Classification of Occlusion
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ANGLE CLASSIFICATION
Edward Angle, the father of modern orthodontics developed the Angle classification system. Though often identified by the relationship between upper and lower first molars, it can be determined by comparing any two opposing teeth (e.g. upper and lower canines). The INBDE questions may try to trip you up by asking the relationship between different reference points (e.g. the distobuccal cusp of the maxillary first molar instead of the mesiobuccal cusp).
Primary molar relationship
The primary molar relationship describes the mesiodistal relationship of the opposing second deciduous molars, which in turn strongly influences the mesiodistal relationship (and Angle classification) of the adult first molars.
Flush terminal plane – is seen in about 90% of patients and is considered the optimum relationship, where the distal surfaces of the deciduous molars line up vertically. The adult molars end up with edge to edge cups tips, but these teeth will most likely shift into a Class I molar relationship due to the larger mandibular leeway space.
Distal step – essentially a deciduous class II.
Mesial step – essentially a deciduous class I.
An equivalent of the Angle class III is almost never seen in the deciduous dentition, the mandible is always lagging behind the maxilla in growth. It can be helpful to predict what molar relationship will result. Patients with a flush terminal plane develop into a Class I in 56% of the cases and into a Class II in 44% of the cases. Cases with a mesial step relationship usually transition into a Class I. A class III outcome is rare but possible.
Angle Class I
Most (70%) of patients will present with an Angle Class I occlusion, where the mesiobuccal cusp of the maxillary first molar lines up with the buccal groove of the mandibular first molar. The maxillary canine lies between the mandibular canine and first premolar. Normal overjet 1-2mm. A Class I malocclusion means an Angle Class I molar relationship but incorrect inter-arch relationships, most likely caused by crowding (excessive mesiodistal widths of teeth compared to arch circumference).
Angle Class II
About a quarter (25%) of patient will present with an Angle Class II occlusion (also known as distoclusion or retrognathism), where the mesiobuccal cusp of the maxillary first molar falls between the mandibular first molar and the second premolar. The maxillary canine will sit mesial to mandibular canine. You can further subdivide the classification into fractions. An overjet of more than 5mm usually suggests this dental relationship. An Angle Class II malocclusion is usually associated with a convex facial profile. A Sunday bite refers to forward posturing of the jaw by a patient with a Class II occlusion in order to improve aesthetics. The Class 2 classification can be further described in two levels.
- Class – molar relationship (or posterior teeth relationship).
- Division – relationship of incisors to each other.
- Subdivision – malocclusion on one side of the arch (asymmetrical).
CLASS II DIVISION 1 (DIV 1) – Maxillary incisors protruded/flared/labioversion.
CLASS II DIVISION 2 (DIV 2) – Maxillary central incisors retruded or nearly normal, laterals tipped mesially and protruded/flared. Deep overbite.
CLASS II DIVISION 1 SUBDIVISION – A class II molar relationship on one side only. The other side may be in a molar class I relationship.
Angle Class III
Also known as mesioclusion, prognathism, or underbite, the Angle Class III occlusion is the least likely to be encountered, seen in around 5% of patients. The mesiobuccal cusp of the maxillary first molar falls between the mandibular first molar and the second molar, the maxillary canine is distal to the mandibular canine. Overjet may be 0mm (edge to edge) or negative (upper incisors sit behind lower incisors in crossbite). Angle Class III occlusions can also be seen with a subdivision (Class III on one side, Class I on the other side), and usually requires surgical correction.
Pseudo Class III
Functional movement during closing results in class III molar relationship. Often the patient’s anterior teeth are edge to edge when closing in centric relation, which is uncomfortable. Muscle memory pushes the lower jaw forward upon closing. This is a milder form of class III malocclusion and treatment is less complicated.
MALOCCLUSION AND SPEECH
Certain occlusal relationships may affect speech because the relationship between tongue, teeth, lips and roof of the mouth is affected. Patients with an anterior open bite may have difficulty making the speech sounds “th, sh, ch”, creating a lisp. S and Z can also be difficult to pronounce with a midline diastema. In Class II div 2, the retruded incisors may interfere with T and D sounds. Class III may make F and V difficult to pronounce. For more, see SOUNDS IN DENTISTRY.
