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ORIGINAL ARTICLE

Midpalatal miniscrews for orthodonticanchorage: Factors affecting clinical success

Young Ho Kim,a Seung-Min Yang,b Seonwoo Kim,c Joo Yong Lee,d Kyu Eok Kim,e Anthony A. Gianelly,f

and Seung-Hyun Kyungg

Seoul, Korea, and Boston, Mass

Introduction: The purpose of this study was to investigate the success rate of midpalatal miniscrews used for

orthodontic anchorage and the factors affecting clinical success. Methods: One hundred twenty-eight

consecutive patients (101 female, 27 male; mean age, 23.4 years), who received a total of 210 miniscrews

in the midpalatal suture area, were examined. Success rates were determined according to 10 clinical vari-

ables. Results: The overall success rates were 88.20% for the total number of patients and 90.80% for the

total number of miniscrews. There were no significant associations among success rate and sex, total period

of treatment with miniscrews, diameter of miniscrews, types of tooth movements, and variables that represent

sagittal and vertical skeletal relationships (ANB, FMA, and Sn-GoGn). The operators learning curve, patients

age, area (midpalatal or parapalatal), and splinting significantly influenced the success rates. After adjustingfor other variables, only 1 splintingshowed a significant effect on the success rate. Conclusions: The join-

ing of 2 miniscrews by splinting, placement of the miniscrew in the midpalatal suture, patients age (especially

.15 years), and operators skill were factors influencing the clinical success of orthodontic miniscrews in the

palate. (Am J Orthod Dentofacial Orthop 2010;137:66-72)

Since the introduction of implants as absolute

anchorage in orthodontic treatment, various

types of tooth movement without patient com-

pliance have become possible with newly developed

miniscrews.1-3

Of the possible placement sites for miniscrews, the

midpalatal area has been reported to be appropriate.4-8

The midpalatal suture is a highly dense structurewith

sufficient bone height up to the cresta nasalis,9,10 and

vertical bone support is somewhat higher (at least 2

mm) than is apparent on cephalograms.7 The midpalatal

area within 1 mm of the midsagittal suture is composed

of the thickest bone available in the whole palate,11 and

the thickness of soft tissues in the midpalatal area isuniformly 1 mm posterior to the incisive papilla,10

ensuring biomechanical stability of the miniscrews.

There are no roots, nerves, or blood vessels to compli-

cate the placement of surgical miniscrews, and there is

no needfor additional surgery because of their easy re-

moval.12 Miniscrews have been placed in the midpalatal

suture area of adults, and the parapalatal area in adoles-

cents to prevent possible developmental disturbances of

the midpalatal sutures.13 This is because the transverse

growthof the midpalatal suture continues up to thelateteens14 and is not fused completely even in adults.15,16

Various attempts to use implants as absolute anchor-age in the midpalatal suture area have been made.17-19

Now, midpalatal miniscrewsare used for retraction of

maxillary anterior teeth,20,21 intrusion,22-25 distaliza-

tion,12 and protraction of maxillary posterior teeth,

making it possible to produce movements that were, at

best, difficult with conventional orthodontic treatment

strategies.

The purpose of this study was to investigate the

success rate of midpalatal miniscrews used as orthodon-

tic anchorage for various types of tooth movements and

factors affecting clinical success.

aAssociate professor, Department of Orthodontics, Institute of Oral Health &

Science, Samsung Medical Center, Sungkyunkwan University School of

Medicine, Seoul, Korea.bAssociate professor, Department of Periodontics, Institute of Oral Health &

Science, Samsung Medical Center, Sungkyunkwan University School of

Medicine, Seoul, Korea.cSenior statistician, Biostatistics Unit, Samsung Biomedical Research Institute,

Samsung Medical Center, Sungkyunkwan University School of Medicine,

Seoul, Korea.dPrivate practice, Seoul, Korea.eResident, Department of Orthodontics, Institute of Oral Health & Science, Sam-

sung Medical Center, Sungkyunkwan UniversitySchool of Medicine, Seoul, Korea.f

Professor and chairman emeritus, Department of Orthodontics, Goldman School ofDental Medicine, Boston University, Boston, Mass; now deceased.gAssociate professor, Department of Orthodontics, Institute of Oral Health & Sci-

ence, Samsung Medical Center, Sungkyunkwan University School of Medicine,

Seoul, Korea; visiting professor, Department of Orthodontics, Goldman School

of Dental Medicine, Boston University, Boston, Mass.

