Lower arch crowding after establishment of the permanent dentition during teen age period (post adolescent crowding) is a common orthodontic problem. This late lower arch crowding is caused by pressure from the back of the arch. Whether this pressure results from a developing third molar, physiologic mesial drift or the anterior component of force derived from the forces of occlusion on mesially inclined teeth is not clear. There is also a school of thought holding the view that in the absence of third molar, the dentition has room to settle distally under anterior pressures caused by late growth or soft tissue changes. Thus the third molar plays, at the very least, a passive role in the development of late lower arch crowding.
Bishara et al calculated the change in lower incisors at the age of 12 to 25 years and again at the age of 45 years. They tried to find out the tooth size arch length discrepancy. It was seen that there is a difference of 2.7mm in males and 3.5 in females. This is due to the decrease in arch length with time.
Margaret Richardson in AJO 92 examined changes in the lower arch crowding in young adults and showed that between the ages of I8 and 21 years, the lower arch is stable in terms of tooth alignment and mesial drift, regardless of third molar status or continuing mandibular growth.
60 dental students were examined. Out of them thirty have unilateral maxillary upper molar present and 27 have the lower third molar absent on one side. 3 students either have lost the third molar or it was altogether absent. It was found that in the quadrant in which third molar is present there is more crowding as compared to the side in which third molar is absent. It is more prominent in mandibular arch as compared to the maxillary arch.
In another study 40 patients with the third molars or wisdom teeth present and the 25 patients with third molar absent were studied. There arch perimeter was measured at the age of 13 and then at the age of 19. It was found that in all the cases the arch perimeter decreased. But it is less in case where the third molars or wisdom teeth are absent.
According to Richardson in AJO89 the Belfast third molar study produced further evidence in support of the 'pressure from behind' theory. A group of 51 subjects with intact lower arches and bilateral third molars or wisdom teeth present were examined at ages 13 to 18 years. On average these cases had an increase in lower arch crowding of slightly more than 1 mm on each side during the five year observation period. In some quadrants the crowding increased by as much as 4mm. Molar space measured along the maxillary horizontal as the distance between the distal contact point of the first molar and the junction of the ramus with the body of the mandible was also examined. The changes in molar space condition were calculated by subtracting the size of second and third molars or wisdom teeth from the measurement of molar space. A significant correlation between increase in anterior crowding and initial degree of molar crowding was found suggesting that a person who lacks adequate space in the molar region in early permanent dentition is likely to show an increase in crowding anterior to first molar in subsequent years. They also showed that conditions in the molar region were only partly responsible for increased crowding. Mesial inclination of lower canine usually is considered to be a sign that the buccal segment has moved forward. Richardson also investigated the effect of second molar extraction on the development of late lower arch crowding and found that there was a slight decrease in crowding (- 1.5mm) and a slight distal movement of first molar (-1.3mm).
Studies Indicating Lack of Correlation between Third Molar or Wisdom Teeth and Crowding
Kaplan in 1974 compared pre-treatment, post treatment and 10 year post treatment study models and lateral cephalograms of 75 orthodontically treated patients. Three groups with third molars or wisdom teeth erupted, impacted and congenitally missing were compared. Kaplan showed that some degree of lower anterior crowding relapse occurred in majority of cases. He concluded that the presence of third molars or wisdom teeth does not produce a great degree of lower anterior crowding or rotational relapse after cessation of retention.
According to Kaplan the theory that third molars or wisdom teeth exert pressure on the teeth mesial to them could not be substantiated.
Ades et al in AJO 1990 studied pre treatment, post treatment and post retention study models and lateral cephalometric radiographs of 97 patients. The subjects were divided into those with lower third molar erupted, impacted, agenesis and extracted at least 10 years before. The study showed that the irregularity in mandibular incisors increases as the inter canine width decreases and it does no depends upon whether the third molars or wisdom teeth are present or not.
These studies indicate that incidence of mandibular incisor crowding increased during adolescence and adulthood in untreated as well as orthodontically treated patients after retention is discontinued.
The third molars or wisdom teeth need to be considered as part of overall treatment planning and this may include a recommendation for extraction. The timing of extractions requires an understanding of the various techniques available.
