Principles of Tooth Preparation: Part-5

Fig. 4.71, 4.72: Putty (A or C-Silicone) is knead into a small ball and adapted over the tooth to be prepared, extending at least one tooth on either side. This index is then cut sagittally across the occlusal surface (over a cusp and not the middle) with a No. 11 blade.

Steps in preparation

Each stage of tooth preparation has its own special considerations and working in a logical sequence each time simplifies the procedure to a routine.

Preparation Sequence:

1. Buccal preparation.
2. Interproximal preparation.
3. Lingual preparation.
4. Occlusal preparation.
5. Functional cusp bevel preparation.
6. Finishing of the preparation.

1. BUCCAL PREPARATION

Desired amount of preparation: Margin 1-1.2 mm; Axial Surface:

1.2-1.5 mm

Margin Architecture:

Modified Shoulder

Bur used:

MS 1.0 (Cluster 1)

Fig. 4.74: The MS 1.0 bur is held parallel to the long axis of the tooth (or along the intended path of insertion) and the tip of the bur is sunken full depth, around the mesio-distal center of the tooth. This creates a 1.0 mm deep groove. The groove should ideally be placed on enamel, slightly above the gingival margin.

Fig. 4.75: Three such depth grooves are made on the buccal aspect. One around the mesio-proximal surface, one around the mesio-distal center and one around the disto-proximal surface of the tooth.

Fig. 4.76, 4.77: With the same bur placed in any one of the three grooves, the hand piece is moved using uni-directional sweeping movements (either push or pull) to amputate the islands between them. The full diameter of the bur is used to reduce the tooth. The tip of the diamond bur should be slightly supra-gingival at this point, even if the final intended position of the finish line is flush with or slightly beneath the gingival margin.

NOTE: It is important to avoid back and forth (continuous) movement of the bur, as this results in undesirable deepening of the depth grooves, thereby resulting in non-uniform reduction.

Fig. 4.78, 4.79: The buccal preparation is then extended as far into the proximal embrasures as possible, without opening the contact or nicking the adjacent teeth. Top: Buccal view; Bottom: Buccal-occlusal view.

2. PROXIMAL PREPARATION

Desired amount of preparation:

Margin: 1-1.2 mm; Axial Surface: 1.2-1.5 mm

Margin Architecture:

Modified Shoulder

Bur used: MI 0.8 (Cluster 3)

Following buccal reduction, the proximal contacts are to be opened. This is one of the trickiest areas to prepare and good hand piece skills are important. At this point, no attempt is made at establishing an interproximal gingival margin. The goal is only to open the interproximal contact without inflicting any iatrogenic damage to the adjacent tooth.

Fig. 4.80: The MI 0.8 bur is placed parallel to the long axis of the tooth and slightly above the interdental papilla. This is important to avoid damage to the ‘col’.

Fig. 4.81: This bur is used to saw through the interproximal surface ensuring a fine sliver of enamel remains between the tooth being prepared and the adjacent tooth. Fig.

Fig. 4.82, 4.83: The procedure is repeated on the other proximal surface. Note, the supra-gingival nature of the margin along the entire interproximal surface. The buccal view (top) and lingual view (bottom) clearly show the constant plane of reduction followed from the buccal through to the lingual and a definite margin formation. This is possible owing to the non-cutting tip of the MI bur.

Fig. 4.84, 4.85: The thin sliver of enamel is then easily flicked away using a straight probe. This opens up the proximal contact, as shown here for the distal surface.

Fig. 4.86, 4.87: The same procedure is then repeated for the mesial contact.

Fig. 4.88: The proximal finish line may be quite irregular at this point, but that can be very easily remedied at the finishing stage of the preparation. The yellow rings outline the separated pieces of proximal tooth enamel (lip).

3. LINGUAL PREPARATION

Desired amount of preparation: Margin: 0.5 mm; Axial Surface: 1.2 - 1.5 mm

Margin Architecture: Deep Chamfer for a lingual metal collar.

Following the proximal surface, attention is paid towards reduction of the lingual aspect, which is the last axial surface to be prepared.

Fig. 4.89: On the lingual surface, just one depth groove is made using the DC 1.0 bur. This bur is sunken to only half its width, in order to avoid formation of a lip of unsupported enamel. This margin should preferably be kept supra-gingival in an attempt to conserve tooth structure.

Fig. 4.90: Using a pull or a push (uni-directional) stroke, a definite margin is created. This is first performed mesially where the lingual margin is merged into the proximal margin.

