GEOMETRIC CONCEPTS OF CROWN OUTLINE
In general all aspects of each tooth crown - except the incisal or occlusal aspects - may be outlined schematically within one of three geometric figures; a triangle, a trapezoid or rhomboid.
Facial and lingual aspects of all teeth
The outline of these surfaces may be outlined by a trapezoids of various dimensions (Fig 1). The shortest of the uneven sides of the trapezoid represent the bases of the crown at the cervices. While the longest of the uneven side represent the incisal or occlusal surfaces and form the approximate point at which the opposing teeth come together when the jaws are closed.
Fig. 1. Outline of labial and lingual surfaces of all teeth is trapezoid. Lower central incisor (Left), lower canine (Middle) and lower first molar (Right).
This arrangement and design brings out the following fundamentals of form:
- Create interproximal spaces to accommodate the interproximal gingival.
- Create a space between the roots of the adjacent teeth which allow enough space for bone and supporting structures required to hold up the gingival tissue to normal level and to provide sufficient circulation of blood to region.
- Provides a contact area between adjacent teeth which help to protect the interproximal gingiva.
- The arrangement of teeth tends to prevent elongation of the antagonists as each tooth has two antagonists in the opposing arch except lower central and upper third molar
Mesial and Distal Surfaces
A. Anterior teeth
These aspects may be included within a triangle, the base of which is represented by the cervical portion of the crown and the apex by the incisal ridge (Fig 2). This design provides:
- A wide base to the crown for maximum strength.
- A tapering outline labially and lingually, narrowing to a relatively thin ridge (incisal ridge) which facilitate cutting and penetration through food materials.
B. Maxillary posterior teeth
The outline of these aspects can be included within a trapezoid with the longest uneven side representing the cervical part of the crown (Fig. 3). The design provides the following fundamentals:
- Since the occlusal surface is constricted, the tooth can be forced into food material more easily
- If the occlusal surfaces were as wide as the bases of the crowns excessive masticatory forces would be transmitted to the roots.
- The form helps self-cleaning of the teeth.
Fig.2. Outline of mesial and distal surfaces of anterior teeth are triangular.
C. Mandibular posterior teeth
These surfaces are somewhat rhomboidal in outline (Fig. 3). The occlusal surfaces are constricted as compared to the bases.
Fig. 3 The outline of the proximal surfaces of upper posterior teeth is trapezoidal (A upper, B & D upper), while that of lower posterior teeth is rhomboidal with lingual inclination (A lower, C and D lower).
The rhomboidal design provides the following fundamentals:
- The crown is inclined lingually which allow proper intercuspation.
- Keep the axis of the crown and root of the teeth of both jaws together.
- Prevent clash of the opposing cusps with one another.■
FUNDAMENTALS AND PREVENTIVE CURVATURE OF TEETH
The teeth have a specific shape, with fundamental curvatures which play an essential role in maintenance of the teeth in the dental arch, preventing diseases, damage, bacterial invasion and calculus building. Also they disperse the excessive occlusal trauma and biting forces and protecting the periodontium and therefore increasing the life span of the tooth within the dental arch. Table 1 list direct and indirect tooth form characteristics that are successful in protecting and preserving the teeth.
Proximal contact areas
The contact areas are located on the mesial and distal surfaces of each tooth. The contacts between the adjacent teeth are not just a point but rather flattened area which are narrow on the anterior teeth and wide on the posterior teeth. (Fig. 4)
The proximal (mesial or distal) contact areas of the teeth are situated in a way that the food debris is prevented from packing between them.
Fig. 4. Contact areas of mandibular teeth as seen from the buccal aspect
Fig. 5. Interproximal spaces of lower anterior teeth (left) and lower posterior teeth (right).
The actual proximal contact areas touch each other so that the surfaces are not large enough to create a build up of excessive amounts of bacteria or food debris, but large enough to effective barrier and prevent food from packing between teeth. Proper contact areas help the following:
- Stabilize the dental arches by anchorage between the adjacent teeth.
