Comparative Study between Two Adjacent Implants Supported Crowns and One Implant Supported Cantilever Fixed Dental Prosthesis: An In Vivo Study

INTRODUCTION

Implant-based rehabilitation in esthetic zone presents a significant clinical challenge. Specifically, achieving softtissue contour, mimicking that of natural teeth, remains difficult to predict.¹

Although there have been improvements in implant dentistry, replacing two lost adjacent anterior teeth remains challenging for clinicians. The tooth-implant papilla fill, when replacing a single tooth, is mostly influenced by the neighboring tooth’s attachment level integrity. For two neighboring implants, it remains challenging to create the necessary presence as well as stability of papillae between the two implant restorations owing to the lack of dentogingival fibers attachment.² Yet, it is essential to maintain at least 1.5 mm spacing between teeth and implants’ surface in order to prevent bone resorption. Additionally, when placing two neighboring implants, they need to be positioned 3 mm apart to provide an ideal papilla between them.³

The maxillary lateral and mandibular incisors are of small diameter. When two neighboring implants are utilized for replacement, they are commonly positioned in close proximity to each other, thus compromising the superior esthetic outcomes. When the distance between implants is less than 3 mm, the loss of crestal bone might impact the bone height between implants. Inter-implant bone loss leads to more papilla loss, which may negatively impact esthetics, thus producing inaccessible areas that are difficult to clean or prone to food accumulation.⁴

Implant-supported single crowns have become a widely accepted and favorable approach for replacing a single tooth loss. When it comes to replacing two adjacent missing teeth, there are two treatment approaches: utilizing two implants with two distinct crowns or using one implant for supporting a cantilever fixed dental prosthesis (FDP) with two units.⁵ Typically, the cantilever design is used to address issues related to alignment, bypassing anatomical limits, requiring large bone grafting, and for esthetic purposes, particularly in cases where there is little space between the anterior as well as posterior teeth.⁶

Utilizing the cantilevers in fixed prosthodontics has created controversies regarding the pontic’s function as a lever, thus probably producing lateral forces that provide a higher risk of overloading in implant-supported FDP. Therefore, it was anticipated that there would be a greater occurrence of biological and prosthetic issues with this particular form of rehabilitation, as opposed to those without cantilevers.⁷ However, research has shown comparable rates of success for implants, regardless of whether they had cantilever extensions or not. Currently, the cantilever extensions’ impact on marginal bone level as well as problems related to biologic or prosthetic factors is not well understood.⁸

Nevertheless, using implant-supported cantilevers has introduced details about the prolonged stability as regards survival rates of implants as well as peri-implant marginal bone loss (MBL). However, in a prospective study that lasted for 7 years with cantilever FDP supported by implants, when controls were absent, the implants’ survival rate indicated 97% while success rates of prostheses reached 98%.⁹ Zurdo et al.¹⁰ addressed that the frequently occurring adverse events for FDP with cantilevers involved veneers’ fracture (10%), abutment screws’ fracture (1.6%), decementation (6.9%), as well as loosening of screw (7.9%).

Polyetheretherketone (PEEK) has excellent chemical resistance and superior mechanical characteristics, while being biocompatible. It exhibits superior compatibility with recent imaging technology. It is a dental implant material that is tooth-colored and often utilized when esthetics is a significant factor.¹¹ Polyetheretherketone could be employed as a superstructure for implants, abutments, as well as implant body or impression post. The Young’s modulus of pure PEEK indicates 3.6 GPa, while that of carbon-reinforced PEEK indicates 18 GPa, as well as glass fiber reinforced PEEK indicates 12 GPa. The Young’s modulus of PEEK is similar to that of cortical bone, thus exhibiting less stress shielding compared with titanium material.¹²

Regardless of the attempts to prevent MBL, it seems to occur more frequently at the implant-neck level after implant insertion and loading. During the first year following the surgical procedure, a dental implant often experiences a certain degree of physiological MBL, which may occur both horizontally and vertically. However, following this period, there is only limited further bone loss yearly.¹³ The process of bone remodeling is a crucial determinant in assessing the success of an implant. The necessary conditions for the successful placement of an implant include an MBL of up to 1 mm during the first year after its insertion, as well as a mean yearly MBL approaching 0.2 mm throughout subsequent monitoring duration. However, ensuring the prolonged esthetic and functional success related to FDP supported by implants requires the maintenance and improvement of soft tissue and bone contour.¹⁴

The study’s null hypothesis is that: there was no variance among the first group of two separated adjacent implants with titanium abutments and two separate metal ceramic crowns, the second group of FDP with a cantilever supported by single implants using titanium abutment as well as two metal ceramic FDP and the third group of FDP with a cantilever supported by single implants using PEEK framework, regarding the evaluated parameters of marginal bone level, stability of dental implant, chipping of veneering as well as screws’ loosening while following up with cases.

