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MiniScrews
A new North Star for orthodontics
By Björn Ludwig, DMD, MSD;
Bettina Glasl, DMD, MSD;
Thomas Lietz, DMD;
Joerg Lisson, DDS, PhD; and
S. Jay Bowman, DMD, MSD
A
ncient mariners used the constellations and their
individual component stars as landmarks from
which to navigate, often in uncharted waters, as they discovered new horizons during their journeys. The advent of
miniscrew anchorage provides a new reference, a kind of
Polaris or “North Star,” that allows today’s orthodontist to
plot courses of treatment that may have been previously unpredictable or even impossible with traditional mechanics.
In view of the plethora of publications, continuing
education courses, and marketing materials available to
our specialty on the subject of skeletal anchorage, it
would seem that the use of miniscrews has become ubiquitous. The reality is much different. In actuality, very few
orthodontists have yet to embrace miniscrew use in daily
clinical practice. Those who have set sail into this unfamiliar territory have employed them sparingly, and many
have elected to have someone else insert them for their
patients. It is the authors’ intended purpose to encourage
hesitant practitioners to come aboard and incorporate
miniscrews on a more routine basis.
are discussing support for orthodontic tooth movement,
then forces that are applied between or across teeth will
act on all of the dental units involved, perhaps resulting in
unintended tooth movement of the so-called “anchor”
teeth. The extent of this movement and also the countermovement depends upon the anchorage support derived
from the individual teeth. In other words, dental anchorage is based on the number of teeth, the length of their
roots, the root surface areas, and the structure of the supporting bone surrounding the teeth.
Anchorage quality in orthodontics can be divided
into three categories: minimum anchorage, medium anchorage, and maximum anchorage. These three categories may be easily described with the example of
conventional canine retraction after removal of a first
premolar (Figure 1). If only the individual teeth involved
in the mechanism provide support, you have minimal
anchorage. Figure 1A demonstrates that a single premolar will not provide sufficient anchorage to retract the canine; rather, the premolar will likely be moved mesially,
and there will be anchorage loss. Figure 1B shows two
Anchorage Concepts
equal anchorage segments where action and reaction are
In order to move an object, some type of anchorage
comparable, and the result is medium anchorage and reor countersupport is required to stabilize a force applied
ciprocal tooth movement. In Figure 1C, the posterior
to that object. In his Third Law, Sir Isaac Newton specified
group of teeth is secured and stabilized with a miniscrew
that every action has an equal and opposite reaction. If we
(indirect anchorage). The canine can then be retracted
without mesial movement of the posterior
teeth as the reactive force is completely absorbed by the anchorage block. This is maximum anchorage.
Apart from the quality of the anchorage,
the location of the anchorage support is also
an important factor. A multitude of different
anchorage devices have been employed
throughout the first century of our specialty.
1C
1B
1A
Methods of dental or desmodontal supFigures 1A–1C: Anchorage quality demonstrated by canine retraction mechanics.
port have included the following:
Expected results for a) minimum; b) medium (reciprocal); and c) maximum anchorage.
•
intraoral, intra-arch devices (such as
34 OrthodonticProductsOnline.com April/May 2008
the Nance holding arch, transpalatal arch, lingual arch,
and lip bumper);
• modification of fixed appliances (buccal root torque to
move roots into the cortical plate, blocking groups of
teeth together, tip-backs or “setting anchorage,” uprighting springs and auxiliaries); and
• interarch mechanics (Class II or III elastics, fixed functional appliances).
Extraoral support has been provided by headgear and
face masks.
The tools of skeletal anchorage support include implants, on-plants, and miniscrews.
This article describes anchorage only within bony
structures; therefore, the terms “skeletal” and “cortical” anchorage are used interchangeably.
History and Overview of Skeletal Anchorage
The era of skeletal anchorage began in 1945 when
Gainsforth and Higley1 inserted screws into jawbone. Many experiments were unsuccessful, and the method had become virtually obsolete by the late 1970s. Starting again in 1980, various
research groups (such as Creekmore and Eklund,2 Roberts et
al,3-5 and Turley et al6,7) took up the subject once more.
