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Principles of removable appliances
Terms in this set (23)
Describe the difference between passive and active appliances
Passive - maintain the position of teeth e.g. Space maintainers
Active - bring about tooth mvt by incorporating active forces within appliance
List the advantages and disadvantages of removable appliances.
Advantages - removable and easy for patient to clean; difficult to apply excessive forces; constructed in a lab, little chairside time
Disadvantages - limited tooth movements possible (tipping mvt only); anchorage may be difficult; retention is more difficult; need cooperation + skill from patient; less tolerable therefore less successful in lower arch
What are the components of removable appliances?
Define the active component of a removable appliance - and give 3 examples
Means by which the forces are applied to teeth to bring about the required mvt
Examples include - springs, bows, screws
Define retention and describe 3 examples
Means by which appliance resists displacement
1. Adams clasps - used on 1st molars, constructed on 0.7mm hard SS wire with arrowheads to engage undercuts on MB and DB aspects of molar, or on premolars for additional retention (can be adjusted with Adams pliers)
2. Southend clasp - for anterior retention and construction in 0.7mm wire
3. Fitted labial bow - additional retention anteriorly, construction in 0.7mm wire and only effective for proclined incisors
Define anchorage and the 3 types of anchorage
Site from which forces are applied, works on Newton's third law "for every action is an equal and opposite reaction" - stabilises wanted movement by balancing it with surface area of roots
1. Simple anchorage - using a large tooth to move a small one
2. Compound anchorage - using a group of teeth as anchorage
3. Reciprocal anchorage - e.g. upper centrals anchoring each other
Describe the baseplate component of a removable appliance.
Made in acrylic, the baseplate supports the wire components, contributes to anchorage, prevents unwanted tooth drifting, transmits forces from active component and may be extended into bite planes (anterior or posterior)
What is the commonest active component? How does it work?
Springs constructed in SS wire
Continuous force applied with correct magnitude exerted over a full range of tooth movement.
What physics equation does the spring work off? What is the width of palatal and buccal springs and how much activation (in mm) do they require?
F = dr4/L3
D = deflection of spring
R = radius of wire
L= length of wire
Increasing the length (incorporation of coils) and decreasing radius can reduce force
Width of palatal springs - 0.5mm, requiring 3mm activation
Width of buccal springs - 0.7 mm, requiring 1mm activation
For a single rooted tooth, what is the correct force application? What happens when force is excessive vs. too minimal
Excessive force - delary tooth movement and cause discomfort
Too little - inadequate rate of tooth movement
What is the ideal rate of tooth movement that can be achieved?
1-2mm per month (with adjustments to springs made monthly)
List 3 different types of palatal springs and 1 type of buccal spring.
Palatal springs - finger spring, Z spring, T spring
Buccal spring - buccal canine retractor
Describe the design and working of a palatal finger spring?
Constructed in 0.5mm wire - a coil is added to increase length and the wire uncoils as the tooth moves. It is activated at junction of coil and free arm. It often needs protection with a guard.
Can canine retraction be achieved with removable appliances?
No - bodily movement is required to prevent gross tilting
Describe the design and working of a palatal Z spring?
Constructed in 0.5mm wire and placed perpendicular to the palatal surface of a tooth to aid proclination of incisors
Describe the design and working of a palatal T spring?
Constructed in 0.5mm wire, pulls springs away from baseplate can move premolars buccally, can vertically displace and therefore needs good retention.
Describe the design and working of a palatal coffin spring.
Construction in heavy wire (1-25mm) to cause upper transverse palatal expansion.
Describe the design and working of a buccal canine retractor. Describe the disadvantages.
Constructed in 0.7mm wire, activated by 1mm, for tilting movement. Disadvantages - may be uncomfortable, difficult to adjust, less flexible than finger spring
What's the difference between active, passive, flexible and less flexible retractors? Name 2 types of bow retractors.
Passive - retainers
Active - retracting incisors
Flexible - reduce overjet
Less flexible - reduce moderate overjet or irregularities
Bow retractors - Roberts
Labial bow with U loops
Describe the Roberts retractor construction, advantages and disadvantages.
A flexible bow constructed in 0.5mm wire with buccal arms sheathed in tubing to provide additional strength; requires 3mm activation with the bow halfway up the incisors and baseplate cut away palatally to accelerate tooth movement.
Advantages - light controlled force, easily adjustable, very flexible bow can be used to reduce large overjets and does not slide gingivally up proclined incisors
Disadvantages - may cause trauma to gingival sulcus, if it breaks it needs to be reconstructed
Describe the Labial bow with U loops retractor construction, advantages and disadvantages.
A less flexible bow constructed in 0.7mm, with 1mm activation by closing U loops
Advantages - suitable for minor overjet alignment or alignment of irregular incisors, suitable as a rigid retainer (Hawley retainer)
Disadvantages - unsuitable for large overjet reduction, small activation produces large force
Describe the screw type appliance, how it works and its disadvantages.
Applies a large intermittent force through a series of small activations of 0.2mm to move individual or groups of teeth, at a rate of 1-2mm per month, by turning the screw 1-2 times per week.
Disadvantages - expensive, bulky and requires pt cooperation.
What are 5 clinical indications for use of a screw type appliance?
1. Bilateral expansion of buccal segments
2. Unilateral expansion of individual buccal segment
3. Buccal movement of individual tooth
4. Distal movement of a molar tooth
5.Labial movement of upper incisors
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