Ebook Orthodontics - Prep manual for undergraduates (3/E): Part 2

SECTION VI

Orthodontic Appliances
OUTLINE
18. Removable appliances
19. Functional appliances
20. Orthopaedic appliances
21. Expansion appliances
22. Fixed appliances


CHAPTER
18


Removable appliances
CHAPTER OUTLINE
❖ Definition and classification of orthodontic appliances
❖ Ideal requirements of orthodontic appliances
❖ Advantages and limitations of removable orthodontic appliances
❖ Mechanical principles involved in designing of removable
appliances
❖ Components/Various parts of removable appliances
❖ Designing of clasps used in orthodontics
❖ Classification of clasps used in orthodontic practice
❖ Free-ended clasps

❖ Circumferential clasps
❖ Triangular clasp
❖ Ball end clasps
❖ Duyzing’s clasp

❖ Lingual extension clasp
❖ Continuous clasp
❖ Jackson’s clasp
❖ Arrowhead clasp
❖ Southend clasp
❖ Adams’ clasp
❖ Delta clasp


❖ Various active components of removable appliance
❖ Springs used in removable appliance
❖ Canine retractors
❖ Labial bows
❖ Screws
❖ Elastics
❖ Orthodontic pliers
❖ Base plate
❖ Hawley appliance and retainer
❖ Insertion of removable appliance
❖ Failures of removable appliance/unsuccessful removable
appliance therapy
❖ Accessory points
❖ Advanced learning

❖ Designing of removable orthodontic
appliances
❖ Fabrication of removable appliance
❖ Treatment of various malocclusions by
removable mechanical appliances

Definition and classification of
orthodontic appliances
Definition
Orthodontic appliances are devices by means of which pressure may
be applied to a tooth or group of teeth to move them in a
predetermined direction.

Classification of orthodontic appliances
There are different methods of classification.
I. Simple classification

Appliances
• Removable
• Mechanical
• Functional
• Fixed appliances
• Mechanical
• Functional
1. Removable appliances: Appliances that are


designed to be taken from the mouth by the
patient.
2. Attached removable appliances maintain a fixed
relationship to the dentition through clasps or
other attachments.
3. Loose appliances fit imprecisely and alter the
neuromuscular activity during function; also
called functional appliances.

4. Passive appliances are used to maintain the
existing occlusion (space maintainers and
retention appliances) and to disocclude the
dentition (bite-blocks). Passive appliances do not
exert force.
5. Mechanical appliances exert force to tooth or
group of teeth through mechanical devices or
active components.
II. Detailed classification

Appliances
1. Removable appliances
• Attached


• Active
1. Headgears
2. Facemask
3. Chin cups
4. Lip bumpers
5. Active plates
– Hawley appliance
– Space regainers
– Schwarz expansion plates
– Anterior spring aligners (Barrer appliance)
– Crozat appliance
– Vacuum formed appliances (invisible appliances)
• Passive
1. Space maintainers
2. Bite planes

3. Retainers
4. Occlusal splints
5. Posterior bite-blocks
• Loose removable appliance/functional appliance
1. Activator
2. Bionator
3. Frankel
4. Twin block
2. Fixed appliances
• Mechanical
1. Edgewise
2. Begg
3. PEA or preadjusted edgewise appliance
4. Lingual orthodontic appliances
• Functional


1. Herbst
2. Jasper Jumper
3. Forsus


Ideal requirements of orthodontic
appliances
Ideal requirements of orthodontic appliances can be studied under the
following headings:

Mechanical aspects

➤ Appliance should deliver light continuous force.
➤ Appliance should have low load deflection rate.
➤ It should have high maximum elastic load.
➤ Should have control over centre of rotation.
➤ Appliance should have self-limiting force, i.e. if the patient misses
appointment, force delivery should not occur.
➤ Ease of fabrication is a requirement.
➤ Appliances should be able to withstand forces from mastication.

Biological aspects
➤ Should be able to produce tooth movement in the desired direction.
➤ Should not restrict normal growth.
➤ Function should not be interfered.
➤ Deleterious effects like root resorption, nonvitality should not be
produced.
➤ Frontal resorption is desirable.


Oral hygienic aspects
➤ Appliance should be comfortable for the patient.
➤ Easily cleansable.

Aesthetic aspects
➤ With more number of adults seeking orthodontic treatment, the
appliance should be acceptable aesthetically.
➤ Should be less visible.

