Malposition of teeth in the horizontal plane
The development of the dentition may be disrupted, leading to variations from the normal positioning of teeth (Figs 5-16 to 5-18). A distinction is drawn between rotation, tipping, and displacement of teeth (Fig 5-19). An incorrect tooth position may be caused by displacement of a tooth germ, an incorrect tooth germ, or external influences during normal eruption, such as thumb sucking, lip biting, tongue thrusting, or persistence of primary teeth.
Rotation means a twisted tooth position relative to a vertical axis of rotation. A distinction is drawn between centric rotation (when the central tooth axis is the axis of rotation) and eccentric rotation (when the axis of rotation lies alongside the center of the tooth). A tooth rotated eccentrically out of the dental arch may be pushed back at the edge that is protruding; a centrically rotated tooth is twisted in the dental arch by diametrically opposed forces at its protruding edges. Rotation can be present with or without a lack of space. Severe rotation in a dentition with gaps often has a hereditary element, and the tooth will often rotate back into the incorrect position. This rotating back is stopped by surgical division of the marginal system of fibers. Rotation in a mouth with crowding, once the lack of space has been resolved, is usually rectified by the influence of the lips, cheeks, and tongue.
If a tooth is tipped beyond an axis that lies within the dental arch, or beyond an axis that is perpendicular to the arch, this gives rise to vestibular, oral, mesial, or distal tipping.
Vestibular tipping of the maxillary anterior teeth may be inherited or caused by thumb sucking. In the posterior region, a persistent primary tooth or displacement of a tooth germ may produce vestibular tipping.
Oral tipping of the maxillary anterior teeth is an inherited characteristic of the complete vertical overlap. Oral tipping of the mandibular anterior teeth occurs after premature loss of primary canines in association with considerable lower lip tension. Oral tipping of the posterior teeth arises from incorrect germ positioning and disrupted exfoliation.
Mesial tipping of the incisors often results from a lack of space. If the lack of space is resolved, the incisors usually right themselves again. Mesial and distal tipping in the posterior region can arise because of teeth displaced into a gap.
Tooth displacement means the shifting of individual teeth or groups of teeth. If a tooth exceeded the occlusal level at eruption or if the occlusal line is not reached, this is deemed to be tooth displacement (high or low). Displacements usually accompany tipping or rotation.
Tooth retention is present when a tooth lies correctly in the jaw but remains buried beyond the normal eruption time. Semiretention arises if a tooth has only managed half of its eruption path because it is squeezed by crowding or because root and alveolar bone are fused.
Crowding can have various causes and take various forms. In the case of primary crowding, there is often a mismatch between tooth size and jaw size, for example, in the form of restricted development in the growth of jaw width and length. Secondary crowding results from pronounced mesial migration of the posterior teeth, so that the anterior teeth twist to form a flat front. A lack of space will produce local tipping and rotation when the crowding affects parts of the dentition or the entire arch. A temporary lack of space will result from primary tooth persistence.
Spaces or gaps can have a hereditary cause, where the teeth are too small in jaws that are too large (see Fig 5-18). A localized space between the central incisors or in other parts of the dental arch is called a diastema. Gaps can arise because of external influences such as supernumerary teeth or cysts intervening, but also when the tongue or foreign bodies are incorporated as a result of habit. Spaces between all the teeth can be resolved if all the teeth are pushed forward toward the midline. Gaps in the maxillary anterior teeth as a result of thumb sucking are reversible if the child gives up the habit. Spaces in the mandibular anterior teeth can be caused by overdevelopment or faulty functioning of the tongue. Tongue function can usually be relearned with the aid of appliances. In the case of overdevelopment, surgical reduction of the tongue may become necessary. Dysgnathia caused by lack of space is more common than faulty development caused by dentitions with gaps.