Biomechanical and histological comparison of self-drilling and self ...

ORIGINAL ARTICLE 

Biomechanical and histological comparison of 

self-drilling and self-tapping orthodontic 

microimplants in dogs 

Yan Chen, a Hong-In Shin, b and Hee-Moon Kyung c 

Hohhot, China, and Daegu, Korea 

Introduction: The purpose of this study was to compare the influences of different implant modalities on 

orthodontic microimplants and surrounding tissues biomechanically and histologically. Methods: Fifty-six 

titanium alloy microimplants placed on the buccal side of the maxillae and the mandibles in 2 dogs were 

divided into 2 groups of 28; one group of microimplants was self-drilling, and the other was self-tapping. 

Approximately 200 g of continuous and constant forces were applied immediately between 2 microimplants 

by stretching closed nickel-titanium coil springs for 9 weeks. Peak insertion torque and removal torque were 

recorded immediately after the implants were placed and when the dogs were killed, respectively. 

Undecalcified sections of the microimplants and the surrounding tissues were studied with light microscope 

and fluorescent microscope. Results: Success rates were higher in the self-drilling group (93%) than in 

self-tapping group (86%). Higher peak insertion torque and peak removal torque values were seen in the 

self-drilling group in both the maxilla and the mandible. A tendency to fracture was found in self-drilling 

group. The percentage of bone-to-implant contact values was greater in the self-drilling group. Conclusions: 

Self-drilling microimplants can provide better anchorage and can be recommended for use in the maxilla and 

in thin cortical bone areas of the mandible. (Am J Orthod Dentofacial Orthop 2008;133:44-50) 

The self-tapping method of microimplant placement 

is an excellent approach in orthodontics, 

1,2 

although it has some shortcomings. Compared 

with the pretapping method, the self-tapping 

method offers quicker placement, less damage to bone, 

3 

and a better grip in bone. A prerequisite for selftapping 

placement is pilot drilling to prepare a hole for 

the implant. 4 Particularly for interradicular microimplants, 

pilot drilling has potential dangers, such as 

damage to tooth roots, drill-bit breakage, overdrilling, 

5-8 

and thermal necrosis of bone. Also, the process takes 

time. 

The growing demand for absolutely rigid orthodontic 

anchorage has led to an expansion of implant 

technology. 8 drilling procedure is simpler and involves less use of 

instruments. 

Recent advances in biomaterials have 

resulted in a new self-drilling implant system for rigid 

8 

internal fixation in maxillofacial surgery. The self- 

9 

Nevertheless, differences in biomechanical characteristics 

and mechanisms between self-drilling microimplants 

(SDIs) and self-tapping microimplants (STIs) 

in terms of how they are anchored to bone have not 

10-12 

been clarified. In previous animal and clinical 

studies, 13-16 osseointegrated dental implants have behaved 

like ankylosed teeth to provide orthodontic 

anchorage. In theory, SDIs might cause less damage to 

bone during placement; therefore, osseointegration 

17 

might happen earlier and be better than with STIs. 

Because primary stability is thought to be essential in 

achieving ideal bone-to-implant contact (BIC), clinical 

studies have shown significantly higher placement 

torque with SDIs, 

18 but no histologic data of BIC 

values have been shown. Based on the limited literature 

and clinical experiences, 

8,19 SDIs might have a relatively 

high anchorage value for the en-masse movement 

of anterior teeth, molar uprighting, and forward movement. 

Perhaps the BIC values at the interface can be 

enhanced with SDIs, 

17 aAssociate professor, Oral Department, Attached Hospital of Inner Mongolia 

Medical University, Hohhot, China. 

bChairman and professor, Department of Oral Pathology, School of Dentistry, 

Kyungpook National University, Daegu, Korea. 

cChairman and professor, Department of Orthodontics, School of Dentistry, 

Kyungpook National University, Daegu, Korea. 

so that they can be expected to 

Reprint requests to: Hee-Moon Kyung, Department of Orthodontics, Dental 

School, Kyungpook National University, 188-1, Sam Duk 2 Ga, Jung Gu, 

Daegu, Korea 700-412; e-mail, hmkyung@knu.ac.kr. 

improve the anchorage of implants, especially in areas 

of low bone density, to increase the success rate. 

Submitted, September 2005; revised and accepted, January 2006. 

