Comparative Assessment of the Primary Stability of Straumann BLX Implant Design Using an In Vitro Sinus Lift–Simultaneous Implant Insertion Model

Sufficient initial mechanical anchorage—or the so-called primary stability—of a dental implant plays a key role in implant survival.^1,2  Moreover, primary stability has become a crucial factor in implant dentistry to meet patients' expectations that necessitates the use of early and/or immediate loading protocols, such as “teeth in a day.”^3,4  However, the restricted bone volume and/or poor bone quality at an implant recipient site could negatively influence the initial stability and jeopardize treatment success.

To ensure a high primary stability, different surgical techniques have been proposed, such as underdimensioned implant bed preparation, cortical anchorage of the implant, or condensation of the bone at the implant recipient site. The geometry and the macro-topography of the implant could also have a direct influence on implant stability.⁵  It has been stated that implants that are greater in length, larger in diameter, that have multiple and deep threads, and that have threads with smaller pitch and decreased thread helix angles could significantly increase primary stability.⁶ 

Recently, Straumann (Institute Straumann AG, Basel, Switzerland) has developed a novel self-drilling implant system (BLX) with double threads that could provide ideal primary stability in all types of bone through uniform and controlled compaction and densification of the peri-implant bone.⁷  However, the number of studies focusing on the primary stability of this novel system is limited.

Because of the restricted residual bone height and poor bone quality, implant insertion in the edentulous posterior maxilla poses a great challenge for dental practitioners. Simultaneous implant insertion with bone augmentation via sinus lift is commonly performed on residual maxillary sinus floors with more than 5 mm of vertical height.⁸  However, an increased risk of implant failure due to the insufficient primary stability that results in implant mobility during healing has been reported.^9,10 

In an in vitro study performed by Romanos et al,¹¹  higher implant stability values have been reported for tapered designed implants (Tapered Effect Straumann, Institute Straumann AG) when compared among 3 Straumann implant designs. The aim of the current study was to comparatively assess the primary stability of Straumann BLX implant systems with Straumann tapered effect (TE) implants using a simultaneous sinus elevation and implant insertion model in freshly slaughtered bovine ribs.

This study was conducted in accordance with the German Animal Welfare Act and with the European Communities Council Directive 2010/63/EU for the protection of animals used for experimental purposes. All samples were prepared using animal body parts, which were commercially purchased at the slaughterhouse Steffen in Muxall, Germany. No animal was harmed or sacrificed to conduct the experiments described herein.

A total of 10 freshly slaughtered bovine ribs were cut into 30-cm-long pieces. Soft tissues and periosteum were completely stripped off the bone. To ensure the interrater and intrarater reliability of the experimental design, the following procedures were followed:

• All samples were prepared by a single researcher (A.G.).

• Implant insertions were performed by 2 experienced surgeons (A.G. and M.E.).

• Two blinded independent observers (E.B. and J.W.) assessed the accuracy of the implant placement.

• The bone quality at the implant recipient site was examined histologically (Y.A.).

• The methodology was reviewed by an independent statistician (B.T., please see the acknowledgments).

Before the samples were prepared, bone biopsies were taken with a trephine burr (2.3 mm) for histological evaluation to determine the bone type at the corresponding implant recipient site (Figure 1a). A bone window of 3 cm in length, 5 cm in width, and 3 cm in depth, resembling the maxillary bone window of the lateral sinus wall with 5 mm of residual bone height, was prepared at the dorsal side of the freshly slaughtered bovine ribs, as described by Gülses et al¹²  (Figure 1b).

The implants were placed according to the manufacturer's guidelines using the complete sequence of drills for each individual implant design (Figure 1c). Four implants—BLX (4.0 × 12 mm), TE (4.1 × 12 mm), BLX (4.5 × 12 mm), and TE (4.8 × 12 mm)—were placed in each window with a peak insertion torque of ≥80 N/cm (Figure 1d). The distance between the implant shoulders was set to a minimum of 5 mm.

Implants (BLX [4.0 × 12 mm], TE [4.1 × 12 mm], BLX [4.5 × 12 mm], and TE [4.8 × 12 mm]) were inserted into the proximal diaphysis of the bovine ribs, according to the drilling protocol and instrumentarium determined by the manufacturer. Similar to the study model, bone biopises were taken at the implant recipient sites prior to implant placement, and the distance between the neighboring implant shoulders was 5 mm (Figure 2). For both groups, a total of 40 implant insertions were performed.

