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A New Three-Quarter Crown Preparation and Bridge Attachment
A NEW THREE-QUARTER CROWN PREPARATION AND BRIDGE ATTACHMENT By
M ic h a e l
A.
N o v ia ,*
H E purpose o f this paper is to present an original idea in prepar ing a three-quarter crown for a bridge attachment. It is a new and simple approach to the old and difficult problem o f replacing one or two posterior teeth, either by the fixed or b y the semi fixed bridge. This new type attachment is designed for reducing the trauma inci dent to preparation of the abutment teeth, differing from the traditional type attachment in that it is inserted lingually rather than vertically. In order to appreciate the principle involved in this new attachment, let us consider the present type three-quarter crown now in use. In the preparation of the present type three-quarter cast crown, the mesial and distal proximal surfaces are sliced, the slices converging toward the occlusal surface. Since the natural tooth presents a greater circum ference at the midportion and the occlu sal portion than at the cervical, much tooth structure must be removed to elim inate undercuts. (Fig. iA .) T h e lingual surface o f the natural tooth presents a convexity which causes an undercut and so must be ground with a slight taper toward the occlusal surface. A groove is then cut across the occlusal surface from mesial to distal aspect, and another groove is placed within the slices from the occlusal groove to the cervical m ar gin. Sufficient tooth structure is also removed from the occlusal surface to allow for the gold o f the three-quarter cast crown. (Fig. iB.)
T
•Assistant professor of prosthetic dentistry, Tufts College Dental School, Boston, Mass.
Jour. A .D .A ., Vol. a8, February 1941
D .M .D ., Worcester, Mass. Because o f the amount of cutting just described, one can readily see that con siderable tooth structure must be removed from the natural tooth in making the conventional three-quarter cast crown. Furthermore, if the tooth to be used as the abutment is tipped either mesially or distally, even more tooth structure must be removed from the proximal surfaces in order to insert the completed bridge in the vertical plane. In contrast, in this new three-quarter cast crown, regardless o f the bell shape of the natural teeth, the mesial and distal proximal slices are made with no attempt to remove the undercuts, but rather allowing the slices to follow the natural inclination of the proximal sur face o f the tooth, and thus the slices will diverge rather than converge at the occlusal portion. (Fig. 2A.) Sufficient tooth structure is then removed from the occlusal surface up to the incisal edge of the buccal cusps to allow for the gold replacement. T h e lingual surface, re gardless of its convexity, is left un touched. Next, the grooves (Fig. 2C) are placed within the limits of the proximal slice, running diagonally from the mesiolinguoclusal point angle to the mesiobuccogingival point angle (Fig. 2B) and, similarly, on the distal slice. It is obvious that the copper band impression must be taken from the lingual aspect. (Fig. 3.) In festooning, the copper band is trimmed to include all but the buccal surface. T o take the impression, the copper band is gently forced into position in the same plane as that of the grooves, and, after the compound is chilled, any
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surplus that extends on the buccal sur face is removed. Figure 4 illustrates the replacement of a lower left first molar. I f a completely fixed bridge is to be constructed, a three-quarter cast crown on the second premolar must be made as in the second molar. However, if a semifixed principle is the choice, a threequarter crown on the premolar is not necessary and a simple distoclusal inlay will suffice. A movable joint in the disto clusal inlay of the second premolar is necessary in order for the bridge to func tion with the new three-quarter cast crown. Obviously, if this new bridge is to be inserted from the lingual aspect, the countersunk occlusal rest, as at present used, cannot be employed as it is made O L d T bftE E -Q i/m E R
D
ental
A
s s o c ia t io n
molar after preparation for the threequarter crown and has a downward inclination; that is, the part o f the groove next to the pontic (Fig. 5A ) is nearer the occlusal surface b y 1 mm. than that part of the groove nearest the tooth itself (Fig. 5B ), so that a definite lock is established which w ill prevent the abut ment teeth from drifting aw ay from each other. T h e construction of this m ovable at tachment is a very simple laboratory procedure. As these castings are made by the indirect method, the amalgam dies o f the abutment teeth are in position on the cast, which is mounted on a small articulator, with a cast o f the teeth of the opposite jaw. 1. T h e wax pattern o f the distoclusal N tw T h re e -Q u a rte r
C row n
Crow n
Bug.
