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A modified lower anterior porcelain-veneer crown preparation
A modified lower crown preparation Gerald Calgary,
anterior
porcelain-veneer
1. Locke, D.D.S. Alberta, Canada
omplete porcelain-jacket (veneer) crowns for lower anterior teeth present a challenge to prosthodontists because of tooth morphology, occlusion, and tissue relationships. The tooth size, shape, and relationship in standard preparations provide for a weak veneer crown at best; the high proximal rises that accommodate the interdental papillae seriously weaken the crown and subject it to the possibility of fracture in either the labiogingival or linguogingival area, or in both (Figs. 1 and 2). With the thin crowns necessaryfor lower anterior teeth, the hydraulic pressure generated during cementation can cause fractures. If the lower incisor is prepared with a shoulder as small as 0.5 mm. placed subgingivally for its full circumference, the remaining tooth structure is often weak and liable to fracture. This is particularly true for tapered incisors with narrow roots or when deep proximal caries or cervical abrasion on the labial surface is the reason for making the restoration c
(Fig. 2). From experience with porcelain half crowns (Figs. 3, 4, and 5)) with their remarkable strength and obvious preservation of tooth structure, there evolved a modified preparation which, in effect, is a half crown with a gingival extension on the labial surface (Fig. 6). This preparation (Fig. 7) is easier to make, and it preserves the cingulum of the tooth for strength. It eliminates the linguogingival fracture. The labiogingival extension makes the lower anterior porcelain crown an esthetic and relatively fracture-free restoration. This crown resistsheavy occlusion even with full function on the anterior teeth in protrusive occlusions without posterior occlusion (Fig. 5). Since it is desirable to preserve the interdental papilla and the gingival third of the lingual surface is almost never carious when there is no prosthesiscovering this surface, there is no need to cover it even when there is rampant caries. A further advantage is the freedom from possible snagging of a lingual margin which may Read
before
The
Pacific
Coast
Society
of Prosthodontists,
Newport
Beach,
Calif.
611
612
J. Prosthet. Dent. June, 1973
Locke
Fig. 1. Diagram of a lower incisor with cervical of the cementoenamel junction in an indented anterior crowns. Fig. 2. Severe tooth reduction and veneer crown preparation is used.
Fig. 3. Anterior teeth of a patient crowns for the lower incisors. Fig.
4. The
half-crown
the most
with
abrasion, narrow
frequent
heavy
anterior
gingival root, all
fracture
recession, of which
sites where
occlusion
prior
and the high rise complicate lower
a standard
to construction
porcelain-
of half
preparations.
in this area if the shoulder on the lingual surface is prepared into the root cementum and dentin. While the usual lower anterior porcelain-veneer crown is weak and the usual preparation weakens the tooth (Fig. 7)) the half crown is esthetically deficient in teeth where the patient fails to keep the uncut labial enamel spotlessly clean. Also, the half crown does not permit restoration of labial defects in the gingival region. The modified porcelain crown preparation suffers none of these deficiencies (Figs. 8 and 9). occur
when
scaling
the
teeth
Volume Number
29 6
Modified
Fig.
Fig.
6.
5.
Preparation
613
Half crowns that have been in function seven years.
Profile of a modified preparation
Fig. 7. A lateral view tooth structure which the standard preparation.
crown
showing
minimal
of the usual preparation. may be preserved by the
tooth
reduction.
The shaded area represents modified preparation when
the amou compared
mt of with
PROCEDURE The preparation may often be completed with one instrument, namely a carbide No. 170 tapered smooth fissure bur in a true-running air-driven contra-angle handpiece. 1. The gingival shoulder is established first (Fig. 10). 2. The depth of enamel to be removed from the labial surface is established by sinking part of the length of the bur into the mesial and distal proximal line angles and into the center of the labial enamel, maintaining the obtuse angle of this surface (Figs. 11 and 6). 3. The distal line angle cuts are continued through the interproximal surface to
614
J. Prosthet. Dent. June, 1973
Locke
Fig. 8. Modified partial service
porcelain-veneer crowns with full denture, and posterior disclusion in protrusive since 1962 without a fracture.
