ARTÍCULOS ORIGINALES

Evaluation of fracture torque resistance of orthodontic mini-implants

 

Fernando Dalla Rosa¹, Paola F. P. Burmann², Henrique C. Ruschel³, Ivana A. Vargas⁴, Paulo F Kramer⁵

¹ Private practice, Caxias do Sul, RS, Brazil.
² Private practice, Santo Ângelo, RS, Brazil.
³ Department of Oral Histology and Department of Pediatric Dentistry, Universidade Luterana do Brasil (ULBRA), Canoas, RS, Brazil.
⁴ Department of Orthodontics, ULBRA, Canoas, RS, Brazil.
⁵ Department of Pediatric Dentistry, ULBRA, Canoas, RS, Brazil.

CORRESPONDENCE Dr.Fernando Dalla Rosa Rua Sinimbu, 1878, sala 1106, bairro Centro 95020002 Caxias do Sul, RS Brazil ferdallarosa@yahoo.com.br

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ABSTRACT

This study sought to assess the fracture torque resistance of miniimplants used for orthodontic anchorage. Five commercially available brands of miniimplants were used (SIN®, CONEXÃO®, NEODENT®, MORELLI®, and FORESTADENT®). Ten miniimplants of each diameter of each brand were tested, for a total 100 specimens. The miniimplants were subject to a static torsion test as described in ASTM standard F543. Analysis of variance (ANOVA) with the Tukey multiple comparisons procedure was used to assess results. Overall, mean fracture strength ranged from 15.7 to 70.4 N·cm. Miniimplants with larger diameter exhibited higher peak torque values at fracture and higher yield strength, regardless of brand. In addition, significant differences across brands were observed when implants were stratified by diameter. In conclusion, larger miniimplant diameter is associated with increased fracture torque resistance. Additional information on peak torque values at fracture of different commercial brands of miniimplants may increase the success rate of this orthodontic anchorage modality.

Key words: Dental implants; Orthodontics; Torque.

RESUMO

Resistência de fratura ao torque de mini-implantes ortodônticos

O objetivo do presente estudo foi avaliar a resistência de fratura ao torque de miniimplantes ortodônticos. Foram utiliza das cinco marcas comerciais (SIN®, CONEXÃO®, NEODENT®, MORELLI® e FORESTADENT®). Para cada diâmetro, de cada marca comercial, foram testados 10 miniimplantes, totalizando 100 amostras. Os miniimplantes foram submetidos a um Ensaio Estático de Torção, conforme a norma técnica ASTM F543. Os resultados foram submetidos à Análise de Variância (ANOVA) complementado pelo teste de compa rações múltiplas de Tukey. Os valores médios de resistência de fratura ao torque variaram de 15,7 a 70,4 N·cm e miniimplantes de maior diâmetro apresentaram maiores valores de torque máximo de fratura e de limite de escoamento, independente da marca comercial. Além disso, foram obser vadas diferenças significativas entre as marcas comerciais quando agrupadas de acordo com o diâmetro. Concluise que miniimplantes de maior diâmetro apresentaram maiores valores de resistência de fratura ao torque. Informações sobre o torque máximo de fratura das diferentes marcas comerciais podem aumentar o índice de sucesso deste método de ancoragem ortodôntica.

Palavras-chave: Implantes dentários; Ortodontia; Torque.

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INTRODUCTION

The control of loads placed on teeth and their bony foundations is one of the principles of orthodontics¹. For every action force there is a reaction force of equal size and opposite direction, which causes movement of the anchorage unit². Therefore, management of orthodontic anchorage, which may be defined as the resistance offered by a group of teeth or extraoral supports when a force is applied, thus preventing or limiting unwanted movement, is essential to the success of orthodontic treatment ^{3, 4}. In recent years, alternative orthodontic anchorage methods have become the focus of substantial research and miniimplants have been introduced into the market, broadening the range of options available ^{5, 6}.
The success of miniimplants is related to their minimally invasive nature, ease of insertion and removal, low cost, immediate loading, versatility, and little discomfort to the patient^6-8. Overall, their success rate is over 80%.⁹ However, failure in the placement of these devices has been reported^{10, 11}. Research into factors that interfere with the stability of these devices and their resistance to fracture at insertion and removal has therefore been encouraged5. Fracture torques of 5 N・cm to 50 N・cm during implant placement have been reported in the literature^{8, 10, 11}, although few manufacturers report such reference values. Studies have also suggested that factors associated with miniimplant design, thread profile, and material may also influence outcomes^12-14. In addition, miniimplants with larger diameter have been found to have superior fracture strength¹⁵. In view of the foregoing, the present study sought to assess the fracture torque resistance of orthodontic miniimplants from different manufacturers.

