Laws of Articulation Hanau quint:
There are five factors involved in eccentric occlusal balance in complete dentures.
- Posterior or Condylar guidance
- Anterior or Incisal guidance
- The occlusal plane
- The compensatory curves
- Cusp angulation or inclination
1. Condylar guidance – it is definite anatomic feature that depends on the inclination of the floor of the glenoid fossa.
2. Incisal guidance – Incisal guidance is the effect of the contact of the upper and lower anterior teeth on the movement of the mandible. It is usually expressed in degrees of angulations from the horizontal by a line drawn in the sagittal plane between the incisal edges of the upper and lower incisor teeth when closed in centric occlusion. If the incisal guidance is steep, it requires steep cusp, a steep occlusal plane, or a steep compensating curve to effect an occlusal balance.
Factors affecting the incisal guidance angle :
Vertical overlap (over bite): the vertical overlap is directly proportional with the incisal angle. For complete dentures the incisal guidance should be as flat as esthetic and phonetics will permit. For the following reasons:
- To guard against loss of posterior teeth contact during protrusive movement.
- To allow the use of posterior teeth with reduced cusp angle and this will reduce the lateral stresses transmitted to the ridge.
- To reduce the downward movement of the mandible during edge to edge position.
3. Plane of Occlusion – the occlusal plane is established in the anterior by the height of the lower cuspid, which is nearly coincident with the commissure of the mouth, and in the posterior, by the height of the retromolar pad. It is also related to the related to the ala-tragus line. Its role is not as important as are the other determinants.
4. Compensating curves — Compensating curve is one of the more important factors in establishing a balanced occlusion so that the occlusal surface results in a curve that is in harmony with the movement of the mandible as guided posteriorly by the condylar path. A steep compensating curve for occlusal balance. A lesser compensating curve for the same condylar guidance would result in a steeper incisal guidance (anterior interference), which would cause loss of molar balancing contacts.
5. Height of cusps on teeth or inclination of cusp less teeth.
Cusp angle – the angle made by the slopes of a cusp with a perpendicular line bisecting the cusp, measured mesio-distally or bucco-lingullay.
Cusp Height – the shortest distance between the tip of a cusp and its base plane. Altering the cusp height by widening or narrowing a tooth alters the length of the cusp incline but does not change the relationship to the mean occlusal plane, i.e. cusp angle is not affected by a change in cusp width.