Principles of Tooth Preparation: Part-6

Conversely, inadequate tooth reduction can result in compromised aesthetics and often chronic gingival inflammation owing to the overhanging margin. Lipstick stains on upper anterior crowns often indicate an under-reduced tooth or an over-contoured prosthesis.

Fig. 4.131: Tooth preparation should follow a two plane reduction in order to fulfill all the three principles of tooth preparation.

Fig. 4.132: Single plane reduction can result in either an aesthetic failure (left) or an over-contoured prosthesis (center) or possible pulpal damage (right).

Fig. 4.133, 4.134: For beginners, it is advisable to fabricate a pretreatment index using putty (A or C-Silicone) extending over the tooth to be prepared and at least one adjacent tooth on either side. This index is sliced mid-sagittally across the tooth being prepared. Optosil C-Silicone from Heraeus Kulzer was used for this documentation.

Fig. 4.135: Using a DC 1.4 bur, a depth groove is made in the gingival plane (in the center of the tooth) by sinking the bur tip to half its diameter. Sinking the bur any deeper carries the risk of developing a lip of unsupported enamel at the margin.

Fig. 4.136: Two additional grooves are then made (in the gingival plane) along the mesial and distal line angles of the tooth. Note, how the depth grooves shallow out (and eventually fade away) as they approach incisally where the facial curvature of the surface is the greatest.

Fig. 4.137: Using the same bur, a depth orientation groove is then made in the incisal plane. Here again, the bur tip is sunken to only half it’s diameter.

Fig. 4.138: Two or more depth grooves are placed in the incisal plane (depending on the width of the labial surface).

Fig. 4.139: Depth grooves along the two planes can be clearly distinguished.

Fig. 4.140: Using the same bur, enamel islands are first removed in the gingival plane. It is important that the preparation margin is placed slightly supra-gingival at this point and that it follows the gingival contour.

Fig. 4.141: The incisal plane is then reduced, ensuring all enamel islands between the depth grooves are reduced completely

Fig. 4.142: It is important to ensure that the preparation is in harmony with the labial contours of the adjacent teeth.

Fig. 4.143, 4.144: The MI 0.5 bur is placed slightly above the interdental papilla and used in a sawing motion to slice through the interproximal surface, ensuring that a fine sliver of enamel remains between the tooth being prepared and the adjacent tooth.

Fig. 4.145, 4.146: The same procedure is then repeated along the other proximal surface. Above: Labial view; Below: Palatal view.

Fig. 4.147: The proximal contacts are then opened by breaking the sliver of enamel. This is done with the help of a straight probe.

Fig. 4.148: The DC 1.4 bur is used to score one depth groove in the center of the palatal surface by sinking the bur tip slightly less than half its width. It is advisable to place this groove slightly supra-gingival in order to conserve tooth structure.

Fig. 4.149: Image depicts the correct angle of the bur, which is relatively parallel to the gingival plane of the labial surface. This helps in achieving maximum resistance from the palatal axial wall.

Fig. 4.150, 4.151: Using uni-directional motion, the preparation is then extended from the palatal groove into the mesial and distal interproximal margin.

Fig. 4.152: The IR 2.0 bur is a self-limiting bur and thus, ideally suited to create depth grooves of 2 mm. It is placed such that the non-cutting shaft sits flush with the incisal edge, allowing only the working tip to be in contact with the tooth.

Fig. 4.153: This bur is then run across facio-palatally along the mid-incisal edge (in one stroke), thereby leaving a 2 mm deep grove.

Fig. 4.154, 4.155: Two additional grooves are made, to the mesial and distal of the mid-incisal groove.

Fig. 4.156: Next, any long bur is used to connect these grooves. Maxillary incisal edge reduction should be done with the bur angled slightly palatally.

Fig. 4.157: It is advisable to reduce the last 0.5 mm of the incisal edge with a coarse Sof-Lex disc from 3M. This prevents the grooves from deepening further, thereby preventing inadvertent over-reduction of the incisal edge beyond 2 mm.

Fig. 4.158: In order to achieve adequate reduction, the CR bur is held such that it’s widest portion sits into the deepest aspect of the palatal concavity. This ensures that desired reduction is achieved in the middle third of the lingual fossa.

Fig. 4.159: Too steep an angle. This would reduce the height of the palatal axial wall, thereby compromising the resistance form and can also cause over-thining of the incisal edge.

Fig. 4.160: Too shallow an angle. This would result in under-reduction of the tooth, thereby causing premature contact in MIP and interferences in excursion.

Fig. 4.161: To reduce this surface, the CR bur is moved mesio-distally across the fossa and as the bur approaches incisally, it is straightened out. This ensures uniform reduction and maintenance of the palatal curvature.

Fig. 4.162: First, the DC 1.8F bur is used to half its depth on the labial aspect of the preparation. The large head of this bur helps remove any lip of enamel that may have formed earlier. At this phase, the preparation margin can be taken closer to the gingival crest (if necessary).

Fig. 4.163: Proximal surface may not have enough clearance to allow the wide DC 1.8F bur to pass through. Thus, a thinner DC 1.2F is used to finish the mesial and distal proximal margins. Here, it is important to follow the pre-existing gingival architecture (interdental col) of the tooth.

Fig. 4.164: The same DC 1.2F is used to finish the palatal axial margin as well.

Fig. 4.165: A Sof-Lex Spiral from 3M can be used to finish the lingual fossa area of the preparation.

Fig. 4.166: Final Preparation: Labial View.

Fig. 4.167: Final Preparation: Palatal View.

Fig. 4.168: Final Preparation: Incisal View.

Fig. 4.169: Final Preparation: Lateral View.

Fig. 4.170: Pre-treatment index can be used to evaluate the uniformity and adequacy of reduction.