The occlusion is evaluated in the three planes of space: antero-posterior, transverse, and vertical. Many methods have been devised to record malocclusion.14 The Ackerman and Proffit method is probably the most widely used comprehensive method. It evaluates the malocclusion in the three traditional planes of space, as well as arch perimeter, alignment and facial esthetics. Angle’s classification of malocclusion, however, is by far the most commonly used for the molars and can be generally descriptive for categorizing of malocclusion. A sagittal evaluation of the dentition focuses on the molar Angle classification and the amount of overjet. Angle’s molar classification is based on the relationship of the first molars as follows:
Class I Occlusion.
Class II Division 1 Occlusion.
Class II Division 2 Occlusion.
Class III Occlusion.
Increments are often added to the classification in an attempt to quantify the severity of the relationship by approximating the fraction of a cuspal width (i.e., one-quarter cusp Class II). In the mixed dentition, a classification of one-half cusp Class II (flush terminal plane) will develop into a Class I relationship in the permanent dentition. The permanent premolars that replace the deciduous molars have a smaller mesio-distal dimension. This extra space is often termed the E space or the leeway space, and is on average about 5 mm in the mandible and 3 mm in the maxilla. This additional space in the mandible allows the permanent mandibular first molars to drift anteriorly to reach a Class I relationship.
Overjet is measured as the distance between the upper and lower incisors at the point of greatest severity. It is measured from the facial surface of the more lingual tooth (usually the mandibular incisor) and the incisal edge of the more facial incisor (usually in the maxilla). A normal overjet is 1-3 mm with the maxillary incisors in front of the mandibular incisors. Excessive overjet and a Class II molar relationship are characteristics of a Class II dental relationship. Negative overjet (anterior crossbite) combined with Class III molars are indicative of a Class III dental relationship. Individual or groups of teeth may also be found to be in anterior crossbites, sometimes referred to as a “scissor bite.” This can cause abnormal wear on the teeth and destruction of the periodontal supporting tissues of those teeth. Zero mm of overjet indicates an edge-to-edge relationship. Some patients may demonstrate an edge-to-edge relationship in CR and slide into a Class III with negative overjet in MI. This is called a pseudo-Class III and may result in excess horizontal growth in those with remaining growth, leading to a more severe Class III skeletal relationship, abnormal wear of the incisors, and TMJ discomfort. For these reasons pseudo- Class III’s should be treated early with the objective of obtaining positive overjet and eliminating the shift.
Pseudo Class III in (A) Centric Relation (B) Maximum Intercuspation.
Vertical evaluation (Figure 15) of the dentition focuses on overbite, or the amount of overlap of the upper and lower incisors, and is usually expressed as a percentage. Ideal overlap is 10-30%. Anterior open bites consist of no overlap, or space, between the upper and lower incisors vertically (Figure 16), and can be indicative of habits such as thumb sucking or tongue thrusts, or of a more serious skeletal imbalance. On the contrary, deep bites (overbite 80-100%) can be found in developing malocclusions. Some deep bites result in impingement of the palate, which can lead to recession on the lingual of the incisors or palatal irritation from the pressure of the mandibular incisors. Many times a patient presenting with deep bite will have accompanying pronounced curve of spee, decreased lower facial height, with or without an associated sagittal component of excessive overjet, all of which are indicative of potential severe problems. Determining whether the problem of deep bite is caused by extrusion of the maxillary anterior teeth or minimized eruption of the posterior teeth is important in planning treatment. It is also important to assess the patient for occlusal cants with differing vertical dimensions on one side of the dentition compared to the other. An extraoral photo with the patient biting on a bite stick with the posterior teeth might improve the assessment of how severe the cant may be.
Vertical evaluation of the dentition focuses on overbite.
Anterior open bite.
Transverse evaluation of the dentition focuses on midline (Figure 17) discrepancies and posterior crossbites (Figures 18 and 19). A midline discrepancy, in the absence of any mandibular shifts, is typically due to asymmetric crowding in one or more quadrants. Alternatively, a lateral mandibular shift may produce a midline discrepancy in MI. These patients often present with the chin being off to one side in MI, a unilateral crossbite, and asymmetric molar classifications on each side. If the midline and chin are on in CR and the crossbite is edge-to-edge or in crossbite bilaterally, this is an indication there is no true skeletal asymmetry and elimination of the shift can address the midline and chin deviation. When found early with growth still remaining, timing of treatment is critical to eliminate the potential for permanent facial asymmetry that could occur from the chin growing more in one direction and reduce abnormal wear of anterior teeth.
Bilateral posterior crossbite.
Another consideration in the development of a diagnosis is the examination of the maxillary and mandibular arches for crowding and overall position of teeth. In the primary dentition, the best indicator of potential crowding is lack of spaces between the primary teeth. Crowding in the primary dentition almost always guarantees crowding in the permanent dentition.
Orthodontists tend to be more critical when it comes to noting abnormalities in the dentition. Their threshold for noticing a midline discrepancy is lower when compared to lay people and general dentists and orthodontists. While orthodontists were found to tolerate up to a 4 mm deviation, lay people and dentists did not notice it until the midline deviation was greater.15 Midline diastemas were noted as unaesthetic at 1 to 1.5 mm by orthodontists, while dentists and lay people found it to be unaesthetic once it reached 2 mm.16 In general, orthodontists are able to detect smaller discrepancies than lay people and general dentists.