Occlusal Schemes and Teeth Selection
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GENERAL TERMS
The following section is also covered in ORTHODONTICS AND OCCLUSION.
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.
- Disclusion of posterior teeth during excursive movements (canine guidance, anterior guidance).
- No interferences during excursive movements.
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 lead 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.
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.
REFERENCE LINES
The interpupillary line serves as a horizontal reference for the position of the maxillary teeth and occlusal plane. The Frankfort Horizontal plane, Camper’s plane and the Palatal plane may be used as reliable guides to establish the occlusal plane in edentulous patients.
- Frankfort horizontal plane can be visualized externally by drawing a line from the bony infra-orbital rim (orbitale) below the eye to the tragus of the ear (porion).
- Camper’s plane is determined by drawing a line from the tip of the anterior nasal spine (acanthion, or inferior border of the ala of the nose) to the superior border of the tragus. The inferior surface of the maxillary occlusal rim is often constructed to be parallel to the Camper’s line.
- The palatal plane is determined by drawing a line from the most anterior point of the maxillary bone, the anterior nasal spine (ANS), to the most posterior limit of the bony palate of the maxilla, the posterior nasal spine (PNS).
The dental midline should ideally (but not necessarily) coincide with the facial midline. Optimum incisal display at rest is 2 mm for males and 3.5 mm for females. With age more of the mandibular incisors are visible at rest. It is important to evaluate the upper lip when smiling (smile line). During an esthetic smile, 1-2mm of gingiva is visible. An excess of 3mm, seen in the case of a high smile line, is termed a “gummy smile”. The inferior maxillary incisal outline should follow the lower lip line. The buccal corridor refers to the “black spaces” between the buccal aspects of the posterior teeth and the corner of the mouth when smiling.
TOOTH WEAR
Attrition refers to the loss of occlusal and incisal surface material caused by tooth to tooth contact. It is generally a normal result of mastication, but can be exacerbated by parafunctional habits or the introduction of an acidic environment (reflux, wine tasters, bruxers etc.).
Abrasion is caused by abnormal wear due to a mechanical process other than tooth to tooth contact. Common sources of abrasion include toothbrush abrasion, tongue piercings, tooth picks, ill-fitting dental appliances, nail biting, or using teeth as tools (cutting fishing line with your teeth etc.). Porcelain restorations cause the accelerated wear of the opposing dentition (as much as 40 times more than gold). Gold is a more favorable material because of its occlusal wear characteristics, especially in the case of parafunction (bruxism).
Erosion is tooth loss caused by chemical means that do not involve bacteria. Erosion may be caused by intrinsic (acid reflux, GERD) and/or extrinsic (diet) factors. Soft drinks are a significant extrinsic cause of erosion. Erosion on anterior teeth often leaves the tooth surface appearing characteristically shiny. Cupping is visible on posterior teeth due to the difference in erosion between enamel and dentine. Restorative materials are relatively unaffected by chemical erosion and are left standing proud of the tooth structure.
Abfraction is often a debated topic but may still be an accepted term in the INDBE. It is caused by biomechanical loading forces leading to flexure fatigue. Abfraction is often seen as a V-shaped notch around the gingival margin on the buccal surfaces of teeth.
OCCLUSAL SCHEMES
A mutually protected occlusion is an occlusal scheme in which the anterior teeth protect the posterior teeth during movement, and vice versa. During vertical loading the posterior teeth absorb most of the force. During protrusion the overbite and overjet relationship cause disclusion of the posterior teeth. During lateral excursions the working side canines disclude all other teeth. This is called canine guidance. If canine guidance is lost and multiple premolars and molars cause the disclusion of non-working side teeth, this is termed group function. When fabricating dentures a balanced articulation is often desired. The dentition is set up in a way to allow for teeth on the working and non-working side to simultaneously contact during vertical loading, protrusion and lateral excursions. A balanced occlusion can be obtained with a flat plane occlusion if the clinician uses balancing ramps. Anterior guidance is avoided in complete denture setup to prevent dislodgement.
HANAU'S QUINT
Hanau’s quint gives the five factors of articulation that govern a balanced occlusion, namely:
- Condylar guidance – anatomically dictated by the TMJ and cannot be changed by the clinician/technician. Condylar guidance will vary from patient to patient, but averages about 30°. It is recorded using a facebow and transferred to an articulator.
- Incisal guidance – minimized in a denture setup to minimize tipping forces. Modification of incisal guidance is also limited by phonetics and esthetics.
- Compensating curve – a three dimensional “curve”, the combination of the curves of Wilson and Spee, formed by the alignment of the occlusal surfaces and incisal edges of the teeth. In the edentulous patient this is entirely under the dentist/technician’s control, allowing to maintain a balanced occlusion when the mandible is protruded.
- Relative cusp height/cuspal inclination – the average slope of the cusp. It is generally reduced in complete dentures to minimize the horizontal forces during function.
- Occlusal plane – the average plane generated with the incisal edges of the anterior teeth and the occlusal surface of the posterior teeth. It cannot be altered substantially since functional requirements dictate the position of the teeth. In complete denture setup the occlusal plane is kept relatively parallel to the denture base.
