Orthodontic Appliances and Tooth Movement
Master the INBDE with Dental Panda: Your go-to resource for expert practice exams and tailored study resources!
ORTHODONTIC MATERIALS
Archwires can be made from stainless steel, chromium cobalt alloys, beta-titanium (also known as TMA, titanium and molybdenum alloy), and Nickel titanium. Arch wires should have:
- high strength (can take a lot of stress before plastic deformation or failure).
- high range of movements.
- high formability.
- low stiffness (low modulus of elasticity, flexible). Stiffness is the extent a material can resist deformation. Stiffness decreases with increased length, and increases with increased diameter.
Stainless steel is a very popular archwire material with good mechanical properties, corrosion resistance, and low cost. These archwires typically consist of 18% chromium and 8% nickel. They exhibit the highest modulus of elasticity (stiffness) and lowest working range.
Nickel-titanium archwires have a very low modulus of elasticity and extremely wide working range. They can also reversibly shift between two atomic arrangements when heated or cooled, from austenite phase to martensite phase. Each phase has different physical properties. This property is also useful in endodontics.
Beta titanium archwires are also known as TMA (Titanium-molybdenum alloy) and offer a combination of good strength and acceptable working range, with physical properties between stainless steel and NiTi.
Changing the length or cross section of a wire changes the major elastic properties. Doubling the length of a wire decreases its strength by half, but gives it 4 times the range and makes it 8 times less stiff (or 8 times more elastic). Similarly, when the diameter of a wire is doubled, strength is increased 8 times, and stiffness is increased 16 times, while its working range is halved.
ORTHODONTIC MOVEMENT
When orthodontic forces are applied the mechanical signals are transformed into biological signals, leading to a response. The tooth moves as force is applied due to osteoclastic activity on the same side (compression side/pressure side) as the direction of the force, and osteoblastic activity on the opposing side (tension side). Breaking and reorganization of PDL fibers accompany bony remodeling. It is not a good idea to try and push a tooth into the cortical bone plates since cortical bone is far more resistant to tooth movement.
Counterintuitively, a heavy force (~300g, or 3N) applied to a tooth does not make the tooth move faster or more efficiently. Initially the bone bending and tooth compression leads to the PDL fluid escaping from the PDL space and the tooth moving within the PDL space only. This can be very painful. What follows is a “lag period”, with no further movement until there is bony resorption. Osteoclasts activate in the area, but no movement can occur until the resorptive process is complete. The compression of the PDL can lead to cellular necrosis and hinder cellular activity. Hyalinization of PDL occurs within hours. Cellular infiltration from the surrounding area is seen 3-5 days later. Only now can secondary movement begin.
A light force (~50g, or 0.5N) is preferred because it leads to smooth and continuous movement without significant damage to the PDL. Partial compression of the PDL alters blood flow which lowers oxygen tension, leading to prostaglandin and cytokine release. After a few hours enzymatic and cellular changes are seen, and certain chemical messengers (parathyroid hormone and calcitonin) are detectable. cAMP increases within hours, cellular differentiation takes place, and resorption starts within a few days
Classification of movement
Rotation often refers to the movement of a tooth around its long axis, but it may refer to any rotational movement without any bodily movement (pure moment). Rotational movement occurs around the center of resistance, which is roughly half way between the crest of the alveolar bone and the apex of the tooth in second and third order rotation. First order rotation moves a posterior tooth in the transverse plane (viewed from occlusal), second order rotation moves in the sagittal plane (viewed from buccal or lingual), and third order rotation moves in the coronal plane (viewed from mesial or distal). There are 2 tension areas and 2 compression areas, one above and one below center of rotation.
Translation is also known as bodily movement. It can only be accomplished when the force vector is through the center of resistance. No brackets are placed on the roots though (where the center of resistance is located) so you need a couple force that rotates the tooth to counteract the tipping movement. This is done via a “prescription” (specific to the tooth) built into the bracket and the use of square wires (in cross-section). Translational forces are slow and more difficult to accomplish.
Tipping forces cause both the crown and the center of resistance to move in the direction of the applied force, while the apex moves in the opposite direction. The apex of the tooth usually moves less than the crown, with the axis of rotation closer to the apex, around the junction of the apical and middle third. Tipping movements are the easiest and fastest type of movement.