0889-5406/$34.00 

Copyright © 2008 by the American Association of Orthodontists. 

doi:10.1016/j.ajodo.2007.01.023 

The objectives of this study were to compare the 

biomechanical properties of microimplants with different 

drilling methods by measuring peak insertion torque 

44

American Journal of Orthodontics and Dentofacial Orthopedics 

Volume 133, Number 1 

(PIT) and peak removal torque (PRT) in an animal 

model, and to investigate the influence of self-drilling 

and self-tapping techniques on BIC values under orthodontic 

force. 

MATERIALS AND METHODS 

Two adult female mongrel dogs (weight, 13 kg) 

were housed in the animal laboratory at Kyungpook 

National University Medical College in Daegu, Korea. 

All permanent teeth were erupted, and maxillary and 

mandibular growth was completed. The treatment of the 

experimental animals was approved by the ethics committee 

of the Medical College at Kyungpook National 

University. 

Fifty-six threaded microimplants (Absoanchor, 

Dentos, Daegu, Korea) made of titanium alloy (titanium, 

aluminum, vanadium) were used for this experiment 

and divided into 2 groups. The head of the 

microimplant was hexagonal to enable placement with 

hand-driver; it had a small hole for threads and ligature 

wires. SDIs, with a specially designed sharp tip of 

conical shape with threads, served as the study group, 

and STIs with an ordinary tip served as the control 

group (28 for each group). Size 1312-07—tapered with 

1.3 mm neck diameter, 1.2 mm diameter near the apex, 

and threaded intrabone part 7 mm—was chosen for this 

experiment (Fig 1). The 2 types of microimplants were 

the same size and shape (except for the tip), had the 

same placement angle and similar force magnitudes, 

and were intended for similar bone densities. 

The buccal side of the bilateral maxilla and mandible 

of the dogs was chosen as the recipient site for 

symmetrically placing 2 groups of microimplants. The 

left side was for SDIs, and the right side was for STIs 

Chen, Shin, and Kyung 45 

Fig 1. Microimplants and hand-driver used in this 

study: A, STI with tip; B, SDI with tip; C, microimplant 

head; D, hand-driver. 

head of the microimplant remained outside the attached 

gingiva for connecting the nickel-titanium springs (Fig 2). 

Twenty-eight STIs were placed with the self-tapping 

method. A pilot hole was drilled first with a 

0.9-mm diameter round bur, as deep as the length of 

in 1 dog (Fig 2). The left maxilla and the right mandiblemicroimplant 

thread part, with a handpiece at a speed 

received SDIs, and the right maxilla and the left of 500 rpm by drilling intermittently. Saline solution 

mandible received STIs in the other dog. All microim- was used to keep the bur and drill cooled and the 

plant placement procedures were performed aseptically placement site lubricated. Then the microimplants were 

with a 90° implant placement angle to the long axis of manually placed with the same long hand-driver, ac- 

the teeth. Oral hygiene was achieved by rinsing each 

20 

cording to the technique described by Kyung et al. 

dog’s mouth biweekly with 0.2% chlorhexidine glu- Approximately 200 g of continuous and constant 

conate solution. 

horizontal force was immediately applied by stretching 

For the placement of the microimplants, the dogs nickel-titanium superelastic closed-coil springs (Tomy, 

were anesthetized with a cocktail composed of ket- Tokyo, Japan) between 2 microimplants for 9 weeks (Fig 

amine (Ketara; Yuhan, Seoul, Korea) at a dose of 10 2). The springs contacted the microimplants directly. 

mg per kilogram of body weight, xylazine (Rompun An operator who has placed more than 100 microim- 

Bayer Korea, Seoul, Korea) at 0.4 mg per kilogram of plants in patients and for animal experiments placed all 

body weight, and saline solution. 

the microimplants. 

Twenty-eight SDIs were placed with the self-drill- PIT—the force of the maximum clockwise moveing 

method. They were manually placed through the ment that stripped bone from screw insertion until it 

attached gingiva with a long hand-driver (Dentos, was perfectly placed—was measured with a precision 

Daegu, Korea) that was especially designed for these digital indicator (EMT, E-mobile tech 17000 series; 

microimplants, without pilot drilling and incision. The SEEC, Seoul, Korea) drilling after placement for all

46 Chen, Shin, and Kyung 

microimplants in the 2 groups. After 9 weeks of observation, 

PRT—the force of the maximum counterclockwise 

movement to loosen the microimplants—was 

measured with the same machine for 12 stable microimplants 

at symmetrical sites in each group. 

