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Guided Infra-Zygomatic Screws IJOI 46
Fig. 1A:
The skeletal anatomy (osteology) of the Infrazygomatic Crest
(IZC) is marked with an ovoid magenta circle.
Fig. 1B:
A practical position for an IZC bone screw is anterior to the
anatomic ridge and buccal to the mesiobuccal root of the
maxillary first molar (MBR of U6). Distal to the anatomic
ridge, the preferred site is buccal to the MBR of U7. These
TAD sites are designated as IZC 6 and IZC 7 (yellow ovoid
circles), respectively.
Guided Infra-Zygomatic Screws:
Reliable Maxillary Arch Retraction
Introduction
The infra-zygomatic crest (
IZC
) is a buccal process on
the maxilla, connecting to the zygoma. Intraorally it
is a crest of bone emanating from the buccal plate
of the alveolar process, lateral to the roots of the
rst and second maxillary molars (
Fig. 1A
). The ridge
of bone extends 2cm or more superiorly to the
zygomatic-maxillary suture, and the inferior portion
can be subdivided into the IZC 6 and IZC 7 areas,
respectively (
Fig. 1B
). The IZC is a common site for
insertion of temporary anchorage devices (
TADs
).
Melsen
1
and Uribe
2
placed routine TADs along the
intraoral anatomical ridge of the IZC, and Villegas
3
used a 25mm long screw to engage the superior
aspects of the IZC, approximating the zygoma.
The amount of alveolar bone buccal to the
maxillary molars is the critical factor for placing
OrthoBoneScrew® (
OBS, Newton’s A Ltd, Hsinchu,
Taiwan
) in an Extra-Radicular (
E-R
) position. Inter-
radicular (
I-R
) TADs are also effective for maxillary
retraction, if the screws avoid the path of distal tooth
movement. This article reviews the relevant anatomy
and clinical procedures for routinely achieving
maxillary retraction with TADs, inserted directly into
alveolar bone of the posterior maxilla. (
Int J Orthod
Implantol 2017;46:4-16
)
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Guided Infra-Zygomatic Screws IJOI 46
Fig. 2:
An IZC anchorage screw penetrates about a 3mm thickness
of attached gingiva and cortical bone. The clearance of the
screw head to the soft tissue should be ~1.5mm, so there
is a distance about 4.5mm between the base of the screw
and the inner surface of the cortical bone. Thus, screws
of 8-12mm length will extend into the non-cortical bone
space (medullary bone or sinus) about 3.5-7.5mm.
Dr. John Jin-Jong Lin,
Examiner of IJOI, Director of Jin-Jong Lin Orthodontic Clinic
(Left)
Dr. W. Eugene Roberts,
Editor-in-chief, International Journal of Orthodontics & Implantology
(Right)
Anatomical Considerations
Soft tissue irritation is a common problem if the
inferior aspect of the screw platform is contacting
or near the mucosa. To control this problem the
IZC TADs are placed in attached gingiva with
~1.5mm of clearance from soft tissue to the base
of the TAD platform.
4,5
It is important to carefully
consider the anatomy of the IZC site to select an
appropriate screw length. The average thickness
of the attached gingiva in the maxillary first molar
is about 1.0mm,
6
and the cortical bone thickness
is about 1.1-1.3mm.
7
The screw threads must
engage cortical bone to insure primary stability.
Generalizing the widths, for soft tissue clearance,
attached gingiva and cortical bone at 1.5mm each
(
Fig. 2
), reveals that 8-12mm IZC screws penetrate
the medullary bone or sinus from 3.5-7.5mm.
Under most clinical conditions, an 8mm screw is
adequate to engage the cortical plate and secure
primary stability (
Fig. 2
).
Liou
8
suggested orienting screws about 55-70
degrees inferior to the horizontal plane to achieve
maximal buccal bone engagement, but it was not
clear whether IZC 6 or 7 was the preferred site
from an anatomic perspective (
Figs. 1-3
). Because
the alveolar bone is thicker on the buccal surface
of the second molar (
Figs. 4 and 5
), the IZC 7 site
is usually preferable for TADs.
9
In Taiwan, most
orthodontists utilize the IZC 7 site (
Fig. 6A
),
10,11
because the buccal plate covering the first molar
roots is too thin. For clinical convenience and the
advantage of attached gingiva, the preferred IZC
bone screw sites are considerably inferior (
Fig. 6B
)
to the anatomic zygomatic crest (
Figs. 1A & B
).
