Dental Crowding and Impaction

Crowding of teeth results either from a lack of space due to reduced growth of the jaws or to increased tooth size. Malnutrition adversely affects jaw growth but bone growth is stimulated by increased function during development. Soft diets are associated with reduced jaw dimensions and less space available for erupting teeth. The incidence of crowding and malocclusion is higher in both domesticated animals and captive animals compared to wild animals. Third molar impaction also increases in human populations where there has been a shift to a soft diet. Loss of space in the jaws also occurs when deciduous teeth are lost early and there is mesial drift of posterior teeth into the space they hold open. Crowded teeth are usually either displaced from the dental arch as they erupt (buccally or lingually) or maintain their developmental packing positions and fail to spread out and align correctly. Crowding among modern humans is common and displaced canines, lateral incisors remaining behind central incisors, premolar rotation and displacement and molar impactions are all frequent occurrences. Among great apes crowding is also frequently observed. Imbricated lower incisors are more common in Pan and Pongo than in Gorilla (8%, 15% and 1.1%, respectively) and may reflect the narrow mandibular dimensions anteriorly. Displaced canines are more common in Gorilla but rare in Pan and Pongo. Premolar displacement and rotation is common (as much as 4.5%) in Gorilla and Pongo but less so in Pan. Early loss of deciduous molars is a likely cause of this. Molar crowding is rare in great apes and usually takes the form of mesial tilting or rotation of mandibular third molars or a distal tilting of maxillary third molars that appear to maintain their developmental inclinations. Impaction, or gross misplacement, of third molars is very rare in great apes.