The use of provisional resin implant-supported complete dentures is a fast and safe procedure to restore mastication and esthetics of patients soon after surgery and during the adaptation phase to the new denture. This study assessed stress distribution of provisional implant-supported fixed dentures and the all-on-4 concept using self-curing acrylic resin (Tempron) and bis-acrylic resin (Luxatemp) to simulate functional loads through the three-dimensional finite element method. Solidworks software was used to build three-dimensional models using acrylic resin (Tempron, model A) and bis-acrylic resin (Luxatemp, model B) for denture captions. Two loading patterns were applied on each model: (1) right unilateral axial loading of 150 N on the occlusal surfaces of posterior teeth and (2) oblique loading vector of 150 N at 45°. The results showed that higher stress was found on the bone crest below oblique load application with a maximum value of 187.57 MPa on model A and 167.45 MPa on model B. It was concluded that model B improved stress distribution on the denture compared with model A.
The most common complications for the Branemark protocol, such as peri-implant bone loss, implant fracture, bar fracture close to the most distal implants, loosening, fracture of prosthetic screws and implant failure due to loss of osseointegration, are related to the most distal implants near the suspended elements. The all-on-4 concept (Nobel Biocare, Gothenburg, Sweden), a variation of the Branemark protocol, uses longer and inclined distal implants to reduce the distal cantilever of implant-supported prostheses, thereby reducing flexural strength of the structure and, consequently, overload on implants.1
Implant-supported prostheses fabricated of resin were used in some studies on immediate implant loading,2,3 but the laboratory phase was not conducted to fabricate the metal bars. Some authors used complete removable prostheses and converted them into an implant-supported structure by adding resin to the internal part.4,5
In some cases of immediate loading, resin has some advantages over other materials because it is easier to work with it and costs less. Bis-acrylic resin shows higher values of resistance to tensile, compressive, and flexural forces and elasticity than acrylic resin. In addition, it is easy to apply because the self-mixing application system significantly reduces clinical time compared with the bonding technique using cylinders and incremental application of acrylic resin with a paint brush.6 Another advantage is the flexibility of the material, which favors adaptation of dentures that present distortions. It is also easy to fabricate, substitute and repair. Acrylization without a supporting structure is the fastest and most economical provisional treatment. The short time between the patient's rehabilitation and delivery of the prosthesis lowers the cost of clinical time.6 Finite element analysis has been used to study the properties of materials and the effect of the suprastructure design in the distribution of stress and force at the implant-bone interface.7
Therefore, the aim of this study was to assess maximum stress and stress distribution on the structures of implant-supported fixed complete prostheses using the all-on-4 concept fabricated using self-curing acrylic resin (Tempron, GC, Kasugai, Japan) or self-curing bis-acrylic resin composite (Luxatemp, DMG, Hamburg, Germany) and peri-implant bone by means of the finite element analysis.
Materials and Methods
After the completely edentulous patient signed a consent form, computed tomographic exams were performed on the mandible and complete prosthesis fabricated of a radiopaque material; images were obtained in the DICOM format and were imported into an image-processing program and three-dimensional digital reconstruction software (Invesalius 2.0, Centro de Tecnologia de Informação Renato Archer, Campinas, São Paulo, Brazil). After virtual reconstruction, the models were exported to the Ansys DesignModeler version 11 software (Ansys Inc, Canonsburg, Pa) to edit the virtual models. All the models were built following the sequence of 4 implants and 2 straight anterior and 2 posterior 30°-tilted implants measuring 4.1 mm in diameter and 13 mm long. To design the framework, a tomographic scan of the complete prosthesis replica was used. The features of the complete prosthesis was: distal extension of 15 mm, straight abutment in the anterior implants and 17°-tilted in the posterior implants, titanium cylinder for the mini-tapered abutment in anterior abutments and posterior abutments were added to the titanium cylinder with a distal bar
Two models were built by replacing the material present in the anterior lingual region of the prosthesis to simulate wear and taking a denture impression of the implants using 2 types of resin; the entire anterior lingual area was replaced up to the vestibular and distal limit of 1 mm from the prosthetic components of the implants (Figure 1). The main advantages of this technique are patient satisfaction, less postsurgical discomfort, and recovery of esthetics and chewing function after the surgery.8
The purpose of this study was to analyze the difference in stress distribution of denture impressions using the bis-acrylic resin Luxatemp (model A) compared with denture impressions using the self-curing acrylic resin Tempron (model B).
