Significance of bone morphology and quality on the primary stability of orthodontic mini-implants: in vitro comparison between human bone substitute and artificial bone

Anchorage plays an essential role in orthodontics. During orthodontic treatment, in accordance with Newton’s third law, forces act on the teeth that always result in a reciprocal force of equal magnitude in the opposite direction [23]. Therefore, a desired tooth movement can provoke undesirable side effects and unwanted tooth movements [2]. It is important to avoid these side effects by counteracting opposing forces using an anchorage unit.

The use of orthodontic mini-implants (OMIs) can provide a skeletally anchored anchorage unit.

The failure rates of these OMIs, which usually have a smooth surface, have been variably reported in the literature. According to Pasch, the overall failure rate of OMIs is 10.1%, with interradicularly placed implants having a higher failure rate of 29–40% [37].

The clinical success of skeletal anchorage systems largely depends on primary stability [40], which should be distinguished from secondary stability [20, 43]. Primary stability is derived from a combination of displacement and compaction of bone during implant placement and creates a tight fit between the implant and the bone [24].

Factors such as insertion site preparation, bone quality, insertion angle, and implant design affect primary stability and, thus, the success of implantation [47, 48].

Local patient factors, such as deep bone defects around dental implants, have been associated with higher rates of periodontal pathogenic bacteria [1] and can lead to implant failure due to inflammation [36]. Circular bone defects have also been linked to the loss of primary stability of dental implants [18].

Depending on the anatomic conditions of the underlying bone, the insertion angle, and depth, OMIs cannot always be fully covered with bone, similar to the bone defects described by Goldman and Cohen [16], and this may lead to reduced primary stability.

To our knowledge, the effect of bone defects on OMIs has not been reported. Another crucial factor for stability is bone quality, which can vary within the jaw and depend on gender and age [7].

Primary stability can be reliably measured using resonance frequency analysis (RFA) [31, 41]. The measuring variable resulting from this analysis is the implant stability quotient (ISQ), with higher values indicating higher stability for the implant. Another value that is often assessed with regard to OMI insertion is the insertion placement torque (IPT), which could be higher for thick compact bone [30] but should not exceed 20 Ncm, as higher torque insertion rates could cause implant breakage [47].

In vitro studies evaluating the mechanical properties of OMIs commonly used solid rigid polyurethane foams in different densities to simulate human bone without comparison to human bone specimen [6, 21, 29, 35].

Thus, this in vitro study evaluated artificial bone models against a human bone substitute to assess the primary stability of OMIs at varying implant sites with different bone morphologies and qualities using RFA and IPT measurements. It also examined the effect of peri-implant defect geometry on primary stability.

A total of 1200 OMI placements of four different types from the same manufacturer (PSM Medical Solutions, Gunningen, Germany) were inserted into five types of bone models. The implants varied in diameter (2.0 and 2.3 mm) and length (9 and 11 mm). The artificial bone models were made from solid rigid polyurethane foam blocks (#1522-05, #1522-04, #1522-03, #1522‑1, #1522-16; Sawbones, Malmö, Sweden) with different densities: D1 (0.64 g/cm³: “very dense bone”), D2 (0.48 g/cm³: “dense bone”), D3 (0.32 g/cm³: “porous bone”), and D4 (0.16 g/cm³: “very porous bone”) [27]. The human bone model (HB) was made up of a block of human femoral bone substitution material (Maxgraft, Botiss Biomaterials, Zossen, Germany), which is considered gold standard. Before inserting the OMIs, the allogeneic bone block was conditioned in sterile saline for 24 h.

For all bone models, regardless of their quality, the same defect preparation according to Shin et al. was carried out [42]. Using trepan drill burs and cylindrical burs, three different defect types were prepared at 4 mm depth and checked with a standard dental probe. Each bone block consisted of four implant sites at a distance of 15 mm: no defect, one-wall defect, three-wall defect, and circular defect.

In accordance with the various parameters (bone quality, implant length/diameter, and bone defect), 80 groups were obtained. Fifteen implantations were performed in each group. Each implant was used for 15 implantations before being changed but was always inserted into freshly prepared implantation sites (Fig. 1).

