Primary stability of orthodontic mini-implants inserted ...

Vol. xx No. x Month 2012

ORIGINAL ARTICLE

Primary stability of orthodontic mini-implants inserted intomaxilla and mandible of swineMatheus Melo Pithon,a Matilde Gonçalves Nojima,b and Lincoln Issamu Nojima,b Bahia and Rio de Janeiro,BrazilSOUTHWEST BAHIA UNIVERSITY AND FEDERAL UNIVERSITY OF RIO DE JANEIRO

Objective. The objective of this study was to assess the primary stability of different orthodontic mini-implants inserted intodifferent maxillary and mandibular regions of swine.Material and methods. One hundred eighty orthodontic mini-implants produced by 5 different manufacturers, all presentingseveral shapes, were divided into 5 groups: Mondeal (M), Neodent (N), SIN (S), INP, and Titanium Fix (T). Fifteen pigs (Susscrofa piau) were used for study and 12 mini-implants were inserted into 3 mandibular and maxillary regions. After insertion,the animals were killed and osseous blocks containing the mini-implants were obtained for mechanical pullout tests to beperformed by a universal test machine at cross-head speed of 0.5 mm/s. Maximum force values (N/cm) for insertion wererecorded and submitted to both analysis of variance and Tukey’s test.Results. The primary stability provided by cylindrical mini-implants (groups M and I) was statistically significantly superior tothat of conical mini-implants (groups N and S). On the other hand, screw-type mini-implants were shown to be statisticallyinferior compared with the others (P � .05). Statistical differences between pullout forces at different oral cavity regions werealso found (P � .05). The mini-implants inserted into palatal suture had lesser stability, whereas those inserted into uppermolar and premolar regions were shown to be more stable.Conclusions. The shape of mini-implants, in association with location of insertion, is directly related to primary stability. (Oral

The use of mini-implants for orthodontic anchorage iswell established in the literature.1-3 This approach hasrevolutionized orthodontics over the years, thus allow-ing stable anchorage to be reliably achieved.4,5 How-ever, to promote adequate anchorage opposing the re-action forces from the orthodontic movement, it isnecessary to insert the mini-implants into bone tissue sothat stability can be achieved.6

Didactically, mini-implant stability can be either pri-mary or secondary. The former involves direct contactbetween mini-implant and bone, whereas the latter (orlate) occurs following a cicatrization.7 Primary stabilityis an important sign of a successful insertion, as mostcases of failure involving mini-implants occur duringthe initial stages following such a procedure.8,9 Thelack of stability may be related to osseous factors10 and

The study was self-supported, but SIN, Neodent, Mondeal, INP, andTitanium Fix provided the materials used in the study.aAuxiliary Professor, Southwest Bahia University, UESB, Bahia,Brazil and Doctor of Orthodontics, the School of Dentistry, FederalUniversity of Rio de Janeiro, UFRJ, Brazil.bAdjunct Professor of Orthodontics, the Faculty of Dentistry, FederalUniversity of Rio de Janeiro, UFRJ, Brazil.Received for publication May 18, 2011; returned for revision Jun 16,2011; accepted for publication Jun 24, 2011.© 2012 Elsevier Inc. All rights reserved.2212-4403/$ - see front matter

mini-implant characteristics, including diameter, screw,11

and dimension of pilot perforation.12-14

When the stability of a given mini-implant has to bemechanically evaluated, one of the increasingly usedmethodologies is the pullout test,15,16 which is widelyused in medical areas, such as orthopedics,17 neurosur-gery,18,19 and cosmetic and maxillofacial surgery,20,21

for testing the primary stability of several screw de-vices. The pullout test consists of extracting the mini-implant from osseous tissue at constant speed, thusenabling the assessment of maximum force needed toremove the implanted device.

Because there are few studies that have assessedmini-implant stability in vivo, the objective of the pres-ent work was to evaluate the biomechanical behavior oforthodontic mini-implants of different shapes and di-ameters inserted in different regions of swine maxillaand mandible. The pullout test was used to relate theshape of mini-implants to their stability at differentregions of the oral cavity.

MATERIAL AND METHODSA total of 180 mini-implants from 5 different manufac-turers (M [Mondeal, Tuttlingen, Germany], N [Neo-dent, Curitiba, Brazil], S [SIN, São Paulo, Brazil], I[INP, São Paulo, Brazil], and T [Titanium Fix, São Josedos Campos, Brazil]), all presenting their own charac-

1

drical;

ORAL AND MAXILLOFACIAL SURGERY OOOO2 Pithon et al. Month 2012

group) as listed in Table I and Figure 1, A and B. Beforeinsertion, the mini-implants were characterized andmeasured by using a profile projector model 6 (Nikon,Tokyo, Japan).

