Chapter 9 Orthodontics

The word orthodontics often is equated with unethical veterinary practices. Certainly, orthodontic manipulation can be used for unethical purposes. However, orthodontic treatment should not be performed to deceive show judges, prospective owners, or buyers of offspring. The veterinarian who performs the procedures should not, inadvertently, become part of a treatment that will result in a deceptive act. The goal is to provide the patient with a pain free, functional bite. Regaining normal, cosmetic occlusion may be the end result of the treatment and is in itself not the result of unprofessional treatment. It is what the owner does with the animal that has had its heritable anatomy altered that determines the ethical or unethical behavior.

OCCLUSAL EVALUATION

• Occlusal (bite) evaluation involves more than just the relationship of the incisors to each other and the number of teeth. The entire mouth and dentition are used to evaluate occlusion properly.

Step 1—Observe the symmetry of the head, face, and dentition.

• The midpoints of the mandibular and maxillary dental arches should be in alignment with the midsagittal plane of the head.

Step 2—Count the teeth.

• All teeth should be present.

Canine Dental Formula

Primary (Deciduous)

Secondary (Adult)

Feline Dental Formula

Primary (Deciduous)

Secondary (Adult)

Step 3—Evaluate the occlusion of the incisors (Fig. 9-1, A). The normal head type in feral dogs is the mesocephalic head as seen in the German shepherd. The line of the teeth is seen as a smooth, symmetrical curve not broken by rotated or misplaced teeth. All other relationships, for scientific purposes, must be considered to be in malocclusion to a greater or lesser degree.¹ The normal incisor occlusion has the large cusps of the lower incisors occluding near the cingulum on the lingual side of the upper incisors (Fig. 9-1, B). The large cusps of the central incisors should be centered with each other. The second and third incisors lose their centered relationship, and the large cusp of the third mandibular incisor should be in the interproximal space between the second and third maxillary incisors.

Step 4—Observe the relationships of the canine teeth (Fig. 9-1, C).

• The mandibular canine tooth should occlude buccal to the gingiva of the maxilla and should be equidistant between the maxillary canine tooth and the maxillary third incisor. The relationship of the mandibular canine to the maxillary canine and third incisor is called the dental interlock. An abnormal dental interlock often signals a current or pending abnormal maxillary-mandibular occlusal relationship.

• This is the most reliable reference point in the mouth.²

Step 5—Observe the relationship of the premolars (Fig. 9-1, D).

• The large cusp on the lower fourth premolar should divide the space between the upper third and fourth premolars, the central cusp pointing interproximally between the two teeth.

Step 6—Observe the occlusal plane of the upper and lower arches (Fig. 9-1, E).

• The premolars should interdigitate from the second premolars back to the cusps of the upper fourth premolar, and there should be overlapping of the cusp tips.

• The molars should occlude to allow the cusps to function in crushing.

• The premolars and molars should be aligned mesial to distal in a smooth curve, with none of the teeth rotated.

Fig. 9-1

Classes of Occlusion

Normal

• Scissor bite: normal occlusion pattern in which the lower incisors occlude on the cingulum on the palatal surface of the upper incisors. The upper and lower premolars are in an anisognathic, shearing relationship with the maxillary teeth buccal to the mandibular teeth. The upper and lower arches are symmetrical.

Abnormal Occlusion

Class 1

• Patients with class 1 malocclusion have generally normal occlusion with one or more teeth out of alignment or rotated. It may occur in any one of four basic formats: (1) a shift in the interdigitating relationship of the maxillary and mandibular premolars, (2) an anterior (or may be also thought of as an incisor) crossbite (see below), (3) a base narrow mandibular canine tooth or teeth, and (4) posterior crossbite of the premolars or molars.

• Anterior crossbite: a common abnormal occlusion in which one or more of the mandibular incisors are anterior to the maxillary incisors (Fig. 9-2,A, arrow) and, most importantly, the rest of the teeth occlude normally.

• Base narrow or lingually displaced canine teeth: one or both of the cusps of the mandibular canine teeth are displaced lingually and occlude on the hard palate (Fig. 9-2, B). Similar trauma may also occur in class 2 malocclusions.