The mesh was generated with quadratic 10-noded tetrahedral elements (Ansys solid 187, Ansys Inc, Canonsburg, Pa), which enables a copy to be made of the irregular geometry present in the models analyzed. The number of knots was 2 652 773 and the number of elements was 1 731 977. All the materials were considered homogeneous, isotropic, and linearly elastic. All contacts between the structures were considered a perfect bond. Rigid supports were added in the posterior region of the mandible and in the insertion region of the masticatory muscles. Two loading patterns were applied on each model.
The first pattern was a right unilateral load of 150 N at 3 different axial points on the long axis of the anterior implants, which corresponded to the occlusal surfaces of the posterior teeth. A right unilateral load of 150 N was also applied on the second pattern, but the load vectors were oblique at an angle of 45° in relation to the long axis of the anterior implants in the lingual-vestibular direction (Figure 2).
The following 2 types of analysis were performed: qualitative analysis, which verifies distribution and points of higher stress intensity in the plotting, and quantitative analysis, which verifies and compares the numerical intensity of the stress peaks of each region at each variation of the research.
For the analysis of stress distribution in the peri-implant bone, the Von Mises equivalent stress criteria was used and for the resins, the Rankine criteria was used. The plotting of maximum principal stress was used and the stress peaks of the peri-implant bone of each implant were individually analyzed.
Tables 1 and 2 show the stress peaks on the different parts of the models in this research. Note that the stresses during axial and oblique loading were lower in the Luxatemp material, better distributing forces on the bone crest around the implants.
In Tables 3 and 4, tensile stresses on axial and oblique loading were lower with Luxatemp resin. In the repair resin, because the flexural strength of the 2 materials is different, a percentage was calculated to equalize the reference values, as shown in Table 3, in which Luxatemp material surpassed Tempron resin by a small percentage.
When the plotting of the peri-implant bone (Figure 3) was analyzed in the different models, the stress peak was found on the bone crest in all models. Higher stress peaks were found on the peri-implant bone located below the load application; the stress concentrations were lower in the other regions. When comparing the different materials for oblique and axial loading, stress distribution was similar among the peri-implant regions.
In Figure 4, stress distribution on the original self-curing denture resin was higher when submitted to oblique loading; Luxatemp resin showed lower stress concentration. In Figure 5, when stresses were analyzed in the repair/impression resins of the prosthesis, concentration was visibly higher for the Luxatemp material, but when the limits of flexural strength were established, Luxatemp proved to be less susceptible to fracture even though it showed higher stress.
Studying materials used in implant dentistry is imperative to better understand the behavior of these materials and predict their clinical application. In this context, the finite element method has been increasingly more applied in dental research. Therefore, based on the study of Sertgoz,9 the present study compared and studied the effect of force distribution of provisional materials used in implants.