- Prevent food impaction between teeth.
- Resistance to teeth displacement.
These are V-shaped spaces between the teeth, formed by the proximal surfaces and their contact areas. The space is wider cervically than occlusally and filled with gingival tissues. (Fig. 5)
When gingival recession occurs between the teeth, the interdental papilla and bone no longer fill the entire spaces, accordingly a void exist cervical to the contact area which is termed “cervical embrasure”. It occurs frequently as a pathological consequence of periodontal or orthodontic causes and offer a place in which bacteria and food debris can accumulate. Table 2 list the functions of the interproximal spaces.
These are spaces between the teeth surrounding the contact areas (Fig. 6). They widen out from the contact area in all direction. The names of the embrasures are facial (labial or buccal), lingual, incisal or occlusal. There are also cervical or gingival embrasures but only when the interproximal space is not occupied by ginigva or bone. Table 3 lists the functions of the embrasures.
Fig. 6. Occlusal view of contact areas and spillways (arrows) of anterior teeth (left) premolars and molars (middle and lright)
Curvature of the cervical line
The curvature of the cervical line on the mesial and distal surfaces depends on the height of the contact area above the crown cervix as well as the diameter of the crown labiolingually or buccolingually.
The periodontal attachments follows the cervical line and connects the gingiva and the cementum. The periodontal ligament attaches the cementum to the bone.
The anterior teeth are narrow labiolingually, accordingly the curvature of the cervical line is high to provide more anchorage and bony support. The posterior teeth on the other hand are wider buccolingually and have more bony support and therefore need not to have this raised portion of bone.
The maxillary anterior teeth show greater amount of curvature of the cervical line. The more anterior the tooth the more the curvature. The mandibular anterior teeth show less curvature (about 1mm) than in the maxillary anterior teeth.
Fig.7. The cervical line of anterior teeth is more concave than in the posterior teeth.
Contours of the facial and lingual surfaces
The degree of these contours vary from tooth to tooth but the following concepts should be considered:
- The location of the buccal contour of anterior and posterior teeth is at the cervical third of the crown.
- The lingual height of contour of the anterior teeth is at the cervical third of the crown (the cingulum)
- The lingual height of contour of posterior teeth is at the middle third.
- The average degree of curvature found on most facial contours is approximately 0.5 mm, and some what less lingually on the anterior teeth.
- The average degree of curvature on the lingual aspects of maxillary posterior teeth is about 0.5 mm, while on the mandibular poaterior teeth it is about 1 mm.
Functions of having the right degree of surface contour are shown in table 4.
Fig.8. The curvature of the facial and lingual surface allow proper food deflection and protect the gingival attachment (left). Lack of curvature allow food impaction into the gingival crevice (right).
If the curvature is absent or too slight, gingival recession will occur. And if the curvature is too high the gingiva will be over protected and food material will pack around the gingival area and result in gingival inflammation.
In young individuals most of the curvatures lie beneath the gingival crest. However, in old persons the cervical line may be visible and may be just under the gingival crest due to the gradual gingival recession and the curvature becomes exposed.
Self-cleaning qualities of the teeth
The smoothness of the enamel which covers the crown of the teeth helps food substance to slip off the crown and aids in the prevention of periodontal diseases by stimulating and cleaning the gingival tissues.
It is evedent that teeth reflects their function as well as their cleaning ability. The premolars are shaped in such a way to deflect food to the occlusal surface where it is ground by cusps.
Pits and fissure provide a method of dissipating the extreme occlusal force that result from interdigitation of cusps during the process of grinding of food. These little pits and fissures act as a spillways on the occlusal surface of the tooth.
