Microsoft Word - Winnie Zhang 2016.docx


	
  

Vol	
  4,	
  No	
  1	
  (2016)	
  
ISSN	
  2167-­‐8677	
  (online)	
  

DOI	
  10.5195/d3000.2016.49	
  	
  

	
  

	
  

	
  

http://dentistry3000.pitt.edu	
  

	
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  University	
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by	
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  Press.	
  

	
  

Third	
  Molar	
  Eruption	
  Mechanisms	
  and	
  Patterns	
  
Winnie	
  Zhang1	
  
	
  
1	
  University	
  of	
  Pi.sburgh	
  School	
  of	
  Dental	
  Medicine,	
  Pi.sburgh	
  PA,	
  USA

Abstract	
  

Third	
  molars	
  are	
  highly	
  variable	
  in	
  their	
  presence	
  and	
  form.	
  This	
  report	
  focuses	
  on	
  
a	
  horizontally	
  impacted	
  third	
  molar	
  and	
  analyzes	
  the	
  poten.al	
  e.ology	
  of	
  this	
  sit-­‐
ua#on.	
  Upon	
  a	
  clinical	
  and	
  radiographic	
  examina#on,	
  it	
  was	
  noted	
  that	
  the	
  pa#ent	
  
had	
   four	
   third	
  molars	
  present.	
  The	
  pa3ent’s	
   third	
  molars	
  began	
  erup3ng	
  around	
  
the	
  age	
  of	
  19.	
  Currently,	
  they	
  are	
  asymptoma!c	
  with	
  incipient	
  caries	
  on	
  the	
  occlu-­‐
sal	
  surfaces.	
  While	
  three	
  of	
  the	
  third	
  molars	
  erupted	
  in	
  a	
  normal	
  orienta4on,	
  one	
  
of	
  the	
  third	
  molars	
  (mandibular	
  le2)	
  erupted	
  in	
  an	
  orienta5on	
  that	
  would	
  be	
  clas-­‐
sified	
  as	
  horizontal	
  and/or	
  mesio-­‐angular.	
  No	
   treatment	
  has	
  been	
  undertaken	
  at	
  
the	
  moment,	
  however	
  surgical	
  extrac4on	
  can	
  be	
  recommended	
  with	
  the	
  progno-­‐
sis	
   being	
   very	
   good.	
   For	
   the	
   clinician	
   that	
   has	
   to	
   treat	
   dental	
   complica6ons	
   that	
  
arise	
   from	
   abnormal	
   tooth	
   erup1on,	
   as	
   seen	
   in	
   numerous	
   gene1c	
   and	
   acquired	
  
disorders,	
   knowledge	
   about	
   the	
   basic	
   molecular	
   mechanisms	
   involved	
   may	
   be-­‐
come	
  extremely	
  important.	
  

	
  

Cita%on:	
  Zhang,	
  W.	
  (2016)	
  Third	
  Molar	
  Erup.on	
  
Mechanisms	
  and	
  Pa-erns.	
  Den$stry	
  3000.	
  
1:a001	
  doi:10.5195/d3000.2016.49	
  
Received:	
  May	
  21,	
  2016	
  
Accepted:	
  	
  June	
  13,	
  2016	
  
Published:	
  	
  September	
  28,	
  2016	
  
Copyright:	
  ©2016	
  Zhang,	
  W.	
  This	
  is	
  an	
  open	
  ac-­‐
cess	
  ar!cle	
   licensed	
  under	
  a	
  Crea!ve	
  Commons	
  
A"ribu%on	
  Work	
  4.0	
  United	
  States	
  License.	
  
Email:	
  wiz5@pi'.edu

	
  

Introduction	
  

Third	
  molars,	
  often	
  re-­‐
ferred	
  to	
  as	
  wisdom	
  teeth,	
  are	
  the	
  
most	
  distal	
  (posterior)	
  teeth	
  of	
  the	
  
three	
  molars	
  in	
  each	
  quadrant	
  of	
  
human	
  dentition.	
  They	
  generally	
  
erupt	
  between	
  the	
  ages	
  of	
  17and	
  
23.	
  Ideally,	
  the	
  third	
  molars	
  should	
  
erupt	
  just	
  like	
  all	
  the	
  other	
  teeth	
  
and	
  be	
  properly	
  aligned.	
  However,	
  
often	
  these	
  third	
  molars	
  are	
  misa-­‐
ligned	
  which	
  can	
  on	
  occasion	
  lead	
  
to	
  crowding	
  of	
  the	
  teeth	
  or	
  dam-­‐
age	
  to	
  adjacent	
  teeth,	
  and	
  rarely	
  
to	
  cystic	
  pathology	
  or	
  root	
  resorp-­‐
tion	
  [1].	
  	
