Convenience Form

The convenience form was conceived by G.V.Black as a modification of the cavity outline form, to establish greater convenience in the placement of intracoronal restorations. In endodontic therapy, the convenience form makes the preparation and filling of the root canal more convenient as well as accurate.

The convenience form modifications’ offers four important benefits:

i) Unobstructed access to the canal orifice.

ii) Direct access to the apical foramen.

iii) Cavity expansion to accommodate filling techniques.

iv) Complete authority over the enlarging instrument.

Unobstructed Access to the Canal Orifice

For the instruments to be placed easily into the orifice of each canal without interference from the overhanging walls, enough tooth structure must be removed. You should be able to see each orifice and reach it easily with the instrument point. Additional canals present in some teeth should also be kept in mind. For example, there is a high incidence of a second separate canal in the mesiobuccal root of maxillary molars. The lower incisors often have additional canals. Also, a second canal is found often in the distal root of mandibular molars. You must keep in mind these variations while preparing the endodontic access cavity. Such variations often result in a modified outline form, to establish greater convenience for instrumentation.

Direct Access to the Apical Foramen

Extensive removal of coronal tooth structure becomes necessary to provide a direct access to the apical foramen and to allow complete freedom of the endodontic instruments so that they can extend down the canal in an unstrained position. This is especially true when the root is severely curved or leaves the chamber in an obtuse angle.

Direct access to apical foramen in severely curved root

Cavity Expansion to Accommodate Filling Techniques

Often it becomes necessary to expand the outline form of the endodontic cavity to make certain filling techniques more convenient. For instance, if a softened gutta percha technique is used to fill the canals, where the rigid pluggers are used in vertical thrust, then the outline form may have to be widely extended to accommodate these heavier instruments.

Complete Authority Over the Enlarging Instrument

It is important that you must have a complete control over the root canal instrument. If the instrument is impinged at the canal orifice by tooth structure that should have been removed, then you will lose control of the direction of the tip of the instrument and the intervening tooth structure will dictate control of the instrument. On the contrary, if you had removed the tooth structure around the orifice so that the instrument is free in this area of the canal, the instrument will then be controlled by two factors:

a) Your fingers on the handle of the instrument, and

b) The walls of the canal at the tip of the instrument.

If the access cavity outline is not properly modified by extending the convenience form, this can lead to failure by either root perforation, “ledge” or “shelf” formation within the canal, instrument breakage, or the incorrect shape of the completed canal preparation often termed “zipping” or apical transportation.