bone compared to the cancellous bone. Although the present study revealed maximum stress in cortical bone compared to cancellous bone in the anterior maxilla, previous studies suggest that under buccolingual load, the strain was concentrated in the cancellous bone.¹⁷

Our study also reveals maximum stress values in the cervical aspect compared to the middle and apical third which was in concurrence with the previous study. Bone qualities D2 and D3 showed maximum stress concentration at the implant neck within the cortical bone surrounding the implant for both vertical and oblique loads.¹ Abrahamsson I et al. concluded that increased load can result in increased stress, thereby increasing marginal bone loss.⁵,⁶

The outcome of our study suggests that PEEK can be a viable option in the treatment modality to be used as an abutment for anterior implants, especially with D2 bone. Despite the best efforts to model the structure accurately, the model has several limitations. Masticatory forces are dynamic, whereas this study was conducted under static loads. Bone is a viscoelastic, anisotropic, and heterogeneous material, whereas in the present study, it was assumed to be linearly elastic and homogeneous. The resultant stress values obtained may not be accurate quantitatively, though they are generally accepted qualitatively. The merging of the colors in the model makes it difficult to ascertain the definitive range, and hence, subjective variation cannot be eliminated.

PEEK abutments were comparable to titanium and zirconia abutments in anterior implant prosthetic rehabilitation, though generalized higher stress was observed. The stress on the bone proportionately increased in both vertical and oblique loads suggesting the influence of mechanical load in crestal bone loss rather than the abutment material.

Acknowledgment

The methodology was reviewed by independent statistician, Dr. S. Venkatesan, MSc, MPhil, PhD, Proprietor, Zigma Analytics, Chennai.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, and financial or non-financial in this article.

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