Photoelastic Analysis of Cemented or Screwed Implant-Supported Prostheses With Different Prosthetic Connections

The success of implant-supported restoration and the health of surrounding tissues are related to the accuracy and fit between the components, stability at implant/abutment interface, and the resistance of this interface submitted to masticatory loads.1,2 

The stability at implant/abutment interface may be influenced by several factors such as connection type1,3 and retention system. Therefore, the selection of the retention system is a widely discussed topic that may be determined by subjective preferences. The option for cemented or screwed prosthesis depends on such aspects as reversibility,2,4–6 predictability of retention,2 esthetics, and complexity of laboratory techniques.

Another factor that influences the force transferring to the bone/implant interface is the connection type between the abutment and the implant. The connections can be external or internal.7 The external connection is a joint in the external hexagon of the implant platform, while the internal connection exhibits the implant/abutment interface into the implant in the same manner as the Morse taper connection. However, the tapered design of the mating surfaces generates more accurate fit with the Morse taper connection.3 

Experimental studies with different connection types have demonstrated significant improvement in the performance for internal connection,7–9 mainly for the Morse taper connection.10,11 Some studies evaluated different implant designs,12–14 and other studies compared the screwed and cemented retention systems.2,4,13,15–18 However, there is a lack of research focusing on stress distribution with different implant connections,3,19 and there is no study associating the different connection types and the prosthetic retention systems.

Therefore, the aim of this study was to evaluate the stress distribution of different retention systems (screwed and cemented) associated with different prosthetic connections (external hexagon, internal hexagon, and Morse taper) in 3-unit implant-supported fixed partial dentures through photoelasticity.

Six blocks (44 × 22 × 10 mm) were fabricated with photoelastic resin PL-2 (Vishay Precision Group Inc, Raleigh, NC) that was manipulated according to the manufacturer's instructions. Each model contained 2 implants of 4.0 × 10.0 mm (Conexão Sistemas de Prótese Ltda, São Paulo, Brazil) with the same connection type (external hexagon, internal hexagon, and Morse taper) (Table 1).

Conventional techniques were used to fabricate the fixed dentures in Ni-Cr alloy (Fit Cast–SB Plus, Talladium do Brasil, Curitiba, Paraná, Brazil). A silicone matrix was used to standardize the crowns. The crowns were fabricated on stone casts to avoid tension in the photoelastic models. The prostheses were screwed or cemented with provisional cement (TempBond, Kerr Corporation, Orange, Calif) on the implant in the photoelastic models.

The photoelastic models were positioned in the circular polariscope, and a universal testing machine (EMIC-DL 3000, São José dos Pinhais, Paraná, Brazil) applied axial and oblique (45°) loads of 100 N in fixed points on the occlusal surface of all crowns. The models were positioned in a device with preestablished angulation of 45° for oblique loading.

The results were recorded by a digital camera (Nikon D80, Nikon Corp, Japan) and analyzed with a graphic software (Abobe Photoshop CS3, Adobe Systems, San Jose, Calif) to allow visualization, comprehension, and interpretation of the localization and intensity/concentration of stress distributed surrounding the implants.

Qualitative analysis was used in the present study as suggested by Caputo and Standlee,20 French et al,21 Clelland et al,22 and da Silva et al.23 According to these authors, the number and order of the fringes indicate stress intensity, while the proximity between them represents stress concentration.

Photoelasticity demonstrates the distribution of force in an object by fringe patterns that appear as a series of successive and contiguous different colored bands (isochromatic fringes), and each fringe order is counted by the passage of fringe:

• fringe of order N  =  0 (black)

• fringe of order N  =  1 (transition red/blue) – low intensity

• fringe of order N  =  2 (transition red/green) – medium intensity

• fringe of order N  =  3 (transition pink/green) – high intensity

The initial photo was recorded with no loading to assess the presence of tensions in the photoelastic model. All models were tension-free, which is necessary to avoid reading mistakes during the experiments.

The analysis was divided according to the number of fringes of high intensity (transition pink/green) and the area of stress concentration. All images were evaluated by the same operator.

