1 Introduction

The level of stability achieved by a dental implant during placement, more commonly referred to as primary stability, is crucial for successful osseointegration [1]. A high level of primary stability creates a buffer-like situation against micromovement, which could compromise the osseointegration process. In addition to the quality and quantity of bone, the surgical technique, the morphology of the osteotomy and the implant design all play an important role in achieving good primary stability [2,3,4,5].

For many years, implants were mostly cylindrical and intended to be placed with profuse irrigation and minimal torque, followed by a mandatory 3–6 month integration period before loading [6, 7]. More recently, implants with more prominent external threads and modifications to the apex have been introduced to increase primary stability and bone to implant contact [8,9,10,11]. As a result, dental implant design can be broadly classified into two macro-geometric types: cylindrical (following the original Brånemark design) and tapered (following the natural shape of the tooth).

Both cylindrical and tapered design implants are intended to achieve maximum stability once fully surrounded by bone. However, it is not always possible for a surgeon to ensure that the implant will be fully surrounded by bone. More specifically, in immediate placement cases this might be challenging or impossible to achieve. Often, the upper coronal section will not be fully surrounded by bone. The majority of the stability must therefore be generated by the lower apical section of the implant.

Reverse Tapered Body Shift design implants have recently been developed for cases where good stability must be achieved by the apical half of the implant. The narrower upper coronal part ensures a gap between the implant and the thin buccal bone plate is maintained [12,13,14,15,16,17]. The purpose behind maintaining this gap is to mitigate, and compensate for, buccal bone plate resorption following extraction [18, 19]. The aim of this in-vitro study was therefore to evaluate the impact of dental implant macrogeometry (conventionally tapered (CT) vs reverse tapered body shift (RTBS)) on insertion torque performance under various bone support scenarios.

2 Materials and Methods

2.1 Insertion Torque Analysis

Comparative insertion torque measurements of Ø4.0 CT implants (IBT13, Southern Implants, Irene, South Africa) and Ø4.0/5.0 RTBS implants (Inverta™; IV-EX40-5013, Southern Implants, Irene, South Africa) were assessed in different scenarios representing various levels of bone support in synthetic bone blocks (density = 0.32 g/cm3, SAWBONES, Vashon Island, WA, USA). Both the CT and RTBS implants had an overall length of 13 mm and a coronal diameter of Ø4.0 mm, while the RTBS implant had a diameter of Ø5.0 mm roughly midway along the length of the body.

Customized synthetic bone blocks were prepared by the manufacturer to simulate scenarios in which a 13 mm long implant would be 100%, 80%, 60%, 40% and 20% surrounded by bone, according to its length (referred to henceforth as scenarios A, B, C, D and E, see Fig. 1).

Fig. 1
figure 1

Control and test groups for 13 mm long conventionally tapered and RTBS implants at 100%, 80%, 60%, 40% and 20% bone support scenarios

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Osteotomies in the various bone support scenario were prepared by two experienced implant surgeons (VC and MG) for each implant type according to the manufacturer’s guidelines (see Fig. 2).

Fig. 2
figure 2

Site preparation protocol for CT (left) and RTBS implants (right) (Southern Implants, Irene, South Africa)

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The implants were then placed by the same implant surgeons (VC and MG) using a dental handpiece set (W&H, Salzburg, Austria). Peak insertion torque measurements required for the final seating positions were recorded using a digital Tohnichi BTGE100CN-G torque gauge (Tohnichi, Tokyo, Japan). The insertion torque measurements from the CT implants in scenario A (100% bone support) were considered the control group, as it represents a classic design implant placed in a healed ridge. From the four remaining bone level support scenarios, 9 test groups were created according to implant type (CT vs RTBS) and bone support level.

2.2 Maximum Bone to Implant Contact

Based on external measurements of each implant type, the approximate surface area in contact with the synthetic bone blocks at each bone support level was calculated, to obtain a maximum theoretical bone to implant contact value. For the sake of comparison, all bone to implant contact values were normalized to that of the CT implants in the 100% bone support condition. However, the extractive nature of the site preparation prevents the RTBS implants from being fully surrounded in bone, due to its narrower coronal section.