  

Third	
  molars	
  can	
  also	
  be	
  
impacted	
  which	
  means	
  they	
  are	
  
enclosed	
  within	
  the	
  soft	
  tissue	
  
and/or	
  jawbone	
  or	
  they	
  only	
  par-­‐

tially	
  break	
  through/erupt	
  through	
  
the	
  gum.	
  Partial	
  eruption	
  of	
  third	
  
molars	
  provides	
  an	
  opening	
  for	
  
bacteria	
  to	
  enter	
  around	
  the	
  
tooth.	
  This	
  could	
  potentially	
  cause	
  
an	
  infection,	
  which	
  results	
  in	
  pain,	
  
swelling,	
  jaw	
  stiffness,	
  and	
  other	
  
complications.	
  Partial	
  eruption	
  
also	
  makes	
  the	
  third	
  molar	
  more	
  
susceptible	
  to	
  caries	
  and	
  perio-­‐
dontal	
  disease	
  because	
  they	
  are	
  in	
  
hard	
  to	
  reach	
  areas,	
  thus	
  making	
  it	
  
hard	
  to	
  brush	
  and	
  floss	
  those	
  are-­‐
as	
  [1].	
  	
  

There	
  are	
  multiple	
  ways	
  to	
  
classify	
  impacted	
  third	
  molars.	
  
One	
  way	
  is	
  based	
  on	
  the	
  nature	
  of	
  
the	
  overlying	
  tissues.	
  Based	
  on	
  the	
  
nature	
  of	
  the	
  overlying	
  tissue	
  im-­‐
paction,	
  	
  

impacted	
  lower	
  wisdom	
  teeth	
  can	
  
be	
  classified	
  into	
  [2]:	
  	
  

1.	
   Soft	
  Tissue	
  Impaction.	
  	
  
When	
  the	
  height	
  of	
  the	
  tooth’s	
  
contour	
  is	
  above	
  the	
  level	
  of	
  the	
  
surrounding	
  alveolar	
  bone	
  and	
  the	
  
superficial	
  portion	
  of	
  the	
  tooth	
  is	
  
covered	
  only	
  by	
  soft	
  (though	
  this	
  
can	
  be	
  dense	
  and	
  fibrous)	
  tissue.	
  	
  
Soft	
  tissue	
  impaction	
  is	
  usually	
  the	
  
easiest	
  type	
  of	
  impacted	
  tooth	
  to	
  
remove.	
  

2.	
   Hard	
  Tissue	
  ('Bony')	
  Impac-­‐
tion.	
  	
  This	
  is	
  where	
  the	
  wisdom	
  
tooth	
  fails	
  to	
  erupt	
  due	
  to	
  being	
  
obstructed	
  by	
  the	
  overlying	
  bone.	
  	
  
This	
  can	
  be	
  sub-­‐divided	
  into	
  (a)	
  
partial	
  and	
  (b)	
  complete	
  bony	
  im-­‐
pactions.	
  



	
  	
  Third	
  Molar	
  Erup.on	
  Mechanisms	
  and	
  Pa5erns	
  

Vol	
  4,	
  No	
  1	
  (2016)	
  	
  	
  	
  DOI	
  10.5195/d3000.2016.49	
  

	
  http://dentistry3000.pitt.edu	
  
2	
  

a.	
   Partial	
  Bony.	
  	
  The	
  superfi-­‐
cial	
  portion	
  of	
  the	
  tooth	
  is	
  covered	
  
only	
  by	
  soft	
  tissue	
  but	
  the	
  height	
  
of	
  the	
  tooth's	
  contour	
  is	
  below	
  the	
  
level	
  of	
  the	
  surrounding	
  alveolar	
  
bone.	
  	
  Apart	
  from	
  cutting	
  the	
  gin-­‐
giva	
  (gum)	
  and	
  possible	
  bone	
  re-­‐
moval	
  from	
  behind	
  the	
  tooth,	
  the	
  
tooth's	
  roots	
  may	
  need	
  to	
  be	
  di-­‐
vided.	
  

b.	
   Complete	
  Bony.	
  	
