Hamid Yazdanpanah Asmarz
EVALUATION OF INTERNAL SURFACE ALTERATIONS IN MORSE TAPER FRICTIONAL IMPLANT (FGM) AND THE REMOVAL TORQUE OF THE PROSTHETIC COMPONENT AFTER MECHANICAL LOADING ASSOCIATED OR NOT TO
BACTERIAL BIOFILM (IN VITRO)
Dissertação submetida ao Programa de Pós-graduação em Odontologia da Universidade Federal de Santa Catarina para a obtenção do título de Mstre em Odontologia
Orientador: Prof. Dr. César Augusto Magalhães Benfatti.
Florianópolis 2019
Hamid Yazdanpanah Asmarz
EVALUATION OF INTERNAL SURFACE ALTERATIONS IN MORSE TAPER FRICTIONAL IMPLANT (FGM) AND THE REMOVAL TORQUE OF THE PROSTHETIC COMPONENT AFTER MECHANICAL LOADING ASSOCIATED OR NOT TO
BACTERIAL BIOFILM (IN VITRO)
Esta Dissertação foi julgada adequada para obtenção do Título de Mestre e aprovada em sua forma final pelo Programa de Pós-Graduação
em Odontologia.
Auditório da pós-graduação do CCS, 25 de Junho de 2019. ________________________
Prof.ª Dr.ª Elena Riet Correa Rivero Coordenadora do Curso Banca Examinadora:
________________________ Prof. Dr. César Augusto Magalhães Benfatti
Universidade Federal de Santa Catarina
________________________ Prof. Dr. Ricardo Souza Magini Universidade Federal de Santa Catarina
________________________ Prof. Dr. Bernardo Born Passoni
Este trabalho é dedicado aos meus colegas de classe e aos meus queridos pais.
AGRADECIMENTOS A Deus, o criador da vida.
À Minha família, especialmente meus pais, que sempre me apoiaram e me incentivaram.
Aos meus professores, que me deram a oportunidade de estudar e aprender coisas novas,
em particular o Prof. Cesar Augusto Magalhães Benfatti.
A meus colegas de pós-graduação, trabalhamos juntos e crescemos profissionalmente, em particular Dr. Abrão Prado Morateli e studante de Doutorado Mariane Beatriz Sordi.
RESUMO
Desgaste no nível da interface implante-pilar devido à instalação do pilar e carga mecânica é inevitável. O objetivo deste estudo in vitro foi avaliar o torque de remoção e os aspectos morfológicos de desgate das superfícies interna de implante cone morse friccional após caregamento mecânico com/sem contato com biofilme. Métodos de estudo: Foram utilizados trinta conjuntos de implantes cone morse e pilares protéticos (FGM), divididos em seis grupos(n=5): Grupo A, sem contato com biofilme, G B, contato com biofilme, G C, 100,000 ciclos de carga mecânica sem contato com biofilme, G D, 100,000 ciclos de carga mecânica e contato com biofilme, G E, 500,000 ciclos de carga mecânica sem contato com biofilme, G F, 500,000 ciclos de carga mecânica e contato com biofilme. Os implantes foram embutidos em resina acrylica transparente através de um padrão para instalar todas as amostras iguais. Os pilares foram ativados através de 3 batidas com o martelete preconizado pela própria FGM. As amostras dos grupos B, D e F ficaram imersos na cultura bacteriana em TSB. Para evitar cargas oblíquas durante a instalação e medição do contra-torque cada bloco das amostras de resina acrílica for colocado em um dispositivo de suporte. Carga mecânica foi realizado a uma força de 80±15 N a 2 Hz à temperatura ambiente. Após a avaliação do torque de remoção do pilar (contra-torque), as superfícies dos implantes foram avaliadas por microscopia eletrônica de varredura (MEV) e por perfilometria óptica. Os resultados foram analisados estatisticamente a um nível de significância de p <0,05, utilizando o software SPSS 17.0 for Windows (Chicago, IL, EUA).
Resultados: A avaliação do contra-torque, os valores médios de torque de remoção aumentaram para as amostras com carga mecânica em comparação das amostras sem carga mecânica. O torque de remoção foi maior nas amostras que estavam sob carga mecânica e simultaneamente em contato com biofilme. Áreas de desgaste nas superfícies de conexão com e sem biofilme foram identificadas por MEV. A análise de desgaste revelou que a diferença entre as amostras dos grupos A e B, sem carga mecânica, não foi estatisticamente significativa. O dano da região da borda da superfície interna dos implantes reduziu em amostras imersas no meio de biofilme durante o carregamento mecânico cíclico. Além disso, aumentar o número de ciclos de carga em amostras com contaminação por biofilme, tanto o terço inferior como o terço médio da superfície interna apresentaram menor taxa de desgaste.
Conclusões: Em base nos resultados do presente estudo, a presença de biofilme não diminui o torque de remoção no sistema cone morse friccional. A análise de desgaste revelou que o biofilme oral não diminui os valores de taxa de desgaste e rugosidade ao longo de toda a superfície interna dos implantes cone morse friccional.
Palavras-chave: conexão implante-pilar, biofilme, desgaste.
ABSTRACT
Wear at the level of the implant-abutment interface due to the installation of the abutment and mechanical loading is unavoidable. The objective of this in vitro study was to evaluate the removal torque and the morphological aspects of wear of the internal surface in morse taper frictional implant after mechanical loading with or without biofilm contamination.
MATERIALS AND METHODS: Thirty sets of morse taper implants and prosthetic abutments (FGM) were divided into six groups (n=5): Group A, without biofilm immersion, G B, with biofilm immersion, G C, 100,000 cycles of mechanical loading without biofilm immersion, G D, 100,000 cycles of mechanical loading with biofilm immersion, G E, 500,000 cycles of mechanical loading without biofilm immersion, G F, 500,000 cycles of mechanical loading with biofilm immersion. The implants were embedded in transparent acrylic resin through a pattern to install all samples equally. The abutments were activated through 3 taps with the special mallet recommended by the FGM. The samples in groups B, D and F were immersed in the medium culture of TSB. To avoid oblique loads during the installation and counter-torque measurement, each block of the acrylic resin samples were placed in a metallic supportive device. Mechanical loading was performed at a force of 80±15 N with a frequency of 2 Hz at room temperature. After removal torque evaluation, internal surface of the implants were evaluated by scanning electron microscopy (SEM) and optical profilometer. The results were statistically analyzed at a significance level of p <0.05, using the software SPSS 17.0 for Windows (Chicago, IL, USA).