MATERIALS AND METHODS

This was a laboratorybased in vitro study and may be described as a static torsion test of bone screws. The study was conducted at Laboratorio de Ensaios Mecanicos - Solucoes em Ensaios de Materiais e Produtos (LEMSCITEC, Palhoca, SC, Brazil), a facility accredited by the Brazilian National Institute of Metrology, Quality and Technology - Inmetro (CRL 0495). Five brands of orthodontic miniimplants commer cially available on the Brazilian market, with fully threaded, cylindrical, solid shafts, were used. The material from which miniimplants are made is defined by ASTM standard specification F136 (Ti 6Al4V). It is a titanium alloy containing 6% aluminum and 4% vanadium used for manufacturing medical and dental implants. Ten miniimplants of each diameter of each brand were tested, for a total 100 specimens. Diameters ranged from 1.3 to 2 mm. All implants had a transmucosal profile of 1 mm (Table 1). All specimens had fully threaded cylindrical shafts 8 to 9 mm in length. The following characteristics were assessed in each specimen: mode and site of failure, angle of rupture, resistance to fracture at insertion, and yield torque. Static torsion testing was performed as described in ASTM standard F543 Standard Specification and Test Methods for Metallic Medical Bone Screws ¹⁶. Each screw was secured in locking pliers to prevent rotation during testing, keeping five threads exposed above the transmucosal profile.

Table 1: Identification of mini-implants evaluated in the study.

Tests were conducted at a constant speed of 1 rpm, under dry conditions, at a temperature of 20 ± 5o C. A torque (N・m)angle (°) curve was plotted for each tested specimen, and the test was terminated at the time of screw failure. The equipment used in torsion testing is described in Table 2.

Table 2: Identification of mini-implants evaluated in the study.

Ten specimens were used as a comparator group for the present study. Taking into consideration a mean fracture torque value of 39.2 N・cm (SD=4), reported in a previous study conducted with 1.7mm miniimplants¹⁷, the present study has 90% statistical power and a 95% confidence level to detect a 15% difference between groups. The collected data were assessed by analysis of variance (ANOVA) with the Tukey multiple comparisons procedure.

RESULTS

Rupture was the characteristic mode of failure for the tested miniimplants. Fractures occurred along the free end formed by the five exposed screw threads, with the fracture angle ranging from 89° to 406.8°. On ANOVA with Tukey multiple comparisons, neither failure site nor rupture angle were significantly associated with miniimplant brand or diameter at the 5% significance level. Table 3 shows the fracture torque resistance and yield torque values of the tested miniimplants. Mean fracture strength at insertion and yield limit ranged from 15.7 to 70.4 N・cm and 9.2 to 53.1 N・cm, respectively. Minimum and peak torque curves obtained during mechanical testing are shown in Figs. 1 and 2.

Table 3: Mean (standard deviation) fracture torque resistance and yield torque of orthodontic mini-implants.

Fig. 1: Torque curve obtained for the 10 specimens with the lowest fracture resistance (Neodent® 1.3x9 mm).

Fig. 2: Torque curve obtained for the 10 specimens with the highest fracture resistance (Conexão® 2.0x8 mm).

Significant differences were observed between brands. In addition, miniimplants with larger diameter exhibited superior fracture strength and yield limits, regardless of brand. The worst performance was observed for the specimen with the narrowest diameter (NEODENTR 1.3) and the best performance for the specimen with the largest diameter (CONEXAOR 2.0). Table 4 shows the results for miniimplants stratified into three groups by diameter: small (1.3 mm/1.4 mm/1.5 mm), medium (1.6 mm/1.7 mm) or large (1.8 mm/1.9 mm/2.0 mm). Mean fracture strength for small, medium, and large specimens was 25.9 N・cm, 33.9 N・cm, and 54.2 N・cm, respectively. In the smalldiameter group, MORELLIR brand miniimplants had the best performance. In the mediumdiameter group, the SINR and NEODENTR brands stood out, whereas in the largediameter group, CONEXAOR brand miniimplants exhibited superior resistance to fracture at insertion.

Table 4: Mean (standard deviation) fracture torque resistance of orthodontic mini-implants, stratified into three groups by diameter.

