Dental Anatomy

Associated and Supporting Structures of the Teeth and Anatomy of the Edentulous Mouth

The proper construction of functional and harmonious dental appliances requires an intelligent knowledge of certain anatomical features.

Tooth loss is accompanied by anatomical changes in structure and function which must be understood before successful replacement can be achieved.

The mouth, or oral cavity is the first portion of the digestive system. One of the primary functions of the tongue and dentition is to begin the digestive process by reducing the food particles into smaller size.

With the help of the chemical substances from the salivary glands, the mechanical and chemical breakdown of food is initiated.

Within the oral cavity, the spaces between the lips and the teeth are called the vestibules (Figures 1 and 2).

In the maxillary vestibule opposite the maxillary second molar the duct of one of the major salivary glands, the parotid gland, empties its secretions.

The roof of the mouth is formed by the palate and the upper teeth.

Anteriorly, the palate is composed of an underlying base of bone covered by soft tissue. This is a very firm area and is referred to as the hard palate (Figures 2 and 3).

Posteriorly, or just behind the hard palate this consistency changes to a softer nature, since the composition is primarily connective tissue and muscle tissue instead of bone. This area quite naturally then is referred to as the soft palate (Figures 2 and 3).

A finger like process, the uvula, (Figure 1), projects down from the posterior aspect of the soft palate.

This is not a consistent anatomical feature in all people, for frequently the uvula will be missing.

The floor of the mouth is formed by the lower teeth, the mylohyoid muscle with its investing structure and the tongue.

When the tongue is elevated into the roof of the mouth, a fold of soft tissue extends from the floor of the mouth to the tip of the tongue. This is known as the lingual frenum (Figures 4 and 5).

On each side of this frenum, in the anterior of the mouth, there are enlargements which represent openings of another major salivary gland, the submandibular gland (Figure 5)

There is not a single opening for this submandibular gland, but rather there are multiple openings through which secretions are discharged into the floor of the oral cavity.

The entire internal surface of the oral cavity is lined with a modified type of skin known as the mucous membrane.

The oral mucous membrane is kept moist by small glands in the deeper layers of the connective tissue and by secretions from salivary glands.

Because of variations in the function of different parts of the mouth and because of differences in structure of the underlying connective tissue, there are differences in the character of the mucous membrane in different areas of the oral cavity.

In the area of the teeth, the mucous membrane is loosely attached to the underlying muscle or bone and has a rather translucent appearance.

This translucency permits diffusion of the colour of the underlying blood vessels and results in a pale pink colour.

In the region of the hard palate the mucosa is thicker and more firmly attached to the periosteum of the underlying bone.

The tissue is firmer and not freely moveable.

More closely associated with the teeth there are two different types of mucosa:

1. the gingival mucous membrane (Figure 6) 2. alveolar or vestibular mucous membrane

(Figures 6 and 8).

Proceeding from the vestibule toward the necks of the teeth, the mucous membrane abruptly changes character.

This line of demarcation divides the alveolar mucosa from the gingival mucosa and the line itself is referred to as the mucogingival junction (Figures 6 and 8).

The gingiva or gum, is that portion of the mucous membrane which is attached directly to the alveolar process and surrounds the necks of the teeth.

Normally, the gingiva is tightly attached to the alveolar bone and is characterized by a stippled texture.

The stipples or depressions represent points of attachment of the gingiva to the underlying alveolar bone.

This appearance of the gingiva is in sharp contrast to the glistening appearance of the alveolar or vestibular mucous membrane.

The gingiva is further divided into free gingiva and attached gingiva (Figure 6).

The free gingiva is actually the unattached marginal tissue which encircles the necks of the teeth; the space between the neck of the tooth and the free gingiva is known as the gingival crevice.

In the normal individual there is an average depth of this crevice of approximately 1/2 to 1 mm.

The attached gingiva extends from the base of the free gingiva to the mucogingival line.

During mastication the attached gingiva bears the load of masticatory forces as the food is moved from the tooth surfaces onto the soft tissue.

Because of its firm attachment, as the name would imply, the attached gingiva is functionally designed to withstand the masticatory forces placed upon it.

The vestibular mucosa (alveolar mucosa) is smooth in texture (without stipples) and is somewhat deeper pink or darker in colour than the gingiva.

