Eruption of Teeth

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					                                      ‫بسم اهلل الرحمان الرحيم‬

Introduction to Dentistry Dent. 101

Lecture number: 4

Development and Eruption of Teeth

By: Dr. Hazza’a

       In dentistry we deal with what we call the “Oral Cavity”, the oral cavity is
composed of two parts: the upper jaw and the lower jaw. The upper jaw is called the
“Maxilla”, and the lower jaw is called the “Mandible”, each jaw is composed of a number
of teeth surrounded by bone and soft tissue.

       Just like animals and any other living creature we have two stages of dentition ,
the first stage is called the primary [deciduous /milk teeth ( ‫ , ])األسنان اللبنيه‬this first
stage of teeth form from the age of 6 months to 2 years, approximately 30 months,
within this time all the primary teeth should have already erupted (‫ )بزغت‬within the oral
cavity. With children; babies especially the jaws are small just as the arms, legs and
head, because the jaw is small the number of teeth is
directly related to the size of the jaw, therefore the
Deciduous (primary) teeth are always 20 teeth , 10 in the
Maxilla (upper jaw) and 10 in the “Mandible”.

       As the child grows larger so does the jaw, which
gives room for more teeth, therefore in the second stage of
Dentition: The permanent teeth (‫ )مرحلة األسنان الدائمه‬the number
of teeth will increase from Twenty to Thirty-two. The
original twenty teeth divided between both jaws, 10 in each
jaw, the 12 new teeth will also be divided between both
jaws giving us a total of 16 teeth in each jaw.

       As we previously mentioned the deciduous teeth
start to erupt at around the age of 6 months, and all twenty
teeth are complete by the age of two, of course these teeth
do not form all at once, but rather tooth after tooth (‫ .)مش مرة وحده، سن ورا سن‬In a bit we
will see the order that these teeth form in the Oral cavity. The first permanent tooth
forms at the age of 6 years, during this stage both deciduous and permanent teeth are
in the mouth at the same time, this stage is called “Mixed Dentition” (‫.)مرحلة األسنان المختلطة‬

Q-What is the difference between permanent and deciduous teeth?

   A- In terms of size and shade (color) there is a difference, the size of permanent
       teeth is always larger than deciduous teeth; the shade of deciduous teeth is
       whiter (lighter) than that of permanent teeth.

Each tooth is composed of two parts:

     a- The part visible within the oral cavity which is called the “crown” (‫)التاج‬

     b- The second part of the tooth is called the “root” (‫ ,)الجذر‬the root cannot be seen
         because it is inside the bone [similar to a nail in a piece of wood, the head of
         the nail being the crown and the rest being the root ( ‫زي المسمار بالخشب، جزء بارز او‬
         ‫ ,)جزء ال‬the root is what stabilizes the tooth within the oral cavity, the longer the
         root the more stable the tooth, the same goes for the number of roots as well
         (‫ . )كل ما زاد عدد الجذور أو طوله بيزداد ثبات السن في الفم‬one root differs both in size and
         shape from the adjacent root, The number of roots also differs from one tooth
         to another; some have only one root others may have two or three. All teeth in
         the back of the mouth (posterior teeth) have more than one root in order to
         increase chewing strength.

     If we took a radiograph (‫ )صورة أشعة‬of a patient who is in the mixed dentition stage;
we would see that underneath every deciduous tooth there is a permanent tooth ( ‫تحت كل‬
‫ .)سن لبني يوجد سن دائم‬Usually all deciduous teeth will be lost naturally, without needing to
be removed by a dentist.

Q- Why do deciduous teeth fallout by themselves?
   A- Because there is a permanent tooth underneath the deciduous tooth’s root
       pressuring it from below, causing the root of the deciduous tooth to decay ( ‫يسبب‬
       ‫ ,)تاكل جذر السن‬because the root which anchors the tooth to the jaw is gone, the
       crown part of the tooth is only connected to the gum which is a soft tissue that
       easily allows the tooth to fall out ( ‫بما أن جذر السن تأكل يبقى فقط التاج مثبت بللثه التي تتكون من نسيج‬
       ‫ .)مرن يسهل سقوط السن منه‬Therefore once the permanent tooth is fully grown the
       deciduous tooth falls out so it can be replaced by the permanent tooth.

   As we already mentioned there are 32 permanent teeth 16 in each jaw, but only 20
deciduous teeth with 10 in each jaw, therefore there are 6 teeth in each jaw that have
no deciduous teeth above them these teeth usually form after all the other permanent
teeth form (the ones with deciduous teeth above them). Then the jaw expands towards
the back of the skull where the 6 extra teeth form behind the last deciduous teeth, 3
on the right end of the upper and lower jaw, and 3 on the left end of the upper and
lower jaw.

      Remember the visible part of tooth is the crown: D

       Each jaw is divided into two equal pieces when divided along the nose-line we
   call this line the “Mid-line”. The mid-line divides the 32 teeth in both jaws to 16 to
   the right, and 16 to the left; every tooth on the right side has a tooth on the left
   side that is similar in shape, shade, and number of roots.

