In vitro flexural strength evaluation of a mini-implant prototype designed for Herbst appliance anchorage

Abstracts

OBJETIVO: o propósito do presente estudo é avaliar o limite de resistência à flexão de um protótipo de mini-implante desenvolvido para ancoragem do aparelho de Herbst. MÉTODOS: após a realização de um cálculo do tamanho da amostra, quatro corpos de prova contendo os protótipos de mini-implantes foram submetidos a uma força de flexão por engastamento simples, utilizando-se uma máquina universal de ensaios mecânicos, sendo calculado o limite de resistência à força de flexão. RESULTADOS: após os ensaios mecânicos, os novos mini-implantes apresentaram o limite de resistência à força de flexão de 98,2kgf, que foi o menor valor encontrado. CONCLUSÃO: os protótipos de mini-implantes desenvolvidos para ancoragem do aparelho de Herbst foram capazes de suportar forças de flexão maiores do que as forças de mordida descritas na literatura.

Aparelhos ortodônticos; Ortodontia; Aparelho de Herbst; Mini-implante

ORIGINAL ARTICLE

^IPhD in Orthodontics, School of Dentistry, University of São Paulo

^IIAssociate Professor in Orthodontics, Department of Pedodontics and Orthodontics, School of Dentistry, University of São Paulo

^IIIProfessor of Orthodontics, Department of Pedodontics and Orthodontics, School of Dentistry, University of São Paulo

^IVProfessor, Nuclear and Energy Research Institute, IPEN - CNEN/SP

^VProfessor and Chair of Orthodontics, Department of Pedodontics and Orthodontics, School of Dentistry, University of São Paulo

Contact address

ABSTRACT

AIM: The purpose of this study was to evaluate the limit of flexural strength of a mini-implant prototype designed for Herbst appliance anchorage.

METHODS: After sample size calculation, four specimens with the new mini-implant were submitted to a single cantilever flexure test using a universal testing machine. The limit of flexural force strength was calculated.

RESULTS: The mini-implant prototype showed a limit of flexural force strength of 98.2 kgf (982 N), that was the lowest value found.

CONCLUSION: The mini-implant prototype designed for Herbst appliance anchorage can withstand flexural forces higher than the maximum human bite forces reported in the literature.

Keywords: Orthodontic appliances. Orthodontics. Herbst appliance. Mini-implant.

INTRODUCTION AND LITERATURE REVIEW

Mini-implants were conceived based on decreasing the length and diameter of existing implants, allowing new areas to be used as orthodontic anchorage according to the bone thickness.⁵ Thus, mini-implants could be used for lower incisors intrusion, when inserted in the mandibular symphysis; for horizontal traction, when inserted in the alveolar crest; for molars intrusion, when inserted between its roots; or for molar distalization, when inserted in the palate.^11,12,13

Other authors have suggested the use of implants as orthopedic anchorage in animals^7,19 and in Class III treatment due to a retrusive maxilla in humans.⁸ On the other hand, in Class II treatments, the Herbst appliance¹⁸ has been used frequently because of its efficiency³ and also because of its positive effects in orthodontic and orthopedic correction.^10,14,15,16 However, some investigators have stated that there is certain lack of control in orthodontic movement (specially exaggerated lower incisor proclination), which could be associated with and increase in mandibular incisor crowding and gingival recession.^2,6

In view of these problems and with the intention of solving them, a mini-implant was created for Herbst appliance anchorage. However, the first doubt was if this new mini-implant would be capable of resisting maximum bite forces (mean of approximately 756 N or 75.6 kgf).¹

So, the purpose of this study was to evaluate, in vitro, the limit of flexural forces strength of a mini-implant prototype designed for Herbst appliance anchorage.

MATERIALS AND METHODS

Four specimens were used in this experiment and each one had three parts: the mini-implant prototype, a brass block which acted as a support for the setting, and a straight shaft from the telescopic tube (Dentaurum) of the Herbst appliance (Figs 1 and 2).

The brass blocks were machined to a rectangular section of 18 mm in width and 20 mm in length. A perforation was performed at the center of the squared section of the block in its long axis with a 1.9 mm diameter drill. A beveller was used on the perforation edge to permit the full adaptation of the conic profile of the mini-implant when it was inserted. For insertion torque standardization of the mini-implant in the brass block, a calibrated torquimeter was used, not exceeding 30 cN.m. The mini-implant prototype was then carefully inserted with the torque key attached to the torquimeter, so that only the mini-implant head was exposed with the screw with 4.0 mm in length. Then, the shaft of the appliance was attached to the mini-implant prototype with a screw.

The mechanical test

The mechanical tests were performed in the IPEN (Nuclear and Energetic Research Institute) in the CCTM (Materials Technologic Research Center) located at USP (University of São Paulo). To perform the mechanical tests, the specimens were placed in the Instron 4400 R test machine, with the brass block attached to the lower clasp and the telescopic tube to the upper clasp, which were connected to a load cell of 100,000 N (Fig 3).