Creekmore and Eklund2 published the first clinically successful
patient treatment case in which a metal post served as anchorage to intrude maxillary incisors and reduce an overbite.
There are now numerous options for cortical anchorage,
ranging from the use of (artificial or pathologically) ankylosed teeth to miniplates (adapted from orthognathic surgery) and even prosthetic osseointegrated implants (Figure
2). Wehrbein and co-workers8-10 were the first to present an
implant system specially designed for orthodontics (the
Orthosystem from Straumann). Another example, the
Midplant (HDC), was designed, like the Orthosystem,
to be inserted into the palate. Both methods have been
demonstrated to be safe and successful.
In recent years, the requirements for cortical anchorage techniques have been carefully defined in the
literature.11-14 Under closer inspection, miniscrews appear to have the edge compared to other methods due
3A
to numerous advantages:
• biocompatibility;
• small size;
• simplicity of insertion and use;
• favorable primary stability;
• capacity for immediate loading;
• adequate resistance against orthodontic forces;
3C
• ability to enhance typical orthodontic mechanics, often independent from patient cooperation;
• improved treatment effectiveness and efficiency;
• ease of removal; and
• cost-effectiveness.
Miniscrew Properties
All types of skeletal anchorage (including miniscrews) are, by definition, implants. It is important,
3D
Figure 2: The range of cortical anchorage options.
however, to differentiate miniscrews from typical dental
implants.
More than 30 different terms for skeletal temporary
anchorage screws are in use in the international literature.
The most common of these are mini-implant, mini-pin,
miniscrew, or TAD (temporary anchorage device). The
term “miniscrew” appears to provide the most accurate
(and most palatable) description of these “miniature
screws,” especially when discussing their use with prospective patients (Figure 3). There are also more than 30 manufacturers of miniscrew systems (Figure 4, page 40), with the
number of different screws offered per system ranging from
two to 154. It can be an overwhelming and perplexing
process to sort through all of these options and select those
3B
Figures 3A–3D: Examples of
miniscrew-supported mechanics
for predictable unilateral space
closure. Miniscrews prevented
the typical reactive side effect of
midline shift or anterior
anchorage loss that would have
occurred with traditional
orthodontic mechanics.
April/May 2008 OrthodonticProductsOnline.com 39
devices that are needed for
daily practice. The following is an overview of the
most important decisionmaking criteria for choosing a miniscrew system.
definitive answer as to the
amount of bone required,
it appears that from between 0.5 mm to 2 mm is
necessary for stabilization
to reduce premature loss.
The amount of bone beFigure 4: Eight examples of the more than 30 miniscrews that are
currently available. From left to right: Ortho Easy from Forestadent,
Materials
tween the roots of teeth,
Miniscrews are typical- Aarhus from American Orthodontics, AbsoAnchor from Dentos, Dual
therefore, defines the maxly fabricated from pure tita- Top from Jell Medical, LomasMondeal, Osas from Dewimed, Spider
imum diameter of screw
Screw from HDC, and tomas from Dentaurum.
nium or from an alloy of
that can be used in a partitanium with aluminum or vanadium. The safe biocomticular site. In short, the total distance between roots
patibility of these materials when in direct contact with
should be at least 1 mm greater than the diameter of the
bone has been clearly proven in the literature.
chosen miniscrew to provide adequate bone support.
It is preferable to place miniscrews through attached
Osseointegration
mucosa. Not only is the insertion easier, but there is also less
Branemark15 was the first to define the concept of osrisk of tissue damage and subsequent tissue irritation or
seointegration. He described it as “a direct functional and
overgrowth of the implant. Publications by Poggio et al,24
structural link between living bone tissue and the surface
Schnelle et al,25 and Costa et al26-27 provide some clues as to the
15-17
of a force-absorbing implant.” Several authors have invertical distance that is typically available between the cedicated that there is no intention to anchor miniscrews by
mentoenamel junction (CEJ) and the mucogingival junction
osseointegration; rather, primary retention is described as
(MGJ). These investigations clearly show that the diameter of
an interradicular miniscrew should not exceed 1.6 mm. An
exception may be miniscrews inserted into the infrazygomatic ridge or on the lingual alveolus between the maxillary first
molar and the second premolar. This, in fact, is the site with
the widest interradicular space28 (Figure 7, page 42).
