口腔解剖生理学之国外PPT--下颌第二磨牙

DESCRIPTIVE ANATOMY OF MANDIBULAR SECOND MOLAR
Prof. A. Elsahn
CHRONOLOGY
Initial calcification 2½ – 3 years
Completion of crown 7 – 8 years
Eruption
Completion of roots
MESIAL ASPECT
THE CROWN
L
O
B C

Geometrically, the crown is similar to the first molar; rhomboidal in shape with lingual inclination, short cervicoocclusally, and wide bucco-lingually.
Prof. A. Elsahn
BUCCAL ASPECT
THE ROOTS

Both roots are flattened mesio-distally and the mesial root is slightly longer than the distal.
M D

Both roots usually are inclined distally more than those of the first molar.
11 – 13 years
14 – 15 years
Prof. A. Elsahn
GENERAL CHARACTERISTICS

This molar is the seventh tooth from the middle line.
7
6 5
o It has four cusps.
4

Its disto-buccal cusp is larger than that found in the lower first molar.
Prof. A. Elsahn
BUCCAL ASPECT
THE CROWN
D C
O
O
M
C
D

As in the first molar,the crown is much larger mesiodistally than cervico-occlusally. The crown appears to be wider at the cervix than the first molar (less taper toward the cervix below the contact areas), due to Prof. A. Elsahn absence of a distal cusp.
There is a depression on the root trunk from the cervical line to the bifurcation.

Prof. A. Elsahn
BUCCAL ASPECT
THE ROOTS

The roots (mesial and distal roots) vary in their length.

LINGUAL ASPECT
THE ROOTS

The short root trunk has a depression between the cervical line and the bifurcation.

The proximal surfaces of the roots converge lingually to a slight degree, therefore, little of the mesial or distal surfaces may be seen lingually. Prof. A. Elsahn

They may be as long or shorter than those of the first molar.

The roots are usually closed together and nearly parallel.
Prof. A. Elsahn
BUCCAL ASPECT
LINGUAL ASPECT
The

crown
The lingual developmental groove separates the two lingual cusps.

It is a short groove, usually end in the occlusal third and is never pitted. The cervical line is irregular mesio-distally, sometimes dropping cervically on the root bifurcation. Prof. A. Elsahn
Prof. A. Elsahn
LINGUAL ASPECT
The

crown
The mesial and distal curvatures on the crown that describe the contact areas are more noticeable from the lingual aspect.
BUCCAL ASPECT
THE CROWN
CERVICAL LINE
It It
is nearly straight.
may point sharply toward the root bifn
BUCCAL ASPECT
THE ROOTS

The root trunk is short (but slightly longer than on lower first molar).
M The crown is usually smaller than the first molar in the same mouth.
1
2
3
B
B 2 4 D D
1
3 L

L
Prof. A. Elsahn
GENERAL CHARACTERISTICS
B D L

M
There is only one buccal groove. The cervical ridge is less pronounced than on the first molars in most instances, but a prominent bulge cervically on the mesio-buccal Prof. A. Elsahn cusp is common.
Prof. A. Elsahn
BUCCAL ASPECT
THE CROWN
OCCLUSAL OUTLINE
M D

The two buccal cusps (mesio-buccal and disto-buccal) and the tips of the two lingual cusps (mesio-lingual and disto-lingual) are visible from the buccal side because the lingual cusps are slightly longer. Prof. A. Elsahn
Prof. A. Elsahn

MESIAL ASPECT
THE CROWN
L B

Lingually, the crest of curvature of the crown is about in the center of the crown and is lingual to the root due to the lingual inclination of the crown. Prof. A. Elsahn
Prof. A. Elsahn
MESIAL ASPECT
THE CROWN

L
B
Buccally, the cervical ridge is in most cases less pronounced although many of them show a prominent bulge cervically on the mesio-buccal cusp only. Occlusal to the cervical ridge, the buccal outline slants lingually.
Prof. A. Elsahn
DISTAL
BUCCAL ASPECT
THE CROWN

Both contact areas are at slightly lower level, especially in the distal area than those of the first molar.
Prof. A. Elsahn

Only the lingual cusps are visible.
LINGUAL ASPECT
The
crown
The lingual cusps are more pointed than the buccal cusps.
D
M
The mesiolingual cusps is slightly wider and longer than the distolingual. Prof. A. Elsahn

BUCCAL ASPECT
THE CROWN
M
D
Mesial outline: Distal outline: