Comparison of stress distribution in a three unit cement retained implant supported prosthesis with different implant abutment connections: A photoelastic analysis

Introduction

Implant abutment connection is the joint between the implant and abutment. A study of the implant abutment connection is of paramount importance as it is primary determinant of strength and stability of the implant supported restoration, which in turn determines the prosthetic stability.

To overcome the inherent deficiencies of original external hex connection, internal connection implants were developed. The goal of new designs was to improve the implant connection stability throughout the function.¹

Internal connection implants can be further divided into passive fit joint, where in space exists between mating components and friction fit (Morse taper) where no space exists between the mating components. Passive fit joint includes internal hexagonal connections and internal octagonal connection. Internal hexagonal connection is the most common type of commercially available internal implant abutment connection. It has six sided geometric figure, that is, hexagon recessed into the body of the implant. As the internal geometry is a hexagon, the abutment can fit over the implant at every 60 degree rotation of the implant over the abutment but not at any intermediate angle. Thus the abutment positioning is possible at six different positions of implant over the abutment.²

Hence, in this in vitro study, internal hexagonal connection was compared with Morse taper connection for stress distribution through photo elasticity.

The internal connection design offers several advantages like reduced vertical height platform for restorative components, distribution of lateral loading deep within the implant, a shielded abutment screw, long internal wall engagements that creates a stiff and unified body, which resists the joint opening. The concept of Morse taper is a true Morse taper implant with an angle of taper 1.5 degree, in the implant. There is a friction fit and cold welding at the implant abutment interface.¹

Many photo elastic studies have been done to compare the stress distribution on implants with different implant abutment connections. Although, previous studies have compared the stress distribution around different implant abutment connections in implant supported cement retained three unit fixed partial denture, there is a lack of research, focussing on stress distribution around different implant abutment connections in implant supported three unit fixed partial denture. Therefore, the purpose of the present study, is to evaluate and compare the stress distribution, in three unit cement retained implant supported fixed partial denture with internal hexagonal implant abutment connection and three unit cement retained implant supported fixed partial denture with Morse taper connection using photo elasticity.³

Materials and Methods

This in vitro study was conducted in the Department of Prosthodontics, Crown and Bridge including Implantology, M.R. Ambedkar Dental College and Hospital, Bengaluru. Photo elastic stress analysis was done in Mechanical and Engineering department, Indian Institute of Science, Bengaluru. For this in vitro study, moulds for two photo elastic resin models were printed from poly amide material, based on scanned picture of the block, of standard dimensions of (44mm x 22mm x 10mm). The moulds of standard dimensions were obtained by 3D printing. Group I samples consisted of three unit implant supported cement retained fixed partial denture with Internal hexagonal implant abutment connection. (Paltop Advanced, Keystone Dental Company, US). Group II samples consisted of three unit implant supported cement retained fixed partial denture with conical 1.5 degree Morse taper connection. (Paltop Conical Active, Keystone Dental Company US).The scanned data was converted to digital file format [Surface Tesellation Language (STL)].

The moulds were subsequently made using Fused Deposition Modelling (FDM) through 3D Printing and moulds,for Group I and Group II samples were obtained. Then, moulds were processed, and were subjected to sand blasting, jet washing and grinding Implant positioning guides were designed in CAD software and subsequently made by Fused Deposition Modelling. They were designed to hold the implants in the moulds. Implants with Internal hexagonal implant abutment connection (Paltop Advanced, Keystone Dental Company, US) and implants with Morse taper connection (Paltop Conical Active, Keystone Dental Company, US) were placed at the distance of 13mm using implant positioning guides ,for three unit implant supported cement retained prosthesis, in moulds of Group I and Group II samples respectively. The position of implants simulated missing mandibular second premolars, first molar and second molar, with first molar acting as pontic (Kennedys class I partially edentulous space). The material used for fabrication of photo elastic model was Epoxy resin C-51 (3222) resin and K6 hardener. The resin and hardener were mixed in the ratio of 10: 1 and blended with low speed, to avoid trapping of air bubbles. The mixture was poured in the prepared moulds, with the implant positioning guides, for Group I and Group II samples and allowed to cure for 6 hours. After curing, Photo elastic models for Group I and Group II samples were retrieved from their respective moulds, and further finishing and polishing was done (Fig2).Standard abutments for implants with internal hexagonal connection of dimensions 8mm and standard abutments for implants with Morse taper connection of dimension 8mm, were tightened with the help of hex, and torque to 30 N cm. The impressions were made for Group I and Group II samples with rim lock stock trays using polyvinyl siloxane impression material and were poured with type IV Die stone. The casts were retrieved. Wax patterns were fabricated for full metal (cobalt chromium alloy) fixed partial prosthesis. Immediately, wax patterns were invested, and casting was done. Three unit metal fixed partial denture frameworks were retrieved, trimmed and polished. Full metal three unit fixed partial prosthesis for Group I and Group II were seated on their respective abutments and fit was checked. After that Zinc phosphate cement was mixed in luting consistency and one third of the crowns were filled with cement and seated on their respective abutments of Group I and Group II. Care was taken that no excess cement residue was left on respective samples. (Fig 3)