It is not common practice for third molar teeth to be enucleated before orthodontic treatment of adolescents. Orthodontists are normally reluctant to make surgical extractions a prerequisite of providing treatment. They may feel that ramus growth and lower third molar eruptive pattern cannot be predicted and take the view that the third molars or wisdom teeth may erupt eventually. Those in favour of enucleating believe that many young adults between the age of I8 and 22 years experience problems with their third molars or wisdom teeth and that at later ages, pathologic changes often occur.
Ricketts et al indicated that removal of the third molar bud at the age of 7 to 10 year is surprisingly simple and relatively atraumatic. Schulhof recommended that enucleation should be considered for any lower third molar which, after careful diagnosis, has a greater than 50% chance of impaction. Enucleation is not, however, a generally accepted procedure. Later caries experience, space condition, and the effect of orthodontic treatment are unknown at thisearly age.
At about age 12 years, the need for third molar extraction may he more obvious. Conventional surgical removal of the calcified crowns, at this age is difficult. The tooth rolls in its crypt with considerable trauma to the adjacent tissues. There is also a risk of gingival damage, with a pocket formation to the distal of the lower second molars due to the ill shaped incision involved. Henry recommended a deep lateral approach, calling it “Lateral trepanation” for third molars or wisdom teeth in an early stage of partial development. Burgess et al and Henry recommended lateral trepanation and an easier technique, with lesser complications and more rapid healing.
If orthodontic treatment includes orthognathic surgery in one or both jaws, and third molars or wisdom teeth also require extractions, surgeons prefer to do it at the same operation. However, some prefer to remove lower third molars or wisdom teeth 6 months before orthognathic surgery, so that bone healing can occur in the surgical site.
Extraction of third molar after orthodontic treatment with a view to prevention of relapse should seldom be necessary. Lindquist and Thilander did a study to determine whether the third molars or wisdom teeth should be extracted as the prophylactic measure before the ortho treatment or not. They extracted the third molars or wisdom teeth on one side and retained on the other side. After 3 years they did a series of X-ray’s to find out the effect of the extraction of third molar but could not find any difference on the side where the third molar was extracted from the other side where the third molar was present.
In another study to determine the contact tightness of the lower teeth in which the third molars or wisdom teeth are not erupted, it was found out that extraction of third molar des not have any effect on the tightness of the contact. Similarly another study was carried out in which24 individuals got there lower third molars or wisdom teeth extracted in te third decade of life . The dental casts were taken before and after one year of extraction. It is seen that there is slight mesial drifting of second molar but nothing significant in relation to the lower anterior.
All the above mentioned studies indicate that the third molars or wisdom teeth do not influence the lower anterior teeth crowding and just to avoid the anterior crowding third molars or wisdom teeth should not be extracted till it is absolutely essential.
By evaluating the various studies the following conclusions are reached.
1) Various factors are responsible for the crowding in the lower incisors like retrusion of the incisors, decrease in the arch length, decrease in the intercanine width etc. so if enough space is present for the eruption of the third molar then there is no need to extract the third molar to relieve the anterior crowding.
2) When ortho treatment is done in the upper ad lower arch some times the 1st and the 2nd molar has to be shifted backwards. In this condition the space for third molar is reduced and causes the impaction of the third molars or wisdom teeth. If clinician predicts this condition then it is advisable to extract the third molars or wisdom teeth before they get impacted.
3) It is advisable to extract the third molar if needed ,as soon as possible. Like before it is fully developed as the post operative complication like pain, swelling is less if the third molar is extracted before the roots are completely formed. So if the clinician sees the condition where third molar has to be extracted, then he should extract it as early as possible.
4) No studies support that the third molars or wisdom teeth help in the development of maxilla or mandible. So if there is absolute necessity to extract the third molar then it should be extracted as early as possible.
5) It is advisable to extract the third molar as soon as possible if there is absolute necessity but at the same time it is not advisable to enucleate the tooth bud of third molar by doing the studies at the age of 7 to 8 years as those measurements are not fully reliable.
Bonding techniques allow placement of fixed lower retainer which allows stabilization of lower incisors, while awaiting further development of third molars or wisdom teeth. Bonding techniques also allow uprighting of some third molars or wisdom teeth. Thus the case for 'wait and see' with third molars or wisdom teeth is stronger than it was, and the case for extractions has become weaker.
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