• Rostrally angled maxillary canine teeth: these can be unilateral or bilateral and are most frequent in the Shetland sheepdog. The maxillary canine tooth erupts at an angle creating interference with the mandibular canine tooth, whereas the rest of the occlusion is generally normal. This occlusion is also known as lance tooth or spear tooth and can be seen in other breeds and in cats.

Fig. 9-2

Class 2

• Patients with class 2 occlusion have the mandibular premolars and molars positioned caudal (distal) to the normal relationship (Fig. 9-2, C and D). This occlusion is also known as mandibular brachygnathism, overjet (often called overshot by breeders),retrusive mandible, or distal mandibular excursion.

• The difference between the often incorrectly used terms overbite/underbite and overjet/underjet, is that the suffix “-bite” refers to the vertical overlapping of the maxillary over the mandibular teeth, whereas the suffix “-jet” refers to the horizontal projection of the maxillary or mandibular teeth beyond the opposite arch.³

Class 3

• Patients with class 3 occlusion have the mandibular premolars and molars positioned rostral (mesial) to the normal relationship (Fig. 9-2, E). This occlusion is also known as prognathism, undershot, underjet (called underbite by breeders), protrusive mandible, or mesial mandibular excursion. A level bite (often called an even bite by breeders) is a mild form of class 3 malocclusion. A reverse scissor bite is a class 3 occlusion that is a little more pronounced. In that case, the incisal edges of the maxillary incisors make contact with the lingual surface of the mandibular incisors. A true underjet is one in which the mandibular incisors are rostral to the maxillary incisors and not in contact with them.

• Level bite: an abnormal occlusal pattern in which the maxillary and mandibular incisors occlude cusp to cusp (edge to edge). This is a very punishing malocclusion, resulting in premature wear to the incisors, a predisposition to inflammatory periodontal problems (secondary to occlusal trauma) and, to a lesser extent, endodontic disease.

Unclassified

• Wry bite: an abnormal occlusion caused by a difference in lengths of the two halves of the mandible and maxilla. This abnormal occlusion is reported to be genetically created and can result in a variety of jaw relationships. It is characterized by asymmetry of the head in which the midline of the maxilla does not align with the midline of the mandible. The asymmetry may be found in either the maxilla or the mandible, or both. In a wry bite, when evaluating occlusion, a clinician may find a normal scissor incisal occlusion, a normal dental interlock, and normal premolar interdigitation on one side of a dog or cat, while the opposite side is afflicted with an underjet or overjet, a malevolent dental interlock, and an abnormal premolar interdigitation. If that patient is then appraised from a frontal view, the face will be seen to be asymmetrical, with the midsagittal plane not truly occurring between both the maxillary and mandibular first incisors. Clinically, this is a very heritable and undesirable occlusion. Among English Bulldog breeders, a wry bite refers to a bowed or twisted hemimandible, not the wry bite described above.

Orthodontic Fundamentals

• Tooth roots are held in the alveolus by the periodontal ligament (PDL), which attaches to the cementum on the tooth and the alveolar bone of the dental arch.

• Osteoclasts and osteoblasts occur in the alveolar bone.

• Forces applied to the crown of the tooth are transmitted by the PDL to the bone.

• Stretching the PDL applies a pull on the alveolar bone and stimulates the osteoblasts to deposit new bone.

• Compressing the PDL and compressing the periodontal space applies compressive pressure to the alveolar bone and stimulates osteoclasts to resorb bone.

• The magnitude of the force applied to the crown of the tooth is critical. If the force exceeds the capillary blood pressure in the PDL, then the PDL will necrose or hyalinize and become cell free. When this happens, remodeling of bone in the acellular area must be accomplished by cells derived from adjacent healthy bone. This causes a delay in the orthodontic movement process.

• Types of movement are created by the way the force is applied to the tooth:

Tipping: one part of the tooth moves a greater distance and direction than another (Fig. 9-3, A) (requires light force).

