Effect Of Crown-To-Implant Ratio of Short Implants on Peri-Implant Stress: A Finite Element Analysis

Aim: Objective of this study was to evaluate the distribution of bone tension around single implants with different crown/implant ratio (1:1, 1.25:1,1.5:1) according to the von Mises criterion.
Materials and Methods: Stress analysis was performed using In Vesalius 3.0, Rhinoceros 3D 4.0 software. Three 3D models of the mandibular segment of the bone block were developed:
model A, implant (6 mm) connected to a crown (6 mm), C/I - 1:1; model B, implant (6 mm), connected to the crown (7.5), C/I -1.25:1; model C, implant (6 mm) connected to a crown (9 mm), C/I -1.5:1. The applied forces were 100 N (axial). The clinical part of the study included 68 patients with 152 short implants restored using various crown/implant ratios (1:1, 1.25:1, 1.5:1). The results were evaluated: the cumulative frequency of implant engraftment, MBL changes around the implant, follow up for 5 years.
Results: Under axial load, the highest stresses were concentrated at the abutment/implant junctions, the highest stresses were 67.94 MPa C/I -1:1, 68.09 MPa C/I -1.25:1 and 68.24 MPa C/ I -1.5:1, respectively. The results of the current study, indicate the increase in crown height showed little difference in load at the crown/implant interface with crown/implant ratios of 1:1, 1.25:1 and 1.5:1 Short implants cumulative success of implantation after 5 years was 97.8% for group 1, 97.6% for group 2, and 97.3% for group 3. Patients were satisfied with the results of treatment.
Conclusions: Use of shorts implants for prosthetic rehabilitation can be considered favorable and well substantiated.

Factors that determine the success of implant osseointegration are: implant biocompatibility, implant surface and geometry, implant diameter and length [1-7]. For effective implantation, a sufficient amount of bone tissue is required. To restore the necessary amount of bone tissue, various labor-intensive multi-stage bone-reconstructive operations are used, including autogenous block, distraction method, nerve reposition; these operations are costly, require high professional skills and have a high risk of complications [8-15]. In order to avoid expensive and time-consuming multi-stage implantation-bone-reconstructive operations in such patients, it is advisable to use a one-stage surgical method using narrow diameter implants or short implants (5 to 6 mm) [16-19]. The use of short implants allows, prevents damage to anatomical structures, reduces time and shows high clinical efficiency [20-26]. 
Optimization of macro- and micro-design of short implants by increasing the implant surface, diameter will help to avoid potential risks and ensure long-term stability [27,28]. 
The height ratio (C/I) of short implants to the prosthesis structure placed on it can be a risk factor in terms of the distribution of biomechanical stresses associated with overload, and may increase the risk of complications [29]. Therefore, it is important to aim for a low C/I ratio, to avoid overstressing the bone. However, some authors argue that treatment with short implants is possible with a C/I ratio greater than 1:1 [30-32]. Predicting the long-term efficacy of short implants requires assessing the effect of length, diameter, and C/I ratio on biological complications [33-37]. Various in vivo methods have been used to predict the distribution of bone load around the implant, Finite Element Analysis (FEM) is the most successful calculation method, and can non-invasively depict the stress distribution in a structure [39-41]. In the scientific literature there are recommendations for a favorable C/I ratios ratio for natural teeth, but recommendations for C/I ratios by implantology have not been established, which may be helpful to determining the prognosis of dental implants. Objective this study was to evaluate the distribution of bone tension around single implants with different crown/implant ratios (1:1, 1.25:1, 1.5:1) according to the von Mises criterion.

The experimental part of the study included modeling use FEM method. Stress analysis was performed using In Vesalius 3.0, Rhinoceros 3D 4.0 software. Three 3D models of the mandibular segment of the bone block were developed: (Figure 1, 2). 