Initially, it was thought that all OBSs (
OrthoBoneScrews,
Newton’s A Ltd, Hsinchu, Taiwan
) placed buccal to
the MBR of U7 were routinely achieving E-R (
Extra-
Radicular
) placement for arch retraction. However,
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Guided Infra-Zygomatic Screws IJOI 46
Fig. 5B:
A similar series of CBCT axial cuts (3-8) from the CEJ apically
(series of upper views), is shaded in the series of lower views
to show available buccal bone for extra-radicular placement
of IZC screws: IZC 7 (green) and IZC 6 (red). See text for
details.
Fig. 4:
The coronal view of a CBCT (red rectangle) documents
alveolar bone thickness on the buccal (B) and palatal (P)
aspects of molar roots. Extra-radicular placement of a TAD is
more predictable for the IZC 7 compared to the IZC 6 sites,
because the bone is thicker over the distobuccal (7DB) and
mesiobuccal (7MB) roots of the second molar (U7), compared
to the corresponding 6DB, 6MB sites for the first molar (U6).
Fig. 3:
The alveolar bone is too thin to place a TAD buccal to the
MBR of 6, even with an increased angulation of 55-70º (Liou’s
IZC 6). The senior author (JJ-JL) proposes placing an IZC
screw lateral to the MBR of 7 (Lin’s IZC 7) to more reliably
achieve an E-R position for maxillary arch retraction.
Fig. 5A:
Five CBCT axial cuts (1-5) in the area demarcated by the
red rectangle show the alveolar bone anatomy for ~1mm
apical to the cementoemamel junction (Cut 1). The teeth are
labeled with Palmer notation 4-8. 5-6mm above CEJ is the
level where IZC screws are usually inserted. Note that the
alveolar bone is much thicker buccal to the U7 compared to
the U6.
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Guided Infra-Zygomatic Screws IJOI 46
Fig. 6A:
IZC 7 interference with maxillary retraction is documented
for four months (4ms) of treatment from 19y4m to 19y8m
of age. Canine cusps are marked with yellow lines in the
upper arch and blue lines in the lower arch. Four months
(4ms) of retraction with IZC 7 bone screws produced ~2mm
of maxillary arch retraction on the patient’s right side (upper
views), but little or no retraction on the left side (lower views).
See text for details.
Fig. 6B:
3D CBCT images of molars (white) and TADs (yellow) are
superimposed on a panoramic radiograph of the patient
illustrated in Fig. 6A. The screw on the patient’s left side
overlaps the root apex and may be interfering with maxillary
arch retraction. See text for details
Fig. 6C:
A horizontal maxillary cut of the CBCT show the IZC 7 screws
are inserted in bone, buccal to the roots of the molars (red
shading).
Fig. 6D:
A similar CBCT cut near the apices of the second molars
shows the screw on the right side penetrated the sinus, but
it is extra-radicular and still provided effective anchorage
to retract the right buccal segment, and the patient had no
symptoms or complaints. The left IZC 7 impinged on the
distal surface of the MBR of the U7, preventing retraction of
the left buccal segment in the direction of the yellow arrows.
CBCT analysis of consecutive patients revealed
some TADs were I-R (
Inter-Radicular
) (
Figs. 6A-D
), and
mostly they were E-R (
Fig. 7
). Consistently placing
IZC bone screws in an E-R, or carefully selected
I-R position, is critical for reliable maxillary arch
retraction.
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Fig. 7A:
IZC 6 interference with maxillary retraction is documented.
A patient 14y1m of age was treated with nine months of
maxillary retraction, that was anchored with IZC 6 and IZC 7
screws. Note there is a further deviation of the upper midline
to the right, which is consistent with more retraction on the
right than the left sides. Little or no progress was noted in
correcting the Class II buccal segment on the left side (lower
views). See text for details.
Fig. 7B:
The right IZC 7 is buccal to the roots, and has provided
effective anchorage for maxillary retraction (yellow arrows).
The MBR of the U6 on the left side interfered with retraction
(Stop!). See text for details.
Anchorage Requirements
Originally, the main purpose for TADs in the
posterior maxillary arch was for maximal retraction
of the anterior segment following extraction of
premolars. Ideal I-R placement was right in the
middle of the roots of upper 2
nd
premolar and
1
st
molar to avoid hitting the roots. In effect, the
TADs provided solid anchorage for the extraction
space closure. Similar I-R placement of TADs did
not work to retract maxillary molars, paradoxically
for the same reason: they blocked the path of
distalizing tooth movement. Thus, substantial
retraction of the entire maxillary arch was not
much or not possible with routine I-R TADs.