Results: Evaluation of counter-torque revealed that the mean values of removal torque increased for the samples with mechanical loading compared to the samples with no mechanical loading. The removal torque was higher in samples that were under mechanical load and simultaneously in contact with biofilm. Wear areas on the connection surfaces with and without biofilm were identified by SEM. Wear analysis revealed that the difference between groups A and B samples, without mechanical load, was not statistically significant. The damage of the border region on the internal surface of the implants reduced in samples immersed in the biofilm medium during cyclic mechanical loading. Moreover, by increasing the number of load cycles in samples with biofilm immersion, both the border and middle third parts of internal surfac showed lower wear rate.
Conclusions: Base on the present study results, presence of biofilm does not decrease the removal torque in morse taper frictional system. Wear analysis revealed that oral biofilm does not decrease the wear rate and roughness values along the whole internal surface of morse taper frictional implants.
Key words: implant-abutment connection, biofilm, wear.
LIST OF FIGURES
Figure 1. Mean and standard deviation of the counter-torque evaluation of morse taper components: with and without mechanical axial load associated or not to bacterial biofilm……….……….40 Figure 2. SEM micrographs of Morse taper implant before I-A connection, obtained by secondary electrons (SE) mode at 15 kV: A. Border to apical internal surface of implant. B and C. Intact border and middle part of internal surface ………...42 Figure 3. SEM micrographs of group A samples obtained by secondary electrons (SE) mode at 15 kV: after I-A disconnection. A. Border to apical internal surface of implant. B and C. Trivial wear area of the apical third part of implant……….42 Figure 4. SEM micrographs of group B samples obtained by secondary electrons (SE) mode at 15 kV: after I-A disconnection. A. Border to apical internal surface of implant. B. and C. Limited wear area along the implant internal surface………..43 Figure 5. A. SEM micrographs of group C samples after I-A disconnection, obtained by secondary electrons (SE) mode at 10 kV: B middle and C. apical third with wear area………...…….…..44 Figure 6. SEM micrographs of group D samples obtained by secondary electrons (SE) mode at 15 kV: after I-A disconnection. B. Remarkable wear area on the middle region in comparison to A. the border and C. apical third region………...44 Figure 7. SEM micrograph of group E samples obtained by secondary electrons (SE) mode at 10 kV: after I-A disconnection. A. and B. Wear area from border to apical region………...45 Figure 8. A. SEM micrographs of group F samples obtained by secondary electrons (SE) mode at 10 kV: after I-A disconnection. B. and C. Wear with lower intensity on the internal surface in comparison to group E samples ………45 Figure 9. Mean and standard deviation of roughness value evaluation at the border third part of morse taper implant. A. arithmetical roughness (Ra) values B. maximum distance from peak to valley (Rt) values…...49 Figure 10. Mean and standard deviation of roughness value evaluation on the middle third part of morse taper implant. A. arithmetical roughness (Ra) values B. maximum distance from peak to valley (Rt) values……….…...50 Figure 11. Mean and standard deviation of roughness value evaluation at the apical third part of morse taper implant. A. arithmetical roughness
(Ra) values B. maximum distance from peak to valley (Rt) values……….…….51
LIST OF TABLES
Table 1. The specifications of the dental implant system used in this study and test condition information related to each group…………...36 Table 2. Removal torque values recorded on morse taper implant-abutment connections. ………...40 Table 3. Results obtained from optical profilometer for morse taper implant-abutment connections considering the arithmetical roughness (Ra) values………..47 Table 4. Results obtained from optical profilometer for morse taper implant-abutment connections considering the maximum distance from peak to valley (Rt) roughness values………..…………48
LIST OF ABBREVIATIONS AND ACRONYMS
SEM-SE Scanning Electron Microscope-secondary electrons mode I-A Implant-Abutment
MT Morse Taper EH External Hexagon
SUMMARY 1. Introduction...17 2. Review of literature...23 3. Objetives...31 3.1. General objective...31 3.2. Specific objectives...31
4. MATERIALS AND METHODS...33
4.1.MATERIALS...33
4.1.1. Sample preparation………...33
4.1.2. Biofilm growth...33
4.2. METHODOLOGY………...35
4.2.1. Removal torque evaluation...36
4.2.2. Analysis the Implant internal surface………...36
4.2.3. Statistical analysis.………...………....39
5. Results ...39
5.1. Removal torque evaluation...39
5.2. Surface analysis………...41
6. DISCUSSION...53
7. CONCLUSION...57
Acknowledgements...59
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1. INTRODUCTION
Dental implant treatment is successful if provides an equilibrium between the mechanical and aesthetic aspects (JUNQUEIRA et al., 2013). In the field of oral rehabilitation, especially dental implants, several factors influence the outcome of treatment throughout a long period, including periodontal and supportive bone conditions, biofilms accumulation, force of occlusion and overload and (BROGGINI et al., 2006; HECHMANN et al., 2006).
The failure of dental implant treatment is multifactorial, as the oral cavity is a complex environment of mechanical and biological actions. It is not possible to consider fixed conditions for the oral cavity as a multifunctional organ. Force of occlusion, temperature, pH, saliva composition, oxygen extent and resident microorganisms are changing and certainly influence the intraoral structures , both natural tissues and restorative materials which are synthetic (PEREIRA et al., 2015). Different types of implant-abutment (I-A) connections are available, including external hexagon (EH), internal hexagon and morse taper (WANG and FENTON, 1996). All types of these connections have mechanical characteristics such as fatigue and wear resistance. These characteristics are important because they can directly affect the internal surface of the I-A connection (RICOMINI FILHO et al., 2010; HANSSON., 2000).
The wear resistance is related to the mechanical characteristics of the materials that the implant system is manufactured. Therefore, wear of the surface at the I-A connection over a long period of mastication force results in a decrease in the mechanical integrity of the implant component. The stability of the I-A joint is different between the external / internal hexagon and the morse taper (MT) system. In the internal and external hexagon, the stability mainly is the result of the friction between the abutment screw and the inner surface of the implant after torque application. Torque application, causes wear at the I-A connection level, which results in plastic deformation and may be aggravated by force of occlusion (ASSUNÇÃO et al., 2011).
In the MT system, a conical surface contact of the inner part of the implant and the outer surface of the abutment lead to friction between them. Thus, the probability of torque loss and occurrence of instability at the I-A connection level decreases (VIANNA et al., 2013). A morse taper frictional implant has a tapered angle from 1 ° to 2 ° at the contact level of the implant and abutment without any screw. In this system, force limited taps are required to connect the related abutment to
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the implant. Friction plays an important role in this type of connection. The wide contact area between two adjacent surfaces, the inner surface of the implant and the external surface of the abutment prosthetic part besides a small angle between them results in a high contact pressure in this system and, finally, the friction increases (ALVES et al., 2016). In the MT system, the diameter of the abutment is slightly larger than the implant hub which causes friction resistance at the I-A interface and the wedge effect (BOZKAYA and MUFTU, 2003). The axial displacement of the abutment occurs in two stages in conical connections. First, at the time of abutment connection to the implant, secondly at the time of applying axial load such as mastication. In the oral environment, mastication through the wedge effect and settling of the abutment increases the friction and anti-rotational stability in the conical connections (YAO
et al.,
2019).On a microscopic scale, two coupling walls are not completely smooth and have irregularities and roughness, the high contact pressure at the I-A coupling level results in cold-welding (RABELO et al., 2015; SANNINO et al., 2013).
