Bacterial Profile and Radiographic Analysis Around Osseointegrated Implants with Morse Taper and External Hexagon Connections: Split-Mouth Model

Bone loss around dental implants is an inevitable event. This loss is considered to be 2 mm in the first year, followed by marginal bone loss of 0.2mm over the following years.¹  Other authors have reported a marginal bone loss in the first year of 1.5mm.^2,3  Marginal bone loss is influenced by several factors,²  such as the presence of microgap that can facilitate bacterial colonization, triggering a host immunoinflammatory response, with consequent bone remodeling.⁴  On the other hand, some researchers propose a correlation with micromovements at implant/abutment interface as a determinant factor for the bone resorption response.⁵  Moreover, implant-abutment connection type was considered one of the main factors that leads to peri-implant bone loss.⁶ 

The introduction of the switching platform concept supports on the hypothesis is that as the discrepancy between the diameter of the implant platform and the base of the abutments increases, there is less bone loss around the implant.^7–10  However, some factors are still not elucidated, especially in relation to the bacterial colonization that may influence bone remodeling around dental implants. Some studies suggest that the difference in bone crest resorption between implants with platform switching, external hexagon, and internal hexagon is not associated with differences in peri-implant microbiota.^11–13  However, the pathogens Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, Bacteroides forsythus, and Treponema denticola were found at higher levels around implants with a marginal bone loss greater than 2mm.¹⁴ 

To assess bone resorption in peri-implant regions, standardized radiographic images are considered fundamental to observe alterations in parameters over time.¹⁵  In addition, the literature also suggests an association between bone resorption and peri-implant microbiota composition.¹¹  Thus, the aim of this clinical study was to evaluate bacterial colonization, marginal bone loss, and optical alveolar density in implants with Morse taper (MT) and external hexagon (EH) connections.

A total of 7 patients were included in this study (mean age: 65.8 ± 6.7 years). All procedures were approved by the Research Ethics Committee registered under number 1841-2007. All patients the study required oral rehabilitation and were signed an informed consent form. Patients were totally edentulous; did not present any medical conditions that compromise the immunological status; wore complete superior and inferior dentures, which were aesthetically and functionally adequate; had enough bone to support 5 dental implants; and were nonsmokers.

Five implants were installed in each patient in the anterior portion of the mandible between the mental foramen, following a split-mouth experimental model, following pre-established positioning parameters: Morse taper on the right side and external hexagon connection implants on the left side. Implants were divided into 2 groups, according to implant platform: G1 - MT connections, positioned 2mm infra-osseous (Titamax Cortical MT, length: 13mm, Ø 3.75mm, Neodent, Curitiba, Paraná, Brazil); G2 - EH connections, positioned at bone level according to the Branemark protocol¹⁶  (Titamax Cortical Ti, length: 13mm, Ø 3.75mm, Neodent). The implant in the middle was not used in the analyses because the distribution of loads on it would not be the same as the equivalent loads on the other implants.

Tomographic planning established the presence of a minimum bone thickness of 2mm in the buccal and lingual bone. The surgical protocol consisted of a full-thickness flap for mandibular bone exposure to the site of the two mental foramina. The two most distal implants were installed 5mm far from each foramen, and a minimum distance of 4mm was observed between implants. The drill sequence was performed according to the implant system recommendations (Neodent). After installation of the implants, tapered mini-abutments (Neodent) were installed with different heights, so that all components had the same final height. Protectors were installed on the tapered mini-abutments and the wound was sutured. After 7 days, the suture was removed and the patient's denture was attached to implants using titanium provisional components with distal bars (Neodent) in both cantilevers' sides.

Radiographic evaluation was performed in mesial and distal sites of each implant. Patients were evaluated at baseline (T0), 21 days (T1), 3 months (T2), 6 months (T3), and 12 months (T4) after implant installation. A radiographic positioner Rinn/XCP (Dentsply, York, Pa) was attached with self-curing acrylic resin (Duralay II; Reliance Dental Mfg Co, Wort, Ill) to a prosthetic component (Neodent distal bar cylinder), which was fixed to the abutment to guarantee the standardization of images.

