This document discusses the creative use of elastics in orthodontic treatment. It begins by introducing elastics as simple yet routine appliances. It then discusses different types of intra-arch and interarch elastics and how they are used at different stages of treatment. The document focuses on maxillomandibular elastics and how they can be used to correct problems, enhance anchorage, and directly address intra-arch or interarch issues. It provides numerous diagrams to illustrate how different elastic placements produce different orthodontic movements through equivalent force systems and moments applied to the dental arches.

1. Creative use of Elastics
in Orthodontics
Elastomeric rings, more commonly called elastics, are routine in orthodontic
treatment and are some of the simplest appliances used. Unfortunately,
however, the mode of operation and the proper selection of
maxillomandibular elastics (also referred to as intermaxillary elastics) are not
well known orpracticed clinically. Different types of elastics are used either
within the same arch (intra-arch elastics) or between arches
(maxillomandibular elastics). One orthodontic sequence is to accomplish intra-
arch alignment early and then later coordinate the arches that do not fit
either because of side effects during intra-arch alignment or because of
remaining original discrepancies. Commonly, maxillomandibular elastics are
needed to correct problems introduced during early states of treatment.
Maxillomandibular elastics can also be used directly to enhance anchorage or
correct intra-arch or maxillomandibular problems.
Dr. Ishfaq Ahmad
BDS, BCS, MCPS, MS
Rajshahi Medical College,
Dental Unite
Long Class II elastic. (a and b) A single force from the elastic (red arrow) is
replaced with an equivalent force system (yellow arrows) at the CRs of the
maxillary and mandibular arches. (c) Both arches rotate synchronously (dotted
curved arrows) in the same clockwise direction because of the same magnitude
and direction of the moments (D1 = D2). (d) Lateral superimposition of
cephalometric radiographs before (black) and after (red) long-term use of Class
II maxillomandibular elastics in an extraction case using round wire. Both the
maxillary and mandibular arches rotated in a clockwise direction, with extrusion
of the maxillary anterior teeth and mandibular posterior teeth.
Short Class II elastic. A single force
from the elastic (red arrows) is
replaced with an equivalent force
system (yellow arrows) at the CR of
each arch. It is also synchronous
because the CR is an equal distance
from the force in each arch (D1 = D2).
The moment is lower and the vertical
component of force is greater than that
with the long Class II elastic.
Anterior vertical elastic. The maxillary
and mandibular arches will rotate in
opposite directions, leading to an
increase in vertical overlap. Most of
the rotation will occur in the maxillary
arch (D1 > D2).
Short Class II elastic placed posteriorly.
(a and b) A single force from the
elastic (red arrow) is replaced with an
equivalent force system (yellow
arrows) at the CR of the mandibular
arch. (c) The moment in respect to the
CR will be different for each arch;
therefore, only the mandibular arch
rotates asynchronously.
Short Class II elastic placed anteriorly.
(a and b) A single force from the
elastic (red arrow) is replaced with an
equivalent force system (yellow
arrows) at the CR of the maxillary
arch. (c) The moment in respect to
the CR will be different for each arch;
therefore, only the maxillary arch
rotates asynchronously
Short Class II elastic force (red arrow) placed anteriorly in a Class II open bite case.
An equivalent force system at the CR of the maxillary arch (yellow arrows)
indicates that a large moment is only produced in the maxillary arch, closing the
open bite and reducing the Class II malocclusion. The cant of the mandibular
plane of occlusion will not change.
Short Class II elastic placed posteriorly in a Class II deep bite case. (a) The single
force (red arrow) is through the CR of the maxillary arch. (b) An equivalent force
system at the CR of the mandibular arch (yellow arrows) produces a large
moment, opening the bite and reducing the vertical overlap. The cant of the
maxillary occlusal plane will not change.
Frontal view of the long Class II elastic
shown in Fig 5-1. The replaced equivalent
force system (yellow arrows) at the CR
shows that the mandibular second molar
(terminal molar) will move in a superior
direction; at the same time, the moment
produced by the elastic at the CR will tip
the molar crown lingually.
Posterior crisscross elastic in proximal view. (a) Force magnitude and direction can
vary depending on jaw opening and hook placement. (b) For simplicity, we will
assume the line of action to be an arbitrary line connecting the points at the
brackets or hooks where the elastic is attached.
Unilateral posterior crisscross elastic in a continuous arch. (a) The forces from
the crisscross elastic (red arrows) are replaced with equivalent force systems at
the CRs (yellow arrows). (b) Asynchronous occlusal plane effects causing an open
bite on the left side are anticipated.