Fig. 9. Modified have provided
Fig. 10. The labial
unilateral free-end precision occlusion, jaw relation. The crowns have been in
lower anterior porcelain-veneer satisfactory service for several
crowns opposed years in a patient
initial
establishes
cut
of the preparation
the
width
by a complete denture. who is a chronic bruxer.
and
often
the depth
They
of the
shoulder.
Fig. 11. Depth gauge marks made by a No. 170 smooth fissure carbide bur. 12. Proximal enamel is removed before removing the remaining labial enamel. Fig. 13. The completed restoration in which the labial margin is usually placed part.way
Fig.
the gingival
into
sulcus.
the lingual surface at the height of or very slightly below the gingival crest sweeping up from its labiogingival shoulder (Fig. 12). 4. The proximal cuts are joined lingually by a cut that establishes the lingual shoulder on or above the cingulum (Fig. 6). 5. Depth gauge cuts are made on the lingual surface either when making the labial cuts or at this time. 6. The remainder of the axial portion of the preparation is completed by joining the depth gauge cuts with lateral sweeps of the bur (Fig. 12).
Modified
crown
preparation
615
7. Any remaining incisal reduction that may be necessary is carried out now. 8. A careful sweep around the shoulder eliminates axial undercuts and smooths the enamel margins at the same time it lowers the shoulder to the desired level relative to the gingiva (Fig. 13). 9. Bastian files may be used for final finishing of the shoulder and axial walls when necessary to remove undercuts or fragile enamel rods. 10. These same steps may be used to provide controlled reduction of tooth structure for any complete crowns on upper incisors or posterior teeth.
SUMMARY A step-by-step procedure for a modified preparation of a lower anterior porcelain crown has been described and illustrated. The finished form of the prepared tooth takes into consideration ease of preparation, uniform and minimal amount of enamel reduction, and a uniform thickness of porcelain in the crown. The modified prosthesis seems to provide a remarkably fracture-free crown. The interdental papilla can be preserved, and excellent esthetic results can be achieved. 479 PALLISER CALGARY, CANADA
SQUARE EAST ALBERTA T2G OP5
anterior
porcelain-veneer
1. Locke, D.D.S. Alberta, Canada
omplete porcelain-jacket (veneer) crowns for lower anterior teeth present a challenge to prosthodontists because of tooth morphology, occlusion, and tissue relationships. The tooth size, shape, and relationship in standard preparations provide for a weak veneer crown at best; the high proximal rises that accommodate the interdental papillae seriously weaken the crown and subject it to the possibility of fracture in either the labiogingival or linguogingival area, or in both (Figs. 1 and 2). With the thin crowns necessaryfor lower anterior teeth, the hydraulic pressure generated during cementation can cause fractures. If the lower incisor is prepared with a shoulder as small as 0.5 mm. placed subgingivally for its full circumference, the remaining tooth structure is often weak and liable to fracture. This is particularly true for tapered incisors with narrow roots or when deep proximal caries or cervical abrasion on the labial surface is the reason for making the restoration c
(Fig. 2). From experience with porcelain half crowns (Figs. 3, 4, and 5)) with their remarkable strength and obvious preservation of tooth structure, there evolved a modified preparation which, in effect, is a half crown with a gingival extension on the labial surface (Fig. 6). This preparation (Fig. 7) is easier to make, and it preserves the cingulum of the tooth for strength. It eliminates the linguogingival fracture. The labiogingival extension makes the lower anterior porcelain crown an esthetic and relatively fracture-free restoration. This crown resistsheavy occlusion even with full function on the anterior teeth in protrusive occlusions without posterior occlusion (Fig. 5). Since it is desirable to preserve the interdental papilla and the gingival third of the lingual surface is almost never carious when there is no prosthesiscovering this surface, there is no need to cover it even when there is rampant caries. A further advantage is the freedom from possible snagging of a lingual margin which may Read
before
The
Pacific
Coast
Society
of Prosthodontists,
Newport
Beach,
Calif.
611
612
J. Prosthet. Dent. June, 1973
Locke
Fig. 1. Diagram of a lower incisor with cervical of the cementoenamel junction in an indented anterior crowns. Fig. 2. Severe tooth reduction and veneer crown preparation is used.