The authors report no commercial, financial, or proprietary interest in the prod-

ucts or companies described in this article.

Reprint requests to: Seung-Hyun Kyung, Department of Orthodontics, Institute

of Oral Health & Science, Samsung Medical Center, Sungkyunkwan University

School of Medicine, #50, Irwon-dong, Gangnam-Gu, Seoul 135-710, Korea;

e-mail,[email protected].

Submitted, August 2007; revised and accepted, November 2007.

0889-5406/$36.00

Copyright 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2007.11.036

66
mailto:[email protected]:[email protected]

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MATERIAL AND METHODS

The subjects were 128 patients (101 female, 27

male; age range, 8.1-56.2 years; mean age, 23.4 6 8.0

years), who received miniscrews for orthodontic

anchorage in the midpalatal area. All miniscrews wereplaced by 1 doctor (S.H.K.) at the Department of Ortho-

dontics, Samsung Medical Center, Seoul, Korea,

between 1999 and 2005. The patients were informed

about the possibilities of inflammation around and

loosening of the miniscrews.

A total of 210 miniscrews were placedwithoutflap el-evation under local anesthesia. They were placed in the

midpalatal sutures and, sagittally, between the mesial

and distal aspects of the maxillary first molar. In adoles-

cents, miniscrews were placed in the parapalatal area in-

stead of the midpalatal areas to prevent possible damage

to the developing sutures. An orthodontic force was ap-plied immediately with elastomeric modules (power

chain), and they were replaced every 3 weeks.

Two types of self-drilled miniscrews of the same

length but different diameters were used (Fig 1). One

was a surgical miniscrew (diameter, 1.5 mm; length,

5.0 mm; KLS-Martin, Jacksonville, Fla) that oral sur-

geons usually use for fixation of bone fragments. The

other was especially designed (diameter, 2.0 mm;

length, 5.0 mm; Orthoplant, Biomaterials Korea, Seoul,

Korea) and developed for orthodontic anchorage at the

midpalatal area. Its head diameter was larger (4.0 mm)

so that it provided a wide contact area between

its head and a screw-supported bonded sheath

(S-sheath), generating higher bonding strength suffi-

cient to resist heavy orthodontic forces. The sheath

was custom-madewith a normal lingual sheath welded

onto metal mesh.25

Although some patients were treated with 1 mini-screw as orthodontic anchorage, most patients, espe-

cially adolescents and young adults, were treated with

2 miniscrews splinted together to ensure stability. The

S-sheath was used because a heavy orthodontic force

was needed to control several maxillary posterior teeth

simultaneously. Splinting was done by simply bonding

the S-sheath on the top of 2 miniscrews with flowablecomposite resin. After placing the miniscrews, 500 to

800 g of force was applied initially for various types

of tooth movement: distalization, mesialization, intru-

sion, or retraction of anterior teeth, either singly or in

combination. Elastomeric chains generally lose 50%

to 70% of their initial force during the first day of

load application and, at 3 weeks, retain only 30% to

40% of the original force.26-28 Therefore, we believed

that 250 to 400 g might be loaded over 3 to 7 teeth after

force degradation of the elastomers.

Figure 2shows the orthodontic mechanics for some

types of tooth movements by single or splinted minis-

crews with or without an extension arm: distalization

of maxillary molars, mesialization of maxillary molars,

intrusion of maxillary molars, and retraction of anterior

teeth.

The procedure was regarded as a clinical success

when a miniscrew remained without loosening until ithad accomplished its purpose. To examine the factors

affecting the clinical success of midpalatal miniscrews

for orthodontic anchorage, 10 clinical variables were

investigated: operators learning curve (determined by

calculating the success rate of miniscrews placed by

the operator over 4 time periods of 18 months each),sex, age, area (midpalatal or parapalatal), total treat-

ment period with miniscrews, splinting (single screw

vs joined screws), diameter of miniscrew (1.5 vs 2.0

mm), types of tooth movements (distalization, mesiali-

zation, intrusion, retraction of anterior teeth, and combi-

nations), and variables representing sagittal and vertical

skeletal relationships (ANB, FMA, and Sn-GoGn). In

previous studies, there was no evaluation of the clini-

cians skill as a factor influencing success rates. There-

fore, this study included an evaluation over time of

whether the operators increasing experience with the

procedure was a factor that affected the stability of

miniscrews.

