Clinical management of the mandibular molars Josh
A.
Canut
Valencia, Spain
E
ver since Angle presented his classifications of malocclusion, the permanent first molars have been the protagonists in orthodontic treatment. Angle’s concept of the stability of the upper first molar as the reference tooth in the ocelusal arrangement lasted several decades. Later, clinical evidence of effective distal movement of this tooth by means of extraoral traction changed the therapeutic concept and the general focus of the treatment of malocclusion. The gradual gain in the popularity of selective extraction in orthodontics obscured the equally effective possibilities of distal movement of the lower first molar, and it continues to be considered as a tooth difficult to move distally within its osseous mandibular framework. This incapacity for clinical purposes seems to be due to confusion concerning the true role of the lower molar in orthodontic treatment and to the alternatives that this large tooth offers in the correction of malocclusion. The purpose of this article is to describe the treatment of certain clinical cases in which proper correction of the mandibular molars will determine the success or failure of the mechanical procedures. A concept of the normal position of the lower first and second molars will be described first. An approach and a technical procedure in the treatment of certain types of malocclusion will then be illustrated. A concept
of normality
The work done by Stollerl and Andrew9 illustrates the importance of moving the upper first molars into a relationship that is slightly distal to the traditional concept proposed by Angle. In the great majority of ideal occlusions, the mesiobuccal cusp of the upper molar overlaps the distobuccal cusp of the lower molar. The maximum transverse width of the maxillary arch is located between the mesiobuccal cusps of the upper first molars. From that point, the parabolic design of the arch converges toward the median line, with the second and third molars being tapered somewhat lingually from the first molars. In order that the occlusion of the first molars may be perfect, the lower first 277
Fig.
1. Lower
Fig.
2. Gingival
molar
position
position
on
of the
human second
mandible molar
(3000 with
respect
B.C.]. to the
lower
first
molar.
molars should also present a similar position with their mesiobuccal cusps. This morphologic characteristic has been observed in anthropologic specimens (Fig. 1). With the mesiobuccal cusp of the lower first molar as a base, the rest of the cusps are situated on an imaginary line of lingual convergence. The maximum width of the lower arch is also found at the level of these mesiobuccal cusps. The first molar may be sa,id to be located normally on the external oblique line. and the third molar on the internal oblique line of the mandible. Stoller and Andrews also stress the need to align the upper first molars with an accentuated mesial inclination. Thus, in the normal occlusion, the distobuccal cusp of the upper first molar is located slightly occlusal to the mesiobuccal cusp of the lower molar. In order to attain optimal occlusion, the distolingual cusp occludes over the mesial marginal ridge of the lower second molar. As a result, the lower second molar adopts a position slightly gingival with respect to the first molar; the crown of the lower second molar, therefore, will appear slightly submerged because it receives the occlusal impact of t,he distolingual cusp of the moderately extruded upper first molar (Fig. 2’). Positional
anomalies
of the
second
molars
Pathologic efects. The supraeruption and buccal displacement of the lower second molars are among the problems that must be treated orthodontically in order to achieve a gnathologic balance of the dentition. The malposition of the
Volume Number
68 3
Fig.
managewed
Clinical
4.
Patient
A.B.
A,
Occlusal
view
before
of qnandi6ular
treatment;
B, after
molars
279
treatment.
lower second molars is a source of trauma on the balancing side of the arches. Their alignment and articulation with the upper molars is often neglected at the end of orthodontic treatment until the orthodontist “discovers” them later on examining the final plaster casts of the case. The loss of the first molars is a cause of future periodontal pathosis. It is due to the tipping of the crowns of the second molars and the second premolars toward the extraction site (Fig. 3). Prosthetic reconstruction is difficult in its construction a.nd uncertain in its prognosis. In these cases, the loss of the first molars (Fig. 4) was accompanied by an open-bite that was corrected before prosthetic restorations were placed. In this way, future osseous and periodontal lesions can be prevented on the teeth that have to act as the basic abutments of the dental reconstructions. Mesinl blocking. The mesial eruptive tendency of the lower posterior teeth explains the frequency with which the lower second molars are blocked in their eruptive path by the crown of the first molars. It is not rare to find that this molar malposition is self-corrective. If more severe wire ligatures have to be placed below the point of contact, it has a lever or wedging effect on the second molar. Finally, there are even more difficult cases in which complete appliances have to be placed to move the second molar into its normal position (Fig. 5). Buccal displacement. This is a transversal malposition that may appear in the final phases of active treatment. In the case shown in Fig. 6, the second molars erupted to the buccal after the extraction spaces had been closed. When this occurs, the second molars are banded and the position of the teeth is corrected with springs, as illustrated in Fig. 6. Clinical observation makes it possible to determine whether it is the second
Fig.
5.
molar;
Patient
A.G,
6, arch
design
a
13-year-old
to upright
girl. this
molar;
A,
Lower C, lateral
arch view
with after
blocked
right
lower
second
treatment.
molal3 that art’ ln~cc~all,~tlisplacctl, 01’ \\-llt$hc>r thcfi cross-l)itcl is cluckto c5cessivt contraction of the lower arch. The frequent association of cross-bites with upper arch compression must al~va~s be c.onsitlered. “(,“riss-csross” elastics from the buccal aspect of the lower molars to the lingual aspccat ot’ the upper molars arc indicated to correct the cross-bite. Therapeutic
value
of the
second
molars
One way to tlctcrminc the quality ot’ the treatment procedures in an office is to observe the number of lower second molars in the patients under treatment. The tedious task of cementing bands 011 the sccontl molars can be justified by t,hc satisfaction gained from the improvctl position of these teeth. It is nevessar? to judge meticulously the most opportune moment iTI the treatment sequencec to place these bands. Extrrrctim chases.In those cases of malocclusion in which premolar cstraction is indicated, the banding of the second molars will be determined in part by the need for posterior anchorage in the lower arch. The value of the second molars as anchoragr is (lcrirccl from their position illltl theira ostensive root surface area. The dense, compact type of bone that surrounds their roots makes them difficult to mope either mesially or distally. Orthodontists should take advantage of their static potential in orcicr to strengthen the anchorage base in the lower arch. In cases of lower protrusion or accentuated (