(a) To balance the moment created by
the unilateral posterior crisscross
elastic in Fig 5-11, a vertical elastic
force (red arrow) is applied on the left
side. The vertical elastic’s equivalent
force system at the CR (yellow arrows)
is equal and opposite to the moment
from the crisscross elastic on the right
side. (b) The yellow arrows are the
resultant from the two elastics. (c)
The resultant force is replaced with
the equivalent force system at each
CR (yellow arrows).
Bilateral crisscross elastics. (a) The resultant (yellow arrows) of the two forces of the
crisscross elastics (red arrows) lies an equal distance (D1 = D2) from the maxillary and
mandibular CRs. (b) The equivalent force systems at the CRs of each arch have
synchronous moments (yellow arrows), rotating the maxillary and mandibular arches
equally so that no lateral open bite will be produced.
Changing the point of force application
of a single force (red arrows) from
crisscross elastics in a continuous arch.
Arch rotation (occlusal view) will be
produced unless the force passes
through the CR. A, clockwise rotation; B,
translation; C, counterclockwise rotation
produced with various positions of the
force.
The location of a crisscross elastic force (red arrow) and arch rotation (occlusal
view). The equivalent force system at the CR is represented by yellow arrows. (a) An
anterior crisscross elastic rotates the arch in a clockwise direction. (b) An elastic placed
at the first molar rotates the arch in a counterclockwise direction.
Anterior crisscross elastic (red arrows).
Equivalent force systems at the maxillary
and mandibular CRs are represented by
yellow arrows. The force system would
aid in midline correction; however, the
canting from the moments of the
maxillary and mandibular occlusal
planes is unavoidable.
Predicted treatment result from an
anterior synchronous crisscross elastic.
The canting of the frontal occlusal planes
would be particularly unesthetic.
Anterior crisscross elastic (red arrows) placed off-center. Equivalent force systems
(yellow arrows) at the CRs show a maxillary occlusal plane that rotates very little (a)
and not at all (b) because D1 is very small. Therefore, the cant of the maxillary
occlusal plane will be maintained. An open bite may occur on the right side because
of the counterclockwise rotation of the mandibular arch due to the large moment at
the mandibular CR (large D2).
Various locations of vertical elastics.
Canting of the occlusal plane will be
produced unless the force is passing
through the CR. A, counterclockwise
rotation; B, no rotation; C, clockwise
rotation.
Vertical elastic placed off-center. The
equivalent force system at the CR
produces an open bite on the left side.
Unilateral Class II elastic. (a) Occlusal view of the maxillary arch showing an
equivalent force system at the CR (yellow arrows). The maxillary arch tends to
rotate around the CR and can produce a buccal reverse articulation. (b) Frontal
view showing an equivalent force system at the CR (yellow arrows), producing an
open bite on the left side. The mandibular arch rotates more because D2 > D1.
(a) Unilateral Class II elastic with a vertical elastic on the opposite side. (b) The
magnitude of the moment at the CR from the left vertical elastic force (purple
arrows) is set to balance the moment (equal and opposite) from the right Class II
elastic (yellow arrows). It may prevent the rotation of the maxillary arch, but the
mandibular moment still exists. (c) Unilateral Class II elastic with a posterior
vertical elastic. The horizontal component of force from the Class II elastic
produces an unwanted lateral reverse articulation. (d) Unilateral Class II elastic
with a posterior crisscross elastic on the opposite side. (e) The posterior
crisscross elastic on the maxillary left side with a downward lingual direction
could produce a synchronous occlusal plane change without an open bite. An
equivalent force system is replaced at the CR (Class II elastic,yellow arrows;
crisscross elastic, purple arrows). (f) Unilateral Class II elastic with a posterior
crisscross elastic. The added component of force from the crisscross elastic
might produce a greater crossbite. (g) A unilateral Class II elastic with a
crisscross elastic from the maxillary palatal to the mandibular buccal. (h) There
will be no occlusal plane rotation in the frontal view, but extrusive forces are
increased. (i) Occlusal view of the force system of the unilateral Class II elastic
and crisscross elastic. There is no lateral component of force. The moment will
probably be ineffective to rotate the entire arch, but it operates in the correct
direction.
(a) Class II elastic on the right side and Class III elastic on the left side. (b) The
resultant forces (yellow arrows) show that both arches will rotate
synchronously in a clockwise direction.
Class II elastic on the right side and Class III elastic on the left side (occlusal view
of the maxillary arch). (a) The purple arrowsare the replaced equivalent force s
ystem of the Class III elastic on the left side, and the yellow arrows are the
replaced equivalent force system of the Class II elastic on the right side. (b) The
final resultant of each force system at the CR is depicted in yellow arrows. It
shows not only a couple but also a lateral and posterior force.