Clinical experience with OBSs placed in the
IZC 6 and 7 areas revealed that maxillary arch
retraction was accomplished more often with
TADs in the IZC 7, rather than the IZC 6 position.
CBCT evaluation demonstrated that the TADs,
successfully anchoring maxillary arch retraction,
were placed in an E-R position of the relatively
thick buccal bone (
Figs. 3-5
).
A truly reliable (
“fail-proof ”
) method for maxillary
retraction must evaluate bone quality as well as
quantity at the TAD site. Chang et al.
12
recently
found an IZC screw failure rate of <7%, compared
to ~20%, as reported by Uribe.
2
The improved
performance of the Chang et al.
12
method may
relate to favorable soft tissue on the more inferior
sites like IZC 6 & 7 (
Fig. 1B
), compared to the more
superior crestal positions of the TADs used by
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Guided Infra-Zygomatic Screws IJOI 46
Fig. 8A:
Maximal anchorage for routine space closure after
premolars extractions is inadequate for correction of a
severe malocclusion in a 16y4m old patient. Retraction of the
maxillary buccal segments is required.
Uribe
2
and Melsen
1
(
Fig. 1A
). From the evidence
reviewed, it is clear that an E-R bone screw is
more reliable for maxillary retraction compared
to high IZC
1,2
or routine I-R TADs.
1
The current
challenge is to develop a clinical approach that
consistently achieves TAD anchorage that reliably
supports posterior retraction of the molars.
Double Film Method
Dr. Leslie Chen
13
suggested that screws can be placed
mesial to the MBR of either U6 or U7 to achieve
maxillary arch retraction. This generalization worked
well for the thick buccal bone of the IZC 7, but the IZC
6 site was problematic. It was more dicult to achieve
E-R anchorage due to thin buccal bone, so the IZC 6
TADs usually produced I-R interference that blocked
the path of distal tooth movement. A CBCT scan is
valuable for confirming that an IZC TAD is unlikely
to interfere with retraction of the maxillary molars,
but 3D imaging is not necessary for placing the TAD.
The Chen
13
double lm method is a 2D radiographic
guide for screw placement in patients requiring IZC
anchorage (
Fig. 8A
).
Two Screw Insertion Procedure
A radiograph after the initial, preliminary positioning
of the TAD serves as baseline for planning its final
position, which is confirmed with a follow-up
radiograph. Either periapical, preferably with the
long cone technique, or panoramic radiographs are
suitable for the double lm procedure:
1. Initial insertion of the TAD with ~1mm of bone
engagement (
Fig. 8B
).
2. Check the position with an initial radiograph
(
Fig. 8C
).
3. The screw position is adjusted according to the
radiographic image.
4. The preferred position for the screw is
immediately mesial to the MBR, for either the
IZC 6 or IZC 7 sites.
5. Take a second periapical film to confirm that
the position is over the mesial of MBR (
Fig. 8C
).
6. Dening the 3D position of the screw relative to
the roots of adjacent teeth requires a CBCT (
Fig.
8D
). If necessary, the TAD can be repositioned.
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Fig. 8D:
Axial view of a CBCT shows the right IZC 6 is between the
roots of the U5 and U6, positioned so the right maxillary
segment can be retracted.
Fig. 8E:
Axial view of a CBCT shows that the left IZC 6 is also in
a favorable position between the roots of the first molar
and second premolar (shaded in red), so the left maxillary
segment can also be retracted.
Fig. 8B:
The double film method is effective with periapical
radiographs. For the initial film (lower left) the screw is
engaged into the bone about 1mm (upper left). The clinical
view after the TAD was placed (upper right) and the final film
(lower right) shows that the screw (arrow) was reinserted in a
more gingival and axial position.
Fig. 8C:
The double film method can also be used with panoramic
radiography. On the initial film (upper), space for screw
placement is noted between the U5 and U6 roots. The
final film shows the screw positioned mesial to the MBR
of the U6s. This configuration is suitable for maxillary arch
retraction.
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Guided Infra-Zygomatic Screws IJOI 46
Advantages:
1. There is usually enough I-R space between
the U5 root and the MBR of the U6 for an IZC
6 TAD. Placing the screw in this area is suitable
for maxillary arch retraction if it’s mesial to the
MBR of the U6 and there is adequate clearance
for distal movement of the root of the U5.
2. If it is desirable to position the TAD over the
mesial of MBR of U7 (
IZC 7
), the chance of extra
radicular is much higher.
3. Local infiltration analgesia is recommended
to control pain, and fortunately this form of
anesthesia does not interfere with the patient's
perception of a screw contacting the root. If
the patient feels the screw touching the root of
a tooth, the TAD can be repositioned.
