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Influences of microgap and micromotion of implant–abutment interface on marginal bone loss around implant neck
Accepted Manuscript Title: Influences of microgap and micromotion of implant–abutment interface on marginal bone loss around implant neck Authors: Yang Liu, Jiawei Wang PII: DOI: Reference:
S0003-9969(17)30243-1 http://dx.doi.org/doi:10.1016/j.archoralbio.2017.07.022 AOB 3960
To appear in:
Archives of Oral Biology
Received date: Revised date: Accepted date:
19-2-2017 19-7-2017 27-7-2017
Please cite this article as: Liu Yang, Wang Jiawei.Influences of microgap and micromotion of implant–abutment interface on marginal bone loss around implant neck.Archives of Oral Biology http://dx.doi.org/10.1016/j.archoralbio.2017.07.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influences of microgap and micromotion of implant– abutment interface on marginal bone loss around implant neck Running title: Microgap and micromotion of IAI Author names： Yang Liu, MD; E-mail: [email protected]. Jiawei Wang, PhD; E-mail: [email protected].
Authors’ affiliation: Department of Prosthodontics, School of Stomatology, Wuhan University, Wuhan, 430079, China.
Corresponding author: Prof. Jiawei Wang, The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST), Key Laboratory of Oral Biomedicine Ministry of Education, School & Hospital of Stomatology, Wuhan University, 237 Luoyu Road, Wuhan 430079, China; e-mail: [email protected].
Conflicts of interest: none
Highlights Elaborated respectively the influence mechanisms of the microgap and micromotion between the implant and the abutment on implant marginal bone loss in detail; The relationship between microgap, micromotion, microleakage and mechanical damage, and their influences on bone resorption around implant neck are particularly explained; Some feasible clinical methods to reduce the bone resorption engendered by the microgap and micromotion at the IAI are recommended.
ABSTRACT Objective: To review the influences and clinical implications of micro-gap and micro-motion of implant-abutment interface on marginal bone loss around the neck of implant. Design: Literatures were searched based on the following Keywords: implant-abutment interface/implant-abutment connection/implant-abutment conjunction, microgap, micromotion/micromovement, microleakage, and current control methods available. The papers were then screened through titles, abstracts, and full texts. Results: A total of 83 studies were included in the literature review. Two-piece implant systems are widely used in clinics. However, the production error and masticatory load result in the presence of microgap and micromotion between the implant and the abutment, which directly or indirectly causes microleakage and mechanical damage. Consequently, the degrees of microgap and micromotion further increase, and marginal bone absorption finally occurs. We summarize the influences of microgap and micromotion at the implant-abutment interface on marginal bone loss around the neck of the implant. We also recommend some feasible methods to reduce their effect. Conclusions: Clinicians and patients should pay more attention to the mechanisms as well as the control methods of microgap and micromotion. To reduce the corresponding detriment to the implant marginal bone, suitable Morse taper or hybrid connection implants and platform switching abutments should be selected, as well as other potential methods. Keywords: Implant-abutment interface, Microgap, Micromotion, Microleakage, Marginal bone loss
KEYWORDS: Implant-abutment interface; Microgap; Micromotion; Microleakage; Marginal bone loss
INTRODUCTION Marginal bone loss around the neck of dental implant is one of the most common complications after implantation and exerts remarkable influence on the future success and long-term stability of the implant. Generally, when the implant is placed into the alveolar bone, the resorption of marginal bone usually begins from the bone cortex (Branemark et al., 1969). Factors contributing to the loss of marginal bone include surgical trauma, periimplantitis, occlusal overload, microleakage, biologic width, and implant anatomy on the crest area (Macedo et al., 2016; Oh, Yoon, Misch, & Wang, 2002). The phrase microleakage of the implant-abutment interface (IAI) was coined in the 1990s, and it describes a microbial leakage between the implant and the abutment, which is attributed to the microgap and micromotion of the IAI. Efforts have been exerted over the last two decades to explore the discrepancy in the microleakage level within different implant systems and the reasons behind this phenomenon. Some scholars considered the microgap responsible for the phenomenon, and others deem it as the result of micromotion. In this article, the keywords were determined as, for instance, implant-abutment interface/implant-abutment connection/implant-abutment conjunction, microgap, micromotion/micromovement, microleakage, and current control methods available. The literatures were searched based on the keywords up to February 2017.To be analyzed in the review, papers had to (i) be written in English, (ii) be published in an international peer-reviewed journal, and (iii) have a clear definition for microleakage and related keywords. The titles and abstracts for eligible papers were screened. If eligibility aspects were present in the title, the paper was selected for further reading. If none of the eligibility aspects were mentioned in the title, the abstract was read in detail and screened for suitability. After selection, full-text papers were read in detail. The search resulted in 4350 records of titles and abstracts. Screening of these titles and abstracts initially resulted in 264 articles. Based on detailed reading of full texts, 181 articles were excluded and 83 studies were identified eligible for inclusion in the literature review. The influences of microgap and micromotion existing between the implant and the abutmentinterface on marginal bone loss are reviewed and clearly illustrated, and their clinical significances are discussed.
MICROGAP, MICROMOTION, AND MICROLEAKAGE Two-piece implant system Given the protection of implant from unwanted load during bone healing phase and the beneficial potential to adjust the prosthetic angle, the two-piece implant system is widely used in clinics. The IAI connection of the two-piece implant system includes two types: external
and internal connections. In a typical external connection, the implant convex extends outside by 1–2 mm, thereby forming an external structure similar to a hexagon or an octagon, which connects to the abutment. External connection is incorporated in some systems and once commonly used worldwide during the period when two-piece implant system was initially used because of its superior antirotational mechanism and ability to orient the abutment in the implant (Davi, Golin, Bernardes, Araujo, & Neves, 2008; Gracis et al., 2012). However, the short and narrow external geometry is particularly vulnerable when off axis loads are applied, which consequently leads to the deformation of the IAI (Binon, 2000; Gracis, et al., 2012). Along with the rapid development of two-piece implant system, internal connection including but not limited to internal hex connection gradually occupies a larger share in the market. Internal connection refers to the abutment stretches of 4–6 mm into the implant cavity. Subsequently, this abutment fixes with the implant and forms a conical, octagonal, hexagonal, trilobe, or spline design. Internal connection includes taper, butt joint, and hybrid connections (Figure 1). The taper connection originates from the concept of Morse taper in mechanical engineering and simply means a cone within another cone (Hernigou, Queinnec, & Flouzat Lachaniette, 2013; Oh, et al., 2002). Therefore, this connection is also called conical connection. Butt joint connection refers to the connecting area between the implant and the abutment without a taper, and it is only retained via a retaining screw. A hybrid connection means that both Morse taper design and the regular polygonal shape of antirotational or guiding grooves are present. Compared with the external connection, internal connection remarkably lowers the rotation center and improves the mechanical stability (Sailer, Sailer, Stawarczyk, Jung, & Hammerle, 2009). Furthermore, when internal connection adopts the form of platform switching, the stress distribution of the peri-IAI bone is reduced (AlvarezArenal et al., 2017; Liu et al., 2014).