The film used was DFL Contrast DV-54 size 0 (22 × 35 mm) (DFL Industria e Comércio SA, Rio de Janeiro, Brazil), with exposure of 0.5 seconds at 70kVp, 10 mA, using a Spectro 70X X-ray generator (Dabi Atlante, Ribeirão Preto, São Paulo, Brazil). Film processing was performed with an AT 2000 XR automatic processor (Air Techniques Inc, Hicksville, NY), with a total dry run time of 8 minutes, using Kodak developer and fixative solutions (Kodak, Rochester, NY).

Two radiographic parameters were evaluated: linear bone loss and optical alveolar density. Intra-examiner concordances were 98% for linear measurements within ±0.1mm and 99% for optical alveolar density measurements ±3.

Radiographs were scanned (Nikon CoolScan IV ED, Japan) and linear digital radiographic subtraction was performed using Emago v.5.0.12 software (Oral Diagnostic Systems, Amsterdam, Netherlands). Subtraction data were expressed through a histogram with mean values of density, assessing areas of interest previously delimited with a standard size of 0.5×0.5cm. The ImageJ software (National Institutes of Health, Bethesda, Md) was used to evaluate the linear bone loss, considering the distance from prosthetic component/abutment interface to the most coronal point of the bone/implant contact, measured in millimeters.

Sample was collected from buccal and lingual surfaces of each implant at 6 months after implant installation. After removal of supragingival plaque, subgingival biofilm was collected with a sterile Teflon curette (IC4R / 4L-HuFriedy, Chicago, Ill), positioned in the most apical portion of the sites. Samples were placed in separate Eppendorf tubes containing 0.15 mL of TE (10 mM Tris-HCl and 1 mM EDTA, pH 7.6). To each plastic tube, 100 μL of NaOH (Labsynth) at 0.5M was added so that the bacterial DNA remained viable for sufficient time. Samples were kept at −20°C until analysis.

Forty bacterial species were determined in each sample using checkerboard DNA–DNA hybridization. A single-blinded examiner read the radiography films twice on two different days. The intensity of each signal produced by a probe in the sample was compared to the signal produced by the same probe in two control lines that contained 10⁵ and 10⁶ bacteria. Therefore, a score of 0 was attributed to a sample when no signal was detected; score 1 corresponded to a signal with intensity lower than 10⁵ (control); score 2 corresponded to 10⁵ cells; score 3 corresponded to between 10⁵ and 10⁶ cells; score 4 corresponded to approximately 10⁶ cells; and score 5 was attributed to more than 10⁶ cells. The analyses were performed as described in Matarazzo et al.¹⁷ 

Statistical analyses were performed using SPSS 23.0 (IBM, Chicago, Ill). Data normality was evaluated by the Shapiro–Wilk test. Mean and standard deviation (SD) or median and the quartiles were used according to data normality. The values for optical alveolar density, peri-implant bone loss and bacterial profile of each group studied according to the bacterial complexes^18,19  were compared by Wilcoxon test. The groups were compared to the mean difference between time intervals. Boxplots were used to represent optical alveolar density and peri-implant bone loss to describe the dispersion of these data. Regarding the bacterial profile, the mean and standard deviation of each bacteria were evaluated through a descriptive statistic, followed by the creation of a line graph of mean. P-values ≤ .05 were statistically significant.

The methodology was reviewed by an independent statistician.

There was no significant difference in the evaluation of the optical alveolar density in groups G1 - MT and G2 - EH, according to the mean difference between time intervals T1-T0 (P = .818), T2-T0 (P = .639), T3-T0 (P = .886), and T4-T0 (P = .421) (Figure 1).

After 3 months, alveolar bone loss was significantly higher in the G2 - EH according to the mean difference between the time intervals T2-T0 (P = .006), T3-T0 (P = .003), and T4-T0 (P = .005) (Figure 2).

The bacterial profile of each group studied according to the bacterial complexes, as defined by Socransky et al¹⁸  and Socransky and Haffajee,¹⁹  can be observed in Figure 3. There were significant differences in Tannerella forsythia (P = .048), Campylobacter showae (P = .038), and Actinomyces naeslundii (P = .027).

The results were reviewed by an independent statistician.

