iv.tooth prep v. occlusion

IV. TOOTH PREPARATION

A. Definition of Tooth Prep

-Tooth Preparation is the mechanical treatment of dental disease or injury to hard tissues that restores a tooth to its original form or contour


B. Objectives of Tooth Prep

1. Reduction of the tooth in miniature to provide retainer support

2. Preservation of healthy tooth structure to secure resistance form

3. Provision for acceptable finish lines

4. Performing pragmatic axial tooth reduction to encourage favorable tissue response from artificial crown contours


C. Principles in Tooth Prep

1. Preservation of tooth structure

2. Retention and resistance form

3. Structural durability of the restoration

4. Marginal integrity

5. Preservation of the periodontium


D. Basic Steps of Tooth Prep

1. Incisal/ Occlusal Reduction

2. Facial Reduction

3. Lingual/ Palatal Reduction

4. Proximal Reduction

5. Gingival Margins/ Finishing Lines

6. Rounding up of Sharp Corners and Line Angles

7. Finishing

BIOLOGIC

Conservation of tooth structure

Avoidance of overcontouring

Supragingival margins

Harmonious occlusion

Protection against tooth fracture

ESTHETIC

Minimum display of metal

Maximum thickness of porcelain

Porcelain occlusal surfaces

Subgingival margins

MECHANICAL

Maximum surface area

Apical extension

Adequate thickness of metal

Bulk at margins

Depth Guides / Orientation Grooves

Help dentists in the preparation of teethPrevent overreduction as well as

underreduction


Anterior teeth – 1.5 – 2mm or 2/3 of the incisal 3rd

Posterior teeth – -metal occlusal 1.5-2mm

-metal and porcelain/ fiber reinforcedFC-1.5-2mmGC-1-1.5mm

2. Labial/ Buccal Reduction

3. Lingual Reduction

3. Lingual Reduction

.75 – 1mm amount of tooth reduction


Degree of Taper

2-5 deg on each side

Average taper of 3 degrees

5-10 degrees combined taper

Average taper of 6 degrees

Different Types of Finishing Lines

ShoulderChamferKnife edgeShoulder Bevel Chamfer Bevel

Different Levels of Finishing Lines

SupragingivalEquigingivalSubgingival

Is it bad to place margins subgingivally?


BIOLOGIC WIDTH

What is Biologic Width?

It is a band of soft tissue attachment

What is its composition?

It is composed of approximately 1mm of junctional epithelium and 1mm of connective tissue fibers.

The dentogingival junction includes the gingival sulcus (A-B) approximately 0.8 mm .The junctional epithelium (B-C) 0.7 to 1.3mm (average 1mm)The connective tissue attachment (C-D) 1.07mm.The biologic width (B-D) averages 2mm in occlusogingival height.

CD

BA

“When you bury the collar,

You attend the funeral of the

periodontium”

BIOLOGIC WIDTH

What is its significant clinical implication?

Crown margins can be placed Crown margins can be placed

subgingivally but should not encroach subgingivally but should not encroach

the Biologic Width.the Biologic Width.

IF VIOLATED …

Inflammation

and

Osteoclastic Activity

Bone Resorption

and

Pocket Formation

Intacrevicular Margin

6. Rounding up of Sharp Corners and Line Angles

7. Finishing

Most Common Errors in Tooth Preparation

Over reductionUnder reductionUndercutsRough tooth preparationsLack of parallelismFailure to contour proximal surfaces of

adjacent teeth

Type of CVC Facial Reduction

Lingual Reduction

Incisal/ Occlusal Reduction

Acrylic Jacket Crown

.75-1mm

shoulder

.75-1mm

shoulder

Ant. 1.5-2 mm

Porcelain Jacket Crown

1.2-1.5mm

shoulder

.75 – 1mm

shoulder

Ant. 1.5-2 mm

Post.

Porcelain Fused to Metal crown

1.2-1.5 mm

shoulder

.75-1mm

chamfer

FC-1.5-2 mm

GC-1-1.5 mm

Fiber Reinforced Metal Crown

1.2-1.5mm

shoulder

.75-1mm

chamfer

Acrylic Fused to

Metal Crown

1.2-1.5 mm

shoulder

.75-1mm

chamfer

Ant. 1.5 – 2 mm

Post. 1-1.5 mm

Complete Veneer Metal Crown

.75-1mm

chamfer

.75-1mm

chamfer

Post. 1-1.5mm

V. OCCLUSION

A joint is a joining together of two bones. The temporomandibular joint (TMJ) is the articulation between the temporal bone and the mandible. It is bilateral, andmovement of the right and left sides are interrelated and function as a single unit..