Fig. 9-3

Translation or bodily movement: all parts of the tooth move the same distance in the same direction in the same amount of time (Fig. 9-3, B) (requires twice the force of tipping).

Rotation: tooth is rotated around its axis (Fig. 9-3, C) (requires light force).

Intrusion: tooth is moved into the alveolus (Fig. 9-3, D) (requires the greatest amount of force).

Extrusion: tooth is moved out of the alveolus (Fig. 9-3, E) (requires the least amount of force).

• Duration of the force also influences the response. The three classes of duration are:

Continuous: force gradually diminishes (but does not reach zero) between adjustments.

Interrupted: force is reduced to zero between adjustments.

Intermittent: force drops to zero when a removable appliance is removed and is regained when the appliance is replaced.

• Anchorage is resistance to unwanted tooth movement.⁴ The object is to create a platform from which an orthodontic force may be exerted that will move the active tooth and only minimally move the anchorage tooth or teeth (unless one also wants to move the anchorage).

• Once tooth movement has been accomplished, the tooth or teeth must be maintained in their desired positions; in veterinary orthodontics this is usually 2 to 4 weeks. This is known as the retention period.

Legal and Ethical Considerations

Before accepting a patient for orthodontic correction, the client should be advised of the potential legal and ethical implications of these procedures. A release, approved by the practitioner’s attorney, should be signed by the client. The following release is a sample only.

Agreement and Consent for Orthodontics

The correction of malocclusions in animals has moral, ethical, and legal implications. In addition, the rules of many breed clubs and organizations state that any animal that has had its heritable anatomy altered is subject to disqualification from showing in conformation classes.

Because many orthodontic conditions are inherited, we strongly recommend that such animals treated for orthodontic conditions not be used for breeding purposes. Such an animal should be neutered, rendering it incapable of being shown in conformation classes.

We believe that all pets are entitled to a comfortable, functional bite. There is nothing wrong with the correction of an acquired malocclusion, but the doctor and clinic staff will not be an accomplice to fraud.

My signature authorizing treatment indicates that I have read and understand the above information.

Signature of Client/Owner and Date

GENERAL ORTHODONTIC TECHNIQUES

Making an Impression Tray

Creating the Impression

Indication

• Any time a model is needed for study, treatment planning, or appliance fabrication.

Contraindication

• Uncooperative patient or patient unable to undergo general anesthesia.

Technique

Step 1—Select an impression tray that fits over all the teeth loosely (Fig. 9-4, A).

Step 2—The alginate is “fluffed” by gently rolling or shaking the container. The powder is allowed to settle to avoid a particle dust cloud when the container is opened (dustless alginate is available).

Step 3—The alginate is measured into the mixing bowl with the measuring spoon provided with the alginate (Fig. 9-4, B). The amount will depend on the size of the tray used. Experiment until you know how much alginate a particular tray size needs (generally two to three scoops for small breeds or anterior teeth only, and four to six scoops for a full tray). The alginate is placed in the rubber mixing bowl (Fig. 9-4, C).

Step 4—Water is measured with the container provided with the alginate (Fig. 9-4, D). If tap water is too cold or too hot, water is used from a storage flask at room temperature to avoid variations in setting time caused by water temperature. Water temperature affects the setting speed of the alginate; warmer water increases setting time, and colder water decreases it. All the water is poured into the bowl with the alginate at the same time (Fig. 9-4, E).

Step 5—The alginate and water are mixed with the spatula (Fig. 9-5, A). Incorporate all the powder first by mixing in the center of the bowl. Smooth the mix by spatulating the mixture against the sides of the bowl (Fig. 9-5, B). The bowl is held in one hand, and the spatula is used to spread the mixture vigorously onto the sides of the bowl. Turn the bowl while spatulating to get a more homogeneous mixture. The entire mixing procedure should be completed in 30 to 45 seconds. The mixing is finished when all the lumps are removed and the consistency is homogeneous.

Step 6—The alginate is placed in the tray; any air bubbles or voids are removed (Fig. 9-5, C).