The alteration in peri-implant bone level is considered an important criterion for evaluating success after installation of dental implants.^1,20  This study showed that MT connections had less bone loss compared to the external hexagon connections. These data are in accordance with those reported by Pessoa et al.¹³  However, in our study, this difference was detected 3 months after implant installation, while Pessoa et al¹³  reported this difference at the first month of follow-up. This difference can be attributed to the fact that in the present study the implants were installed 2mm infra-osseous, while in Pessoa et al,¹³  the implants were positioned 0.5mm below the bone crest. Vetromilla et al²¹  and Caricasulo et al²²  also suggested that MT presented better survival, success rates, and lower peri-implant bone loss. The reason for these results may be due to MT connections biomechanical characteristics such as less microgap formation, seal performance, torque maintenance, and reduced micromovements during loading.²³ 

The main difficulty of methodology of this study is to make the radiographic bone measurements related to implant platform and bone crest in implants with different designs (MT and EH) and different insertion technique. As a result, we seek to use the same methodology already published by Pessoa et al.¹³  Was used to evaluate the linear bone loss, considering the distance from prosthetic component/abutment interface to the most coronal point of the bone/implant contact, measured in millimeters.

The concept of switching platform, as presented by Lazzara and Porter,⁸  is still not fully understood in relation to bone remodeling. Several theories have been suggested to explain this phenomenon, among them the influence of biomechanical factors. This theory proposes that the switching platform configuration moves the area of stress concentration away from the bone-implant interface,²⁴  which can explain the significant difference in bone height between MT and EH, as found in the present study.

In addition, the presence of a microgap formed between the implant and the abutment has been suggested as one of the possible factors responsible for bone loss around dental implants.^25,26  Thus, we analyzed the bacterial profile in the groups, seeking a possible relationship between bone remodeling and bacterial colonization. The species used in this study were periodontal pathogens that form the bacterial complexes described by Socransky and Haffajee.¹⁹  The results showed similar profiles between studied groups, but the count of T forsythia, C showae, and A naeslundii, showed a significant difference between the MT implant and EH. However, it was not possible to establish a correlation between the bacterial profile and alveolar bone remodeling. Pessoa et al¹³  also did not observe significant microbiological differences between the MT and EH connections after 6 months, though the bone loss was significantly different between the groups (smaller for the MT implant). This result is similar to that of Canullo et al,^11,27  who also did not find statistically significant differences regarding bacterial colonization between implants.

In our study, 40 bacterial species were determined in each sample using checkerboard DNA–DNA hybridization.¹⁷  With this, we evaluate a limited number of bacterial species. But we know that modern sequencing techniques have been shedding light on the peri-implant microbiome complexity.²⁸ 

Mechanical and biological factors have been identified as possible etiological causes for the marginal bone resorption process, horizontally and vertically, around dental implants.²⁵  The present study revealed less peri-implant bone loss in MT implants, without a correlation with the bacterial profile. This indicates that mechanical issues were probably more preponderant in the significant differences that were presented. In a mechanical analysis of finite elements, Macedo et al²⁹  found that MT implants had a higher peri-implant bone volume in response to low magnitude stress compared to EH implants, thus exhibiting better biomechanical behavior.

This study had some limitations, such as the small sample size and short follow-up period. Despite the limitations, this is a clinical study with follow-up and analysis of measurements that are difficult to measure in patients. However, the literature is scarce with respect to in vivo studies of bone remodeling in implants with different platforms. This does not discourage us from continuing to study on this topic and for future research, we would like to increase the sample and longer follow-ups. In this way, we would be able to investigate if these results and their clinical implications will be maintained.

Based on the results of this study, and taking into account their limitations, it can be concluded that there was less peri-implant bone loss in MT when compared to EH connections, but microbiological profile did not seem to influence bone changes.

Abbreviations

DNA:

deoxyribonucleic acid

EH:

external hexagon connections

MT:

Morse taper connections

This study was carried out at the Faculty of Dentistry of the State University of Rio de Janeiro. The authors thank all collaborators of this study. The methodology and results were reviewed by an independent statistician, Luciane de Souza Velasque, PhD, from the Department of Quantitative Methods, Rio de Janeiro State Federal University (UNIRIO), Rio de Janeiro, Brazil.

The authors declare no potential conflict of interest.