After thorough research of literature, it was found that the studies comparing the two adjacent implants supported crowns and one implant supported cantilever FDP are scarce.

MATERIALS AND METHODS

RESULTS

Thirty-six patients were assessed for eligibility; six patients did not meet the criteria, and three patients refused to participate in the study. The remaining 27 patients were randomly allocated into three equal groups (9 patients in each). All allocated patients were followed-up and analyzed statistically (Fig. 1).

Regarding MBL, no significant variance was observed among all groups up to 6 months, and then there was a significant variance among them up to 18 months. Mean marginal bone level exhibited a significant rise from baseline to 18 months for all groups (Tables 1 and 2 and Figs 2 and 3).

Regarding implant stability, no significant variance was observed among all groups for 18 months (Tables 3 and 4 and Figs 4 and 5).

Regarding prosthetic complications, no veneer fracture was observed among the three studied groups. Only one case from second group (cantilever metal ceramic group) came with screw loosening.

The implants were placed by using a handpiece according to the manufacturer’s instructions (maximum speed of 15 rpm and torque of up to 90 Ncm). The implants were placed with an insertion torque of at least 45 Ncm. The implant or plate form to be located at the bony level and reevaluated by periapical radiograph. After 4 to 6 months healing period, the patients were called back for the second stage surgery. Healing abutments were tightened for 15 days and evaluated radiographically. Three months later, another closed tray impression was done using polyvinyl siloxane impression material (two-phase one-step technique) through impression post and implant analog to transfer the hard and soft tissue relationship to the laboratory technician for fabrication of a definitive metal ceramic restoration. Then, final abutment was tightened using 35 Ncm torques and checked the need for angled abutment or not in addition to evaluation of implant abutment connection clinically and radiographically. Ceramic try was done to evaluate the restoration before glazing. The final metal ceramic crowns were examined clinically and radiographically. Clinically by checking the seating of the crown, margin and occlusion, anatomical features, contours, and color matching. Radiographically by checking the marginal adaptation between abutment finish line and margin of the restoration (Fig. 6).

Another case of single implant supporting two units (cantilever design) as the case attended with missing upper right canine and adjacent lateral incisor and restored with single implant supporting two units of metal ceramic FDP over titanium abutment (Fig. 7).

DISCUSSION

The presence of neighboring missing maxillary or mandibular anterior teeth remains challenging while placing implants. The diameters of the upper laterals along with all lower incisors are pretty small. When two implants are utilized to replace them, they are often placed very close to each other, thus potentially leading to bone loss as well as soft tissue recession that may not be ideal for the esthetic outcomes.³

Significant crestal bone loss was observed around implants close to the cantilever in comparison with other implants within 12 and 18 months follow-up. These findings fell within the normal and accepted clinical range. As regards the implant stability, no significant variance was documented among the studied groups.

Marginal bone level is a key factor since bone remains crucial for a successful implant. Our research addressed that implant close to the cantilever exhibited a slight increase as regards the average MBL, indicating 1.35 ± 0.21 mm for PFM cantilever group, while 1.26 ± 0.08 for PEEK cantilever group after 1.5 years, which revealed a significant variance in comparison with the first group of two implants with separate crowns. Nevertheless, all bone loss measurements remained within the accepted clinical range.

Our research’s findings supported several prior studies. Wennström et al.²² along with Hälg et al.²³ addressed a reduced peri-implant bone loss for an implant cantilever FDP (ICFDP), which exhibited no significant variance in comparison with that around the implant within controls. The same findings were observed by Kim et al.,²⁴ revealing no significant variance as regards overall bone loss among ICFDPs and non-cantilevered FDPs supported by implants.