The FGM implant system takes advantage from an internal taper connection with a 1.5 degree taper angle between the implant and abutment surfaces. In this type of connection, the friction between the implant and the abutment unites them perfectly without any screws. The titanium alloys (Ti6Al4V, ELI, Grade 5) and stainless steel (18Cr14Ni2.5Mo) are used in the construction of FGM Implant and abutment, respectively. In Ti6Al4V alloy, vanadium plays a role of corrosion resistance, while aluminum increases the strength of the alloy and reduces its weight. (SYKARAS et al., 2000) In nickel-based alloys, chromium plays a role in the formation of oxide layer to protect the alloy from corrosion. In addition, molybdenum is another metal that increases the corrosion resistance of the alloy (BRUNE., 1996). The elastic modulus of the Ti alloy is approximately 110 GP, while stainless steel has an elastic modulus greater than 200 GP (GONZÁLEZ et al., 2012). Therefore, the probability of wear of Ti alloy is high when it has a close contact to stainless steel(SERHAN et al., 2004; SEOL et al., 2015).
The mastication force causes micro-movements at the level of the I-A connection(KOUTOUZIS et al., 2014). Micro-movements result in wear and corrosion of the materials of the implant structure. Thus, the probability of formation of microgaps at I-A connection level is high (PEREIRA et al., 2015). Micro-movements can cause pumping effect
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and facilitate the leakage of bacteria through the I-A interface (KOUTOUZIS et al., 2014).
Studies have shown that conical connection systems compared to external / internal hexagon systems could reduce micro-movement and microgaps significantly at the level of the I-A interface, even if they did not eliminate micro-movement and microgaps (CARICASULO et al., 2018; SCHMITT et al., 2014).
The microgap at I-A connection can act as a reservoir of bacteria, therefore, its impact on long-term biological success of implant treatment could not be neglected. The microgaps act as retentive areas for dental plaque. The infiltration of bacteria through microgaps is directly related to the disease of the peri-implant tissues (PEREIRA et al., 2015, GIL et al., 2014).
A mature dental biofilm is a combination of microbial mixture and extracellular matrix composed of polysaccharides, proteins, nucleic acids and water. The main microbial biofilm is bacteria. Biofilm bacteria is the main cause of tooth diseases (TOVE LARSEN and NILS-ERIK FIEHN, 2017). Composition of the biofilm is alterable in relation to nutrition and personal hygiene level (PEREIRA et al., 2015).
The role of biofilm due to its bacterial content in periodontal disease is affirmed. The bacterial composition of the oral biofilm from one person to another is variable (QUIRYNEN, DE SOETE and VAN STEENBERG, 2002). The composition of the biofilm is affected by the environmental conditions of each person, such as nutritional habits, oral hygiene, types of food that every person feed(MARSH and BOWDEN, 2000).
Saliva acts as a natural lubricant in the oral cavity. Some molecules such as mucin and low molecular weight proteins in saliva play a key role in lubrication. (MYSTKOWSKA et al., 2018) The surface of soft and hard tissues in the oral cavity can be covered by a conditioning film that is essential for the biofilm construction. This layer is the combination of glycoproteins, ions and water with 1-10 μm of thickness. The conditioning film plays an important role in microorganism's adherence to the oral surface. In addition, it protects the oral cavity against wear resulting from chewing. Streptococcus species are the greatest primary colonizers. They adhere to the surface through various mechanisms. Initially, Streptococcus Mutans forms electrostatic bonds with the calcium ion mediated salivary glycoprotein receptor. The microorganisms adhere to the glycoprotein receptors in the conditioning film layer through their adhesins. Some bacteria, such as Streptococcus mutans, produce hydrated extracellular polysaccharides
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from sucrose metabolism. The structure of extracellular polysaccharides is an essential factor for agglutination and agglomeration of S. mutans. Different microorganisms and species co-aggregate through various mechanisms, including glycoproteins in saliva and gingival fluids (eg C. albicans and S. mutans), and also the accumulation of cells into cells can occur via adhesin-receptor binding and some late colonizers, such as Prevotella intermedia and Porphyromunas gingivalis adhere to the mass through their filaments. The nature of cell-to-cell interactions is based on the hydrophobic, electrostatic and Van der Waals forces. Finally, a multilayer and multispecies biofilm mass is established. Biofilm has different functions. Its viscoelastic characteristic can act against elastic deformation under shear stress and distribute loads. In addition, biofilm affects friction and contact pressure on the surface of the oral environment (SOUZA et al., 2015). Resistance against movement between two moving surfaces in close contact is called friction, which is the ratio of wear , loss of volume and thickness in the material of the friction surfaces (ZHENG and ZHOU., 2007).
Although titanium has corrosion resistance due to its ability to form a protective layer of oxide (TiO2) within 5-6 nm thickness, its wear resistance is low. This passive layer is not permanent and can be influenced by mechanical and chemical changes. The film layer of stainless steel is 2-3 nm. On the other hand, the oral cavity is a complex biological organ that undergoes repetitive chemo-mechanical changes. (CHATURVEDI, 2009; HAYES, 2010) Lucas and Lemons, in their study in 1992, demonstrated the instability of the Ti oxide layer under dynamic conditions such as mechanical loading (LUCAS and LEMONS, 1992).
Lipopolysaccharide is one of the fundamental molecules in biofilm formation. The wear and corrosion of commercially pure titanium and the Ti alloy in the presence of lipopolysaccharide are accelerated (MATHEW et al., 2012) The worn and corroded surface is susceptible to more wear in the presence of lipopolysaccharide (MYSTKOWSKA et al., 2018). After destruction of the Ti oxide passive layer, lipopolysaccharide accelerates Ti corrosion by ion exchange between the new Ti surface and saliva. Corrosion of Ti alloy is influenced less than commercially pure titanium by lipopolysaccharide (MATHEW et al., 2012) The biofilm decreases friction, moreover, it has a negative effect on the stability and integrity at the I-A interface (SOUZA et al., 2010).
The purpose of this in vitro study is to evaluate the removal torque and internal surface morphological alterations of morse taper
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frictional implants (FGM) after cyclic mechanical loading associated or not to the bacterial biofilm.
