Insertion Angle Impact on Primary Stability of Orthodontic Mini-Implants
Objective: To analyze the impact of the insertion angle on the primary stability of mini-implants.
Materials and Methods: A total of 28 ilium bone segments of pigs were embedded in resin. Two different mini-implant sizes (Dual-Top Screw 1.6 × 8 mm and 2.0 × 10 mm) were inserted at seven different angles (30°, 40°, 50°, 60°, 70°, 80°, and 90°). The insertion torque was recorded to assess primary stability. In each bone, five Dual-Top Screws were used to compensate for differences in local bone quality.
Results: The angle of mini-implant insertion had a significant impact on primary stability. The highest insertion torque values were measured at angles between 60° and 70° (63.8° for Dual-Top 1.6 mm and 66.7° for Dual-Top 2.0 mm). Very oblique insertion angles (30°) resulted in reduced primary stability.
Conclusions: To achieve the best primary stability, an insertion angle ranging from 60° to 70° is advisable. If the available space between two adjacent roots is small, a more oblique direction of insertion seems to be favorable to minimize the risk of root contact.Abstract
INTRODUCTION
In many orthodontic cases, proper anchorage is crucial for a successful treatment outcome. Skeletal anchorage provided by orthodontic mini-implants has attracted great attention in recent years because of its versatility, minimal surgical invasiveness, and low cost.1–6
The most frequently used insertion site is the alveolar ridge. However, tooth injury represents a risk that is not to be underestimated.7–9 To avoid root damage, Park et al10 introduced an oblique instead of a perpendicular mini-implant insertion because more space was available near the apical region10–12 (Figure 1). Clinicians have assumed that the reduced stability and inferior anchorage quality associated with the obliquely inserted mini-implant are due to the minor insertion depth (Figure 2).
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
The actual impact of different insertion angles on mini-implant stability, however, remains unknown. Clinical evidence suggests that the primary stability of an implant determines its survival rate and reliability.13–15 In recent years, this has been proved for mini-implants as well.16
The aim of the present study was to analyze the impact of the insertion angle on the insertion torque and hence primary stability of mini-implants.
MATERIALS AND METHODS
The ilium of country pigs was chosen as the bone model. Compacta thickness of the bone segments ranged from 0.5 to 1.0 mm on the side toward the iliosacral joint and from 2.0 to 3.0 mm toward the hip joint. These values are comparable with the compacta thicknesses encountered in the human maxilla and mandible (Figure 3). Twenty-eight bone segments were embedded in resin (Probase, Ivoclar Vivadent, Schaan, Liechtenstein), and curing was performed under cooling water to avoid overheating of bone by polymerization energy.
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Two different mini-implant sizes were used for this study: Dual-Top Screw (Jeil Medical Corporation, Seoul, Korea) 1.6 × 8 mm and 2.0 × 10 mm (Figure 4). The pilot drillings were performed in the direction of the planned mini-implant insertion by a bench-drilling machine (Opti B 14 T; Rexon Europe GmbH, Hilden, Germany) at 915 rpm with the predrills of the Dual-Top System (Jeil). The drilling depths were adjusted at 3 mm with the following predrilling diameters: 1 mm for 1.6 mm implants and 1.3 mm for 2.0 mm implants.
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Prior to the time of measurement, the implants were manually inserted at seven different angles (α = 30, 40, 50, 60, 70, 80, and 90) with the use of a handheld screwdriver (Jeil) until the bone-to-collar distance of the mini-implant reached 0.7 mm (Figures 5 and 6). In each bone segment, five Dual-Top Screws (1.6 × 8 mm) were used as references to establish compatibility between the bone segments.
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Afterward, final screwing by another 0.2 mm up to the definite insertion depth (bone-to-collar distance = 0.5 mm; Figure 6) was performed by the robotic measurement system (RMS). A central component of the measuring system is a precision robot RX60 (StäubliTec-Systems GmbH, Bayreuth, Germany), which was equipped with a precision potentiometer (WHALE 300; Contelec, Biel/Bienne, Switzerland) that functioned as an angle sensor and as a torque sensor (8625-5001; Burster Präzisionsmesstechnik GmbH, Gernsbach, Germany). The moment sensor was coupled with the mini-implant with the use of the driver shaft of the Dual-Top System. The analog signals delivered by the sensors were digitized by the multichannel measuring device Spider 8 (Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) and were stored in a personal computer. The software of the measuring system was programmed such that the robot arm performed a rotation of 80° within 2 seconds (Figures 7 and 8).
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Each combination of angle and mini-implant size was repeated 34 times, and overall, 616 torque measurements were conducted. All maximum insertion torques were transferred to a pivot table (Excel 2003; Microsoft Corporation, Redmond, Wash, USA) and were categorized according to the parameters of implant size and insertion angle. The significance of mean value differences was evaluated by Kruskal-Wallis tests (Statistical Package for the Social Sciences [SPSS], version 12.0; SPSS Inc., Chicago, Ill, USA) and regression analysis with OriginPro 7.5 (OriginLabs, Northampton, Mass, USA). The maximum error was limited to P < .05.
RESULTS
Mini-implants with a 2 mm diameter showed significantly higher insertion torque when compared with mini-implants with a 1.6 mm diameter.
