Dental Occlusion and Guidance
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DEVELOPMENTAL STAGES
- Gum pad stage – birth to 6 or 7 months, ends when the first incisor appears.
- Primary dentition – starts with the eruption of the first baby tooth, and ends with the appearance of the first adult tooth. Adult incisors are usually 25% bigger than the deciduous teeth and the lower ones take up about 6mm of extra space.
- Mixed dentition – starts with the first erupted adult tooth, usually the lower molar around the age of 6. There is roughly a 6 month lag between the exfoliation of the deciduous tooth and the eruption of the adult tooth.
- Permanent stage – begins when the last primary tooth is lost.
ERUPTION SEQUENCE
Eruption sequences are covered in Tooth development
ERUPTION PATTERNS
The ugly duckling stage is seen around age 7 or 8, before the eruption of the permanent canines, when the upper central and lateral incisors are tipped laterally due to the crowding created by the unerupted canines. This produces a midline space (diastema). The ugly duckling phase auto-corrects when the canines erupt. A large midline diastema could indicate a mesiodens (supernumerary).
Permanent teeth normally sit apical and lingual to primary incisors, and then move in a buccal and coronal direction during eruption. The lower adult canines are positioned more facially, often right in line with the deciduous teeth. This means the adult canines could be displaced either lingually or labially. During active eruption, there is apposition of bone on all surfaces of alveolar crest and on the walls of the bony socket.
Many things could cause a failure of eruption. Localized causes include congenital absence, supernumerary teeth, lack of space, dilacerated roots, and an abnormal position in the crest. Systemic causes include Down’s syndrome, Rickets, hereditary gingival fibromatosis, or issues with the thyroid or parathyroid hormones. Hyperparathyroidism can cause the premature loss of baby teeth.
GENERAL TERMS
Occlusion refers to the contact relationships of teeth, edges or points touching other edges, points or areas. The following dictate the structure of a patient’s occlusion:
- Anterior determinants – the teeth are their contacting surfaces. These can be changed.
- Posterior determinants – the left and right temporomandibular joints, and the associated musculature and suspensory ligaments. These are fixed.
The four requirements for occlusal stability are:
- Stable stops on all teeth during centric occlusion.
- Synergy between anterior guidance and border movements.
- No interferences during excursive movements.
- Disclusion of posterior teeth during excursive movements (canine guidance, anterior guidance).
Multiple posterior teeth on the working side may contact, termed group function. This is an acceptable occlusion.
Functional occlusion refers to the occlusion during normal function: chewing, eating, swallowing etc.
Physiological occlusion refers to the arrangement of teeth that is stable and can be maintained by the balance of natural oral forces (musculature of tongue, cheeks, lips, opposing teeth, eruptive force).
Pathological occlusion refers to an arrangement of teeth that leads to damage. Possible consequences of a pathological occlusal setup include excessive tooth wear (attrition), temporomandibular disorders (TMD), pulpal changes leading to pulpal symptoms, or periodontal changes. Clinically, vertical facial positions can be measured by marking any two arbitrary points on the face, one above and one below the mouth, and measuring the distance.
The vertical dimension of occlusion (VDO) refers to this vertical measurement when the teeth are positioned in centric occlusion/maximum intercuspation. If a prosthesis is set up with an excess vertical dimension of occlusion, it may lead to:
- Decreased freeway space.
- Excessive display of mandibular teeth.
- Muscle fatigue.
- Clicking of the posterior teeth during speech.
- Incompetent lips.
- Discomfort.
- Excessive soft tissue trauma.
- Gagging.
- Difficulty swallowing.
If a prosthesis is set up with an insufficient vertical dimension of occlusion, it may lead to:
- Increased freeway space.
- Cheek biting.
- Poor masticatory force.
- Angular cheilitis in the folds of the corner of the mouth.
- Aged appearance.
- “Collapse” of facial tissues, deepening of the nasolabial angle, loss of the labiodental angle, prognathic jaw appearance (overclosure), decrease in horizontal labial angle, narrow lips, and increased columella-philtral angle.
