A malocclusion is a misalignment or incorrect relation between the teeth of the two dental arches when they approach each other during mouth closure. Malocclusions are common, although usually not serious enough to require treatment. Severe malocclusions may involve craniofacial anomalies and require orthodontic or sometimes surgical treatment (orthognathic surgery) to correct the problem. Correction of malocclusion may reduce the risk of tooth decay and prevent damage to the temporomandibular joint. Orthodontic treatment is also used to correct minor misalignments for aesthetic reasons.

Malocclusions can involve teeth that are mispositioned, tilted, rotated, deformed extranumerary or missing, and with dozens of teeth in the mouth, diagnosis can be a complicate process. Angle (1899) produced one of the oldest and simplest way of classifying malocclusions. It focuses on the position of the teeth. Several other classification systems have been proposed later to account for the limitations of Angle's system. This includes classifications by Martin Dewey (1915), Benno Lischer (1912, 1933), Simon (1930, the first 3D system), Jacob A. Salzmann (1950, based on skeletal structures) and James L. Ackerman and William R. Proffit (1969).

The performance of an organism in biting and chewing is strongly influenced by the alignment of the teeth. It takes a precisely tuned developmental program not only to form each tooth and bring it into position, but also to account for the replacement of deciduous teeth without significant loss in functionality. The fine positioning of the teeth is based on remodeling of the alveolus in the supporting bone, and adjustment to the periodontal ligaments, the collagen fibers that attach the tooth to the bone. This mechanism remains active throughout life, constantly adjusting the position of the teeth in response to any changes in the forces experienced during mastication. Orthodontics is the study of tooth movement and alignment. It includes normal and abnormal tooth movement, bone growth, tooth eruption and shedding of baby teeth. Orthodontists study these mechanisms to make adjustments to the alignment of the teeth. This chapter explains the processes above, introduces the most common problems and treatment in humans and reviews dental alignment in other mammals.

Dental occlusion

Occlusion, in dentistry, means the contact between teeth. With the teeth ideally positioned, all mandibular teeth should touch maxillary teeth at the same time during mouth closure. The real condition tends to be very close to this ideal due to feedback-driven adjustment of tooth position. Tooth eruption progresses until it is inhibited by pressure received from the opposing tooth at the occlusal surface. Similarly, the position of the tooth along the mesio-distal axis is also adjusted such that the alveolar bone tends to grow in areas of high pressure between teeth and be resorbed in areas of low pressure between teeth. These processes tend to position teeth to spontaneously maintain tight occlusion along the entire dentition and to approximate even spacing among teeth along the arcades.

The occlusal plane is the imaginary surface formed by the occlusal surfaces of the teeth with the mouth closed. This surface is actually curved in humans. The Curve of Spee is the curvature of the mandibular occlusal plane beginning at the tip of the lower incisors and following the buccal cusps of the posterior teeth, continuing to the posterior molar. According to another definition the curve of Spee is an anatomic curvature of the occlusal alignment of the teeth, beginning at the tip of the lower incisor, following the buccal cusps of the natural premolars and molars and continuing to the anterior border of the ramus. It is named for the German embryologist Ferdinand Graf von Spee (1855–1937), who was first to describe the anatomic relations of human teeth in the sagittal plane.

The pull of the main muscle of mastication, the masseter, is at a perpendicular angle with the curve of Spee. This directs force to the teeth in alignment with their longitudinal axis, which is structurally favorable. The longitudinal axis of each lower tooth is also nearly parallel with its arch of closure. The curve of Spee is, essentially, a series of sloped contact points between upper and lower teeth. It is of importance to orthodontists as a flat or mild curve of Spee is essential to an ideal occlusion.

The curve of Spee is at a right angle with the curve of Wilson, which is the upward (U-shaped) curvature of the maxillary and mandibular occlusal planes in the coronal plane. The occlusal surfaces of the mandibular molars are slightly higher on the vestibular side than on the lingual side because the teeth are tilted inwards. Both curves (Spee and Wilson) are believed to be relevant for stability of the temporomandibular joint and for the appropriate distribution of the forces of mastication on the teeth and supporting bones.

Alignment of anterior teeth

In a normal dentition, the incisive edge of the inferior incisor occludes against the cingulum of the superior incisor. The incisive edges of the two teeth do not touch but move past each other, creating some overlap between the teeth. The horizontal overlap is called overjet, whereas the vertical overlap is called overbite.

