Failure rates of 958 interradicular mini-screws for maxillary total arch distalization following a well-established insertion protocol

Abstract

Objectives

Maxillary Total Arch Distalization (MTAD) with interradicular mini-screws (MSs) in combination with Completely Customized Lingual Appliances (CCLAs) is an effective concept for Class II correction in adolescents and adults which does not require any additional supraconstructions. The aim of this study was to investigate the failure rates of MSs after strictly following a defined insertion protocol.

Patients and methods

From November 2015 to August 2025, a total of 958 interradicular MSs were inserted in the maxilla for MTAD in combination with a CCLA in one orthodontic specialist practice. The surgical procedures were performed by 8 different clinicians (6 orthodontist specialists/2 postgraduate students in orthodontics) on 283 patients (female: 199/male: 84) with an average age of 27.6 years, ±13.3. Premature loss was defined as any instance in which a MS required removal due to mobility or was lost spontaneously on a yes/no basis.

Results

Of the 958 interradicular mini-screws, 14 (1.5%) failed before schedule (palatal 13/buccal 1). No significant influence of the experience level of the clinicians (P = 0.394) on the mini-screw loss rate was observed. The highest loss rate was observed in the patients > 45 age group.

Limitations

One limitation is the retrospective study design, but no patient meeting the inclusion criteria was excluded for any reason.

Conclusions

Interradicular mini-screws used as temporary anchorage devices for maxillary total arch distalization can exhibit comparably low failure rates (1.5%) to those placed in the anterior palate, provided that a well-established clinical protocol is strictly followed.

Introduction

Correction of Class II malocclusion remains a routine challenge in clinical orthodontic practice. When residual growth potential is present, removable functional appliances are considered an appropriate treatment option [1]. However, in the permanent dentition, particularly in cases with minimal or no remaining growth potential and where a surgical approach is either not indicated or not desired, the selection of an appropriate treatment concept becomes more complex. Fixed vestibular appliances or more esthetic approaches to Class II correction, such as lingual fixed appliances, may incorporate intermaxillary elastics [2] as well as rigid [3, 4] or flexible bite-jumping appliances [5], which primarily exert mandibular effects. In cases requiring substantial retraction of the maxillary teeth, Class II correction can be achieved either through premolar extraction or via maxillary distalization which can be supported by temporary skeletal anchorage [6–9].

As early as 1996, Wehrbein et al. introduced a specially designed orthodontic implant for skeletal anchorage in Class II correction, placed in the palate [10]. One year later, Kanomi, 1997, was among the first to introduce surgical mini-implants with interradicular insertion for skeletal anchorage into routine clinical orthodontic practice [11], and their use has steadily increased since then. In one of the earliest reviews on this topic, Crismani et al., 2010, reported that orthodontic mini-screws (MS) offer advantages such as ease of insertion and removal, relatively low cost, and low complication rates [12]. Recently, Barry et al., 2025, have confirmed that insertion-related discomfort is generally low and that overall patient discomfort is manageable [13]. Today, common insertion sites for maxillary distalization include interradicular regions and the anterior palate (e.g. paramedian or midpalatal locations), where MSs are frequently combined with supraconstructions such as sliders [14].

The first prospective clinical investigation on loss rates of MSs in orthodontics was initiated in 2003 by Wiechmann et al. [6]. In this study, a total of 133 MSs with different diameters (thin 1.3 mm and large 1.6 mm) were inserted interradicularly in various regions of the maxilla and mandible in 49 patients. The results demonstrated considerable variability in loss rates, with more favorable outcomes observed for insertions in the buccal side of the maxilla. In a follow-up investigation involving an additional 105 MSs and a loading protocol optimized based on animal studies, Berens et al. were able to further improve success rates [15–17]. The authors reported MS failure rates of < 5% for interradicular insertion in the buccal parts of the maxilla [17]. In the following years, numerous efforts were made to achieve similarly high success rates for insertions in the mandible and in other regions of the maxilla (median and paramedian). In addition to more sophisticated diagnostic approaches, such as periapical radiography and cone-beam computed tomography (CBCT), the use of surgical guides and jigs to enhance the precision and safety of screw placement was introduced [18, 19]. Furthermore, operator experience has been emphasized for achieving high MS survival rates [20].