Microgap The implant and the abutment cannot be accurately matched because of the precision limit during production (Alves, de Carvalho, Elias, Vedovatto, & Martinez, 2016). The IAI microgap, defined as the microscopic space between implant and corresponding abutment, exists (Scarano, Mortellaro, Mavriqi, Pecci, & Valbonetti, 2016). The microgap between the titanium abutment and the titanium implant is smaller than that between the zirconia abutment and the titanium implant. Moreover, the IAI microgaps of zirconia abutments increase significantly when torque values less than those of manufacturer-recommended values are applied (Hernigou, et al., 2013; Rack, Zabler, Rack, Riesemeier, & Nelson, 2013). Concerning the manufacturing technique, the premachined abutments exhibit smaller microgaps than those of cast on and castable abutments (Harder et al., 2010; Rismanchian, Hatami, Badrian, Khalighinejad, & Goroohi, 2012).
From the perspective of IAI connection style, Morse taper connection is sealed better than butt joint connection (Khorshidi, Raoofi, Moattari, Bagheri, & Kalantari, 2016). Fixing of taper connection depends on friction. Thus, the fitting degree of IAI connection mainly relates with the taper degree and connecting area. When the taper degree of the internal cone is larger than 5.8°, the removing force is smaller than the tightening force (Bozkaya & Muftu, 2004). On the contrary, the small taper degree results in large removing force and tight IAI. Additionally, the IAI under cyclic loading increases and becomes close with time, thereby achieving a metal–to–metal cold welding (Norton, 1999). Currently, except for one type of taper connection that is totally fixed by 1.5° Morse taper and a large contact surface of the implant and the respective abutment (Broggini et al., 2003; Dibart, Warbington, Su, & Skobe, 2005), all other connections need a certain preloaded screw to achieve and maintain the close connection of IAI (Aloise et al., 2010).
Micromotion/micromovement For two-piece implant system, although the micromotion in the IAI connections decreases due to a precise fabrication of the implant and the abutment, the current production process cannot avoid the micromotion when chewing between the abutment and the implant (Binon, 1996; Vigolo, Fonzi, Majzoub, & Cordioli, 2006). The micromotion of IAI includes the microabrasion and relative microshift between the implant and the abutment and the microrotation of the abutment relative to the implant (Figure 2). The micromotion size generally ranges from 1.52 µm to 94.00 µm (Karl & Taylor, 2014). According to the IAI connection style point, butt joint connection tends to fret, and taper connection is likely to rotate. In addition, oral environment and liquid also influence the wear mechanism to a certain extent. The peri-implant microenvironment (Ericsson et al., 1995), which is composed of blood, saliva, and biofilm, acts as a lubricant (Karl & Taylor, 2014) that connects the implant internal cavity and the peri-implant oral environment and decreases the friction resistance. In an antirotation study, the rotational micromotion degree of the internal trilobe connection and internal taper connection is the lowest and highest, respectively; moreover, such degree of internal hexagonal and octagonal connections, which produce similar patterns of micromotion and stress distribution, is the middle value (Saidin, Abdul Kadir, Sulaiman, & Abu Kasim, 2012). Consequently, for single implant without adjacent teeth, the increased number of guiding polygons lessens the resistant to rotational micromotion. Furthermore, Semper and colleagues (Semper, Kraft, Kruger, & Nelson, 2009) found that regular polygons display lower antirotation than that of rounded polygonal profile. Compared with the taper connection, butt joint connection exhibits better antirotational micromotion, but is more prone to fretting wear.
Microleakage
For most two-piece implant systems, the microgap size ranges from 0.1 µm to 10 µm after connection of the two components and prior to loading; this size may increase after cyclic loading. However, most oral bacteria are within the width of 0.2–1.5 µm and length of 2–10 µm (Nascimento et al., 2016). Therefore, bacteria and endotoxin can freely pass through the IAI microgap and enter the implant internal cavity, which results in the biomaterial exchange between the implant internal cavity and the peri-implant oral environment (Teixeira, Ribeiro, Sato, & Pedrazzi, 2011). The passing of bacteria, bacterial toxic byproducts, and small molecules through the IAI microgap and penetration into the implant internal cavity or vice versa is defined as the IAI microleakage (Ericsson, et al., 1995; Gross, Abramovich, & Weiss, 1999; Quirynen, Bollen, Eyssen, & van Steenberghe, 1994; Quirynen & van Steenberghe, 1993). Broggini and colleagues (Broggini et al., 2006) found that the infiltration of neutrophils near IAI increased with increasing implanting depth; additionally, the peak concentration of neutrophils was constantly around the IAI, regardless of the implant position. The authors believed that the IAI microleakage could cause a persistent inflammatory process, which ultimately led to alveolar bone destruction (Oh, et al., 2002; Siar et al., 2003). Whether under static condition without load (Tesmer, Wallet, Koutouzis, & Lundgren, 2009) or under dynamic cyclic load condition (Koutouzis, Wallet, Calderon, & Lundgren, 2011), the microleakage of the Morse taper connection is smaller than that of the butt joint connection. Along with the taper degree, the connecting area considerably affects the IAI connection intimacy (Baggi, Di Girolamo, Mirisola, & Calcaterra, 2013; Blum et al., 2015; do Nascimento, Pedrazzi, Miani, Moreira, & de Albuquerque, 2009). In different implant systems, the taper degree and connecting area are different, which are mostly responsible for the differences in bacterial penetration (Scarano et al., 2016). The torque value applied is also important (Ranieri et al., 2015). Commonly, a large connecting area results in small taper degree, and a large torque value translates to the low microleakage level of bacteria (Ranieri, et al., 2015).
Disadvantage of microgap and micromovement Microgap leads to microleakage The internal cavity of implant is similar to a reservoir (Nayak et al., 2014; Orsini et al., 2000; Proff et al., 2006). When the abutment is removed and replaced, bacteria can enter the implant internal cavity, where they reside and proliferate. The bacteria with their toxic by-products and small nutritious molecules can freely penetrate into the implant internal cavity or reverse through the IAI microgap. Thus, for two-piece implants, bacteria come from both the periimplant and implant internal cavity (Broggini, et al., 2006). Moreover, the internal cavity of implant is characterized by easy entrance but difficult eradication for bacteria, thereby leading to the continuous existence of bacteria and their toxic by-products around the IAI. In addition to the toxic bacterial by-products, endotoxin, which is a small molecule complex of lipopolysaccharides and a component of gram-negative bacterial cell walls, plays an
important toxic role on marginal bone resorption processes (Nair et al., 1996). With smaller sizes, these endotoxins can penetrate gaps that are considerably smaller than a bacterium. After its release from the implant internal cavity, endotoxin can induce alveolar bone destruction via the osteoclast-activating pathway. Furthermore, a detectable immunological response in human whole blood has been observed (Harder, Quabius, Ossenkop, & Kern, 2012). The internal cavity of each implant system was inoculated with endotoxin, and the implant and corresponding abutment were connected together and stored under static conditions. Endotoxin contamination could be observed immediately at 5 min after inoculation in the supernatant of pyrogen-free water, which stores the inoculated and bolted implants (Harder, et al., 2010).