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2. LITERATURE REVIEW
Titanium and its alloys are one of the common metal materials for dental clinical use, such as dental implants, orthodontic appliances. As Ti and its alloys form a protective oxide on its surface, they are biocompatible and have high corrosion resistance (YAMAZOE et al., 2007).
SERHAN et al(2004) evaluated corrosion and wear of spinal implants. A total of four components were used in this study. They studied mixed screw and nut structures which were made from stainless steel and titanium alloy. Each screw and nut complex received 5 million cycles of mechanical compression load. They applied 30-300N of force at 5Hz, at room temperature. All tests were performed in a saline bath (PH 7.4). They found that close contact of titanium alloy component (Ti-6Al-4V) to stainless steel component under load condition causes fretting and pitting of titanium. Also they observed that stainless steel in nigh contact with the same alloy is more susceptible to corrosion than Ti-SS adjacency.
Despite the remarkable progress in the dental implant field, studies have shown 5-10% of the failure and have reported infection and overload as the main causes of failure of dental implant treatment (SCARANO et al., 2016). The abutment generates its stability from torque application which deals with high friction of the surface of the joint. The dental implant and its components will be affected by occlusion force, oral hygiene and pH alterations, as the oral cavity is a biological medium (PRADO et al., 2016).
Applying the recommended torque to the abutment or abutment screw to install the abutment to the implant is called preload. Preload is the result of compression and friction. The required torque to remove an abutment or an implant-supported prosthesis is called counter-torque or removal torque. A digital or analog torque wrench is used to measure torque and counter-torque.
The preload is essential for abutment proper installation. Also maintaining this preload over time is an important factor for the success of dental implant treatment. If the preload decreases below the critical level, although the I-A connection surfaces are in contact, the prosthetic component becomes more susceptible to micro-movements. Loss of preload can happen in two steps. External forces, such as mastication force or lateral force during chewing, mainly lead to sliding movements and preload reduction. If preload loss remains below the critical level, micro-movements increase, and its consequence is the loss of function
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of the prosthetic component. This can be found in the control or maintenance time as a "loosening" of the prosthetic component (CIBIRKA et al., 2001).
Complications in dental implant treatments, regardless of unitary or multiple, have different reasons, including soft tissue inflammation, fracture of the component or prosthesis screw and more common reason is loss of stability (GIL et al., 2014).
The external and internal hexagon implant gain stability at the level of implant-abutment connection by a screw. In recent years, some changes in the dental implant field have occurred, such as metal alloys modifications, surface treatment of the screw and application of appropriate torque values. In the morse taper implant system, the close contact between conical surfaces, the inner part of the implant hub and the external surface of the abutment prosthetic part, result in the friction between them (JUNQUEIRA et al., 2013).
The wear and fatigue of the implant-abutment connections can alter the internal surface of the implants and results in increased microgaps (HANSSON, 2000).
Several studies revealed that internal connections, such as the morse taper, with greater contact surface between the implant and the abutment, not only makes biofilm infiltration difficult but also the larger contact surface helps a better stress distribution. However, in screw-retained abutment systems, lateral forces can lead to preload reduction and consequently misalignment of the components (DELBEN et al., 2014).
The exposure of I-A structure to the oral biofilm is inevitable. On the other hand, all types of dental implants have connections at the level of I-A and abutment-prosthesis. The agglomeration and development of microorganisms in the retention areas and microgaps of the dental implant structure was reported by SCARANO et al (2005). The distance between the implant and abutment, between abutment and the prosthesis are crucial to prevent microbial accumulation and corrosion products in the areas of the connections (PEREIRA et al., 2015). Many new technologies are currently being targeted to prevent the formation of biofilms on implant surfaces. A wide variety of materials have been applied for hermetic seal in the Implant-abutment connections, including: adhesive, wax, silicon tightness components and chlorhexidine 2% solution. No long-term efficient results are available (ANTONIO et al., 2006).
An evaluation on microgap size was done via scanning electron microscopy. In this research, 4 morse taper implants and 4 related
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abutments before and after cyclic loading were examined. Each sample received 345,600 cycle of axial load along the I-A complex with fatigue test machine. Applied load was 80 N with the frequency of 4 Hz. Notable difference between before and after loading was observed. The average gap size after loading decreased. The SEM images revealed a better and more adequate adjustment between the implant and abutment walls at the connection level. Cyclic loading leads to invasion of the abutment and increased friction between implant and abutment. The final result was better sealing capacity and better integration at the I-A contact level (GEHRKE and PEREIRA FDE, 2014).
SCRANO et al. (2016) performed an in vivo study with the objective of evaluating the seal provided by healing abutment. In their investigation, 15 patients (3 males and 12 females, aged 28 to 60 years) participated with total or partial mandibular edentulism. They used 23 morse taper dental implants and 14 internal hexagon system implants. After 3 months the second stage of the surgery was performed and the healing abutments were placed to the implants. All patients after 10 days were evaluated . After the removal of the healing abutments and washing the internal surface of the implants, the healing abutments were repositioned with the manufacturer's recommended torque (20 N). Immediately after the removal of the healing abutments, Volatile Organic Compounds (VOCs) test were performed at the implant site. (VOCs) evaluates the presence of volatile products of bacterial metabolism and fluid in the implant directly in vivo and in real time. They concluded improved sealing ability and resistance to bacterial infiltration in the morse taper connection in comparison to the internal hexagon implant system.
The correlation between removal force and number of tapping in morse taper frictional connection was assessed in a study. Ten MT frictional abutments were connected to their related implants via 1-5 times of compression force. The taper angle of I-A connection was 1.23 degree. Removal force was measured by a universal testing machine. They observed a positive correlation between the number of applied force and the removal force value. An increase in the number of tappings resulted in a greater intrusion of the abutment shaft into the implant socket and higher removal force value (Zielak et al., 2011). In an in vitro study to evaluate the removal torque and bacterial colonization at the implant-abutment level, KOUTOUZIS et al, compared two types of implants, including morse taper (gp 1, n = 14) and implants with internal conical connection with four-grooves connected to multi-base abutments (gp 2, n = 14). Samples were fixed in
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autopolymerizing resin. Both groups were immersed in a bacterial culture of BHI with Escherichia coli and simultaneously loaded with 500,000 cycles of 15N force in a wear simulator.
The comparison of torque before and after cyclic loading showed an increase in samples of MT group, but the
conical group with four grooves after dynamic loading demonstrated torque reduction. In the MT group, after disconnection of I-A complex, one of the samples developed multiple Colony Forming Units of E. coli. In group of internal conical connection with four-grooves, 12 out of 14 samples demonstrated multiple Colony Forming Units (KOUTOUZIS et al., 2011).