This alveolar mucosa is loosely arranged and permits a wide range of movement of the cheeks during the processes of mastication and speaking.

In both the maxillary and mandibular vestibules there are crescent shaped elevations of the alveolar mucosa which connect to the gingiva.

A fold of such tissue is known as a frenum (plural-frena).

These frena are composed of connective tissue and are devoid of any muscle fibres.

Within each dental arch we are able to identify 3 such frena; the exact location, however, may vary.

The most consistent location of the frena in both the maxillary and mandibular arch is in the median line between the maxillary and mandibular central incisors. Lateral to this area, in each quadrant, 2 other frena attach in the area of the canine or occasionally in the area of the premolar teeth.

Not infrequently, in the maxillary arch there is an accentuated or overdeveloped median-line frenum.

This firm fold of tissue passes from the facial gingiva across the interdental papilla (Figure 6) between the maxillary central incisors to attach on the lingual aspect. Accompanying such overdevelopment, there is a separation (diastema) of the maxillary central incisor.

The tongue is a very muscular organ consisting of several sets of muscles whose principle fibres course in different directions.

This arrangement permits a wide range of tongue movements.

There are basically 2 sets of muscles involved with the tongue.

The first set, intrinsic muscles of the tongue, actually compose the body of the tongue and are responsible for the change in shape of the tongue.

The second set of muscles, the extrinsic muscles of the tongue, which support and maintain the body of the tongue are attached to bone.

These extrinsic muscles are primarily responsible for changes in the position of the tongue which are necessary in:

Speech Mastication Swallowing

Additionally, changes in position of the tongue must accompany mandibular movements such as:

Extension

Depression

Retrusion

The wide range of movements contribute several important functions to tongue:

Manipulation of food in such a way that each bite is pushed between the teeth to be chewed

Shaping the food into a spherical mass called a bolus

Initiation of swallowing by forcing the bolus of food into the pharynx

Formation of words in speech by its apposition to the roof of the mouth and the teeth

Contribution to our sensation of taste

The tongue completely fills the floor of the mouth and confined only by the teeth and palate. This points out a very practical problem for the dental technician and the denture wearer.

A person who has been missing teeth for an extended period of time will experience some difficulty in adjusting to a denture which once again places confining limits on the tongue.

On the upper surface (dorsum) of the tongue there are a number of tiny projections called (papilla).

3 types of these papilla exist:

filiform

fungiform

circumvalate

Filiform papilla are the smallest and the most numerous, but contain no taste buds.

The fungiform and the circumvalate papilla are less numerous, but do contain the taste buds.

Landmarks of the Edentulous Mandible

The mandibular alveolar ridge (Figures 4 and 7) is the remnant of the alveolar process which originally contained the alveoli (sockets) for the teeth.

In most cases, the alveolar ridge shrinks (resorbs) after the teeth are removed. The amount of resorption determines the size of the residual ridge.

The retro-molar pad (Figure7) is composed of soft fatty and glandular tissue and is located at the distal end of the mandibular alveolar ridge.

The retro-molar pad is an important landmark, since its level in the mouth remains relatively constant regardless of the amount of alveolar bone loss. Because of its relatively permanent location, the retro-molar pad serves 2 important purposes:

1. it serves as a guide in establishing the level (occlusal plane) of the posterior teeth.

2. when covered by a denture (complete or partial), resorption of the alveolar ridge may be decreased. The constant location of the pad serves as distal support and also as the distal portion of the border seal of dentures.

A frenum is a fold of mucous membrane that limits movements.

There are 4 frena in the mandibular arch.

1. The labial frenum (Figure 8) extends from the inner surface of the lip to the anterior surface of the alveolar gingiva. It may attach as high as the crest of the ridge or it may be absent. The labial frenum contains no muscle fibres.

2 & 3

There are two buccal frena (Figures 2, 5 and 7). They are usually found in the cuspid or premolar region and extend from the buccal mucosa to the alveolar mucosa.

The buccal frena contain fibres of the triangularis muscle (one of the muscles of facial expression.)

4.The lingual frenum (Figures 4 and 5) can be seen when the tongue is raised. It extends from the floor of the mouth to the lingual alveolar mucosa.