2.3 Statistical Analysis

Data analysis was performed in IBM SPSS® Statistics 27.0 (IBM Corp. Armonk, NY, USA). Statistically significant differences in insertion torque performance between the two implant types in each relative bone support scenario were assessed using paired samples t-tests (i.e., control group vs group 5, group 1 vs group 6, etc.). Finally, the influence of implant type and relative bone support on insertion torque performance was assessed using 2-way analysis of variance (ANOVA) with Welch’s test for unequal variance and Dunett’s T3 multiple comparison test. The level of significance was set to 0.05.

3 Results

3.1 Insertion Torque Analysis

A total of 200 insertion torque measurements were recorded, consisting of 10 groups (1 control group and 9 test groups) of 20 measurements each. The mean insertion torque (Ncm) for the CT implants in scenario A was 39.9, 95% CI [38.38, 41.38], which was considered as the gold standard as this scenario represents a classic design implant placed in a healed ridge. The mean insertion torque measurements for the CT implants in the remaining support scenarios (B, C, D and E) were 31.8 [30.91, 32.73], 17.0 [16.40, 17.62], 10.3 [9.80, 10.70] and 4.3 [3.96, 4.55], respectively. Similarly, the mean insertion torque measurements for the RTBS implants in scenario A, B, C, D and E were 49.7 [47.54, 51.87], 50.1 [47.42, 52.79], 45.5 [42.99, 48.09], 23.6 [22.11, 25.13] and 7.3 [6.72, 7.89], respectively (see Fig. 3). Statistically significant (p < 0.001) differences in the insertion torque results were found when comparing pairwise between the CT and RTBS implants at each relative bone support level.

Fig. 3
figure 3

Estimated marginal means of implant type and relative bone support conditions on insertion torque performance

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The implant type and relative bone support level had a significant effect on the insertion torque performance (p < 0.001); however, homogeneity of variances was violated (p < 0.001). Post-hoc testing found statistically significant differences when comparing between the CT implant insertion torque values at all relative bone support scenarios (p < 0.001). Similarly, significant differences were found when comparing between the CT and RTBS insertion torque results at all relative bone support levels (p < 0.05). Significant differences were also found when comparing between the RTBS implant insertion torque results at the 40% and 60% bone support levels (p < 0.001). Non-significant differences were found when comparing between the RTBS insertion torque values at the 100% and 80% bone support levels (p = 1.00); the 100% and 60% bone support levels (p = 0.391); and the 80% and 60% bone support levels (p = 0.410).

3.2 Maximum Bone to Implant Contact

Maximum bone to implant contact values, normalized to that of the CT implants at the 100% bone support condition, at each bone support level were calculated and are shown in Table 1.

Table 1 Maximum Bone to implant contact (BIC) and mean insertion torque for CT and RTBS implants at each relative bone support level
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4 Discussion

Due to the novelty of the RTBS implant, only a proof-of-concept animal study [13], a cadaver study [12] and a few case series studies have been conducted [16, 17]. In all of the clinical studies, the focus was more on the aesthetic outcome and survivability rather than the mechanical performance. To the best of our knowledge, this is the first study to examine the technical performance of this novel implant design.

The aim of this in-vitro study was to evaluate the insertion torque performance of CT implants versus RTBS implants in different relative bone support conditions. Both implant types had the same coronal diameter, the same grit-blasted surface, and the same overall length, while RTBS implants have a wider mid-section to achieve good stability when there is limited bone support at the coronal diameter (such as in an immediate placement case). The results of the present study support this intention, as the RTBS implants achieved significantly higher (p < 0.001) insertion torque than the CT implants at all bone support levels tested.

As one would expect, the surface area of a CT implant in contact with the bone decreases in proportion to the bone support scenario. This is in contradiction to an RTBS implant, where the bone support scenario has minimal influence on the surface area in contact with the surrounding bone. This relationship is also reflected in the torque values for the two implant types. Bone to implant contact has an effect on the stability of dental implants [2, 20, 21]. Due to the specific shape of RTBS implants, this influence is only important when 40% (or less) of the length of the implant is in bone compared to CT implants, in which the bone to implant contact has an effect in every bone support scenario. It may seem paradoxical that the RTBS implants achieve only 67% bone to implant contact in the 100% bone support condition, however this is due to the wider middle section preventing the upper coronal section contacting the bone. The wider middle section creates a steeper taper angle in the apical bottom half and greater surface area to engage the bone when support for the coronal section is not available. There is a clear drop in insertion torque performance and bone to implant contact by the CT implants as the bone support level decreases, however the RTBS implants maintains a higher insertion torque performance despite having lower bone to implant contact at all bone support levels.

