TOOTH PREPARATION FOR CROWN

INTRODUCTIONTooth preparation is defined as the mechanical treatment of dental disease or injury to hard tissue that restores a tooth to original form. Perceptive diagnosis & disciplined tooth preparation can determine the success of a fixed prosthodontic. Reduction of the tooth structure is preceded by a mental image of the design of the artificial crown.Sound preparation design enhance longevity for the majority of cast restoration. The current focus is an conservative preparation that is noninvasive & the minimally involves dentin. Dentistry is changing from macro preparation to an environment of molecular chemistry I.e. esthetic bonding.Diagnosis & disciplined tooth preparation are essential to successful fixed prosthodontic.

HISTORYEgyptian are the first known efforts to enamel a substructure with a ceramic veneerChinese ceramists developed porcelain which was characterized by verification, translucency, Hardness and impermeabilityIn seventeenth century Europeans attempted of developing porcelain of similar quality. This lead to the knowledge of basic component kaolin and feldsparIn the second half of the eighteenth century Fauchard and others attempted to use porcelain for dental applications.

HISTORYEnd of 1800:porcelain was successfully used for dental prosthesis.Mid 1950s-a dental porcelain was developed with a coefficient of thermal expansion similar to the exisisting dental casting alloysLater 1950-metal ceramic restoration became commercially available1970s-1980s the term Porocelain fused to metal was commonly used

FINISH LINE CONFIGURATION CHAMFERThe finish gingival line for veneer metal restoration is the chamfer.Finish line shows least stress.It can be cut by the tip of a round end diamond.The margin of the cast restoration that fits against it combines an acute edge with a near by bulk of metal.

HEAVY CHAMFERThe heavy chamfer is used to provide a 90degree cavosurface angle with a large radius rounded internal angle.Created by round end diamond.It provide better support to ceramic crown.

SHOULDERShoulder has been the choice for all ceramic crown.The wide ledge provides resistance to occlusal force & minimizes stress that might lead to fracture of the porcelain.Produces space for healthy restoration contours & maximum esthetic.Require more destruction of tooth.

RADIAL SHOULDERIts a modified form of shoulder finish line.After preparing of shoulder, a small radius rounded internal angle is instrumented by an end cutting parallel sided carbide finish bur & finish is completed by modified bin angle chisel.Cavosurface angle is 90degree and width is slightly lessened by classic shoulder.Support for ceramic restoration wall is good.Destruction of tooth is equal to that required for classic shoulder.

SHOULDER WITH BEVELIts utilized as a finish line for 3/4th crowns, facial finish line of metal crown where esthetic is not critical or where shoulder is already present.Its a good finish line for preparation with extremely short wall.By adding the bevel, it is possible to create an acute edge of metal at the margin.Should not be used routinely for all ceramic crown.

KNIFE EDGEUltimate finish line for an acute margin of metal but can create a problem.The thin margin of the restoration that fits this finish line may be difficult to accurately wax or cast.The use of knife edge can result in over countered restoration.Have to be used on the lingual surface of mandibular posterior teeth, teeth with very convex axial surface & on the surface toward which a tooth may have tilted.

PORCELAIN JACKET CROWN

PORCELAIN JACKET CROWNThe PJC is the most esthetic restoration for duplicating individual anterior teeth.Adequate tooth restoration is created to achieve space for the porcelain bulk required for strength of the restoration.Pulpal & occlusal relationships are carefully examined before the PJC is prepared. Porcelain can withstand with compressive strength but susceptible to fracture from shearing force. Size & position of the pulp is determined before selecting PJC.

INCISAL REDUCTIONThe incisal reduction is perpendicular to the long axis of the tooth & 2mm apical to contemplated edge of the finish restoration.

Initial the reduction was preformed in the flat plane perpendicular to the long axis of the tooth.It is now modified to the inclination of the mandibular teeth, usually at 45degree angle to the long axis of tooth in a normal occlusion relationship.

AXIAL REDUCTIONThe mesial & distal areas are first reduced to 2-5degree taper with establishing a shoulder at this time with long taper diamond. This help to isolate the tooth.

The facial reduction is done by flatened diamond or No.700 carbide bur to remove the labial surface while establishing a preliminary shoulder. The incisal 2/3rd of the facial surface should be inclined to provide uniform porcelain & ensure suitable esthetic.