Figure 1: Models A, B and C resulting in a C/I ratio of 1/1, 1.25/1 and 1.5/1

Figure 2: Three-dimensional (3D) finite element modeling
model A, a short implant (6 mm) connected to a crown (6 mm), C/I - 1:1; model B, a short implant (6 mm), connected to the crown (7.5), C/I -1.25:1; model C, a short implant (6 mm) connected to a crown (9 mm), C/I -1.5:1.
The abutment and implant body were treated as a single unit, which prevented the potential effect of micromotion between the components. images of implants and abutments were reconstructed using CT pro 2.0 software (Metrics, Belgium). The bone model represented a mandibular bone block in the region of the second molar of quality type II bone in the center of the trabecular bone surrounding 1 mm of cortical bone. The surrounding bone around each implant was modeled as a cylinder 15 mm in diameter and 18 mm high. Bone block information was obtained using a CT database of implants and abutment geometries. After wards, modeling solids geometries have been exported to FEA pre- and post-processing software (FEMAP 11.2, Siemens PLM) to get the grids tetrahedral parabolic solid elements. 3D computer-aided design software (Solid Works 2010; USA) and Rhinoceros 4.0 CAD were used to calculate the Mises stress distribution. CAD software was used to define the mesh control of the 3D models. The color changes from yellow to blue, where blue represents the maximum stress value expressed in megapascals (MPa). The stress analysis was performed under the following conditions: the model contained an implant preloaded to 35 N, an abutment, and a cemented crown. An axial force of 100 N was applied to the occlusal surface of the crown. Bone density was modeled by changing the elastic modulus of tissue with elastic moduli of 13,7 GPa, the titanium implant component alloy with an elastic modulus of 112 GPa, and the superstructure feldspathic Ceramic of the model was designed as a restoration with an elastic modulus of 82,8 GPa.20. All materials modeled in this study are presented in Table 1.

Table 1: Mechanical properties of the simulated materials

68 patients with severe vertical bone atrophy in the masticatory region and the need for restoration with fixed implants prostheses were included in the clinical part of the study. The patients had 152 ultraviolet functionalization (UV) short implants (length 5-6 mm, diameters 4,5-5mm) and restored with different C/I crown/implant ratios (1:1, 1.25:1, and 1.5:1). The mean C/I ratio was 1.37. To clinical assessment the effect of C/I ratios of short implants and MBL there were formations of 3 groups. Group 1 included 48 short implants subjected to ultraviolet functionalization (UV) with a C/I ratio of 1:1. Group 2 included 51 short implants subjected to ultraviolet functionalization (UV) with a C/I ratio of 1,25:1. Group 3 included 53 short implants subjected to ultraviolet functionalization (UV) with a C/I ratio of 1.5:1. The results were evaluated: failures of the prosthesis and implant, any complications, implants marginal bone loss (MBL) the follow up for 5 years. 
Statistical Analysis 
Statistical analysis was performed using SPSS. P values <0>

Under axial load, the highest stresses were concentrated at the abutment/implant junctions, the highest stresses were 67.94 MPa C/I -1:1, 68.09 MPa C/I -1.25:1 and 68.24 MPa C/ I -1.5:1, respectively (Figure 3).

Figure 3: von Mises stress distribution on implant/crown set — axial loading. (A) C/I -1:1 (B) C/I 1.25:1 (C) C/I-1.5:1 in type II bone with 100N axial load.

The increase in crown height showed little difference in load at the crown/implant interface with crown/implant ratios of 1:1, 1.25:1, and 1.5:1 respectively (Figure 4).

Figure 4: Model C showed little difference in load at the with crown/implant 1.5/1 ratios

During implant placement and in the follow up period, no serious biological or prosthetic complications were recorded. Аfter 1 years in Group 1 mean MBL  0.86±0.4 mm, after 3 years MBL was 0,98±0.2 mm, after 5 years MBL was 1.05±0.3 mm. After 1 years in Group 2 mean MBL  0.97±0.3 mm, after 3 years MBL was 1,07±0.4 mm, after 3 years MBL was 1.12 ±0.6 mm. Аfter 1 years in Group 3 mean MBL 1,08 ±0.6 mm, after 3 years MBL was 1,19±0.2 mm, after 5 years MBL was 1.27±0.3 mm (table 2).