Fig. 4.91: The same is then repeated to create a definite distal-lingual margin. Here again, careful attention needs to be paid to ensure correct bur orientation along the proposed path of placement.

Fig. 4.92: Lingual reduction as seen from the lingual view. Note, the supra-gingival nature of the prepared margin.

4. OCCLUSAL PREPARATION

Desired amount of preparation: Functional Cusp: about 2mm; Non- Functional Cusp: about 1.5 mm

Burs used: OR 1.6 and OR 1.2 (Cluster 4)

It is often said, a preparation should look like a miniature of the original and this mainly holds true for the occlusal aspect.

When reducing the occlusal aspect, it is imperative to follow the pre-existing cuspal inclines and contours.

The occlusal surface of a mandibular first molar has a small distal cusp. This cusp often gets reduced as part of the proximal preparation and no special efforts are made at recreating this cusp during the preparation.

For those who find working this aspect of the preparation difficult, it would help to visualize (or even mark) the occlusal ridges and desired reduction contours on the tooth.

Fig. 4.93: Two dots are marked on the buccal aspect which represent the future cusp tips.

Fig. 4.94: These two dots are joined together in the form of a valley. This represents the outline of the desired reduction in this area.

Fig. 4.95: The marks then are extended towards the proximal surface to develop a two mountain pattern that roughly represents the architecture of the final occlusal reduction desired.

Fig. 4.96: Three marks are made on the occlusal aspect along the mesiobuccal cusp ridge, the buccal developmental groove and the distobuccal cusp ridge. We shall be using these three marks to make depth grooves by sinking the bur along their length.

Fig. 4.97: All markings made as seen from an angular view. Once all the marks are made, it is time to use these to help anatomically reduce the occlusal aspect.

Fig. 4.98: The occlusal reduction bur should be oriented along the preexisting incline of the tooth.

Fig. 4.99: In case the bur orientation flattens out, the occlusal reduction begins groove. This often culminates into metal exposure or porcelain fracture along this under-reduced area.

Fig. 4.100: As the functional cusp requires 2 mm reduction, the OR 1.6 bur is used to create a depth orientation groove extending from the cusp tip to the base/central developmental groove. This bur needs to be sunken full depth.

Fig. 4.101: This creates a groove that is 1.6 mm in depth. For descriptive purposes, we shall label this groove as “B”.

Fig. 4.102: The same step is repeated on the mesiobuccal and distobuccal cusp ridges. For descriptive purposes, we shall label the mesial groove as “A” and distal groove as “C”.

Fig. 4.103: All three grooves as seen from the buccal view.

Fig. 4.104: Following the anatomic contour of the occlusal surface and without changing its angle or depth, the bur is moved from “A” downward and inward to connect into “B” and from “C” downwards and inwards to also connect into “B” (as depicted by the yellow arrows). This produces a valley in the mid-buccal region.

Fig. 4.105: Keeping the same angle and depth, the bur is next moved from “A” downward and mesially to exit into the proximal area and from “C” downward and distally to exit into the proximal area (without nicking the adjacent teeth). This results in the creation of two mountain peaks with uniform reduction. These peaks would eventually become the mesio-buccal and the disto-buccal cusp tips of the final prosthesis.

Fig. 4.106: The overall buccal-occlusal reduction as seen from an angular view. The reduction should reproduce the general occlusal morphology.

Fig. 4.107: The overall buccal-occlusal reduction as seen from the lingual view. Note, the reduction follows the general occlusal morphology.

Once the gross buccal reduction is completed, the lingualocclusal surface is reduced following similar principles.

Fig. 4.108: Three marks are made on the lingual-occlusal aspect along the mesiolingual cusp ridge, the lingual developmental groove and disto-lingual cusp ridge.

Fig. 4.109: As the amount of desired reduction is 1.5 mm, the OR 1.2 bur is sunken full depth along the three markings made. These again can be labeled in one’s head as “A” “B” and “C”. By following the arrows as depicted in the image, uniform occlusal reduction can be achieved along the lingual occlusal surface.

Fig. 4.110: Occlusal reduction can be done one cusp at a time, allowing the other cusp to help gauge the amount of reduction achieved. As shown here, first the disto-lingual cusp is reduced.

Fig. 4.111: The mesio-lingual cusp is reduced next. This completes the gross lingual occlusal preparation.

