Mandibular incisors differ in size, with the central incisor being smaller than the lateral incisor. The roots of the mandibular incisors are so small and slender that they cannot be used as abutment teeth and should not have crowns placed on them. The size and morphology of these teeth are such that preparation of the tooth may damage the pulp.
The maxillary central incisor (dens incisivus me-dialis) (Figs 4-19 to 4-21) is the largest incisor tooth, having a rectangular to rhomboid, triangular, or oval vestibular surface, which can harmonize with the shape of the face.
The anterior teeth are the four incisors and two canines in the maxilla and mandible. These 12 teeth form a group of antagonists that produce an interlocking pattern in a normal dentition. When viewed approximally, the maxillary anterior teeth are anterior to and occlude vertically over the mandibular anterior teeth without any contact. The overlap in the horizontal plane is sometimes called the overjet and ranges from 0.1 to 2.0 mm; the vertical overlap is sometimes called the overbite and can be between 2.0 and 3.0 mm (Fig 4-14).
When dental technicians have to fabricate a prosthetic tooth crown, they must be familiar with the normal form (norm) and then individualize it to match the given case. The alternative—knowing all the individual cases and picking out the right one as needed — is simply not feasible. Any description of tooth forms is based on statistical mean values, which are used to work out the normal shape of the tooth concerned, ie, an ideal form.
The previous descriptions of dental substances and the tooth attachment apparatus (periodontium) referred to the law of form and function. The connection between form and function also applies when describing the different tooth forms. It explains the extent to which the teeth and the tissue involved form a unit, having developed and grown for their particular functions and hence taken on a particular form. Every characteristic feature of the form of dental tissue therefore indicates a particular significance.
The fundamental importance of the periodontium in the functioning of the tooth has already been mentioned. However, this relationship needs to be explained in more detail. Every restoration that is fixed in some way to another tooth and hence puts extraordinary strain on that tooth will also influence the periodontium. This is why a precise understanding of the attachment apparatus is important for every dental technician.
The terms dental periosteum or desmodontium can also be applied to the
periodontal ligament (or membrane). This is a firm, fiber-rich connective tissue between the surface of the tooth root and the inside walls of the alveoli with which the tooth is joined to the
alveolar bone.The periodontal space is where the tissue structures of the periodontal ligament are found. Its volume varies from single-rooted teeth (30 to 100 mm3) to multirooted teeth (65 to 150 mm3) and corresponds to the volume of the particular cementum.
The bony makeup of the alveolar process (os al-veolare) has a typical bone structure (Fig 3-20):
- An extremely dense outer bone plate that is covered with periosteum
- An inner bony layer lining the tooth sockets that is greatly perforated
- Spongy bone, which lies between the compact layers
The tooth is anchored in the jawbone by a complicated attachment mechanism. Previously it was assumed that the teeth were fused into the bone of the jaw. More detailed analysis of tissue sections, however, revealed a narrow gap between bone and cementum—the periodontal space—in which the tooth is joined to the jawbone by a fibrous apparatus. In descriptive anatomy, this attachment mechanism is called syndesmosis.
The dental pulp (pulpa dentis) lies in the pulp cavity and in the dentinal root canals inside the tooth. This cavity is very large in young people but later becomes progressively narrower as dentin production continues. The pulp is made up of jelly-type connective tissue that contains blood vessels, lymph, and nerve endings. The pulp is generally referred to as the "nerve" of the tooth.