Hanau’s quint can be expressed as a formula, also known as Thielemann’s formula:
The equation can be used to show how changing one parameter needs to be compensated for by adjusting another. For example, increasing the incisal guidance will require compensating with metric under the line (adjusting the compensating curve, cuspal inclination, occlusion plane, or a combination of the three). If, say, the dentist notices a separation of posterior teeth during excursion, the problem can be corrected by increasing the compensating curve. The compensating curve can allow a dentist to change the effective cusp angulation without changing the form of the manufactured teeth.
TEETH SELECTION
Though porcelain teeth are available, most dentures are fabricated using acrylic resin teeth, which come in different shapes (rectangular/square, tapering/triangular, ovoid), sizes and shades. Posterior teeth are selected according to the occlusal scheme being implemented, and the cusp angle sought. Anatomical teeth will have a cusp angle between 33 and 45 degrees. Semi-anatomical teeth will have a cusp angle between 10 and 20 degrees. Both of these are set up in a balanced occlusion, where there is a harmonious relationship between the occluding surfaces in centric and eccentric positions during functional movement. Meaning, during lateral excursions opposing cusps contact on the working and non-working side. Balanced occlusion is the goal in complete denture fabrication, improving dentures stability during function.
A denture with non-anatomical/flat teeth will carry a monoplane occlusion. These teeth are flat or flat with a compensating curve. Non-anatomical teeth are more commonly used in patients with a skeletal Class II or III profile. A monoplane occlusion is more adaptable in Class II or III case, or cross-bite cases, but leads to poor functional and esthetic outcomes. It is best to use the simplest setup to fulfill the patient’s needs.
Anatomic and semi-anatomic teeth allow for better food penetration (steeper cusps) and improved esthetics, but the setup is technically complicated. A lingualized occlusal scheme uses more anatomical maxillary teeth and flatter mandibular teeth, utilizing a balanced type of occlusion. Though the masticatory efficiency is reduced, the setup is easier to achieve.
Supporting cusps (aka working cusps, stamp cusps, centric cusps) are the cusps in contact with the opposing tooth’s marginal ridge or central fossa. These cusps tend to be more round, better suited to crush food, and support the vertical dimension of the face. In the ideal (“normal”) occlusal scheme, the maxillary lingual and mandibular buccal cusps are considered supporting cusps. Centric stops are areas of contact that a supporting cusp makes with its opposing neighbour. For example, the mesio-lingual cusp of the maxillary first molar (supporting cusp) contacts the central fossa (centric stop) of the mandibular first molar.
The maximum distribution of occlusal stresses should be in centric relation (if possible), borne by the long axis of the tooth. If there is surface-to-surface contact the clinician should consider changing this to a point-to-surface contact. Once centric occlusion is established, never take the teeth out of centric occlusion.
Non-supporting cusps (aka guiding cusps, shearing cusps, balancing cusps, non-centric cusps) usually overlay the opposing neighbor in centric occlusion. They are narrower and sharper than supporting cusps, better suited to shear food. The inner occlusal inclines that face the supporting cusp are guiding inclines. In the ideal (“normal”) occlusal scheme, the maxillary buccal and mandibular lingual cusps are considered non-supporting cusps. In a case with a posterior crossbite, the guiding and supporting cusps would switch. Reduction of occlusal interferences, or selective grinding, is almost always done before a prosthesis is made, carefully so as not to destroy cusp height and affect the VDO. One common instance when selective grinding may be best completed after the prosthesis is in place: if a fixed or removable partial denture is to be constructed for a space opposing an over erupted tooth. The device is often fabricated to the ideal plane of occlusion and the opposing tooth is adjusted after insertion.
Only grind cusp tips of the maxillary buccal and mandibular lingual (BULL rule) side if they are premature in centric, lateral, or protrusive movements. The same rule is used for working side interferences, buccal cusps and inner inclines of upper teeth and lingual cusps and inner inclines of lower teeth. Selective grinding should result in harmonious cups to fossa contacts for all upper and lower fossa. Whenever interferences exist in centric but not in lateral excursions, the fossa/marginal ridge opposing the premature cusp is deepened.
For the best esthetics, maxillary anterior teeth are usually positioned facial to the ridge. Setting teeth up directly over the ridge will lead to poor aesthetics. Sufficient anterior bulk is needed to provide proper lip support. For most patients, the labial surface of the maxillary central incisors should be about 8mm anterior to the center of the incisive papilla. The long axis of the teeth should be about perpendicular to the occlusal plane. Maxillary central incisors are the most important teeth in terms of esthetics, their placement controls the midline, speaking line, lip support and smile line.
The occluso-gingival lengths of posterior teeth will largely depend on the amount of interarch space available. Generally the bucco-lingual widths of denture teeth are less than that of the natural teeth they are replacing. This possibly reduces the stress transferred to the denture support area during function. Reducing the bucco-lingual widths of teeth also increased the tongue space.