Intrusion refers to vertical movement, where the tooth is moved along its long axis into the alveolar socket. There will be compression at the apex. Intrusion is a difficult movement to accomplish.
Extrusion refers to vertical movement, where the tooth is moved along its long axis out of the alveolar socket. There will be tension at the apex. Extrusion is an easy movement to accomplish and will likely cause alveolar bone to “extrude” with the tooth.
In orthodontics, torque refers to the “dragging” of the tooth root through the alveolar bone without much movement of the crown. It is also known as uprighting. To torque a tooth you would need a large couple force. This is the slowest and most difficult movement.
Orthodontic anchorage
Orthodontists cannot cheat Newton’s Third Law, so the sum of forces and moments need to equal zero. Anchorage is the resistance to movement, proportional to the root surface area used. Reciprocal movement is seen when equal and opposite forces are applied to units with equal anchorage. Anchorage can be increased by using more teeth, headgear, inter-arch elastics, or bony implants.
REMOVABLE APPLIANCE COMPONENTS
Removable appliances can be used in some limited orthodontic treatment. They are best suited for:
- growth modification (though there is debate about the actual utility).
- limited movements like tipping.
- retention after conventional orthodontic treatment.
You need to know the basic components of removable appliances:
- baseplate – usually acrylic, but metal base plates can be used. The baseplate provides anchorage for the other components, adding stability and support.
- retentive components – provides retention. Examples include ball clasps, Adams clasps, C Clasps, and arrow clasps.
- active components – springs, elastics, jack screws etc.
- anchorage component – counters forces generated by the active components. A base plate, labial bow, or framework may provide anchorage.
The advantages of removable appliances include better hygiene, improved comfort, and reduced chair time. The main disadvantage is that treatment depends on patient compliance. Generally removable appliances can only accomplish tipping movements. If more complicated movements are required fixed appliances are used.
ORTHODONTIC APPLIANCES
The Hawley retainer is the most common retainer, utilizing clasps on the molar teeth and a bow with adjustment loops canine to canine. The acrylic base provides anchorage. A Hawley retainer can be useful in many clinical applications, including retracting flared incisors, but is most commonly used as a retainer. A bite plane can be included to control excessive overbite. You can make a lower Hawley appliance but they are fragile and not very popular.
Hyrax appliance – metal framework with expansion screw, bands are cemented to premolars and molars.
Y-plate (Schwartz) – similar to Hyrax but allows simultaneous expansion and anterior movement.
Haas appliance – bands around premolars and molars, two acrylic pads with midline jackscrew. The pads add force to the bony structures under the soft tissue (not just force to the teeth), but hygiene and inflammation is an issue.
Hawley-type removable appliance – compliance will be an issue.
Quad Helix/W arch – Heavy stainless steel wire with four (quad helix) or three (W arch) provide the force instead of a screw. May be fixed or removable. Created tipping of teeth, suggested for small movements.
Transpalatal arch (TPA) – similar to quad helix, but just one bow. Often used to maintain expansion.
ORTHODONTIC HEADGEAR
A headgear device can be used to provide supplemental anchorage, distalize maxillary teeth or restrict maxillary or mandibular growth. It needs to be worn a minimum of 8 hours per day, preferably 10-12 hours a day (normally orthodontists suggest 14 hours per day). For orthopedic effects, 250g to 500g of force is used for each quadrant. For dental movements only, 100g to 200g per side is common. Headgear can achieve its effects on one arch without impacting the opposing arch.
- High pull headgear – distal and intrusive forces used to restrict anterior and downward maxillary growth. It can help with molar distalization, intrusion, as well as controlling maxillary molar eruption.
- Straight pull headgear – distal force.
- Cervical pull headgear – distal and extrusive force used to restrict anterior maxillary growth. It can be used to help distalize and erupt maxillary molars.
- Reverse pull headgear – mesial force, usually used in Class III cases where there is maxillary deficiency (retrognathic maxilla).
- Chin cup – restrains the mandible to try and inhibit excessive mandibular growth.
