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RETENTION FORM 135
mal part, to the box form with its line and point angles
(see Figs. 148, 149, 150, 151, 162 and 163).
Seat.—In simple cavities the bottom or floor of the cavity,
either the axial or pulpal wall.
Step.—The auxiliary portion of the compound mortise
form, consisting of the axial and pulpal walls in complex
cavities.
(c) Retention Form.—The so shaping of the cavity that
the filling may be enabled to resist tipping or lifting stress.
(i) In pit and fissure and gingival third cavities^ where the
depth of the cavity is greater than its diameter, no extra
retention form is needed, the filling being retained by its
contact with the lateral walls of the cavity. The accomplish-
ment of this condition, though, is not often admissible, owing
to the proximity of the pulp. In the majority of cases, then,
retention is provided for by transforming the simple mortise
form into the dovetail mortise form (Fig. 136). In doing
this, one, or at most two, opposite walls of the cavity are
slightly dovetailed. (2) In proximal cavities on incisors and
cuspids not involving the angle, retention is provided for by
making more acute the labio-gingivo-axial, lingo-gingivo-axial
and the incisal point angles (Figs. 154 and 156). (3) In
proximal cavities on incisors and cuspids involving the angle,
the gingival and incisal point angles are deepened as in
^^(2)," for the simpler cases. In the incisal step form, the
entire step, which has been given a flat floor, is dovetailed
on its labial and lingual walls, and slightly deepened at its
extremity (Figs. 159 and 164). In the lingual step form
the step is dovetailed throughout, and its two point angles
rendered more acute (Fig. 160). (4) In proximo-occlusal
cavities on bicuspids and molars the gingival point angles are
rendered more acute in both the simple mortise form and the
compound mortise form (Figs. 145, 162 and 163). The
mal part, to the box form with its line and point angles
(see Figs. 148, 149, 150, 151, 162 and 163).
Seat.—In simple cavities the bottom or floor of the cavity,
either the axial or pulpal wall.
Step.—The auxiliary portion of the compound mortise
form, consisting of the axial and pulpal walls in complex
cavities.
(c) Retention Form.—The so shaping of the cavity that
the filling may be enabled to resist tipping or lifting stress.
(i) In pit and fissure and gingival third cavities^ where the
depth of the cavity is greater than its diameter, no extra
retention form is needed, the filling being retained by its
contact with the lateral walls of the cavity. The accomplish-
ment of this condition, though, is not often admissible, owing
to the proximity of the pulp. In the majority of cases, then,
retention is provided for by transforming the simple mortise
form into the dovetail mortise form (Fig. 136). In doing
this, one, or at most two, opposite walls of the cavity are
slightly dovetailed. (2) In proximal cavities on incisors and
cuspids not involving the angle, retention is provided for by
making more acute the labio-gingivo-axial, lingo-gingivo-axial
and the incisal point angles (Figs. 154 and 156). (3) In
proximal cavities on incisors and cuspids involving the angle,
the gingival and incisal point angles are deepened as in
^^(2)," for the simpler cases. In the incisal step form, the
entire step, which has been given a flat floor, is dovetailed
on its labial and lingual walls, and slightly deepened at its
extremity (Figs. 159 and 164). In the lingual step form
the step is dovetailed throughout, and its two point angles
rendered more acute (Fig. 160). (4) In proximo-occlusal
cavities on bicuspids and molars the gingival point angles are
rendered more acute in both the simple mortise form and the
compound mortise form (Figs. 145, 162 and 163). The