The lingual frenum may show on an impression as a sharp line or as a broad band of tissue.

The buccal shelf (Figure 7) is present only in the edentulous mouth as it is produced by a decrease in the size of the alveolar process.

The buccal shelf is located in the molar and premolar regions. It extends from the crest of the residual ridge to the external oblique line.

The buccal shelf provides the primary support for lower dentures when the alveolar ridge is small.

A buccal shelf cannot be seen when the alveolar ridge is large. In such cases, the alveolar ridge provides the primary support.

The alveolingual sulcus (Figure 4) (lingual sulcus) is the groove formed by the floor of the mouth as it turns onto the lingual aspect of the alveolar ridge.

When constructing a lower denture ensure the lingual border contacts the bottom of the sulcus.

The mylohyoid ridge (Figure 7) is a bony ridge on the lingual surface of the mandible.

The mylohyoid muscle, which forms the floor of the mouth, attaches to this ridge.

Most dentists contour their impressions so the mylohyoid ridge will not irritate the patient. Some dentists request that the disto-lingual border of lower dentures be finished at the mylohyoid ridge.

The retromylohyiod fossa (Figure 7) is a depression which lies distal and below the mylohyoid ridge (and to the mylohyoid muscle which attaches to the mylohyoid ridge).

A good impression of this area produces an eminence on the resulting denture which aids retention.

Some dentists avoid using this area.

The mental foramen (Figure 9) normally located in the premolar region, has no visible landmark.

In cases of extensive resorption of the alveolar process the foramen will have located near the crest of the ridge and some denture relief will be necessary to avoid pressure on the nerve fibres which exit from this foramen.

Relief is space provided between the fitting surface of the denture and the soft tissue to reduce or eliminate excessive pressure on certain anatomical structures.

Landmarks of the Edentulous Maxilla

As in the mandibular arch, the maxillary alveolar ridge (Figures 2 and 7) (residual ridge) is a remnant of the alveolar process.

The maxillary alveolar ridge will usually appear to be broader and larger than the mandibular alveolar ridge.

The maxillary tuberosity (Figures 2 and 7) is found on the distal portion of the maxillary alveolar ridge.

Usually the size of the tuberosity corresponds with the size of the alveolar ridge.

When the tuberosity is extremely large, problems may arise in the technical phases of making dentures.

For example, with an unusually large tuberosity there may be insufficient vertical space for the denture base and / or artificial teeth.

The hamular notch (Figures 2 and 7) is a depression distal to the maxillary tuberosity.

The hamular notch can be found on a maxillary edentulous model by following the depth of the border groove around the tuberosity; the hamular notch will be medial to the depth of the most posterior portion of the border groove.

The hamular notch is important as it forms one of the landmarks which determine the posterior border of an upper denture.

The palatine foveae (Figures 2 and 7) appear as 2 small indentations, one on each side of the mid-line, near the posterior border of the hard palate.

The foveae are in close proximity to the posterior border of a denture. In conjunction with the hamular notches, they form a series of guides for determining the posterior limit of maxillary dentures.

The vibrating line (Figure 2) is an imaginary line which denotes the junction of the hard and soft palates.

The dentist should mark this line on the model of the maxillary edentulous arch.

This line, extending from one hamular notch to the other and including the palatine foveae, will designate the location of the posterior palatal seal for the maxillary denture.

The exact location of this seal may vary in different patients, as its location depends on the contour of the soft palate, and the degree of reflection of the soft palate during function.

There are 2 frena in the maxillary arch:

1. The labial frenum (Figure 10) is similar to the mandibular frenum in location and in that it contains no muscle fibres.

2. The buccal frena (Figures 2 and 3) are in corresponding positions to their counterparts in the mandible.

The rugae (Figures 2 and 11) are irregular ridges of fibrous tissue found in the anterior part of the palate.

The rugae aid in the production of vocal sounds and serve as a secondary bearing area for dentures.

The incisive papilla (Figures 2 and 11) is found behind the 2 maxillary central incisors (in a person with teeth).

The position of the incisive papilla on the alveolar ridge of an edentulous person serves as a guide in determining the labiolingual position of the maxillary anterior teeth.

In denture construction, this area may necessitate some relief because excessive pressure often causes the patient to experience a burning sensation.