Statistical analysis

Success rates related to the numbers of subjects and

miniscrews were calculated. The success rate was also

Fig 1. Two types of miniscrews used in the study: left,

cylindrical type (diameter, 1.5 mm; length, 5.0 mm;

head diameter, 3.0 mm); right, tapered type (diameter,

2.0 mm; length, 5.0 mm; head diameter, 4.0 mm).

American Journal of Orthodontics and Dentofacial Orthopedics Kim et al 67Volume137,Number1

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presented for each category of clinical variable. A

continuous clinical variable was categorized to provide

the success rate. Logistic regression analysis was used

to examine the influence of each of 10 clinical variables

(categorical variables and continuous variables without

categorization) on success. Multiple logistic regressionanalysis was also used to investigate the influence of

each variable when the effects of other variables were

controlled. The odds ratio (OR) for each factor was

also calculated.

RESULTS

Fifteen of the 128 patients had at least 1 miniscrew

failure, for a success rate of 88.20%. Eighteen of the 197

miniscrews failed, for a success rate of 90.80%. The

average time after placement for miniscrew failurewas 3.5 months.

Logistic regression analysis showed no significantassociation between the success rate and each of follow-

ing variables: sex, total period of treatment with minis-

crews, diameter of miniscrew, types of tooth movement,

and variables representing sagittal and vertical skeletal

relationships (ANB, FMA, and Sn-GoGn).

Six of 51 miniscrews placed in 27 male patients

were recorded as failures, for a success rate of 88.2%.

In 101 female patients, 12 of 146 miniscrews failed, rep-

resenting a success rate of 91.8%. There was no statisti-

cally significant difference in the success rates between

the sexes (OR 5 0.70;P 5 0.4517).

The total period of treatment with miniscrews varied

according to the purpose of orthodontic treatment.

Although some patients were treated for less than

6 months with miniscrews, other treatments lastedmore than 18 months. When a miniscrew failed before

it achieved its purpose, it was replaced. In these cases,

the total period of treatment with miniscrews was calcu-

lated by the sum of the periods during which the 2 min-

iscrews were loaded. In the group of patients with

relatively short treatments (\6 months), 3 of 11 minis-crews failed, for a success rate of 72.7%. In the group

with longer treatments (.18 months), some patients

needed more than 1 miniscrew because of the early

loss of the first miniscrew, and 11 of 111 miniscrews

failed, for a success rate of 90.1%. Even though the dif-

ference in success rates between the groups with shorterand longer treatment periods (72.7% and 90.1%-95.1%,

respectively) was high, there was no statistical signifi-

cance between the treatment period and the success

rate (OR 5 0.93;P 5 0.2386).

Similarly, the success rate for miniscrews with

a 2.0-mm diameter (91.9%) was slightly higher than

for those with a 1.5-mm diameter (89.0%), but the diam-

eter of the miniscrews was not a significant factor.

With the midpalatal miniscrews, various kinds of

tooth movements could be made: distalization, mesiali-

zation, intrusion, retraction of anterior teeth, and

Fig 2. Midpalatal miniscrew placements:A, distalization of maxillary molars; B, mesialization of max-

illary molars;C, intrusion of maxillary molars; D, retraction of anterior teeth.

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combinations of these. Although these movements

require heavy forces of over 500 g, the success rates

varied from 88.7% to 96.0%; there were no statistically

significant differences.

The success rate according to the ANB differencerepresenting sagittal skeletal relationships varied from

89.1% (Class I) to 97.1% (Class III), but there was no

significant association between the ANB value and the

success rate (OR 5 1.05). In patients whose FMA and

Sn-GoGn represented vertical skeletal relationships,

those with high angles had high success rates of

97.4% (FMA) and 97.1% (Sn-GoGn), but there were

also no significant associations (OR 5 1.03 [FMA)];

1.02 [Sn-GoGn]).

There were significant associations between the suc-

cess rate and the following variables: operators learn-

ing curve, age, area, and splinting. The operators

learning curve, which indicated his skill or experience

over time, had a significant association with the success

rate; the longer his learning experience, the higher the

success rate (OR 5 1.60;P 5 0.0132). During the first

18 months, when the operator was not accustomed to the

procedure for placing miniscrews in the palate, 9 of 36

miniscrews failed, a success rate of 75%. This was much

lower than the rates of later periods (91.2%-97.9%); the

success rate increased to more than 95% after the third

period of 18 months.