Miniscrew Lengths
The length of the various miniscrews on the market
ranges from 5 mm to 14 mm. Typically, the length of the
miniscrew refers only to the shaft or shank (the threaded
section). As with the diameter, the selection of the length of
a miniscrew is dependent upon the amount of bone availFigure 5: Loads to miniscrews as determined by diameter.
able. Depending on the region, the total thickness of the
alveolus is between 4 mm and 16 mm.29
The length of a screw, however, is of secondary impora “skeletal resistance block.”18,19 In the opinion of Cope20
21
and Bumann and co-workers, miniscrews are specifically
tance when it comes to secure anchorage—the diameter is
anchored by mechanical stabilization and not by osseoinmuch more critical. Various investigations have shown that
tegration. Since typical dental analogue implants and
the thickness of the cortical plate plays the most important
miniscrews are fabricated from the same materials, there
role in miniscrew stability.30-32 For example, FEM analyses
appears to be nothing to preclude osseointegration, except
have demonstrated that the typical orthodontic load is apthe lack of surface coating and the smaller surface area of
plied only in the region of the cortical bone.33,34
the miniscrew.
When selecting the length of a miniscrew, the depth
of the gingiva must also be considMiniscrew Diameters
ered. The average depth of the ginThe stability of a miniscrew in the
giva is about 1.25 mm except in a
bone depends primarily on its diamefew locations such as the retromolar
ter, and not on its length (Figure 5).22-23
region. The ratio between the length
The diameter of available miniscrews
of the head (the part of the minisvaries between 1.2 mm and 2.3 mm. In
crew outside the bone) and the
length of the threaded shaft (the
this case, “diameter” is the outside dipart of the screw inside the bone)
ameter of the threads. For safe and seshould be at least 1:1. Consequently,
cure primary mechanical stabilization,
Poggio et al24 recommended minisa minimum amount of bone is re- Figure 6: A radiograph demonstrating the
crews lengths of 6 mm to 8 mm, and
quired around the shank of the minis- range of available interradicular space for
Costa26,27 suggested that miniscrews
crew (Figure 6). Although there is no placement of a miniscrew.
40 OrthodonticProductsOnline.com April/May 2008
potential access route for microorganisms, thereby posing a risk of perimucositis or peri-implantitis, one of the
main causes of premature loss of
miniscrews.36-37 During the immediate
postoperative phase, the mucosa
should adapt as closely as possible to
the screw to seal the area.38 The most
advantageous shape for that to occur
appears to that of a cone, as this shape
naturally results in a safe seal without
a pressure zone. A cone-shaped transFigure 7: Larger-diameter (2-mm) miniscrews may be inserted between the maxillary first
gingival collar appears to seal the mumolars and second premolars to provide more stable anchorage support for molar distalization
cosal perforation wound, much as a
(without anchorage loss that can result from standard dental anchorage) for the Horseshoe Jet
cork seals a bottle, and it may help to
(a modification of the Distal Jet from AOA Laboratories, Racine, Wis). Since the roots of the
reduce bleeding. Some miniscrews
second premolars are angled toward the buccal, they will bypass 8-mm miniscrews that are
feature no transgingival collar so that
inserted at an angle to the cortical bone. This precludes the need to reposition miniscrews after
the screw resembles a pan-head screw.
the molars have been distalized (as is required when they are inserted into the buccal alveolus).
The distal set-screw is then locked to convert the Horseshoe Jet to a miniscrew supported
These types of miniscrews may result
holding arch, and retraction of the remaining teeth is initiated.
in a void between the screw and the
soft tissue that could provide a site for
should be from 6 mm to 10 mm. On the basis of these in- plaque or debris accumulation.
vestigations, it would appear that longer screws are unnecessary except in unusual circumstances. This has also been Simplification of Inventory
confirmed by numerous, anecdotal clinical experiences.