CROSSBITE
A crossbite describes a situation where teeth are positioned on the wrong side of their counterparts. It can be caused by a skeletal, dental, or functional discrepancy, the latter describing a shift into crossbite when closing. A crossbite is often seen when there is a disparity in maxillary and mandibular jaw sizes.
A class III occlusion can cause a reverse/negative overjet. Kids with a class III skeletal growth pattern may present with an edge to edge or Class III deciduous incisor relationship (very rare), often accompanied by visible attrition. It is not until the adult teeth come through that the patient presents with a Class III/pseudo class III anterior profile. Though usually found in children with a class III tendency, anterior crossbites may be seen in normal skeletal relationships in cases of crowding or ectopic eruption. Early extraction of a deciduous tooth may result in spontaneous correction, but an appliance may be needed to help the tooth into its correct place. A removable appliance with a simple finger spring is used. Tipping movement will be seen, and will only work if there is enough space for the tooth in the arch.
Prolonged thumb sucking, pacifier use or similar habits can cause a crossbite or open bite. The habit will have to be stopped before correction can begin. If this is done before the eruption of the adult teeth, an open bite will likely spontaneously correct. Crowding, deciduous tooth retention or ectopic eruption can lead to a tooth being trapped in cross bite. The classic profile of digit sucking habit: anterior crossbite, proclination of upper incisors, narrow upper arch, retroclination of lower incisors, and a Class II malocclusion.
A crossbite can be classed as unilateral or bilateral, anterior or posterior, or a combination. A complete crossbite is where the teeth entirely miss each other. A Scissor bite or bilingual crossbite results from an extreme mismatch in width between the maxilla and mandible.
Anterior and posterior crossbites are preferably treated before skeletal growth has stopped (as soon as possible). In fact this treatment is the most common active treatment to be started in the primary dentition because the transverse dimension of growth stops first. A palatal expander is usually used. A rapid palatal expansion (RPE) device can be activated at varied intervals and amounts widening the maxilla about 0.5mm/day. A RPE device is usually activated once or twice a day, one quarter mm (0.25mm) per turn, with forces as high as 100N. The length of treatment depends on the final result sought, usually one to two months. The expander may remain in the mouth for 3-6 months after the expansion phase of the treatment is complete to allow for bony fusion along the mid palatal suture (retention).
A midline diastema is often seen after palatal expansion, and will likely autocorrect within a few weeks. Rapid maxillary expansion also results in the expansion of the nasal floor. Mild anterior crossbites of one or more teeth should be fixed as soon as possible, but be sure there is enough mesio-distal space for the tooth/teeth to move in to. More severe crowding is usually reserved for second phase (conventional fixed appliance) treatment. Anterior correction will usually not need retention, the bite will retain the new position.
OPEN BITE
An open bite can be caused by skeletal or dental discrepancies, but it is most commonly associated with a habit (thumb sucking, pacifier etc.). Non-nutritive sucking habits lead to malocclusion only if it continues during the mixed dentition stage, resulting in the protrusion of upper anterior teeth and retrusion of lower incisors. A tongue thrust is often seen in association, but is likely a result of the malocclusion, not the cause. The digit sucking force presses the buccinator muscles in on the maxilla and can cause narrowing of the upper arch, leading to a posterior crossbite.
An open bite can be classified as an anterior or posterior open bite. Anterior open bites are more commonly seen in African Americans patients, whereas deep bites are more common in Caucasians patients. Individuals with an anterior open bite usually have a convex facial profile, but their profile becomes less convex as they mature. Treatment starts by discouraging the functional habit, done with or without appliances like a tongue crib or bluegrass. This can be followed by:
- Posterior bite planes.
- High pull headgear.
- Orthodontic treatment with fixed appliances.
- Orthognathic surgery may be required in severe cases of skeletal open bite, such as patients who show excessive anterior vertical facial height, steep mandibular plane angle (see cephalometric angles), or long facial heights.
Treatment is followed by retention.