Age was also associated with the success rate, and

the logistic regression analysis showed that older

patients had higher success rates (OR 5 1.01; P 50.0249). Notably, in the group of patients less than 15

years of age, 9 of 31 miniscrews failed, for a success

rate of 71.0%, which was much lower than rates for

the older groups (92.9%-100%).

In the parapalatal area, 5 of 24 miniscrews failed,

whereas in the midpalatal area, only 13 of 173 failed.

Thus, the miniscrews in the parapalatal area showed

a significantly lower success rate (79.2%) than those

in the midpalatal area (92.5%) (OR 5 2.77;

P 5 0.0426).

Splinting the 2 miniscrews produced a higher suc-

cess rate (95.9%) than use of 1 miniscrew (82.4%)(OR 5 0.23; P 5 0.0033), and splinting was also the

only clinical variable that showed a significant associa-

tion with the success rate (OR 5 0.09;P 5 0.013) after

controlling for the effects of the other variables (Table).

DISCUSSION

The criterion used to define the success rates for

miniscrews in previous studies was how long they lasted

under loadingeg, 6 months,29 10 to 12 months,30 and

1 year.31,32 However, the criterion in this study

depended on the achievement of purpose, not on the

amount of time that the miniscrews lasted as anchorage.

According to this criterion, 5 miniscrews were recorded

as successes, although they lasted less than 6 months.

On the other hand, 4 miniscrews were regarded as fail-ures even though they lasted more than 1 year because

they did not complete their missions.

Most previous studies reported success rates on the

basis of the total number of miniscrews; however, the

success rate based on the total number of patients is

also meaningful. In this study, the success rates were

88.20% for the total number of patients and 90.80%

for the total number of miniscrews; these rates are

similar to those reported for buccal miniscrews.29-33

The applied force was initially 500 to 800 g per mini-

screw, although a miniscrew placed in the buccal bone

can withstand 200 to 250 g. Therefore, considering

the amount of applied force and the success rates in

this study, we recommend the midpalatal area as an

appropriate site to obtain a strong orthodontic anchor-

age for group movements of maxillary teeth.

Many studies have reported the success rates for

miniscrews placed during several years without consid-

ering improvements of the operators skills. When the

operator in this study (S.H.K.) was a novice in placing

the midpalatal miniscrews, his success rate for the first

18 months was 75%. Thereafter, his success rate

increased to over 90% and remained over 90% until

the last period of 18 months. This result indicates that

the operators skill or experience is critical to the suc-cess rate, and this finding might also be true for labial

miniscrew applications.

Many studies have found no significant differences

between success rate and age, but, in this study, younger

patients, especially those less than 15 years ofage, had

a higher failure rate than did older age groups.29-32 This

might be attributed to a difference in bone density

because calcification of bone is not completed in adoles-

cents, or a difference in area because miniscrews were

usually placed in the parapalatal area in adolescents.

The midpalatal area has sufficient bone height for the

placement of miniscrews, although, even in adults, thereis low bony obliteration or fusion of the midpalatal

suture.15,16 However, in growing children and adoles-cents, the parapalatal area is recognized as an alterna-

tive.34,35 Miniscrews placed in the parapalatal area

had a significantly higher failure rate, and there were

3 failures in 1 adolescent patient. Thus, although cau-

tion is required in the placement of miniscrews in the

parapalatal area in adolescents, this procedure is not

contraindicated in patients younger than 15 years old.

The most important factor contributing to the

success rate of miniscrews in the midpalatal area was


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Table. Success rates and number of loosened miniscrews according to 10 clinical variables

Logistic regression Multiple logistic regression

Clinical variable Miniscrews (n) Loosened miniscrews (n) Success rate (%) OR Pvalue OR Pvalue

Operators learningcurve (mo)

1.60 0.0132 1.33 0.2911

First 18 36 9 75.0

Second 18 68 6 91.2

Third 18 47 1 97.9

Fourth 18 46 2 95.7

Sex 0.4517 0.7164

Male 51 6 88.2 0.70 0.71

Female 146 12 91.8 1.00 1.00

Age (y) 1.01 0.0249 3.23 0.1269

\15 31 9 71.0

\20 32 0 100.0

\30 112 8 92.9

.30 22 1 95.5

Area 0.0426 0.2716

Midpalatal 173 13 92.5 2.77 3.23

Parapalatal 24 5 79.2 1.00 1.00

Total treatment period

usingminiscrews (mo)