Table 2: 3 GRUPS Implants marginal bone loss (MBL) after 1,3,5 year

In CT observations in patients with different C/I ratio after 5 years of functional load, there were no significant differences in peri-implant marginal bone tissue (Figure 5). 

Figure 5: CT in three models after 5 years functional load, no significant differences in the peri-implant marginal bone tissue were noted

He use of short implants for fixed-denture restoration in patients with edentulous and severe vertical bone atrophy in the posterior regions of the mandible has shown good results.  Short implants cumulative success of implantation after 5 years was 97.8% for group 1, 97.6% for group 2, and 97.3% for group 3. Patients were satisfied with the results of treatment.

Dental implants are widely used for prosthetic rehabilitation in patients with various forms of adentia.42 In recent years, short implants have become a predictable alternative for patients with vertical bone resorption (≤ 8 mm in length) [43-45]. Brown-to-implant ratio (C/I) is an important factor in successful oral restoration with short implants. Analysis of the occlusal load force at the bone-implant interface is important for the success or failure of an implant. One of the factors contributing to bone resorption around the implant body is overload which can lead to bone resorption and complications [46]. Various in vivo methods are used to assess bone stress around an implant. FEM is widely used to study biomechanics in various industries including mechanical engineering, civil engineering and aircraft industry [47]. Computerized Finite Element Analysis (FEM) is widely used in implantology to study the nature of the load on the bone around the implant and to predict the success of implantation in the clinical setting [48-50]. The results of Finite Element Analysis (FEM) by some authors indicate that the C/I ratio is not so important for the effective functioning of short implants and does not affect the loss of alveolar bone around the implant [51-53]. Tawil G et al in his research has shown that short implants are clinically successful regardless of the crown C/I ratio [54].  Schulte et al. compared the survival of single implants with different C/I crown-to-implant ratios and compared it to the crown-root ratio for natural teeth. The implant survival rate was 98.2% suggesting that the guideline used for natural teeth should not be applied to implant-supported restorations [55]. FEA computerized in vitro study, in which it is impossible to fully reproduce the clinical condition and should be supplemented with clinical assessment. 
Long-term in vivo and in vitro studies are needed to objectively assess the effect of C/I ratio of short implants success. In this study, we performed a biomechanical test on short implant implants using 3D finite element analysis to elucidate the distribution of bone stresses around implants with crowns of different heights (crown/implant ratio 1:1, 1.25:1 and 1.5:1, respectively) according to the von Mises stress distribution. The area of bone around the implant was used to compare stress distribution across models. The crown/implant ratio (C/I) is an important factor in the successful restoration of the oral cavity with short implants. FEA is a computerized in vitro study in which it is impossible to fully reproduce the clinical condition. In this study, we performed a biomechanical test on short implants using 3D finite element analysis to elucidate the distribution of bone stresses around implants with crowns of different heights (crown/implant ratios 1:1, 1.25:1 and 1.5:1 respectively) according to the von Mises stress distribution. The bone area around the implant was used to compare stress distribution between models. Maximum underlying stress criteria have been used to evaluate cortical and trabecular bone (brittle materials), these criteria make it possible to distinguish between tensile and compressive stresses. The unit of measurement in this study was the megapascal (MPa). The results of the current study, are consistent with the results of our previous study results and indicate the increase in crown height showed little difference in load at the crown/implant interface with crown/implant ratios of 1:1, 1.25:1 and 1.5:1 respectively [56-59].

Study showed that short implanters are clinically successful regardless the C/I ratio. Use of shorts implants for prosthetic rehabilitation can be considered favorable and well substantiated.

Competing Interest
The author declares that he has no competing Interest. None of the authors have relevant financial relations with a commercial interest.
Funding
The work was not funded.
Ethical Approval and Consent to participate
The study was reviewed and approved by University Ethical Committee (Approval number N12, Date 17.11.2021) and in accordance with those of the World Medical Association and the Helsinki Declaration. 
Consent for publication
Patients were informed verbally and in writing about the study and gave written informed consent. 
Availability of data and materials
All data generated or analysed during this study are included in this published article.