Table 2 shows models 3 (screwed 3-unit fixed partial denture with Morse taper connection) and 6 (cemented 3-unit fixed partial denture with Morse taper connection) exhibited the highest number of fringes, and models 2 and 5 presented the lowest number of fringes for both axial and oblique loading.

Comparing the models with screwed and cemented prostheses, the models with cemented prostheses exhibited a lower number of fringes. Considering the number of fringes of high intensity, models 3 (screwed 3-unit fixed partial denture with Morse taper connection), 6 (cemented 3-unit fixed partial denture with Morse taper connection), 1 (screwed 3-unit fixed partial denture with external hexagon connection), 4 (cemented 3-unit fixed partial denture with external hexagon connection), 2 (screwed 3-unit fixed partial denture with internal hexagon connection), and 5 (cemented 3-unit fixed partial denture with internal hexagon connection) presented the highest to lowest stress intensity, respectively.

According to models 1 (screwed 3-unit fixed partial denture with external hexagon connection) and 2 (screwed 3-unit fixed partial denture with internal hexagon connection) (Figures 1a and b, and 2a and b), the loading on the premolar, pontic, and molar generated stress concentration on the middle and apical thirds of the implant (premolar), with higher stress when the load was applied on the premolar. The largest area of stress concentration between the implants occurred with loading on the pontic.

In model 3 (screwed 3-unit fixed partial denture with Morse taper connection) with loading on the premolar and pontic (Figure 3a and b), a large area of fringe concentration was observed surrounding the body of the implant (premolar), mainly from the middle to the apical third, and a small area of stress at the apex of the implant (molar). When the load was applied on the molar (Figure 3c), the stress was distributed from the middle to the apical third of both implants with a larger area in the implant (molar). The largest area of stress between the implants was observed with loading on the premolar.

Comparing the 3 models (Figures 1 through 3), model 3 (screwed 3-unit fixed partial denture with Morse taper connection) exhibited the highest concentration of stress.

In models 1 (screwed 3-unit fixed partial denture with external hexagon connection) and 2 (screwed 3-unit fixed partial denture with internal hexagon connection) (Figures 4a and b, and 5a and b) with loading on the premolar and pontic, the stress was distributed in the photoelastic model with concentration in the apical third of the implant (premolar) and a small area at the apex of the implant (molar). For the loading on the molar (Figures 4c and 5c), the number of fringes increased and was concentrated at the side opposite to the load.

In model 3 (screwed 3-unit fixed partial denture with Morse taper connection) with loading on the premolar and pontic (Figure 6a and b), stress concentration was observed in the middle and apical thirds of the implant (premolar) with lower intensity surrounding the body of the implant (molar). For the loading on the molar (Figure 6c), the stress was concentrated in the middle and apical thirds of the implant, with greater quantity at the side opposite to the load. A large area of stress was observed between the implants with loading on the premolar.

Comparing the 3 models (Figures 4, 5, and 6), the highest area and concentration of stress were shown in model 3.

In models 4 (cemented 3-unit fixed partial denture with external hexagon connection) and 5 (cemented 3-unit fixed partial denture with internal hexagon connection) (Figures 7a and b, and 8a and b) with loading on the premolar and pontic, stress was concentrated in the middle and apical thirds of the implant (premolar), with a small area of stress concentration at the apical level of the implant (molar). For the loading on the molar (Figures 7c and 8c), stress was concentrated surrounding the body of the implant with the highest intensity at the apex.

In model 6 (cemented 3-unit fixed partial denture with Morse taper connection) (Figure 9a, b, and c) with loading on the premolar, stress of high intensity was observed surrounding the body of the implant (premolar) and a small area at the apex of the implant (molar). For the loading on the pontic, stress of high intensity was concentrated in the apical region of both implants. For the loading on the molar, a large area of stress concentration was noticed surrounding the body of the implant and a small area of stress was exhibited at the apex of the implant (premolar).

Comparing the 3 models (Figures 7, 8, and 9), model 3 (screwed 3-unit fixed partial denture with Morse taper connection) presented the highest area and concentration of stress.