After implant insertion, implant stability quotient (ISQ) was measured by performing resonance frequency analysis via Osstell device (Osstell Mentor, Integration Diagnostics Ltd, Göteborg, Sweden). For each implant design, a suitable transducer was inserted into the implant body (SmartPeg, Osstell; Figure 3). Measurements were taken in 4 different directions perpendicular to the SmartPeg, according to the manufacturer's guidelines. The mean value of the 4 measurements was calculated to determine the final ISQ of each implant. To ensure the homogeneity of the data, all ISQ measurements were performed in triplicate by a single researcher (M.E.).

Clinical validity was implemented using the histological classification of bone density at the implant recipient site. The tissue samples were evaluated according to the technique described by Donath and Breuner¹³  and later developed by Açil et al.^14,15  Briefly, tissue samples were placed into 10% neutral buffered formalin for fixation for 4 days and embedded in methacrylate prior to sawing and grinding. Sawing and grinding were performed, and the samples were placed in glass vessels filled with monometric resin solution and incubated at 37°C to 40°C for 2 to 4 days for resin impregnation. The sample was longitudinally precut with a band saw (Exakt, Norderstedt, Germany), and sections of about 100 μm were obtained via an oscillating diamond saw (Exakt), grounded with the Saphir 360 E grinder (ATM, Altenkirchen, Germany) and highly polished with silicon carbide paper (grades 500, 1200, 2400, and 4000). Stainings were made by using toluidine blue. The ground surface was decalcified with 0.1% formic acid, and 20% methanol was applied for better cell and soft-tissue staining. The samples were rinsed in distilled water and stained in a toluidine blue solution for 2 minutes. Relative amounts of bone area vs total tissue were as described by Takahashi et al¹⁶  (% bone area/total tissue is: % bone area [BA]/total area [TA]; Figure 4). The preparations were digitally photographed using Nikon photomicrography with a magnification of ×509 and assessed manually with the Leica QWin Imaging program (McBain Systems, Westlake Village, Calif).

A power analysis was carried out by using paired t-test power calculation to calculate the sample size for each group. A statistically significance with a power of >0.80 was tested to reject the null hypothesis. Considering the variance of the results, the different analyses, and treatment groups, estimation matched the sample size of previous studies with experimental background regarding the investigation of primary stability in a bovine rib model.^11,12  Blind data analysis via Python (open-source programming language) was performed by an independent statistician (B.T.). The mean and standard deviation for each group were calculated. The Shapiro-Wilk test was performed to examine the distribution of the parameters. Before the statistical evaluation of the subgroups, the Levene test was used to determine the homogeneity of variances. One-way analysis of variance and independent t test were used to compare the groups. Statistical results were evaluated at a 95% confidence interval and P < .05 significance level. All results were reviewed by an independent statistician (B.T., please see the acknowledgements)

The mean residual bone height in the study model was 5.01 ± 0.02 mm. Histological examination revealed that 9.81% of the bone samples correlated with D3 (% BA/TA), whereas 81.19% of the specimens were identical to D4 bone type (% BA/TA).

In the study group, TE implants showed relatively higher ISQ values compared with BLX in both 4.0-mm BLX vs 4.1-mm TE and 4.5-mm BLX vs 4.8-mm TE subgroups. The difference between BLX 4.5 mm and TE 4.8 mm was statistically significant (P = .040; Figure 5)

The 4.0-mm BLX implants in the control group showed significantly higher ISQ values compared with the 4.1-mm-diameter TE implants (P = .014) The superiority of the BLX implants with a diameter of 4.5 mm over TE implants with a diameter of 4.8 mm was statistically insignificant (P = .116; Figure 6).

After examining the differences in the ISQ values of each implant design between the control and experimental groups, all implants showed lower ISQ values. The primary stability of the BLX implants in the control group was significantly higher than that of the experimental group in both widths (P = .018 for 4.0-mm BLX and P = .002 for 4.5-mm BLX, respectively). However, despite showing lower ISQ values in the study model, the differences among the TE implants were statistically insignificant for both diameter (P = .840 for 4.1-mm TE and P = .588 for 4.8-mm TE; Figure 7).

The number of studies focusing on the clinical aspects of the BLX concept were limited for case reports.^17,18  Moreover, the literature survey revealed only 2 experimental studies on BLX implants. Among these, Bergamo et al¹⁸  assessed the probability of survival for narrow implants under anterior physiologic masticatory forces by using Active (Nobel Biocare), Epikut (S.I.N. Implant System), and BLX implants and reported a high probability of survival for all examined implant systems. Only 1 recently published article focusing on the mechanical anchorage of BLX implants by Ibrahim et al¹⁹  evaluated the primary stability of different implant designs including BLX and TE in various defects created in artificial bone blocks with different bone densities. According to their results, there was a significant reduction in primary stability, especially in circular defects; however, the authors could not determine a significant difference regarding the implant design. To the best of our knowledge, the current article is the first study to highlight the mechanical anchorage behavior of the BLX implant system.