RETENTION & RESISTANCE FORMThe lingual surface of the tooth is reduce in two planes

A cingulum shoulder is placed with a flat- ended tapered diamond to create 0.75mm shoulder in the cingulum with 2-5degree taper.

A flame or wheel shaped diamond is used to form the lingual concavity of the anterior teeth.

If the preparation is formed on canine, this portion of reduction has commonly two concave due to canine lingual ridge.

MARGINAL DEVELOPMENT & REFINEMENTAn end cutting bur held perpendicular to the shoulder is indicated for lowering margins.A sharp chisel is an excellent instrument for removal of undermined enamel & finishing line.

The axial walls are smoothed to the complete the preparation, while sharp angles are rounded.The labial gingival finish line is rarely supra gingival, so special attention is directed to the soft tissue.

COMMON ERRORS IN PJC PREPARATIONExcessive taper of proximal surface.Insufficient tooth reduction on the facial surface.Inadequate tooth reduction of the lingual surface for clearance & diminished strength for porcelain.Variable shoulder width.Shoulder not placed far enough apically.Under cuts at the axis gingival line angle.Rough tooth surface preparation.

FULL METAL CROWN

FULL METAL CROWNWhen all the axial surface of a posterior tooth have been attacked by decalcification or caries or when those surface have been previously restored. The tooth is candidate for a full metal crown.The preparation of full metal crown is less destructive than that required for either metal ceramic or all ceramic crown.The full veneer crown is a restoration that replace lost structure & imparts some measure of structural support of tooth.

OCCLUSAL REDUCTIONReduction begun by creating about 1.5mm of clearance on the functional cusp & 1mm on the nonfunctional cusp.Depth orientation grooves are placed on the occlusal surface of the tooth to provide an easy reference to determine when reduction is sufficient.A round ended tapered diamond bur is used to place the grooves.The tooth structure remaining between the grooves is removed to accomplished the occlusal reduction.

FUNCTIONAL CUSP BEVELA wide bevel is placed on the functional cusp, with rounded ended tapered bur.Depth orientation grooves are also helpful to obtain the bevel.The functional cusp bevel placed on the buccal inclines of mandibular buccal cusp & lingual inclines of maxillary lingual cusp.The functional cusp bevel are placed with a no.171L bur.

BUCCAL & LINGUAL REDUCTIONThe buccal & lingual walls are reduced with a torpedo diamond.A definite, even finish line is necessary to enable the fabrication of a well fitting restoration & the chamfer is the best for providing the bulk needed for strength while still allowing good adaptation.

The proximal cuts are made with a short needle diamond.Thin diamond is worked through the proximal area in an occlusogingival or buccolingual sawing motion. Torpedo diamond is introduced to plane the walls while simultaneously forming a chamfer as the interproximal gingival finish line.

FINAL PREPARATIONAll the axial surfaces are smoothed with carbide finishing bur.Special care should be taken in rounding the corners to insure that the finish line will be smooth & continuous.

The final step in the preparation is to placement of a seating groove.The groove is placed with a no.171L bur & is placed on the buccal surface of mandibular & lingual surface of the maxillary preparation.

METAL CERAMIC CROWNSAlso called porcelain fused to metal restoration, consists of a ceramic layer bonded to a thin cast metal.Such restoration combines the strength & accurate fit of a cast metal crown with a the cosmetic effect of ceramic crown.Friedlander et al found the metal ceramic crown 2.8 times strong & more longevity than ceramic alone.

ANTERIOR METAL-CERAMIC CROWNA uniform reduction of approximately 1.2mm is needed over the entire facial surface. To achieve adequate reduction, the facial surface must be prepared in two planes. If reduced in one plane, that is extension of gingival plane, the incisal edge will protruded, resulting in a bad shade match.If its prepared in the incisal aspect, the facial surface will be over tapered & too close to pulp.If index is prepared before the preparation is begun, help to +ve check on reduction produce by preparation.If the contour of the exiting tooth is are correct the index can be prepared intraorally if not should be made from diagnostic cast.

TOOTH PREPARATIONLabial grooves should be cut in two sets.One set parallel with the gingival half of the labial surface & one set parallel with the incisal half of the labial surface.Grooves should be 1.2mm deep.