0.93 0.2386 0.94 0.4569

\6 11 3 72.7

6-12 41 2 95.1

12-18 34 2 94.1

.18 111 11 90.1

Splinting 0.23 0.0033 0.09 0.013

Single 74 13 82.4

Splint 123 5 95.9

Miniscrew diameter (mm) 0.74 0.4975 0.33 0.2674

1.5 73 8 89.0

2.0 124 10 91.9

Tooth movements 0.7875 0.6193Distalization 55 4 92.7 0.55 0.56

Mesialization 10 1 90.0 0.40 0.28

Intrusion 36 4 88.9 0.36 0.39

Retraction of anterior

teeth

71 8 88.7 0.36 0.57

Combination 25 1 96.0 1.00 1.00

Sagittal skeletal

relationship

ANB () (mean) 1.05 0.5302 1.16 0.2195

Class I (3.28) 128 14 89.1

Class II (7.42) 35 3 91.4

Class III (1.91) 34 1 97.1

Vertical skeletal

relationship

FMA (

) (mean) 1.03 0.3814 1.08 0.4444Low angle (20.16) 36 4 88.9

Middle angle (29.5) 122 13 89.3

High angle (39.8) 39 1 97.4

Sn-GoGn () (mean) 1.02 0.5129 1.02 0.8381

Low angle (28.3) 30 3 90.0

Middle angle (38.0) 138 14 89.9

High angle (48.4) 34 1 97.1


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splinting. In the multiple regression analysis, splinting

was the only clinical variable that showed a significant

difference in the success rate, and this result strongly

suggests that the stability of midpalatal miniscrews

can be further enhanced by splinting 2 miniscrews.Although in some patients 1 of the 2 splinted minis-

crews loosened, no patient had both splinted miniscrews

loosen simultaneously. It was difficult to detect loosen-

ing of the splinted screws because the 2 miniscrews

were splinted firmly with composite resin and an

S-sheath. The only sign indicating loosening was the

continuous growth of inflammatory tissue around the

S-sheath. We found that, unlike inflammation from

poor oral hygiene, inflammation caused by loose minis-

crews was not controlled with improved oral hygiene.

Therefore, we believe that inflammation or swelling

around a miniscrew might be a result of its loosening

rather than a cause.

When screws were placed in the midpalatal area,

there was no significant association between success

rate and sex; this agrees with previous reports.29-32

The stability of a miniscrew increases as its diameter

increases; theoretically, this is because the applied force

can be distributed over more bone area, resulting in

decreased pressure. Petrie and Williams36 emphasized

that the diameter of the implant was critical in decreas-

ing the amount of crestal strain. In this study, no signif-

icant difference was seen between miniscrews of 1.5

and 2.0 mm in diameter; this might be because excellent

bone quality and quantity could have masked the differ-ence in diameters. The total period of treatment with

miniscrews was included as a variable in this study to

investigatewhether the duration of exposure to an ortho-

dontic force influenced their success rates. If so, more

miniscrews might be expected to fail as the treatment

period increased. However, our results indicated that

the length of the period under loading did not influence

the success rate of the miniscrews.

During treatment, alterations of force vectors were

needed when the teeth did not move as expected.

Force vectors could be adjusted easily by bending or

refabricating the mesially extended-TPA and distalarms, which were removable and placed in the S-

sheath.25 The type of tooth movement determinedthe direction of force received by the miniscrews.

Therefore, the type of tooth movement was selected

as a clinical variable to determine whether the specific

directions of the force vectors are associated with the

stability of miniscrews. The results indicated that they

were not a factor related to the miniscrew success rate;

this meant that the miniscrews were fixed evenly in 3

dimensions and were not more resistant to any partic-

ular direction of load.

There have beenreports that themandibular plane

angle may be related31 or not related32 to the success

rates of miniscrews. In this study, sagittal and vertical

skeletal variables also did not show significant differ-

ences in the success rate, supporting the midpalatalapproach for miniscrews used as absolute anchorage,

regardless of a patients sagittal and vertical skeletal

pattern.

CONCLUSIONS

The overall success rates of midpalatal miniscrews

were 88.20% for the total number of patients and

90.80% for the total number of miniscrews under an

initial load of 500 to 800 g per miniscrew. The midpala-

tal area is appropriate for miniscrews, and midpalatal

miniscrews can serve as absolute orthodontic anchorage

for various types of tooth movements with high success

rates. Factors that influenced the clinical success

of miniscrews in the palate were splinting 2 miniscrews,

placement of the miniscrews in the midpalatal suture,

the patients age (especially .15 years), and the opera-

tors skill.

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