While implant stability is important in immediate placement cases, it must be counterbalanced with the need to maintain a gap between the thin buccal bone plate and the implant (the buccal gap) of at least 2 mm [22,23,24]. It may be possible to use a wide diameter CT implant to achieve similar insertion torque results to those of the RTBS implants reported herein, however this may result in a smaller buccal gap being created and a greater risk of mucosal recession [19, 22,23,24,25]. An investigation into the potential buccal gap distance available in extraction sockets with RTBS implants was conducted by Christiaens et al. [12], who reported an average buccal gap distance of 2.8 mm.

While synthetic bone blocks may not fully represent human bone found in clinical practice, their use in the current study provide the opportunity to record repeated stability measurements in a consistent medium which is not ethically possible in human patients. Furthermore, the bone blocks allowed the insertion torque performance to be evaluated at different bone support levels in a controlled manner, which is also physically impossible in a clinical situation since human bone differs in every patient. The blocks used in the current study conform to ASTM F1839 (“Standard specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments”) and have been used in previous in-vitro assessments of insertion torque for dental implants [1, 26, 27]. The insertion torque results from the control group (CT implant in 100% bone support) is in a similar range to those reported by Brown and colleagues [28] for 12° co-axis Ø4.0 mm and Ø4.7 mm implants from the same manufacturer in a prospective clinical trial (mean = 36.7 Ncm, range: 20–45 Ncm). Similarly, in a prospective case series of RTBS implants, Levin and colleagues reported a mean clinical insertion torque value of 51.42 Ncm [14], which is also very close to the insertion torque results reported herein for the RTBS implants.

Insertion torque is not the only way to assess implant stability. Alternative methods include Percussion Testing, Pulsed Oscillation Waveform, the Impact Hammer Method and Resonance Frequency Analysis (RFA) [29]. Of these, RFA is one of the most widely used clinical methods to monitor stability over time, by measuring the Insertion Stability Quotient (ISQ) [30]. However, its use in the present study design would have been- limited to a single measurement. Aparicio et al. [31] found that a single ISQ measurement does not define bone or interface characteristics nor provide a quantitative evaluation of the bone tissue integration. For this reason, it was not considered for use in the present study.

The focus of the present study was on primary stability (i.e. directly after placement), which is an important clinical parameter to determine whether or not an immediate placement or immediate loading procedure can be performed. Of course, stability during the bone remodelling phase (i.e. secondary stability) is also an important consideration for a successful outcome in the longer term. Secondary stability enhancing strategies such as fluorapatite coated implants and the addition of periodontal tissue stem cells into the osteotomy have been proposed [32, 33], however the in-vitro nature of the present study prevented further investigation of these options.

An interesting finding from the present study is that the only point at which the RTBS implants achieved less insertion torque than the gold standard (CT implant in 100% bone support condition), was when the RTBS implants were tested in the 40% and 20% bone support conditions (scenarios D and E). The results would suggest that RTBS implants can consistently achieve good stability, even when the bone surrounding the lower apical section of the implant is less than half the length of the implant.

The favourable insertion torque results achieved by the RTBS implants herein should be examined more closely in an appropriately powered clinical study.

5 Conclusion

RTBS implants appear to provide superior primary stability to CT implants in scenarios in which the bone support around the upper coronal section of the implant is limited. However, a randomized controlled trial is required to validate these results in a clinical setting.

6 Summary Box

What is known

  • Numerous dental implant designs are available on the market, each with their advantages and disadvantages

  • Many implant systems struggle to achieve adequate primary stability when support around the upper coronal section of the implant is limited.

What this study adds

  • A comparative stability evaluation of a different implant design that might be helpful in specific clinical scenarios