Disadvantages:
1. Additional wounds occur if screw positioning is
changed.
2. Repositioning of the screw is associated with
saliva contamination.
Pin Head Soft Tissue Penetration Method
Dr. Mala Ram Manohar
14
presented this innovative
method at the 8
th
World Implant Orthodontics
Congress in Goa in 2016. The distinct advantage
over the Chen
13
double film method is the lack of
saliva contamination associated with reinserting
the TAD. The procedure is as follows:
1. Use stainless steel dot pins (
Fig. 9-1
); cut o the
heads (
Fig. 9-2
) leaving about a 1mm piece of
the shank (
Fig. 9-3
).
2. Canker sore patches (
Fig. 9-4
) used to cover
aphthous ulcers (
Fig. 9-5
) are thin, opaque strips
or circular patches; and position a pin head in
the center of the patch, with the point up (
Fig.
9-6
).
3. Following topical and then local infiltration
anesthesia, the circular patch covering the
pin-head point is pressed firmly into place,
penetrating the soft tissue in the anticipated
position of a TAD.
4. Image the area with a 2D radiograph, and
reinsert the TAD if needed into a more
desirable position; use the soft tissue mark left
by the pin head as a landmark.
5. Take a follow-up radiograph to check the nal
position of the screw, and adjust the TAD
position as needed.
Advantages:
1. No saliva contamination of the screw, unless
it must be repositioned after the follow-up
radiograph.
2. Avoids multiple screw placement wounds.
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Guided Infra-Zygomatic Screws IJOI 46
Transparent Adhesive Patch for Double Film Technique
1. Source a transparent, adhesive canker sore patch for oral use. It is important that the patch remains
rmly attached to soft tissue in the presence of saliva (
Fig. 10A-1,2
).
2. Cut 5mm rectangular patches, remove the protective lm, and press the back side of the pin head into
the center of the adhesive side of the patch (
Fig. 10A-3
).
3. Under local anesthesia, press the pin head into the soft tissue as before (
Fig. 10A-5
).
4. The transparent patch allows the clinician to see the exact position of the pin head (
Fig. 10A-4,6
).
Fig. 9:
The double film method employs a soft tissue marker that is secured to the soft tissue with an opaque canker sore patch.
1. Source standard steel dot pins (SS is preferred for sterilization).
2. Cut off the pin head.
3. Leave about 1mm of the pin shaft to serve as a soft tissue indentation marker.
4. Choose a canker sore patch.
5. Large patches are inappropriate because they are easily distorted by movable mucosa.
6.Select the round canker sore patch and place a pin head in the center of the circle.
7. Position the patch with a pin head on the tip of a gloved finger.
8. Under local anesthesia, press the pin head into the soft tissue; the patch should maintain the position of the pin head on the
soft tissue during the initial radiograph.
9. When the patch is removed, the soft tissue indentation of the pin head serves as the mark to reposition the TAD.
10. The initial film (x-ray with pin head) has pin head dots marking the soft tissue penetration points.
11. The final film shows the “Correct TADs” as placed based on the positions of the initial pin heads.
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Guided Infra-Zygomatic Screws IJOI 46
Fig. 10A:
Testing of multiple canker sore patches revealed that ComfortBrace
®
strips are a better material for the double film method
initial film (1).
1. Protective strips to cover braces have the advantage of soft tissue adhesion up to 24 hours.
2. The transparent material is packed in strips with a protective layer that is peeled off prior to application.
3. Place the pin head on the adhesive side, with the pin shaft pointed out.
4. The very sticky material is best positioned with two sets of forceps.
5. After anesthetizing the soft tissue, press the pin head attached to the strip firmly into the mucosa to produce a mark to serve
as reference for planning the penetration point for the screw.
6. The pin head is held securely by the adhesive strip in preparation for the initial radiograph.
After an extensive search, ComfortBrace® strips (
Lighthouse Point FL
), originally designed for preventing xed
appliance irritation, were selected as the ideal product because they:
1. Are clear, so the position of the pin head is readily apparent (
Fig. 10A-1
).
2. Can be cut into small 5mm rectangular pieces to be easily positioned on the attached gingiva, and do
not move (
Fig. 10A-2,3
).
3. Are made with FDA approved ingredients for use in the oral cavity: polyvinylpyrrolidone, acrylates
copolymer, sorbitol, water, hydroxyethyl cellulose, carbomer 980, and mint (
www.comfortbracestrip.com/
safety.asp
).