In models 4 (cemented 3-unit fixed partial denture with external hexagon connection) and 5 (cemented 3-unit fixed partial denture with internal hexagon connection) (Figures 10a and b, and 11a and b) with loading on the premolar and pontic, stress concentration was exhibited in the middle and apical thirds of the distal region of the implant (premolar) and a small area of stress of low intensity was observed at the apex of the implant (molar). For the loading on the molar (Figures 10c and 11c), a large area of stress surrounding the body of the implant and a higher number of fringes concentrated at the side opposite to the load were observed.

In model 6 (cemented 3-unit fixed partial denture with Morse taper connection) (Figure 12a and b) with loading on the premolar and pontic, stress was concentrated in the distal apical third of both implants. This stress was higher in the implant (molar) for the loading on the pontic. In the molar, the stress was concentrated surrounding the body of the implant (molar) with greater concentration and intensity at the side opposite to the load.

Comparing the 3 models (Figures 10, 11, and 12), the highest area and concentration of stress were noticed in the implant (molar) for the loading on the molar.

Considering the results obtained with the screwed and cemented prostheses for both axial and oblique loading, the cemented prostheses presented a more favorable situation for stress distribution. This result is in accordance with the laboratory and clinical research15,16 that compared the biomechanical behavior of multiple cemented and screwed prostheses. This may result from the damping effect of the cement between the prosthesis and the abutment.

Opposite results were observed by Weber et al18 in a 3-year prospective study evaluating the peri-implant condition and esthetics of cemented and screwed prostheses. The authors reported that the peri-implant tissues surrounding the screwed implant-supported prostheses exhibited better condition than those surrounding the cemented prostheses, and there was no recession of soft tissue for both prostheses. Although the cemented implant-supported prostheses were preferred by the dentists, the patients reported no preference. However, this study does not allow analysis of stress transference at the peri-implant interface.

Considering the stress distribution with different implant connections, the internal connection generated better results than the external and Morse taper connections in the present study. This fact results from the stability at the implant/abutment interface with the internal connections that exhibit contact between the surface of the abutment and the internal surface of the implant, which decreases the micromovement during loading.7,9,24 

Nevertheless, Çehreli et al,3 using photoelasticity and strain gauge methodologies for external, internal, and Morse taper connections under axial and oblique (20°) loading, did not find differences between the systems for both loadings. In this study, there was no consistency due to the different sizes of the implants (length, diameter), shapes (conical and cylindrical Branemark, conical and cylindrical Astra Tech implants, and ITI solid screw implant), surface (machined, turned surface, TiO₂-blasted, and blasted large-grid acid-etched), and thread design (microthreads in the collar region, V-thread, buttress thread), which may have influenced the results.

For the Morse taper connection, the results indicated better stress distribution according to a biomechanical standpoint. Studies with different methodologies10,14,25 evaluating the external and Morse taper connections concluded that the Morse taper connection supports and distributes lateral forces better than the implants with external connection. However, these studies evaluated single prostheses instead of multiple prostheses as in the present study. This difference may have modified the biomechanical behavior of the Morse taper connection due to splinting of implants, which generated higher stress at the bone/implant interface. Although the Morse taper connection seems to be the best option for single prostheses, additional studies are necessary to assess the stress distribution in fixed dentures.

Considering the load type, the results of the present study demonstrated increased stress for oblique loading for all systems tested, which is in agreement with previous studies.1,7,24,26 In addition, there was higher torque at the prosthesis/implant/bone assembly.26,27 High stress concentration was observed at the side of the load at the cervical level in all models with oblique load. This indicates a compressive area that was also observed by the studies of Çehreli et al3 and French et al21 using photoelasticity. However, clinically the bone is strongest under compressive forces, weaker under tensile loads, and even weaker yet in shear loads.27 

According to the methodology, it was concluded that:

• The cemented implant-supported prostheses exhibited better distribution and lower intensity of stress.

• The internal hexagon connection generated better stress distribution for both prostheses types.

• Stress increased with oblique loading in all models.

Foundation to Support to Research of the State of Sao Paulo – FAPESP, Process Number: 07/55061-3. Conexão Sistemas de Prótese Ltda.