Fig. 3. Anterior teeth of a patient crowns for the lower incisors. Fig.
4. The
half-crown
the most
with
abrasion, narrow
frequent
heavy
anterior
gingival root, all
fracture
recession, of which
sites where
occlusion
prior
and the high rise complicate lower
a standard
to construction
porcelain-
of half
preparations.
in this area if the shoulder on the lingual surface is prepared into the root cementum and dentin. While the usual lower anterior porcelain-veneer crown is weak and the usual preparation weakens the tooth (Fig. 7)) the half crown is esthetically deficient in teeth where the patient fails to keep the uncut labial enamel spotlessly clean. Also, the half crown does not permit restoration of labial defects in the gingival region. The modified porcelain crown preparation suffers none of these deficiencies (Figs. 8 and 9). occur
when
scaling
the
teeth
Volume Number
29 6
Modified
Fig.
Fig.
6.
5.
Preparation
613
Half crowns that have been in function seven years.
Profile of a modified preparation
Fig. 7. A lateral view tooth structure which the standard preparation.
crown
showing
minimal
of the usual preparation. may be preserved by the
tooth
reduction.
The shaded area represents modified preparation when
the amou compared
mt of with
PROCEDURE The preparation may often be completed with one instrument, namely a carbide No. 170 tapered smooth fissure bur in a true-running air-driven contra-angle handpiece. 1. The gingival shoulder is established first (Fig. 10). 2. The depth of enamel to be removed from the labial surface is established by sinking part of the length of the bur into the mesial and distal proximal line angles and into the center of the labial enamel, maintaining the obtuse angle of this surface (Figs. 11 and 6). 3. The distal line angle cuts are continued through the interproximal surface to
614
J. Prosthet. Dent. June, 1973
Locke
Fig. 8. Modified partial service
porcelain-veneer crowns with full denture, and posterior disclusion in protrusive since 1962 without a fracture.
Fig. 9. Modified have provided
Fig. 10. The labial
unilateral free-end precision occlusion, jaw relation. The crowns have been in
lower anterior porcelain-veneer satisfactory service for several
crowns opposed years in a patient
initial
establishes
cut
of the preparation
the
width
by a complete denture. who is a chronic bruxer.
and
often
the depth
They
of the
shoulder.
Fig. 11. Depth gauge marks made by a No. 170 smooth fissure carbide bur. 12. Proximal enamel is removed before removing the remaining labial enamel. Fig. 13. The completed restoration in which the labial margin is usually placed part.way
Fig.
the gingival
into
sulcus.
the lingual surface at the height of or very slightly below the gingival crest sweeping up from its labiogingival shoulder (Fig. 12). 4. The proximal cuts are joined lingually by a cut that establishes the lingual shoulder on or above the cingulum (Fig. 6). 5. Depth gauge cuts are made on the lingual surface either when making the labial cuts or at this time. 6. The remainder of the axial portion of the preparation is completed by joining the depth gauge cuts with lateral sweeps of the bur (Fig. 12).
Modified
crown
preparation
615
7. Any remaining incisal reduction that may be necessary is carried out now. 8. A careful sweep around the shoulder eliminates axial undercuts and smooths the enamel margins at the same time it lowers the shoulder to the desired level relative to the gingiva (Fig. 13). 9. Bastian files may be used for final finishing of the shoulder and axial walls when necessary to remove undercuts or fragile enamel rods. 10. These same steps may be used to provide controlled reduction of tooth structure for any complete crowns on upper incisors or posterior teeth.
SUMMARY A step-by-step procedure for a modified preparation of a lower anterior porcelain crown has been described and illustrated. The finished form of the prepared tooth takes into consideration ease of preparation, uniform and minimal amount of enamel reduction, and a uniform thickness of porcelain in the crown. The modified prosthesis seems to provide a remarkably fracture-free crown. The interdental papilla can be preserved, and excellent esthetic results can be achieved. 479 PALLISER CALGARY, CANADA
SQUARE EAST ALBERTA T2G OP5