Micromotion destroys the stability of hard and soft tissues and aggravates microleakage The destruction of the IAI micromotion is mainly displayed in two aspects. First, micromotion interferes the attachment of soft tissue around the implant neck and disrupts the stability of soft tissue that has completed integration (Passos, Gressler May, Faria, Ozcan, & Bottino, 2013). Second, micromotion causes a micropumping effect (Ericsson, et al., 1995), which intensifies the leakage of bacteria and their toxic by-products and accelerates the blood, saliva, and proteoglycans (including the extracellular matrix and mucus layer) into the internal cavity of implant (Baixe, Tenenbaum, & Etienne, 2016). The latter provides nutrients for bacteria, aggravates bacteria colonization and proliferation, and decreases the removal torque values of abutment by creating a slippery environment (Sahin & Ayyildiz, 2014). King and colleagues (King, Hermann, Schoolfield, Buser, & Cochran, 2002) welded abutment and implant into a whole piece and observed that marginal bone absorption was considerably reduced. Relative to the non-welded two-piece implants, the size of the IAI microgap of the welded whole piece showed no change, but the micromotion was eliminated. Hence, micromotion is also an important cause of bone destruction.
Microgap synergizes with micromotion and causes mechanical damage Mechanical damages of microgap and micromotion include fretting wear, adhesive wear, and screw loosening (Jorn, Kohorst, Besdo, Borchers, & Stiesch, 2016; Sakamoto et al., 2016). Fretting wear refers to microfracture and chipping between the IAI, whereas adhesive wear is defined as the plastic deformation in the IAI (Blum, et al., 2015). Generally, for most of two-piece implants, the abutments should be fixed with implants through a screw according to the recommended torque value. Both of the implants and the abutments will transfer the occlusal loads from prosthetic suprastructure to the surrounding bone tissue through the IAI. Nevertheless, the IAI connection with poor margin fitness can cause undesired rapid stress, consequently leading to the loosening of the screw when masticating (Binon, 2000; Jung et al., 2008). Furthermore, Sahin and colleagues (Sahin &
Ayyildiz, 2014) demonstrated that large IAI microgap resulted in high microleakage degree and small removal torque value. The removal torque value should be the same as or higher than the tightening torque value (Barbosa et al., 2008; Spazzin et al., 2010), and it should be maintained at this state as long as possible. The reduction of removal torque values means that screws are prone to loosen, that is, the IAI microgap will promote the loosening of screw through causing microleakage (Sahin & Ayyildiz, 2014). In addition, during chewing, the IAI of all two-piece implants exhibits chipping and plastic deformation, which suggested that both fretting wear and adhesive wear occur (Blum, et al., 2015). In a study carried by Blum and colleagues, particles were found to be embedded in the layer of the IAI connecting surfaces or suspended within the microgap. The size and form of the wear particles varied depending on its location of IAI and which implant system it belonged to. In general, the sizes ranged from 2 to 30µm, and presented in various formations such as flat shape or round shape, etc. Meanwhile in all implant systems examined, plastic deformation could be observed in different degree (Blum, et al., 2015). In view of the Morse taper degree point, the small taper degree will result in close IAI connection and low level of fretting wear and plastic deformation when functioning (Rack, et al., 2013). In the perspective of material properties, when zirconia abutment is fixed on the titanium implant and functions together, the deformation energy further tends to distribute to the component with low Young’s modulus, that is, the implant (Saidin, et al., 2012; Stimmelmayr et al., 2012). Consequently, zirconia abutments are more likely to cause fretting wear and deformation on the implants than those of titanium abutments. The overall amount of microwear debris generated by zirconia abutments is also more than that of the titanium abutments (Stimmelmayr, et al., 2012). This observation may be attributed to that the interface of different rigidity materials is inclined to incur pure fretting wear, which is in contrast to that of the same rigidity material tends to cause both fretting wear and deformation. Furthermore, the bone mode might influence the mechanism of the wear at the IAI when under fatigue loading because the surrounding bone with different resiliencies will provide different buffering effects; afterward, the force transferred to the bone might change (Blum, et al., 2015).
Relationship between microgap, micromotion, and microleakage Under static condition, the bacteria can enter and proliferate in the implant internal cavity. The microgap provides the nutrition supply for the bacteria inside the implant internal cavity and causes them to migrate from the implant internal cavity to the surrounding tissue continuously. Therefore, the bacteria around the IAI exist continually. Rismanchian and colleagues (Rismanchian, et al., 2012) conducted a research on the microleakage of four ITI abutments with different microgap sizes.
They observed that the bacterial microleakage of the abutments after 5 h differs, but were not significantly different after 24 h. This result indicated that with extension of time, the influence of the microgap size on bacterial microleakage is gradually reduced, until no significant difference exists. Moreover, when functioning, the mismatch of the abutment and the implant, namely, the microgap, will incur a relative microwear and microshift between two components, which is collectively known as micromotion. The micromotion will in turn lead to fretting wear and plastic deformation, which further reduces the precise adaptation and increases the microgap size between them (Blum, et al., 2015). Microwear and plastic deformation will result in enlargement of microgap at a load of 98 N within 1 million cycles, during which the highest inclination of increase of the microgap was within the first 200,000 cycles, and then gradually increased (Blum, et al., 2015). Under cyclic loading, the microgap size would increase, the micromotion level would expand, and the interaction between the two factors would increase the degree of microleakage and damage the mechanical properties of the IAI connection (Rack, et al., 2013).
In summary, microgap permits the bacterial microleakage to persist around the IAI (Scarano, Lorusso, Di Giulio, & Mazzatenta, 2016) and further aggravates the micromotion when in function. Additionally, micromotion and microleakage both lead to fretting wear, plastic deformation, and screw loosening. These mechanical destructions will increase the micromotion and microgap (Gratton, Aquilino, & Stanford, 2001), thereby increasing the microleakage and mechanical damage and causing a malignant circulation. Thus, microgap is the fundamental cause of microleakage, and micromotion is the key factor for microleakage. Both components influence and promote each other. Consequently, they synergize the microleakage of bacteria and endotoxins around the IAI, which ultimately induces the marginal bone loss around the neck of the implant. Micromotion will also destruct the osseointegration by causing mechanical damage (Figure 3).