The aim of the present study was to evaluate the actual failure rates of interradicular MSs exclusively used for Maxillary Total Arch Distalization (MTAD), which were inserted and loaded according to a standardized protocol, with particular consideration given to the experience level of the operator. The application of this concept does not require any additional supraconstructions (Fig. 1).

Figure 1.

Adult patient with MTAD protocol (a). At the beginning of maxillary distalization the curve of spee was leveled and a correct upper incisor inclination was established (b). Interradicular insertion of four MSs and loading with power chains (150–200 cN initially, depending on the soft-tissue thickness). A critical tip moment of 600 cN mm should not be exceeded (c). Final occlusion after MTAD (d).

Patients and methods

This retrospective study was approved by the Ethics Committee of Hannover Medical School, Hannover, Germany (3151-2016).

Patients

Inclusion criteria were consecutively debonded patients who underwent orthodontic treatment using a completely customized lingual appliance system (CCLA; WIN, DW Lingual Systems, Bad Essen, Germany) in combination with interradicular MSs for MTAD as part of Class II malocclusion correction of up to more than one full unit [7] (Fig. 1). All patients were treated in one orthodontic specialist practice (Prof. Wiechmann, Dr. Beyling and colleagues, Bad Essen, Germany) between October 2014 and April 2026. No patients were excluded from the sample for any reason. Specifically, systemic conditions, smoking, a history of previous orthodontic treatment, missing teeth, incomplete records, or poor compliance (e.g. inadequate oral hygiene, insufficient Class II elastic wear, or missed appointments) were not considered exclusion criteria.

A total of 283 patients received maxillary interradicular MSs for Class II correction. The sample consisted of 70% female and 30% male patients. Patient age at the time of treatment start ranged from 11.5 to 73.5 years, with a mean age of 27.6 years. Baseline characteristics at the patient level are summarized in Table 1.

Table 1.

Baseline characteristics.

			Age group				Observation period
							First Half	Second Half
Patient	N	All  N = 283a	<18  N = 103a	18–30  N = 78a	30–45  N = 64a	>45  N = 38a	N  =  84a	N  =  199a
Age (years)	283	27.6 (13.27) [11.50, 73.5]	15.1 (1.51) [11.5, 17.8]	24.4 (3.64) [18.1, 29.9]	37.2 (4.29) [30.4, 44.9]	52.2 (6.13) [45.0, 73.5]	29.4 (13.61) [13.4, 73.5]	26.9 (13.09) [11.5, 68.1]
Tx time (months)	283	31.80 (11.94) [11.99, 71.45]	35.48 (11.79) [14.00, 71.45]	31.40 (12.46) [11.99, 69.00]	27.63 (10.82) [12.00, 63.00]	25.74 (9.64) [14.00, 62.00]	38.9 (14.43) [15.37, 71.45]	28.82 (9.25) [11.99, 66,95]
Female	283	199 (70%)	54 (52%)	66 (85%)	48 (75%)	31 (82%)	57 (68%)	142 (71%)
Male		84 (30%)	49 (48%)	12 (15%)	16 (25%)	7 (18%)	27 (32%)	57 (29%)

^aMean (SD) [Min, Max]; n (%).

Methods

MS placement was performed between 2015 and 2025. Patients received between one and eight interradicular MSs placed on the palatal (DualTop S16-G2–010 N, Jeil Medical, Seoul, Republic of Korea) and buccal sides (Abso Anchor SH 1312–10, Dentos, Daegu, Republic of Korea). In total, 958 MSs were inserted (496 DualTop and 462 Abso Anchor). Both screw types were self-drilling and 10 mm in length; DualTop screws had a diameter of 1.6 mm, whereas Abso Anchor screws measured 1.2 mm at the tip and 1.3 mm at the neck of the screw. Pilot drilling to a depth of ∼10 mm was routinely performed using a 1.0 mm diameter pilot driller.

Interradicular MS insertion protocol

All instruments were sterilized according to the manufacturer's instructions. The most recent panoramic radiograph was reviewed to assess potential abnormalities in root morphology and positioning. In the absence of contraindications, buccal and palatal infiltration anesthesia was administered in the maxilla, preferably in the region of teeth U5 and U6 (Ultracain DS, Hoechst, Frankfurt am Main, Germany). Subsequently, the patient rinsed for one minute with a 0.2% chlorhexidine mouthwash to achieve oral disinfection.

