Skeletal anchorage gained popularity for expanding the biomechanical modalities of orthodontic treatment and expanding the scope of orthopaedic treatment. It can be accomplished with mini-plates, mini-implants (TADs), or orthodontic implants. Among these, mini-implants are favoured by many clinicians. The ease of clinical use, reasonable cost, easy insertion and removal, and the possibility of immediate loading based on sufficient primary stability were the reasons for their frequent use in orthodontic practice [1,2,3,4,5].
The survival rate of mini-implants in the anterior palate is reported to be 97.9% in contrast to interradiculary inserted mini-implants with a failure rate of 10–30% [6, 7]. Within the so-called T-Zone, the anterior palate offers bone with high quality and is covered with thin mucosa [8,9,10,11]. Nonetheless, until now, no quantitative data documents the long-term stability of mini-implants and how the stability develops over time. From in vivo studies in dental implantology, it is well known that dental implant stability is subject to changes up to 20 months after the insertion [12,13,14,15,16].
The gold standard technique to measure dental implant stability, namely resonance frequency analysis (RFA), was successfully transferred and established to measure the stability of mini-implants [13, 17,18,19,20,21,22]. So far, mini-implant stability in the anterior palate was followed for 6 weeks through examination of the transition for primary to early secondary stability [20,21,22]. To our best knowledge, the long-term stability of mini-implants using RFA had not been studied in vivo.
The mini-implant length (9 mm vs 11 mm) does not seem to affect primary and early secondary stability in the anterior palate [20]. Until now, no long-term data exists about the influence of the implant length on the long-term stability of mini-implants inserted in comparable areas in the anterior palate. Hence, the aim of this study was to investigate:
- 1.
The stability of midpalatal orthodontic mini-implants after orthodontic treatment.
- 2.
The influence of the implant length on the long-term stability of mini-implants inserted in the midsagittal suture of the anterior palate.
- 3.
To compare the primary and early secondary stability data from previous prospective clinical trials with the RFA values prior to removal of the mini-implants.
It is hypothesized that the stability of the mini-implants can be attributed to the change after the initial healing period of 6 weeks and that the implant length has no significant impact on the long-term stability of the implants.
Subjects and methods
The stability of 9-mm and 11-mm mini-implants after orthodontic treatment was assessed by RFA (long-term group 9—LT9, long-term group 11—LT11).
Pieces of data were compared with each other, as well as with the data from a matched initial healing period group (initial healing group 9—IG9, initial healing group 11—IG11), to assess the initial and early secondary stability of mini-implants in a repeated cross-sectional study design [20].
All the patients that underwent treatment employing median mini-implants in the anterior palate of 2 × 9 mm or 2 × 11 mm (Benefit, PSM, Tuttlingen, Germany) were consecutively asked to participate in the study. The distance between the mini-implants is given by the connecting plate which is clinically ranged between 7 mm and 9 mm. Further inclusion criterion was the previous use of sliding mechanics for sagittal molar movement, 200 cN each side (Fig. 1). The implants and soft tissues were examined. Exclusion criteria were patients with systemic diseases affecting the bone metabolism, cleft patients, and patients showing signs of peri-implant inflammation. Visual inspection, performed before the removal of the implant, included detection of infection-related reddening and swelling. The tests for bleeding on probing were performed with a peri-odontal probe on four sites at each implant. Positive bleeding on probing without signs of any marginal bone loss around the implant was recorded as peri-mucositis. Informed consent was obtained from all the participants of this study. Therapeutic success was not a selection criterion.
Mini-implant insertion was carried out using a standardized protocol in all the groups. After predrilling with a burr of 1.3 mm in diameter to a depth of 3 mm, the mini-implants were inserted perpendicular to the palatal surface. The 2 × 9 mm mini-implants were inserted distal to the third ruga palatina, while the 2 × 11 mm implants were inserted slightly more anterior. The gingival thickness was measured using a dental probe after local anaesthesia. The appropriate gingival thickness was defined between 1 and 2 mm. The insertion and predrilling were performed using a surgical machine (ElcoMed SA 200C, W&H, Bürmoos, Austria). The identical exclusion criteria were applied to both groups. The study was conducted in accordance with the Declaration of Helsinki guidelines on experimentation involving human subjects and was approved by the local ethics committee.
After the removal of the mechanics and prior to the removal of the implants (T4), RFA was performed using the Osstell ISQ device (Osstell, Gothenburg, Sweden):
The removal of the mini-implant was carried out manually using a manual-driven contra-angled hand piece without local or topical anaesthesia.
The data obtained from these patients was compared with the matching groups from the previous prospective clinical trials examining primary and early secondary of mini-implants [20]. In this study, the stability of 2 × 9 mm median (IG9) and 2 × 11 median (IG11) mini-implants was observed during the healing phase over a period of 6 weeks. Mini-implant stability was measured on four different occasions using RFA:
T0—immediately after the insertion
T1—2 weeks after the insertion
T2—4 weeks after the insertion
T3—6 weeks after the insertion
To ensure the comparability of the groups (IG9 + 11 and LT9 + 11), the mini-implant diameter, insertion site, insertion protocol, vertical bone height, and the orthodontic appliance were nearly identical.
In total, 78 implants were investigated in this study. The distribution of the sample in each group presented the following composition: LT9, n = 21; LT11, n = 18; IG9, n = 19; and IG11, n = 20.
In the LT11 group, two implants were dropped out due to signs of peri-mucositis. In the LT9 group, one implant was dropped out due to implant loosening with clinical signs of mobility. In the IG groups, three dropouts were reported respectively [20].
Statistics
The sample size for the long-term groups was derived from a previous clinical trial [20]. Our data was compared with the data from that clinical trial. In addition, we retrospectively investigated the confidence intervals to confirm the relevance of the measurements (Table 3).
Group matching regarding age at the mini-implant insertion was tested with the Kruskal–Wallis test based on non-normal distribution (Shapiro–Wilk test). Gender matching was tested with the chi-square test. The vertical bone height was tested with a univariate ANOVA based on normal distribution (Shapiro–Wilk test).
The treatment time was compared using the Student’s t test for independent samples based on the normal distribution of the parameters (Shapiro–Wilk test).
Mean ISQ (implant stability quotient) values and standard deviations were calculated.
The ISQ values prior to the removal of the implants (2 × 9 mm and 2 × 11 mm) at T4 were compared with each other, as well as with the data from a previous prospective clinical trial examining primary and early secondary stability. Based on a test for normal distribution for small sample sizes (Shapiro–Wilk test), a univariate ANOVA was carried out to perform intra-group comparisons, followed by a Tukey post hoc test, wherever appropriate. Inter-group differences for each measurement time were tested with the Student’s t test for independent samples. The statistical analysis was carried out using SPSS Statistics 23 (IBM, Chicago, USA).