The mini-implants were implanted into the maxilla andmandible of 15 adult male pigs (Sus scrofa piau), eachweighting on average 75 kg at 18 months of age. The animalswere healthy and were fed water and ration ad libitum.

The regions selected for insertion of the mini-im-

Figure 1. A, Microphotograph of mini-implants after me-chanical assay (magnification �15). B, Mini-implants evalu-ated. Images are available in color at www.ooooe.net.

Table I. Main characteristics and dimensions of mini-M N

Type Self-drillingScrew design CI COCorporation Mondeal NeodLength, mm 10 10Body length, mm 7 7Screw length, mm 6 7Screw diameter, mm 1.5 1Alloy

M, Mondeal; N, Neodent; S, SIN; I, INP; T, Titanium Fix; CI, cylin

plants were situated between the lateral incisors and

canines, second premolar and first molar (upper andlower ones), and in the median palatal suture and man-dibular retro-molar area (Figure 2).

Before insertion, the animals were deeply anesthe-tized. First, 0.1 mg/kg of acepromazine maleate (Ace-pran 1%, São Paulo, Brazil) and 2 mg/kg of merepidine(Dolosal, São Paulo, Brazil) were administered intra-muscularly as preanesthetic medications. After 15 min-utes, vascular access (cephalic vein) was achievedand 20 mg/kg of sodium ampicillin was immediatelyadministered. Anesthesia was induced with intrave-nous propofol (2 to 4 mg/kg) and maintained duringorotracheal intubation using 100% oxygen throughan inhalation anesthetic system. The insertion proce-dures were performed with the animals under deepanesthesia.

Four groups of mini-implants (M, N, S, and I) wereself-drilling screws, thus not requiring previous corticalperforation; however, mini-implants from group T re-quired pilot perforation of 1.1 mm in diameter beforeinsertion.

The insertion of mini-implants was performed byusing a surgical leader made of thermoactive resinmaterial and synthetic rubber (CTRBS, Tramontina,Canoas, Brazil), aimed to mold the occlusal surfaces,and a rectangular metal wire of 0.19 � 0.024 inch(Moreli, Sococaba, Brazil), serving to locate and markthe optimal local insertion. Periapical radiographs weretaken according to the surgical leader so that the centralradius was in parallel to the interproximal area. Afterlocalization, the insertion area was cleaned with a so-lution of 0.12% chlorhexidine digluconate. After local-izing and disinfecting the area, a small gingival incisionwas made to facilitate the insertion procedure, as pigshave thicker gingiva.

The mini-implants were inserted by using a torquescrewdriver mounted on a digital calliper, thus allowingboth insertion and torque measurement to be perpen-dicularly performed in relation to the osseous surface.

Next, all 15 animals were killed under deep generalanesthesia followed by administration of potassiumchloride until cardiorespiratory failure was achieved.

nts used in the present studyS I T

ScrewlikeCO CI CISIN INP Titanium Fix

10 10 107 8 56 6 4.51.6 1.5 1.5

Ti-6AI-4V

CO, conic.

impla

ent

.6

The animals’ maxillas and mandibles were dissected so

OOOO ORIGINAL ARTICLEVolume xx, Number x Pithon et al. 3

that bone blocks containing the mini-implant could beobtained. The samples were immersed in saline solu-tion and then stored at –15°C for 15 days. After thisperiod of time, the bone blocks were left at roomtemperature for mechanical assay.

The test was performed using a universal test ma-chine (Emic DL 10.000, São José dos Pinhais, Brazil).

Figure 2. A–C, Regions selected for insertion of the mini-implants. Images are available in color at www.ooooe.net.

A claw-shaped device was confected and mounted on

the upper part of the machine so that the mini-implantcould be removed. Another device served as a base forboth fixing the bone block and keeping the mini-im-plant in a perpendicular position during the tests, thusavoiding momentum creation (Figure 3). Mechanicalassay was performed at a crosshead speed of 0.5 mm/sfor removing the mini-implant from osseous bone.Load and displacement values were recorded, as well asthe maximum force (Fmax) for posterior evaluation.

Following mechanical assays, the bone blocks werestained with eosin so as to differentiate the corticalmedullar bone and then covered with acrylic resin(Clássico, São Paulo, Brazil). The blocks were sec-tioned by using a diamond disk and the cortical bonethickness was measured with a digital calliper (Starlet,São Paulo, Brazil) and �16 magnifying stereoscopicglass (Carl Zeiss, Göttingen, Germany). In this way, themean cortical values for the 6 regions containing mini-implants could be obtained in vivo, thus allowing acorrelation between pullout test and cortical thicknessvalues.

