Full metal crown construction

“Casting procedures”

Definition: Casting procedure is the process of converting the wax pattern into a metal restoration ready for cementation.

-After preparation is completed, tissue retraction is done and an impression is taken. A stone working cast will be obtained. Wax pattern and spuring is carried out on the removable die.

-Investing, burning out of wax and casting of metal, then finishing and polishing of the cast crown will be the last step in construction of all metal crown.

Restorations are often made too bulky. Naturalteeth are rarely more than 1 mm wider at theirheight of contour than at the CEJ. This should notbe exaggerated when recreating a tooth in wax.

The tooth surface gingival to its height of contourimmediately adjacent to the gingival soft tissues,

sometimes called the emergence profile,18 is usuallyflat or concave. Creation of a convexity in thisarea or a shelf or ledge19 makes bacterial plaque removaldifficult and has been shown to cause inflammatoryand hyperplastic changes in the marginalgingiva. Before dental plaque was identifiedas the direct etiologic agent in periodontal disease,

2° an excessive axial contour was considerednecessary to keep food from entering the gingivalsulci." However, there is no evidence to supportthis concept. Indeed, artificially reduced axial contours

(as when a prepared tooth is left unprotectedfor an extended period)

I- Wax pattern

Definition:

It is the process of rebuilding the prepared teeth, on the removable die, by wax using direct and\or indirect technique.

Methods of wax pattern construction

There are three methods for construction of wax pattern:

1)Direct wax pattern:

-Indicated in case of post crown and inlay retainer.

-Done and adjusted and carved in the patient’s mouth.

2 )Indirect wax pattern:

-Indicated in all types of retainers.

-Done on the die and working cast.

3)Indirect-direct wax pattern:

The wax pattern is constructed on the die and then the margins are refined and rechecked in the patient mouth.

Steps of wax pattern construction (indirect method)

1-lubricate the die by any separating medium.

2-using inverted cone bur, a circumferential groove might be cut around the die, 0.3 mm below the preparation margin.

N.B. this process is called Ditching of the die it act as:

a) A guide in carving the wax pattern.

b) Checking the seating of the crown on the die.

c) Finishing of the cast crown at the margin.

3-the wax is softened and placed on the die increment by increment or by dipping the die in a container that contains molten wax several times.

4 -after hardening of the wax, carving is carried out by blunt and slightly hot wax carver. This is important to minimize the internal stresses inside the wax

5-the buccal and lingual surfaces: should be carved to reproduce proper contour and convexity mesio -distally and occluso -gingivally.

6-the mesial and distal surfaces: should have their proper contour and contact restored with the adjacent teeth.

7 -the occlusal surface:a) Its proper carving without any high cusps.

b) Its proper intercuspation with the opposing teeth.

c) After complete carving and checking, smoothening of the wax pattern is done by a piece of silk cloth.

d) The pattern is then sprued and invested as soon as possible to prevent stress relaxation and deformity of the pattern

Preparation of the die prior wax pattern construction

1 )Trimming the die:

If the die has a representing free gingiva at a higher level than the finish line, trimming away must be done with a stone to define the margin.

2 )Correction of defects:

There are may be small effects in the die resulting from caries or previous restorations that are under cut with respect to the path of withdrawal of the new restoration. It is convenient to block them out on the working die with zinc phosphate cement.

3 )Shaping of the die handle:

Shaping of the die handle by using an acrylic bur, then ditching the die below the finish line by 0.3mm using inverted cone bur, this ditching is a circumferential groove act as a guide during wax pattern construction at the gingival margin and help also during

finishing of the cast restoration .

4 )Marking the margin:

The finish line of the preparation should be colored with color bright red pencil without any scratches and to facilitate the carving of the wax at this margin accurately.

5 )Provision of an adequate cement space:

Die spacer ( similar to model airplane paint) is used to maintain good marginal adaptation. The produce space (20-40 micron) is necessary to provide room for the luting agent and permit complete seating of the restoration during

cementation .

After that the die is thoroughly lubricated with a water-soluble lubricant to allow the die to soak in for several minutes, remove the excess lubricant with gentle stream of compressed air. The lubricant will reduce the surface tension and allow the wax to flow more freely.

The casting wax requirements

1 )Has different color than the used die material.2 )Flow readily

3 )Easily carved without shipping or flaking4 )Accept addition and carved easily and precisely.5 )Evaporate without leaving any debris or residual

ashes, which can contaminate the produced cast restoration.

6 )Take the fine and sharp details of the pattern and preserve it till investing without any distortion.

7 )Has a minimum rate of stress relaxation after carving and before investing to prevent any distortion.

8 )Can be burnished and polished.

9 )Can compensate part of the metal shrinkage during its solidification.