Conditions of malocclusion can exaggerate the overlap, reduce it or even invert the positions of the teeth. The inferior incisors have most commonly 30-50% of their height overlapped by the superior incisors. Both increased and reduced overlaps may be considered malocclusions. An excessive overlap is also called deep bite whereas a lacking overlap is called open bite.

Maxillary or alveolar prognathism and mandibular retrognathism tend to increase the overjet. Mandibular prognathism, on the other hand, reduces the overjet and can result in negative overjet, with the mandibular incisors occluding anteriorly to the maxillary ones. The condition of negative overjet is also called underbite or anterior crossbite. Notice that overbite is a quantitative property of the alignment of incisors, whereas underbite refers to a condition of malocclusion characterized by negative overjet.

Teeth are dynamically positioned

Each of our teeth articulate at an alveolus of the maxillary bone or mandible. The shape and position of the alveolus is adjusted by our body through the process of bone remodeling. This process is most intense during development allowing, the growth of jaw bones and the replacement of the primary dentition.

Alveolar remodeling occurs constantly throughout our lives, however. Adjustments are made in response to external forces, particularly occlusal forces. Bone is removed from areas where it is no longer needed and added to areas where it is needed. Osteoblasts are found in large numbers in the areas of the alveolus where tension is high, whereas osteoclasts are found in large numbers where the tissues are being compressed. These forces also influence the density and alignment of trabeculae inside the bone. The bony trabeculae are aligned in the path of tensile and compressive stresses to provide maximum resistance to occlusal forces with a minimum of bone substance. When forces are increased the bony trabeculae also increase in number and thickness.

The fibers of the periodontal ligament hold the tooth in the alveolus, allowing for a minimal amount of movement of the tooth. They also rely on mechanical stimulation to preserve their structure. Within physiologic limits, the fibers become thicker in response to increased stresses. If occlusal forces are reduced, the fibers become thin. This phenomenon is called disuse atrophy.

Tooth mobility

Healthy teeth are not completely immobile inside the alveolus. It is normal for them to move about 0.25 mm in response to pressure in the bucco-lingual direction. This is because the tooth is not fused to the bones of the jaws but is connected to the alveolus by the periodontal ligament. This slight mobility accommodates forces exerted on the teeth during chewing without damaging them. Milk (deciduous) teeth also become looser naturally just before their exfoliation. This is occurs through gradual resorption of their roots and periodontal ligaments when stimulated by the developing permanent tooth underneath.

Tooth mobility is evaluated by applying pressure with the ends of 2 metal instruments and trying to rock a tooth gently in a bucco-lingual direction. Multiple classifications of tooth mobility have been proposed:

Grace & Smales Mobility Index

• Grade 0: No apparent mobility
• Grade 1: Perceptible mobility 1 mm in bucco-lingual direction
• Grade 2: 2 mm
• Grade 3: > 2 mm or can be depressed in the alveolus

Miller Classification

• Class 1: 1 mm (horizontal)
• Class 2: >1 mm (horizontal)
• Class 3: > 1 mm (horizontal+vertical)

Teeth become loose when they loose their attachments or when they are exposed to abnormal mechanical forces. Loss of attachment includes periodontal disease, and dental abscesses. Abnormal mechanical forces include those produced in bruxism (tooth grinding or clenching), dental trauma (blow), or when a new filling or crown is too prominent and concentrates the pressure of the bite on a single occlusal surface.

Edward Angle used the position of the first molars to classify malocclusion. Molars tend to be more stable than front teeth and the first molars erupt early. The choice of the first molars for reference resulted in a system that is still widely used today. According to Angle, the mesiobuccal cusp of the upper first molar should align with the buccal groove of the mandibular first molar. The teeth should all fit on a line of occlusion which, in the upper arch, is a smooth curve through the central fossae of the posterior teeth and cingulum of the canines and incisors, and in the lower arch, is a smooth curve through the buccal cusps of the posterior teeth and incisal edges of the anterior teeth.

He classified malocclusions in three major types:

Class I: Neutrocclusion

The molars are correctly positioned as described above, but there are misalignment problems involving the front teeth.

Class II: Distocclusion

The lower first molar is distal to the neutral position. Usually the mesiobuccal cusp of the upper first molar rests in between the first mandibular molar and second premolar. There are two subtypes:

• Class II Division 1: The anterior teeth are protruded (tilted anteriorly).
• Class II Division 2: The central anterior teeth are tilted inwards.

This condition commonly results from the development of a short mandible.