Romeo and Storelli found similar findings to ours. Following a mean period approaching 3 years of function, the peri-implant bone loss amount near the cantilever extension of FDPs showed significant variance in comparison with the peri-implant bone loss within FDPs without cantilevers.²⁵

Roccuzzo et al.⁵ revealed that the mean MBL remained stable (<1.5 mm) during the observation time in both the groups. These findings differ from those described by Van Nimwegen et al.,⁶ who reported an increased “MBL at the distal side of the central implant (cantilever side) compared with the mesial side (noncantilever side) in the implant-cantilever group” at 5-year follow-up. This discrepancy could be caused by the different location of the cantilever (mesial vs distal) or explained by a wide range of follow-up times (7−64 months) in the present study. However, Wu et al.²⁶ reported a mean MBL of 1.35 mm at the 3-year follow-up, with no significant difference between the mesial and distal aspect, which is consistent with data from the present study.

The mean ISQ measurements for osseointegrated implants has fallen between 65.4 and 72. In our research, the Osstell results revealed that within 18-months period, the stability related to implant near the cantilever exhibited greatly reduced values in comparison with that of the other implants without cantilever, but without a significant variance. Though a significant variance was observed for the same implants as regards MBL after 12 months, it had no effect on implant stability. The findings of MBL as well as stability tests are not generally correlated with each other since other factors like the local bone quality and quantity often influence the implants’ stability.²⁷

Taha et al.²⁸ revealed that the average MBL for implants near the cantilever was significantly higher than that for the other implants in the two designs, with a value of 1.8 mm at the 2-year follow-up examination, which was also within the typical range of bone loss per year.

Utilizing cantilever implants within the anterior regions has many benefits. First, it helps in preventing inter-implant bone loss. Additionally, it allows for soft tissue grafting, which may result in a tissue height extending 6 mm beyond the crestal bone. This, in turn, enables the construction of an ovate pontic with full papillae.²⁹

Over time, it has been evident that implants exhibit distinct behaviors and possess superior resistance to force compared with natural teeth. As stated by Schulte et al.,³⁰ the implant survival rates do not seem to be influenced by the concept of crown-to-root ratio. Another example is cantilevers; implants seem to have a higher tolerance for cantilevers.

Roccuzzo et al.⁵ found that the implant survival rate was 100% for a two-unit cantilever FDP after a mean of 33 months of prosthetic loading, which is consistent with studies with a follow-up from 1 to 5 years. Jung et al.³¹ reported in a systematic review with meta-analysis on 46 included studies 5-year results of single implants with single implant-supported crowns without a cantilever. They reported a 5-year implant survival of 97.2% and a 5-year crown survival of 96.3%. Jensen-Louwerse et al.³² reported that the implant survival and restoration survival were both 100%.

Our findings align with the conclusions of prior research. In a pilot trial, Tymstra et al.¹⁸ did not address significant variance among inserting one implant and two neighboring implants when it came to replacing lost maxillary central and lateral incisors. This finding was seen throughout a 10-year follow-up period.

A systematic review and research with a follow-up period lasting for 10 years by Meijer et al.³³ determined that using a restoration of two units supported by one implant may be a feasible option instead of placing two separate crowns supported by one implant in the esthetic zone.

There were no instances of veneers chipping, retention loss, screws’ breakage, or implants’ loss seen after 18 months. Out of the second group (specifically the cantilever metal ceramic group at the 3 months follow-up visit), just one case had screw loosening, which may be attributed to an insufficient screw torque or premature contact; however, when torqued again with correct torque and the occlusion was adjusted, no screw loosening was observed. Nevertheless, longer periods could influence the outcome. So, we cannot definitively confirm that ICFDP placement could show prolonged success when replacing anterior teeth.³⁴

Palmer et al.³⁵ revealed that the incidence of abutment screw loosening in their study was higher than that experienced in single-tooth replacements.

The limitations of this study included that this research confined to missing anterior two adjacent teeth; however, there was no standardization regarding the specific implant site or diameter. Second, the low number of participants with some dropouts may have affected the results. Finally, follow-up times need to be more extended. Comparisons between the two treatment alternatives should be made with caution because of the randomized nature of the study.