Experimental data from pullout tests were statisti-cally analyzed using SPSS software v13.0 (SPSS Inc.,Chicago, IL). Maximum force values (in N/cm2) weresubmitted to analysis of variance and Tukey’s test,wherein the former was applied to determine any sta-tistical difference among the groups. The results werefound to be statistically significant at P less than .05.

RESULTSThe highest pullout values were achieved in group M,involving almost all combinations except for the lowerincisor region, whereas group I had the highest meanvalues (P � .835). The lowest values, on the otherhand, were obtained in group T in all regions evaluated(Tables II and III).

With regard to those regions containing mini-im-plants, the highest pullout values were observed in theupper molar and lower molar, in the median palatalsuture, which had the lowest values (Tables II and III).

The mean values of cortical thickness were 1.58 �0.14 (between incisors and upper canines), 2.56 � 0.08(premolars and upper molars), 0.92 � 0.13 (medianpalatal suture), 1.4 � 0.1 (lower incisor and canine),2.44 � 0.167 (lower premolars and molars), and 2.24 �0.08 (lower retro-molar) (Figure 4).

DISCUSSIONThe objective of the present study was to evaluate thebiomechanical behavior of orthodontic mini-implantsof different shapes inserted in various regions of max-illa and mandible for the pullout test. Such information

in colo

statistic

ORAL AND MAXILLOFACIAL SURGERY OOOO4 Pithon et al. Month 2012

the choice of specific mini-implant based on the regionfor insertion.

The pullout test consists of extracting the mini-im-plant from osseous tissue perpendicularly and at con-stant speed. This method, which is extensively used inseveral areas of medicine,17-21 has been increasinglyused in orthodontics since an article by Huja et al.16

was published.Despite the nontraction force being applied to mini-

implants, the values obtained during mechanical assayshow “imbrication” between the screw-part of mini-

Table III. Mean pullout force of mini-implants inserteGroups Incisor/Canine Tukey* Pr

M 169.64 � 14.62 AC 271.0N 159.56 � 21.32 AC 238.4S 148.01 � 21.83 AC 221.09I 182.57 � 17.00 A 252.8T 136.54 � 22.44 C 204.8

M, Mondeal; N, Neodent; S, SIN; I, INP; T, Titanium Fix.

Figure 3. A, Support device for bone block and mini-implmini-implant, and z � support device. Images are available

Table II. Mean pullout force of mini-implants insertedGroups Incisor/Canine Tukey* Pr

M 249.54 � 9.56 A 285.0N 172.94 � 8.07 B 253.7S 161.26 � 10.1 B 227.7I 178.79 � 15.7 B 264.8T 86.91 � 20.03 C 180.9

M, Mondeal; N, Neodent; S, SIN; I, INP; T, Titanium Fix.*Statistical analysis in which equal letters correspond to absence of

*Statistical analysis in which equal letters correspond to absence of statistic

implants and the osseous tissue in which they are in-serted.14

Piau pigs (Sus scropha) were the experimental ani-mal model used in the present study. This choice wasbased on the similarity between humans and swine interms of maxillary bone, a fact that is well known andwell established in the literature.22,23

The present study assessed the force exerted on mini-implants inserted into different osseous regions of theoral cavity of swine. The animals were killed after themini-implant insertion and consequently no healing

ifferent mandibular regionsrs Tukey* Retromolar Tukey*

.91 A 250.91 � 12.48 A

.54 B 247.30 � 12.25 A

.77 BC 221.8 � 10.21 B

.2 AB 250.15 � 8.57 A

.00 C 180.76 � 19.90 C

, Mechanical assay; y � load cell, x � device for mountr at www.ooooe.net.

ifferent maxillary regionsrs Tukey* Suture Tukey*

.65 A 180.40 � 5.09 A

.40 BC 95.144 � 2.93 C

.34 B 86.47 � 2.63 C

.42 AC 114.63 � 17.66 B

.73 D 79.62 � 5.78 C

al differences (P � .05).

d in de/Mola

6 � 107 � 106 � 167 � 192 � 22

ant. B

in de/Mola

3 � 144 � 138 � 148 � 134 � 15

al differences (P � .05).

taining

OOOO ORIGINAL ARTICLEVolume xx, Number x Pithon et al. 5

process could change the results. Therefore, the initialstability between different mini-implants in differentareas of oral cavity was reliably achieved.

