Essentials of Occlusion Department of Endodontics, Prosthodontics, and Operative Dentistry University of Maryland, Baltimore

Preview Factors affecting distribution of occlusal forces Occlusal schemes of the natural dentition Defining occlusal interferences How to evaluate an occlusion

Degree of Force Direction of Force

1. Forces applied along the long axes of teeth are generally well tolerated Compare to pounding the top of a fencepost With proper contact, posterior teeth receive force along the long axis in MI

2. Forces applied at an angle to the long axis have potential to cause harm Compare to forces used to remove a fence post May cause mobility, wear, or fracture Anterior teeth take non-axial forces in all positions Posterior teeth take non-axial forces in all excursive contacts and with unreciprocated incline contacts in MI

3. During excursions, the farther forward contact occurs, the less force is being applied Compare to applications of a nutcracker Williamson and Lundquist study shows elevator muscle contraction decreasing as posterior teeth disocclude Rationale for anterior protection and progressive disocclusion in group function

4. The greater number of teeth in simultaneous contact, the less force applied to individual teeth Rationale for group function, but more significant in MI Becomes more significant as tooth loss occurs

5. In excursions, friction intensifies forces applied to individual teeth Wear facets produce friction during excursions As wear facets increase in area, friction and force intensify

Occlusal Schemes in the Natural Dentition

Anterior Protection Mutual Protection Canine Protection Anterior Group Function

Canine Guidance: Lateral Movement

Anterior Protection All teeth contact in MI Posterior teeth protect anterior teeth from non-axial forces in MI Anterior teeth protect posterior teeth from non-axial forces during all excursions By far the most favorable scheme for restorative dentistry

Group Function Posterior teeth contact on working side during excursions No contact on non-working side during excursions No posterior contact during protrusive movements All teeth contact in MI

Progressive Disocclusion

Partial Group Function

Bilateral Balance

Effects of Occlusal Curves

Anteroposterior Curve

Mediolateral Curve

Occlusal Interferences Prematurities Deflective occlusal contacts Interferences to healthy occlusal schemes

Interferences to Healthy Occlusal Schemes Centric interferences Working side interferences Non-working side interferences Protrusive interferences

Centric Interferences Arc of closure interferences Line of closure interferences

Arc of Closure Interference

Line of Closure Interference

Excursive Interferences

Working side interferences Excursive contacts on other than inner inclines of maxillary buccal cusps Excursive contacts disallowing progressive disocclusion Excursive contacts producing broad wear facets

Non-Working Side Interferences Any contact on the non-working side during excursions is an interference Contact causes elevator muscle contraction as condyle translates Lateral pterygoid is placed in competition with elevator muscles

Protrusive Interferences Any posterior contact during protrusion Occurs with steep antero-posterior curve or steep cuspal inclines May occur after loss of mand. 1st molar Potentially most damaging because of wear on anterior teeth/ loss of anterior guidance

Evaluating an Occlusion

Favorable Conditions Strong anterior guidance Relatively flat occlusal curves Intact dentition with no third molars Posterior cuspal inclines less than anterior guidance inclines All teeth contacting in MI

Unfavorable Conditions Most are conditions used to create a balanced occlusion in complete dentures

1. Little or no anterior guidance No vertical overlap No contact in maximum intercuspation

Steep occlusal curves Steep medio-lateral curve makes non-working side interferences likely Steep antero-posterior curve makes protrusive interferences likely Missing posterior teeth with tipping or supraeruption may produces changes in occlusal curves

3. Third Molars Occlusal curves are exaggerated in third molars

4. Wear Facets Incisors- Wear on incisal edges reveals protrusive bruxing Canines- Wear on incisal edge indicates lateral bruxing. Look for non-working interferences on the contralateral side Posteriors- Learn how wear facets on inclines are formed

Wear Facet Analysis

5. Excessively steep posterior cuspal inclines If the inclination of posterior cuspal inclines exceeds the inclination of anterior guidance, posterior inclines cannot clear during excursions

6. Excessively steep orientation of the occlusal plane If the inclination of the occlusal plane equals or exceeds the inclination of horizontal condylar guidance, the posterior teeth may not be able to disocclude in protrusive excursions

The End!