Operative dentistry aje qualtrough, jd satterthwaite la morrow, pa brunton

Design considerations
Preparation form
A straight-line access for the root canal instruments should be
achieved whenever possible. Undue bending of instruments during
insertion renders them more liable to distortion or fracture and makes
access to some part of the pulp space problematical. Having to force
an instrument around curves makes a perforation more likely.
Furthermore, sufficient freedom of movement of an instrument, when
in the canal, is necessary to permit effective cleaning and shaping of
all parts of the pulp walls.
The access cavity itself should therefore have a rounded outline 
and be symmetrically placed about fissures and away from crown
margins. This will reduce the incidence of fracture of porcelain
crowns. Palatal and lingual openings on anterior teeth are preferred
for aesthetic reasons but lingual inclination, especially of lower 
anteriors (and sometimes premolars), may indicate access cavity
preparations involving the labial or buccal surface (Fig. 3.3).
Preparation is related to the form and position of the pulp chamber
and the root canals extending from it rather than to the overlying
fissure pattern.
Unnecessary tissue removal should be avoided because of the func-
tional need to preserve the strength and integrity of root and crown
structure. An access cavity through the roof of the pulp chamber
significantly weakens the tooth – in the extreme case of a molar with a
mesio-occluso-distal preparation, tooth tissue is left only in the furca-
tion area leading to a high risk of fracture if remaining cusps are still 
in high functional load. Occlusal reduction of a molar undergoing
endodontic treatment may be helpful. Subsequent placement of a cus-
pal coverage restoration will permit restoration of occlusal function.
Therefore, the access cavity should be of sufficient size only to satisfy
endodontic objectives.