It has been clearly shown in the literature that TE implants presented higher stability values compared with conventional implant designs (with respect to macro-design), and these are suggested to be the most appropriate option when immediate loading is attempted.^20–22  In the current study, TE systems with a 4.8-mm diameter demonstrated significantly higher ISQ values compared with BLX in the sinus lift model. This could be attributed to the compression by the TE design to the cortical residual bone, which was also previously shown by O'Sullivan et al²²  at implant recipient sites with poor bone quality. Moreover, the main superiority of the BLX design, in terms of primary stability, was suggested to be due to the double-thread design and the presence of 2 sharp and highly engaging threads at its apex. In the experimental model used herein, the implant apex corresponded to the defect area resembling the maxillary sinus and therefore could not have influenced the primary stability of the implant. Contrary to this, BLX implants in the control group showed higher ISQ values compared with the TE system, most likely because of their aggressive thread design, which ensured a superior bone-to-implant contact and therefore resulted in higher mechanical anchorage.

When the differences in ISQ values of each implant design between control and experimental groups were examined, all implants showed lower ISQ values. This result might be explained by the fact that mechanical anchorage could be jeopardized in sinus lift procedures and also highlighted the accuracy of the current experimental model. In addition, the ISQ values of the BLX implants in the control group were significantly higher compared than those of the experimental group. However, despite the observation of lower ISQ values in the study model, the difference among TE implants was statistically insignificant. This finding also endorses that the advanced primary stability of the TE design is more likely a consequence of its form; thus, the upper third of the implant body fits firmly into (with) the surrounding bone.

The one-staged simultaneous implant placement with sinus lift is a widely used treatment modality in cases with insufficient bone volume at the posterior maxilla. However, because of the poor bone quality and the lack of ideal bone height, obtaining ideal primary stability in this area will not always be possible.^23–25  The experimental model used herein resembles a possible indication for simultaneous implant placement with sinus lift regarding the residual bone height of 5 mm. However, the residual bone height of >5 mm, where the simultaneous implant placement with the sinus lift is indicated, might be the subject of further studies.

The use of bovine ribs has been an appropriate model for evaluating the mechanical stability of various implant systems.^11,25–27  In addition, the current experimental model, which resembles the maxillary bone window of the lateral sinus wall with 4 mm of residual bone height, has been used previously in the study by Gülses et al.¹²  However, the influence of the bone graft placed during the procedure on the primary stability could also be regarded as a limitation of this model.

It is obvious that bone density is the major determinant of implant stability in maxillary sinus augmentation procedures with simultaneous implant placement.²⁸  Pommer et al²⁹  proclaimed that the preoperative assessment of bone density may help avoid stability-related complications in single-stage implant treatments of the atrophic posterior maxilla. In addition, problems related to differences in bone density in experimental bovine rib models have been previously reported, and the use of identical sites of the ribs has been recommended.¹²  To overcome the bone density–related discrepancies in the current study, bone biopsies were taken from corresponding implant beds and evaluated histologically. The histological evaluation was used to determine the bone type at all implant areas as D3 or D4, which coincides with the posterior maxilla of the humans.^30,31 

The results obtained from the current research revealed the following:

• The BLX design shows elevated ISQ levels for standard implant insertion protocols.

• Both tapered and BLX designs could ensure appropriate primary stability values at the implant recipient sites with poor bone volume and quality.

• Tapered implants with a wide diameter provide the highest ISQ values in the posterior maxilla, if in simultaneous implant placement with sinus lift is performed.

The results of this experimental research showed that the use of BLX Straumann TE wide-diameter implants when used in procedures with simultaneous sinus augmentation and implant placement could present higher ISQ values. Therefore, these implants might be a more appropriate option for implant insertions for implant recipient sites with poor bone volume and quality. In addition, the BLX design could be beneficial for achieving primary stability with standard implant insertion protocols. Further studies with long-term outcomes are needed to elucidate the complete clinical benefits of the BLX system.

Abbreviations

BA:

bone area

ISQ:

implant stability quotient

TA:

total area

TE:

tapered effect

The authors would like to thank Mr. Burak Tunca MSc, Jigsaw Academy, School of Analytics, University of Chicago, Illinois, for reviewing the statistical methodology and conducting the data analysis of the study.