\ B uccal
P roximal
Fig. 1.— Diagrammatic illustration of old, or conventional, three-quarter crown. A , buc cal view, showing mesial and distal slices converging toward occlusal surface, and amount of tooth structure removed (indicated by diagonal lines). B, lingual view, showing taper of lingual surface toward occlusal sur face, amount of tooth structure removed and proximal and occlusal grooves.
to function with vertical insertion o f the bridge. Figure 5 shows the type o f groove necessary for the movable joint in the distoclusal inlay of the second premolar. This groove, which is placed m idway between the marginal ridge and the cer vical m argin (Fig. 5B ), is about 2 mm. deep, 0.5 mm. wide and in length ex tends four-fifths of the buccolingual di mension of the distoclusal inlay. I t is made parallel with the groove on the
A
L1M6
/
B
Bu c c a l
P ro v im a l
Fig. 2.— Diagrammatic illustration of new three-quarter crown. A , buccal view, show ing mesial and distal slices diverging toward occlusal surface, and amount of tooth struc ture removed (indicated by diagonal lines). B, proximal view, showing diagonal groove ( C ) , within limits of slice, and the unaltered lingual surface.
inlay o f the premolar is carved to ana tomic form and articulation. 2. About 1 mm. o f w ax is added to the proximal portion o f the inlay for greater convexity to give added bulk for the depth o f the groove. 3. W ith the groove o f the molar pre pared for the three-quarter crown as a guide, a parallel line is inscribed on the wax pattern of the premolar with a pointed instrument. 4. Gold that is purchased in rectangu lar form is about 0.5 mm. thick and is
N o v ia — T
h r e e -Q u a rte r
G r o w n P r e p a r a t io n
285
Fig. 6.— Lingual view of completed bridge showing movable joint (A ) in position.
Fig. 3.— Upper molar with copper band impression partly seated. T h e arrow on the copper band indicates the angle of insertion parallel to the groove partly visible on the proximal surface.
Fig. 7.— Geometric figure with opposite sides parallel. A , only possible path of insertion for three-quarter casting.
Fig. 4.— Replacement of lower left first molar. A, groove on distoclusal inlay parallel to groove on new type three-quarter crown of molar.
Fig. 5.— Diagrammatic illustration of de tails of movable joint. T h e angulation of groove A , on the distoclusal inlay of the bicuspid, and of C , on the gold portion of the pontic, permits locking them together in seating and prevents drifting of the abutment teeth.
Fig. 8.— Geometric figure with two oppo site sides converging. A , B, C , paths of in sertion for three-quarter casting.
286
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o urnal of th e
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the instrument used to make this groove. By heating this gold slightly and placing it on the inscribed line with a back and forth motion, a uniform, smooth groove
A
D e n t a l A s s o c ia t io n
5. T h e porcelain pontic is now selected and ground. T h e w ax pattern for the gold portion of the pontic is carved to anatomic form and occlusion and placed in position on the working cast. 6. A piece about 4 by 3 mm. is cut
B
Fig. 9.— A , proximal view of lower first bicuspid, showing groove farther from hori zontal than groove of B, on lower second bicuspid, because proximal surfaces of A con verge toward lingual more than toward proxi mal surfaces of B.
Fig. 10.— Buccal view of upper first molar illustrating divergence of slices at occlusal sur face.
is developed, which is cast with the in lay. (Fig. 4A.)
Fig.
1 1.— Lingual view of upper molar.