The condyle of the mandible articulates with the mandibular (GLENOID) fossae of the temporal bone. The specific location is the posterior slope of the articular tubercle and the anterior portion of the mandibular (glenoid) fossae. The condyle does not fit into the center of the mandibular fossae but rests closer to the articular tubercle. The condyle and articular eminence do not actually touch, the articular disc (meniscus) rests between them. This disc is a pad of dense fibrous connective tissue that is thickest at the posterior ends, thinnest in the middle and thicker again at the anterior ends. The articular disc, in effect, separates the temporomandibular joint into upper and lower joint spaces. Laterally and medially, the disc is attached to the condyle itself, so that whenever the condyle glides forward and backward, the disc moves with it.

The condyle and articular eminence are covered by dense collagenousconnective tissue, which contains no blood vessel or nerves. Synovial fluids bathes this structures, providing nourishment and lubrication that enables the bones to glide over each other without friction.

A thick fibrous capsule surrounds and encloses the entire joint. The

disc and capsule are fused anteriorly, and some fibers of the lateral pterygoid muscle insert into the disc. Posteriorly, the disc and capsule are not directly attached but are connected by means of a retrodiscal pad, a pad of loose connective tissue that allows for anterior movement of the joint. Nerve and blood supply- Innervation is supplied by two nerves, the auriculotemporal and ,masseteric nerves, which are branches of the mandibular nerve (V3), blood supply is provided by branches of the superficial temporal and maxillary arteries.Movement- TMJ movement within the temporomandibular joint is essentially of two types: Hinge (swinging) motion and gliding movement.

The condyle of the mandible articulates with the mandibular (glenoid) fossae of the temporal bone. The specific location is the posterior slope of the articular tubercle and the anterior portion of the mandibular (glenoid) fossae. The condyle does not fit into the center of the mandibular fossae but rests closer to the articular tubercle . The condyle and articular eminence do not actually touch, the articular disc (meniscus) rests between them. This disc is a pad of dense fibrous connective tissue that is thickest at the posterior ends, thinnest in the middle, and thicker again at the anterior ends. The articular disc in effect, separates the teemporomandibular joint into upper and lower joint spaces. Laterally and medially, the disc is attached to the condyle itsel, so that whenever the condyle glides forward and backward, the disc moves with it. The condyle and articular eminence are covered by dense collagenous connective tissue, which contain s no blood vessel or nerves. Synovial fluid bathes these structures, providing nourishment and lubrication that enables the bones to glide over each other without friction. A thick fibrous capsule surrounds and encloses the entire joiunt. The disc and capsule are fused anteriorly....(contiued above)

Mandibular movementMandibular movement can be broken down into a series of motions that occur around three axes:2.Horizontal This movement, in the saggital plane occurs when the retruded mandible produces a purely rotational opening and closing movement around the hinge axis, which extends through both condyles.

2. Vertical The movement occurs in the horizontal plane when the mandible moves into a lateral axcursion. The center for this rotation is a vertical axis extending through the working side condyle.

Sagittal When the mandible moves to one side, the condyle on the side opposite from the direction of movement travels forward. As it does, it encounters the eminentia of the glenoid fossa and moves downward simultaneously. When viewed in the frontal plane, this produces a downward arc on the side opposite the direction of movement, rotating about an anteroposterior (sagittal) axis passing through the other condyle.

Various mandibular movements are comprised of motions occuring about one or more of the axes. The up and down motion of the mandible is a combination of two movements...

...There is a purely rotational component produced by the condyle rotating in the lowercompartment of the temporomandibularjoints.

...There is also some gliding movement in the upper compartment of the jaw.

When the mandible slides forward so that the maxillary and mandibular teeth are in an end to end relationship, it is in a protrusive position. Ideally, the anterior segment of the mandible will travel a path guided by contacts between the anterior teeth.

