Mandibular Molars Comparison

Question 1

Mesiodistal Crown Width

Answer 1

• The mesiodistal
  crown width is
  greatest on first
  molars and less
  on second molars.
• Most of the experts
  say that third molars
  are smaller mesiodistally
  than second
  molars, a couple say
  they are the same,
  and one says that
  third molars are
  actually larger than
  second molars.
• Due to the
  variability in
  the size of third
  molars, all of
  these are true
  possibilities.

Question 2

Occlusocervical Crown Height

Answer 2

• Some of the
  experts say that
  the occlusocervical
  crown
  height is greatest
  on first molars and
  less on second
  molars.
• Others say they
  are the same, and
  still others say that
  the crown height of
  the second molar
  is actually greater
  than the first.
• Likewise, there is
  disagreement about
  third molars, where
  some experts say
  they are shorter
  than second molars
  and others say they
  are the same.
• In fact all of
  these are true
  possibilities.

Question 3

Crown Symmetry

Answer 3

• Second molars are
  more symmetric
  than first molars
  (second molars
  don’t have a distal
  cusp).

Question 4

Third Molar Distal Crown Height

Answer 4

• The distal half of
  third molar crowns
  is noticeably
  shorter occlusocervically
  than the mesial half.

Question 5

Number of Buccal Cusps

Answer 5

• First molars have
  3 buccal cusps
  whereas second
  and third molars
  usually have only
  2 buccal cusps.
• Second and
  third molars will
  occasionally have
  3 buccal cusps and
  will look like small
  first molars.

Question 6

Occlusocervical Cusp Height and Form

Answer 6

• Second molar
  cusps are taller
  and more pointed
  than the shorter,
  blunted first molar
  cusps.

Question 7

Number of Buccal Grooves

Answer 7

• First molars have
  2 buccal grooves
  whereas second
  and third molars
  usually have only
  1 buccal groove.

Question 8

Root Length

Answer 8

• First molars have
  longer roots than
  second molars.
  Second molars
  have longer roots
  than third molars.

Question 9

Root Separation

Answer 9

• The greatest
  mesiodistal root
  separation is on
  first molars, less
  on second molars,
  and the least
  separation (even
  fusion) on third
  molars.

Question 10

Root Parallelism

Answer 10

• The roots are
  more parallel to
  one another on
  second molars
  than first molars.

Question 11

Root Apex Curvature

Answer 11

• The mesial root
  is usually curved
  to the distal on
  first molars,
  and sometimes
  strongly curved in
  the apical one third.

Question 12

Root Trunk Dimensions

Answer 12

• The cervico-apical
  dimension (height)
  of the root trunk is
  greatest on third
  molars, less on
  second molars,
  and is the shortest
  on first molars.

Question 13

Root Inclination

Answer 13

• The distal
  inclination of the
  roots is greatest on
  third molars, less
  on second molars,
  and the least distal
  inclination occurs
  on first molars.

Question 14

Root Apex Form

Answer 14

• Second molars
  have more pointed
  root apices than
  first molars.

Question 15

Buccolingual Crown Width

Answer 15

• When viewed
  occlusally, the
  buccolingual
  width is greatest
  on first molars
  and less on
  second molars.
• Third molars
  may be narrower,
  wider, or the
  same as second
  molars.

Question 16

Crown Outline Shape

Answer 16

• From an occlusal
  view, crowns of
  first molars are
  pentagonal in
  shape whereas
  second molars
  are more
  rectangular.
• Third molars
  are rectangular
  or ovoid.

Question 17

Mesiobuccal Prominence

Answer 17

• There is a
  mesiobuccal
  prominence
  when second
  molars are
  viewed
  occlusally.

Question 18

Crown Form Variations

Answer 18

• Third molars
  vary from their
  “normal” form
  more often than
  other teeth.
• One common
  variation is an
  extra-large crown
  with short, poorly
  formed roots.
• The existence
  of the extra-large
  crown probably
  accounts for
  some of the
  expert’s disagreements
  about the sizes
  of these teeth.

Question 19

Proximal Crown Form

Answer 19

• From an occlusal
  view, the mesial
  and distal
  surfaces of
  first molars are
  straighter than
  the corresponding
  surfaces on
  second molars.
• Comparing
  second molars
  to third molars,
  third molar
  surfaces would
  be more convex.
• For each individual
  tooth the mesial
  surface would be
  straighter (flatter)
  than its distal
  surface, a remnant
  of the evolution of
  mammalian teeth
  from their reptile
  ancestors.

Question 20

Proximal Crown Convergence

Answer 20

• The crown
  tapers lingually
  more on first
  molars than
  second molars.

Question 21

Number of Cusps

Answer 21

• Mandibular first
  molars usually
  have 5 cusps
  whereas second
  and third molars
  usually have 4
  cusps.
• Five-cusp
  third molars
  are common,
  however, and
  they may
  occasionally
  have more
  than 5 cusps.
• If you find 5-cusp
  third molars in
  your extracted
  teeth next week,
  save them for
  mounting, do
  not cut them up.

Question 22

Central Developmental Groove Form

Answer 22

• The central
  developmental
  groove has a
  “zigzag” mesiodistal
  form on first molars
  whereas it is
  straighter on
  second molars.

Question 23

Developmental Groove Alignment

Answer 23

• The buccal
  and lingual
  developmental
  grooves are not
  aligned with each
  other on the first
  molar whereas
  they are aligned
  on second molars.
• Anthropologists
  have referred to
  the pattern on
  first molars as
  “Y 5” and the
  pattern on second
  molars as “+ 4”.
• There is no set
  pattern on third
  molars, as they
  may resemble
  either first or
  second molars.

Question 24

Supplemental Grooves

Answer 24

• Second molars
  generally have
  more supplemental
  grooves than first
  molars, and third
  molars have more
  supplemental
  grooves than
  second molars.
• It’s not uncommon
  for third molars
  to have many
  supplemental
  grooves, and
  a “wrinkled”
  occlusal surface.

Question 25

Buccolingual Cusp Proximity

Answer 25

• The buccal and
  lingual cusps of
  third molars are
  closer to each
  other than the
  cusps of the
  other molars.
• Third molars
  therefore have a
  smaller occlusal
  table than first or
  second molars.

Question 26

Crown Form

Answer 26

• Third molars
  have more
  bulbous crowns
  than first or
  second molars.

Question 27

Buccolingual Root Width

Answer 27

• The buccolingual
  width of
  the mesial root
  is greater on
  first molars than
  second molars.