Furthermore, the angle of mini-implant insertion influenced the measured insertion torques: For the Dual-Top Screw 1.6 mm × 8 mm, the highest mean value (101 Nmm ± 31) resulted in insertions with an angle of 70°. The lowest insertion torque (78 Nmm ± 33) was found at very oblique insertions in the group with an angle of 30° (Figure 9). The differences were statistically significant (P < .05).
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Regression analysis (R2 = 0.68) revealed a maximum of the curve near the angle of 63.8° (Figure 10). The Dual-Top Screw 2 mm × 10 mm showed the highest mean value (167 Nmm ± 62) at insertions with an angle of 70°. As was obvious for the smaller Dual-Top Screw, very oblique insertions (angle = 30°) led to very low insertion torques (109 Nmm ± 46) (Figure 11). The differences were statistically significant (P < .01). Regression analysis revealed a maximum of the curve near the angle of 66.7° (Figure 12).
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
Citation: The Angle Orthodontist 78, 6; 10.2319/100707-484.1
DISCUSSION
As has been shown in previous studies,17–18 diameter has a great impact on the insertion torques of orthodontic mini-implants. The Dual-Top Screws with a 2 mm diameter showed significantly higher insertion torques when compared with mini-implants with a smaller 1.6 mm diameter. The insertion torques in this study of an animal bone model were similar to clinically measured values.
Even though the insertion depth is less after an oblique insertion than after a perpendicular insertion, the highest torques were measured when the mini-implants were inserted slightly obliquely at angles between 60° and 70° (63.8° for the mini-implant with a diameter of 1.6 mm and 66.7° for the mini-implant with a diameter of 2.0 mm). The reason for this may be the longer distance through cortical bone when the mini-implant is inserted in an oblique direction.
Therefore, should mini-implants generally be inserted obliquely? Probably not, because of the following drawbacks of the oblique insertion:
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Severe angulation during insertion may create slippage of the mini-implant at its first contact with bone. This leads to the need for predrilling even with drill-free mini-implants.
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Obliquely inserted mini-implants may expose a greater lever arm if forces are applied with the risk of higher failure rates.19
Besides the possibility of implant fracture, does a very high insertion torque also increase the risk of implant loss caused by excessive bone damage/compression? This phenomenon is known from dental implantology20 but has not yet been thoroughly investigated for mini-implants. Motoyoshi et al16 reported higher loss rates when the insertion torque exceeds 10 Ncm (100 Nmm) for mini-implants with a diameter of 1.6 mm. Presumably, this could be the reason for higher implant loss rates with mini-implants at very high insertion torques in the mandible.16–21 In the maxilla, however, it is difficult to obtain sufficiently high insertion torque because of the reduced bone quality, especially in the distal parts of the alveolar process. Because oblique insertion has major biomechanical advantages over perpendicular insertion, it is worth knowing that at least primary stability is not reduced. The findings of this study suggest that oblique insertion leads to slightly greater primary stability, which is advantageous in regions with reduced bone quality.
CONCLUSIONS
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Implant diameter has a great impact on insertion torque and hence primary stability of orthodontic mini-implants.
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To achieve a higher insertion torque, an insertion angle ranging from 60° to 70° is advisable.
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If the available space between two adjacent roots is small, a more oblique direction of insertion seems to be favorable to minimize the risk of root contact.
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Very high insertion torques may lead to higher failure rates caused by excessive bone compression; the appropriate ratio between implant and predrilling diameter is crucial.
Because of root divergence, more space is evident near the apical region
Obliquely inserted mini-implant (Dual-Top Screw, 1.6 × 8 mm) between the right upper lateral incisor and the canine. The treatment goal was mesial movement of the first molar with the missing second premolar
Ilium segment of a pig. The compacta thickness of the bone segments ranged from 0.5 mm toward the iliosacral joint up to 3.0 mm toward the hip joint
Tested mini-implant types: Dual-Top Screws 1.6 × 8 mm and 2.0 × 10 mm
Manual insertion with a handheld screwdriver up to a distance between bone and collar of 0.7 mm
Final screwing (another 0.2 mm insertion depth) by the robot (bone-to-collar distance = 0.5 mm). The mini-implants were inserted at different angles α (bone-to–mini-implant axis)
Construction of the measurement system, which comprises a precision potentiometer that functions as an angle sensor, a torque sensor, and the driver shaft
Measuring the final insertion torque with the robotic measurement system (RMS). The mini-implants were inserted at seven different angles (α = 30°, 40°, 50°, 60°, 70°, 80°, and 90°). Illustrated is an insertion angle of 40°
Insertion torques of the Dual-Top Screw 1.6 mm × 8 mm compared with the angle of insertion. The differences were statistically significant (P < .05)
Regression analysis of the insertion torques (Dual-Top Screw 1.6 × 8 mm) compared with the angle of insertion
Insertion torques of the Dual-Top Screw 2 mm × 10 mm compared with the angle of insertion. The differences were statistically highly significant (P < .01)
Regression analysis of the insertion torques (Dual-Top Screw 2 × 10 mm) compared with the angle of insertion
Contributor Notes
Corresponding author: Dr Benedict Wilmes, Department of Orthodontics, University of Duesseldorf, Moorenstr. 5 Duesseldorf, Germany 40225 Germany (wilmes@med.uni-duesseldorf.de)