Fig. 1 a.— Proximal view of upper molar il lustrating parallelism of groove with lingual cusps and unaltered lingual surface with con vexity.
from the rectangular sheet of casting gold mentioned above. T h e piece is heated and placed in the groove o f the
N o v ia — T
h ree-Q uarter
distoclusal inlay and, with slight force, guided to position in the w ax pattern of the pontic. This portion is now cast as an integral part of the pontic pattern, and any surplus that extends lingually can be polished in finishing the bridge. T h e usual procedure is followed in as sembling and soldering the bridge. F ig ure 6A shows the attachment in position. S U M M A R Y AND CO N CLUSIO N
In the form of the posterior teeth, Nature has presented us with a con venient geometric figure that makes this new three-quarter crown possible. I f we wish to make a casting to cover four surfaces o f the geometric figure in Figure 7 to simulate a three-quarter crown, there is only one path of insertion for that
Fig. 13.— Buccal view of completed bridge showing small amount of gold visible on buc cal surface.
casting, and that path of insertion will be in the horizontal plane indicated by the line A . I f we alter the geometric form o f Figure 7 so that two opposite sides converge (Fig. 8), we have a geo metric figure that truly represents the form o f the posterior teeth. Now, if a casting is to be made for this geometric figure to cover four surfaces, we have not only a horizontal path of insertion, but also the paths o f insertions indicated by the lines A, B, and C and any other lines that m ay be drawn between them. From the above-described geometric design, we deduce these fa c ts: 1. T he
C r o w n P r e p a r a t io n
287
greater the convergence o f two opposite sides which represent the proximal sur faces, the farther from the horizontal will the grooves be placed. 2. T h e less the convergence of two opposite sides, the nearer the horizontal will the grooves be placed. These facts are illustrated in Figure 9. A shows a lower first pre molar whose proximal surfaces converge more than those o f B, a lower second premolar. Therefore, the proximal sur faces of A will be farther from the horizontal than those of B. I f advantage is taken of the path o f insertion farthest from the horizontal, less grinding o f the lingual cusps will be necessary. This method o f bridge construction offers m any advantages over the old : 1. Less tooth structure is lost.
Fig. 14.— Lingual view of completed bridge partly seated to indicate path of insertion governed by groove visible on second molar.
2. T he operating time is greatly re duced. 3. There is less trauma to the tooth. 4. Less pain is experienced. 5. There is better cervical adaptation, and so less irritation to soft tissue due to overhang o f gold and food traps. 6. Less gold is visible on the buccal surface. 7. No thermal shock is felt after the bridge is cemented, because there is no deep grinding. 8. Regardless of the tilt of the tooth, the preparation is the same as though the tooth were vertical. 1 20 Front Street.
M ic h a e l
A.
N o v ia ,*
H E purpose o f this paper is to present an original idea in prepar ing a three-quarter crown for a bridge attachment. It is a new and simple approach to the old and difficult problem o f replacing one or two posterior teeth, either by the fixed or b y the semi fixed bridge. This new type attachment is designed for reducing the trauma inci dent to preparation of the abutment teeth, differing from the traditional type attachment in that it is inserted lingually rather than vertically. In order to appreciate the principle involved in this new attachment, let us consider the present type three-quarter crown now in use. In the preparation of the present type three-quarter cast crown, the mesial and distal proximal surfaces are sliced, the slices converging toward the occlusal surface. Since the natural tooth presents a greater circum ference at the midportion and the occlu sal portion than at the cervical, much tooth structure must be removed to elim inate undercuts. (Fig. iA .) T h e lingual surface o f the natural tooth presents a convexity which causes an undercut and so must be ground with a slight taper toward the occlusal surface. A groove is then cut across the occlusal surface from mesial to distal aspect, and another groove is placed within the slices from the occlusal groove to the cervical m ar gin. Sufficient tooth structure is also removed from the occlusal surface to allow for the gold o f the three-quarter cast crown. (Fig. iB.)
T
•Assistant professor of prosthetic dentistry, Tufts College Dental School, Boston, Mass.