  The	
  tooth	
  
is	
  completely	
  encased	
  in	
  bone	
  so	
  
that	
  when	
  the	
  gingiva	
  is	
  cut	
  and	
  
reflected	
  back,	
  the	
  tooth	
  is	
  not	
  
seen.	
  	
  Bone	
  removal	
  (large	
  
amounts)	
  together	
  with	
  root	
  sec-­‐
tioning	
  will	
  be	
  needed	
  to	
  remove	
  
the	
  tooth.	
  	
  These	
  are	
  often	
  the	
  
most	
  difficult	
  teeth	
  to	
  remove.	
  

Another	
  way	
  to	
  classify	
  
impacted	
  third	
  molars	
  is	
  using	
  
Winter’s	
  Classification	
  which	
  is	
  
based	
  on	
  the	
  inclination	
  of	
  the	
  im-­‐
pacted	
  third	
  molar	
  to	
  the	
  long	
  axis	
  
of	
  the	
  second	
  molar	
  [2].	
  Each	
  type	
  
of	
  impaction	
  has	
  some	
  definite	
  
path	
  of	
  withdrawal	
  of	
  the	
  teeth.	
  
Mesially	
  impacted	
  teeth	
  are	
  (can	
  
be)	
  easier	
  to	
  remove	
  whereas	
  dis-­‐
tally	
  impacted	
  teeth	
  are	
  (can	
  be)	
  
the	
  hardest	
  to	
  remove.	
  Bucally	
  
positioned	
  maxillary	
  teeth	
  are	
  eas-­‐

ier	
  to	
  remove	
  as	
  the	
  bone	
  covering	
  
the	
  tooth	
  is	
  thinner	
  whereas	
  the	
  
palatally	
  positioned	
  tooth	
  requires	
  
bone	
  removal	
  and	
  hence	
  makes	
  
the	
  extraction	
  difficult	
  [2].	
  

1.	
   Mesio-­‐Angular	
  (Figure	
  1):	
  
The	
  impacted	
  tooth	
  is	
  tilted	
  to-­‐
ward	
  the	
  2nd	
  molar	
  in	
  a	
  mesial	
  
direction. 	
  

2.	
   Disto-­‐Angular	
  (Figure	
  2):	
  
The	
  long	
  axis	
  of	
  the	
  3rd	
  molar	
  is	
  
angled	
  distally	
  or	
  posteriorly	
  away	
  
from	
  the	
  second	
  molar.	
  	
  

3.	
   Horizontal	
  (Figure	
  3):	
  The	
  
long	
  axis	
  of	
  the	
  third	
  molar	
  is	
  hori-­‐
zontal.	
  

4.	
   Vertical	
  (Figure	
  4):	
  The	
  long	
  
axis	
  of	
  the	
  third	
  molar	
  is	
  par-­‐
allel	
  to	
  the	
  long	
  axis	
  of	
  the	
  
second	
  molar.	
  

5.	
   Buccal	
  /	
  Lingual	
  
Obliquity	
  (Figure	
  5):	
  	
  The	
  
tooth	
  can	
  be	
  buccally	
  (tilted	
  
towards	
  the	
  cheek)	
  or	
  lin-­‐
gually	
  (tilted	
  towards	
  the	
  
tongue)	
  impacted.	
  

6.	
   	
  Transverse	
  (Figure	
  6):	
  	
  
This	
  type	
  is	
  where	
  the	
  tooth	
  is	
  in	
  
effect	
  horizontally	
  impacted	
  but	
  
in	
  the	
  cheek	
  or	
  tongue	
  direction.	
  

7.	
   Inverse	
  (Figure	
  7):	
  Orienta-­‐
tion	
  of	
  the	
  impacted	
  tooth	
  in	
  
completely	
  inverted	
  in	
  reference	
  
to	
  the	
  occlusaly	
  immediate	
  oppo-­‐
site	
  tooth.	
  

Familial	
  Aggregation	
  

Observations	
  of	
  a	
  patient	
  and	
  fam-­‐
ily	
  members:	
  