On the buccal side, the root of the maxillary second premolar and the mesiobuccal root of the first molar were palpated at the alveolar process using a periodontal probe (PCP 11, Aesculap, Tuttlingen, Germany; Fig. 2a). The insertion site was determined midway between the roots of teeth U5 and U6, slightly coronal to the mucogingival junction within the attached gingiva. The site was marked with the periodontal probe after local infiltration anesthesia (Fig. 2b). A pilot hole was drilled to a depth of 10 mm at an insertion angle of <40° indicating a pronounced upward orientation relative to the alveolar process. The MS was then inserted manually and slowly with a screwdriver, following the angulation of the pilot hole. Both pilot drilling and screw insertion were performed under continuous irrigation with 0.9% saline solution (Fig. 2c).

Figure 2.

Insertion of the buccal interradicular MSs. Determination of the insertion site using a periodontal probe (a and b). Manual insertion with a screwdriver after pilot drilling (c).

On the palatal side, the midpoint between the palatal roots was identified using a periodontal probe (Fig. 3a). The probe was positioned beneath the archwire, creating a visible indentation on the gingiva centered between teeth U5 and U6. In patients with healthy periodontal structures, the insertion site was marked at least 10 mm apical to the archwire and 1.5 mm distal to the midpoint between U5 and U6 (Fig. 3b). In patients with reduced bone height, the distance from the archwire was increased, accordingly. Following local anesthesia, a pilot hole was drilled at a 90° angle to the alveolar process. The drill angulation was verified from an occlusal perspective using a mirror to prevent mesial or distal deviation and to ensure accurate positioning. Insertion of the palatal MS was performed using a 30:1 reduced handpiece with the appropriate adapter. The MS was inserted slowly and without excessive pressure, following the angulation of the pilot-hole. Both pilot drilling and MS insertion were carried out under continuous irrigation with 0.9% saline solution (Fig. 3c).

Figure 3.

Insertion of the palatal interradicular MSs. Determination of the insertion site using a periodontal probe (a and b). Insertion of the MS using a handpiece after pilot drilling (c).

The standard insertion site for MTAD was the interradicular region between the maxillary second premolar and first molar on both the buccal and palatal sides. When the available interradicular space became insufficient during distalization, or when further distal movement was required, in a limited number of cases additional buccal and palatal MSs were placed between the maxillary first and second molars.

In most patients four MSs were inserted, and the most frequent insertion sites were located between the premolars and first molars on both the buccal and palatal sides (Table 2). All MSs were loaded immediately with a force of 150–200 cN after insertion according to the MTAD protocol.

Table 2.

Number of MSs per patient and MS position.

Number of MSs	All	1 MS	2 MSs	3 MSs	4 MSs	5 MSs	6 MSs	7 MSs	8 MSs
Patients, N (%)	283 (100%)	3 (1%)	86 (30%)	6 (2%)	180 (64%)	5 (2%)	2 (0.7%)	0 (0%)	1 (0.4%)

Position of MSs	All	MB UR6	MB UL6	MP UR6	MP UL6	DB UR6	DB UL6	DP UR6	DP UL6
MS, N (%)	958 (100%)	224 (23%)	229 (24%)	233 (24%)	244 (25%)	4 (0.4%)	5 (0.5%)	7 (0.7%)	12 (1.3%)

Abbreviations: DB UL6, distobuccal of upper left molar; DB UR6, distobuccal of upper right first molar; DP UL6, distopalatal of upper left molar; DP UR6, distopalatal of upper right molar; MB UL6, mesiobuccal of upper left first molar; MB UR6, mesiobuccal of upper right first molar; MP UL6, mesiopalatal of upper left first molar; MP UR6, mesiopalatal of upper right first molar.

Operators and operator experience

For the orthodontic protocol, including loading of the MSs, the procedure described by Beyling et al. was followed in full detail [7]. MS insertion was performed by six orthodontic specialists and two postgraduate students within the same orthodontic practice. All clinicians received structured training and supervision before independently performing MS insertions. Based on their experience (number of MSs inserted) the practitioners were divided into three groups (Table 3).

Table 3.

Operator experience.

Placed MSs	<50 MS	50–200 MS	>200 MS
Operators, N (%)	4 (50%)	3 (37.5%)	1 (12.5%)
Total MS per group, N (%)	80 (8%)	3312 (33%)	566 (59%)
Total MS by operator, N	36, 30, 12, 2	160, 78, 74	566

Outcome assessment

Patients were scheduled for follow-up visits at ∼4 to 6-week intervals. At each visit, MS stability was assessed, and power chains were replaced in accordance with the protocol proposed by Beyling et al. [7].