Stress Analysis

Two jigs were fabricated (for axial and oblique loading) to enable stable positioning of the photo elastic model, on the Circular Polaris cope (Fig 1 ). A biaxial servo hydraulic frame was used to apply maximum static loads of 100 N in the second premolar, first molar and second molar region, in both vertical and 45-degree oblique directions, using a universal testing machine (EMIC-DL 3000, Universal Test System). The prostheses were loaded under 100 N for 10 sec for every test, after which the model was relieved from the stresses and loaded again for the next test. As the setup was standardized for all the loadings, ten tests were done for the Group I samples and ten tests were done for the Group II samples. Monochromatic light of wavelength 565nm was emitted from the light source which in turn, passed through the photo elastic model.

Images were captured for each loading, with a digital camera (Canon EOS1300D) with a resolution of 5,184 x 3,456 pixels, while the prostheses were under load for 10 secs. The changes in coloured fringe patterns on load application were captured. The fringe orders were determined by tardy method of compensation. 5 points were considered for determining the stresses- mesio-cervical, mid-mesial, apical, mid-distal disto-cervical at implant bone interface at premolar region, pontic region and molar region. Stresses at the marked points were determined using the stress-optic law. Stress distribution data were generated for Group I and Group II samples under both axial and oblique loading. Mean was taken for the stress values at reference points, for comparison between the two groups, respectively (Fig 4,5,6,7).

Results

Comparison of stress distribution for Internal hexagonal connection and Morse taper connection in a three unit implant supported cement retained prosthesis under axial loading, showed that, there was significant difference in stresses between the Group I samples and Group II samples, under axial loading, at premolar, pontic and molar area, (Table 1).

Comparison of stresses generated under oblique loading, showed that, there were no significant differences between Group I samples and Group II samples, at premolar, pontic and molar area. Stresses increased in both groups under oblique loading, (Table 2).

Comparison of stresses generated in Group I sample, under axial and oblique loading have shown, more stresses under oblique loading but that was not statistically significant, (Table 3). Comparison of stresses generated in Group II sample, under axial and oblique loading has shown no significant differences in stresses. Increased stresses were seen under both axial and oblique loading, (Table 4).

Results revealed that magnitude of stresses were lower in three unit cement retained implant supported prosthesis having Internal hexagonal implant abutment connection under axial loading. Stresses were seen more in apical region. Under oblique loading, there was no significant difference seen between two groups. Stresses were seen more in the direction of application of loads and the apical area.

Within the limitations of this in vitro study, it can be concluded that the stresses under axial loading were more for Morse taper connection in three unit fixed partial prosthesis when compared to internal hexagonal connection, while there was no difference under oblique loading. However, there was uniform stress distribution for both groups.

Discussion

Implantology has made it possible for osseointegrated implants to be used in partially edentulous patients. Despite the high success rate of implant supported prosthesis, failures are still observed. Implant failures can largely be classified into four main categories: Loss of integration, Positional failures, Soft tissue defects, and biomechanic failures. Prosthesis failure is mainly due to the biomechanical aspects, such as the number of implants, location and angle of the implants, and the type of implant abutment connection.⁴

To improve stress distribution between the implant and the bone, several implant connection systems have been introduced. Some basic implant abutment connections are passive fit connections, where space exists between the implant and abutment like internal hexagonal,internal octagonal connections. The most recent development is Morse taper connection, where there is no space between implant and abutment. Morse taper connection is friction fit connection with different implant abutment angles like 11.5, 8, 1.5 degrees.¹

Studies have shown, internal connections are easier for abutment attachment and provide protection for the abutment screw because abutment walls contact the internal surface of the implant, which reduce micro movements and complications such as screw loosening and fracture during loading. Internal connections also present high stability and anti rotational mechanism due to wide connection area, greater resistance to lateral loads as a result of low rotational centre which generates better stress distribution.⁵

Among Internal connections, internal hexagonal connection has the advantage of distribution of axial as well as lateral loads effectively. Axial loads acting on the implant are distributed to the step plateaus, whereas lateral forces are dissipated to the enveloping surfaces. The internal hexagon connection provides a 60 degree indexing and rotational resistance.¹

The Morse taper configuration has been described as stable, strong and appropriate for transference of lateral loads. This connection type protects the screw, increases the resistance to loosening due to gear extension and taper angulation, and generates an interface of bacterial sealing. The mechanical design of Morse taper connection is significant for maintenance of the bone implant interface not only for biomechanics and stress distribution, but also biologically, considering the bacterial leakage at the connection interface. All these factors may influence the long term success with implant rehabilitation.⁶ Morse taper connections are incorporated with different implant abutment angles like 11.5, 8, 1.5 degrees. 1.5 degree is a true Morse taper connection where implants connects to abutment by friction fit and cold welding.6 Hence, in present study three unit implant supported cement retained prosthesis with Internal hexagonal connection and three unit implant supported cement retained prosthesis with Morse taper connection are compared for stress distribution.