A	
  23	
  year-­‐old	
  Asian	
  fe-­‐
male	
  with	
  no	
  significant	
  medical	
  
history	
  has	
  received	
  regular	
  
dental	
  check-­‐ups	
  every	
  six	
  
months	
  throughout	
  her	
  life	
  and	
  
had	
  orthodontic	
  treatment	
  from	
  
age	
  14	
  to16.	
  The	
  patient	
  has	
  
class	
  I	
  occlusion.	
  Upon	
  a	
  clinical	
  
and	
  radiographic	
  examination,	
  
it	
  was	
  discovered	
  that	
  the	
  pa-­‐
tient	
  had	
  four	
  third	
  molars	
  pre-­‐
sent	
  (see	
  Figure	
  8).	
  The	
  pa-­‐

tient’s	
  third	
  molars	
  began	
  erupting	
  
around	
  the	
  age	
  of	
  19.	
  Currently,	
  
they	
  are	
  asymptomatic	
  with	
  incip-­‐
ient	
  discoloration	
  on	
  the	
  occlusal	
  
surfaces.	
  While	
  three	
  of	
  the	
  third	
  
molars	
  erupted	
  in	
  a	
  normal	
  orien-­‐
tation,	
  one	
  of	
  the	
  third	
  molars	
  
(mandibular	
  left)	
  erupted	
  in	
  an	
  
orientation	
  that	
  would	
  be	
  classi-­‐

fied	
  as	
  horizontal	
  and/or	
  mesio-­‐
angular.	
  	
  

	
  



	
  	
  Third	
  Molar	
  Erup.on	
  Mechanisms	
  and	
  Pa5erns	
  

Vol	
  4,	
  No	
  1	
  (2016)	
  	
  	
  	
  DOI	
  10.5195/d3000.2016.49	
  

	
  http://dentistry3000.pitt.edu	
  
3	
  

The	
  patient’s	
  father	
  had	
  
four	
  erupted	
  third	
  molars,	
  all	
  with	
  
normal	
  eruption	
  orientation.	
  The	
  
patient’s	
  mother	
  has	
  congenitally	
  
missing	
  third	
  molars.	
  The	
  patient	
  
has	
  a	
  younger	
  sibling	
  who	
  is	
  19	
  
and	
  third	
  molars	
  have	
  not	
  erupted	
  
but	
  one	
  of	
  them	
  is	
  slightly	
  mesially	
  
tipped,	
  and	
  impacted	
  (Figure	
  9).	
  	
  	
  

If	
  one	
  considers	
  third	
  mo-­‐
lar	
  impaction	
  and	
  agenesis	
  as	
  vari-­‐
ation	
  of	
  the	
  same	
  clinical	
  presen-­‐
tation,	
  this	
  family	
  would	
  fit	
  an	
  au-­‐
tosomal	
  dominant	
  mode	
  of	
  inher-­‐
itance,	
  suggesting	
  a	
  major	
  gene	
  
effect.	
  

Mechanisms	
  Controlling	
  Tooth	
  
Eruption	
  

The	
  etiology	
  of	
  the	
  mecha-­‐
nisms	
  behind	
  eruption	
  patterns	
  of	
  
third	
  molars	
  is	
  currently	
  not	
  very	
  
well	
  understood.	
  Tooth	
  eruption	
  is	
  
a	
  complex	
  and	
  tightly	
  regulated	
  
process	
  that	
  involves	
  cells	
  of	
  the	
  
tooth	
  organ	
  and	
  the	
  surrounding	
  

alveolus.	
  The	
  genes	
  
that	
  are	
  involved	
  in	
  the	
  
process	
  of	
  tooth	
  erup-­‐
tion	
  are	
  found	
  all	
  over	
  
the	
  human	
  genome.	
  	
  

Mononuclear	
  
cells	
  (osteoclast	
  precur-­‐
sors)	
  must	
  be	
  recruited	
  
into	
  the	
  dental	
  follicle	
  
prior	
  to	
  the	
  onset	
  of	
  

eruption.	
  These	
  cells,	
  in	
  turn,	
  fuse	
  
to	
  form	
  osteoclasts	
  that	
  resorb	
  
alveolar	
  bone,	
  forming	
  an	
  eruption	
  
pathway	
  for	
  the	
  tooth	
  to	
  exit	
  its	
  
bony	
  crypt	
  [3].	
  There	
  are	
  many	
  
different	
  genes	
  that	
  are	
  involved	
  
in	
  tooth	
  eruption.	
  Some	
  of	
  the	
  
molecules	
  possibly	
  involved	
  in	
  the	
  
signaling	
  cascades	
  of	
  eruption	
  
have	
  been	
  proposed	
  in	
  studies	
  
from	
  null	
  mice,	
  osteopetrotic	
  ro-­‐
dents,	
  injections	
  of	
  putative	
  erup-­‐
tion	
  molecules,	
  and	
  cultured	
  den-­‐
tal	
  follicle	
  cells.	
  In	
  particular,	
  re-­‐
cruitment	
  of	
  the	
  mononuclear	
  
