Third molars or wisdom teeth, as they are more commonly known, are the teeth which are most often missing, impacted and with altered morphology. Advances in dental anthropology states that there is a reduction in the number of teeth and size of jaws on evolutionary basis for the past 100000 years. Third incisors, third premolars and fourth molars have disappeared already. At present human third molars or wisdom teeth or wisdom teeth often fail to develop, which indicate that these teeth may be on their way out. Although there are some who advice the early removal of the third molars or wisdom teeth, many strongly believe that the retention of asymptomatic third molars or wisdom teeth may be useful in later years as a substitute for badly decayed teeth or may even be useful as a transplant.
Development and Eruption of Wisdom Teeth or Third Molars
There is great variation in the timing of development, calcification, and eruption of third molars or wisdom teeth. Development of wisdom teeth may begin as early as 5 years or as late as 16 years, with the peak formation period at 8 or 9 years. Calcification can start at age 7 years in some children and as late as age 16 years in others. Enamel formation is normally complete between 12 and 18 years and root formation is normally completed between 18 and 25 years. Hellman reported that the average age of eruption was 20.5 years. In 1962, Fanning reported that average ages of eruption of 19.8 years for females and 20.4 years for males. Early formation of third molars or wisdom teeth is generally regarded as predictive of early maturation but not always of early eruption. Most surveys report that more than 17% of lower third molars or wisdom teeth become impacted. Lower third molars or wisdom teeth normally have their occlusal surfaces tilted slightly forwards and lingually during early calcification. As the mandible increases in length, with bone resorption at inner angulation between body and the ascending ramus of the mandible, the third molars or wisdom teeth become more upright. In contrast, upper wisdom teeth erupt downwards, backwards, and often outwards. There is, therefore, a possibility of crossbite, but tongue pressure on lower crowns and buccinator pressure on upper crowns will often correct this. If there is a lack of space, then normal eruptive paths cannot be followed, and crossbite can result.
Eruptive Pathways of Third molars or Wisdom Teeth
Richardson investigated the development of lower third molars or wisdom teeth between ages 10 and 15 years, using models and four cephalometric radiographs (90 degree left lateral, straight posteroanterior and 60 degree' left and right lateral views). She found that the angle of the occlusal surface of the lower third molars or wisdom teeth to the mandibular plane was 41 degree on average she found this decreased by 11 degree by age 15. Successful eruption of the lower third molar or wisdom teeth occurs by the tooth continuing to decrease its angle to the mandibular plane and moving occlusally into sufficient space.
J .N. Fayad et al in AJO 2004 determined the relationship between the maxillary molar’s sagittal inclination and the eruption of third molars or wisdom teeth using CT scans. In their study the sagittal inclination of maxillary first and second molars were greater in the subjects with erupted maxillary third molars or wisdom teeth than in those with impacted third molars or wisdom teeth and particularly in the younger subjects. They concluded that the vertical position of the first maxillary molar in the sagittal plane is a predictor of the eruption of the adjacent third molar and that the sagittal inclination of the maxillary molars increases with age which could be the effect of mesial drift.
Kahl et al when valuated ortho pantomograms of orthodontically treated 58 individuals in which the third molar was asymptomatic. He found out that with time some third molars or wisdom teeth became more upright while others showed more inclination either towards the mesial side or the distal side. They concluded that with third molars or wisdom teeth nothing can be predicted. It is not influences by age, amount of space, amount of bone, developmental stage.
Assessment of Space for Third molars or Wisdom Teeth
Shortage of space between the second molar and the ramus has long been identified as a major factor in the etiology of lower third molar or wisdom teeth impaction. Henry and Morant suggested a technique for predicting impaction of lower third molar or wisdom teeth using their third molar space index obtained by expressing the mesiodistal width of the third molar as a percentage of the space available measured on bimolar radiographs, If this index, exceeded a value of 120 for a person at maturity, impact ion could be predicted. This index decreases as the growth continues and the space available increases. Ledyard, studying lateral jaw radiographs, found that less than a 2 mm increase in space between the lower second molar and the ramus could be expected after the age of 14 years and a negligible increase after 16 years.
Ricketts claimed that, if 50% of the third molar crown lies ahead of the external oblique ridge at maturity, there is a 50% chance of eruption. Schulhof in 1976 claimed that growth prediction can estimate the adult dimension from Xi point to the mandibular second molar on a lateral cephalogram taken at the age of 9 years with a standard error of 2.8 mm. Schulhof suggested that lower third molars or wisdom teeth could not be classified as likely to erupt to good occlusion if the Xi point to lower second molar was measured less than 25mm.
Richard Olive et al in AJO 81 in a study on dried human skulls, examined the reproducibility of estimates of a space width ratio (space available between lower second molar and the ramus divided by mesiodistal width of mandibular third molar or wisdom teeth) on the lateral cephalogram, rotational tomograms (OPG), intraoral bitewings and 60 degree Cephalograms.
A template of cellulose acetate was prepared with a right angle T drawn in line through the lips of the most superior, anterior and posterior cusps of the first premolar to second molar section of buccal segment. The template was placed over the radiograph with the horizontal part of the T on the occlusal plane and the vertical pan touching the most distal part of second molar crown. The space available (AB), the mesiodistal width of the lower third moIar (CD) and space width ratio (AB/CD) were computed from the Digitized data. If this ratio is less than 120% then chances of third molar or wisdom teeth impaction are more. The rotational tomogram, intraoral bitewing and 60 degree rotated cephalogram were superior to the lateral cephalogram on the basis of reliability of results and reproducibility of radiograph technique for estimating the space width ratio.
The results suggested the difficulties in landmark location on lateral cephalogram. Locating anterior border of the ramus on lateral cephalogram is often difficult, which militates against good reproducibility of results. It was shown that the reliability for the lateral cephalogram technique alone was not as good as for the other techniques. The rotational tomograms yielded the most accurate estimates of space width ratio. Intraoral bitewings yielded the next best estimates.