A single cantilever flexure test was performed, in which the flexure force application occurs with a distance from the brass block, generating a momentum. Flexural traction was applied at 0.5 mm per minute until the maximum strength was reached. The values were recorded, generating a strength x dislocation graph using a specific software provided by the equipment manufacturer (Fig 4).

Statistical analysis

Initially, a sample size calculation was carried out using a pilot study with three mechanical tests. The mean maximum resistance strength and standard deviation of the mini-implant prototype was calculated for each specimen using the software Instron Series IX (Table 1). Then, the following statistical²⁰ formula was used:

where n = number of specimens, Z = number of standard deviations from normal distribution, s = standard deviation and D = difference between a variable value and the average of values from this variable. As Z = 1.96 (95% of confidence interval), s = 8 and D = 8 (998 - 990). Calculating, n = 3.84 ≅ 4. The number of specimens needed for the study was 4.

RESULTS

After the maximum flexure resistance tests performed on the specimens, a mean of 989 N was found, with a standard deviation of 6, a minimum value of 982 N and a maximum value of 998 N (Table 2).

Figure 4 shows the performance of the four specimens tested. It can be observed that there was a constant dislocation until the peak was reached. At this moment, the mini-implant started to be pulled out of the brass block or the telescopic tube had fractured.

DISCUSSION

Figure 5 shows a specimen in lateral view after the test, depicting the bending of the mini-implant and the telescopic tube of the Herbst appliance. The maximum force criteria used in this study is based on the indication that when plastic deformation occurs on the mini-implant or on the telescopic tube they are incapable of maintaining the loads, resulting in lower force levels. None of the tested mini-implants fractured, but all of them suffered permanent deformation.

Some modifications were done in the mini-implant prototype in relation to the mini-implants mostly used nowadays, which measure approximately 1.6 mm in diameter and 6 mm in length. These changes had the purpose of increasing the contact of the mini-implant prototype to the bone and, consequently, its resistance. So, the diameter was increased to 2.0 mm which is compatible with the mesio-distal distance between the roots of the upper first molars and second premolars and between the lower canines and first premolars (suggested location for the insertion of the mini-implant prototypes for Herbst appliance anchorage), which is 3-8 mm of distance from the alveolar crest.¹⁷

Although Miyawaki et al¹² did not find a significant association between the success rate and the mini-implant length, Brettin et al⁴ concluded that bicortical mini-implants provide superior anchorage resistance, reduced cortical bone stress, and superior stability when compared with monocortical mini-implants. Therefore, in order to achieve bicortical anchorage, the length was increased to 10 mm.

In relation to the sample, it could be questioned the few specimens used in this study that is not common in Orthodontics. However, this number is common in other areas, such as Engineering and Physics, because of the smaller difference among the studied variables. So, a pilot study with three specimens allowed a sample size calculation.²⁰ This calculation showed that the number of specimens needed was four. In this case just one other mechanical essay was necessary.

Another issue to be discussed regarding the sample could be on its selection. Due to the fact that the mini-implant prototype was machined by a company specifically to be used as anchorage for the Herbst appliance and it is not commercially available, no random sample selection was performed, by selecting different mini-implants from different manufacturing batches, as is recommended for in vitro studies. Only one batch existed and this could explain why maximum force values found is this study were so similar, since different batches could present differences in titanium alloy composition or in dimensions, which could lead to slighter different results.

Another question that could arise would be the use of a brass block, rather than bone, to attach the mini-implant, which would be the material in which the mini-implants would regularly be inserted. It should be emphasized that the mechanical tests performed were technological essays, where the results are valid only for the specimen tested. Therefore, this study aimed only to test the mini-implant prototype resistance. It was necessary to insert the mini-implant into a rigid material, so as to evaluate only the performance of the mini-implant prototype. The brass was the better metal to be used, because the aluminium was not sufficiently resistant (an essay with a aluminium block showed 750 N of maximal mini-implant resistance and the hole of the block deformed). The steel was considered too resistant (three mini-implants fractured when inserted to steel blocks).

In relation to the mechanical essay performed, it was necessary to submit the mini-implant to a force perpendicular to its long axis, simulating the force received when it is used as Herbst appliance anchorage. Because there was a distance of 4 mm between the block base and the point of force application, the shear force test was not indicated because in this test this distance should not exist. The traction test also was not indicated because in this test the force is loaded parallel to the mini-implant long axis, not reproducing the force perpendicular to the long axis applied when the mini-implant is used as anchorage to the Herbst appliance. Therefore the single cantilever flexure test was the one which best simulated the usage conditions of the mini-implant, taking into consideration the momentum created by the distance between the point of force application and the brass block.

The mean maximum resistance strength of 989 N found in the present study and even the minimum strength of 982 N, that would be the resistance limit, were higher than the highest value found in the literature for human bite maximum force mean of 756 N¹. It suggests that the mini-implant prototype could resist the maximum bite forces when used as a Herbst appliance anchorage. However, questions about mini-implant failure as Herbst anchorage are related to bone/mini-implant interface suggesting that, before clinical studies, other in vitro tests should be performed to evaluate the resistance of the mini-implant prototype when it is inserted in bone.

