This study evaluated the influence of bone cortical thickness on the maximum torque required for insertion and removal of orthodontic mini-implants of different shapes. Five different types of orthodontic mini-implants were examined Cylindrical 1 (CYM), Cylindrical 2 (CYI) and Cylindrical 3 (CYT) Conical 1 (CON), Conical 2 (COS). Insertion and removal torque tests were performed in mini-pig medullary bone (8 mm thick) and cortical bone 1, 2, 3 and 6 mm thick. A digital torque meter measured the torque forces; the maximum values of insertion and removal were obtained (N/cm). There were no statistically significant differences between the different implants in the torque forces required for insertion and removal from medullary bone ( P > 0.05). During insertion into 1–2 mm cortical bone, COS, CON and CYT had torque values statistically higher, but CON had higher torque values compared with the others when 3–6 mm cortical bone was used ( P < 0.05). The removal torque values were significantly lower for CYM and CYI. Conical type mini-implants require a greater torque force for insertion and removal compared with cylindrical types. Torque values were directly related to cortical thickness.
Anchorage control is important for successful orthodontic treatment . Titanium mini-implants are increasingly being used to provide anchorage . They might be the method of choice for providing anchorage in patients who are difficult to manage using conventional orthodontic approaches, such as those with severe tooth loss, deficient osseous support, or who refuse to wear extra-oral appliances . Correct insertion of mini-implants into bone is essential for stable anchorage. The initial stability of a mini-implant is important because most failures occur at the early stages following insertion . Initial instability might lead to gingivitis during the peri-implantation period, which could compromise the interface between bone and mini-implant . The most common causes of instability or loss of primary stability are reduced mini-implant shape and diameter incompatible with the available bone thickness, decreased bone density and cortical thickness, and faulty insertion into the bone marrow . The most common methods for assessing implant stability at the osseous surface are measuring the torque or torsional forces required for insertion and removal of the mini-implants, and the pull-out test . Earlier studies suggested that bone density and the design of screws are important factors influencing mini-screw stability . The aim of the present study was to measure the maximum torque force required for insertion and removal of orthodontic mini-implants into cortical mini-pig bone. The authors examined the relationship between the force required and two factors: the shape of the mini-implant (conical or cylindrical); and the thickness of the cortical bone.
Materials and methods
250 titanium mini-implants from five different manufacturers were used in this study ( Fig. 1 ). The details of the mini-implants and the abbreviations given to each group are given in Table 1 . A profile projector (Nikon, Tokyo, Japan) was used to verify the shape and dimensions of the implants prior to the study. The 50 mini-implants from each manufacturer were subdivided into five subgroups ( n = 10) that were inserted into mini-pig cortical bones 0, 1, 2, 3 and 6 mm thick. They were categorized as Groups 0–6, according to thickness.
|Screw design||Cylindrical (CY)||Conic (CO)||Conic (CO)||Cylindrical (CY)||Cylindrical (CY)|
|Manufacturer||Mondeal Tuttlingen, Germany||Neodent, Curitiba, Brazil||SIN, São Paulo, Brazil||INP, São Paulo, Brazil||Titanium Fix, São José dos Campos, Brazil|
|Length||10 mm||12 mm||10 mm||10 mm||10 mm|
|Body length||7 mm||7 mm||7 mm||8 mm||5 mm|
|Thread length||6 mm||7 mm||6 mm||6 mm||4.5 mm|
|Thread diameter||1.5 mm||1.6 (1.4) * mm||1.6 (1.4) * mm||1.5 mm||1.5 mm|
The osseous tissue used in the study was obtained from the ribs of the mini-pig ( Sus scrofa piau ), the main characteristic of which is the presence of cortical bones of different diameters in the same animal. After being killed, the animal was dissected and the ribs removed. Each rib was sectioned into blocks of approximately 50 mm long, 30 mm wide and 30 mm high. Each osseous block had a different cortical thickness of 0, 1, 2, 3 or 6 mm. These measurements were checked using digital callipers (Starret, São Paulo, Brazil).
The prepared osseous samples were attached using a custom-made device designed to secure the sample and stabilize the digital torque meter during insertion of the mini-implants ( Fig. 2 ), which was undertaken using a wrench mounted onto the digital torque calliper (Lutron TQ-8800, Taipei, Taiwan). Mini-implants were placed manually by rotating the digital torque meter clockwise until full insertion was achieved. The value for the maximum torque achieved was recorded. The value for maximum counter-torque was recorded as the digital torque meter was rotated anti-clockwise. Based on the insertion and removal torque values, it was possible to obtain the value of torque loss: the difference between the maximum insertion and maximum removal torques. The self-drilling mini-implants were inserted directly without drilling a pilot hole. For the self-tapping mini-implants (CYT group) a hole was provided using a 1.1 mm diameter pilot drill prior to placement.