SEOL et al. (2015) evaluated the removal torque and wear of I-A complex after cyclic loading. They studied three types of connections, including EH, MT (one piece) and MT with octagonal connection (two pieces), (n = 7). All samples were fixed in a metallic support device to avoid oblique loads at the moment of installation and the counter-torque measurement.
After abutment installation on the basis of the manufacturer's recommendations, the samples were fixed in a metallic holding device to apply vertical load along the long axis of the implant-abutment complex. A custom made mechanical loading device was used in this study to apply axial load of 150N, 1,000,000 cycle at 3Hz of frequency. Removal torque decreased in all samples. The samples of morse taper (one piece) presented greater removal torque than group of morse taper with octagonal connection, due to friction between implant and abutment in conical connections.
The internal surfaces of implants and external surfaces of abutments were observed by scanning electron microscope. Implant and abutment in this study were made from Ti grade 4 alloy and Ti grade 5 alloy, respectively. Since the elastic modulus of Ti grade 4 alloy is less than Ti grade 5 alloy, the wear on the internal wall of the implants was greater in comparison to the abutment surfaces. Sliding and micro-movements cause fretting wear and loss of material. Another type of wear, adhesive wear, was observed on the internal surface of morse taper implants, specifically in morse taper group (one piece). In this type of connection, plastic deformation occurs at contact points due to high stress.
In another study 10 morse taper dental implants were evaluated. After installation of the screw retained abutment according to the manufacturer's recommendations, I-A complexes were immersed in biofilm medium for 72 hours. The biofilm medium contained a mixture of 2ml of BHI solution with 5% sucrose and 5μl of diluted human
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saliva. The solution was incubated for 72 hours in a micro-aerophilic condition of 5% CO2 and 37 ° C. The torque wrench was connected to a supportive metallic device to avoid oblique loads during torque and counter-torque application and measurement. After measuring the removal torque with a digital torque gauge, the abutments were removed.
Firstly, the inner surface of each dental implant was analyzed with SEM. Then, the arithmetic roughness (Ra) and the maximum distance from peak to valey (Rt) were evaluated by optical perfilometer after cutting the samples. The internal surface of samples were analyzed in three parts, including edge, middle and apical third. They observed reduced roughness at internal surface of MT implants immersed in biofilm. Mechanical integrity decreased in these samples as well. They also reported a significant decrease in the mean value of removal torque in the samples with immersion in biofilm. (PRADO et al., 2016).
One study evaluated the removal torque after mechanical loading in two types of connections, screw retained morse taper and external hexagon. In this study 60 implants were divided into 2 groups. The implants were embedded in acrylic resin that had an elastic modulus close to the maxillary cancellous bone. Saliva was collected from 4 individuals during 4 days and diluted in phosphate buffered saline (PBS) each day. The optical density of the initial solution was approximately 1x108 CFU /mL. A mixture of brain-heart infusion medium (BHI) enriched with 5% sucrose in combination with 5μL from the initial solution was used for biofilm growth.
A metallic holding device was used to avoid oblique loads during torque application and measurement. The samples were immersed in 2mL of BHI solution with human saliva for 72 hours at 25 ° C. Fatigue tests were performed at a force of 50N, at 1.2 Hz at 30 ° C. Each sample received 500,000 cycles of axial load in a fatigue test machine while it was immersed in biofilm.
The removal torque after mechanical loading decreased in both groups, screw retained morse taper and external hexagon. Torque evaluation revealed that torque lost in MT samples were less than EH due to friction at the implant-abutment interface level. The biofilm acts as a lubricant that can reduce the friction between two sliding surfaces and consequently the removal torque. (Pereira et al., 2016).
PRADO et al. (2017) conducted a study to compare the wear between two implant systems, including external hexagon and morse taper. In this study 40 implants (20 screw retained MT and 20 EH) were divided into four groups (n=10). The abutments installed to the related
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implant based on the torque recommended by the manufacturer. One group from each type of connection was immersed in biofilm containing solution. The other group of each connection system remained without biofilm immersion.
Biofilm culture was prepared by diluting natural saliva in BHI solution (1: 5). The samples were immersed in biofilm medium containing a mixture of 2 ml BHI solution and 5 μl of diluted human saliva with 5% sucrose. The solution was incubated for 72 hours in a micro-aerophilic condition of 5% CO2 at 37 ° C.
A digital torque wrench was used to measure torque and removal torque of the abutments. Both groups of EH revealed lower removal toreque in comparison to the initial torque. Although, both groups of MT samples presented higher removal torque than the initial torque, the samples immersed in biofilm showed lower removal torque than the groups without immersion in biofilm, in both systems. This result showed a negative effect of the biofilm on the removal torque.
Scanning electron microscope analysis showed increased roughness at the I-A interface in both groups. In the groups immersed in biofilm, less damage to the internal surface of the implants was observed, whereas the result was reduced integrity at the connection level due to the biofilm lubrication effect. In samples of EH without biofilm immersion, values of Ra and Rt were higher than those samples were immersed in biofilm medium.
The internal surface of MT implants and platform of external hexagon implants were analyzed under optical profilometer. Internal surface of the MT samples were assessed in three parts, including edge, middle and apical third.
The roughness values (Ra and Rt) of the samples immersed in biofilm, at the border and apical region, were lower than in the group without immersion in medium containing biofilm. However, roughness values were higher in the middle region than two other regions in biofilm contaminated samples.
Reduced damage to the connection of implants after immersion in the biofilm medium suggests a biofilm lubrication effect that reduces friction between surfaces and can cause loss of integrity at I-A connections.
KOFRON et al (2019) compared stability of implant-abutment connections under in-vitro force application. They evaluated 6 different internal connections in their study, including: friction-fit (screwless conical connection with 1° taper of hex portion), cross-fit (screwless, 15° cylindrical connection with 4 grooves), conical + hex #1 (screw
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retained, 1.5mm hex flats), conical + 6 indexing slots (screw retained, 5 symmetrically slots and one additional slot which provides only one position for abutment connection), conical + hex #2(screw retained, hexagon with indexing every 60°) and internal tri-channel(screw retained, internal connection with indexing every 120° (n=5). After connection of related abutments in every system, force of 200N at 30° angle to the implant vertical axis was applied to the implant-abutment complexes. Each sample submitted 1000 cycles of force via the Instron E3000 materials testing system.
The abutment pull out force was measured after cyclic loading and unscrewing the screw retained abutments. The highest required force to separate I-A connection was observed in friction fit system at the range of 81.8 to 150.6N which indicated the high stability of friction-fit connection. The conical + Hex #2 system and cross-fit connection demonstrated almost equal pull out force but significantly lower than friction fit system. The remaining systems showed zero pull out force, which means their stability is provided just through screw engagement. Also horizontal micro-movement under loading condition was evaluated in this study. They found the minimal horizontal micro-motion in the friction fit and cross-fit system in comparison to other samples. Samples of the conical + 6 indexing slots demonstrated the greatest amount of horizontal micro-motion.