CLINICAL IMPLICATIONS Appropriate application of chlorhexidine As a surface-active agent with strong effects of broad-spectrum bacteriostasis and sterilization, chlorhexidine is generally used for treating periodontal diseases (Koutouzis, Gadalla, Kettler, Elbarasi, & Nonhoff, 2015), due to its efficient antimicrobial and antifungal function (Sheen & Addy, 2003). In a research conducted by Paolantonio and colleagues (Paolantonio et al., 2008), clinical and microbiological parameters were recorded three months after completion of prosthodontic restoration. The internal cavity of the implant was filled with 1% chlorhexidine gel, and then the implant was connected with the abutment. After six months, the clinical and microbiological parameters were re-examined. The test group effectively prevented bacterial colonization and caused significant improvements for
clinical parameters in contrast with the control group. Similarly, Groenendijk and colleagues (Groenendijk, Dominicus, Moorer, Aartman, & van Waas, 2004) reported that 0.2% of chlorhexidine solution inhibited bacterial growth, and the beneficial effect could last for six weeks. Conversely, some scholars held that only irrigation by chlorhexidine solution exerted no significant effect on bacterial penetration into IAI (Romanos, Biltucci, Kokaras, & Paster, 2016; Wennstrom, Dahlen, Grondahl, & Heijl, 1987). For instance, Koutouzis and colleagues (Koutouzis, et al., 2015) inoculated the internal cavity of the implant with 0.2% chlorhexidine solution and sterile saline respectively. Subsequently, the abutments were replaced, and the implants were cyclic loaded. Endotoxins were detected in both groups, and no statistically significant differences were observed at all of the testing points. The differences in the above results may be due to the different exposure time to chlorhexidine when located in the implant internal cavity and subgingival area (Ready et al., 2015). To summarize, the benefit of chlorhexidine is hard to justify in light of the opposing evidences. Therefore, the using of chlorhexidine in the oral implantation is suggested with possible benefits, and the appropriate dosage and form of chlorhexidine which could make its bactericidal effect more durable still need further studies.
Selection of Morse taper connection implants and corresponding platform switching abutments Mangano and colleagues (Mangano et al., 2009) evaluated the survival rate and related clinical indexes of 1920 Morse taper connection implants. They revealed that the use of tapered abutments minimized the IAI microgaps and enhances the mechanical stability, which eventually reduced the crestal bone loss and prosthetic complications. Furthermore, the use of platform switching abutments keeps the microleakage and micromotion at the IAI distant from the alveolar ridge. This technique directly reduces the pollution of bacteria and endotoxins (Canullo, Pace, Coelho, Sciubba, & Vozza, 2011; Wang, Kan, Rungcharassaeng, Roe, & Lozada, 2015) and transfers the microenvironment, which is detrimental to the integration of implant and peri-implant tissue away from the IAI and close to the implant center (Passos, et al., 2013). The large platform width results in improved periimplant tissue repair and rebuilding. For the same implant system and abutment, the larger implant diameter, i.e., wider platform, results in less marginal bone loss (Cumbo et al., 2013), which is also beneficial for the formation of a relatively thin and uniform connective tissue sealing (Canullo, Fedele, Iannello, & Jepsen, 2010). Therefore, Morse taper connection implants and platform switching abutments could be used to reduce the resorption of the alveolar crest (Canullo, et al., 2010; Farronato et al., 2012; Hurzeler, Fickl, Zuhr, & Wachtel, 2007; Novaes, de Oliveira, Muglia, Papalexiou, & Taba, 2006).
Selection of abutments and retained mode As for the biofilm mass on the surfaces of abutments, some scholars regarded that zirconia abutments accumulate small amounts of biofilm mass and bacteria because the zirconia material can lower the susceptibility to microorganism adhesion (Hisbergues, Vendeville, & Vendeville, 2009; Nascimento et al., 2014; Nascimento, et al., 2016; Scarano, Piattelli, Caputi, Favero, & Piattelli, 2004). However, within this clinical research, either the implants are tested without prosthetic suprastructure and load (de Oliveira et al., 2012; Nascimento, et al., 2014) or zirconia abutments are placed in the anterior region; additionally, titanium abutments are placed in the posterior region, despite the availability of prosthetic suprastructures (Nascimento, et al., 2016). Thus, a defect exists since the effect of masticatory force on the microleakage of IAI is not compared when these two abutments are functioning. Consequently, unless necessary, such that the missing teeth are in the esthetic zone (usually in anterior region), or the patients express high aesthetic requirements, zirconia abutments are not recommended to reduce the microgap and microleakage of IAI. When zirconia abutments should be selected, the manufacturer-recommended torque values should be strictly followed (Smith & Turkyilmaz, 2014). For the same reason, premachined abutments, rather than custom abutments, are suggested because premachined abutments show smaller microgaps than those of customized abutments. Regarding the retained mode of prosthetic suprastructure, all advantages and disadvantages of cement-retained and screw-retained modes should be synthetically considered to connect the abutment and prosthetic suprastructure appropriately. Regardless of whether the test is in vivo (Keller, Bragger, & Mombelli, 1998) or in vitro (Passos, et al., 2013; Penarrocha-Oltra et al., 2016), the microleakage at the IAI of cement-retained prosthesis is smaller than that of screwretained prosthesis. Lemos and colleagues (Lemos et al., 2016) conducted a systematic review and meta-analysis and concluded that cement-retained prosthesis exhibited less marginal bone loss and higher survival rate than those of screw-retained prosthesis during follow-up, which ranges from 12 months to 180 months. Nevertheless, some deficiencies associated with cement-retained prosthesis still exist. Among these deficiencies, the most important is the difficulty in clearing residual cement, which would promote biofilm formation (Busscher, Rinastiti, Siswomihardjo, & van der Mei, 2010), increase the peri-implant gingival sulcus bacterial loads (Penarrocha-Oltra, et al., 2016), and lead to peri-implantitis (Korsch, Obst, & Walther, 2014). In general, when the IAI is deep in subgingival, such as when implants are placed deep, or the gingival tissue is thick, screw retained prosthesis is preferred, especially when the patient presents a history of periodontal disease. On the contrary, when the IAI is not deeply located, cement-retained prosthesis is favored provided that the remaining cement can be cleared away meticulously.
SUMMARY AND CONCLUSIONS
To date, two-piece implant systems are widely used in the clinic. However, microgap and micromotion inevitably exist at the IAI. This phenomenon results in both microleakage and mechanical damage, which finally incur bone resorption around the implant neck. Clinicians should sufficiently recognize and understand the characteristics of the two-piece structures. To reduce the bone resorption engendered by the microgap and micromotion at the IAI, the first and most important thing is to select suitable Morse taper or hybrid connection implants and platform switching abutments, which not only reduce the pollution of bacteria and endotoxins directly, but also transfer the harmful microenvironment away from the IAI and close to the implant center. Moreover, chlorhexidine solution or gel may provide some benefits, zirconia and custom abutments should be chosen sparingly, and appropriate retain mode should be selected for suprastructure.
ACKNOWLEDGEMENT This study was financially supported by the grants from National Natural Science Foundation of China (81570956) and the Bureau of Science and Technology of Wuhan ([2014]160, 2015060101010051).
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Figr-4Fig 1. (a) Butt joint connection. (b) Tapered connection. (c) Hybrid connection. Fig 2. Green arrow refers to the microabrasion and relative microshift between the implant and the abutment; blue arrow refers to microrotation of the abutment relative to the implant. (a) Fretting wear means the microfracture and chipping between the IAI. (b) Adhesive wear is defined as the plastic deformation in the IAI. Fig 3. Influences of microgap and micromotion of IAI on marginal bone loss around implant neck.