MSs were routinely removed once a slight overcorrection of the Class II relationship (up to 2 mm) had been achieved to compensate for minor immediate relapse. MSs were also routinely removed if they interfered with further distalization during treatment. This occurred more frequently with buccal screws after 3–5 months, primarily due to limited interradicular space. In contrast, palatal screws, positioned more distally (1.5 mm distal to the midpoint between U5 and U6), generally remained functional until the end of MTAD [8]. In a small number of cases, additional MSs were placed one segment further distal (U6/U7) to allow continued distalization.

Premature failure was defined as spontaneous loss of an MS or removal due to clinically detectable mobility before the screw had fulfilled its intended anchorage function. In contrast, stable MSs that were removed because they interfered with further distalization due to limited interradicular space were not classified as failures, as these screws had remained clinically stable and had served their intended function until removal. Accordingly, MSs were classified as successful if they maintained sufficient clinical stability and remained in situ until the end of their possible anchorage function, including cases in which removal became necessary because the screw position impeded further distalization.

Besides actual screw loss, hardware-related complications (e.g. fracture of the predriller or mini-screw during insertion or removal) and soft tissue inflammation was additionally documented.

Statistical analysis

All data were summarized descriptively on the patient level for using mean, ±standard deviation (SD), median, and range (min, max). Count data were shown using absolute counts and percentage values. For the analysis of the primary endpoint MS loss (yes/no), all MSs were considered as independent. To assess the potential impact of categorical variables on the failure rates a Fisher Exact Test with a significance level of α = 0.05 was used due to the low number of lost Pins observed. In case a variable had more than 2 expression levels and the Fisher Exact Test was significant (P < 0.05), post-hoc pairwise comparisons were performed with a P-value adjustment for multiple testing using the Bonferroni-Holm method. A P-value P < 0.05 was considered as statistically significant. Based on the explorative nature of this study no additional alpha correction was applied. All statistical analyses were conducted using the statistical software R studio 4.5.1 (R Foundation for Statistical Computing, Vienna, Austria).

Results

The average duration between insertion and removal of the MSs was 6.9 months (±4.18, min: 0.4, max: 32.8). Buccally inserted MSs were removed earlier, after a mean of 4.8 months (±2.31, min: 0.69, max: 18.17), whereas palatally inserted MSs remained in situ for nearly twice as long (8.9 months, ±4.54, min: 0.4, max: 32.8). Of the 958 interradicular inserted MSs, 14 (1.5%) were lost prematurely (Table 4). The operator's level of experience was not a predictive factor for MS loss (P = 0.4). While only one of 462 buccally inserted MSs required premature removal (0.2%), this occurred in 13 palatally inserted MSs (2.6%), representing a significantly higher rate (P = 0.002). A significant association was found between patient age and MS failure (P = 0.006), with the highest loss rate (4.4%) observed in patients older than 45 years. Post-hoc pairwise comparison revealed a significant difference between the 30–45 age group and the >45 age group (adjusted P = 0.019). Although the mean failure rate for insertions distal to the first molars was considerably higher than the average (7.1%), no statistically significant difference compared with mesial insertions was observed, likely due to the small group size (n = 28; P = 0.060). Neither gender nor the side of insertion (first or second quadrant) had a significant influence on the screw loss rate.

Table 4.

Mini-screw loss rates.

	Total	Loss	P-valuea
Total	958	14 (1.5%)
Doctor's experience			0.394
Low	80	0 (0.0%)
Medium	312	3 (1.0%)
High	566	11 (1.9%)
Buccolingual position			0.002
Buccal	462	1 (0.2%)
Palatal	496	13 (2.6%)
Age			0.006b
< 18	349	3 (0.9%)
18–30	262	5 (1.9%)
30–45	212	0 (0.0%)
> 45	135	6 (4.4%)
Mesiodistal position			0.060
Mesial	930	12 (1.3%)
Distal	28	2 (7.1%)
Gender			0.372
Female	692	12 (1.7%)
Male	266	2 (0.8%)
Quadrant			0.598
1st	468	8 (1.7%)
2nd	490	6 (1.2%)

^aFisher's exact test.