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3. OBJECTIVES 3.1. General objective
Evaluation of removal torque and morphological aspects of internal surface in morse taper frictional implant -abutment connection after mechanical loading associated or not to bacterial biofilm.
3.2. Specific objectives
Evaluate the counter-torque or removal torque of prosthetic abutments in morse taper frictional implant system(FGM) after cyclic loading.
Assess the removal torque of morse taper frictional abutments after simultaneous cyclic loading with biofilm immersion.
Analyze the internal surface of morse taper implants using scanning electron microscopy and optical profilometer.
Analyze changes on the internal surfaces of the morse taper implants in contact with biofilm under SEM and optical profilometer.
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4. MATERIALS AND METHODS 4.1. MATERIALS
4.1.1. SAMPLE PREPARATION
Thirty 3.8 x13mm MT frictional implants (FGM, Joinville-SC Brazil) and thirty related morse taper abutments (3.5 trans-mucosa x 2.5 x 4mm) were divided into 6 groups (n = 5).
The implants were embedded in transparent acrylics resin (Clássico Ltda., SP, Brasil). A plastic mold with the size of 25 x 20mm was used to prepare the samples. A FGM implant installation instrument was attached perpendicular to the center of the plastic mold cap to standardize all samples in an accurate vertical position. As in 3 groups, the I-A interface must be submerged by biofilm, the pattern provided this space by installing the implant platform 1 mm above the level of the acrylic resin. Thus the I-A connections were 1 mm above the level of the acrylic resin which allowed the samples to be immersed in the bacterial culture inside the cycling machine. After mixing the liquid and powder of the acrylic resin base on the factory recommendation, the acrylic resin was poured into the plastic mold. Each implant through the standardized pattern was transferred to the acrylic resin until hardening process of acrylic resin finished.
In this study, to simulate the biomechanical characteristic of the maxillary trabecular bone, the acrylic resin used in the preparation of the samples had an elastic modulus of 3.5-4 GPa close to the maxillary cancellous bone(1-10 GPa), (PEREIRA et al., 2016). After curing the acrylic resin blocks, to avoid oblique loads during torque application and measurement, the samples were placed in a metallic holding device. The abutments were activated through 3 taps with the specific FGM mallet.
4.1.2. Biofilm growth
Five ml of human saliva from two healthy individuals ranging from 20 to 31 years old obtained and diluted (1:5) in culture medium (TSB, Sigma-Aldrich, USA). This solution was diluted in TSB and incubated 48 hours at 35° . The optical density of the initial solution was measured by spectrophotometry (BIOTEK, Brazil) at 630 nm which is approximately equal to 0.5 McFarland Standard or 1x108 colony forming units per mL - CFU / mL. (DO NASCIMENTO et al., 2008). Individuals for saliva collection were in good dental and oral health, with no history of antibiotic treatment during the previous 6 months.
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4.2. METHODOLOGY
This study is characterized as an in vitro laboratorial study. In Table 1, six groups of samples based on the number of cyclic loading and biofilm contamination are described.
Groups A and B without cyclic loading were control samples. The samples of group A only received torque application and after 72 hours, their removal torque was measured. Group A samples did not have contact with biofilm, as a negative control group. The samples of group B as a positive control group, after abutment installation, were immersed in biofilm for 72 hours and, finally, removal torque of the abutment by counter-torque application was measured.
Groups C and D received 100,000 cycles of mechanical axial load. Group C samples had no contact with biofilm and only received 100,000 cycles of axial mechanical load, at 2Hz. The samples of group D were in contact with biofilm and at the same time received 100,000 cycles at frequency of 2 Hz. The force of 80±15 N which is in the range of physiologically clinical load, was applied to both group samples (RICHTER., 1995).
Groups E and F received 500,000 cycles of axial load. Group E had no contact with biofilm and group F samples were in contact with biofilm during the load application. Both group received a force of 80±15N at 2Hz of frequency.
Samples of groups D and F were immersed in biofilm for 72 hours before mechanical load. Biofilm media was replaced every 4 hours in groups B, D and E samples.
In all groups, the removal torque was evaluated with portable digital torque wrench. The internal surface evaluation was done with SEM and optical Profilometer.
Biopdi pneumatic mechanical cyclic loading machine (Biocycle V2, Biopdi, Sao Paulo, Brasil) was used to apply the mechanical axial load in this study.
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Table 1. The specifications of the dental implant system used in this study and test condition information related to each group.
4.2.1. Removal torque evaluation
A digital torquemeter (Lutron TQ8800, Lutron, Taiwan) was used to measure the removal torque of the prosthetic abutments. Each of the I-A assemblies was placed in a metallic support device to avoid oblique loads during counter-torque measurement.
Removal torque was measured in group A and B samples after 72 hours and samples from groups C and E were evaluated after cyclic loading according to the Table 1. The counter-torque of samples in groups D and F was measured after immersing in biofilm and simultaneous cyclic loading according to the Table 1.
4.2.2. Analysis the implant internal surface
After removal torque evaluation, the abutments were removed. The samples were washed in an ultrasonic bath containing 70% isopropyl alcohol for 10 minutes, then analyzed by scanning electron microscopy (SEM) secondary electron mode at 10-20 kV (JEOL JSM- 6390LV, USA). The samples were evaluated by SEM under magnification of X30 to X500.
After SEM analysis, the implants were sectioned transversely by the Exakt System (EXAKT Technologies, Oklahoma, USA) to evaluate the
Group Implant Cycle Ambient effect
A Morse Taper 0 Without biofilm contamination B Morse Taper 0 With biofilm contamination C Morse Taper 100.000 cycles Without biofilm contamination D Morse Taper 100.000 cycles With biofilm contamination E Morse Taper 500.000 cycles Without biofilm contamination F Morse Taper 500.000 cycles With biofilm contamination
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internal surface roughness of the morse taper implants by an optical profilometer (DektakXT, Bruker, Germany). Prior to surface evaluation, the samples were washed with phosphate buffered saline (PBS) and then in an ultrasonic bath containing 70% isopropyl alcohol for 10 minutes. Optical profilometry was done base on the following parameters: 1 mm length, cut at 0.25mm and speed at 30 mm/s. Two parameters of profilometer, including arithmetic roughness (Ra) and maximum peak-to-valley distance (Rt) were analyzed.
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4.2.3 Statistical analysis
The results were statistically analyzed using the Anova test, with a significance level less than 5% (p <0.05), using SPSS 17.0 for Windows software (Chicago, IL, USA).