The stability of the microimplants was measured 

with dental tweezers; failure was defined if a microimplant 

could be moved in any direction after the 9-week 

observation period. 

A vital staining procedure was used for the animals. 

Oxytetracycline (15 mg per kilogram per day) was 

given intramuscularly for the first 4 days after the 

microimplants were placed. A subcutaneous injection 

of 1% calcein (15 mg per kilogram per day) was given 

at the fourth week, and 0.16% alizarin red (75 mg per 

kilogram per day) was injected intramuscularly at the 

seventh week. 

The anesthetized dogs were killed by intracardial 

perfusion with neutral 10% formaldehyde solution. The 

maxillary and mandibular bones containing microimplants 

were dissected. The specimens were fixed in the 

same solution for more than 24 hours. After being washed 

with distilled water, each implant area was separated with 

a saw (D-54518, Proxxon; Woo Sung E&ICo,Ltd, 

Tokyo, Japan). 

Fourteen microimplants with surrounding tissue were 

selected for histologic evaluation from the 2 groups at 

symmetrically interradicular areas of the maxillae and the 

mandibles of the 2 dogs. These specimens were infiltrated 

with vilanueva bone stain propylene oxide from a starting 

solution of 70% alcohol and stained. This procedure 

included 11 steps and lasted more than 3 weeks. 

Before being embedded in vilanueva bone stain 

propylene oxide, the specimens were dehydrated and 

defatted with graded ethanol. Each embedded implant 

with bone was sectioned sagittally with a low-speed 

digital saw (Accutom-50; Struers, Ballerup, Denmark) 

into thicknesses of 500 m. These sections were 

ground with a grinding machine (Rotopol-35; Struers) 

until the implants and the surrounding tissues could be 

seen clearly. Microscope slides, prepared by mounting 


January 2008 

Fig 2. Placement of microimplants in dog jaw with force applied by nickel-titanium coil springs: A, 

SDI; B, STI. 

Table I. Comparison of peak insertion torque values 

between groups (in Ncm) 

Group (n 14 each group) STI SDI P 

Maxilla 3.5 2.1 5.6 1.1 .017 

Mandible 7.4 1.1 8.7 2.3 .029 

Significance, P .05. 

the flattened sections with low fluorescence, were 

observed with a light microscope and a fluorescent 

microscope. 

Histomorphometric analysis was performed on the 

digitized images from the microscope with a camera 

system (Olympus, Tokyo, Japan). The images obtained 

with 100-times magnification were combined into 1 

figure including the entire implant surface and analyzed 

with image analysis software (Image & Microscope 

Technique, Goleta, Calif). For each microimplant, the 

bone contact sections were measured and expressed as 

percentages of BIC. 

Data are expressed as means and standard deviations. 

The differences between the SDI and STI groups 

were evaluated with the Student t test. A P value less 

than .05 was considered significant. 

RESULTS 

Not all the microimplants remained firm enough 

during the experimental period. In the STI group, the 

overall success rate was 86%; 4 microimplants loosened 

and tipped. Two failures occurred in the SDI 

group; 1 microimplant loosened, and another in the 

distal region of the mandibular second molar fractured 

in the fourth week. The success rate was 93%. 

Mild inflammation and swelling of the alveolar 

gingivae were observed on both sides of the maxilla 

and the mandible. They were more serious on the left 

side in both dogs. 

The comparison of the PIT values taken at the 

beginning of the experiment showed significant differ-



Table II. Comparison of peak removal torque values 

between groups (in Ncm) 

Group (n 6 each group) STI SDI P 

Maxilla 5.7 2.3 6.5 2.2 NS 

Mandible 6.1 0.5 7.1 2.0 NS 

NS, P .05. 

Fig 3. Bone implant interfaces: A, SDI; B, STI (20X 

original magnification). 

ences between the 3 groups in the maxilla and the 

mandible (Table I). 

The comparison of mean PRT showed no statistically 

significant difference between the 2 groups after 9 

weeks (Table II). 