• The surface is smoothed and lightly dampened with water.

Step 7—With the patient in sternal recumbency for the maxilla and dorsal recumbency for the mandible, the tongue is rolled back into the pharynx and the lips are held back. While the patient’s mouth is held open, the tray with the alginate is placed carefully in the mouth (Fig. 9-5, D). Contact with the teeth is made first in the caudal portion of the mouth, and the tray is rocked forward gently to incorporate the teeth in the anterior portion of the mouth (Fig. 9-5, E).

• Once the tray is in position, it is held steady until the alginate has set. The amount of time depends on the alginate used and the temperature of the water (usually 3 to 7 minutes).

• The setup is tested by pressing a finger on exposed alginate. When the finger does not stick or leave an indentation, the alginate has set.

Step 8—The tray with the alginate is removed from the mouth by firmly snapping the tray off the teeth (Fig. 9-5, F).

• The impression is examined for voids, flaws, or bubbles. If there is an imperfection in the impression in a critical area, the impression should be retaken. If the model is not going to be poured immediately, wrap the alginate and tray in a damp paper towel (Fig. 9-5, G). (Do not soak the tray and alginate in water because it will absorb water and distort the impression.) For accurate representation, laboratory stone should be poured into the alginate impressions within 30 minutes.

Step 9—Bite registration is critical for helping the dental laboratory determine the proper relationship of the two jaws so as to be able to manufacture an appliance or a crown. To record the correct articulation after the impressions are made and just before the patient is awakened, the patient is extubated, and the bite registration is taken. A sheet of bite wax, which is often made of beeswax, is softened in warm water, placed in the mouth, and the patient is made to bite down on it. As an alternative to the bite wax, special bite impression registration materials are available that can be injected into the occlusal space.

Fig. 9-4

Fig. 9-5

Complications

• Inadequate mixing will leave lumps and voids in the alginate. To prevent this, mix the alginate thoroughly.

• Too thin a mixture will not have enough body and will tend to run out of the tray and not stay around the teeth. Follow the manufacturer’s instructions on the mix and adjust only if necessary, adding small increments.

• Too firm a mixture will not conform to the teeth and mouth well and, thus, will provide poor detail. Correct by creating a thinner mix.

• If the tray is not filled properly, voids (bubbles) may be incorporated into the impression. Fill the tray completely and smoothly.

• Bubbles can be trapped when the tray is placed in the mouth. To prevent bubbles, slowly seat the tray, caudally to rostrally.

• Movement while the alginate is setting will cause distortion. Because movement usually is caused by the patient, adequate sedation or general anesthesia is suggested.

• The alginate may tear when removed from the mouth. This is especially a problem in the mandibular canine teeth, which are splayed at their coronal end. If the canines are severely divergent and cause tearing, slow removal of the impression, as well as a slight rotational movement as opposed to snapping the impression material off the teeth, may help prevent tearing. A minimal tear may not cause distortion and can be corrected on the model later.

• Alginate will become distorted if the water content changes between the time when the impression is taken and when the model is poured. Alginate will become distorted, also, if the tray is not solid.

Making a Model

General Comment

• Once the impression is made, a cast model should be poured as soon as possible.

Technique

• It is often beneficial to mix a small amount of stone with a more fluid consistency first to allow flowing of the stone into the indentations made by the teeth when placed on the vibrator. When these areas are filled, a second, thicker batch of stone can be used for the rest of the model.

Step 1—The water is measured and placed in the bowl (Fig. 9-6, A).

Step 2—The stone is measured, ideally by weighing (Fig. 9-6, B), and added to the water all at once.

Step 3—Mixing is started in the center of the bowl (Fig. 9-6, C). After all the water is incorporated into the stone, the mixture is spatulated on the sides of the bowl to remove air bubbles and to remove lumps of unmixed powder. The bowl is turned, and spatulation is performed in one direction to minimize bubbles.

Step 4—The bowl with the stone can be placed on the vibrator during this process to help remove air bubbles (Fig. 9-6, D).

Step 5—The impression is rinsed gently with water. Excess water is removed with compressed air.

