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

(BQ) Part 1 book Orthodontics - Prep manual for undergraduates has contents: General considerations in treatment planning, methods of gaining space, removable appliances, removable appliances, removable appliances, removable appliances, removable appliances, preventive orthodontics, preventive orthodontics,... and other contents.

C H A P T E R

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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

❖ Various active components of removable appliance

❖ Springs used in removable appliance

❖ Hawley appliance and retainer

❖ Insertion of removable appliance

❖ Failures of removable appliance/unsuccessful removableappliance 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

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

– Schwarz expansion plates

– Anterior spring aligners (Barrer appliance)

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 thefollowing 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 missesappointment, 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 beproduced

➤ Frontal resorption is desirable

Oral hygienic aspects

➤ Appliance should be comfortable for the patient

➤ 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 byremovable appliance

➤ It is used along with fixed appliances (e.g posterior bite-block) toeliminate occlusal interferences

➤ Deep bite correction can be done satisfactorily by anterior biteplate

➤ 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 theappliance 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 caseswhich 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 ofposterior 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 mandibularappliances

➤ 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 Theyare supported either on one side or both sides Wires or appliancessupported on one side act as cantilever beams, e.g springs projectingfrom 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 analysedwith 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

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 thewire 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 toothmovement

2 Load deflection rate: It denotes the force produced per unit

activation Active members of the appliance should have low loaddeflection 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:

Designing of clasps used in

Good retention is essential for proper delivery and direction of force

by the active component

➤ 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 maximumbulge or circumference of the crown

➤ Undercuts are portion of the tooth surface which is below theheight of contour of contours the area between height of contourand the anatomical neck of the crown

➤ Clasps make use of these undercuts for retention purpose.

➤ There are differences between mesial, distal and labiolingual

➤ Circumferential clasp engages one interdental undercut and buccalundercut

➤ A line drawn from one side clasp to the other side should passthrough 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 Buccal and lingual

Begins below the contact points Less extensive

Accessible immediately after eruption Not accessible till full eruption

Clasp using this undercut is most useful Less useful

More efficient, e.g Adams’ clasp, triangular clasp 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 usagemust be possible

➤ Clasps should provide retention in partially erupted and deciduoustooth 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

Classification of clasps used in

• 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

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 distalbuccal 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

FIG 18.6. Modification in clasp design.

➤ Retentive ability is inferior when compared to Adams’ clasp

➤ Can be used only for retainers and not active removableappliances

➤ Cannot be used in partially erupted tooth

➤ Modifications are not possible for different applications likeAdams’ clasp

Triangular clasp

➤ Wired used: 0.7 mm or 21 gauge hard stainless steel wire.

Clasp design ( fig 18.7 )

➤ This is a triangular-shaped clasp with the open end of the trianglealways placed distal

➤ It is actually a single arrow on a wire crossing the contact point

➤ Apex of the triangle engages the proximal undercuts between twoteeth

➤ Usually placed between two premolars

FIG 18.7. Triangular clasp.

Advantages

➤ This clasp provides excellent retention

➤ No irritation to the gingival tissues

➤ Easily fabricated

➤ Preformed clasps are available.

➤ Triangles can be made in advance and adjusted according to thetooth

➤ Can be used to engage elastics

Disadvantages

➤ Not a versatile clasp like Adams’ clasp

➤ Mainly used for additional retention

Ball end clasps

➤ Synonym: Ball clasps

➤ Wire used: 0.7 mm or 21 gauge hard stainless steel wire

Clasp design ( fig 18.8 )

➤ Clasp extends across the embrasure between adjacent teeth andengages interdental undercut on the buccal surface

➤ End of the clasp is fabricated in the form of a ball or knob

➤ They are available as ready-made forms

➤ Instead of the ready-made ball, the end of the wire can be recurved

to make it fit into the interdental undercut

FIG 18.8. Ball end clasps.

➤ Relatively stiff because of short length

➤ Provides only limited retention

➤ Trauma to interdental papilla is possible

Duyzing’s clasp

➤ Wire used: 0.7 mm or 21 gauge hard stainless steel wire.

Clasp design ( fig 18.9 )

➤ This is made by two wires emerging from the acrylic plate, onecrosses the mesial and the other crosses the distal contact point

➤ Each wire goes above the height of contour or greatest

circumference till the middle of the tooth.

➤ From there, it is curved back upon themselves so that the lowerpart lies below the maximum contour in the undercut area

FIG 18.9. Duyzing’s clasp.

Advantages

➤ Possible to use only one-half of the clasp

➤ One-half may be made to extend fully to the anterior or posteriorpart of the tooth

➤ Does not irritate the gingiva

Disadvantages

➤ Provides limited retention

➤ Accumulation of food debris is more in this type of clasp

Lingual extension clasp

➤ Wire used: 0.5 mm or 23 gauge wire.