5. Results
5.1 Removal torque evaluation
In group A, the samples without mechanical load and biofilm contamination, the average removal torque value was 23 ± 9.4N. Although group B samples, immersed in biofilm without mechanical load, showed higher counter-torque values in comparison to group A, 27.2 ± 7.46N, the difference between group A and B was not statistically significant.
Groups C and D (100,000 cycles of mechanical load) showed a statistically significant difference in removal torque values compared to those recorded on group A and B. The average removal torque in group C, samples without biofilm immersion, was 83.8 ± 15.83N. A statistically significant increase in counter-torque value was noticed in group D samples. Group D samples, with biofilm contamination, demonstrated the mean removal torque of 160.6 ± 16.21N which was almost two times of the group C samples.
The average removal torque value in groups E and F (500,000 cycles of mechanical load) was significantly higher than groups A and B and C but lower than group D samples. Mean removal torque in group E, the samples without biofilm contamination, was 147±29.31N. Samples of group F with biofilm contamination, 154.5±14.01N, had no significant difference to group E.
While Group D samples received lower number of loading cycles, they exhibited somewhat higher removal torque value than groups E and F, The difference of removal torque value between group D, E and F was not statistically significant.
One of the samples in group F was omitted from this study. The sample did not show any removal torque value and surface alteration due to I-A mal-adaptation at the time of abutment installation.
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Table 2. Mean and standard deviation values of the removal torque evaluation recorded on morse taper I-A connections. Values represented with statistical significance (p <0.05). Groups C, D, E and F (samples with mechanical loading) presented a statistical difference to groups A and B (samples with no mechanical loading). Also, the difference of removal torque in groups D, E and F were statistically significant compared to group C. (n: number of samples, Std: Standard deviation, Min: minimum, Max: maximum)
Groups n Mean Torque (Ncm) Std Min Max Group A 5 23 9.4 13 32 Group B 5 27.2 7.46 17 35 Group C 5 83.8 15.83 67 106 Group D 5 160.6 16.21 133 175 Group E 5 147 29.31 105 175 Group F 4 154.5 14.01 145 175
Figure 1. Mean and standard deviation of the counter-torque evaluation of morse taper components: with and without mechanical axial load associated or not to bacterial biofilm.
Rem o v a l to rque Groups Standard deviation Pattern error Mean value
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5.2 Surface analysis
Images obtained by SEM from the internal surfaces of the morse taper implants are shown in Figures 2 to 8, respectively. Areas of plastic deformation were observed in the images as marks of abrasion on the inner surface of the morse taper implant. Wear areas were found in both groups, immersed or not in human biofilm culture.
The results of roughness analysis of morse taper implants by optical profilometer are presented in Tables 3 and 4.
Although group B showed higher values of roughness than reference group and group A, the difference between them was not statistically significant. Moreover, the number of samples in group A and B was equal (n=5) but statistical analysis of the reference group was done just on one sample.
Roughness values (Ra) and (Rt) of the group C (free of medium containing biofilm) in border region were higher than those values in group D. In contrary, group D samples (immersed in biofilm) in middle third had greater wear rate in comparison to group C and demonstrated higher Rt value or maximum distance from peak to valley. In apical third both groups had almost equal (Rt) value though group D exhibited greater mean arithmetic (Ra) value.
In group D, both Ra and Rt values were statistically lower than those in group C at the border region. However, the roughness values were higher than those recorded on the middle and apical regions.
Roughness values (Ra) and (Rt) at border and middle parts of the group group E samples, 500,000 cycles of load without biofilm immersion, was higher than group F and showed a statistically significant difference compared to those recorded on group F. On the apical third region, despite disparate results, the difference of roughness values was not statistically significant in groups E and F.
The difference of roughness values for group D compared to groups E and F were not statistically significant on all three regions.
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Figure 2. SEM micrographs of Morse taper implant before I-A connection, obtained by secondary electrons (SE) mode at 15 kV: A. Border to apical internal surface of implant. B and C. Intact border and middle part of internal surface.
A
B C
Figure 3. SEM micrographs of group A samples obtained by secondary electrons (SE) mode at 15 kV: after I-A disconnection. A. Border to apical internal surface of implant. B and C. Trivial wear area of the apical third part of implant.
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A
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Figure 4. SEM micrographs of group B samples obtained by secondary electrons (SE) mode at 15 kV: after I-A disconnection. A. Border to apical internal surface of implant. B. and C. Sparce wear areas along the implant internal surface. C C B
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*
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Figure 5. A. SEM micrographs of group C samples after I-A disconnection, obtained by secondary electrons (SE) mode at 10 kV: B middle and C. apical third with wear area.
A
Figure 6. SEM micrographs of group D samples obtained by secondary electrons (SE) mode at 15 kV: after I-A disconnection. B. Remarkable wear area on the middle region in comparison to A. the border and C. apical third. A
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Figure 7. SEM micrograph of group E samples obtained by secondary electrons (SE) mode at 10 kV: after I-A disconnection. A. and B.
Wear area along the Internal surface from border to apical region.
B C
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A B
Figure 8. A. SEM micrographs of group F samples obtained by secondary electrons (SE) mode at 10 kV: after I-A disconnection. B. and C. Wear with lower intensity on the internal surface in comparison to group E samples.
A
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B C
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Table 3. Results obtained from optical profilometry for morse taper implant-abutment connections considering the arithmetical roughness (Ra) values. Values presented with statistical significance (p <0.05). Groups D and F samples (with immersion in biofilm) in comparison to groups C and E showed statistical difference in Ra value at the border region on the implant internal surface. The difference of Ra values in groups E and F (with 500,000 cycles) was statistically significant to groups C and D (with 100,000 cycles) on all three regions.
Group n Ra (μm) Std dev Min Max Border Initial 4 0.134 0.200 0.11 0.16 Group A 20 0.117 0.133 0.10 0.15 Group B 20 0.144 0.438 0.10 0.24 Group C 20 0.150 0.490 0.10 0.28 Group D 20 0.129 0.205 0.11 0.18 Group E 20 0.239 0.109 0.10 0.50 Group F 16 0.207 0.999 0.10 0.44 Middle Initial 4 0.142 0.007 0.14 0.15 Group A 20 0.114 0.096 0.10 0.14 Group B 20 0.144 0.053 0.10 0.29 Group C 20 0.131 0.040 0.10 0.25 Group D 20 0.135 0.412 0.11 0.25 Group E 20 0.219 0.809 0.10 0.39 Group F 16 0.206 0.910 0.10 0.40 Apical Initial 4 0.123 0.141 0.10 0.13 Group A 20 0.118 0.127 0.10 1.15 Group B 20 0.144 0.563 0.10 0.27 Group C 20 0.126 0.247 0.10 0.19 Group D 20 0.145 0.406 0.10 0.24 Group E 20 0.210 0.673 0.11 0.30 Group F 16 0.224 0.104 0.11 0.44
(n: number of samples. Std dev: Standard Deviation, Min: Minimum, Max: Maximum)
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Table 4. Results obtained from optical profilometry for morse taper I-A connections considering the maximum distance from peak to valley (Rt) roughness values. Values presented with statistical significance (p <0.05). The groups D and F (with immersion in biofilm) in comparison to groups C and E showed statistical difference in Rt at the border region. Also, the difference of Rt values in groups F and C samples were statistically significant compared to group E and D on the middle third part. The difference of Rt values in groups E and F was statistically significant to groups C and D on all three regions.