S0003-9969(17)30243-1 http://dx.doi.org/doi:10.1016/j.archoralbio.2017.07.022 AOB 3960
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Archives of Oral Biology
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19-2-2017 19-7-2017 27-7-2017
Please cite this article as: Liu Yang, Wang Jiawei.Influences of microgap and micromotion of implant–abutment interface on marginal bone loss around implant neck.Archives of Oral Biology http://dx.doi.org/10.1016/j.archoralbio.2017.07.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influences of microgap and micromotion of implant– abutment interface on marginal bone loss around implant neck Running title: Microgap and micromotion of IAI Author names： Yang Liu, MD; E-mail: [email protected]. Jiawei Wang, PhD; E-mail: [email protected].
Authors’ affiliation: Department of Prosthodontics, School of Stomatology, Wuhan University, Wuhan, 430079, China.
Corresponding author: Prof. Jiawei Wang, The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST), Key Laboratory of Oral Biomedicine Ministry of Education, School & Hospital of Stomatology, Wuhan University, 237 Luoyu Road, Wuhan 430079, China; e-mail: [email protected].
Conflicts of interest: none
Highlights Elaborated respectively the influence mechanisms of the microgap and micromotion between the implant and the abutment on implant marginal bone loss in detail; The relationship between microgap, micromotion, microleakage and mechanical damage, and their influences on bone resorption around implant neck are particularly explained; Some feasible clinical methods to reduce the bone resorption engendered by the microgap and micromotion at the IAI are recommended.
ABSTRACT Objective: To review the influences and clinical implications of micro-gap and micro-motion of implant-abutment interface on marginal bone loss around the neck of implant. Design: Literatures were searched based on the following Keywords: implant-abutment interface/implant-abutment connection/implant-abutment conjunction, microgap, micromotion/micromovement, microleakage, and current control methods available. The papers were then screened through titles, abstracts, and full texts. Results: A total of 83 studies were included in the literature review. Two-piece implant systems are widely used in clinics. However, the production error and masticatory load result in the presence of microgap and micromotion between the implant and the abutment, which directly or indirectly causes microleakage and mechanical damage. Consequently, the degrees of microgap and micromotion further increase, and marginal bone absorption finally occurs. We summarize the influences of microgap and micromotion at the implant-abutment interface on marginal bone loss around the neck of the implant. We also recommend some feasible methods to reduce their effect. Conclusions: Clinicians and patients should pay more attention to the mechanisms as well as the control methods of microgap and micromotion. To reduce the corresponding detriment to the implant marginal bone, suitable Morse taper or hybrid connection implants and platform switching abutments should be selected, as well as other potential methods. Keywords: Implant-abutment interface, Microgap, Micromotion, Microleakage, Marginal bone loss
KEYWORDS: Implant-abutment interface; Microgap; Micromotion; Microleakage; Marginal bone loss
INTRODUCTION Marginal bone loss around the neck of dental implant is one of the most common complications after implantation and exerts remarkable influence on the future success and long-term stability of the implant. Generally, when the implant is placed into the alveolar bone, the resorption of marginal bone usually begins from the bone cortex (Branemark et al., 1969). Factors contributing to the loss of marginal bone include surgical trauma, periimplantitis, occlusal overload, microleakage, biologic width, and implant anatomy on the crest area (Macedo et al., 2016; Oh, Yoon, Misch, & Wang, 2002). The phrase microleakage of the implant-abutment interface (IAI) was coined in the 1990s, and it describes a microbial leakage between the implant and the abutment, which is attributed to the microgap and micromotion of the IAI. Efforts have been exerted over the last two decades to explore the discrepancy in the microleakage level within different implant systems and the reasons behind this phenomenon. Some scholars considered the microgap responsible for the phenomenon, and others deem it as the result of micromotion. In this article, the keywords were determined as, for instance, implant-abutment interface/implant-abutment connection/implant-abutment conjunction, microgap, micromotion/micromovement, microleakage, and current control methods available. The literatures were searched based on the keywords up to February 2017.To be analyzed in the review, papers had to (i) be written in English, (ii) be published in an international peer-reviewed journal, and (iii) have a clear definition for microleakage and related keywords. The titles and abstracts for eligible papers were screened. If eligibility aspects were present in the title, the paper was selected for further reading. If none of the eligibility aspects were mentioned in the title, the abstract was read in detail and screened for suitability. After selection, full-text papers were read in detail. The search resulted in 4350 records of titles and abstracts. Screening of these titles and abstracts initially resulted in 264 articles. Based on detailed reading of full texts, 181 articles were excluded and 83 studies were identified eligible for inclusion in the literature review. The influences of microgap and micromotion existing between the implant and the abutmentinterface on marginal bone loss are reviewed and clearly illustrated, and their clinical significances are discussed.
MICROGAP, MICROMOTION, AND MICROLEAKAGE Two-piece implant system Given the protection of implant from unwanted load during bone healing phase and the beneficial potential to adjust the prosthetic angle, the two-piece implant system is widely used in clinics. The IAI connection of the two-piece implant system includes two types: external
and internal connections. In a typical external connection, the implant convex extends outside by 1–2 mm, thereby forming an external structure similar to a hexagon or an octagon, which connects to the abutment. External connection is incorporated in some systems and once commonly used worldwide during the period when two-piece implant system was initially used because of its superior antirotational mechanism and ability to orient the abutment in the implant (Davi, Golin, Bernardes, Araujo, & Neves, 2008; Gracis et al., 2012). However, the short and narrow external geometry is particularly vulnerable when off axis loads are applied, which consequently leads to the deformation of the IAI (Binon, 2000; Gracis, et al., 2012). Along with the rapid development of two-piece implant system, internal connection including but not limited to internal hex connection gradually occupies a larger share in the market. Internal connection refers to the abutment stretches of 4–6 mm into the implant cavity. Subsequently, this abutment fixes with the implant and forms a conical, octagonal, hexagonal, trilobe, or spline design. Internal connection includes taper, butt joint, and hybrid connections (Figure 1). The taper connection originates from the concept of Morse taper in mechanical engineering and simply means a cone within another cone (Hernigou, Queinnec, & Flouzat Lachaniette, 2013; Oh, et al., 2002). Therefore, this connection is also called conical connection. Butt joint connection refers to the connecting area between the implant and the abutment without a taper, and it is only retained via a retaining screw. A hybrid connection means that both Morse taper design and the regular polygonal shape of antirotational or guiding grooves are present. Compared with the external connection, internal connection remarkably lowers the rotation center and improves the mechanical stability (Sailer, Sailer, Stawarczyk, Jung, & Hammerle, 2009). Furthermore, when internal connection adopts the form of platform switching, the stress distribution of the peri-IAI bone is reduced (AlvarezArenal et al., 2017; Liu et al., 2014).
