Ortho quiz questions

Question 1

What is a hawley retainer and what are its components? When would you use it?

Answer 1

A Hawley retainer is an active removable maxillary appliance commonly used in contemporary orthodontics following fixed orthodontic treatment
• Comprised of the following:
o Clasps on molar teeth
 Adams
 Ball
 Circumferential
o Labial bow to control incisors
 Spans 3-3
 Includes U-loop for adjustment
• Passes interproximally through distal embrasure of canine
o Acrylic over the palate
 Can incorporate a bite plate lingual to upper incisors to control overbite
o Finger spring to activate tipping force
 allows for minor adjustments at the finishing stage or to correct relapse e.g.
• proclining incisors
• correcting localised cross-bite to create a better line of occlusion

Question 2

What are the uses and methods of rapid maxillary expansion? What age should it be used?

Answer 2

2. Rapid maxillary expander (RME) – device used to correct posterior cross-bites in cases where there is a transverse maxillary deficiency
   • Case selection
   o The morphology of midpalatal suture in human autopsy material has guided the timing of expansion treatment
    Infants and young children: open, straight-line structure
    Early mixed dentition: convolutions
   • Pre-adolescent  expansion lingual arches (W-arch or quad helix)
   • NB RME not indicated as it also affects structures higher up in the skeletal midface  expansion of nasal bridge  not recommended in young children because of risk of distorting pliable face structures
    Late childhood/early adolescence: interdigitation of suture  more tortuous at ~ age 10, so would need to microfracture bone spicules along the suture line if commencing treatment at this point
   • Late adolescent  MRE, because heavy force required to open
    After adolescent growth spurt: suture completely fused
   • Surgical intervention required
   • Principle
   o Activation of expansion screw: 0.5-1 mm/day
    Applies force to posterior teeth  force transferred to the midpalatal suture  rapid opening  max skeletal changes, but insufficient time for dental changes through tipping of max posterior teeth  only move minimally relative to the supporting bone
   • Initially, the midsagittal space created is filled with haemorrhage and tissue fluids
   • NB Widens more rapidly anteriorly (due to buttressing of bone in posterior maxilla)
    Can achieve >10 mm bony expansion in 2-3 weeks, but it also creates
   • Midline diastema
   • Temporary speech interference
    However, 3-4 months of retention are required
   • Allows filing of suture with organised bone, or else the stretched palatal mucosa will cause skeletal relapse during stabilisation and healing
   o Dental expansion maintained, but two halves of maxilla move back toward each other because the teeth move laterally on their supporting bone
   o Can also use the same appliances to conduct slow expansion if the interval between activation is increased
    0.25 mm/ ¼ turn every other day  more physiological response (bone laid down at rate equal to the expansion of the suture)
   • No midline diastema or haemorrhage created
   o Both approaches produce similar results (50% skeletal and 50% dental expansion)

Question 3

What is the difference between skeletal and dental crossbites and how can you tell the difference?

Answer 3

A cross-bite is a condition in which single or multiple anterior and posterior teeth occlude against the opposing teeth in an abnormal anteroposterior or transverse relationship, respectively.

• Posterior cross-bite: max posterior teeth occlude lingual to the mandibular posterior teeth
o Thus, can be diagnosed by assessing jaw widths on dental casts
 Skeletal posterior crossbite can be caused by
• Narrow maxilla and palatal vault and/or wide mandible
o Maxillary teeth will be buccally inclined
• OR Mismatch of upper and lower arches
• Mandibular asymmetry
o True
o Due to dental interferences that result in a lateral shift  unilateral crossbite
 If arches are of normal width and are relatively well-matched, more likely to have a dental cause
• Maxillary teeth will tilt lingually

• Anterior cross-bite: max incisors occlude lingual to the mandibular incisors
o Can be identified by assessing facial profile via facial form analysis
o Skeletal anterior cross-bites
 Exist due to either
• Class III skeletal malocclusion caused by
o maxillary deficiency and/or mandibular excess, but jaws have normal transverse proportions
 may also result in posterior crossbite!
• OR a vertically deficient maxilla with mandible rotated up and forward
 Teeth = reasonably well-related to their supporting bone
• However, dental compensations are frequent
• If child presents with multiple teeth in cross-bite, likely to be skeletal
o Dental anterior cross-bites
 Teeth are displaced relative to their supporting bone, but the skeletal bases are in normal positions
• i.e. mandibular teeth are anteriorly positioned relative to the mandible AND/OR maxillary teeth positioned posteriorly cf maxilla
 Usually due to deflection of the path of eruption of teeth because of crowding within the dental arch
• Thus, lack of space for permanent incisors is the most common aetiological factor in the development of dental anterior cross-bite
o Permanent tooth buds for max incisors develop palatally to 1* incisors
o If shortage of space  permanent teeth remain palatally
 Can also be due to lingual tipping of incisors e.g. due to trauma/malpositioned tooth buds independently of crowding
 Single or multiple teeth may be involved

Question 4

What are the types of anchorage?