Mandibular movement to one side will place it in a working, or laterotrusive relationship on that side and a nonworking or mediotrusive relationship on the opposite side;e.g., if it moves to the left, the left side is the working side, and the right side is the nonworking side. In this type of movement, the condyle on the nonworking sidewill arc forward and medially (A). Meanwhile, the condyle on the working side will shift laterally and usually slightly posteriorly (B). This bodily shift of the mandible in the direction of the working side was first described by Bennet.

The presence of an immediate or early side shift has been reported in 86% of the condyle studied. In addition to demonstrating the predominant presence of early side shift, Lundeen and Wirth have shown its median dimension to be approximately 1.0, with a maximum of 3.0mm. Following the immediate side shift, there is gradual shifting of the mandible.

The determinants of mandibular movementThe two condyles and the contacting teeth are analogous to the three legs of an inverted tripod suspended in the cranium. The determinants of the movements of that tripod are:-posteriorly, the right and left temporomandibular joints;-anteriorly, the teeth of the maxillary and mandibular arches; - And overall, the neuromuscular system.

The dentist has no control over the posterior determinants, the temporomandibular joints.they are unchangeable.However, they influence the movements of the mandible, and of the teeth, by the paths which the condyles must travel when the mandible is moved by the muscles of mastication. The measurement and reproduction of those condylar movements is the basis for the use of the articulator. The anterior determinant, the teeth, provides guidance to the mandible in several ways. The posterior teeth provide the vertical stops for mandibular closure. They also guide the mandible into the position of maximum intercuspation, which may or may not correspond with the optimum position of the condyles in the glenoid fossae. The anterior teeth (canine to canine) help to guide the mandible in right and left lateral excursive movements and in straight protrusive movements.Dentists have direct control over the tooth determinant by orthodontic movement of teeth; restoration of the occlusal surfaces ;and equilibration, or selective grinding, of any teeth which are not in harmonious relationship. Intercuspal position and anterior guidance can be altered, for better or for worse, by any of these means.

The Determinants of OcclusionThe closer to a determinant that a tooth is located, the more it will be influenced by the determinant. A tooth placed near the anterior region will be influenced greatly by anterior guidance, and only slightly by the temporomandibular joint. A tooth in the posterior region will be influenced partially by the anterior guidance.

The neuromuscular system, through proprioceptive nerve endings in the periodontium, muscles, and joints, monitors the position of the mandible and its paths of movement. Through reflex action, it will program the most nearly physiologic paths of movement possible under the set of circumstances present. Dentist have indirect control over this determinant. Procedures done to the teeth may be reflected in the response of the neuromuscular system.

The Determinants of Occlusion

Condylar Guidance Anterior/Incisal GuidanceOcclusal PlaneOcclusal CurveCusp Height

The Determinants of Occlusion

The Types of Occlusal Interferences

Centric InterferenceWorking InterferenceNon-Working InterferenceProtrusive Interference

One of the objectives of restorative dentistry is to place the teeth in harmony with the temporomandibular joints. This will result in minimum stress on the teeth, and only a minimum effort need be expended by the neuromuscular system to produce mandibular movements. When the teeth are not in harmony with the joints and with the movements of the mandible, an interference is said to exist.

Occlusal interferencesInterferences are undesirable occlusal contacts which may produce deviation during closure to maximum intercuspation, or which may hinder smooth passage to and from the intercuspal position. There are four types of occlusal interferences:

5.Centric

7.Working

9.Nonworking

4. Protrusive

The centric interference is a premature contact which occurs when the mandible closes wit the condyles in a retruded, superior position in the glenoid fossa. It will cause deflection of the mandible in a forward and/ or lateral direction.

A working interference may occur when there is contact between the maxillary and mandibular posterior teeth on the same side of the arches as the direction in which the mandible has moved. If that contact is heavy enough to discludeanterior teeth, or interfere with the smooth progress of the nonworking side condyle, it is an interference.

A nonworking interference is an occlusal contact between maxillary and mandibular teeth on the side of the arches opposite the direction in which the mandible has moved in a lateral excursion. The nonworking interference is of a particularly destructive nature. The potential for damaging the masticatory apparatus has been attributed to changes in the mandibular leverage, the placement of forces outside the long axes of the teeth, and disruption of normal muscle function.