cells	
  to	
  the	
  follicle	
  may	
  require	
  
colony-­‐stimulating	
  factor-­‐one	
  

(CSF-­‐1)	
  and/or	
  
monocyte	
  chemo-­‐
tactic	
  protein-­‐1	
  
(MCP-­‐1)	
  [3].	
  If	
  the	
  
recruitment	
  of	
  
mononuclear	
  cells	
  
to	
  the	
  follicle	
  were	
  
misguided	
  and	
  the	
  
dental	
  follicle	
  of	
  a	
  

third	
  molar	
  were	
  disoriented	
  to	
  
begin	
  with,	
  it	
  could	
  potentially	
  re-­‐
sult	
  in	
  the	
  tooth	
  erupting	
  in	
  a	
  hori-­‐
zontal	
  orientation.	
  	
  

Paracrine	
  signaling	
  by	
  par-­‐
athyroid-­‐hormone-­‐related	
  protein	
  
and	
  interleukin-­‐1α,	
  produced	
  in	
  
the	
  stellate	
  reticulum	
  adjacent	
  to	
  
the	
  follicle,	
  has	
  also	
  been	
  found	
  to	
  

potentially	
  play	
  a	
  role	
  in	
  regulating	
  
eruption	
  [3].	
  This	
  is	
  an	
  example	
  of	
  
how	
  a	
  gene	
  seemingly	
  unrelated	
  
to	
  tooth	
  development	
  can	
  influ-­‐
ence	
  a	
  very	
  crucial	
  step	
  in	
  denti-­‐
tion	
  development.	
  Therefore,	
  it	
  is	
  

possible	
  that	
  a	
  mutation	
  anywhere	
  
along	
  the	
  genome	
  can	
  easily	
  influ-­‐
ence	
  a	
  tooth	
  to	
  develop	
  and	
  erupt	
  
in	
  an	
  abnormal	
  pattern	
  such	
  as	
  
horizontal	
  eruption.	
  	
  

Osteoblasts	
  might	
  also	
  in-­‐
fluence	
  the	
  process	
  of	
  eruption,	
  
the	
  most	
  important	
  physiologic	
  
role	
  likely	
  being	
  at	
  the	
  eruptive	
  
site,	
  in	
  the	
  formation	
  of	
  osteo-­‐
clasts	
  through	
  signaling	
  via	
  the	
  
RANKL/OPG	
  pathway.	
  If	
  this	
  sig-­‐
naling	
  pathway	
  were	
  interrupted	
  
by	
  another	
  protein,	
  osteoblasts	
  
function	
  could	
  potentially	
  be	
  al-­‐
tered	
  and	
  interfered	
  with.	
  This	
  
could	
  subsequently	
  affect	
  how	
  
bone	
  was	
  deposited.	
  If	
  bone	
  was	
  
deposited	
  only	
  underneath	
  the	
  
distal	
  portion	
  of	
  the	
  third	
  molar,	
  it	
  
could	
  potentially	
  cause	
  the	
  tooth	
  
to	
  tip	
  over,	
  giving	
  it	
  the	
  horizontal	
  
orientation	
  it	
  is	
  in	
  now.	
  Evidence	
  
thus	
  far	
  supports	
  a	
  role	
  for	
  an	
  os-­‐
teoblast-­‐specific	
  transcription	
  fac-­‐
tor,	
  CBFA1	
  (RUNX2),	
  in	
  molecular	
  
events	
  that	
  regulate	
  tooth	
  erup-­‐
tion	
  [3].	
  In	
  the	
  case	
  presented	
  



	
  	
  Third	
  Molar	
  Erup.on	
  Mechanisms	
  and	
  Pa5erns	
  

Vol	
  4,	
  No	
  1	
  (2016)	
  	
  	
  	
  DOI	
  10.5195/d3000.2016.49	
  

	
  http://dentistry3000.pitt.edu	
  
4	
  

above,	
  one	
  possibility	
  is	
  that	
  since	
  
neither	
  of	
  the	
  patient’s	
  parents	
  
had	
  any	
  third	
  molar	
  eruption	
  ab-­‐
normalities,	
  the	
  genetic	
  factors	
  
that	
  caused	
  this	
  horizontal	
  orien-­‐
tation	
  was	
  probably	
  a	
  polygenic	
  
manifestation	
  that	
  resulted	
  from	
  a	
  
unique	
  combination	
  of	
  the	
  par-­‐
ents’	
  genetic	
  make-­‐up.	
  The	
  pa-­‐
tient’s	
  mother	
  had	
  congenitally	
  