The physiological rest position (postural position) of the mandible is seen when the mandible and all its supporting structures (muscles of mastication, suprahyoid and infrahyoid muscles) are in their resting posture. It is a muscle-guided position of the mandible caused by the tonic stretch reflex (myotatic reflex) of the mandibular elevators. There are no teeth contacting during this equilibrium.
The vertical dimension of rest (VDR) refers to the vertical dimension measurement when the mandible is in this resting position, when the elevator and depressor muscles are in a state of equilibrium. VDR>VDO.
Freeway space (FS) or interocclusal distance is the distance between VDO and VDR, that is, the difference in vertical distance between the teeth in maximum intercuspation and the jaw hanging freely in its rest position. Freeway space averages about 2-6mm, compared to maximum opening (MO) which is about 40-50mm. When a patient is edentulous, the space between the occlusal rims are used to establish the correct vertical dimensions. The patient is guided into a physiological rest position by the clinician and VDR recorded. The normal freeway space distance is then subtracted, to arrive at a correct VDO. Once determined, the correct VDO can be verified by examining phonetics and facial esthetics.
Curve of Spee describes the natural arch curvature in the sagittal (mesiodistal) plane, the curve of Wilson describes the natural arch curvature in the coronal/frontal (buccolingual) plane. Both are necessary for proper occlusion, and both are concave above the curve, convex below the curve.
Functional contacts are contacts made during normal occlusal functional movements, and parafunctional contacts are abnormal or unwanted contacts. Parafunctional contacts are typically area to area contacts resulting from habits such as bruxism. Protrusive contacts are usually edge to edge contacts when the mandible has moved anteriorly from centric occlusion. Protrusive interferences are generally between the distal inclines of the maxillary posterior cusps and mesial inclines of the mandibular posterior cusps. More specifically, between the distal inclines of the buccal cusps of the maxillary posterior teeth and the mesial inclines of the buccal cusps of the mandibular posterior teeth.
Working side contacts (laterotrusive contacts) are contacts on the side towards which the mandible has moved from centric occlusion. If the mandible is shifted to the right, working side contacts will be on the right side. In lateral movements, the working condyle moves down, forward and laterally. Non-working side contacts (mediotrusive contacts, balancing side) are contacts on the opposing side to which the mandible has moved. If the mandible is shifted to the right, non working side contacts will be on the left side. In lateral movements, the non-working side condyle moves down, forward and medially.
Overbite refers to the vertical overlap of maxillary and mandibular anterior teeth. A negative overbite is termed an open bite, a large overbite can be termed a “deep” bite. Overbite can be recorded as a linear measurement, usually in millimeters, or as a percentage of overlap. Normal overbite is 10-40%. Overjet refers to the horizontal overlap of maxillary and mandibular anterior teeth. A negative overjet is termed a crossbite.
The working cusp is charged with supporting the vertical dimension of the face and holding the occlusion. When viewing posterior teeth in the buccolingual plane (from mesial and distal perspective) the maxillary palatal and mandibular buccal cusps are the working cusps (in an optimal occlusion). They oppose the central fossa or marginal ridges of the opposing teeth and are usually broader and rounder in appearance. The non-working cusp is charged with guiding movements during function. The maxillary buccal and mandibular lingual cusps are the non-working cusps. They oppose the embrasure spaces and grooves of the opposing teeth and are usually sharper in appearance.
Centric relation (CR) is the most stable, reproducible mandibular position in relation to the base of the skull. It is defined as the most anterior and superior position of the mandibular condyles within the glenoid fossa (mandibular fossa) of the temporal bone. CR can refer to a position or a range of movement without translation of the condyles (only rotational movement can occur). Because the mandible appears to rotate around a transverse axis the movement is referred to as hinge axis movement, with the condyles in the terminal hinge position. CR is a bone-to-bone relationship, independent of tooth contacts, a ligament-guided position.