Microgap The implant and the abutment cannot be accurately matched because of the precision limit during production (Alves, de Carvalho, Elias, Vedovatto, & Martinez, 2016). The IAI microgap, defined as the microscopic space between implant and corresponding abutment, exists (Scarano, Mortellaro, Mavriqi, Pecci, & Valbonetti, 2016). The microgap between the titanium abutment and the titanium implant is smaller than that between the zirconia abutment and the titanium implant. Moreover, the IAI microgaps of zirconia abutments increase significantly when torque values less than those of manufacturer-recommended values are applied (Hernigou, et al., 2013; Rack, Zabler, Rack, Riesemeier, & Nelson, 2013). Concerning the manufacturing technique, the premachined abutments exhibit smaller microgaps than those of cast on and castable abutments (Harder et al., 2010; Rismanchian, Hatami, Badrian, Khalighinejad, & Goroohi, 2012).
From the perspective of IAI connection style, Morse taper connection is sealed better than butt joint connection (Khorshidi, Raoofi, Moattari, Bagheri, & Kalantari, 2016). Fixing of taper connection depends on friction. Thus, the fitting degree of IAI connection mainly relates with the taper degree and connecting area. When the taper degree of the internal cone is larger than 5.8°, the removing force is smaller than the tightening force (Bozkaya & Muftu, 2004). On the contrary, the small taper degree results in large removing force and tight IAI. Additionally, the IAI under cyclic loading increases and becomes close with time, thereby achieving a metal–to–metal cold welding (Norton, 1999). Currently, except for one type of taper connection that is totally fixed by 1.5° Morse taper and a large contact surface of the implant and the respective abutment (Broggini et al., 2003; Dibart, Warbington, Su, & Skobe, 2005), all other connections need a certain preloaded screw to achieve and maintain the close connection of IAI (Aloise et al., 2010).
Micromotion/micromovement For two-piece implant system, although the micromotion in the IAI connections decreases due to a precise fabrication of the implant and the abutment, the current production process cannot avoid the micromotion when chewing between the abutment and the implant (Binon, 1996; Vigolo, Fonzi, Majzoub, & Cordioli, 2006). The micromotion of IAI includes the microabrasion and relative microshift between the implant and the abutment and the microrotation of the abutment relative to the implant (Figure 2). The micromotion size generally ranges from 1.52 µm to 94.00 µm (Karl & Taylor, 2014). According to the IAI connection style point, butt joint connection tends to fret, and taper connection is likely to rotate. In addition, oral environment and liquid also influence the wear mechanism to a certain extent. The peri-implant microenvironment (Ericsson et al., 1995), which is composed of blood, saliva, and biofilm, acts as a lubricant (Karl & Taylor, 2014) that connects the implant internal cavity and the peri-implant oral environment and decreases the friction resistance. In an antirotation study, the rotational micromotion degree of the internal trilobe connection and internal taper connection is the lowest and highest, respectively; moreover, such degree of internal hexagonal and octagonal connections, which produce similar patterns of micromotion and stress distribution, is the middle value (Saidin, Abdul Kadir, Sulaiman, & Abu Kasim, 2012). Consequently, for single implant without adjacent teeth, the increased number of guiding polygons lessens the resistant to rotational micromotion. Furthermore, Semper and colleagues (Semper, Kraft, Kruger, & Nelson, 2009) found that regular polygons display lower antirotation than that of rounded polygonal profile. Compared with the taper connection, butt joint connection exhibits better antirotational micromotion, but is more prone to fretting wear.
Microleakage
For most two-piece implant systems, the microgap size ranges from 0.1 µm to 10 µm after connection of the two components and prior to loading; this size may increase after cyclic loading. However, most oral bacteria are within the width of 0.2–1.5 µm and length of 2–10 µm (Nascimento et al., 2016). Therefore, bacteria and endotoxin can freely pass through the IAI microgap and enter the implant internal cavity, which results in the biomaterial exchange between the implant internal cavity and the peri-implant oral environment (Teixeira, Ribeiro, Sato, & Pedrazzi, 2011). The passing of bacteria, bacterial toxic byproducts, and small molecules through the IAI microgap and penetration into the implant internal cavity or vice versa is defined as the IAI microleakage (Ericsson, et al., 1995; Gross, Abramovich, & Weiss, 1999; Quirynen, Bollen, Eyssen, & van Steenberghe, 1994; Quirynen & van Steenberghe, 1993). Broggini and colleagues (Broggini et al., 2006) found that the infiltration of neutrophils near IAI increased with increasing implanting depth; additionally, the peak concentration of neutrophils was constantly around the IAI, regardless of the implant position. The authors believed that the IAI microleakage could cause a persistent inflammatory process, which ultimately led to alveolar bone destruction (Oh, et al., 2002; Siar et al., 2003). Whether under static condition without load (Tesmer, Wallet, Koutouzis, & Lundgren, 2009) or under dynamic cyclic load condition (Koutouzis, Wallet, Calderon, & Lundgren, 2011), the microleakage of the Morse taper connection is smaller than that of the butt joint connection. Along with the taper degree, the connecting area considerably affects the IAI connection intimacy (Baggi, Di Girolamo, Mirisola, & Calcaterra, 2013; Blum et al., 2015; do Nascimento, Pedrazzi, Miani, Moreira, & de Albuquerque, 2009). In different implant systems, the taper degree and connecting area are different, which are mostly responsible for the differences in bacterial penetration (Scarano et al., 2016). The torque value applied is also important (Ranieri et al., 2015). Commonly, a large connecting area results in small taper degree, and a large torque value translates to the low microleakage level of bacteria (Ranieri, et al., 2015).
Disadvantage of microgap and micromovement Microgap leads to microleakage The internal cavity of implant is similar to a reservoir (Nayak et al., 2014; Orsini et al., 2000; Proff et al., 2006). When the abutment is removed and replaced, bacteria can enter the implant internal cavity, where they reside and proliferate. The bacteria with their toxic by-products and small nutritious molecules can freely penetrate into the implant internal cavity or reverse through the IAI microgap. Thus, for two-piece implants, bacteria come from both the periimplant and implant internal cavity (Broggini, et al., 2006). Moreover, the internal cavity of implant is characterized by easy entrance but difficult eradication for bacteria, thereby leading to the continuous existence of bacteria and their toxic by-products around the IAI. In addition to the toxic bacterial by-products, endotoxin, which is a small molecule complex of lipopolysaccharides and a component of gram-negative bacterial cell walls, plays an
important toxic role on marginal bone resorption processes (Nair et al., 1996). With smaller sizes, these endotoxins can penetrate gaps that are considerably smaller than a bacterium. After its release from the implant internal cavity, endotoxin can induce alveolar bone destruction via the osteoclast-activating pathway. Furthermore, a detectable immunological response in human whole blood has been observed (Harder, Quabius, Ossenkop, & Kern, 2012). The internal cavity of each implant system was inoculated with endotoxin, and the implant and corresponding abutment were connected together and stored under static conditions. Endotoxin contamination could be observed immediately at 5 min after inoculation in the supernatant of pyrogen-free water, which stores the inoculated and bolted implants (Harder, et al., 2010).