Histologic assessment showed good histocompatibility, 

with various bone structures interspersed with 

fibrous connective tissues around the microimplants in 

both groups (Fig 3). Better osseous tissue formation 

around the microimplants and greater original bone 

were seen in the SDI group than in STI group, as 

evidenced by calcein and oxytetracycline labeling. 

Remodeling and apposition were seen more in the SDI 

group than in STI group (Fig 4). 

BIC values are shown in Table III. 

DISCUSSION 

After the 9-week period, an important finding in 

this study was the high success rates of the SDI and the 

STI groups. The results confirmed our original deduction 

of greater success in the SDI group (93%) than in 


the STI group (86%). Two microimplants failed in the 

SDI group; 1 loosened, and the other fractured in the 

fourth week. In a similar animal experiment, Kim and 

Chang 21 observed a higher success rate in miniscrews 

in the drill-free group than in the drill group with a 

1-week healing period. 

The main reason for the failures might have been 

the poor initial stability caused by early loading and the 

exposed healing situation. This was confirmed in the 

19 

pig experiment of Heidemann et alwith 

a 100% 

success rate in the groups of self-drilling screws (SDS) 

and self-tapping screws (STS), which healed in a closed 

situation and without force. 

Because of early loading in our study, the microimplants 

stayed in the bone with mechanical interlock 

without bone and implant osseointegration at the be- 

1 

ginning of the experiment. Roberts et demonstrated 

al 

that immediate loading was deleterious to implant 

stability in their animal study. However, SDIs have 

been proven to have greater strength and highly mechanical 

friction with bone to keep initial stability and 

to resist micromovement. 

19 Initial stability was be - 

lieved to play an important role in the success of our 

interradicular microimplants that healed in exposed 

situations. 22 

PIT, which exhibited the holding strength to resist 

loosening and loss of the implants, was measured at the 

beginning of the experiment for each microimplant in 

both groups. The mean PIT values of the SDIs in the 

maxilla and the mandible were 5.6 and 8.7 Ncm, 

respectively. In the STI group, PIT values were 3.5 

Ncm for the maxilla and 7.4 Ncm for the mandible. PIT 

values were found to be significantly different between 

the SDI and STI groups, both in the maxilla and the 

mandible, but lower in the maxilla. Our results were 

similar to those from the clinical experiment of Schon 

et al 23 for craniomaxillofacial procedures; they found 

that insertion torque of the SDS was higher compared 

with that of the STS in humans. Microimplants with 

high PIT might have closer initial contact with bone 

and might be good for bone remodeling. From an anchorage 

perspective, the greater the contact between the 

surface of the microimplant threads and the cortical bone, 

24,25 

the higher the initial grip of microimplants would be. 

Fracture is a disadvantage of microimplants even 

when they are placed without excessive force. The 

results showed high PIT values for thin-diameter microimplants, 

and high bone density might be dangerous 

to SDIs. According to the principle of conservation and 

transformation of energy, SDIs have higher pressure 

than STIs, and pretapped implants were placed after the 

pilot holes were drilled. Even though SDIs have many 

advantages, if the bone is dense, they should not to be


Fig 4. Fluorescent microscope views of middle part of peri-implant bone at first molar area 

of 1 dog’s mandible: A and B, SDI; C and D, STI (100X original magnification). 

Table III. Comparison of BIC values between groups 

Group SDI STI 

Maxilla 39.28% (n 4) 27.96% (n 4) 

Mandible 47.44% (n 3) 26.35% (n 3) 


January 2008 

in the mandible in the STI group. Removal torque, as a 

biomechanical method to measure anchorage or endosseous 

integration in which greater forces are required to 

remove microimplants, might be associated with increases 

in the strength of osseous integration. 

Compared with STIs, SDIs showed good resistance 

to shear force with bone growing into the threads of the 

microimplants. The secondary bone healing response 

resulted in biomechanical bone interlocking, which 

developed sufficient interface to resist shear strength 

and was sufficient to withstand orthodontic loads. 

However, there was no statistically significant difference 

in PRT between the 2 drilling methods. This 

chosen. STIs should be considered instead. In the 

maxilla and areas with thin cortical bone in the mandible, 

microimplants would penetrate easily. Failure 

due to stripping of bone was infrequent, so pilot drilling 

was not necessary. 