Cost factor
➤ Appliance should be affordable for the patient.
➤ It should not be expensive.

Advantages and limitations of
removable orthodontic appliances
Advantages of removable appliances
➤ Malocclusions which require tipping can be efficiently treated by
removable appliance.
➤ It is used along with fixed appliances (e.g. posterior bite-block) to
eliminate occlusal interferences.
➤ Deep bite correction can be done satisfactorily by anterior bite
plate.
➤ Fabrication of removable appliance is easier.
➤ Requires less chairside time.
➤ Does not require extensive training like fixed appliance.
➤ Can be removed by the patient for cleaning of teeth.
➤ Appliance can be cleaned by the patients.
➤ When the appliance is damaged, patients can easily remove the
appliance before it causes damage to the tissues.
➤ Aesthetically more pleasing when compared to fixed appliance.
➤ Economically cheaper when compared to fixed appliance.
➤ Used for space maintenance.
➤ Used as retention appliances.


Limitations of removable appliances
➤ Removable appliances produce only tipping movement. So cases
which are already tipped cannot be treated with removable
appliance.
➤ Rotation correction cannot be achieved.
➤ Multiple tooth movements cannot be carried out.

➤ Not suitable for closure of extraction spaces by mesial movement of
posterior teeth.
➤ Difficult to correct ectopic teeth.
➤ Difficult to obtain tight proximal contact between teeth with
removable appliance.
➤ Patient tolerance is not good, more specifically in the mandibular
appliances.
➤ Patients with complex problems cannot be treated.
➤ Patient co-operation is highly important.
➤ Appliances get damaged or broken, if they are not worn.


Mechanical principles involved in
designing of orthodontic appliances
Orthodontic archwires or springs can be considered as beams. They
are supported either on one side or both sides. Wires or appliances
supported on one side act as cantilever beams, e.g. springs projecting
from the removable appliance.
Appliances supported on both sides include labial bows and
archwire (Fig. 18.1).

FIG. 18.1. (A) Cantilever beam, and (B) supported beam.

When a force is applied to a beam, its response can be analysed
with the diagram (Fig. 18.2) and definitions.


FIG. 18.2. Application of force. Relationship between stress,
strain and resilience, formability.

Force: It is an act or load applied to an object which tends to change
the position of object.

Force delivered for a given deflection
depends on the wire length (L), radius (r )
and elastic modulus (E)

➤ Increasing the size or diameter by 2 times increases the stiffness by
16 times.
➤ Increasing the length by 2 times reduces the stiffness by 8 times.
Stress: Force per unit area in a body which resists an external force.


Strain: Can be defined as the internal distortion produced by load
or stress. Strain can be elastic or plastic.
Elastic limit: It is defined as the greatest stress to which a material
can be subjected to, so that it will return to its original dimension
when the forces are released.
Proportional limit: It is the point at which permanent deformation
is first observed.
Yield strength: It is at the point at which 0.1% of deformation is
observed.
Ultimate tensile strength: Maximum load a wire can sustain. This
determines the maximum force a spring can deliver (Fig. 18.3).

FIG. 18.3. Force–deflection curve. Diagram shows the
location of proportional limit, yield point and failure point.

Springiness: This depends on the elastic or proportional limit. More
horizontal the slope, the more springiness.

Range: This is defined as the distance the wire will bend elastically


before permanent deformation occurs.
Resilience of the wire: It is the area under stress–strain curve up to
proportional limit. It represents the mechanical energy stored in the
wire. It is a combination of strength and springiness.
Formability: It is the amount of permanent deformation a wire can
withstand before it breaks.
Fatigue: This is the fracture of the wire due to repeated stress.
Effect of incorporating a coil: Introduction of a coil into a cantilever
increases the length of spring. Spring becomes more flexible.

Spring characteristics
Burstone enumerated three important features of the orthodontic
appliance:
1. Moment to force ratio
2. Load deflection rate
3. Maximal elastic moment.
These three properties put together are found within the elastic
range of an orthodontic wire. These three properties are called spring
characteristics.
1. Moment to force ratio: It determines the centre of rotation of tooth.
Varying the moment to force ratio produces different types of tooth
movement.
2. Load deflection rate: It denotes the force produced per unit
activation. Active members of the appliance should have low load
deflection rate which implies light continuous force.
3. Maximum elastic moment: It is the greatest force or moment that
can be applied to the appliance without producing permanent

deformation. This will prevent distortion of the appliance during
activation or accidental overloading during a chewing.


