As students of the dental profession, you will be concentrating your studies on the head and neck and more specifically on the structures that make up the oral cavity. It is imperative that you are extremely familiar with the normal makeup and structural components of this area. Therefore this chapter has been set forth to serve as an introduction to your studies of the head and neck region.
The oral cavity is the upper end and the beginning of the digestive system and at its posterior end forms a common pathway with the respiratory system. The oral cavity begins at the lips and cheeks and extends posteriorly to the area of the palatine tonsils, which are usually referred to as the tonsils. These lie on the sides of the throat between two folds of tissues, one in front and one in back, called the tonsillar pillars. Posterior to the tonsillar pillars the oral cavity ends and the oral pharynx, a pathway shared by the digestive and respiratory systems, begins. In the area from the oral pharynx to the laryngeal pharynx, the digestive system continues to share a common pathway with the respiratory system and then goes on to the esophagus to the rest of the digestive system. The respiratory system starts at the nasal cavity and includes the nasal pharynx, oral pharynx, and laryngeal pharynx (the last two of which are shared spaces) and then continues on into the larynx, trachea, bronchi, and lungs.
The oral cavity can be logically divided into two parts, the vestibule and oral cavity proper. The vestibule is the space or potential space that exists between the lips or cheeks and the teeth. In an edentulous person (one without teeth), it would extend between the lips or cheeks and the alveolar ridges where the teeth were at one time or will be if it is an infant. Additionally, the oral cavity proper is the area surrounded by the teeth or alveolar ridges back to the area of the palatine tonsils. This includes the region from the floor of the mouth upward to the hard and soft palates.
In considering the vestibular area, you should begin by examining the lips. The lips are the junction between the skin of the face, which is a dry tissue, and the mucosa of the oral cavity, which is a moist tissue. Between these two areas lies a transitional zone of reddish tissue known as the vermilion zone of the lip. It is along the border between the skin and the vermilion zone that one commonly encounters cold sores, which are generally caused by a herpesvirus. The skin of the upper lip has an indentation at the midline known as the philtrum, which is derived from the embryonic medial nasal processes (Fig. 1-1). It is at the lateral junction of this philtrum that a cleft lip might be formed.
By elevating the mandible so that the teeth are in contact and then retracting the lips and cheeks, you can see the vestibule. It is bounded anteriorly by the lips (labia) and laterally by the cheeks (bucca). A finger placed in the posterior portion of the vestibule will be impeded by two obstacles, the bony anterior border of the ramus of the mandible and the soft tissue. The cheek is formed to a great extent by the buccinator muscle, covered with skin on the outside and moist mucous membrane on the inside. This muscle extends back from the corners of the mouth to join with the muscles of the upper throat wall. As it passes backward, it crosses in front of the mandibular ramus from a lateral position to a medial position, limiting the posterior extent of the vestibule. As you run your finger in the upper posterior vestibular space, you can feel the ridge of bone that is the beginning of the anterior part of the zygomatic arch (cheekbone). This is often referred to as the zygomaticoalveolar crest. Run your finger along the cheek area of the vestibule and note the landmarks and structures just mentioned.
The point at which the mucosa of the lips or cheeks turns to go toward the gingival or gum tissue is known as the mucobuccal fold or mucolabial fold. The mucosa lying against the alveolar bone is loosely attached and movable and known as alveolar mucosa. This mucosa is generally reddish because of the presence of blood vessels underneath the relatively thin mucosa. The point at which it becomes tightly attached to the bone is the beginning of the gingiva. This is known as the mucogingival junction (Fig. 1-2). The normal color of the gingiva is pink because the mucosal layer is thicker and therefore the blood vessels do not impart as much color. In patients with darker skin color, generally some pigmentation to the gingiva is evident.
Pulling outward on the lips or corners of the mouth shows several areas where the tissue is attached in folds to the alveolar mucosa. At the midline in both the upper and lower lips, a fold of connective tissue known as the labial frenum can be found. The frenum contains no muscle tissue, only connective tissue. The upper frenum is usually more pronounced than the lower, but problems may occur with either one. The attachment of the upper (maxillary) frenum may extend to the crest of the alveolar ridge and even over the ridge. This band of tissue is so firm that the erupting central incisors might not penetrate it but may be pushed slightly aside so that a space exists between them. This space is known as a diastema (Fig. 1-3, A). Correction of a diastema usually involves the surgical removal, or cutting, of the frenum tissue between the teeth. After this the teeth will generally move together into normal contact. If they do not come back into normal contact, minor orthodontic treatment may be required. This procedure is best done when a child is 6 to 12 years old.
The mandibular labial frenum seldom extends up between the teeth, but it often extends close enough to the gingiva to contribute to gingival recession in that area by pulling downward on the tissue when the lip is tensed (Fig. 1-3, B). In this instance the frenum attachment needs to be incised with possible periodontal follow-up to restore the original gingival contours.
Less well-defined frena are evident in the maxillary and mandibular canine areas. These can be seen in Fig. 1-2 at the area labeled mucobuccal fold and in a similar area above it in the maxillary arch. Although these are not as well developed, they and the midline frena still have to be taken into consideration in the construction of a denture. If a groove is not reproduced in the flange, or edge, of the denture at that point, the appliance will cause irritation and possible ulceration of the frenular tissue.
As we continue to consider the structure of the vestibule in relation to clinical dentistry, it is interesting to note what happens to the vestibule when the mouth is opened wide. Place the teeth together, with the lips and cheeks relaxed. Position your index finger in the posterior-superior part of the vestibule, adjacent to the maxillary third molar area, move your finger as far posteriorly as you can, and open the mouth wide. You can feel your finger being pushed anteriorly out of the area. This is happening because the coronoid process of the mandible is moving into that vestibular space. The coronoid process is of important consideration for several clinical reasons.
In radiology, for example, you can take two periapical films of the maxillary molar area—one using a bisecting angle technique with the mouth open and the patient holding the film and the other using a paralleling technique with the mouth closed on a film-holding device. The coronoid process intrudes into the vestibular space on the film taken with the mouth open, making it difficult to get a clear image on film. However, on the second film, taken with the mouth closed, the coronoid process does not impinge on the space, thereby demonstrating the benefit of a film-holding device, which eliminates exposure to radiation of the finger, and a much more stabilized and accurate film.
The coronoid process may also cause some problems when you are trying to take maxillary study models. When the mouth is open wide, the coronoid process may tend to push on the posterior part of the impression tray and cause it to be displaced, making it difficult to obtain a good impression of the third molar and the maxillary tuberosity regions. It may also impinge on the posterior-lateral portion of a patient’s maxillary denture and cause possible dislodgment of the denture. It is necessary to remove as much bulk as possible from both the impression tray and denture in that area so this does not happen.
Other problems arise in edentulous patients (see Figs. 26-12 and 26-15; dotted lines mark the alveolar process, or alveolar bone). When teeth are lost, some loss of the alveolar bone that formed the sockets for the teeth occurs. It tends to happen to a greater extent in the mandible than in the maxilla. If the bone loss is too great, problems occur in constructing a lower denture that will be stable. You will learn many ways to solve these problems as you progress in your studies.
Study the texture of the inner surface of the lip. Pull the lower lip down, dry it with a tissue and stretch it. Notice the small drops of fluid on the lip, indicating the openings of many small salivary glands. These of course are also found in many other areas of the oral cavity (see Chapter 25).