This section will focus on the characteristics of the buccal, lingual walls and gingival floor of the proximal box.

Buccal and Lingual Extension

The bucco-lingual wall margins are extended to break contact with adjacent tooth.

If you are afraid that your preparations are already somewhat getting big but still have not yet achieved clearance from the adjacent tooth, you still needs to be broken. There are no exceptions regarding breaking contacts. Especially on the lingual, buccal and gingival aspects of the proximal box, all contacts should be cleared. To avoid getting your preps big, you might want to use hand instruments, in this way, you will smooth walls while you break the contact.

Gingival Floor

Generally, the gingival floor is perpendicular to the long axis of the tooth. This to establish a good resistance form when the occlusal force is directed onto the restoration and the tooth.

The width of the gingival floor is: (1) Molar: 1.2-1.5mm; (2) Premolar: 1-1.2mm

The gingival floor is beveled slightly to remove any unsupported enamel in this area. There are some schools which clearly defines the bevel should be very clear, some would not and will ask you to bevel it slightly.

In the case of Lower First Premolar MO, the angulation of the pulpal floor of the occlusal aspect and the gingival floor of the proximal box is NOT parallel with each other.

Since this tooth surface is within the esthetic zone, creating a very lingually inclined gingival floor will translate a risk to overextending the bucco-gingival line angle too much the buccal area. This will result to amalgam showing when the patient smiles.

So to as to still create an even floor and be able to break the buccal contacts, change your bur angulation to a slightly upright position. So your aim here is to only break the contact area and thus will prevent you from over-extending your prep away too much buccally from the contact area. Sometimes, the buccal wall will conform to a rather upright or in a slight angle leaning lingually, which is totally fine, as long as your lingual wall is converging towards it.

Do not worry yourselves too much about the buccal wall whether you had create a convergence or not because inherently if you just only maintain the bur angulation, the wall will just follow the inverted pear shape of the 330 bur. So ALWAYS maintain your bur angulation and that’s it, the wall will just naturally form a convergent wall for you.

Refining Walls and Floor

When refining the walls and the floor it is a good advice not to use a bur when doing so because burs are very aggressive in taking up too much tooth structure. Use hand instruments instead like an enamel hatchet to remove unsupported enamel and smoothen the walls and floor.

When defining the buccuaxiogingival and linguoaxiogingival point angle, a gingival marginal trimmer is helpful. Because burs are naturally round, it will not help with defining these structures. However, do not overdo it! The point angles should be slightly rounded still but well-defined. A good way is to start from the walls first and leave the inner angles untouched.

Convergence of Buccal and Lingual Walls

The shape of the 330 and 245 is naturally inverted pear shaped, it is wider at the tip. You have to fully utilize the shape of this bur and reflect that to make your buccal and lingual walls to be divergent.

Maintaining the angulation parallel to the long axis of the tooth is important. As long as your angulation is consistent, the shape the bur will just do the convergence nicely.