The protrusive interference is a premature contact occurring between the mesial aspects of the mandibular posterior teeth and the distal aspects of maxillary posterior teeth. The proximity of the teeth to the muscles and the oblique vector of the forces make contacts between opposing posterior teeth during protrusion potentially destructive.

The protrusive interference is a premature contact occurring between the mesial aspects of mandibular posterior teeth and the distal aspects of maxillary posterior teeth. The proximity of the teeth to the muscles and the oblique vector of the forces make contacts between opposing posterior teeth during protrusion potentially destructive.

Normal versus pathologic occlusion

In only slightly more than 10% of the population is there complete harmony between the teeth and the temporomandibular joints. Only in that small group do the teeth achieve maximum intercuspation when the mandible is in a retruded position with the condyles in the optimal superior retruded position in the fossae.

In the other nearly 90% of the population, the position of maximum intercuspation is 1.25+mm forward of the retruded position.

There may be an occlusal disharmony (shaded bar) which is not ideal, but which is tolerated by the patient because it is below his threshold of perception and discomfort.

If the threshold is lowered, the disharmony which had been previously tolerated may produce symptoms in the patient. (a normal occlusion can become a pathologic occlusion). Simple muscle hypertonicity may give way to muscle spasm, with chronic headaches and localized tenderness.

Treatment is then then rendered by first raising the patient’s threshold,

And then decreasing or eliminating the disharmony

ARTICULATORS-is a mechanical device which of the simulates the movements of the mandible

The principle employed in the use of articulators is the mechanical replication of the paths of movement of the posterior determinants, the twmporomandibular joints.The instrument is then used in the fabrication of fixed and removable dental restorations which are in harmony with those movements.

As the mandible closes around the hinge axis ( m h a ), the cusp tip of each mandibular tooth moves along an arc

The large dissimilarity between the hinge axis of the small articulator ( a h a ) and the hinge axis of the mandible ( m h a ) will produce a large discrepancy between the arcs of closure of the articulator (broken line) and of the mandible (solid line).

A major discrepancyexists between thenonworking cusp path on the smallarticulator (a) and that in the mouth

The dissimilarity between the hinge axis of the full size semi-adjustable articulator ( a h a ) and the mandibular hinge axis( m h a ) will cause a slight discrepancy between the arcs of closure of the articulator

(broken line)and of themandible(solid line)

There is only a slight difference between cusp paths on a full size articulator(c)and those in the mouth (m), even though the castmounting exhibits a slight discrepancy

The condyle travels a curved path in mandibular movements ( A )This is reproduced in semi-adjustable articulators as a straight path ( B ).

The angle between the condylar inclination and the Occlusal plane of the maxillary teeth remains constant between an open (A) and a closed (B) articulator <a1=<a2.

.

..However, the angle changes between an open (C) and a closed (D) nonarcon instrument <a3 not equal to <a4. For the amount of opening illustrated, there would be a difference of 8 degrees between the condylar inclination at an open position ( where the

articulator settings are adjusted ) and a closed position (at which the articulator is used ).

Transfer of the tooth hinge-axis relationship

When a precision face-bow transfer is made, both side arms are adjusted so that the stylus at the end of each arm is located over the hinge axis (arrow). A third reference point, such as the plane indicator shown here, is used.

Two caliper-style face-bows are in use at the present time:

the Quick mount Face-bow

the Slidematic Face-bow

An air activated pantograph for recording mandibular movements

Different Styles/Schemes of Occlusion

Fully Bilateral Balanced OcclusionUnilateral Balanced Occlusion

– (Group Function)

Canine Guidance– (Mutually Protected Occlusion)

FULLY BILATERAL BALANCED OCCLUSION

UNILATERAL BALANCED OCCLUSION ( GROUP FUNCTION )

CANINE GUIDANCE ( MUTUALLY PROTECTED OCCLUSION )

Definition of Terms:

*Centric Relation

*Centric Occlusion – Centric Relation of Occlusion

*Maximum Interdigitation / Intercuspation

* Vertical Relation• Vertical Dimension/Relation at Rest

• Vertical Dimension/Relation of Occlusion

• Interocclusal Distance/ Freeway Space

*Bennett Movement

*Protrusive Movement

Movements

The Axes of Mandibular Movements

The Types of Occlusal Interferences

iv.tooth prep v. occlusion

Health & Medicine