Permanent mandibular second molar is the 7th
tooth from the midline. Its tooth number in FDI system is 3 7 and 4 7, in Universal System
18 and 31 and in Zsygmondy Palmar system it is 7 with appropriate quadrant brackets.
It develops from 4 developmental lobes- mesio-buccal, disto-buccal, mesio-lingual
and disto-lingual, which give rise to 4 cusps respectively. It has 2 roots- mesial root
and distal root. Its function is grinding. In chronology, the tooth erupts at the age of
11 to 13 years. Due to this, it is sometimes also called as the '12-year molar'. Coming to
its dimensions. The cervico-occlusal length of the crown is 7 millimeters and the
length of both roots is 13 millimeters. The mesio-distal diameter of the crown is 10.5
millimeters and at cervix it is 8 millimeters the bucco-lingual diameter of the crown is 10
millimeters and at cervix it is 9 millimeters. The curvature of cervical line on mesial side
is 1 millimeter and on distal side it is flat. The morphology of a tooth is studied
under 5 aspects. Let us start with the buccal aspect. The outline form of
the crown is trapezoidal in shape. Both buccal cusps are seen in front, mesio-buccal
cusp and disto-buccal cusp. Which are almost equal in size. Both lingual cusps- mesio-lingual cusp
and disto-lingual cusp are also seen partially. Both contact areas, mesial contact area and
distal contact area are in the middle third. Both outlines, mesial outline and distal outline are
convex. However, the cervical line is irregular. A buccal developmental groove is seen
running between the two buccal cusps. A faint cervical ridge runs
mesio-distally in the cervical third. From this aspect, both roots are visible. The
root trunk is seen bifurcating into the mesial root and distal root. Both roots are almost
parallel to each other with a distal tilt. Next, the lingual aspect. All the features
of the outline form, from the lingual aspect, are same as that in the buccal aspect.
Moreover, both lingual cusps are seen- mesio-lingual cusp and disto-lingual cusp and
both are of equal size. A lingual developmental groove is seen running between the lingual cusps.
Next, the mesial aspect. The outline form of the crown is rhomboidal in shape. Both mesial cusps
are seen, mesio-buccal cusp and mesio-lingual cusp. Both buccal and lingual outlines are convex,
with crest of curvature of lingual outline in the middle third and buccal outline in the cervical
third, because of the cervical ridge. The cervical line is irregular. The mesial marginal ridge
runs horizontally between the mesial cusps. Coming to the root. The mesial root is seen
in front, which is conical in shape and has a developmental depression. Distal root is
seen partially. Next, the distal aspect. The features of the outline form, from the distal
aspect are same as that from the mesial aspect. However, the distal marginal ridge is lower
than the mesial marginal ridge, because of which more of occlusal surface is visible. From the
distal aspect the distal root is seen in front, which is conical in shape and has a developmental
depression. Mesial root is seen partially. Finally, the occlusal aspect. The outline form
of the crown is roughly rectangular in shape. 4 cusps are seen- mesio-buccal cusp, disto-buccal
cusp, mesio-lingual cusp and disto-lingual cusp. They are all of almost the same size. Ridges. Each cusp has two cuspal ridges- 1
mesial cuspal ridge and 1 distal cuspal ridge. 4 triangular ridges, 1 running from each cusp
tip towards the center. 2 marginal ridges- mesial marginal ridge on the mesial border and
distal marginal ridge on the distal border. Fossa. 1 central force, almost circular in
shape in the center of the occlusal surface. Mesial triangular fossa, next to the mesial
marginal ridge and distal triangular fossa adjacent the distal marginal ridge. Pits.
There is a central pit in the central fossa, a mesial pit in the mesial triangular fossa
and distal pit in the distal triangular fossa. Grooves. The central developmental groove
extends from the mesial pit, runs through the central pit and ends in the distal pit. The buccal
developmental groove runs from the central pit, between the two buccal cusps on to the buccal
surface and the lingual developmental groove, runs from the central pit between the
lingual cusps onto the lingual surface. These 3 grooves make a cross pattern,
dividing the occlusal surface into 4 almost equal parts. Also, numerous
supplementary grooves are present. In clinical considerations. Fixed
orthodontic treatment is usually done after the age of 13 years, that is,
after the eruption of the second molars.