Clasp design ( fig 18.10 )

➤ Clasp starts from the lingual aspect of the acrylic plate

➤ The clasp extends into the lingual embrasure between the teeth

➤ Does not cross through the interdental contact area

FIG 18.10. Lingual extension clasp (A) Lingual extension clasp in place and (B) close-up view of lingual extension

clasp.

Advantages

➤ Does not interfere with occlusion

➤ Used for retention appliances

Disadvantages

➤ Adjustments are difficult

➤ Prone to breakage

➤ Causes tissue irritation.

➤ If active, can cause separation of teeth

Continuous clasps

Jackson’s clasp

➤ This clasp was introduced by Jackson VH in 1906

➤ Synonyms: Full clasp, U clasp, molar clasp.

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

Clasp design ( fig 18.11 )

➤ Palatal portion of the wire is embedded in acrylic

➤ Wire is closely adapted along the buccocervical and proximalundercuts (both mesial and distal)

➤ Placed usually for first molars

➤ Crosses interdentally on both sides of first molar to end in acrylicplate

FIG 18.11. Jackson’s clasp (A) Jackson’s clasp in place and

(B) close-up view of Jackson’s clasp.

➤ Adjustments are not easy

➤ Cannot be used in semierupted teeth

➤ Modifications are not possible.

Arrowhead clasp

Introduced by AM Schwarz

➤ Synonym: Schwarz clasp.

➤ Wire used: 0.7 mm or 21 gauge hard stainless steel wire.

Clasp design ( fig 18.12 )

➤ This is the oldest clasp

➤ The anterior arm of the clasp emerges from the acrylic plate andcrosses through the interdental area between premolar and molar

➤ From here two or three arrows are formed These arrows fit into theinterproximal area

➤ Posterior arms extend up to the last fully erupted tooth

➤ Crosses interdentally and inserted into the acrylic plate

FIG 18.12. Arrowhead clasp (A) Clasp in position and (B)

close-up view.

Advantages

➤ Because of the vast length of the wire, clasp is more elastic

➤ Adjustments can be done in the arrows individually

➤ Conveniently used in combination with posterior bite-block

➤ Facilitates further eruption of buccal teeth

Disadvantages

➤ Needs great care in formation.

➤ Requires special pliers

➤ Breakage chances are more

➤ Occupies more space

➤ Soft tissue injury is more

➤ Tends to separate the teeth

Pliers used for arrowhead clasp fabrication

➤ Arrowhead-forming pliers

➤ Tischler’s pliers

➤ Optical pliers

Southend clasp

➤ Wire used: 0.7 mm or 21 gauge hard stainless steel wire.

Clasp design ( fig 18.13 )

➤ Southend clasp extends to two adjacent margins of the teeth

➤ A small U-loop engages the interdental undercut

➤ Wire is adapted along the buccal cervical margins of the two teeth

➤ Distal ends of the clasp crossover the occlusal embrasure and isembedded into the acrylic plate

FIG 18.13. Southend clasp.

➤ Adams’ clasp was introduced by Philip Adams

➤ It is a modification of Schwarz’s arrowhead clasp

➤ Synonyms: Modified arrowhead clasp, liverpool clasp, universal

clasp

➤ Wire used: 0.7 mm or 21 gauge hard stainless steel wire, 0.6 mm is

used for canines

Clasp design ( fig 18.14 )

➤ Adams’ clasp is the most versatile clasp It engages the mesiobuccaland distobuccal undercuts of posterior teeth.

➤ Adams’ clasp consists of three parts: Arrowhead, bridge and tag onretentive part

1 Arrowhead

• The tip of the arrowhead should be placed below the greatest circumference of the tooth mesially and distally.

• If the tooth is partially erupted, the plaster should

be trimmed in the gingival papilla region This

helps in correct positioning of the arrowhead.

• Arrowheads slope to correspond with the contour

of the gingival margin into the interdental area.

• Arrowheads should be made parallel to each other.

2 Bridge

• Bridge connects the two arrowheads.

• It should be straight and not curved.

• The angulation between the bridge and the tooth surface should be 45°.

• Bridge should be fitted against the buccal surface.

• There should be a gap of 1 or 2 mm between the bridge and tooth surface.

• Bridge should be halfway between the cervical and occlusal margins of the buccal surface.

3 Tags or retentive part

• Fit closely across the contact point.

• It should not create occlusal interferences.

• Therefore, it is best adapted interdentally.

• On the palatal end of the tag, there should be space between the wire and plaster cast This facilitates proper embedding of the tags in the acrylic plate.

FIG 18.14. Adams’ clasp.

Advantages of adams’ clasp

➤ It is the most versatile of all the clasps