^bSignificance difference only between the 30–45 and >45 groups (P_adj = 0.019).

Fractures of the pilot drill occurred in three patients. The fractures occurred within a short time interval and involved pilot drills from the same production lot. Following replacement with a new lot of pilot drills, this complication did not recur. A fracture of one MS during removal was observed in one patient probably due to deformation during MS insertion [16]. Soft tissue inflammation was detected around the head and the neck of 47 palatal and 4 buccal MSs; in nearly all cases, the condition resolved following improved oral hygiene combined with the use of a bacteriostatic mouth rinse. None of the 14 lost MSs had to be replaced, as the planned distalization was mainly achieved or could be continued on the second screw in the same quadrant. In eight patients, more than four MSs were placed because the interradicular distance was insufficient to achieve complete correction of the Class II malocclusion (Table 2). To enable full correction in these pronounced disto-occlusions, the anterior screws were removed, and new screws were inserted distal to the first molars. All these events did not compromise the success of the MTAD procedure and the Class II correction. No clinically apparent root damage was documented in the patient records or routine radiographic controls.

Discussion

This study comprises an unprecedentedly large sample size of 958 MSs in the context of maxillary distalization. Furthermore, the insertion of the optimal type of screws in combination with a precise insertion procedure was performed over a period of more than 10 years by different clinicians with varying levels of experience. In addition to the consistent surgical approach maintained throughout the entire observation period, certain aspects of the orthodontic protocol were successfully optimized over time. This inevitably contributed to a reduction in overall treatment duration and to an increased acceptance of the method (Table 1). The exceptionally low mean failure rate reported [21], with only 14 failures observed among 958 cases (1.5%), can be attributed to a combination of factors, which are examined and discussed in detail.

Surgical protocol

The surgical protocol applied is largely based on studies by Büchter et al., Wiechmann et al., and Berens et al., whose findings were published nearly 20 years ago [6, 15–17]. The authors reviewed the findings of the initial investigations on interradicular MSs in orthodontics, conducted primarily by European and Asian research groups [11, 22–25], and refined the proposed protocols in several key aspects. In particular, the prospective, controlled animal studies with subsequent histological analyses have contributed substantially to the understanding and classification of the cellular-level responses of the hard tissue alveolar process in the context of skeletal anchorage using interradicular MSs [15, 16]. The recommendations established at that time remain valid and have been consistently supported by numerous, predominantly retrospective clinical studies investigating the causes of premature failure of these screws conducted in the intervening years [14, 21].

Based on histological analyses of the osseointegration rate relative to the screw surface following force application, Büchter et al. described a critical tip moment of 500–600 cN mm for the 10 mm screws used in their study. The calculation of this tipping moment is based on the lever arm length, defined by the distance between the point of force application at the screw head (e.g. via a power chain) and the cortical bone. From this, an optimal loading force of 150–200 cN immediately after insertion can be derived. Given the increase in osseointegration rates under optimal loading conditions, reaching over 80% after 10 weeks of loading as reported by the authors, this force may be increased to 200–250 cN during the course of distalization, depending on the soft-tissue thickness. Assuming seven teeth per side and a minimum force requirement of at least 50 cN per tooth for bodily distalization, the use of two screws per side is recommended to avoid overloading the MSs while achieving maximal distalization rates, as also suggested by Bechthold et al. [26].

Predrilling prior to mini-screw (MS) insertion has proven highly advantageous over the years [17]. It should be acknowledged that MS insertion without predrilling may preserve bone compression and thereby enhance primary stability, particularly in low-density maxillary bone. However, the present protocol deliberately included pilot drilling because the screws were inserted interradicularly and without surgical guides. In this setting, predrilling facilitates both manual buccal insertion and machine-assisted palatal, insertion, making the procedure safer and more straightforward. It facilitates control of the insertion path, reduces excessive insertion torque, and may decrease the risk of unintended deviation of MS or root contact. The increased force required during manual insertion without predrilling may lead to unintended deviations from the optimal position; during machine-assisted insertion, tactile feedback regarding potential resistance, for example from a root, may be partially diminished, thereby increasing the risk of root injury [17]. The very low failure rate observed in the present cohort suggests that the potential reduction in bone compression did not negatively affect clinical stability when pilot drilling was performed according to the described protocol. Table 5 summarizes the key recommendations for the use of interradicular MSs in the context of MTAD, which have subsequently been supported by numerous authors [14, 21]. This protocol was meticulously followed in all 283 included patients.