For implantation, predrilling was necessary using a surgical unit (Implantmed SI-1023, W&H, Bürmoos, Austria) and twist drills of 1.4 and 1.8 mm in diameter, followed by implantation of the OMIs of 2.0 and 2.3 mm, respectively. The drilling speed was set to 300 rpm, the axial load was 20 N, and the predrilling depth was 5 mm.

Implantation was performed by one investigator, who inserted the OMIs immediately after osteotomy using a surgical handpiece (WS-56L W&H, Bürmoos, Austria) to simulate clinical conditions. The insertion was limited by the length of the implant and was visually controlled by the operator.

Maximum torque was recorded using the surgical unit applied during the insertion of the OMIs.

RFA was conducted using hand-screwed smart pegs (type 8; Osstell, Gothenburg, Sweden) to measure the achieved primary stability. Measurements were carried out in two planes at a 90° angle to each other. For the one- and three-wall defects, the measurement direction was toward the highest bone defect at the implant site, followed by a measurement in the perpendicular direction. The smart pegs were adjusted in consultation with the manufacturer to provide an ideal fit with the internal thread of the OMIs. The ISQ values ranged from 0–100 and were measured between 3500 Hz and 8500 Hz. The values were interpreted as low (ISQ < 60), medium (ISQ = 60–70), and high stability (ISQ > 70), according to the manufacturer [34]. The measurement was repeated three times for each implant. The average values were calculated.

The areas for the insertion of OMIs can be the maxillary tuberosity [46], retromolar [38], or palatal [7, 49] region, the mandibular buccal shelf [33], and the interradicular alveolar bone [9].

Exposure of a part of the implant surface is commonly seen in dentistry, especially when inserting dental implants in post-extraction sites [5]. As OMIs can be inserted into the tuberal area after extraction of a wisdom tooth (e.g., for distalization purposes), implant exposure of OMIs also cannot be ruled out, as the tuberal bone is reduced after wisdom tooth extraction [19] or can even fracture during the extraction procedure [8].

The sole application of conventional two-dimensional (2D) radiology for orthodontic implant planning is insufficient for detecting a local defect [15]. A narrow palate, which cannot be assessed using 2D cephalometry, may not allow full utilization of the implant length and therefore may also be a cause of coronal exposure of paramedian-inserted OMIs resembling a circular defect, as it was simulated in our study.

To previsualize implant position and minimize the risk of implant exposure, a 2D cephalogram can be superimposed with a digital three-dimensional model of the patient, and a surgical guide can be created utilizing computer-aided-design/computer-aided-manufacturing procedures [28].

The experimental setup of this study was based on a similar setup used in the study of Ibrahim et al., who investigated dental implants [18]. Primary stability was assessed using IPT and RFA. Both procedures have been applied previously and seem to be appropriate for measuring implant stability [11].

ISQ measurements were taken constantly from the largest defect area of the implant site whenever possible because of differing measurement directions could vary the results [42].

The clinical threshold for ISQ values for the primary stability of dental implants is 62.0 ± 1.1 [4]. A similar value was found in this study for the 2.3 mm × 9 mm implant in the HB with no defect (63.23 ± 2.57). Note that OMIs with values as low as 35.4 ± 2.67 already have shown to present primary stability [31]. These differing values could indicate the necessity of a new ISQ scale adapted to OMIs.

Using RFA measurements, Nienkemper et al. found that 2 mm × 9 mm OMIs provided sufficient primary stability and that there was no statistical difference for an OMI with a length of 11 mm and the same diameter [32]. Other studies also did not show any significant differences in implant stability in soft bone in relation to implant length [45], but the difference of the tested implant lengths was only 1 mm.

In contrast, the current study showed an effect of implant geometry on primary stability. Diameter and length affected the primary stability of the groups with no bone defects for D1, D2, and D3. The IPT measurements showed that the effect of implant length should be higher than that of implant diameter to achieve primary stability. This result was also confirmed by Möhlhenrich et al. [29].