CONCLUSION

The mini-implants prototypes designed for Herbst appliance anchorage are capable to withstand single cantilever flexure forces of 982 N when fixed in a brass block and can withstand higher forces than the maximum human bite forces found in the literature.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Dr. Antonio Muench from the Department of Dental Materials of University of São Paulo and Rogério Ruscitto for the statistical analysis orientation and Neodent Inc. (Curitiba, Brazil), specially Dr. Ana Cláudia and Alexsander Luiz Golin for the confection of the mini-implant prototype described in this manuscript.

REFERENCES

• 1. Anusavice KJP. Materiais dentários. 11Ş ed. Rio de Janeiro: Elsevier; 2005. p. 66.
• 2. Artun J, Krogstad O. Periodontal status of mandibular incisors following excessive proclination. A study in adults with surgically treated mandibular prognathism. Am J Orthod Dentofacial Orthop. 1987 Mar;91(3):225-32.
• 3. Bremen JV, Pancherz H. Efficiency of Class II division 1 and Class II division 2 treatment in relation to different treatment approaches. Semin Orthod. 2003;9(1):87-92.
• 4. Brettin BT, Grosland NM, Qian F, Southard KA, Stuntz TD, Morgan TA, et al. Bicortical vs monocortical orthodontic skeletal anchorage. Am J Orthod Dentofacial Orthop. 2008 Nov;134(5):625-35.
• 5. Costa A, Raffainl M, Melsen B. Miniscrews as orthodontic anchorage: A preliminary report. Int J Adult Orthodon Orthognath Surg. 1998;13(3):201-9.
• 6. De Loor P, De Smit AA, Adriaens PA. Periodontal changes induced by orthodontically changed incisor inclination. Eur J Orthod. 1995;17:57.
• 7. De Pauw GA, Dermaut L, De Bruyn H, Johansson C. Stability of implants as anchorage for orthopedic traction. Angle Orthod. 1999 Oct;69(5):401-7.
• 8. Enacar A, Giray B, Pehlivanoglu M, Iplikcioglu H. Facemask therapy with rigid anchorage in a patient with maxillary hypoplasia and severe oligodontia. Am J Orthod Dentofacial Orthop. 2003 May;123(5):571-7.
• 9. Graber TM, Rakosi T, Petrovic AG. Dentofacial orthopedics with functional appliances. 2^nd ed. St. Louis: C.V. Mosby; 1997. p. 336-66.
• 10. Hansen K. Treatment and posttreatment effects of the Herbst appliance on the dental arches and arch relationships. Semin Orthod. 2003 Mar; 9(1):67-73.
• 11. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod. 1997;31(11):763-7.
• 12. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, Takano-Yamamoto T. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dentofacial Orthop. 2003 Oct;124(4):373-8.
• 13. Ohmae M, Saito S, Morohashi T, Seki K, Qu H, Kanomi R, et al. A clinical and histological evaluation of titanium mini-implants as anchors for orthodontic intrusion in Beagle dog. Am J Orthod Dentofacial Orthop. 2001 May;119(5):489-97.
• 14. Pancherz H. Treatment of Class II malocclusion by jumping the bite with the Herbst appliance: a cephalometric investigation. Am J Orthod. 1979 Oct;76(4):423-42.
• 15. Pancherz H, Hansen K. Occlusal changes during and after Herbst treatment: a cephalometric study. Eur J Orthod. 1986 8(4):215-28.
• 16. Pancherz H, Hansen K. Mandibular anchorage in Herbst treatment. Eur J Orthod. 1988 10(1):149-64.
• 17. Poggio PM, Incorvati C, Velo S, Carano A. "Safe zones": a guide for miniscrew positioning in the maxillary and mandibular arch. Angle Orthod. 2006 Mar;76(2):191-7.
• 18. Proffit WR. Ortodontia contemporânea. 3Ş ed. Rio de Janeiro: Guanabara Koogan; 2002.
• 19. Smalley WM, Shapiro PA, Hohl TH, Kokich VG, Brånemark PI. Osseointegrated titanium implants for maxillofacial protraction in monkeys. Am J Orthod Dentofacial Orthop. 1988 Oct;94(4):285-95.
• 20. Weyne GR. Determinação do tamanho da amostra em pesquisas experimentais na área de saúde. Arq Med ABC. 2004 jul-dez;29(2):87-90.

In vitro flexural strength evaluation of a mini-implant prototype designed for Herbst appliance anchorage

Klaus Barretto-Lopes^I; Gladys Cristina Dominguez^II; André Tortamano^III; Jesualdo Luiz Rossi^IV; Julio Wilson Vigorito^V

Publication Dates

• Publication in this collection
  18 Aug 2010
• Date of issue
  Aug 2010

History

• Received
  Oct 2008
• Accepted
  June 2009