Answer 4

• As per Newton’s Third Law, the activation of an appliance generates an equal and opposite reaction force. Anchorage provides resistance to unwanted tooth movement.
• Sources of anchorage
o Other teeth (usually)
 Anchorage values proportional to tooth size and thus PDL volume
• Molars > canines > incisors
 Pressure in PDL of tooth to be moved: anchor tooth should be at least 3:1 ratio
• The larger the ratio, the better the anchorage
o Palatine ruggae (only occasionally – not v effective)
o Extraoral anchorage – headgears
o Skeletal anchorage
 Temporary anchorage devices (TADs) in the bone
• Bone screws (through gingiva into alveolar bone)
• Bone anchors (in basal bone beneath soft tissues esp at zygomatic buttress)
• Types of anchorage:
1. Simple
• Tooth of larger root area used to move a tooth with a smaller root area in the same dental arch
o Generally results in tipping
• This is in contrast to compound anchorage, whereby multiple anchor teeth are engaged as follows:
2. Reciprocal anchorage
 Application of equal force to teeth/groups of equivalent anchorage values  identical PDL pressure  equal movement of both
• Can refer to the movement of
 One tooth pitted against an identical tooth
• e.g. two centrals connected by an active spring to close a diastema
 Groups of teeth
• e.g. 1-3 pitted against the 5 and 6 (i.e. the anchorage unit for anterior teeth) by placing a spring to close the first premolar extraction site with equal and opposite forces
• However, this is not truuuuly reciprocal, because the posterior teeth have a slightly higher combined anchorage value. Has the same effect though!

3. Reinforced anchorage
   o For differential retraction created by moving a tooth of lower anchorage value (e.g. anterior tooth) by anchorage to structure of larger anchorage value
   • Intraorally
   o e.g. adding an additional molar  change the relative root surface areas  reduced pressure on anchorage units  moved down the pressure-response curve
    more retraction of the anterior teeth
    less forwards movement of the posterior segment
    Extraoral force
   • Headgear  to control forward movement of posterior anchorage by counteracting some of the reaction force from anterior teeth
   o doesn’t work in reality, because force is too heavy and discontinuous
4. Stationary anchorage
   • Takes advantage of the different loading of PDL for different types of movement by pitting bodily movement of (e.g. posterior anchor) teeth against tipping (e.g. of canine + incisors)
    e.g. to close premolar extraction site
   • Posterior segment moves bodily, as it is being pulled from the centre of resistance (similar to reinforced anchorage)
   o Force for bodily movement is distributed over twice the PDL area cf the tipping force
   o Thus, if the PDL areas of anterior and posterior segments were equal  reaction force to tip anterior teeth = ½ that required for optimal bodily movement  anteriors lingually tipped twice as much as anchor teeth displaced by bodily movement
    NB anterior roots will have to be repositioned later  two-stage movement, which can reduce the strain on anchorage
   • Two-stage motion required for two reasons
   1) Anchorage control more difficult when sliding along archwire (additional force also has to overcome resistance to sliding)
   2) No resistance to sliding within the loop)
    Only works if tooth kept below optimum force for movement of anchor unit
   o 60:40 ratio of anterior to posterior tooth movement is ideal (any more and lip support and facial appearance will be adversely affected)
    NB using excessive force can disastrously undermine this method of anchorage, because both units will move up the pressure-movement curve  loss of differential

 

Question 5

What is the Mc/Mf couple?

Answer 5

• A moment is a measure of the tendency of an object to rotate around a point
o Generated by a force acting at a perpendicular distance from the centre of resistance of an object
 Calculated to be of magnitude F x s
• If this force is unopposed, pure tipping will result (see column 1)
• However, the moment due to the force applied to move the tooth (MF) can be counterbalanced by another moment referred to as a couple (Mc) to control the tooth movement
o i.e. a parallel force of equal magnitude and opposite direction to cancel out the rotational effect of the force applied to effect tooth movement
• Thus, the ratio of these two moments determines the type of tooth movement

Question 6

What does an Mc/Mf ratio of 0 indicate?

Answer 6

Pure tipping; tooth rotates around the center of resistance = center of rotation