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Guided Infra-Zygomatic Screws IJOI 46
Indication for IZC 6
1. At least a 5mm gap is required between the roots of the U5 and the MBR of U6 to avoid root contact
with an I-R TAD.
2. Small oral cavities are often more convenient to place IZC 6 rather than placing the IZC 7.
3. A buccal frenum on or near the site can induce laceration, inammation and screw failure; fortunately,
there is usually no buccal frenum at the IZC 6 site.
4. The 5mm width of attached gingiva is adequate for most IZC 6 TADs.
5. Avoid placing TADs between the roots of teeth where the sinus oor is low because these areas usually
have low density bone and a thin cortical plate.
Fig. 10B:
Double film method using ComfortBrace
®
strips in preparation for TAD placement (2).
1. Initial film shows the dot image of the radio-opaque pin-head.
2. Peel off the clear strip of ComfortBrace
®
covering the pin head.
3. Remove the pin head, and the soft tissue indentation mark is apparent.
4. Based on the pin head reference, derived from the first film, the TAD is positioned more superior and posterior.
5. The self-drilling screw is installed in the final position.
6. Final film shows the screw positioned mesial to the MBR, as intended.
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Guided Infra-Zygomatic Screws IJOI 46
Indication of IZC 7
1. At least 5mm width of attached gingiva is
advantageous.
2. Access to the IZC 7 area requires a large oral
cavity, as well as lip reflection for adequate
access to the screw insertion site.
3. Avoid placing between the roots of teeth where
the sinus oor is low because this area has a thin
cortical plate.
Failure rate
According to a recent study by Chang et al.
12
the IZC
screw failure rate is <7%. Most of the failures are due
to:
1.Poorbonequality: Unfortunately there is no
reliable method for evaluating bone quality.
The sensation for poor bone quality, beneath
a sound layer of cortical bone, is like punching
through an egg shell, followed by a lack of screw
stability. Unless the TAD can be stabilized by
deeper penetration, it is best to remove it and
try another site.
2.Immediateloading: SS screws are excellent
TADs because they do not osseointegrate
and are easily repositioned to another site, if
necessary.
3.Sinusfloor: A low sinus between the roots of
teeth is undesirable for an IZC TAD site.
4.Movablemucosa: Unattached alveolar mucosa
at the TAD site is usually undesirable.
4,5
However,
Chang et al.
12
found no signicant dierence in
the failure rate between movable mucosa and
attached gingiva if the platform of the screw is
at least 5mm away from the soft tissue surface.
The disadvantages of the latter approach are a
longer screw is required (
~12mm
) and it must be
carefully positioned for patient comfort.
Buccal Shelf Bone Screws
The senior author (
JJ-JL
) previously introduced
mandibular buccal shelf bone screws, which were
usually placed by periodontists or oral surgeons,
using the apically positioned ap to provide attached
gingiva at the TAD site.
15
When the mandibular
buccal shelf is steep and if patients require an extra-
radicular placement, with an apically repositioned
ap of attached gingiva, an experienced surgeon is
needed. Flap surgery is more expensive and tends to
be painful postoperatively, particularly if a TAD must
be repositioned. Currently, a skillful orthodontist can
produce a good result with a self drilling screw by
using the double lm method to place the I-R buccal
shelf screw (
right mesial to the MBR of L7
). Reliable
retraction of the mandibular arch can be done.
Conclusions
1. The double film method is advantageous for
installing TADs in the three most common sites:
IZC 6, IZC 7, and mandibular buccal shelf.
2. ComfortBrace® strips have proven superior for
maintaining a pin head in a stable position
relative to the soft tissue.
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IJOI 46 LIVE FROM THE MASTER
3. The double film method is indicated for
selecting the appropriate point of entry for IZC
or buccal shelf screws.
4. Extensive experience with the double film
method has demonstrated it is an advantageous
approach for reliably placing IZC 6 & IZC 7 bone
screws to retract the dentition.
5. The double film method has significant
advantages for both the clinician and the
patient.
Acknowledgement
1. Thanks to Dr. Leslie Yen-Peng Chen for the
innovative idea leading to the current double
film method for accurately placing TADs for
orthodontic anchorage.
2. Thanks to Dr. Mala Ram Manohar for the practical
idea of using pin heads with a 1mm shaft to mark
the initial point of tissue penetration for a TAD.
3. Thanks to Dr. Po-Jung Chen for the CBCT cross-
sectional evaluation of the IZC 6 vs. IZC 7 sites as
shown in Fig. 4.
4. Thanks to Dr. Po-Jan Kuo for the CBCT information
in Fig. 5 which illustrates the buccal anatomy of
IZC 6 & 7 sites.
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th
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