Micromotion destroys the stability of hard and soft tissues and aggravates microleakage The destruction of the IAI micromotion is mainly displayed in two aspects. First, micromotion interferes the attachment of soft tissue around the implant neck and disrupts the stability of soft tissue that has completed integration (Passos, Gressler May, Faria, Ozcan, & Bottino, 2013). Second, micromotion causes a micropumping effect (Ericsson, et al., 1995), which intensifies the leakage of bacteria and their toxic by-products and accelerates the blood, saliva, and proteoglycans (including the extracellular matrix and mucus layer) into the internal cavity of implant (Baixe, Tenenbaum, & Etienne, 2016). The latter provides nutrients for bacteria, aggravates bacteria colonization and proliferation, and decreases the removal torque values of abutment by creating a slippery environment (Sahin & Ayyildiz, 2014). King and colleagues (King, Hermann, Schoolfield, Buser, & Cochran, 2002) welded abutment and implant into a whole piece and observed that marginal bone absorption was considerably reduced. Relative to the non-welded two-piece implants, the size of the IAI microgap of the welded whole piece showed no change, but the micromotion was eliminated. Hence, micromotion is also an important cause of bone destruction.
Microgap synergizes with micromotion and causes mechanical damage Mechanical damages of microgap and micromotion include fretting wear, adhesive wear, and screw loosening (Jorn, Kohorst, Besdo, Borchers, & Stiesch, 2016; Sakamoto et al., 2016). Fretting wear refers to microfracture and chipping between the IAI, whereas adhesive wear is defined as the plastic deformation in the IAI (Blum, et al., 2015). Generally, for most of two-piece implants, the abutments should be fixed with implants through a screw according to the recommended torque value. Both of the implants and the abutments will transfer the occlusal loads from prosthetic suprastructure to the surrounding bone tissue through the IAI. Nevertheless, the IAI connection with poor margin fitness can cause undesired rapid stress, consequently leading to the loosening of the screw when masticating (Binon, 2000; Jung et al., 2008). Furthermore, Sahin and colleagues (Sahin &
Ayyildiz, 2014) demonstrated that large IAI microgap resulted in high microleakage degree and small removal torque value. The removal torque value should be the same as or higher than the tightening torque value (Barbosa et al., 2008; Spazzin et al., 2010), and it should be maintained at this state as long as possible. The reduction of removal torque values means that screws are prone to loosen, that is, the IAI microgap will promote the loosening of screw through causing microleakage (Sahin & Ayyildiz, 2014). In addition, during chewing, the IAI of all two-piece implants exhibits chipping and plastic deformation, which suggested that both fretting wear and adhesive wear occur (Blum, et al., 2015). In a study carried by Blum and colleagues, particles were found to be embedded in the layer of the IAI connecting surfaces or suspended within the microgap. The size and form of the wear particles varied depending on its location of IAI and which implant system it belonged to. In general, the sizes ranged from 2 to 30µm, and presented in various formations such as flat shape or round shape, etc. Meanwhile in all implant systems examined, plastic deformation could be observed in different degree (Blum, et al., 2015). In view of the Morse taper degree point, the small taper degree will result in close IAI connection and low level of fretting wear and plastic deformation when functioning (Rack, et al., 2013). In the perspective of material properties, when zirconia abutment is fixed on the titanium implant and functions together, the deformation energy further tends to distribute to the component with low Young’s modulus, that is, the implant (Saidin, et al., 2012; Stimmelmayr et al., 2012). Consequently, zirconia abutments are more likely to cause fretting wear and deformation on the implants than those of titanium abutments. The overall amount of microwear debris generated by zirconia abutments is also more than that of the titanium abutments (Stimmelmayr, et al., 2012). This observation may be attributed to that the interface of different rigidity materials is inclined to incur pure fretting wear, which is in contrast to that of the same rigidity material tends to cause both fretting wear and deformation. Furthermore, the bone mode might influence the mechanism of the wear at the IAI when under fatigue loading because the surrounding bone with different resiliencies will provide different buffering effects; afterward, the force transferred to the bone might change (Blum, et al., 2015).
Relationship between microgap, micromotion, and microleakage Under static condition, the bacteria can enter and proliferate in the implant internal cavity. The microgap provides the nutrition supply for the bacteria inside the implant internal cavity and causes them to migrate from the implant internal cavity to the surrounding tissue continuously. Therefore, the bacteria around the IAI exist continually. Rismanchian and colleagues (Rismanchian, et al., 2012) conducted a research on the microleakage of four ITI abutments with different microgap sizes.
They observed that the bacterial microleakage of the abutments after 5 h differs, but were not significantly different after 24 h. This result indicated that with extension of time, the influence of the microgap size on bacterial microleakage is gradually reduced, until no significant difference exists. Moreover, when functioning, the mismatch of the abutment and the implant, namely, the microgap, will incur a relative microwear and microshift between two components, which is collectively known as micromotion. The micromotion will in turn lead to fretting wear and plastic deformation, which further reduces the precise adaptation and increases the microgap size between them (Blum, et al., 2015). Microwear and plastic deformation will result in enlargement of microgap at a load of 98 N within 1 million cycles, during which the highest inclination of increase of the microgap was within the first 200,000 cycles, and then gradually increased (Blum, et al., 2015). Under cyclic loading, the microgap size would increase, the micromotion level would expand, and the interaction between the two factors would increase the degree of microleakage and damage the mechanical properties of the IAI connection (Rack, et al., 2013).
In summary, microgap permits the bacterial microleakage to persist around the IAI (Scarano, Lorusso, Di Giulio, & Mazzatenta, 2016) and further aggravates the micromotion when in function. Additionally, micromotion and microleakage both lead to fretting wear, plastic deformation, and screw loosening. These mechanical destructions will increase the micromotion and microgap (Gratton, Aquilino, & Stanford, 2001), thereby increasing the microleakage and mechanical damage and causing a malignant circulation. Thus, microgap is the fundamental cause of microleakage, and micromotion is the key factor for microleakage. Both components influence and promote each other. Consequently, they synergize the microleakage of bacteria and endotoxins around the IAI, which ultimately induces the marginal bone loss around the neck of the implant. Micromotion will also destruct the osseointegration by causing mechanical damage (Figure 3).
