Efficacy of Various Implant Abutment Screw Access Channel Sealing Materials in Preventing Microleakage: A Systematic Review

Dental implants are widely used to rehabilitate completely and partially edentulous arches. Studies have shown a high success rate of these implant supported prosthesis, making them a reliable and preferred treatment option.^1–3  Various factors play a vital role in the success of these osseointegrated implants. These factors can be broadly divided into (a) operator dependent, (b) patient dependent, and (c) implant component related factors.^3–5 

Failure of dental implants can be attributed to peri-implant mucositis and peri-implantitis, which may be related to systemic conditions, poor oral hygiene, smoking, and microbial leakage between various components of implant assembly.^6–9  Apart from implant-abutment interface, microleakage and microbial colonization, in a 2-piece dental implant, can also take place through the abutment screw access channel (ASAC).^10–13  Studies by Quirynen et al¹²  and do Nascimento et al¹⁴  showed bidirectional microbial leakage and fluid diffusion, which act as bacterial reservoirs in the internal parts of implants. These are predominantly anaerobic proteolytic oral bacteria, which also produce unpleasant smell, commonly encountered after unscrewing healing abutment or a previous abutment from a functional implant.^15–17 

Studies have shown that hollow spaces of ASAC may act as microleakage junction and as a reservoir of these anaerobic bacteria.¹⁸  Thus, it is imperative to seal the ASAC, to prevent or to minimize this microleakage. The existing studies used various materials like zinc oxide eugenol, gutta-percha (GP), cotton pellet, autopolymerizing acrylic resin, Panacea resin, composite resin, vinyl polysiloxane (VPS), and polytetrafluoroethylene (PTFE) tape to seal ASAC and to protect the abutment screw head.^17–25  None of the studies could clearly recommend any 1 material as an ideal material for sealing.^26–32  This deficient information obligates the dentist to depend upon their own clinical experience for selecting the material for sealing the ASAC.

There is no systematic review to date that assess the various materials used for sealing the ASAC, to prevent microleakage. The findings are vital as these can guide the dentist in selecting the best sealing material, and thus consequently reducing peri-implant mucositis and peri-implantitis and delaying the failure of the dental implant. Thus, the aim of the current study is to conduct a systematic review, through the analysis of the articles, on the evaluation of efficacy of various implant ASAC sealing materials, in preventing microleakage. The hypothesis framed is that all ASAC sealing materials are equally effective in preventing microleakage.

The recommendations specified by the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) were applied for constructing framework and reporting the current review.³³ 

Inclusion and exclusion criteria are mentioned in Table 1.

The exposure of interest for the current study was different materials used for sealing dental implant ASAC, irrespective of type of implant connection and cyclic loading. The outcome was the effectiveness of these sealing materials in preventing microleakage or penetration of various microbial species. The focused PICO (Participant, Intervention, Comparison, Outcome) question was: “Which material (C) is more effective in sealing (I) implant ASAC (P) in terms of causing minimal microbial leakage (O)”.

Indexed English literature published up to February 20, 2021 was systematically searched using PubMed/Medline, Web of Science and Cochrane data bases. Various combinations of medical subject heading terms (MeSH) and supplementary non-MeSH terms were employed for searching. Reference lists from the pertinent articles were searched by hand to identify secondary reference articles. The search terms used are listed in Table 2. Minor modifications were done in search strategy and terms according to the database searched.

Two reviewers (SJ and AA) independently screened all the identified titles and abstracts. Predefined inclusion and exclusion criteria were applied to shortlist relevant records (Table 1). Titles with conflicts were discussed to resolve the disagreements. Kappa scores (Cohen kappa coefficient) (k = 0.933) indicate near perfect agreement between the 2 reviewers.³⁴  Final selection of appropriate studies (based on predetermined inclusion and exclusion criteria) was done after reviewing the full text of the remaining articles. References of these articles were searched manually for any additional relevant articles.

Self-designed table was used to tabulate data extracted from these articles. The data extracted were as follows: author's name, year of publication, study design (in vitro or in vivo), sample size used, sealing material used, type of implant connection, incubation media, microbial species studied, parameters assessed, duration of usage/immersion, occlusal implant loading and simulation, authors suggestions/conclusion.