Generally speaking when tooth is well formed histologically and morphologically, properly arranged in the dental arch, has normal relation to the opposing tooth and adjacent tooth, it is considered a sel-cleaning tooth. ■
The mandible is the strongest bone of the facial skeleton. It is a U-shaped bone composed of a body and two rami. Figures 5-7 shows the anatomical landmarks of the outer and inner surface of the mandibular body and ramus.
Fig. 9. The mandible: lateral view (left upper), medial view (left lower) and superior view (right).
12. Body of mandible, 13. Base of the mandible , 15. Mental protuberance,16. Mental tubercle, 17. Gnathion - the lowest medial point of jaw, 18. Mental foramen, 19.External oblique ridge., 20. Digastric fossa,21. Mental spine 22. Mylohyoid line, 23. Mandibular torus, 24. Sublingual fovea, 25. Submandibular fovea, 27. Alveolar bone, 28. Dental alveoli, 29. Interalveolar septa, 30. Interradicular septa, 31. Alveolar juga - prominence on the external surface of bone caused by roots of the teeth.
Age changes of the mandible
To study the changes which occur on the human mandible over age we divide the age periods into: at birth, after birth (childhood), adult period and old age.
- At birth the size of the mandible is very small and body is formed of two separate halves connected by fibrous tissues at the symphysis menti. (Fig. 10 A)
- The body of the mandible contains the developing deciduous teeth in their crypts.
- The mandibular canal runs near the lower border of the mandible.
- The mental foramen opens below the crypt of the lower deciduous first molar near the lower border.
- The coronoid process is higher than the level of the condyle.
- The angle between the body and ramus is obtuse (about 170o).
- The two halves become united at the symphysis from below upwards and become fully united at the end of the first year.
- The hight of the body increase by bone deposition on the inferior border and beginning of development of the alveolar process.
- The mandibular canal runs above the level of the mylohyoid line.
- Mental foramen positioned on a higher level and below the deciduous first molar. (Fig. 10 B)
- The sigmoid notch become more deeper.
- The mandibular angle is reduced by the growth of the ramus and become 140o.
- The mandible reaches maximum size by complete eruption of the permanent teeth and formation of the alveolar process.
- The mental protuberance is well developed to give the characteristic chin appearance.
- The mandibular canal runs nearly parallel to the mylohoid line. (Fig. 10 C)
- The mental foramen opens midway between the upper and lower borders of the body of the mandible and between the apices of the lower first and second premolars.
- The condyle is at a higher level than the coronoid process.
- The sigmoid notch is much deeper.
- The agle of the mandible diminishes till it reaches 110o-130o.
Fig. 10. Morphological age changes of the mandible. At birth (upper left), at childhood (upper right), at adulthood (lower left) and at old age (lower right)
- The mandible is reduced in size after loss of teeth and resorption of the alveolar process.
- The mandibular canal and the mental foramen are closer to the upper border of the body of the mandible. (Fig. 10 D)
- The condylar head is bent backward till it become at a lower level than the coronoid process. This is due to loss of teeth and the continuous trial of the individual to bring the upper and lower jaws near to each other during mastication.
- The sigmoid notch is shallower than in adult life.
- The angle of the mandible increase to 140o and the ramus assume an oblique direction.■
The osseous structures that support the teeth are the maxilla and the mandible. The maxilla consists of two bones, right and left maxilla sutured together at the midline. Both maxilla are joined to the base of the skull The mandible, on the other hand, has no osseous union with the skull and is movable. (Fig.11 and 12)
Each jaw, maxilla and mandible, is formed of alveolar process and basal bone. The alveolar process is that portion of the bone which surrounds the roots of the teeth and give them their osseous support.
Fig. 11. Maxilla and alveolar process.
1. Palatine process, 3. Premaxilla, 4. Incisive canal, 5. Incisive suture, 6. Palatine spine, 7. Palatine sulcus, 9. Alveolar process, 10. Dental alveoli, 11. Interdental septa, 12. Interradicular septs, 13. Alveolar juga, 15. Palatine bone - horizontal plate, 21. Pyramidal process - inserted into the pterygoid notch, 22. Lesser palatine foramen, 31. Posterior nasal spine, 33. palatine crest
Fig. 12. Alveoli of the upper central, lateral and canine (left), upper premolars (middle) and upper molars (lower).