It seems readily apparent that choosing a specific
Color-coding of miniscrews for different lengths or di- miniscrew that could be used for the majority of clinical apameters helps to facilitate the selection process. This color- plications would simplify the process of incorporating
coding is accomplished by an anodized surface coating them into daily practice. Miniscrews with a head design fea(such as that used by Forestadent’s Ortho
turing either some type of cross-slot or
Easy). This oxide layer also appears to en- For more information on miniscrews,
tube can be used for both direct and inplease see Mini-implants in Orthodontics:
hance retention of the miniscrew.35
direct anchorage mechanics. In addition,
Innovative Anchorage Concepts, edited by
screws of 1.4 mm to 1.6 mm in diameter
Ludwig, Baumgaertel, and Bowman, and
Miniscrew Head Design
provide for adequate strength and suffipublished by Quintessence.
Some suppliers have a special head
cient primary stability (dependent upon
variation for every potential orthodontic application in- available bone), and only upon occasion will larger-diamecluding:
ter screws be needed (for placement in the infrazygomatic
• hook tops;
ridge or lingual alveolus between the upper first molars and
• ball-shaped heads;
second premolars). Miniscrews 8 mm in lengths are suit• eyelets;
able for the majority of applications; however, a few screws
• simple slots;
of 6 mm and 10 mm length could be kept on hand for spe• cross-shaped slots; and
cific locations.28 These guidelines should help you to
streamline your selection process and maintain a cost-ef• universal heads.
fective inventory.
The screw head must be very small and compact (low
profile) to ensure that the patient experiences
minimal discomfort and tissue irritation (Figure
8). On the other hand, the head must also be
large enough so that the coupling elements (such
as coil springs, wire segments, auxiliaries, and
elastic chains) can be easily and securely fastened
to it (Figure 9, page 44).
Transgingival Portion
The transgingival portion, also known as
the gingival neck, is a most important part of a
miniscrew. Perforation of the gingiva occurs
when a miniscrew is inserted. This provides a
8A
8B
Figures 8A and 8B: Height or profile difference for two clinical situations.
41 OrthodonticProductsOnline.com April/May 2008
October 2007 OrthodonticProductsOnline.com 41
9A
9C
9B
9D
Figures 9A–9D: For practical reasons, it may be advisable to use
miniscrew systems that offer a single, universally applicable head
design. It would seem feasible to connect all types of force
elements (elastic threads, elastic chains, round wires, square
wires, coil springs, auxiliaries, etc) to one ideal head design,
facilitating the application of direct and/or indirect anchorage
support to the same miniscrew design.
It has been more than 75 years since the concept of
skeletal anchorage was conceived, but now the imagination
and inventiveness of today’s orthodontists are making its
application feasible, safe, and reliable for daily clinical use.
Certainly, significant due diligence and careful continuing
education are a necessity prior to adopting miniscrew anchorage. Perhaps miniscrew anchorage will soon be seen as
a new reference point or “North Star” on which a constellation of predictable orthodontic mechanics may be based. z
Björn Ludwig, DMD, MSD, is in private practice in TrabenTrarbach, Germany. He can be reached at bludwig@
kieferorthopaedie-mosel.de.
Bettina Glasl, DMD, MSD, is in private practice in TrabenTrarbach, Germany. She can be reached at bglasl@
kieferorthopaedie-mosel.de.
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Conclusions
Although the application of skeletal anchorage concepts Thomas Lietz, DMD, is in private practice in Neulingen,
to traditional orthodontic mechanics has opened new vistas Germany.
of treatment predictability, effectiveness, and efficiency, universal adoption of miniscrews throughout our specialty will Joerg Lisson, DDS, PhD, is the head of the orthodontic deendure some growing pains. Although miniscrews are not partment at the University of Homburg/Saar, Germany.
vital for the majority of patient care, they can certainly imS. Jay Bowman, DMD, MSD, is
prove many aspects of specifReferences for this article are
in private practice in Portage,
ic treatment mechanics and
available with the online version at
Mich. He can be reached at
help to reduce reliance upon
O r t h o d o n t i c P r o d u c t s O n l i n e . c o m.
[email protected].
patient compliance.
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