Components/various parts of
removable appliances
Components of the removable appliances:

Active component
1. Springs
2. Labial bows
3. Screws
4. Elastics

Fixation or retention
1. Clasps
2. Bows

Anchorage
1. Clasps
2. Contact of base plate with nonmoving part
3. Headgears
4. Intermaxillary elastic

Base plate
1. Forms the framework.


Designing of clasps used in
orthodontics

Introduction
➤ Clasps are the retentive component of removable orthodontic
appliance.
➤ Retention of appliance implies resistance to displacement of the
appliance.

Importance
Good retention is essential for proper delivery and direction of force
by the active component.
If retention is inadequate:
➤ It leads to difficulty in appliance wear and poor patient
cooperation.
➤ Active components will not work effectively, if there is poor
retention.
➤ Chances of appliance breakage are more, if the retention is less.

Principles of clasp function
➤ Height of contour of a tooth is the line encircling the maximum
bulge or circumference of the crown
➤ Undercuts are portion of the tooth surface which is below the
height of contour of contours the area between height of contour
and the anatomical neck of the crown.


➤ Clasps make use of these undercuts for retention purpose.
➤ There are differences between mesial, distal and labiolingual
undercuts (Fig. 18.4). The differences between mesial, distal and
labiolingual undercuts are enumerated in Table 18.1.
➤ Circumferential clasp engages one interdental undercut and buccal
undercut.

➤ A line drawn from one side clasp to the other side should pass
through the centre of the appliance.

FIG. 18.4. Undercuts: (A) buccal and lingual undercuts and
(B) mesial and distal undercuts.

Table 18.1.
Undercuts, mesial and distal, and buccal and lingual–differences
Mesial and distal
Begins below the contact points
Accessible immediately after eruption
Clasp using this undercut is most useful
More efficient, e.g. Adams’ clasp, triangular clasp

Buccal and lingual
Less extensive
Not accessible till full eruption
Less useful
Less efficient, e.g. Jackson’s clasp, circumferential clasp

This principle should be followed while designing an appliance.


Ideal requisites for a clasp
➤ Clasps should provide adequate resistance against displacement.
➤ Clasps should be passive. They should not produce unwanted
tooth movement.
➤ Should be easy to fabricate.
➤ Adjustments should be easy.
➤ It should not get distorted easily due to frequent removal and

insertion of the appliance.
➤ Clasps should not interfere with occlusion.
➤ Clasps should be versatile, i.e. modification according to usage
must be possible.
➤ Clasps should provide retention in partially erupted and deciduous
tooth also.
➤ They should not irritate the soft tissues.
➤ Should function as anchorage part also, if required.

Types of clasps based on using the undercuts
Clasps
1. Clasps using mesial/distal undercuts:

• Adams’ clasp
• Triangular clasp


• Ball end clasps
• Arrowhead/Schwarz clasp
• Crozat clasp
2. Using buccal/lingual undercuts:

• Jackson’s clasp
• Southend clasp
• Duyzing’s clasp
3. Using both the proximal and buccal lingual undercuts:

• ‘C’ clasp

Classification of clasps used in
orthodontic practice
The various clasps used in removable orthodontic appliances can be
classified under two broad headings. They are:
1. Free-ended clasps (one end of the clasp embedded in acrylic)

• C clasp
• Triangular clasp
• Ball end clasp
• Duyzing’s clasp
• Crozat clasp
• Arrow pin clasp
• Wrought Roach clasp
• Visick’s clasp
• Lingual extension clasps
2. Continuous clasp (both ends of the clasp embedded in acrylic)

• Jackson’s clasp


• Arrowhead clasp
• Eyelet clasp
• Adams’ clasp
• Delta clasp
• Southend clasp


Free-ended clasps
Circumferential clasps
➤ Synonyms: ‘C’ clasps, three-quarter clasps.

➤ Wire used: 19 gauges or 0.9 mm hard stainless steel wire.

Clasp design (fig. 18.5)
➤ Palatal portion of the wire is embedded in acrylic.
➤ From the palatal, wire is bent in interdental undercut between
second premolar and first molar.
➤ Wire passes buccogingivally below the undercut towards the distal
buccal interdental undercut of first molar where it ends.
➤ Utilizes one proximal undercut and buccal undercut.

FIG. 18.5. Circumferential clasps extending into the
distobuccal undercut.

Modifications (fig. 18.6)
➤ The ‘C’ clasp can be modified to engage the mesial proximal
undercut.