CONCLUSION

Implant cantilever FDPs could be successful while replacing missing anterior teeth. The cantilever design had no negative influence on the clinical outcome. The cantilever FDP design facilitated prosthesis fabrication among those having laterals of narrow diameters. Further research is required to investigate such a particular concern due to a limited sample size in our research.

REFERENCES

1. Ji S, Choi Y. Microbial and host factors that affect bacterial invasion of the gingiva. J Dent Res 2020;99(9):1013–1020. DOI: 10.1177/002203 4520922134.

2. Tribble GD, Lamont RJ. Bacterial invasion of epithelial cells and spreading in periodontal tissue. Periodontol 2000 2010;52(1):68–83. DOI: 10.1111/j.1600-0757.2009.00323.x.

3. Van Dyke TE, Bartold PM, Reynolds EC. The nexus between periodontal inflammation and dysbiosis. Front Immunol 2020;11:511. DOI: 10.3389/fimmu.2020.00511.

4. Choi YS, Kim YC, Ji S, et al. Increased bacterial invasion and differential expression of tight‐junction proteins, growth factors, and growth factor receptors in periodontal lesions. J Periodontol 2014;85(8):e313–e322. DOI: 10.1902/jop.2014.130740.

5. Ji S, Choi YS, Choi Y. Bacterial invasion and persistence: Critical events in the pathogenesis of periodontitis? J Periodontal Res 2015;50(5):570–85. DOI: 10.1111/jre.12248.

6. Marchesan JT, Girnary MS, Moss K, et al. Role of inflammasomes in the pathogenesis of periodontal disease and therapeutics. Periodontol 2000 2020;82(1):93–114. DOI: 10.1111/prd.12269.

7. Park E, Na HS, Song YR, et al. Activation of NLRP3 and AIM2 Inflammasomes by Porphyromonas gingivalis Infection. Bäumler AJ, editor. Infect Immun 2014;82(1):112–123. DOI: 10.1128/IAI.00 862-13.

8. Su X, Zhao S, Song Y. Expression of NLRP3 and AIM2 inflammasome in peripheral blood in Chinese patients with acute and chronic brucellosis. Sci Rep 2022;12(1):15123. DOI: 10.1038/s41598-022-19 398-9.

9. Baek K, Ji S, Choi Y. Complex Intratissue Microbiota Forms Biofilms in Periodontal Lesions. J Dent Res 2018;97(2):192–200. DOI: 10.1177/ 0022034517732754.

10. Baek K, Lee J, Lee A, et al. Characterization of intratissue bacterial communities and isolation of Escherichia coli from oral lichen planus lesions. Sci Rep 2020;10(1):3495. DOI: 10.1038/s41598-020-60449-w.

11. Rajakaruna GA, Negi M, Uchida K, et al. Localization and density of Porphyromonas gingivalis and Tannerella forsythia in gingival and subgingival granulation tissues affected by chronic or aggressive periodontitis. Sci Rep 2018;8(1):9507. DOI: 10.1038/s41598-018-27766-7.

12. Bartold PM, Van Dyke TE. An appraisal of the role of specific bacteria in the initial pathogenesis of periodontitis. J Clin Periodontol 2019;46(1):6–11. DOI: 10.1111/jcpe.13046.

13. López-Martínez J, Chueca N, Padial-Molina M, et al. Bacteria associated with periodontal disease are also increased in health. Med Oral Patol Oral y Cir Bucal 2020;e745–e751. DOI: 10.4317/medoral.23766.

14. Netea MG, Schlitzer A, Placek K, et al. Innate and adaptive immune memory: An evolutionary continuum in the host’s response to pathogens. Cell Host Microbe 2019;25(1):13–26. DOI: 10.1016/j. chom.2018.12.006.

15. Netea MG, Domínguez-Andrés J, Barreiro LB, et al. Defining trained immunity and its role in health and disease. Nat Rev Immunol 2020;20(6):375–388. DOI: 10.1038/s41577-020-0285-6.

16. Prigot-Maurice C, Beltran-Bech S, Braquart-Varnier C. Why and how do protective symbionts impact immune priming with pathogens in invertebrates? Dev Comp Immunol 2022;126:104245. DOI:10.1016/j.dci.2021.104245.