CROWN PREPERATION معالجة اسنان \ الخامس د. طلال السلمان م(1+2) د. طلال السلمان م(1+2) 29-11-2106 CROWN PREPERATION Dr. Talal Alsalman

Objectives of crown preparation 1: Eliminate all the undercuts 2: Preservation of healthy tooth structure 3: Provision of acceptable finishing lines 4: providing enough space for the restoration :

Principles of tooth preparation A: Biological principles B: Mechanical principles C: esthetic principles

A: Biological principles: 1: Prevention of damage during tooth preparation: A: Adjacent teeth. B: Soft tissues. C: Pulp.

How to preserve pulp health during preparation ? A: the temperature of the pulp should be maintained. B: using impression material & cements that have no chemical action on the pulp & dentin. C: prevent any bacterial entrees to the pulp

2: Conservation of tooth structure Using a partial veneer crown rather than the full veneer crown whenever possible. Avoid excessive tapering of the axial walls of the preparation Preparation of the occlusal surfaces should follow the anatomical planes Select the most conservative finishing line.

Mechanical principles 1: Retention form 2: Resistance form

Retention form Is the form of the preparation that prevent the cast crown restoration from removal along its path of insertion.

Factors affecting the retention of crown restoration: 1: Amount of dislodging force: it depend on the stickiness of food & the surface area & texture of the restoration subjected to the dislodging force.

Geometry of tooth preparation:2 Parallelism Surface area Surface texture of the preparation Type of preparation Roughness of internal surface of cast restoration Material used for crown construction The type of luting (cementing) agent Film thickness of luting agent

Resistance form Is the form of crown preparation that prevent the restoration to dislodge during functional (mastication) or para-functional forces like the horizontal or lateral forces & rotational forces

the resistance form of preparation may depend on: A: Amount & direction of dislodging force B: Geometry of tooth preparation

Amount & direction of dislodging force the biting force may reach to 4350 N (445 kg). the restoration should be designed to withstand forces approaching such magnitude. In a normal occlusion, biting forces is distributed over all the teeth, most of it axially directed. If a fixed prosthesis is carefully made with a properly designed occlusion, the force should be well distributed & favorably directed. However, some patient has a biting habits like pipe smoking or music playing, it may difficult to prevent large oblique forces from being applied on the restoration, so the complete tooth preparation & restoration must be able to withstand considerable lateral & oblique forces as well as the normal apical force.

Geometry of tooth preparation preparation must be so shaped that particular areas of the axial wall will prevent the tooth rotation of the crown. Studies showed that the increased preparation taper & sever roundation of line angles tend to reduce resistance, short prepared tooth with large diameter found to have very little resistance form. A partial veneer crown has considerably less resistance than the complete crown because it has no buccal resistance areas, so resistance must provided by boxes or grooves.

THE KEY OF RESISTANCE IS THE EVEN DISTRIBUTION OF FORCES NOTE THE KEY OF RESISTANCE IS THE EVEN DISTRIBUTION OF FORCES

Prognosis of FPD Prosthesis The prognosis is an estimation of the likely course of a disease. It can be difficult to make, but its importance to patient understanding and successful treatment planning must nevertheless be recognized.

The prognosis of dental disorders is influenced by: general factors (age of the patient, lowered resistance of the oral environment) and local factors (forces applied to a given tooth, access for oral hygiene measures).

:General Factors - Patient's understanding and comprehension of plaque-control measures. -Systemic problems analyzed in the situation of the patient's age and overall health provide important information. Diabetics are prone to a higher incidence of periodontal pathology

:General Factors - If a patient's previous dental care has been successful over a period of many years, a better prognosis can be expected than when apparently properly fabricated prostheses fail or become dislodged within a few years of initial placement.

:Local Factors - Over all caries rate of the patient's dentition indicates future risk to the patient if the condition is left untreated. - Some patients are capable of an extremely high bite force, and multiple intraoral wear facets are observed. - Individual tooth mobility, root angulation, root morphology, crown-to-root ratios, and many other variables all have an impact on the overall fixed prosthodontic prognosis.

Indications of full metal crown: 1: Teeth with extensive caries, with large amalgam restoration in order to protect the remaining tooth structure & amalgam from fracture. 2: As a bridge retainer. 3: Tooth in the most posterior position.

Contraindications of full metal crown: 1: When esthetic is needed. 2: When caries index is low.

Disadvantages of full metal crown: 1: esthetic problem due to display of metal. 2: Difficulty to test vitality of the tooth especially the electrical pulp testing.

Contraindications of partial veneer crown: 1: Anterior teeth with very thin labio-lingual dimension especially the lower anterior teeth & upper lateral incisor. 2: Teeth with short clinical crown because the retention will be very poor. 3: Teeth with a tapered crown. 4: Grossly carious tooth especially when the margins of preparation may include no sound tooth structure.