Step 6—An edge of the tray with the impression is placed on the vibrator (Fig. 9-6, E).

• A small amount of mixed liquid stone is placed in the center of the impression with a small spatula.

• With the aid of the vibrator, the stone is made to flow into the depressions created by the teeth. Take enough time to allow the stone to move into the voids without trapping air. This is the most critical part of the process. With the impression resting on the activated vibrator, a small cylindrical applicator stick may be used to stir the liquid stone material in each tooth depression, to aid in removing bubbles.

• For smaller teeth, a disposable brush may be used to place a small amount of mixed stone into the individual tooth impressions; avoid bubble formation, especially at the cusps of the teeth.

Step 7—After the stone is spread into the intricacies of the impression and any air bubbles have been vibrated out, the vibrator can be turned off, and additional stone of a thicker consistency is placed on the impression to make the model thick enough to minimize fragility (Fig. 9-6, F).

• Two options are available to make the base of the stone. The first layers of stone that fill in the impression tray can be allowed to harden slightly, and additional stone or plaster is placed on this first layer to create a base as described below. If sufficiently thick stone is used, a one-step model and base can be made without waiting for the stone to set initially.

Step 8—The impression is placed, with the stone in it, on a flat surface and allowed to set for 10 to 15 minutes (Fig. 9-6, G). Additional stone can now be added to the model to create a base. Start by mixing another batch of stone or plaster the same way as previously described.

• Place a layer of the stone onto the bottom of the hardened stone (Fig. 9-6, H). It can be shaped to form a flat base, or the prehardened model can be placed upside down onto a block of plaster on a glass slab or countertop and leveled. Excess stone can be removed before it hardens to provide a shaped model when removed from the impression tray, if a model cutter is not available. The stone should be allowed to set for at least 45 minutes.

Step 9—The hardened stone is removed from the alginate impression after 45 to 60 minutes (Fig. 9-7, A), usually after the model has cooled following the exothermic cycle of setting. If the alginate is too dry, separation is more difficult. A laboratory knife can be used to free the margins. The model is separated gradually upward from the alginate. Do not rock the model, because this may lead to fracture of a canine tooth on the model (Fig. 9-7, B).

• Another technique is to remove the impression tray first. With the stone down and the alginate on top, a sagittal wedge of alginate is removed with a laboratory knife from the center of the impression. Transverse cuts are then made so that the alginate can be removed segmentally with the knife.

Step 10—After the alginate is removed from the models, the edges of the stone can be trimmed with a model trimmer. The models are placed in occlusion with the bite registration in place, and the caudal aspect and sides of the models are placed against the moving trimmer wheel to align the caudal edges of the upper and lower models. To realign the models, they can be placed on their caudal aspect and gently slid together.

Step 11—Label the model with the patient’s and client’s name, date made, and the clinic’s name. It is also helpful to identify, for the laboratory, on opposing arches and teeth cusps, places of occlusal contact. This can be accomplished with a Sharpie marker pen.

Fig. 9-6

Fig. 9-7

Complications

Incorrect Mix Ratios

• The stone will not flow if the mixture is too thick. Correct by using a thinner mixture, but changing the proportions from the manufacturer’s instructions may cause distortion. A swab stick may be used to stir the thinner mix to help it fill the cusps of the teeth.

• Too thin a mixture will be weak and not as dimensionally stable. Correct by mixing more thickly.

Inadequate Mixing

• Lumps of unmixed stone will be found in the mix. Mix more thoroughly.

Flexible Trays

• Allow the impression to become distorted before the stone is poured or set.

Desiccation of the Alginate

• The time between taking the impression and pouring the stone is too long. The stone should be poured quickly after the impression is taken. Wrapping the impression in a damp paper towel until it is poured or when sent to a dental laboratory will diminish this distortion. If the impression is sent to a laboratory before the stone is poured, the alginate wrapped in a damp paper towel should then be placed in a plastic bag until picked up by the laboratory. This will allow the delay of pouring the stone.