The conversion technique, introduced by Balshi in 198510 , involves the modification of a removable complete prosthesis to a transitional, fixed, implant-supported prosthesis, which may be fabricated soon after surgery. The main advantage of the conversion technique using Luxatemp material for the capture is to eliminate the need to make an impression of the implants after surgery; this avoids the laboratory stage of fabricating the provisional prosthesis, thereby reducing clinical time after surgery and avoids handling of the prosthesis on the recently operated area when the effect of the anesthetic has worn off and pain and the edema stage of tissues have begun.11 The use of a provisional prosthesis during the healing period and later replacement provides the necessary time for the tissues to accommodate and for the clinician to fabricate a definite prosthesis with a metal substructure following the gingival edge.9
The success of using 4 implants to support the fixed complete prosthesis, originally described as the “all-on-4 technique,” has also been described in the literature by such authors as Maló et al,6 Ferreira et al,12 and Khatami and Smith.13 The distal inclination of the implants is to reduce deleterious effects of the cantilever. In this connection, the studies of Sertgöz9 using the finite element method showed that higher stresses were also concentrated on the distal implant on the same loading side and the presence of the cantilever increased stress around implants. However, according to the photoelasticity study of Begg et al,14 15°- and 30°-tilted implants showed little stress difference, whereas 45°-tilted implants may favor occlusal overload. The use of the all-on-4 configuration becomes favorable as the cantilever is reduced or avoided.
Clinical studies show that the application of immediate loading for fixed prostheses with acrylic resin and/or a metal framework that rigidly connects 4 or more implants between the mental foramina region with cross-arch stability is a predictable and well-documented procedure.15 In addition to the fast fabrication after surgery, the benefits that make the acrylic resin use more attractive are the easy use of resins, reduced time of clinical practice, low cost, and immediate delivery of the prosthesis to the patient.16
With both materials, stress on the peri-implant bone with axial and oblique loading were 3 to 4 times higher on the side of the load application, though generating similar stress, and the oblique loading increased stress 2 times. Because repair resins are materials with different values of flexural strength, a direct comparison of the results between Tempron and Luxatemp resins should not be made. However, in accordance with the literature, the mean values found for flexural strength of the materials under 3-point flexural tests were 64.31 MPa (SD 7.29 MPa)17 for Tempron resin and 114.6 MPa (SD 26.6 MPa)18 for Luxatemp resin, which was used as a reference according to the standardized tests of the American Dental Association. Next, a ratio percentage in relation to the results was applied to be equivalent.
Despite the fact that Luxatemp resin provided more rigidity to the structure and therefore presented higher stress peaks, it is less susceptible to fracture than Tempron resin if forces are increased. This advantage may be related to the difference that the different characteristics of the materials. Luxatemp has 2 times the higher flexural strength and modulus of elasticity than Tempron resin, which provides higher rigidity and strength to the structure (ie, twice as much force is needed for the material to suffer the same degree of deformation and return to its original shape when force is removed, but the supporting structures are more protected from these stresses).19 Another important characteristic of the materials is the composition. Tempron contains copolymers of methyl methacrylate, whereas Luxatemp resin has one multifunctional methacrylate matrix. The longer the chain of the polymer, the greater the amount of framework (temporary connections) that can be formed between the chains. Therefore, the longer the chain, the higher the properties of rigidity and strength.20
When comparing the materials from a structural point of view, Luxatemp resin presented a small advantage because it preserved the superstructure from higher stress concentrations and was less susceptible to fracture, which could clinically reduce the chance of success. Furthermore, a material with higher rigidity favors bonding and primary stability of the implants, that is, it reduces the change of causing micromovement of the implants that might be deleterious and harm the osseointegration process. As new materials are developed using different chemical compositions, the material is structurally improved and increasingly better bonding is expected. Some studies used the finite element method to assess the effect of a rigid superstructure, but no studies have assessed the provisional prosthesis without rigid bonding of implants. Although prostheses without a rigid structure are completely safe, their use is indicated only in the healing stage of implants. Therefore, it does not replace the definite denture and no clinical article was found in which the denture was not replaced later. The clinical application of the resin prosthesis shows an acceptable esthetic characteristic and may be easily, safely, and rapidly fabricated after surgery, which provides greater comfort for the patient. A resin prosthesis does not require the taking of impressions of the implants or the laboratory phase. The ease of the application system and, consequently, the reduction of clinical time fabricating the immediate prosthesis suggest that denture impression and filling with Luxatemp resin is feasible.
It may be concluded that Luxatemp resin provided better force distribution on the prosthesis in comparison with self-curing acrylic resin. However, forces generated on the peri-implant tissue on both models studies were the same.