Table 5.

Recommended protocol for insertion of two MSs per side for MTAD.

	Palatal	Buccal
MS length	10 mm	10 mm
MS diameter	1.6 mm	1.2–1.3 mm
Insertion guide	No	No
Predrilling, diameter	Yes, 1.0 mm	Yes, 1.0 mm
Insertion angle to alveolar process	90°	30°–40°
Insertion height/tissue	10 mm from wire, attached gingiva, verified by probing	Attached gingiva, next to mucogingival junction
Segment 1st and 2nd choice	Mesial U6/ Distal U6	Mesial U6/ Distal U6
Insertion mode (slow)	Handpiece 30:1 with adapter	Manual screwdriver
Loading (immediate/after 10 weeks)	150–250 cN	150–250 cN
Average time of removal	9 months	5 months

Orthodontic protocol

In addition to the ten points concerning the surgical component, five further prerequisites in the field of orthodontic mechanotherapy must be fulfilled for successful implementation of MTAD. In particular, bite opening with leveling of the mandibular curve of Spee, required in nearly every Class II correction, is a ‘sine qua non’ condition. This can, however, be reliably achieved with fixed appliances, especially lingual ones, without requiring additional patient compliance [27, 28]. Equally important, both at the beginning and throughout maxillary distalization, is reliable inclination control in the maxillary anterior region [29]. Due to the high precision of their bracket slot geometry, completely customized lingual appliances (CCLAs) are particularly well suited to achieve this level of inclination control [29–34].

In addition to these two general requirements for the orthodontic appliance in use, which are indispensable prerequisites for any Class II correction, there are three further aspects that are specifically relevant to the MTAD protocol. A longer insertion period of interradicular MSs is inevitably associated with a higher loss rate [6]. A segmented approach, in which molars are distalized first and the anterior teeth are subsequently retracted, under maximum, MS-supported anchorage similar to an extraction case, will therefore lead to higher failure rates. The short average MS-insertion-time of 6.9 months observed in the present study therefore contributes substantially to the low loss rate of 1.5% [7, 8, 17]. A favorable side effect of anterior torque control for Class II correction is intrusion of the posterior maxillary teeth with a slight clockwise rotation of the occlusal plane [7, 8, 26]. This effect is further enhanced by the position of the force vector of the elastic chain used for distalization, which lies caudal to the center of resistance of the maxillary dentoalveolar complex [35]. When lingual fixed appliances are used, the intrusive force acts palatal to the center of resistance of the posterior teeth and is therefore associated with palatal tipping, which in turn leads to a narrowing of the maxillary dental arch [7, 8]. This effect should be neutralized by the Orthodontist using stainless steel archwires with expansion (in lingual appliances) or compression (in buccal appliances) bends [36]. The potential posterior bite opening due to maxillary intrusion can be prevented by night-time wear of intermaxillary elastics extending from the maxillary canine to the first or second mandibular molar in order to create lower molar extrusion [7, 8].

Overall, the present study demonstrates that, when an optimized protocol is applied, very low MS failure rates comparable to those achieved with routine insertion in the anterior palate can be attained [20, 37]. More substantial deviations from this protocol, which was optimized over a period of more than ten years (2002–2014), appear to inevitably result in higher failure rates. In their systematic review and meta-analysis including 3250 mini-screws from 41 studies, Alharbi et al. [38]. reported a mean failure rate of 13.5% following interradicular MS insertion. Similarly, Mohammed et al. [21]. observed major differences in the mean failure rates between MSs inserted in the anterior palate and an interradicular parapalatal insertion (1.3% and 5.5%). In contrast, the continuous optimization of the clinical protocol presented here has led to progressively lower failure rates over time. Beginning with the study by Wiechmann et al. [6], (observation period: 2002–2004; mean failure rate: 13.3%), already Berens et al. were able to achieve reduced failure rates following an initial process optimization (2004–2005; 5%). In subsequent years, the surgical and the orthodontic protocol were further refined, resulting in even lower failure rates as reported by Beyling et al. (2016–2019; 2.9%) and Janssens et al. (2019–2023; 2.1%), which are consistent with the findings of the present investigation [6–8, 17].