Quality of articles was assessed with modified CONSORT scale for in vitro studies.³⁵  Criteria for assessment are as follows: (1) Structured abstract; (2a) Scientific background and explanation of rationale; (2b) Specific objectives and/or hypothesis; (3) Intervention for each group; (4) How and when outcomes were assessed; (5) Sample size determination; (6) Method used to generate the random allocation sequence; (7) Mechanism used to implement the random allocation sequence; (8) Who generated random allocation sequence; (9) who was blinded after assignment to intervention and how; (10) Statistical methods used to compare groups for primary and secondary outcomes; (11) Precision of results obtained; (12) Study limitations; (13) Sources of funding; (14) Access provided to full study protocol (Table 3).

The initial electronic database search resulted in 2503 titles. Two-hundred four titles that were found to be common in these databases were eliminated before screening the abstracts. Two reviewers independently screened the abstracts (keeping in mind the inclusion and exclusion criteria) and filtered unrelated titles, resulting in 10 remaining articles. After discussion between the reviewers, 9 articles were selected for full-text assessment. No other relevant articles were found, when references of these articles were searched manually. One study discussed about susceptibility of various sealing materials to bacterial or fungal adhesion.²⁹  Two studies inform only about the effect of screw hole filling on retention of implant crowns,^20,36  whereas 1 study informed only about the Influence of sealing of the screw access hole on the fracture resistance of implant-supported restorations.³⁷  So these 4 studies were excluded. Thus, at the end, a total of 5 studies were selected after final screening of full text of the articles on applying inclusion and exclusion criteria and PICO question handling. Search outcomes are illustrated in Figure 1.

All the studies that qualified the selection criteria for this review were in vitro studies. One 1 of 5 article is considered as low quality as it fulfilled only 5 items out of 15. Three were considered moderate quality as they fulfilled 8 to 10 items, whereas 1 was considered as high quality as it fulfilled 12 items out of 15. Results of modified CONSORT scale are displayed in Table 3. All the studies included have adequate abstract, which comprised sufficient details to facilitate proper understanding of the study (item 1). Appropriate introduction, explaining the study's scientific background, rationale (item 2a), specific objectives and/or hypothesis (item 2b) was included in all the studies. All authors reported the approach used in the experiment with specific information on type of intervention (item 3). They also stated the primary (and secondary) outcome(s) of the proposed experiment (item 4). Four out of 5 studies used rigorous statistical approach to compare the outcomes (item 10), had trial limitations (item 12), and had statements defining no conflict of interest (item 13). Two out of 5 studies explained the method used to generate random allocation sequence (item 6) and mechanism used to implement it (item 7). Only 1 study report the precision of results as confidence intervals (item 11), explained the method of determination of sample size (item 5), and how blinding was done (item 9). None of the studies gave details about randomization process related to people involved in allocation and implementation (item 8) and where the full trial protocol could be accessed (item 14).

All papers included (n = 5) were in vitro studies, which were published between the year 2006 to 2017 (Table 4). All the studies evaluated the microleakage of the screw retained implant prosthesis with different sealing materials. Sample size researched in these studies varied from n = 12²⁶ to n = 120.²⁸  Four out of 5 studies used only internal conical/Morse taper implant abutment connection,^18,26,27,30  whereas 1 study used both external hexagon and Morse taper implant abutment connection.²⁸  Materials compared for efficacy to act as sealant varied in each study. Study by Proff et al²⁶  compared GP with no sealant. Park et al²⁷  compared cotton pellet, silicone, VPS, and GP. Cavalcanti et al²⁸  compared GP with PTFE tape. Nascimento et al¹⁸  compared combination of various sealing materials like PTFE tape + composite resin, PTFE Tape + GP, PTFE tape + light-polymerized provisional composite, Cotton pellet + GP, and cotton pellet + light-polymerized provisional composite. Whereas Alshehri et al³⁰  compared GP, PTFE, and VPS.

Four out of 5 studies used microbial species to assess the microleakage,^18,26,28,30  whereas 1 study used basic fuchsin dye for the same.²⁷  Out of total 5 studies, 2 studies quantified the microbial species in terms of genome counts,^18,30  2 assessed the turbidity of media,^26,28  whereas 1 study measured the concentration of basic fuchsin dye for microleakage evaluation.²⁷  Duration of usage/immersion varied from 24 to 72 hours in 1 study,²⁶  7 days in 3 studies^18,27,30  to 14 days in 1 study.²⁸  In 4 out of 5 studies there was no occlusal implant load stimulation,^18,26,28,30  whereas in the study by Park et al,²⁷  cyclic load of 21 N, 16,000 times, at 1-Hz frequency was applied parallel to the longitudinal axis of the prosthesis.