Incisal reduction is done with flat end tapered diamond, so that its parallel the inclination of the unprepared incisal edge.Allow to easy instrument access to axial surface & gingival finish line.

Reduction of the incisal portion of the labial surface is done by flat end tapered diamond bur.All tooth structure is planed off to the depth of orientation grooves.

The gingival portion of the labial surface is likewise reduced to the depth of the groove with flat-end tapered diamondThe reduction is carried around the labioproximal angle to a point 1mm lingual to the proximal contacts

The lingual surface is reduced with a small wheel diamond to obtain a minimum of 0.7mm of clearance with the opposing teeth.Those portion of the lingual surface that will have a ceramic veneer should have 1mm clearance.The junction between the cingulum & the lingual wall must not be over reduced.

A long needle diamond is used to break the contact and complete access through the proximal areas to minimize the chances of nicking the adjacent teethLingual aspect of the proximal wall as well as the lingual surface reduced by Torpedo diamond.Smoothed with the torpedo bur, accentuating the chamfer on the lingual & proximal surface at the same time.

An H158-012 radial fissure bur is used to smooth the labial surface. All the angles & edges are rounded with the side of the bur.At the same time that the labial surface is being planed by the side of the bur, the end is forming a radial shoulder finish line.

POSTERIOR METAL CERAMIC CROWN The use of metal-ceramic crowns on posterior teeth allows the creation of an esthetic restoration on a posterior tooth.Maxillary premolars, 1st molar & mandibular premolars are in esthetic zone.Maxillary 2nd molar, mandibular molars may required metal ceramic crown if patient will not accept full metal crown.Before the preparation is begun, silicon putty will add to all the tooth surface to prepare index for guidance for tooth preparation.

OCCLUSAL REDUCTIONBegun by making depth orientation grooves with a round-ended diamond.In the area where ceramic coverage will be, reduction should be 1.5-2mm.The occlusal reduction will be complete by removing strips of intact enamel between grooves.

FUNCTIONAL CUSP REDUCTION Required uniform bulk of restoration on the lingual inclination on the maxillary lingual cusp & facial inclination on mandibular facial cusp.Depth orientation grooves should be reduce 1.5mm if coverage will be metal or 2mm if ceramic.The functional cusp bevel is completed by removing tooth structure between grooves.No.172L bur used to smooth the occlusal reduction.

FACIAL REDUCTIONFlat-end tapered diamond is used to placed three vertical grooves in the occlusal portion of the facial surface & two orientation grooves are placed near the line angle of the tooth.The full diameter of the bur must cut into the tooth.All tooth structure between the grooves in the occlusal segment of the facial surface should remove with flat-end tapered diamond.The gingival portion of facial surface is reduced & extend into the proximal surface.Reduction should be 1.2- 1.4mm

AXIAL REDUCTIONThe proximal reduction is begun with short needle diamond.Narrow diameter allows interproximal reduction with out niking adjacent teeth by up & down motion of the facial accept of interproximal or it can be used on the occlusal portion with a faciaolingual movement.Initially objective is to achieve separation between the teeth with out over tapering the parallel walls.

The lingual axial wall is reduced with a torpedo diamond.Enough structure is removed from both the lingual & proximal axial wall to create a distinct chamfer finish line & axial surface adjacent to it are smoothed with a torpedo carbide finish bur.Axial surface veneered with only metal, finish with chamfer & that with ceramic finished with shoulder with H158-02 radial fissure bur.At the lingual most extension of the facial reduction , lingual to proximal contact, result vertical wall or wing of the tooth structure.

The radial shoulder, started with flat end tapered diamond at the facial reduction, is finished now with radial fissure bur.The 1mm wide shoulder is smoothed by placing it with the RS-1 modified binangle chisel which will preserve the rounded internal angle created by the radial fissure bur.

Occasionally shoulder with bevel is the finish line choice.When esthetic needs are not as critical or dental technician is unable to consistently produce a precise all ceramic margins.A narrow bevel of 0.3mm can be placed on the shoulder with the tip of a flamed shaped diamond.The bevel is finished with an H48L-010 flame shaped carbide finishing bur to create a finish line that is as clear as possible.