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LECTURE TWO-ROOT STRUCTURE AND SUPPORTING TISSUE

Each tooth is supported directly by its roots and indirectly by the tissues which surround the roots. This lecture will discuss roots and their supporting tissues.

Section 2:1

We discussed in the previous section how the crowns of the teeth varied in shape depending on their function. The same is true of the roots of the teeth. Some teeth have a single delicate root while others have heavy multiple roots. The following chart will show you which teeth have which number of roots. The root numbers indicated here are general guidelines on root number however it should be noted that considerable individual variation exists and almost anything is possible in exceptional cases.

INCISORS CANINES PREMOLARS MOLARS

MAXILLARY 1 ROOT 1 ROOT 2,1 ROOTS 3 ROOTS

MANDIBULAR 1 ROOT 1 ROOT 1 ROOT 2 ROOTS

Teeth that have two roots are said to be bifurcated. Teeth that have three roots are said to be trifurcated. Maxillary first premolars are usually bifurcated while the maxillary second premolar usually has just one root. It is important to understand the number and position of roots since they often will influence such things as root canal treatments and extractions.

The position of the roots varies by tooth. The maxillary first premolar with two roots will have one root positioned towards the facial and the second on the lingual. Maxillary molars have three roots, two roots on the facial and one on the lingual. The mandibular molars have two roots, one root positioned on the mesial and one on the distal. (see figure 2.1)

In the primary dentition the root numbers and location are similar to those of the adult teeth. Maxillary primary molars have three roots in the same location as permanent maxillary molars. Primary mandibular molars have two roots in the same location as the permanent mandibular molars. Primary incisors and cuspids are the same in root numbers as the permanent teeth.

Inside of each root is a small canal through which the nerves and vessel pass. This canal is call the root canal. The normal setup is to have one canal is each root. The exception to this is the mandibular molars where the mesial root contains two canals rather than one. In this case the mesial root will have one facial canal and one lingual canal.

Section 2:2

In discussing the shape and location of teeth and their roots it is helpful to know which angle you are viewing the tooth from. The view angles are described as if you were sitting looking at the tooth from angle named. In a facial view for example you see the teeth as if you were glued to the persons face. In a lingual view you see the tooth as one sitting on the patients tongue. The other views are mesial, distal and occlusal or incisal.

At certain times it is helpful to make sections through the teeth and their roots to better see their shape or location. The three sections that we use are horizontal, facio-lingual, mesio-distal. A horizontal section is the same as a cross section. In this type section you are cutting through the tooth separating the top from the bottom. To remember the other two sections it is helpful to think of the knife making the section as lying in the plane being cut. In other words, in a facio-lingual section the Handle of the knife is towards the facial and the tip towards the lingual. In a mesio-distal section the handle is towards the mesial and the tip towards the distal.

Section 2:3

When making a facio-lingual or mesio-distal section you can see a lengthwise outline of where the nerve and vessels run inside the tooth. As we told you before the narrow path inside the root is called the root canal. The nerve usually will continue up into the inside of the crown where it expands somewhat in its width. The expanded area inside the crown is called the pulp chamber. The walls of the pulp chamber are given names by the surface that they lie closest to. So a pulp chamber will have a facial, lingual, mesial and distal wall. In addition the top of the chamber is called the roof and the base of the chamber, if there is one, is called the floor.

In the roof of the pulp chamber it is common to encounter small projection towards the surface of the tooth. These usually are located below cusps or other projections on the surface of the tooth. These small extensions are called pulp horns and can become a problem especially if you slip while drilling on a tooth. Usually it is a pulp horn into which your drill drops.

The pulp chamber of a tooth is a dynamic environment. In addition to having a constant blood flow and nervous signals, there is also a constant repair mechanism taking place. Inside the pulp chamber small cells exist called odontoblasts. These cells are responsible for producing new dentin. When a tooth is subject to irritation the odontoblasts become active and begin to produce new dentin in the area of irritation. The original dentin laid down when the tooth is formed is called primary dentin. Dentin laid down after that time is called secondary dentin. This phenomenon takes place very slowly and because of that, sometimes the tooth cannot lay down new dentin as fast as the decay is progressing. When this occurs the decay may eventually enter the pulp of the tooth and kill it. However, if the decay moves slowly or the irritation is not too acute, the tooth will lay down layer after layer of dentin until there is a barrier of adequate thickness which isolates the odontoblasts from the irritation. This is part of what is going on when a new filling is placed. When a tooth is drilled on and a new filling is placed the tooth is irritated both by the drilling and the new filling. The tooth will initially be tender and possibly cold sensitive but with time inflammation in the pulp chamber will decrease and the tooth will build a layer of insulation between the nerve and the new filling.