Fracturing of Crowns on Model Teeth

• This often is caused by trying to remove the stone from the impression too early (Fig. 9-7, B). Be patient. Even the most patient practitioner will break a crown occasionally as the model is removed from the impression. Most of the time the fractured piece can be placed back in position and cemented. Allow the model and fragment to dry completely. Cut the impression material away from divergent mandibular canines to reduce the incidence of these teeth being broken at the time of removal of the impression material.

• After the model is dry, the fractured piece can be repaired either with a glass ionomer cement or with polymethacrylate (Fig. 9-7, C and D)

• For very long teeth with narrow diameters, a small piece of orthodontic wire (24 to 26 gauge) may be placed in the impression before pouring the model. This will support the teeth and will help prevent fracture.

• The dry model may also be coated with a liquid model hardener to reduce breakage during shipment and handling.

Direct Bonding of Bands, Brackets, and Buttons

General Comments

• Brackets or buttons are bonded to teeth to attach devices for applying forces to achieve orthodontic movement.

• Bands can be used alone for attachment of elastics or arch wires, or are incorporated into an appliance for fixation to the teeth.

Contraindication

• Patients that cannot be orally controlled or that chew on hard objects.

Technique

Step 1—The teeth that are to have brackets attached are scaled to remove any calculus (Fig. 9-8, A).

Step 2—A flour pumice is used to polish and remove any plaque (Fig. 9-8, B).

• Do not use prophy paste, because it may contain fluoride, waxes, essential oils, and glycerin that may inhibit the bond.

Step 3—The bracket or button is chosen and trial placed on the tooth (Fig. 9-8, C). The baseplate should fit the contour of the tooth. If it does not fit, three-pronged pliers are used to bend the bracket or button baseplate to conform to the tooth surface.

Step 4—Phosphoric acid etching gel is placed on the tooth to etch the surface for 30 to 60 seconds (Fig. 9-8, D). The time depends on the acid concentration and whether the active or passive technique is used. With the active technique, the operator scrubs the tooth surface with a sponge or brush and acid-etches for a prescribed time. The passive technique allows the phosphoric acid to coat the tooth undisturbed for a prescribed time. Whether the active or passive technique is used depends on the manufacturer’s instructions for the bonding agent. These instructions should be followed.

Step 5—Using a three-way syringe, water is used to rinse off the acid (Fig. 9-8, E). It is best to make sure rinsing is complete by rinsing for 20 to 30 seconds.

Step 6—The tooth is dried using air from the three-way syringe (Fig. 9-8, F) or from a handheld hair dryer on a low setting. For the bond to be solid, it is critical that this air be free of moisture and oil. For this reason, it is safest for the clinician to wear procedure gloves when handling the bracket.

• At this point the area on which the bracket will be placed should have a dull, chalky appearance.

• If contamination of the prepared tooth surface occurs from saliva, blood, oil, or other substance, the preparation should begin again from step 2, but reducing the etching time by 75%.

Step 7—The unfilled bonding agent is mixed and applied to the surface of the tooth (Fig. 9-9, A) and to the baseplate of the button or bracket (Fig. 9-9, B).

Step 8—The filled bonding agent is mixed (Fig. 9-9, C) and is placed on the back side of the bracket (Fig. 9-9, D). (Each manufacturer has specific instructions on the way its products should be mixed and handled. These materials are very technique sensitive.)

Step 9—The bracket is placed on the tooth in the position desired and pressed firmly against the tooth so that some of the bonding agent exudes around the edges of the bracket (Fig. 9-9, E).

Step 10—Excess bonding agent is removed with a hand scaler before it has set (Fig. 9-9, F). An ultrasonic scaler might vibrate the bracket loose. Wait to put a force on the bracket until the bonding agent has had the required time to finish polymerizing (Fig. 9-9, G). The time is stated in the package insert of the bonding agent.

• Test the strength of the bond by putting pressure on the bracket with college pliers. If the bond is not sufficient, it is preferable to test it and to rebond it while the patient is under anesthesia than to have the procedure fail, requiring a subsequent visit and additional anesthetic.²