Operator experience

A systematic review investigating barriers to the clinical use of MS reported that clinicians often have concerns regarding costs and potential risks, frequently related to limited clinical experience [39]. This appears to be different when the protocol described here is followed [7, 17]. The results of the present study demonstrate that the insertion of interradicular MSs is straightforward to learn and, when performed according to a strict clinical protocol, represents a safe and reliable method for MTAD, with low failure rates even for less experienced clinicians.

The operator's level of experience was not significantly associated with MS loss in the present cohort. This finding suggests that the described interradicular insertion protocol can be taught and implemented reliably when structured training and supervision are provided. The use of clearly defined anatomical landmarks, pilot drilling, controlled insertion angulation, and standardized loading may reduce the influence of individual operator experience.

Furthermore, both technical complexity (chair-time) and costs are limited, as no supra-construction is required for distalization. Therefore, if there are no major concerns regarding the patients profile the following indications for nonsurgical Class II correction with MS-assisted MTAD might be considered:

1. Patients presenting with a fragile gingival phenotype, including pre-existing gingival recession or generalized thin gingiva.

2. Cases requiring maxillary anterior retraction in which extraction of the upper premolars is contraindicated.

3. Patients with poor compliance, limiting the effectiveness of conventional orthodontic mechanics.

4. Patients exhibiting relapse following dentoalveolar Class II correction, particularly when primarily originating from the mandible.

5. Relapse after previous mandibular advancement surgery.

6. Patients presenting with a Class II ‘Sunday bite’.

A high-quality occlusion at the end of active orthodontic treatment remains, then as now, one of the primary treatment objectives [40]. This is particularly important in ensuring the long-term stability of treatment outcomes, as discussed by Pancherz and Cornelis et al. [41, 42]. Compromises in the occlusal relationship generally diminish the overall quality of the treatment result and may even call into question the value of the therapy itself. Achieving perfect correction of a Class II malocclusion is often challenging, not least due to issues related to patient compliance. In such cases, this may represent one of the principal indications for the use of interradicular MS-assisted MTAD.

Limitations

One limitation of this study is its retrospective design, which entails potential selection bias, and limited control over confounding variables. However, the present investigation spans a period of 10 years, thereby allowing the inclusion of an unusually large patient cohort. Furthermore, no patient meeting the inclusion criteria was excluded for any reason, reducing the risk of additional selection bias.

All patients were treated in a single orthodontic specialist practice in Germany with extensive experience in lingual orthodontics. While this may limit the generalizability of the findings to other populations or clinical settings, it can also be regarded as a strength. The fact that all treatments were performed in a highly experienced orthodontic practice ensures standardized treatment protocols and minimizes variability in the therapeutic approach, thereby enhancing internal validity.

Although the standard insertion site was the interradicular region between the maxillary second premolar and first molar, additional screws were placed more distally in selected cases. Furthermore, although the applied forces were kept within the clinically recommended range, exact force levels and the resulting mechanical load may have varied slightly according to the insertion site, soft-tissue thickness, and treatment stage. These factors may represent potential confounders affecting MS stability.

Smoking status, systemic conditions, and oral hygiene status were not available in a standardized manner and therefore could not be included in the statistical analysis. Because these factors may influence mini-screw survival, especially through their effect on peri-implant soft tissue inflammation and bone remodeling, they represent potential confounders of the present retrospective analysis.

Root damage was not systematically assessed using CBCT or a predefined radiographic protocol. Therefore, the present study can only state that no clinically apparent root damage was documented in the available records; it cannot exclude minor or subclinical root contact.

Finally, the present study primarily focused on post-treatment outcomes, providing only limited insight into the long-term stability of the treatment effects or the potential for relapse over time.

Conclusions

Interradicular mini-screws used as temporary anchorage devices for MTAD can exhibit comparably low failure rates (1.5%) to those placed in the anterior palate, provided that a well-established clinical protocol is strictly followed.

Author contributions

Lara Bettenhäuser-Hartung: Project administration, Data curation, Investigation, Writing—original draft. Frauke Beyling: Data curation, Investigation, Visualization. Rainer Schwestka-Polly: Supervision, Conceptualization, Writing—review and editing. Christos Katsaros: Supervision, Conceptualization, Writing—review and editing. Jonas Q. Schmid: Supervision, Data curation, Investigation, Conceptualization, Statistical analysis, Writing—review and editing.

Funding

None declared.

Data availability

Data underlying this publication will be shared on reasonable request to the corresponding author.