Jour. A .D .A ., Vol. a8, February 1941
D .M .D ., Worcester, Mass. Because o f the amount of cutting just described, one can readily see that con siderable tooth structure must be removed from the natural tooth in making the conventional three-quarter cast crown. Furthermore, if the tooth to be used as the abutment is tipped either mesially or distally, even more tooth structure must be removed from the proximal surfaces in order to insert the completed bridge in the vertical plane. In contrast, in this new three-quarter cast crown, regardless o f the bell shape of the natural teeth, the mesial and distal proximal slices are made with no attempt to remove the undercuts, but rather allowing the slices to follow the natural inclination of the proximal sur face o f the tooth, and thus the slices will diverge rather than converge at the occlusal portion. (Fig. 2A.) Sufficient tooth structure is then removed from the occlusal surface up to the incisal edge of the buccal cusps to allow for the gold replacement. T h e lingual surface, re gardless of its convexity, is left un touched. Next, the grooves (Fig. 2C) are placed within the limits of the proximal slice, running diagonally from the mesiolinguoclusal point angle to the mesiobuccogingival point angle (Fig. 2B) and, similarly, on the distal slice. It is obvious that the copper band impression must be taken from the lingual aspect. (Fig. 3.) In festooning, the copper band is trimmed to include all but the buccal surface. T o take the impression, the copper band is gently forced into position in the same plane as that of the grooves, and, after the compound is chilled, any
383
284
T
he
Journal
of th e
A
m e r ic a n
surplus that extends on the buccal sur face is removed. Figure 4 illustrates the replacement of a lower left first molar. I f a completely fixed bridge is to be constructed, a three-quarter cast crown on the second premolar must be made as in the second molar. However, if a semifixed principle is the choice, a threequarter crown on the premolar is not necessary and a simple distoclusal inlay will suffice. A movable joint in the disto clusal inlay of the second premolar is necessary in order for the bridge to func tion with the new three-quarter cast crown. Obviously, if this new bridge is to be inserted from the lingual aspect, the countersunk occlusal rest, as at present used, cannot be employed as it is made O L d T bftE E -Q i/m E R
D
ental
A
s s o c ia t io n
molar after preparation for the threequarter crown and has a downward inclination; that is, the part o f the groove next to the pontic (Fig. 5A ) is nearer the occlusal surface b y 1 mm. than that part of the groove nearest the tooth itself (Fig. 5B ), so that a definite lock is established which w ill prevent the abut ment teeth from drifting aw ay from each other. T h e construction of this m ovable at tachment is a very simple laboratory procedure. As these castings are made by the indirect method, the amalgam dies o f the abutment teeth are in position on the cast, which is mounted on a small articulator, with a cast o f the teeth of the opposite jaw. 1. T h e wax pattern o f the distoclusal N tw T h re e -Q u a rte r
C row n
Crow n
Bug.
\ B uccal
P roximal
Fig. 1.— Diagrammatic illustration of old, or conventional, three-quarter crown. A , buc cal view, showing mesial and distal slices converging toward occlusal surface, and amount of tooth structure removed (indicated by diagonal lines). B, lingual view, showing taper of lingual surface toward occlusal sur face, amount of tooth structure removed and proximal and occlusal grooves.
to function with vertical insertion o f the bridge. Figure 5 shows the type o f groove necessary for the movable joint in the distoclusal inlay of the second premolar. This groove, which is placed m idway between the marginal ridge and the cer vical m argin (Fig. 5B ), is about 2 mm. deep, 0.5 mm. wide and in length ex tends four-fifths of the buccolingual di mension of the distoclusal inlay. I t is made parallel with the groove on the
A
L1M6
/
B
Bu c c a l
P ro v im a l
Fig. 2.— Diagrammatic illustration of new three-quarter crown. A , buccal view, show ing mesial and distal slices diverging toward occlusal surface, and amount of tooth struc ture removed (indicated by diagonal lines). B, proximal view, showing diagonal groove ( C ) , within limits of slice, and the unaltered lingual surface.
inlay o f the premolar is carved to ana tomic form and articulation. 2. About 1 mm. o f w ax is added to the proximal portion o f the inlay for greater convexity to give added bulk for the depth o f the groove. 3. W ith the groove o f the molar pre pared for the three-quarter crown as a guide, a parallel line is inscribed on the wax pattern of the premolar with a pointed instrument. 4. Gold that is purchased in rectangu lar form is about 0.5 mm. thick and is
N o v ia — T
h r e e -Q u a rte r
G r o w n P r e p a r a t io n
285
Fig. 6.— Lingual view of completed bridge showing movable joint (A ) in position.
Fig. 3.— Upper molar with copper band impression partly seated. T h e arrow on the copper band indicates the angle of insertion parallel to the groove partly visible on the proximal surface.
Fig. 7.— Geometric figure with opposite sides parallel. A , only possible path of insertion for three-quarter casting.
