| SECTION I | IDEAL OCCLUSION VERSUS MALOCCLUSION |
OBJECTIVES
This chapter is designed to prepare the learner to perform the following:
- Define Angle’s class I, II, and III relationships.
- List and describe types of tooth and jaw malocclusions.
- Describe and locate (on a skull) the articulating parts of the temporomandibular joint (TMJ).
- Describe the location and functions of the articular disc.
- Palpate the lateral and posterior surfaces of the condyle of the mandible during movement of the jaws.
- Describe and demonstrate mandibular movement within the lower joint space (rotation) and within the upper joint space (translation).
- Describe mandibular dislocation (luxation) and demonstrate how to alleviate this problem with appropriate mandibular manipulation.
- List and describe signs and symptoms of malocclusion (including the possible effects of premature contacts and parafunctional movements).
- Describe and recognize the following jaw relationships: maximal intercuspal position (MIP), centric jaw relation, and occlusal vertical dimension.
- Describe and recognize the following horizontal eccentric movements: protrusive movement (including the effect of horizontal and vertical overlap on incisal guidance) and lateral movement (including the effect of canine overlap on canine-protected occlusion).
- Define and recognize tooth relationships during lateral movements on the working and nonworking articulation.
- Describe the relationship of teeth and adjacent oral structures during eating.
- Describe (and sketch) an ideal envelope of motion from the facial and sagittal views and label mandibular tooth positions or movements for each segment of the envelope.
- Define and provide examples of parafunctional movements.
- List and describe possible methods of treatment for bruxing, myofunctional trigger points (pain), and temporomandibular disorders (TMDs) including the steps for construction of an occlusal device (bite guard).
- Describe a method for accurately recording a centric relation (CR) position of the mandible.
- Sketch, from memory, the tooth crown outlines on one side of the mouth in ideal class I occlusion.
Ideal occlusion is the harmonious static and dynamic relationship of teeth and jaws that dentists would like to reproduce when restoring a patient’s entire mouth to good form and function. Malocclusion, on the other hand, is literally a “bad” occlusion, or a deviation from the ideal. Malocclusion can be due to an improper alignment of the teeth within an arch or a lack of harmony between the size and shape of the jaws that prevents teeth from fitting together ideally.
Dr. Edward Angle first defined three classes of jaw relationships in 1887. An ideal or normal front-to-back (anteroposterior) relationship between the upper and lower jaws is known as class I occlusion. In contrast, persons with class II or class III jaw relationships have a malocclusion because of a considerable difference in size, or the abnormal positional relationship, of the mandible relative to the maxillae. Each class of occlusion is defined by the relationship of the first teeth to erupt in the adult dentition, namely, the maxillary and mandibular first molars, or, if the first molars are absent, by the relationship between the maxillary and mandibular canines.
The teeth of a person with ideal occlusion are aligned within each arch so that they fit together and function harmoniously, and the jaws are in a class I relationship. Recall from Chapter 1 that class I occlusion (also called neutroclusion or normal occlusion) is defined as the relationship of permanent first molars where the tip of the mesiobuccal cusp on the maxillary first molar is aligned with the mesiobuccal groove on the mandibular first molar (Fig. 9-1A and B) and the maxillary canine fits into the facial embrasure between the mandibular canine and the first premolar (Fig. 9-1A). (This class I relationship occurs in approximately 72% of the population.)
FIGURE 9-1. Angle’s class I occlusion. A. Lateral view of tooth models with the teeth aligned in class I occlusion. B. The first molar relationship showing the mesiobuccal cusp of the maxillary first molar aligned with the mesiobuccal groove of the mandibular first molar. C. Normal anterior relationship of incisors. D. The normal, orthognathic profile of a person having class I tooth relationships.
The facial profile of a person with class I occlusion tends to form a rather straight line from the top half of the face to the anterior border of the mandible (chin) and is called orthognathic [OR thog NA thik], where “gnathic” pertains to the jaw and “ortho” means a straight or normal jaw profile in Figure 9-1D. Compare the word orthognathic that refers to a straight jaw profile and orthodontics that means tooth straightening. This profile may also be called mesognathic (not mesiognathic) and is characterized by the position of the resting mandible relative to the maxillae: no obvious protrusion (the lower jaw is not positioned anteriorly to the normal) and no obvious retrusion (the lower jaw is not positioned posteriorly to the normal).
Ideal occlusion also required the perfect and maximal fitting together (interdigitation) of the upper and lower teeth as described here.
- The incisal edges of maxillary teeth are labial to the incisal edges of mandibular teeth. The amount of this normal horizontal overlap (also called normal overjet) is noted by the horizontal arrow in Figure 9-2A.
FIGURE 9-2. Angle’s class I occlusion: Incisor relationship, horizontal and vertical overlap. A. Normal horizontal overlap has the incisal edge of the maxillary incisors anterior to the incisal edge of the mandibular incisors, also known as normal overjet (denoted by the horizontal arrow). B. Normal vertical overlap has the incisal edge of the maxillary incisors overlapping (hiding from view) the incisal third of the mandibular incisor, also known as normal overbite (denoted by the vertical arrow).
- The incisal edges of mandibular incisors are hidden from view by the overlapping maxillary incisors. The amount of this normal vertical overlap (also called normal overbite) is noted by the vertical arrow in Figure 9-2B.
- Buccal cusps and buccal surfaces of the maxillary posterior teeth are buccal to those in the mandibular arch, whereas the lingual cusps and lingual surfaces of the mandibular posterior teeth are lingual to those in the maxillary arch (Fig. 9-3A).
FIGURE 9-3. Normal molar relationship in cross section. A. The buccal cusps of maxillary molars are facial to the buccal cusps of the mandibular molars, and the lingual cusps of the mandibular molars are lingual to the lingual cusps of the maxillary molars. B. The lingual cusps of maxillary molars occlude with the fossae in mandibular molars, and the buccal cusps of mandibular molars occlude with fossae in maxillary molars.
- Lingual cusps of maxillary posterior teeth rest in occlusal fossae of the mandibular teeth, whereas the buccal cusps of the mandibular teeth rest in occlusal fossae of the maxillary teeth (Fig. 9-3B).
- The vertical long axis midline of each maxillary tooth is positioned slightly distal to the vertical axis of the corresponding mandibular tooth (due, in part, to the wider maxillary than mandibular incisors). For example, in Figure 9-4, the center of the maxillary canine (#11) is distal to the mandibular canine (#22), the center of the maxillary first premolar (#12) is distal to the mandibular first premolar (#21), and so forth.
FIGURE 9-4. Ideal tooth alignment in Angle’s class I occlusion. The center axis of each tooth in the maxillary arch is aligned just distal to the center axis of the same type of tooth in the mandibular arch. For example, look at the two opposing canines: #11 is just distal to #22.
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