The alveoli (Teeth Sockets)
These cavities are formed by the facial and lingual bony plates of the alveolar process and by connecting septa of bone placed between the two plates. The form and depth of each alveolus is regulated by the form and length of the root it supports.
Central incisor: The periphery of the alveolus is regular and rounded and its interior is evenly cone shaped.
Lateral incisor: It is generally cone shaped but it is narrower mesiodistally than labiolingually. It is smaller in cross section but deeper than the alveolus of the central incisor. Sometimes it is curved distally at its upper end.
Canine: It is much larger and deeper than that of the incisors. The periphery is oval with the labial width greater than the lingual. The socket is flattened mesially and somewhat concave distally. The labial plate of bone is very thin over the canine eminence. Generally the facial cortical plate of bone over the anterior teeth is thin.
First premolar: The alveolus is kidney shaped with the cavity partially divided by a spine of bone which fits in the mesial developmental croove of the root. If the root is bifurcated as is often the case, the terminal portion of the cavity is separated into buccal and lingual alveoli. The socket is flattened distally and wider buccolingually than mesiodistally.
Second premolar: It is also kidney shaped but the curvatures are the reverse of the first premolar alveolus. The septal spine is located at the distal side instead of the mesial. The tooth usually has one broad root with blunt end but occasionally bifurcated at the apical third.
First molar: There are three distinct alveoli widely separated. The lingual (palatal) one is the largest and is round, regular and deep. It extends palatally having a lingual plate over it which is very thin. The mesiobuccal and distobuccal alveoli has no special characteristics except that the buccal plates are thin. The mesiobuccal alveoli is broad buccolingually with the mesial and distal walls flattened. The distobuccal alveoli on the other hand, is rounded and more conical.
Second molar: The alveoli are closer together as the roots of this tooth are not as divergent as those of the first molar.
Third molar: It is similar to that of second molar except that it is somewhat smaller in all dimensions. Usually the roots are fused. The inter-radicular septum changes accordingly, if the roots are fused a sptal spine will appear in the alveolus at the point of fusion of the roots.
Central incisor: The periphery of the alveolus often dips down labially and lingually making an interdental spine out of the interdental septum separating the alveoli of the mandibular central incisors. The alveolus is flattened on its mesial surface and is somewhat concave distally to accommodate the developmental groove on the root.
Lateral incisor: Similar to that of central incisor with two variations; the socket is larger, deeper and its periphery dose not dip down on the lingual but may dip more on the labial. (Fig 12)
Canine: It larger, oval and deeper to accommodate the root of the tooth. The mesial and distal walls are irregular to accommodate developmental grooves on the root. The lingual plate is stronger and much heavier than over the alveoli of the incisors, but the labial plate is thin.
First and second premolars: Are similar in outline, which is smooth and rounded although greater buccolingually than mesiodistally. The alveolus of the second premolar is larger than that of the first premolar. The buccal plate is relatively thin but the lingual is heavy.
First molar: The socket is divided by an interradicular septum which is thick and regular. The alveolus of the mesial root is kidney shaped, much wider buccolingulally than mesiodistally and constricted in the center to accommodate the developmental grooves found mesially and distally on the mesial root. The alveolus of the distal root is evenly oval with no constriction, confirming to the rounded shape of the distal root.
Second molar: May be divided into two alveoli as that of the first molar or it may appears as one compartment near the periphery of the alveolus, but divides into two compartments in the deeper portion.
Third molar: Usually the socket of the third molar is irregular in outline. It is much narrower toward the distal than the mesial aspect. In addition it may have interradicular septa or septal spine to accommodate the irregularity of the root. ■