Section 2:4

The bone that supports the teeth is called collectively the alveolar bone. The depressions in the alveolar bone that cradle the teeth are called alveolar sockets or alveolus. The tooth is not actually in direct contact with the bone. Instead, the tooth is held in place by a short ligament called the periodontal ligament or PDL for short. This ligament connects from the bone to the cementum covering the root of the tooth. Essentially teeth hangs in the sling of the PDL and are allowed to move freely in any direction though the amount of movement is small. When a tooth is extracted we are essentially tearing the ligament that connects the tooth to the bone.

The PDL becomes very important when we are performing orthodontics. When pressure is applied to a tooth in any direction it stretches the ligament on the side where the pressure is coming from and compresses the ligament on the side away from the pressure. Contained in the bone are cells that are capable of making new bone (osteoblasts) or breaking down existing bone (osteoclasts). These cells become active in response to pressure. When you pull on bone, as in a stretched PDL, odontoblasts will begin laying down new bone in response. When bone is not pulled on, as is the case in a compressed PDL, osteoclasts will begin to break down the bone in the area. So during orthodontic treatment, through the actions of osteoclasts and osteoblasts, the tooth essentially floats through the bone to its new location.

Section 2:5

Covering the alveolar bone and lining all the inside of your mouth is a tissue broadly referred to as oral mucosa. The environment in the human mouth is a harsh one. Especially in some mouths. The linings of the mouth have adapted to this environment by being rich in keratin, a substance that strengthens the surface, and by shedding off the outermost layers on a regular basis. This is the same mechanism that the body uses to deal with other harsh environments such as the lining of the digestive tract or the skin. The linings of the mouth are turned over approximately every week to ten days. Certain areas in the mouth are subject to greater stress than others and these contain greater amounts of keratin. This can be noted visually by the color changes in the tissue. The areas that receive the greatest stress are the roof of the mouth (palate) and the gums (gingiva) immediately surrounding the teeth. If you look in your mouth you will see that these areas are much pinker than other areas such as the linings of the cheeks.

For now we will focus on the structure of the gingiva. The gingiva that is positioned immediately around the neck of the tooth is called the unattached gingiva (also called the marginal or free gingiva). It receives this name because it does not connect directly into the tooth or root but is rather a projection from the structures below. You can demonstrate in your own mouth this lack of attachment by placing a piece of floss around your tooth and slowly moving it apically. You will see that the floss moves with little or no resistance beneath the gum line. The floss is now sitting in a space called the sulcus. This is a trough like depression that circles each of your teeth between the gums and the tooth. The sulcus is important because it is here that pockets of bacteria first collect and begin the process of gum disease we will talk about later on.

In a healthy mouth the sulcus will be less than 3.5 mm in depth all around the tooth. At the bottom of the sulcus is where the PDL begins it's attachment to the root of the tooth. The gingiva is bound down to the bone by small fibers. These fibers reach nearly through the gingiva to the surface. Where they attach to the gingiva you can see small depressions in the gingiva itself. This will give the gingiva an fine "orange peel" appearance called stippling. This is especially apparent in the upper areas of the gingiva because it is bound to the bone more tightly.

The small pointed projection of gum that goes between the teeth is called the papilla. Below the level of the PDL attachment the gingiva is then referred to as attached gingiva. In this area the gingiva is attached directly to the bone. The unattached gingiva and the attached gingiva together form a band of tissue collectively called the gingiva which is from 3-6 mm wide.

Where the gingiva ends the alveolar mucosa begins. The junction between the two is seen by a color change. The gingiva being a light pink and the alveolar mucosa being a deeper red. (see figure 2.2)

Perhaps the most common disease of the mouth is gum disease. This is caused because bacteria are able to establish a colony on the teeth and from there can attack the gums surrounding the teeth. The gingiva immediately surrounding the necks of the teeth is most prone to this attack. Early episodes of gum disease are called gingivitis. This is when the upper most level of the gums and the linings of the sulcus become infected and inflamed. The clinical evidence of this is not difficult to detect since the gingiva takes on a deeper color or red rather than pink and the gingiva bleeds much more easily when touched. As the disease progresses you will see the redness in the unattached gingiva increasing, the papilla becoming inflamed and puffy. Also you will notice that stippling in the gums is lost as the bacteria attack and destroy the fibers that bind the gingiva to the bone.

In later stages the PDL itself is attacked and destroyed. This will lead to the sulcus depth increasing. When sulcular depth reaches 3.5-5.5 mm it is considered to be a case of moderate gum disease. Sulcular depth greater than 5.5 mm is considered an advanced case of gum disease. Good home care, early detection, and regular cleaning are the most effective way to prevent or control the disease. Once the PDL is destroyed it is difficult if not impossible to regenerate.

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