missing	
  third	
  molars,	
  so	
  it	
  is	
  possi-­‐
ble	
  that	
  some	
  of	
  the	
  genes	
  which	
  
contribute	
  to	
  missing	
  third	
  molars	
  
also	
  play	
  a	
  role	
  in	
  tooth	
  eruption	
  
and	
  influenced	
  the	
  eruption	
  of	
  the	
  
horizontal	
  third	
  molars	
  in	
  her	
  off-­‐
spring.	
  The	
  etiology	
  of	
  third	
  molar	
  
impaction	
  in	
  the	
  population	
  is	
  
most	
  likely	
  complex	
  with	
  multiple	
  

genes	
  influencing	
  the	
  final	
  out-­‐
come	
  of	
  a	
  horizontally	
  oriented	
  
third	
  molar,	
  whereas	
  the	
  case	
  
presented	
  here	
  exemplifies	
  one	
  
of	
  the	
  instances	
  a	
  single	
  gene	
  
form	
  of	
  inheritance	
  is	
  in	
  play.	
  	
  

	
   Hence,	
  an	
  alternative	
  
hypothesis	
  for	
  the	
  etiology	
  of	
  
this	
  case	
  could	
  be	
  that	
  the	
  hori-­‐
zontal	
  impaction	
  of	
  the	
  third	
  
molar	
  is	
  a	
  variation	
  of	
  tooth	
  
agenesis.	
  The	
  mother	
  of	
  the	
  

patient	
  described	
  in	
  this	
  case	
  ex-­‐
hibits	
  agenesis	
  of	
  the	
  third	
  molars.	
  
The	
  agenesis	
  alleles	
  could	
  have	
  
been	
  passed	
  on	
  to	
  the	
  patient,	
  
which	
  then	
  exhibited	
  a	
  variable	
  
phenotype	
  of	
  agenesis:	
  horizontal	
  
impaction	
  of	
  the	
  third	
  molar.	
  This	
  
hypothesis	
  is	
  further	
  supported	
  by	
  
the	
  fact	
  that	
  the	
  patient’s	
  sister	
  

also	
  has	
  a	
  mesially	
  tipped	
  and	
  im-­‐
pacted	
  third	
  molar,	
  which	
  could	
  
likely	
  be	
  another	
  variable	
  pheno-­‐
type	
  of	
  the	
  tooth	
  agenesis	
  allele	
  
passed	
  down	
  from	
  the	
  mother.	
  	
  If	
  
this	
  were	
  the	
  case,	
  a	
  pedigree	
  
drawn	
  of	
  this	
  particular	
  case	
  study	
  
would	
  strongly	
  suggest	
  an	
  auto-­‐

somal	
  dominant	
  mode	
  of	
  inher-­‐
itance	
  (Figure	
  10).	
  	
  

In	
  a	
  research	
  study	
  done	
  
on	
  patterns	
  of	
  third-­‐molar	
  agene-­‐
sis	
  and	
  associated	
  dental	
  anoma-­‐
lies,	
  it	
  was	
  found	
  that	
  permanent	
  
tooth	
  agenesis,	
  microdontia	
  of	
  
maxillary	
  lateral	
  incisors,	
  and	
  total	
  
dental	
  anomalies	
  are	
  more	
  fre-­‐
quently	
  associated	
  with	
  agenesis	
  
of	
  all	
  four	
  third	
  molars	
  than	
  with	
  
their	
  presence	
  [4].	
  Horizontal	
  im-­‐
paction	
  of	
  the	
  third	
  molar	
  could	
  be	
  
categorized	
  as	
  a	
  dental	
  anomaly.	
  
This	
  association	
  could	
  suggest	
  a	
  
variable	
  phenotypic	
  expression	
  of	
  
the	
  same	
  genes	
  or	
  set	
  of	
  genes	
  for	
  
tooth	
  agenesis.	
  	