       * The next part is explained while displaying the picture below:
    Naming of the

    The first tooth is
called the “Central
Incisor” right or left
(depending on which
side of the mouth it’s
on), the next tooth is
the “Lateral Incisor”
then the “Canine” then
the “First Pre-Molar”
and the “Second Pre-
Molar” the last three
teeth are called the
“First Molar” “Second
Molar” and the “Third
Molar”. All these teeth
are referred to as right
or left.

     EX: right second
Each tooth has its’ own specific
anatomy, each tooth also has 5
different sides.

   a- Occlusal plane: the
       grinding/biting side
   b- Labial plane: the side of the
       tooth facing outward
   c- Palatal plane: the side of the
       tooth facing the throat and
       inside of the mouth
   d- Mesail plane: the side of the
       tooth closet to the mid-line
       and the front of the mouth
   e- Distal plane: the side of the
       tooth facing towards the back
       of the mouth

Some the anterior teeth have a sharp point used for eating hard food such as carrots,
the occlusal plane in these teeth is called the “Incisal” plane. This will all be discussed
in more detail later on.

*The third molar is sometimes called “The Wisdom Tooth” (‫)طاحونة العقل‬
  Each tooth regardless if it’s a deciduous or permanent tooth is composed of three
layers, the first layer (the one that is visible to us) is called “Enamel” ( ‫طبقة المينا: وهي عبارة‬
                                  ‫)عن طبقة عاجيه تعكس الطبقة التحتها‬

       The next layer is called “Dentin” this is the sensitive part of the tooth, that if it
is exposed can cause pain from eating or drinking hot things, which is what happens
when someone gets a cavity; the cavity eats the Enamel layer revealing the Dentin layer
which can then lead to toothaches and pain. Also underneath the Enamel is the “Pulp”,
the pulp is the part responsible for feeding the tooth with the blood supply, if a
problem occurs in the pulp then the tooth will not last long because it isn’t getting any
nutrients which the pulp is supposed to deliver, this causes the sometimes brown or
black color on peoples teeth which we shall also discuss later on. These three layers
make up the crown, the root is separated from the crown by a part called the “Neck of
the Tooth”

       In the root of the tooth the outer layer is called the “Cementum”, which is
instead of the Enamel, the rest of the root is similar to the crown consisting of Dentin
and Pulp. In teeth with multiple roots the pulp feeds both roots instead of just one.

       The roots go all the way from the neck of the tooth to inside the jawbone; it is
covered by soft tissue or the “Gingiva” (the red tissue we see from inside our mouths,
this applies to both
the Maxilla and the
Mandible. The upper
teeth are also known
as Maxillary teeth
while the lower teeth
are known as
Mandibular teeth. In
the lower jaw we
have the Mandibular
canal which connects
to the pulp, along
with some nerves.
        The anterior teeth are from canine right to canine left, the rest of the teeth are
called the posterior (from the first pre-molar to the third molar). All 6 anterior are
single rooted, the premolars differ from person to person in the number of roots,
sometimes one root other times two roots, the first and second Molars in the Maxilla
have 3 roots. The third Molar differs from person to person to reach even 6 roots;
some people don’t have a third molar at all ( ‫ممكن اكون لطاحونت العقل جذر واحد أو إثنين أو ثالث، وممكن‬
‫.)إنه مايكون موجود أصال‬

        In the Mandible the anterior teeth all have only one root as do the pre-molars,
while all the Molars have 2 roots.

        *The difference in the number of roots between the upper Molars and lower
Molars means that the upper Molars are more stable than the lower ones.

The deciduous teeth are also split into a left and right side by the same Mid-line as
the permanent, spiting them into 5 on the right and 5 on the left. They are also named
similarly, the first being the “Central Incisor” then the “Lateral Incisor” then the
“Canine”, but in the deciduous teeth there are no Pre-Molars at all, there is the “First
Molar” and “Second Molar”. To distinguish between the permanent Molars and the
deciduous Molars we call them by “First Deciduous Molar” or “Lateral Deciduous

*The only difference between the two types of teeth here is that the deciduous teeth
have no Pre-Molars, and of course there is no third Molar (wisdom tooth).

        Eruption time of the deciduous teeth:       VERY IMPORTANT

                          Deciduous Teeth                                             Eruption Time

                           Central Incisor                                           6 to 12 months

                           Lateral Incisor                                           9 to 13 months

                             First Molar                                            13 to 19 months

                              Canine                                                16 to 23 months
                    Second Molar                                         23 to 33 months

The actual time of eruption differs between children some babies are born with
incisors while others take a much longer time, and the actual eruption time depends
on a number of factors that will be discussed later.

       Eruption of permanent teeth:   VERY IMPORTANT
The first Permanent tooth to erupt is the First Molar, it appears behind the second
deciduous Molar, the First Permanent Molar appears at around 6 years of age, it is also
called “‫ ”السادس‬because it comes at the age of 6.

 Permanent Teeth                                    Eruption Time

 First Molar                                        6 to 7 years

 Central Incisor                                    6 to 8 years

 Lateral Incisor                                    6.5 to 9 years

 First Premolar                                     8 to 12 years

 Second Premolar                                    8.5 to 13 years

 Canine                                             9 to 12 years

 Second Molar                                       11 to 14 years

 Third Molar (Wisdom Tooth)                         17 to 25 years

Done by: Ali Al-Qudsi

Description: Begginers course to an introduciton to dentistry from JUST UNiversity