According to the studies, the analysis of the single implant prosthesis revealed the superiority of the Morse taper connection that exhibited lower stress concentration and intensity under axial loading. Pellizer et al, did the study that concluded Morse taper implants presented more favourable stress distribution followed by Internal hexagonal connection in single unit implants while external hex connection showed highest stress concentrations.⁷

However, the evaluation of models with 3 unit implant supported cement retained prosthesis revealed that the Morse taper connection presented the highest stress concentration and intensity. As there is lack of literature regarding the stress concentration in splinted or three unit implant supported fixed partial denture, the purpose of the present study is to compare stress distribution in three unit cement retained implant supported prosthesis with Internal hexagonal implant abutment connection and three unit implant supported cement retained prosthesis with Morse taper implant abutment connection through photo elasticity.

The photo elastic method consists of investigating the distribution of stresses in transparent bodies with the assistance of polarised light. When the polarised light is passed through a photo elastic material, it produces the phenomenon of optical interference and fringe pattern is obtained. By studying the fringe pattern, one can determine the state of stress in the material. Photo elastic analysis provides good qualitative information about localisation and concentration of stresses. This method is useful in evaluating the biomechanical behaviour of implants and can approximate the actual clinical situation.⁸

The situation of missing teeth simulated a classic situation of a class I Kennedy partially edentulous space in mandible, where patients routinely seek dental professionals with the goal of installing implants in the edentulous region, eliminating free-end removable partial dentures.⁸

The retention system between fixed partial denture and osseointegrated implants, could be screwed or cemented or by combination of two methods. The cemented FPD has the advantage of reproducing the gingival contour, aesthetics, favourable stress distribution.

Guichet et al reported that presence of cement acts as the dissipater of stress. Cemented FPDs have proven to have better stress distribution from biomechanical point of view as presence of cement between FPD and abutment compensates for the misfit that FPD may have acquired during casting process. However it is believed that presence of cement in sulcus may cause inflammation.⁹

Two jigs were fabricated (for axial and oblique loading) to enable stable positioning of the photo elastic model on the Circular polariscope. A biaxial servo hydraulic frame was used to apply maximum static loads of 100 N in the second premolar, first molar and second molar region, in both vertical and 45-degree oblique directions, using a universal testing machine (EMIC-DL 3000, Universal Test System).

The 100N load selected represents a load relative to a standard bite force for a patient with an implant supported FPD and it was a load that the photo elastic model could repeatedly withstand without deforming. It corresponds to the average chewing force required for most food types. Stresses at the marked points were determined using the stress-optic law. Stress distribution data were generated for both models under both axial and oblique loading. These data were averaged for the reference points for comparison between the two groups respectively.¹⁰

In Group I sample, for loading on premolar and pontic, stresses were concentrated at medial and apical third of implant body (premolar). For loading on pontic stresses were seen on apical thirds of both implants. For loading on molar stresses were more in medial third towards apical third of the implant molar.

In Group II sample, with loading on premolar and pontic, stresses were concentrated around implant body and apex. For loading on molar stresses were seen on medial and apical thirds of the implants.

While comparing the stress generation in model having implants with internal hex connection (Group I) under axial and oblique loading, stresses under oblique loading were more but that was not statistically significant. The increase in stresses under oblique loading may be due to lateral forces on the bone due to oblique loading.

While comparing the stress generation in models with Morse taper connection under axial and oblique loading, no significant difference was seen. However, increase stresses were seen under both axial and oblique loading.

This study had few limitations too. The condition of the contact between the implant and the photoelastic resin, which simulates the bone tissue was considered 100 %, therefore this did not simulate accurately the real situation of osseointegration. The photo elastic analysis is a complex procedure and quantitative data derived can be subjected to human error. Lastly, this in vitro study was limited in its ability to replicate clinical occlusal forces.

Conclusion

Within limitations of this in vitro study, it can be concluded that Internal hexagonal connection showed less stresses as compared to Morse taper connection in three unit implant supported prosthesis. However, stresses were uniformly distributed in both groups, Internal hexagonal connection and Morse taper connection in three unit implant supported cement retained prosthesis.

Data availability statement:

Data used to support the findings of this study are included within the article. The prior studies have been cited at relevant places within text as references. The study was done in Indian Institute of Sciences, Bangalore. There was no funding involved in the study.