  

In	
  addition,	
  third	
  molar	
  
agenesis	
  seems	
  to	
  predispose	
  for	
  
reduced	
  size	
  of	
  the	
  remaining	
  
teeth	
  and	
  the	
  delayed	
  develop-­‐
ment	
  of	
  certain	
  teeth	
  and	
  has	
  also	
  
been	
  linked	
  to	
  diminished	
  stability	
  
of	
  specific	
  molar	
  cuspal	
  patterns	
  
[5,6].	
  A	
  relationship	
  between	
  
tooth	
  agenesis	
  and	
  abnormal	
  
morphology	
  of	
  remaining	
  teeth	
  
has	
  been	
  observed	
  within	
  other	
  
types	
  of	
  agenesis.	
  This	
  goes	
  to	
  fur-­‐
ther	
  support	
  the	
  fact	
  that	
  these	
  
dental	
  anomalies	
  are	
  all	
  interre-­‐
lated	
  and	
  thus	
  likely	
  to	
  have	
  a	
  sim-­‐
ilar	
  genetic	
  pathway	
  or	
  mode	
  of	
  
inheritance.	
  It	
  is	
  very	
  possible	
  that	
  
third	
  molar	
  agenesis	
  is	
  a	
  trait	
  that	
  
shows	
  varying	
  phenotypes,	
  such	
  
as	
  the	
  anomalies	
  listed	
  above	
  and	
  
perhaps	
  horizontal	
  impaction	
  of	
  
third	
  molars.	
  	
  

Management	
  of	
  the	
  Condition	
  

The	
  optimal	
  treatment	
  
plan	
  for	
  this	
  case	
  would	
  be	
  extrac-­‐



	
  	
  Third	
  Molar	
  Erup.on	
  Mechanisms	
  and	
  Pa5erns	
  

Vol	
  4,	
  No	
  1	
  (2016)	
  	
  	
  	
  DOI	
  10.5195/d3000.2016.49	
  

	
  http://dentistry3000.pitt.edu	
  
5	
  

tion	
  of	
  the	
  horizontal	
  third	
  molar.	
  
Third	
  molars	
  often	
  cause	
  crowding	
  
in	
  the	
  mouth	
  and	
  therefore	
  are	
  
recommended	
  for	
  extraction.	
  Indi-­‐
viduals	
  with	
  impaction	
  showed	
  
more	
  moderate	
  to	
  extreme	
  
crowding	
  than	
  those	
  with	
  agenesis	
  
[7].	
  In	
  addition,	
  the	
  horizontal	
  
eruption	
  of	
  the	
  mandibular	
  left	
  
third	
  molar	
  has	
  caused	
  the	
  mesial	
  
marginal	
  ridge	
  of	
  the	
  tooth	
  to	
  
come	
  in	
  contact	
  with	
  the	
  distal	
  
cervical	
  portion	
  of	
  the	
  crown	
  on	
  
the	
  mandibular	
  left	
  second	
  molar.	
  
The	
  orientation	
  of	
  the	
  second	
  and	
  
third	
  left	
  mandibular	
  molars	
  cre-­‐
ates	
  a	
  food	
  trap	
  that	
  could	
  poten-­‐
tially	
  lead	
  to	
  caries	
  on	
  the	
  distal	
  of	
  
the	
  second	
  molar	
  or	
  periodontal	
  

inflammation.	
  Therefore,	
  a	
  possi-­‐
ble	
  treatment	
  for	
  this	
  case	
  is	
  sur-­‐
gical	
  extraction	
  of	
  the	
  third	
  molar.	
  	
  

Discussion	
  and	
  Practical	
  
Implications	
  

While	
  the	
  mecha-­‐
nisms	
  and	
  etiology	
  of	
  third	
  
molar	
  eruption	
  are	
  not	
  well	
  
understood	
  currently,	
  con-­‐
ducting	
  more	
  research	
  in	
  
this	
  aspect	
  can	
  yield	
  bene-­‐
fits	
  for	
  both	
  the	
  field	
  of	
  
dentistry	
  as	
  well	
  as	
  medi-­‐
cine	
  as	
  a	
  whole.	
  Skeletal	
  
biologists	
  view	
  the	
  process	
  
of	
  tooth	
  eruption	
  as	
  a	
  valu-­‐

able	
  model	
  to	
  study	
  bone	
  remod-­‐
eling,	
  since	
  the	
  emergence	
  of	
  a	
  
tooth	
  into	
  the	
  oral	
  cavity	
  involves	
  
both	
  coupled	
  and	
  uncoupled	
  bone	
  
turnover	
  events	
  [3].	
  For	
  cell	
  and	
  
molecular	
  biologists,	
  tooth	
  erup-­‐
tion	
  provokes	
  several	
  questions	
  
concerning	
  the	
  tightly	
  pro-­‐
grammed	
  series	
  of	
  signaling	
  inter-­‐
actions	
  between	
  cells	
  of	
  the	
  con-­‐
nective	
  tissue	
  sac	
  surrounding	
  the	
  
tooth	
  (dental	
  follicle)	
  and	
  the	
  sur-­‐
rounding	
  alveolus	
  [3].	
  	