Class III: Mesiocclusion

The lower first molar is mesial to the neutral position. The mesiobuccal groove of the mandibular first molar lies anterior to the mesiobuccal cusp of the maxillary first molar. The lower front teeth are frequently more prominent than the upper front teeth. This condition commonly results from the development of a large mandible.

Angle’s classification offers a useful framework and terminology for diagnosis of the general alignment of the dentition. A complete diagnosis of malocclusion involves further characterization of each tooth in position, tilt, rotation and level of eruption.

• Dislocation: Describes the position of the tooth. It can be mesial or distal, buccal or lingual.
• Inclination: Describes the tilt angle of the tooth. It can be mesial or distal, buccal or lingual, depending on which direction the crown of the tooth is leaning.
• Rotation: Describes the turning of the tooth around its longest axis. It is mesiolingual the mesial aspect of the tooth is turned toward the tongue or it is distolingual if the distal aspect of the tooth is turned toward the tongue.
• Eruption: Describes the vertical position of the occlusal surface of the tooth in relation to the occlusal plane formed by the rest of the teeth. In an infraocclusion, the tooth has not erupted enough to reach the occlusal plane, whereas in a supraocclusion the tooth has overerupted.

While the maxillary first molars (used in Angle’s classification) provide a reasonably stable position reference for the other teeth, they can also be mispositioned. It is therefore important to consider the dimensions of the mandible and maxilla, and the position of the entire dentition in relation to the skull. A cephalometric analysis is conducted to accomplish this step.

Cephalometry is the the study of the dimensions of the head. In a dental cephalometric analysis, molds of the dentition are combined with x-ray or tomographic images of the head to analyze the position and size of all the facial structures involved in mastication.

This allows for the diagnosis of developmental issues involving the maxilla, mandible and other support tissues in addition to the teeth. Most commonly, the analysis involves the identification of a series of standard landmarks on the images, measurement of distances and angles between them and comparison to the normal distribution of such measurements in the population. Various methods of analysis have been devised, and they differ in which landmarks are used. With today’s computing power, several methods involve digitization of the landmark positions and generation of 3D models of the patient’s skull and teeth to facilitate a comprehensive diagnosis of malocclusion.

Prognathism and retrognathism

Cephalometric analysis allows for comparison of the positions of the mandible and maxilla in relation to the skull. Prognathism is when either of the jaws protrudes beyond an imaginary plane (usually that of the forehead) that is parallel to the coronal plane of the skull. The word prognathism derives from Greek (pro = forward and gnáthos = jaw). Prognathism may result in malocclusion.

Prognathism can involve a projecting maxilla, mandible or both and it is usually the result of extensive growth of these bones. Not all alveolar prognathism is anomalous, and significant differences can be observed among ethnic groups. Prognathism, if not extremely severe, can be treated in growing patients with orthodontic functional or orthopaedic appliances. In adult patients this condition can be corrected by means of a combined surgical/orthodontic treatment.

Alveolar prognathism is a special case in which the maxilla and the mandible have normal size and position but the incisors are tilted labially. This condition can be exaggerated or caused by thumb sucking or tongue thrusting. Functional appliances can be used by growing children to help modify behavioral habits and avoid this condition. Otherwise, alveolar prognathism can be corrected with fixed orthodontic therapy.

Retrognathism is a condition in which the maxilla or mandible, particularly the mandible, has an abnormally posterior position relative to the rest of the skull.

This is a malocclusion in which at least one tooth is closer to the tongue or to the cheek (or lips) than the normal position, making it occlude in an abnormal position. It can involve anterior or posterior teeth and it can involve one or both sides of the arcade. A crossbite can be caused by dental issues such as crowding or delayed loss of primary teeth. It can also result from skeletal issues affecting the development of the maxilla or mandible.

Functional appliances

Malocclusions involving the structure of the jaws in children are usually treated with facial growth modification through the use of orthopedic devices. A variety of devices called functional appliances is available to address malocclusions. These appliances can be fixed (ex: Herbst appliance) or removable (chin cup, face mask, head gear) and they apply forces to the jaws that modify the growth of the bones, altering their final length and position.

In adults, orthognatic surgery may be required in combination with orthodontic treatment. For instance, a segment of bone may be surgically removed from the mandible of an adult with mandibular prognathism. The bone is exposed through an incision in the gums to leave no externally visible scars. The mandible is then shifted and the cut ends are held against each other by a small plate and screws as the mandible heals.

Space for the teeth on the arch can be managed with or without the extraction of permanent teeth. The discussion of the advantages and disadvantages of each approach has been controversial, and research has failed to provide evidence of a clear advantage to either approach in the function, aesthetics or stability of the final result of the treatment.