Fig. 4.— Replacement of lower left first molar. A, groove on distoclusal inlay parallel to groove on new type three-quarter crown of molar.
Fig. 5.— Diagrammatic illustration of de tails of movable joint. T h e angulation of groove A , on the distoclusal inlay of the bicuspid, and of C , on the gold portion of the pontic, permits locking them together in seating and prevents drifting of the abutment teeth.
Fig. 8.— Geometric figure with two oppo site sides converging. A , B, C , paths of in sertion for three-quarter casting.
286
T
he
J
o urnal of th e
A
m e r ic a n
the instrument used to make this groove. By heating this gold slightly and placing it on the inscribed line with a back and forth motion, a uniform, smooth groove
A
D e n t a l A s s o c ia t io n
5. T h e porcelain pontic is now selected and ground. T h e w ax pattern for the gold portion of the pontic is carved to anatomic form and occlusion and placed in position on the working cast. 6. A piece about 4 by 3 mm. is cut
B
Fig. 9.— A , proximal view of lower first bicuspid, showing groove farther from hori zontal than groove of B, on lower second bicuspid, because proximal surfaces of A con verge toward lingual more than toward proxi mal surfaces of B.
Fig. 10.— Buccal view of upper first molar illustrating divergence of slices at occlusal sur face.
is developed, which is cast with the in lay. (Fig. 4A.)
Fig.
1 1.— Lingual view of upper molar.
Fig. 1 a.— Proximal view of upper molar il lustrating parallelism of groove with lingual cusps and unaltered lingual surface with con vexity.
from the rectangular sheet of casting gold mentioned above. T h e piece is heated and placed in the groove o f the
N o v ia — T
h ree-Q uarter
distoclusal inlay and, with slight force, guided to position in the w ax pattern of the pontic. This portion is now cast as an integral part of the pontic pattern, and any surplus that extends lingually can be polished in finishing the bridge. T h e usual procedure is followed in as sembling and soldering the bridge. F ig ure 6A shows the attachment in position. S U M M A R Y AND CO N CLUSIO N
In the form of the posterior teeth, Nature has presented us with a con venient geometric figure that makes this new three-quarter crown possible. I f we wish to make a casting to cover four surfaces o f the geometric figure in Figure 7 to simulate a three-quarter crown, there is only one path of insertion for that
Fig. 13.— Buccal view of completed bridge showing small amount of gold visible on buc cal surface.
casting, and that path of insertion will be in the horizontal plane indicated by the line A . I f we alter the geometric form o f Figure 7 so that two opposite sides converge (Fig. 8), we have a geo metric figure that truly represents the form o f the posterior teeth. Now, if a casting is to be made for this geometric figure to cover four surfaces, we have not only a horizontal path of insertion, but also the paths o f insertions indicated by the lines A, B, and C and any other lines that m ay be drawn between them. From the above-described geometric design, we deduce these fa c ts: 1. T he
C r o w n P r e p a r a t io n
287
greater the convergence o f two opposite sides which represent the proximal sur faces, the farther from the horizontal will the grooves be placed. 2. T h e less the convergence of two opposite sides, the nearer the horizontal will the grooves be placed. These facts are illustrated in Figure 9. A shows a lower first pre molar whose proximal surfaces converge more than those o f B, a lower second premolar. Therefore, the proximal sur faces of A will be farther from the horizontal than those of B. I f advantage is taken of the path o f insertion farthest from the horizontal, less grinding o f the lingual cusps will be necessary. This method o f bridge construction offers m any advantages over the old : 1. Less tooth structure is lost.
Fig. 14.— Lingual view of completed bridge partly seated to indicate path of insertion governed by groove visible on second molar.
2. T he operating time is greatly re duced. 3. There is less trauma to the tooth. 4. Less pain is experienced. 5. There is better cervical adaptation, and so less irritation to soft tissue due to overhang o f gold and food traps. 6. Less gold is visible on the buccal surface. 7. No thermal shock is felt after the bridge is cemented, because there is no deep grinding. 8. Regardless of the tilt of the tooth, the preparation is the same as though the tooth were vertical. 1 20 Front Street.