  

For	
  a	
  clinician	
  that	
  has	
  to	
  
treat	
  dental	
  complications	
  that	
  
arise	
  from	
  abnormal	
  tooth	
  erup-­‐
tion,	
  as	
  seen	
  in	
  numerous	
  genetic	
  

and	
  acquired	
  dis-­‐
orders,	
  
knowledge	
  about	
  
the	
  basic	
  molecu-­‐
lar	
  mechanisms	
  
involved	
  is	
  ex-­‐
tremely	
  im-­‐
portant.	
  Knowing	
  
the	
  etiology	
  can	
  
not	
  only	
  help	
  the	
  
clinician	
  formu-­‐

late	
  a	
  better	
  treatment	
  plan	
  for	
  
the	
  patient,	
  but	
  it	
  can	
  help	
  den-­‐
tists	
  better	
  educate	
  the	
  patient	
  in	
  
their	
  dental	
  health.	
  

References	
  

1.	
  Prevalence	
  of	
  impacted	
  teeth	
  
and	
  associated	
  pathologies-­‐-­‐a	
  ra-­‐
diographic	
  study	
  of	
  the	
  Hong	
  Kong	
  
Chinese	
  population.	
  Chu	
  FC,	
  Li	
  TK,	
  
Lui	
  VK,	
  Newsome	
  PR,	
  Chow	
  RL,	
  
Cheung	
  LK.	
  Hong	
  Kong	
  Med	
  J.	
  
2003	
  Jun;9(3):158-­‐63.	
  
PMID:12777649.	
  

2.	
  Mandibular	
  third	
  molar	
  impac-­‐
tion:	
  review	
  of	
  literature	
  and	
  a	
  
proposal	
  of	
  a	
  classification.	
  

Juodzbalys	
  G,	
  Daugela	
  P.	
  J	
  Oral	
  
Maxillofac	
  Res.	
  2013	
  Jul	
  1;4(2):e1.	
  
PMID:24422029.	
  

3.	
  	
  Cellular,	
  molecular,	
  and	
  genetic	
  
determinants	
  of	
  tooth	
  eruption.	
  
Wise	
  GE,	
  Frazier-­‐Bowers	
  S,	
  D'Sou-­‐
za	
  RN.	
  Crit	
  Rev	
  Oral	
  Biol	
  Med.	
  
2002;13(4):323-­‐34.	
  
PMID:12191959.	
  

4.	
  Patterns	
  of	
  third-­‐molar	
  agenesis	
  
and	
  associated	
  dental	
  anomalies	
  
in	
  an	
  orthodontic	
  population.	
  Ce-­‐
likoglu	
  M,	
  Bayram	
  M,	
  Nur	
  M.	
  Am	
  J	
  
Orthod	
  Dentofacial	
  Orthop.	
  2011	
  
Dec;140(6):856-­‐60.	
  
PMID:22133951.	
  

5.	
  Third	
  molar	
  agenesis	
  and	
  size	
  
reduction	
  of	
  the	
  remaining	
  teeth.	
  
Garn	
  SM,	
  Lewis	
  AB,	
  Kerewsky	
  RS.	
  
Nature.	
  1963	
  Nov	
  2;200:488-­‐9.	
  
PMID:14076752.	
  

6.	
  Third	
  molar	
  agenesis	
  and	
  varia-­‐
tion	
  in	
  size	
  of	
  the	
  remaining	
  teeth.	
  
Garn	
  SM,	
  Lewis	
  AB,	
  Kerewsky	
  RS.	
  
Nature.	
  1964	
  Feb	
  22;201:839.	
  
PMID:14161232.	
  

7.	
  Third	
  molar	
  impaction	
  and	
  
agenesis:	
  Influence	
  on	
  anterior	
  
crowding.	
  Esan	
  T,	
  Schepartz	
  LA.	
  



	
  	
  Third	
  Molar	
  Erup.on	
  Mechanisms	
  and	
  Pa5erns	
  

Vol	
  4,	
  No	
  1	
  (2016)	
  	
  	
  	
  DOI	
  10.5195/d3000.2016.49	
  

	
  http://dentistry3000.pitt.edu	
  
6	
  

Ann	
  Hum	
  Biol.	
  2016	
  Feb	
  8:1-­‐29.	
  
PMID:26856343.