Groups S and N showed mean values smaller thangroups M and I for all regions evaluated, although nostatistical differences were observed. The shape ofmini-implants had enormous influence on such differ-ences. Conical-shaped mini-implants, for instance, hadsmaller values because of the displacement toward abroader region, thus resulting in a lower resistance. Onthe other hand, mini-implants from groups M and I hadthe highest pullout values for all insertion regions, afinding certainly related to their format. The cylindricalshape enabled the same bone area to be in contact withthe screw part of the mini-implant during the mechan-ical test; that is, there was greater osseous contact.

The diameter of mini-implants did not have a deci-sive influence on primary stability, as groups M and Ihad mean values greater than those of groups N and S,although the former had a small diameter (1.5 mm)compared with the latter (1.6 mm).

Group T was included to evaluate the behavior ofself-tapping in comparison with self-drilling. Com-pared with other groups, one can observe that primarystability was inferior in all regions containing thesemini-implants. Because self-tapping mini-implants donot have self-drilling characteristics (e.g., small threadwidth), the osseous contact is rather unstable. Anotherfactor possibly contributing to these differences wouldbe pilot perforation. These results are in accordance

Figure 4. Mean values of cortical thickness in the regions con

with findings by Yano et al.,24 who assessed histolog-

ically the osseous contact with both self-drilling andself-tapping mini-implants. According to the authors,there should be a healing period before using self-tapping mini-implants for anchorage.24

The mean pullout force values between the mini-implants inserted in all the regions of the oral cavityranged from 79.62 to 295.03 N. The lowest meanvalues were observed in group T, which had mini-implants inserted in the region of median palatal suture,whereas the highest mean values were observed ingroup M, which had mini-implants inserted betweenthe upper molar and premolars. Other authors, who alsoevaluated in vivo the primary stability by using thepullout test,16 corroborate the values found in the pres-ent work.

As the pullout test was initially performed in vivoand then in vitro, a concern exists regarding storage ofthe samples and time elapsed between when the pigswere killed and the mechanical test. Earlier studies onpullout force demonstrated force variation over time;that is, between insertion and pullout assay. Roe etal.,25 who tested 1-week samples stored at –20°C, re-ported lack of statistical differences when the test wascarried out immediately after the animals were killed.Other works report a decrease in pullout force as stor-age time was extended from 4 to 8 weeks. In the presentwork, the samples were dissected immediately after theanimals were killed, and then stored in saline solutionfor 15 days at –15°C. The procedures followed were in

mini-implants. Image is available in color at www.ooooe.net.

accordance with other studies on orthodontic mini-

ORAL AND MAXILLOFACIAL SURGERY OOOO6 Pithon et al. Month 2012

implants.15,16 On the 15th day, the samples were left atroom temperature to gradually unfreeze.

To fix the bone fragment during the mechanical test,a metallic device was confected that was mounted on auniversal test machine. The inferior part of the devicewas made to keep the mini-implant perpendicularlypositioned on the base without having to apply resin tothe osseous block, as suggested elsewhere.15,16 Thisdecision was made because of the reduced size of thesample and the possibility of resin penetration into theosseous tissue, which might mask the results.

Studies evaluating screw pullout performed immedi-ately following insertion and 8 weeks later found nostatistical differences.26 In another study on titaniumimplants inserted in facial bones, only a modest gain inthe pullout strength was measured after comparing thehealing periods of 1, 2, 3, 4, 6, 9, 12, and 32 weeks.Based on these works, the secondary stability requiredduring the treatment is directly related to the primarystability achieved during insertion of mini-implants.However, further studies are needed to evaluate theprimary and secondary stability of mini-implants with-out force application.

In general, the optimal forces required for orthodon-tic movement range from 0.3 to 4.0 N. The pulloutforces found in the present study were significantlygreater than those for clinical purposes, which validatesthe oral cavity regions studied as well as the differenttypes of mini-implants.

Because cortical thickness is recognizably importantin terms of primary stability of mini-implants, it wasnecessary to measure it so as to qualify and quantify theregions chosen for insertion. The median palatal suturehad the least thickness, with mean value of 2.56 � 0.08.These results are directly related to the primary stabilityachieved by using different types of mini-implants andare also supported in the literature.16,27

CONCLUSIONS

X Cylindrical mini-implants were found to havegreater primary stability compared with the conicalmini-implants.X The primary stability of mini-implants is directly

related to their shape and the regions of the oralcavity in which they will be inserted.

X The thicker the alveolar cortical bone, the greaterthe stability of mini-implants.

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Reprint requests:

Matheus Melo PithonSouthwest Bahia University, UESBAv. Otávio Santos, 395, Sala 705Centro Odontomédico Dr. Altamirando da Costa Lima.Vitória da Conquista, Bahia, Brazil