Braces

The teeth can be aligned through the use of fixed appliances (braces) or removable devices (clear aligners). Braces became popular during the 20th century, especially after 1970, when it became feasible to hold small brackets against the teeth using removable adhesives. The treatment involves the attachment of one bracket to the surface of each tooth and the installation of an archwire that goes through all brackets in the arch, connecting them. A different shape of bracket is designed for each tooth in the dentition. Arch wires vary in diameter and shape (round or square in cross-section). The choice of the precise placement for the bracket and the selection of wire diameter and shape make it possible to control the dislocation of each tooth and adjust its inclination and rotation. Elastics (rubber bands) may also be attached to the brackets to help position the teeth.

The appliance is adjusted with repositioning of brackets and archwires as the teeth move and new applied forces are necessary to conduct the teeth to the desired position. Several aspects of malocclusion may be dealt with at overlapping time frames. In general, the teeth are leveled and aligned to form a smooth curve of Spee, then class II and III malocclusions are corrected, the upper and lower arches are coordinated to achieve proper overbite and overjet, space is created and used to align the teeth and the final position, inclination and rotation of each tooth is fine-tuned. The treatment may also involve the use of other appliances, such as a headgear to promote anteroposterior jaw or tooth adjustments or a transpalatal bar or lingual arch to adjust the width of the arch and/or produce space anteriorly. Variations of braces include ceramic braces in which the tooth-colored brackets are less visually disruptive, lingual braces in which the brackets and arch wire are attached to the lingual surfaces of the teeth to conceal the entire appliance, and gold-plated or titanium braces for patients allergic to the nickel in the stainless steel.

Clear aligners

Clear aligners have been conceived in the middle of the 20^th century and became popular at the turn of the millennium. They are a less aesthetically disruptive alternative to braces. Treatment with clear aligners was initially only applicable when the necessary changes were small and restricted to the front teeth, but the development of the technique has greatly expanded its applicability to posterior teeth and to complex and profound corrections of tooth positioning. Its effect on skeletal elements is reduced, however, especially when lateral adjustments are needed. Hyrax or Herbst appliances are commonly included in the treatment to promote such adjustments.

A precise digital 3D model of the mouth of the patient is built and software is used to calculate the movements needed in each tooth for the dentition to reach the targeted alignment. The model is usually built after digitization of a cast of the patient's dentition. Intraoral scanners (wands with light and camera) are becoming available, however, for direct digitization at the clinic, eliminating the need for a cast.

Software is used to estimate the forces needed to be applied to each tooth. It then balances the forces and designs a series of aligners that will gradually reposition all teeth. Each clear aligner covers multiple teeth and has a fixed shape although it is slightly flexible. Small patches of composite material called “attachments” may be added at specific positions on some teeth to improve the anchorage of the aligner and facilitate specific tooth movements. The patient wears each aligner for a certain amount of time and switches to the next one, reaching aligned teeth at the end of the series. The treatment may be interrupted near the middle for a reevaluation with new aligners being produced based on a new scan of the mouth. This accounts for eruption or loss of teeth, modification of their shape due to restorative treatment (cavities), noncompliance of the patient or changes in plans. A second interruption (called refinement) with scanning and production of new aligners may take place near the end of the treatment. The goal is to correct for teeth that did not respond exactly as predicted to the applied forces. This final adjustment usually results in a fine-tuned alignment that closely matches the targeted one.

Retainers

Relapse is a tendency of teeth to return to their original position after being dislocated through orthodontic treatment. It is a concern both when braces or clear aligner are used in the orthodontic treatment. Tooth relapse may occur due to recoil of periodontal fibers, pressure from surrounding soft tissues, occlusal forces, and the patient’s continued growth and development. Orthodontic retainers are custom-made devices, usually made of wires or clear plastic, that hold teeth in position after they reach their targeted positions. By using retainers to hold the teeth in their new position for some time, the surrounding tissues are allowed to stabilize in the new condition and reduce the risk of relapse. The advisable duration of retainer usage depends on many factors involving the extent and type of orthodontic procedure, and the physiology and behavior of the patient. Some patients are advised to wear retainers for life.

The most common retainers (Hawley type) are movable. They should be worn at night and the need to wear them during the day has not been established. They consist of wires that surround the anterior teeth and insert into an acrylic base. They are custom-molded for each patient. Other types of retainers include vacuum-formed removable retainers, which are made or plastic and can be transparent like clear aligners, and fixed retainers.