Group n Rt (μm) Std dev Min Max Border Initial 4 1.128 0.253 0.91 1.49 Group A 20 1.141 0.545 0.75 2.4 Group B 20 1.426 0.478 0.82 2.4 Group C 20 1.873 0.594 0.89 2.9 Group D 20 1.634 0.424 0.98 2.5 Group E 20 2.482 0.673 1.3 3.9 Group F 16 2.103 0.629 1.29 3.1 Middle Initial 4 1.190 0.470 0.65 1.89 Group A 20 0.979 0.212 0.92 1.56 Group B 20 1.609 0.679 0.85 3.78 Group C 20 1.387 0.559 0.82 2.8 Group D 20 1.908 0.479 1.16 3.06 Group E 20 2.499 0.687 1.37 3.77 Group F 16 2.170 0.802 1.33 3.86 Apical Initial 4 1.231 0.425 0.87 1.80 Group A 20 1.171 0.403 0.75 2.37 Group B 20 1.521 0.490 0.84 2.58 Group C 20 1.819 0.441 1.03 2.7 Group D 20 1.832 0.470 0.98 2.61 Group E 20 2.407 0.791 1.22 4.2 Group F 16 2.437 0.922 1.26 4.29
(n: number of samples. Std dev: Standard Deviation, Min: Minimum, Max: Maximum)
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Figure 9. Mean and standard deviation of roughness value evaluation at the border part of morse taper implant. A. arithmetical roughness (Ra) values B. maximum distance from peak to valley (Rt) values.
Standard deviation Pattern error Mean value A B
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Figure 10. Mean and standard deviation of roughness value evaluation on the middle third part of morse taper implant. A. arithmetical roughness (Ra) values B. maximum distance from peak to valley (Rt) values.
A
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Figure 11. Mean and standard deviation of roughness value evaluation at the apical third part of morse taper implant. A. arithmetical roughness (Ra) values B. maximum distance from peak to valley (Rt) values.
A
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6.Discussion
This study assessed the mean value of removal torque and wear of the MT frictional implants after cyclic axial loading associated or not to biofilm. Various factors such as taper angle, contact length, material characteristic, coefficient of friction, density of oral biofilm in the I-A interface and microgap, insertion depth and implant internal diameter in relation to the external diameter of the abutment in MT connection can affect the removal force, the distribution of stress in the components and wear (BOZKAYA and MUFTU 2003; PEREIRA., 2016).
Evaluation of contra-torque revealed that the mean values of removal torque increased for the samples with mechanical axial load compared to the samples without mechanical load. The removal torque was higher in samples that submitted mechanical load and simultaneously were immersed in biofilm compared to the samples with the same number of loading cycles without biofilm immersion.
In conical system, large contact area between the internal surface of the implant socket and the external surface of the abutment prosthetic part besides a minute cone angle between them results in high contact pressure; therefore, the friction increases (ALVES et al., 2016).The high friction between the tapered surfaces in morse taper implant is responsible for increasing the removal torque values. Friction and wear are indispensable incidences. Moreover, the abutment is composed of stainless steel alloy with modulus elasticity of around 200 MP, while implants are synthesized from grade 5 titanium alloy, have a modulus elasticity value within the range of 100-120 GP. Thus wear of softer material, Ti alloy, is imminent (SERHAN et al., 2004). Due to the friction and the difference in modulus elasticity, all three steps including abutment installation, mechanical loading and removal of the abutment can generate wear areas on the inner implant surface.
The removal torque in group B, D and F, samples immersed in biofilm, increased in comparison to group A, C and E samples. PRADO and his colleagues in two different studies demonstrated contrary results. They reported a diminished removal torque in screw retained MT samples which were immersed in biofilm after abutment connection. They concluded that the presence of the medium containing biofilm from human saliva negatively affected the removal torque values of the dental implant-abutment connections (PRADO et al.,2016; PRADO et al., 2017).
Group C samples had remarkably lower removal torque than group D samples, though both of them submitted equal cycles of axial load, 100,000 cycles. Group D samples were immersed in biofilm culture.
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Also group E samples demonstrated lower removal torque in comparison to group F with the same number of load cycles. The removal torque value in group D, E and F had a slight difference. KOUTOUZIS et al got a similar result about removal torque value in biofilm contaminated group after cyclic loading while PEREIRA et al (2016) reported removal torque decrease in morse taper implant samples with biofilm immersion and simultaneously axial loading. The exact reason of this difference is not obvious but various factors such as biomechanics of the systems, biofilm presence and elastic properties of materials can impact the removal torque value (KOUTOUZIS et al., 2011).
Contrary results indicate the importance of biomechanics. In screw retained morse taper system, abutment gains its retention from friction between screw and internal surface of the implant. Morse taper frictional system gains its stability from friction between implant internal surface and outer surface of abutment shaft. In MT frictional implant friction is the result of large contact area and small taper angle at I-A connection.
Schmitt et al (2014) in a systematic review study showed that in spite of the superiority of MT system to other types of I-A connection in microgap formation and seal performance, hermetic seal is not achievable in none of the connection types. MCFARLANE and TABOR(1950) found strong adhesion when two metal surfaces are in close contact with trapped thin layer of liquid between them. There is a reverse relation between thickness of the liquid and vigor of adhesion. Besides the impact of axial mechanical load and friction impact, the adhesion resulted from liquid surface tension leads to an increase of removal torque in groups immersed in biofilm than groups without biofilm immersion. In MT frictional system, the result of abutment shaft insertion into the implant hub is an intimate contact at I-A connection level and wedging. Continuous loading cycles increases wedging which not only prevents preload lost but also raises the required removal force. Although the removal force value increases by greater number of loading cycles, it is not equal in whole process. The higher the number of applied force, the lower the amount of removal force increase (ZIELAK et al., 2011). BOZKAYA and MUFTU (2003) demonstrated that deeper insertion of abutment in tapered interface does not incidentally leads to infinite increase in removal torque or pull out force. Present study revealed that removal torque in MT frictional system follows an ascending trend base on the number of load cycles as well but it is not unlimited. Besides wedging and mechanical proceedings, smallest amount of trapped biofilm between two metal surfaces can act