The differences between the torque values for each mini-implant group and for the different cortical bone thicknesses were examined using a two-way analysis of variance (ANOVA) using the SPSS software (version 13.0; SPSS Inc., Chicago, IL, USA) program. A ranking order was established using Tukey’s test.
|Groups||Type||0 mm||1 mm||2 mm||3 mm||6 mm||Tukey’s*|
|CYM||CY||1.86 SD 0.28||4.28 SD 0.3||5.34 SD 0.51||5.7 SD 0.69||13.14 SD 0.72||0 < 1 < 2 = 3 < 6 mm|
|CON||CO||2.6 SD 0.25||7.82 SD 0.55||14.28 SD 1.59||23.78 SD 2.04||31.04 SD 1.46||0 < 1 < 2 < 3 < 6 mm|
|COS||CO||2.48 SD 0.39||7.76 SD 0.78||10.72 SD 1.13||20.58 SD 1.35||23.3 SD 1.94||0 < 1 < 2 < 3 < 6 mm|
|CYI||CY||2.32 SD 0.62||5.36 SD 1.13||5.88 SD 0.65||9.88 SD 1.09||10.58 SD 0.85||0 < 1 = 2 < 3 = 6 mm|
|CYT||CY||1.88 SD 0.08||7.36 SD 0.48||10.38 SD 0.43||18.1 SD 1.25||21.54 SD 2.27||0 < 1 < 2 < 3 < 6 mm|
|Tukey’s *||CYM = CON = COS = CYI = CYT||CYM = CYI < COS = CON = CYT||CYM = CYI < CYT = COS < CON||CYM < CYI < CYT = COS < CON||CYI = CYM < CYT = COS < CON|
|Groups||Type||0 mm||1 mm||2 mm||3 mm||6 mm||Tukey’s *|
|CYM||CY||1.0 SD 0.07||2.08 SD 0.19||2.8 SD 0.21||3.76 SD 0.23||8.34 SD 1||0 < 1 = 2 < 3 < 6 mm|
|CON||CO||1.3 SD 0.28||5.18 SD 0.27||10.2 SD 0.88||13.54 SD 1.31||18.96 SD 0.94||0 < 1 < 2 < 3 < 6 mm|
|COS||CO||1.2||4.84 SD 0.28||6.34 SD 1.18||13.36 SD 1.34||18.2 SD 1.42||0 < 1 = 2 < 3 < 6 mm|
|CYI||CY||1.28 SD 0.16||2.58 SD 0.25||3.18 SD 0.46||4.06 SD 0.7||6.6 SD 0.39||0 < 1 = 2 < 3 < 6 mm|
|CYT||CY||1.3 SD 0.35||3.2 SD 0.25||5.42 SD 0.53||10.42 SD 1.47||13.9 SD 1.91||0 < 1 < 2 < 3 < 6 mm|
|Tukey’s *||CYM = CON = COS = CYI = CYT||CYM = CYI < CYT < COS = CON||CYM = CYI < CYT = COS < CON||CYM = CYI < CYT < COS = CON||CYM = CYI < CYT < COS = CON|
|Groups||Type||0 mm||1 mm||2 mm||3 mm||6 mm||Tukey’s *|
|CYM||CY||0.92 SD 0.28||2.2 SD 0.40||2.54 SD 0.68||1.94 SD 0.71||4.8 SD 0.35||0 < 1 = 2 = 3 < 6 mm|
|CON||CO||1.3 SD 0.46||2.64 SD 0.69||4.08 SD 1.90||10.3 SD 2.88||12.08 SD 1.14||0 = 1 = 2 < 3 = 6 mm|
|COS||CO||1.28 SD 0.39||2.88 SD 0.55||4.38 SD 1.08||7.22 SD 2.38||5.1 SD 3.05||6 = 3 > 0 = 1 = 2 mm|
|CYI||CY||1.04 SD 0.72||2.78 SD 1.20||2.7 SD 0.96||5.82 SD 0.97||3.98 SD 0.84||0 = 1 = 2 < 3 = 6 mm|
|CYT||CY||0.98 SD 0.20||4.16 SD 0.53||4.96 SD 0.89||7.68 SD 1.57||7.64 SD 1.31||0 < 1 = 2 < 3 = 6 mm|
|Tukey’s *||CYM = CON = COS = CYI = CYT||CYM = CON = COS = CYI < CYT||CYM = CON = COS = CYI < CYT||CYM < CON = COS = CYI = CYT||CON > CYM = COS = CYI < CYT|