FORESTADENTR brand miniimplants, despite having a larger diameter than MORELLIR brand specimens, were less resistant to fracture at insertion. Fig. 3 illustrates the relationship between fracture torque resistance and yield torque values and the different diameters of the tested miniimplants. Both variables increased with increasing implant diameter, in similar distribution patterns. The results show that the yield torque is immediately below the fracture limit.

Fig. 3: Mean torque values for fracture and yield limit and their association with miniimplant diameter.

DISCUSSION

Conventional orthodontic anchorage systems have biomechanical limitations and are dependent on patient compliance¹⁸. The ease of insertion and removal and the high success rate of miniimplants have encouraged their adoption as an efficient method for skeletal anchorage^5,19-21. However, differences in torsional strength and peak fracture torque between different commercial brands have prompted additional research, with a view to enhancing clinical safety and reducing failure rates ^22-24.
The usual length of orthodontic miniimplants ranges from 5 to 12 mm, while diameter and transmucosal profile usually range from 1.2 to 2 mm and from 0 to 3 mm, respectively^10,11,25. Studies have shown that a progressive increase in implant diameter provides improved primary stability due to increased bone contact area4,2630, but also increases the risk of damage to surrounding structures, particularly to the roots of adjacent teeth^31-33. Miniimplants with smaller diameter and length, however, have increased risk of fracture due to lower mechanical resistance. Despite its importance to periimplant health, the transmucosal profile has no influence on resistance to fracture at implant insertion ^34,35. In the present study, static torsion testing was performed in accordance with ASTM standard F543. This method ensures replicability of the study and prioritizes mechanical analysis of the specimen, regardless of substrate. Mechanical data are obtained from the specimen alone, without external interference, as the implant is isolated and secured in a clamp. Conversely, studies performed in acrylic, porcine bone, and artificial bone are subject to interference from other variables ^{32,33, 36, 37}.
In addition, a previous study evaluated titanium alloy quality and microstructure of the miniimplant brands tested herein¹⁴. According to the authors, these devices were free from internal structural defects and compliant with current standards¹⁴. In the present study, the characteristic mode of failure was miniimplant rupture. Site of failure along the exposed threads and angle of rupture were not associated with brand or diameter of the devices evaluated. Technically, fracture sites may occur randomly, as all screw threads are subject to the same strain condition and intensity. The rupture angle should preferably be high, as this would allow the practitioner to detect during insertion that the implant is undergoing elastic deformation and not driving into bone, halt the procedure, and alter the technique accordingly before fracture occurs. Mean fracture torque resistance ranged from 15.7 to 70.5 N・cm. These values are consistent with those reported in studies that employed similar methods. Lima et al.¹⁵ observed values ranging from 30 to 36 N・cm in 1.6mm NEODENTR implants, whereas Wilmes et al.¹¹, in a study of 41 commercially available brands 1.3 to 2 mm in diameter, reported values ranging from 10.9 to 64.1 N・cm.
Our results also showed that fracture strength is directly related to miniimplant diameter. Miniimplants with larger diameter exhibited higher fracture torque resistance, regardless of manufacturer. In the present study, the CONEXAOR 2.0mm miniimplants performed best overall. According to Barros et al.¹³, a 0.1mm increase in miniimplant diameter significantly reduces the risk of fracture. Toyoshima and Wakabayashi38 also observed that increasing diameter improves fracture torque resistance. Yield torque represents the time point at which alloy deformation shifts from elastic (reversible) to plastic (irreversible). Optimally, in clinical practice, dentists should always work within the elastic limit of the alloy, thus preventing permanent deformation of the device. The higher the yield limit of a device, the greater its ability to resist plastic deformation. According to the results obtained, the yield limit behaves similarly to and is immediately below the fracture limit of these devices. Therefore, manufacturers should adopt this limit as a reference value, as it represents the point at which fatigue and deformation occur; devices torqued beyond this limit may be at increased risk of fracture during removal. Further research into this mechanical parameter is warranted before this paradigm can be adopted and thus increase operator safety. Stratification of the miniimplants into groups by diameter revealed differences across the tested brands. MORELLIR and CONEXAOR brand devices exhibited the best fracture torque resistance in the smalldiameter and largediameter groups, respectively.
The use of miniimplants for orthodontic anchorage is effective and widespread in clinical practice. However, the success of this method depends largely on primary stability. Fracture torque thus plays a critical role in clinical protocols involving placement of these devices. Precise information on the peak fracture torque and yield limit of miniimplants should be made available by manufacturers. In addition, torquesensing instruments should always be coupled to miniimplant drivers in order to ensure measurement of the forces applied.

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