5. FUNCTIONAL CUSP BEVEL PREPARATION

The functional cusps withstand the bulk of occlusal forces and thus, need to be reinforced with greater thickness. To achieve this, a functional cusp bevel is incorporated into the preparation. This step helps in avoiding the creation ofan occlusal interference in excursion and/or porcelain fracture . The average functional cusp bevel is at a 45° angle and is approximately 1.5 mm wide. However, there are considerable variations around these values. Fortunately, all teeth in one quadrant present with the same functional inclines and this can be used to our advantage.

Fig. 4.112: Any long bur is held along plane 2 of the adjacent tooth to help determine the angle of the functional cusp bevel.

Fig. 4.113: This angle is simply carried to the tooth being prepared in order to achieve the desired amount of functional bevel reduction.

Fig. 4.114: The functional cusp bevel in itself helps accomplish the buccal reduction in two planes.

6. FINISHING OF THE PREPARATION

Burs used: EC 1.4F, MS 1.3F and DC 1.2F (Cluster 6 and 1)

Fig. 4.115: First, the EC 1.4F bur is used to plane the proximal margins that have considerable unsupported enamel at this time. As this bur has diamonds only at the tip, there is no fear of iatrogenic damage to the adjacent tooth. It is important that this bur be used with pull strokes only, as a push stroke carries the risk of inadvertent ploughing of the bur into the margin.

Fig. 4.116: Next, the MS 1.3F bur is used to finish the buccal margin, the buccal axial wall and the functional cusp bevel. The buccal margin can be lowered from a supragingival to an equi-gingival margin using this finishing bur (if needed). As this bur provides greater control during reduction, it decreases the possibility of traumatizing the marginal gingiva owing to overzealous preparation. The same bur is also used to finish both the proximal margins, giving them a modified shoulder design.

NOTE: When finishing the proximal margins, it is important to follow the contours of the interdental col (which is saddle shaped). Unnecessary extension into proximal gingival tissue leads to bleeding and inflammation. This is one of the leading causes for food lodgement between teeth post cementation.

Fig. 4.117: Next, the DC 1.2F bur is used to finish the lingual margin of the preparation extending it into the proximal margin, thereby blending the two margins together. A slight step/wing would be present here as the wide modified shoulder margin merges with the narrower deep chamfer margin. This wing guides the technician towards the mesio-distal extension of the proposed lingual metal collar.

Fig. 4.118: The same bur is then used to round off the plane 2 on the lingual-axial surface.

Fig. 4.119: The same bur is also used to finish the entire occlusal preparation, ensuring that the cuspal architecture is maintained.

FINAL LOOK of the preparation

Fig. 4.120: The final preparation should look smooth and should be free of undercuts. All 360° of the margin should be continuous and visible when seen occlusally. This should be done with one eye closed, because an undercut can be perceived as near parallel surface when seen with both eyes open.

Fig. 4.121: Final preparation: Lingual angular view. Special care should be taken to check the junction between the proximal and the buccal/lingual reductions, as these are common sites for undercut formation.

Fig. 4.122: Final preparation: Lingual view.

Fig. 4.123: Final preparation: Buccal angular view.

Fig. 4.124: Molar reduction in accordance with the principles of tooth preparation.

Fig. 4.125: Molar reduction in accordance with the principles of tooth preparation.

Fig. 4.126: Molar reduction in accordance with the principles of tooth preparation.

Fig. 4.127: Molar reduction in accordance with the principles of tooth preparation.

Fig. 4.128: Overall reduction can be checked with the pre-treatment sagittal index. Producing consistent preparations is a matter of mental discipline.

IMPLICATIONS OF UNDER-REDUCTION

Fearing pulpal exposure or post preparation sensitivity, a clinician can make a very grave mistake of under-preparing the tooth.

Under-reduction at the margin area results in cervical overhangs in the final prosthesis. Such over-contoured restorations harbor plaque, which in time results in gingival irritation/recession, microleakage and secondary caries, with potential irreversible pulpal damage. Thus, the very act of under-reducing the tooth as an attempt to protect it from exposure eventually hampers its vitality.

In addition, under-reduction of axial surfaces converts into bulky restoration which increases the possibility of cheek/tongue bite following cementation.

Fig. 4.129: Left: Note, prosthesis shows the desired emergence profile owing to adequate amount of axial reduction. Right: Note, the presence of cervical overhang. Such an over-contoured prosthesis is common where axial width of the underlying preparation is deficient. Yellow arrows indicate area of cervical undercut that tend to harbor plaque.

Fig. 4.130: Left: Desired occlusal reduction is one that follows the original architecture, thereby ensuring adequate prosthetic material thickness in all critical areas. Right: A flat occlusal preparation may result in under-reduction around the central fossa area, often perpetuating into occlusal pre-maturities.