CLINICAL IMPLICATIONS Appropriate application of chlorhexidine As a surface-active agent with strong effects of broad-spectrum bacteriostasis and sterilization, chlorhexidine is generally used for treating periodontal diseases (Koutouzis, Gadalla, Kettler, Elbarasi, & Nonhoff, 2015), due to its efficient antimicrobial and antifungal function (Sheen & Addy, 2003). In a research conducted by Paolantonio and colleagues (Paolantonio et al., 2008), clinical and microbiological parameters were recorded three months after completion of prosthodontic restoration. The internal cavity of the implant was filled with 1% chlorhexidine gel, and then the implant was connected with the abutment. After six months, the clinical and microbiological parameters were re-examined. The test group effectively prevented bacterial colonization and caused significant improvements for
clinical parameters in contrast with the control group. Similarly, Groenendijk and colleagues (Groenendijk, Dominicus, Moorer, Aartman, & van Waas, 2004) reported that 0.2% of chlorhexidine solution inhibited bacterial growth, and the beneficial effect could last for six weeks. Conversely, some scholars held that only irrigation by chlorhexidine solution exerted no significant effect on bacterial penetration into IAI (Romanos, Biltucci, Kokaras, & Paster, 2016; Wennstrom, Dahlen, Grondahl, & Heijl, 1987). For instance, Koutouzis and colleagues (Koutouzis, et al., 2015) inoculated the internal cavity of the implant with 0.2% chlorhexidine solution and sterile saline respectively. Subsequently, the abutments were replaced, and the implants were cyclic loaded. Endotoxins were detected in both groups, and no statistically significant differences were observed at all of the testing points. The differences in the above results may be due to the different exposure time to chlorhexidine when located in the implant internal cavity and subgingival area (Ready et al., 2015). To summarize, the benefit of chlorhexidine is hard to justify in light of the opposing evidences. Therefore, the using of chlorhexidine in the oral implantation is suggested with possible benefits, and the appropriate dosage and form of chlorhexidine which could make its bactericidal effect more durable still need further studies.
Selection of Morse taper connection implants and corresponding platform switching abutments Mangano and colleagues (Mangano et al., 2009) evaluated the survival rate and related clinical indexes of 1920 Morse taper connection implants. They revealed that the use of tapered abutments minimized the IAI microgaps and enhances the mechanical stability, which eventually reduced the crestal bone loss and prosthetic complications. Furthermore, the use of platform switching abutments keeps the microleakage and micromotion at the IAI distant from the alveolar ridge. This technique directly reduces the pollution of bacteria and endotoxins (Canullo, Pace, Coelho, Sciubba, & Vozza, 2011; Wang, Kan, Rungcharassaeng, Roe, & Lozada, 2015) and transfers the microenvironment, which is detrimental to the integration of implant and peri-implant tissue away from the IAI and close to the implant center (Passos, et al., 2013). The large platform width results in improved periimplant tissue repair and rebuilding. For the same implant system and abutment, the larger implant diameter, i.e., wider platform, results in less marginal bone loss (Cumbo et al., 2013), which is also beneficial for the formation of a relatively thin and uniform connective tissue sealing (Canullo, Fedele, Iannello, & Jepsen, 2010). Therefore, Morse taper connection implants and platform switching abutments could be used to reduce the resorption of the alveolar crest (Canullo, et al., 2010; Farronato et al., 2012; Hurzeler, Fickl, Zuhr, & Wachtel, 2007; Novaes, de Oliveira, Muglia, Papalexiou, & Taba, 2006).
Selection of abutments and retained mode As for the biofilm mass on the surfaces of abutments, some scholars regarded that zirconia abutments accumulate small amounts of biofilm mass and bacteria because the zirconia material can lower the susceptibility to microorganism adhesion (Hisbergues, Vendeville, & Vendeville, 2009; Nascimento et al., 2014; Nascimento, et al., 2016; Scarano, Piattelli, Caputi, Favero, & Piattelli, 2004). However, within this clinical research, either the implants are tested without prosthetic suprastructure and load (de Oliveira et al., 2012; Nascimento, et al., 2014) or zirconia abutments are placed in the anterior region; additionally, titanium abutments are placed in the posterior region, despite the availability of prosthetic suprastructures (Nascimento, et al., 2016). Thus, a defect exists since the effect of masticatory force on the microleakage of IAI is not compared when these two abutments are functioning. Consequently, unless necessary, such that the missing teeth are in the esthetic zone (usually in anterior region), or the patients express high aesthetic requirements, zirconia abutments are not recommended to reduce the microgap and microleakage of IAI. When zirconia abutments should be selected, the manufacturer-recommended torque values should be strictly followed (Smith & Turkyilmaz, 2014). For the same reason, premachined abutments, rather than custom abutments, are suggested because premachined abutments show smaller microgaps than those of customized abutments. Regarding the retained mode of prosthetic suprastructure, all advantages and disadvantages of cement-retained and screw-retained modes should be synthetically considered to connect the abutment and prosthetic suprastructure appropriately. Regardless of whether the test is in vivo (Keller, Bragger, & Mombelli, 1998) or in vitro (Passos, et al., 2013; Penarrocha-Oltra et al., 2016), the microleakage at the IAI of cement-retained prosthesis is smaller than that of screwretained prosthesis. Lemos and colleagues (Lemos et al., 2016) conducted a systematic review and meta-analysis and concluded that cement-retained prosthesis exhibited less marginal bone loss and higher survival rate than those of screw-retained prosthesis during follow-up, which ranges from 12 months to 180 months. Nevertheless, some deficiencies associated with cement-retained prosthesis still exist. Among these deficiencies, the most important is the difficulty in clearing residual cement, which would promote biofilm formation (Busscher, Rinastiti, Siswomihardjo, & van der Mei, 2010), increase the peri-implant gingival sulcus bacterial loads (Penarrocha-Oltra, et al., 2016), and lead to peri-implantitis (Korsch, Obst, & Walther, 2014). In general, when the IAI is deep in subgingival, such as when implants are placed deep, or the gingival tissue is thick, screw retained prosthesis is preferred, especially when the patient presents a history of periodontal disease. On the contrary, when the IAI is not deeply located, cement-retained prosthesis is favored provided that the remaining cement can be cleared away meticulously.
SUMMARY AND CONCLUSIONS
To date, two-piece implant systems are widely used in the clinic. However, microgap and micromotion inevitably exist at the IAI. This phenomenon results in both microleakage and mechanical damage, which finally incur bone resorption around the implant neck. Clinicians should sufficiently recognize and understand the characteristics of the two-piece structures. To reduce the bone resorption engendered by the microgap and micromotion at the IAI, the first and most important thing is to select suitable Morse taper or hybrid connection implants and platform switching abutments, which not only reduce the pollution of bacteria and endotoxins directly, but also transfer the harmful microenvironment away from the IAI and close to the implant center. Moreover, chlorhexidine solution or gel may provide some benefits, zirconia and custom abutments should be chosen sparingly, and appropriate retain mode should be selected for suprastructure.
ACKNOWLEDGEMENT This study was financially supported by the grants from National Natural Science Foundation of China (81570956) and the Bureau of Science and Technology of Wuhan ([2014]160, 2015060101010051).
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Figr-4Fig 1. (a) Butt joint connection. (b) Tapered connection. (c) Hybrid connection. Fig 2. Green arrow refers to the microabrasion and relative microshift between the implant and the abutment; blue arrow refers to microrotation of the abutment relative to the implant. (a) Fretting wear means the microfracture and chipping between the IAI. (b) Adhesive wear is defined as the plastic deformation in the IAI. Fig 3. Influences of microgap and micromotion of IAI on marginal bone loss around implant neck.