All 5 studies included in this review evaluated the sealing ability of various materials. Proff et al²⁶  concluded that GP sealed group showed less microbial growth as compared with the nonsealed group but sealing with GP is ineffective in preventing penetration of the periodontal pathogen. Park et al²⁷  in their study concluded that when sealing the access holes, GP or VPS will help to reduce microleakage. Maximum microleakage was seen in cotton pellet followed by silicone sealing material, VPS, with GP showing the least microleakage. Cavalcanti et al²⁸  concluded that sealing implant abutment access hole channel with GP is significantly more effective in minimizing microbial leakage as compared with PTFE tape. The effectiveness against microbial leakage of the PTFE tape group was equivalent to that of the group with no sealing in both systems. Nascimento et al¹⁸  concluded that all the tested sealing materials showed some microbial leakage through the ASAC. The mean total genome count values (microbial count) of the studied materials were organized as follows: PTFE Tape + Composite Resin = PTFE Tape + GP < PTFE Tape + Light-Polymerized Provisional Composite = Cotton Pellet + GP < Cotton Pellet + Light-Polymerized Provisional Composite. Alshehri et al³⁰  concluded that microleakage of all microbes assessed was observed only in the group sealed with GP. PTFE and VPS demonstrated less microleakage as compared with GP.

The current study reviews the available literature to evaluate the efficacy of various implant ASAC sealing materials in preventing microleakage. As per the searched indexed English literature, this study is the first review of its kind. All the 5 included papers were prospective randomized controlled trials.^{18,26–28,30}  The findings based on the 5 selected studies support that the use of ASAC sealing materials decrease the microleakage, but the efficacy of each material differs. Thus, the hypothesis framed can be rejected.

Included studies had contrasting results related to the efficacy of various ASAC sealing materials.^{18,26–28,30}  In this perspective, when comparing GP and PTFE tape as material for sealing ASAC, the study by Cavalcanti et al²⁸  showed that GP is significantly more effective as compared with PTFE tape, whereas in the study by Alshehri et al,³⁰  PTFE tape was found to be more effective as compared with GP. When comparing polyvinyl siloxane with GP and/or PTFE as material for sealing ASAC, the study by Park et al²⁷  showed that GP is more effective as compared with VPS, whereas in the study by Alshehri et al,³⁰  PTFE tape was found to be equally effective as VPS. When comparing cotton with GP and/or PTFE tape and/or VPS as material for sealing ASAC, both the studies^18,27  concluded that cotton is least effective in preventing microleakage. The objective of using these sealing materials is to prevent growth of microorganisms, which can cause implant peri-implant mucositis and peri-implantitis and thus delaying the failure of the dental implant. Debatably there are multiple other factors, which are critical for the success of the dental implant.^3–5 

Materials tested as sealants for ASAC have their own advantages and disadvantages. Cotton pellets, PTFE tape, and silicone sealing materials are clinically easy to place and remove.^24,27  On the contrary it is argued that twisted PTFE tape fails to create hermetic seal throughout the abutment, as there is no chemical bond even after compaction.²⁸  GP can be compacted easily and there is a chemical bond formation in it. But placement of GP is not as simple, and once it hardens after cooling down, its removal from ASAC is a time-consuming procedure.²⁸  Raab et al²⁹  did a study to compare the bacterial and fungal adhesion on various materials used for sealing ASAC. They concluded that cotton pellets demonstrated highest potential of bacterial and fungal adhesion followed by PTFE tape, whereas least bacterial and fungal adhesion was demonstrated by GP. More studies should be conducted in the near future to investigate combination of materials like PTFE cylinders along with adhesives and GP with endodontic sealers in preventing microleakage.

The outcomes from the present review are also dependent on different methodologies employed by the included studies. More previous studies^26–28  measured turbidity of nutrient media, whereas latest studies^18,30  used genome counting as assessment criteria. Only 1 study considered occlusal loading while evaluating microleakage. Also, there was no consistency in the materials tested. Due to these shortcomings, meta-analysis was not feasible. Comprehensive article selection approach used is the highlight of this review. All articles mentioning implant abutment and microleakage were evaluated for their inclusion in this review, thus ensuring that no pertinent article is left.

The results of this systematic review indicate that the sealing capacity against microleakage should be considered as 1 of the most important criteria while selecting the material to fill implant ASAC. Definitive conclusions asserting superiority of a single material over others are difficult to draw, due to nonhomogeneity in study design of the included papers.

The author would like to thank Aparna Aggarwal (A.A.) for her help in the screening process of the reviewed articles.