Heidemann et al 

19 reported the increase of PIT and 

shearing forces on the screw itself; fracture of the 

screws was caused in thick cortical bone in their 

in-vitro test. Ellis and Laskin 

24 believed that, if a screw 

was stressed sufficiently in tension or torsion, it ultimately 

would be broken. They thought that, if screws 

were subjected to torsion and flexion stress, they would 

fail at a lower tensile stress value. 

This was validated by an in-vitro trial of screws 

with diameters of 0.8 to 2.0 mm; the screw diameter 

was the major predictor for holding and breaking 

strength. 26 23 

was supported by the results of Schon et in al which 

the PRT value of the SDIs was similar to that of the 

STIs at the same location: 4 Ncm in the maxilla and 12 

Ncm in the mandible. The PRT values in the SDI and 

STI groups might suggest that certain factors associated 

with microimplants stimulate in-vivo incorporation of 

bone. Several previous investigations pointed out that 

removal torque measurement usually gave concomitant 

29-31 

results with histomorphometric evaluation of BIC. 

The authors of that study emphasized that As time went on, the PRT values in the SDI and STI 

the larger the diameter of the screw, the better its groups might be similar, and it was not expected that 

holding strength in thick bone. 

SDIs would have greater PRT values than STIs. In 

The use of reverse torque for measuring the shear contrast to previous reports, 

force required to rupture the bone-microimplant interface 

provided important information concerning bone 

growing into the fixture surface. Our PRT values were 

6.5 Ncm in the maxilla and 7.1 Ncm in the mandible in 

the SDI group, and 5.7 Ncm in the maxilla and 6.1 Ncm 

27-30 our lower PRT values 

might be explained by the small size of the microimplants 

and the short observation time. 

It has been widely accepted that osseointegrated 

implants can provide absolute anchorage for tooth 

movement. The BIC value is a parameter for the linear



surface of microimplants that is directly in contact with 

bone matrix and is calculated with software and expressed 

as a percentage of the total microimplant surface. In this 

study, the mean BIC values in the SDI group were 39.28% 

in the maxilla and 47.44% in the mandible. In the STI 

group, the mean BIC values were 27.96% and 26.35% in 

the maxilla and the mandible, respectively. Because the 

BIC values were higher in the SDI group than in the STI 

group, this showed that the drilling method had an effect 

on bone-to-microimplant contact; this was further confirmed 

by PRT values and histologic findings of more 

original and new bone in the SDI group. 

BIC values of 93.8% in the SDS group and 81% in 

the STS group were found after 6 months of observation 

in pigs. 

19 Heidemann et al 

19 demonstrated that the 

BIC values in the SDS group were superior to those in 

the STS group because of the greater amount of original 

bone in the threads of the drill-free screws. Similarly, in 

21 

the study of Kim and Chang, the mean BIC value 

(43.68%) in the drill-free group was higher than in the 

drill group (23.41%) in dogs. Obviously, compared 

with previous studies, our results indicate that using 

SDIs without chemical and biologic modification 

caused less damage to bone, and the time of bone 

modeling and remodeling was reduced because the 

bone debris was transported and deposited on the bone 

surface around the implant tip because of the conical 

shaft. 

19 

Compared with the results of Heidemann et al, the 

BIC values in our study were lower. This difference 

could be an internal response of bone to the implant 

31 

time and the healing condition. Melsen and Costa 

observed BIC values of 10% to 58% after 6 months of 

forcing without a healing period for miniscrews. They 

found that the BIC ratio increased with implantation 

time but was independent of the type of bone and the 

magnitude of force. Our results have elucidated the 

mechanism behind the behavior of cells on implant 

surfaces, and we report an increase in BIC contact by 

using different drilling methods. 

Because the numbers of dogs and microimplants 

were limited, this study might be regarded as a pilot. 

However, the results were unequivocal because the 

tested microimplants were placed symmetrically. Further 

studies with different force magnitudes after a 

short healing period are required for more discussion 

and definite results. 

CONCLUSIONS 

By comparing the biomechanical and histologic 

properties between SDIs and STIs, without a healing 

period, we found high success rates in both groups. 


PIT, PRT, and BIC values were higher in the SDI group 

than in the STI group. 

We recommend using SDIs in the maxilla and the 

thin cortical bone areas of the mandible because there is 

less damage and use of instruments. In areas of dense 

cortical bone, STIs and SDIs with larger diameters can 

be recommended. 

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