To get a patient into centric relation, precise manual manipulation may be required. Alternatively a deprogramming device (leaf gauge, acrylic resin jig) will disclude the teeth, and when left for a determined period of time can “deprogram” the existing muscle memory to assist in manipulating the mandible into centric relation. When a centric relation record is taken in the natural dentition, impressions of the teeth should be confined to cusp tips only.
From here the movements of the mandible can be broken into two categories (though they occur together): rotational and translational. The mandible moves through three planes, the frontal (coronal), sagittal (longitudinal) and horizontal (transverse). Mandibular movement can be mapped in three dimensions, called Posselt’s envelope.
Centric occlusion (CO, or intercuspal position, IP) refers to the maximum intercuspation of the teeth in opposing arches. It’s purely a tooth-guided position. Centric occlusion (maximum intercuspation) can occur in centric relation (bones of the TMJ in CR) but is only seen in about 1 in 10 patients. Recall, the vertical dimension of occlusion (VDO) is measured with the teeth in CO. During empty mouth swallowing the lower jaw is usually braced in the intercuspal position.
Long centric or freedom-in-centric is the occlusal harmony with an anterior slide from centric relation (CR) to centric occlusion (CO), as well as lateral movement to accommodate the Bennett movement. If the patient presents with long centric, restorative work needs to reestablish long centric.
PDL health is maintained by directing occlusal forces appropriately. Anterior teeth should only lightly contact (or make no contact) during maximum intercuspation. The occlusal table of the tooth is less than sixty percent of the overall faciolingual width of the tooth and is generally positioned at right angles to the long axis of the tooth. The crowns of the mandibular molars are inclined 15-20° lingually.
Protrusive movement refers to anterior movement of the mandible. In a normal occlusion the incisal contacts of the incisors cause disclusion of the posterior teeth. This is known as anterior guidance and is a protective component of the preferable occlusion. When the teeth are edge to edge, the posterior teeth are the furthest apart. Christensen’s phenomenon refers to the space created distally between the maxillary and mandibular occlusal surfaces when the mandible is protruded. It is caused by the downward and forward movement of the condyles.
Lateral movement of the mandible will preferably contain a similar protective feature termed canine guidance, the disclusion of the posterior teeth as the jaw moves side to side. Canine guidance occurs on the working side. Any premature contacts on the same side as the direction of movement is termed working side interferences. Any premature contacts on the opposing side as the direction of movement is termed non-working side interferences. If this protective feature is lost multiple posterior teeth can guide lateral movements. This is called group function. A mutually protected occlusion is an occlusal scheme in which the anterior teeth protect the posterior teeth, and vice versa.
During lateral movement the condyle on the non-working side protrudes and moves medially. The working side condyle also experiences lateral movement towards the working side, though only to a small extent (maybe only 0.5mm). This slight lateral movement of the working side condyle is called Bennett movement (aka lateral shift or immediate side shift). For example, if the jaw is moved to the right, the right condyle moves slightly lateral, to the right. This movement would be small compared to the left condyle. The Bennett angle is the angle formed by the sagittal plane and the path of the mandibular condyle during lateral movement when viewed in a horizontal plane. Lateral displacement of the Bennett movement will result in an average Bennett angle of about 15 degrees
PRIMATE SPACE
Kids usually have spacing between their teeth during the primary dentition stage which helps with tooth alignment. In the maxilla, the primate space is the space between the upper lateral incisor and canine. In the mandible, the primate space is between the canines and first deciduous molars. Space loss generally does not occur when a deciduous incisor is lost, but likely will occur when an adult incisor is lost.
LEEWAY SPACE
When the deciduous canines and molars are lost there is usually a surplus of space created for the adult canines and premolars. This is the Leeway space. There is more space created in the mandible (imagine a Class I molar relationship where the lower molar has to shift further ahead of the upper molar). In the maxilla the Leeway space is 2-2.5mm (~1mm per quadrant), in the mandible 3-4mm (~2mm per quadrant). The space allows the late mesial shift of the permanent first molars.
(mesiodistal length of primary canine + primary molars) – (mesiodistal length of adult canine and premolars)