FUNCTIONAL APPLIANCES
Functional appliance treatment is used to modify the growth of the jaws and is commonly employed during the mixed dentition phase of development. Functional appliances are thought to cause dental and skeletal effects by:
- holding the jaw(s) in restrained positions.
- imparting forces on the associated muscles fibers.
- shifting the balance of soft tissue forces on the surrounding skeleton and dentition.
Most functional appliances are removable and rely on patient compliance. Some focus on soft tissue modification and can be referred to as tissue-borne appliances. They are generally used to treat Class II malocclusions.
The Frankel functional appliance is the only tissue-borne functional appliance, used to adjust facial soft tissue to affect the balance of forces on the teeth. With the lip or cheek out of the way the teeth are free to move in that direction. The Frankel functional appliance postures the mandible forward.
The Lip bumper has a similar function to the Frankel functional appliance, pushing the lip out of the way.
The Herbst appliance splints the maxilla and mandibular together via a pin and tube device that holds the mandible forward. It is most often a fixed appliance.
The Activator is made with an acrylic body that covers part of the palate and the lingual aspect of the mandibular alveolar ridge, advancing the mandible to an edge to edge position. The maxillary teeth are blocked from erupting by the acrylic shelf, but mandibular teeth are free to erupt.
The Bionator is similar to the Activator but less bulky (supposedly more comfortable to wear). It is used for correction of a Class II malocclusion, designed to advance the mandible to an edge-to-edge position in order to stimulate mandibular growth.
The Twin block appliance can be removable or fixed, and has a two part design which should be easier on patients. Inclined guide planes guide the mandible forward.
FIXED APPLIANCES
Fixed appliances are still regarded by many clinicians as the gold standard in orthodontic treatment because of the control and flexibility they provide. Fixed appliances allow for controlled three dimensional movement of teeth. There are four components: brackets, wire, bands, and auxiliaries (elastics, ligatures, springs etc.). Preadjusted edgewise/straightwire systems are the most commonly used.
The dimensions of the edgewise bracket slot allows for the use of wires with different cross-sectional shapes and sizes. The two most common brackets are 0.018 × 0.025 inch and 0.022 × 0.028 inch, a rectangular slot and wire.
- Beggs appliance – slot vertically positioned, round wires. No longer popular.
- Edgewise appliance – slot horizontally positioned.
- Straight-wire appliance – a modification of the Edgewise design, slots are horizontally positioned and are designed with a prebuilt “prescription” in the bracket, accommodating natural tooth anatomy. Generally these appliances require less wire bending.
The shape of the bracket and wire allows for automatic rotational control, horizontal control (negates the need for first order bends), mesiodistal tip control (negates the need for second order bends), and torque (negates the need for third order bends).
The placement of bands and brackets utilizes either resin bonding protocol (prophy clean, phosphoric acid etch, bond, bracket) or glass ionomer cements. You can use a custom tray for accurate, controlled placement (indirect bonding method) but the procedure is more complicated and technique sensitive. It’s often used in lingual placement. Bands are stronger and used in cases where:
- There is an increased risk of patients breaking a bracket.
- Short clinical crowns make bracket placement difficult.
- The tooth movement is expected to be more difficult, or additional anchorage is required.
- Teeth may need both lingual and labial attachments.
- A compromised tooth surface does not guarantee a good bond (enamel defect e.g. amelogenesis imperfecta).
Brackets can be metal (strong but unaesthetic), ceramic (aesthetic but more fragile, more friction), and self ligating (no ligature placement required, less friction, shorter treatment time).
ELASTICS
Orthodontic elastics apply forces within or between arches.
- Class I elastics (intramaxillary elastics) – traction of teeth/groups within the same arch.
- Class II elastics (intermaxillary elastics) – connects an anterior maxillary tooth (usually an upper canine) to a lower posterior tooth (usually the permanent first molar).
- Class III elastics (intermaxillary elastics) – connects a posterior maxillary tooth (usually the first molar) to lower an anterior tooth (usually the canine).
- Crossbite elastics – connects the palatal of one or more maxillary teeth to the buccal of one or more mandibular teeth to correct crossbites.