10 )Has acceptable strength and rigidity after cooling.

Different methods of wax pattern construction

1 )Dipping method: to develop a thin, uniform and adapted layer of thin wax on the die. This is done by dipping the die into wax that has been thoroughly melted

2 )Addition method:

To obtain a bulk of the wax pattern, then carving is carried out. This is done by melting the wax and dropping it on the die using a heated carver until complete bilding of the pattern, and then carved by sharp carver. The main drawback of this technique is due to strain collected from multiple addition of wax tends to release with time and subsequently distort the wax pattern.

Wax instrument

P.K.Thomas wax instrument consist of:

1 -wax addition.

2 -wax carving.

3 -wax burnishing.

P.K.Thomas no.1 and no.2 are wax addition instruments.

P.K.Thomas no.3 is aburnisher for refining occlusal anatomy.

P.K.Thomas no.4 and no.5 wax carver.

Wax is added by heating the instrument in the bunset flame touching it to the wax and quickly reheating its shank in the flame. wax flows away from the hottest part of the instrument, so that if the shank is heated, a bead of wax will flow off the tip. PKT no.1 instrument is used for large increment, the smaller no.2 is used for lesser additions. No.7 A waxing spatula is useful for adding

large amount of wax .

Electric wax instrument

Wax spatula instrument

Internal Surface.Forming a closely adapted internalsurface is the first step in waxing. The wax mustreproduce all retention features of the restoration.

The complete and accurate wax pattern

A- Proximal surfaces:1 )A proximal surface of the pattern cervical

to the proximal contact should be flat or slightly concave for interdental papilla.

2 )Contact area located more occlusally and buccally leading to wide lingual embrasure ,

While between the first and second molars is centrally placed

3 )Too narrow contact allows fibrous food to wedge between teeth, while wide contact do not deflect food.

4 )To ensure that the finished restoration with enough bulk and slight pressure in contact with the adjacent teeth, without creating an open contact, grind small amount of stone from the proximal surface of the adjacent teeth on the cast of both sides

Buccal and lingual surfaces

1 )The surfaces corresponding to surfaces of the adjacent teeth in the arch.

2 )Height of contour lies the cervical 1/3 except on the lingual surfaces of the mandibular posterior teeth, which occur at the middle 1/3

3 )Height of contour extends 0.5mm. Away from the level of cementoenamel junction except the lingual height of lower second premolar (0.75mm) and lingual of the lower molar (1mm)

4 )The line between the base of gingival sulcus and height of contour described as emergence profile, which is a straight line to faciliate cleaning by toothbrush. This creating a space between a) 0.7mm in ceramic restoration b) 0.35mm in metallic one.

5 )Over contouring produces gingival inflammation As it promotes the accumulation of food debris and plaque, also hyperplastic changes of the gingiva may occur, while under contouring may cause gum recession due to the forced deflection of food direction or in many cases no significant changes.

Occlusal surface morphology

1 )During centric closure of normal dentition, lingual cusp of upper and buccal of lower called functional cusps, which occluded to fossa or marginal ridges. While other cusps (non functional) act as rim to prevent over flow of food and protect the cheek and tongue away from the functional cusps.

2 )Occlusal anatomy and cusps follows the curve of wilson and anterioposterior curves (curve of spee).

3 )Small concavities placed on the distal inclination of buccal cusp of upper premolars on lateral movements.

4 )Two types of grooves are found, the primary grooves which represent the union of the lobes and form the major part of the main pits and groove pattern, while secondary grooves are to be characterized by its shallower position and not concerned with tooth development. The secondary grooves located at the inclined planes, increase the cutting and shearing efficiency during mastication.

3 )Cervical margin

1 )The margin should be checked and corrected. Some prefer to readapt the margin by burnishing immediately before investing.

2 )The margin is not over extended may affect the gingival after applying the restoration leading to gum recession, or under extended which may cause caries around the margins. The bulge margin

also is biologically not accepted .

Restorations are often made too bulky. Naturalteeth are rarely more than 1 mm wider at theirheight of contour than at the CEJ. This should notbe exaggerated when recreating a tooth in wax.

The tooth surface gingival to its height of contourimmediately adjacent to the gingival soft tissues,

sometimes called the emergence profile,18 is usuallyflat or concave. Creation of a convexity in thisarea or a shelf or ledge19 makes bacterial plaque removaldifficult and has been shown to cause inflammatoryand hyperplastic changes in the marginalgingiva. Before dental plaque was identifiedas the direct etiologic agent in periodontal disease,

2° an excessive axial contour was considerednecessary to keep food from entering the gingivalsulci." However, there is no evidence to supportthis concept. Indeed, artificially reduced axial contours

(as when a prepared tooth is left unprotectedfor an extended period)