The implant geometry effect seemed to be relevant only in dense bone (p < 0.001; D1, circular defect; 2.0 mm × 9 mm vs. 2.3 mm × 11 mm) and seemed to play a minor role in low-density bone (p > 0.05; D4, circular defect; 2.0 mm × 9 mm vs. 2.3 mm × 11 mm) if looking at the IPT values. This was confirmed by a decrease in the statistically significant differences for the implants inserted into D1–D4 (Fig. 3) for the IPT. Bayarchimeg et al. also found higher IPT values in bone models of higher density [3]. OMIs with different geometries with a cylindrical or conical design but with the same length and diameter showed no significant difference in performance when inserted into bovine pelvic ilium bone with densities of 0.36 (±0.02) g/cm³ and 0.32 (±0.02) g/cm³ [10]. This is similar to the D3 model (0.32 g/cm³) in our study, which was the only one where significant differences between the implant types were found, indicating that implant length and diameter might affect primary stability. Holm et al. found a significant effect of bone density and OMI diameter on primary stability [17]. With regard to RFA measurements, primary stability was reduced in low-density bone in this experimental setup, consistent with other studies showing a correlation between bone density and primary stability [25].

A study on dental implants revealed that not only defect size but also the location of the defect affected primary stability. A more coronal location of the defect led to reduced primary stability of dental implants [26]. Due to the individual anatomic conditions of the palate or the alveolar ridge and, if necessary, additional oblique angulated insertion, the OMI may not be inserted fully into bone. Therefore, this condition may resemble a coronal defect.

In our study, the ISQ values decreased with an increase of the size of the bone defect. This is consistent with the study of Yao et al., who detected bone defects of 2‑mm depth via a decrease in ISQ values using RFA [50]. Similarly, the IPT measurements decreased for greater bone defects and reduced bone density. These results are in accordance with studies that analyzed dental implants in bone defects [18].

Our findings clearly showed that in the sites with a circular defect in the bone model D4, the implants had significantly lower ISQ values and this result confirmed the effect of implant geometry on the stability of OMIs in sites presenting bony defects and low-density bone. According to the ISQ values, geometry could have a greater effect than the IPT measurement. This fact is evidenced when comparing the pattern of significant values for IPT and ISQ measurements in Fig. 4.

Only isolated significant differences were observed when comparing three-wall and one-wall defects in terms of the pattern of significant values for the IPT. The pattern was similar in terms of the ISQ values when comparing the same defect, but it was clearly different for both measurements if comparing the one-wall and circular defects (Fig. 3). Surprisingly, the IPT values were partially higher for the 2.0 mm OMIs than for the 2.3 mm OMIs. This effect could be due to the different pilot drills affecting stability. A pilot drill with a smaller diameter was shown to be associated with higher success rates in 2.0 mm OMIs [44].

According to our findings, the two methods applied to assess primary stability showed different results. RFA was more sensitive to bone quality with low density, while IPT measurements provided more sensitive information on bone with high density. In this context, Lages et al. reported no correlation between these two measurement methods, which seemed to be independent of one another and not comparable [22].

Bone model D2 demonstrated the highest correlation with the human bone substitute for the IPT measurements, and for model D1 the highest correlation with HB for the RFA measurements was observed. However, the r value for model D2 was only 0.03 smaller, also indicating a very strong correlation with HB.

Therefore, if both methods are regarded, D2 (0.48 g/cm³) demonstrated the best correlation with the human bone substitution material. The human mandible owns a typical mean bone density of 1.1 g/cm³, and the bone of the anterior maxilla displays a mean density of 0.55 g/cm³ in the anterior, 0.31 g/cm³ in the posterior, and 0.45 g/cm³ in the hard palate [12]. This confirms that the artificial sawbones D2 as well as Maxgraft can be used for OMI experiments to simulate the insertion of OMIs into the hard palate, which is a favorable region for implant insertion [7].

Nevertheless, the results of this study should be evaluated with caution, as in vitro studies have limited transferability to clinical settings.

Bone defects and bone quality affect the primary stability of implants. It may be favorable to use an implant with greater length and diameter in bone that has lower density. The measurement methods for primary implant stability used in this study have specific sensitivities in different areas. The solid rigid polyurethane foam block D2 is very well comparable in its behavior to the human bone substitute that was used in this study.