The residual
ridge
in partially
edentulous
patients
Jaime Pietrokovski, Dr.C.D., MS.,* Saul Sorin, Dr.Odont.,** and Zvia Hirschfeld, D.M.D.*** The Hebrew University, Hadassah School of Dental Medicine, Jerusalem, Israel, and Tufts University, School of Dental Medicine, Boston, Mass.
I
n fixed partial denture construction, most considerations on the relation between the residual ridge and the pontic are directed to the form of the contacting surface of the pontic and/or the material to be used for the ponticl+ The form of the residual ridge is an important factor in determining the contour of the contacting pontic surface. The formation and structure of the residual ridge have been studied mainly in experimental animals,‘-” but not enough information is available as to its anatomic and topographic configurations in man.12-20 Elbrecht’” described four types of residual ridges in Kennedy’s Class I or Class II partially edentulous arches according to their shape and the angle the ridge forms with the distal surface of the tooth adjacent to the edentulous ridge. They were: Group A, a horizontal ridge; Group B, a descendent distal ridge; Group C, an ascendent distal ridge; and Group D, a concave configuration. Pendleton’” and Rebosio’” made individual descriptions of residual ridges with their main emphasis on the tissue components of the ridges of the maxillae and mandible. Pietrokovski and Masslerlz found that from the occlusal aspect, the center of the residual ridge shifts palatally in the maxillae and lingually in the mandible as related to the original position of the tooth before its extraction. The forms of residual ridges and dental arches were studied in completely edentulous patients.‘?, Is Pietrokovski and Sorin17 found that parabolic, tapered, and square-shaped ridges and high, medium, and low ridges were present at the different regions of the maxillae and mandible. Thus, the size and the shape of the residual ridge vary considerably according to the specific region measured, *Senior Lecturer, Department of Oral Rehabilitation, School of Dental Medicine; Visiting Associate Professor, Tufts University, School of Dental Medicine. **Clinical Lecturer, Department School of Dental Medicine. ***Lecturer, Department Jf Dental Medicine.
150
of Oral Rehabilitation,
of Oral Rehabilitation,
The Hebrew University, Department of Restorative The Hebrew
The Hebrew
University,
University,
Hadassah Dentistry, Hadassah
Hadassah School
$0’~~~ u
36
Residual
2
ridge
in the partially
edentulous
151
Fig. 1. The contour of the natural adjacent teeth (broken line) is superimposed on the edentulous residual ridge to determine the position of the crest of the residual ridge (solid line) in relation t’o the position formerly occupied by the natural teeth. Fig. 2. The contour of the adjacent clinical crowns (broken lines) is superimposed on the edentulous space to determine the amount of tissue resorption in the gingival direction.
Since a systematic classification of residual ridges in partially edentulous patients is not available, the purpose of this work was to study the morphology of the residual ridge clinically and radiographically. From this information, the form of the residual ridge and, by inference, the contour that should be given to the pontic surface according to each residual ridge can be determined.
MATERIALS AND METHODS Tlvo-hundred adult patients were selected with one or several adjacent missing teeth in the incisor, premolar, or molar regions of either jaw. The teeth had been missing for a minimum of three months, and the patients had not worn prostheses on the residual ridge to be studied. Occlusal and lateral clinical intraoral photographs were made of the residual ridge and the adjacent teeth to determine the shape of the residual ridge as related to the adjacent teeth. Periapical radiographs of the residual ridge were made to determine the nature and morphology of the bony residual ridge. The bases of dental casts made from alginate (irreversible hydrocolloid) impressions were ground parallel to the occlusal plane of the remaining teeth. The occlusal aspect of the cast was projected on a sheet of translucent tracing paper with an opaque object projector.* The crest of the edentulous ridge and the adjacent teeth were traced with a pencil on the translucent paper so that the amount of resorption of the edentulous ridge was evaluated as measured between the projection of the alveolar process of the adjacent teeth and the actual crest of the residual ridge (Fig. 1) Tissue remodeling from the lateral aspect vvas evaluated similarly by tracing the gingival contour of the adjacent teeth and the contour of the residual ridge (Fig. 2). The form and size of the residual ridge from the buccolingual aspect were de*Comparator,
Bausch & Lomb, Inc., Chicago,
III.
152
Pietrokovski,
Sorin,
and Hirschfeld
Fig. 3. The crest of the residual ridge is palatal to the original position of the missing left and right prernolars and molars. Fig. 4. The crest of the residual ridges is placed lingual to the place formerly occllpied by the nattlral teeth.
termined by sectioning the casts through the center of the residual ridge in a plane perpendicular to the occlusal plane of the remaining teeth. Using an opaque object projector, the contour of each section was traced on calibrated paper and its shape and size were recorded as in a previous work.‘7 FINDINGS Occlusal view. From the occlusal aspect, the crest of the residual ridge was found to be lingual to the position previously occupied by the natural teeth. This finding was common for both maxillary and mandibular specimens regardless of the number of missing teeth or their location in the dental arch (Figs. 1, 3, and 4). The lingual position of the crest of the residual ridge was especially noticeable when one or a small number of teeth were missing, as the adjacent natural teeth with their periodontal tissues made this position apparent (Figs. 1 and 3). However, when all the molars were missing, the lingual position of the crest of the residual ridge was more pronounced (Fig. 4). The remodeling of the supporting tissues after the extraction of the molars MYIS evaluated by comparing it with the homologous structures on the opposite side of the jaw region where the molars were still present using a previously described method.” Lateral view. From the lateral aspect, the reduction in height of the alveolar tissues in an apical direction \vas evident in all the specimens studied (Figs. 2 and 5 tog). After screening, the specimens were separated into two groups. One group was the mandibular edentulous distal-extension ridge specimens (58 patients), and the second group was the maxillary edentulous distal-extension ridge specimens (32 patients) and the maxillary and mandibular tooth-bounded ridge specimens (110 patients). In the mandibular edentulous distal-extension specimens with all molars missing (first group), the residual ridge was concave, running down from the distal end of the marginal gingivae of the last natural tooth, toward the center of the residual ridge., and upward again in the direction of the ascending mandibular ramus (Fig. 5). The concavity varied in its sharpness and angulation in each specimen, but the
Residual
ridge
in the partially
edentulous
153
Fig. 5. A marked concavity is formed by the residual ridge and the ascending mandibular ramus. A large amount of tissue is missing after the extraction of the molars and subsequent tissue remodeling.
Fig. 6. Lateral view of a maxillary edentulous distal-extension ridge. The loss of substance created a concave ridge between the gingival margin of the second premolar and the maxillary tuberosity. Fig. 7. L,ateral view of a maxillary residual ridge. The ridge has a concave shape running tween the gingival crests of teeth adjacent to the edentulous space.
Fig. 8. Note the U-shaped form of the residual residual ridge. Fig. 9. The ridge forms a straight the ridge.
line between
ridge on a tooth-bounded the gingival
margins
anterior
be-
mandibular
of the teeth adjacent
to
154
Pietrokovski,
Sorin,
and Hirschfeld
Fig. 10. Diagrams sections.
of the three
Fig. 11. Diagrams section.
of the three typical
Table
I. Distribution
Casts Maxilla
typical
patterns
of maxillary
residual
ridges in buccolingual
of mandibular
residual
ridges in buccolingual
of cross sections of the residual Region of crosssection Incisor
ridge according
to shape
No. of specimens Square
Parabolic
Tapered
Total
5 7
4 5 2
18 21 57
16
96
Molar
10
Total
22
9 9 40 58
8
6
5
19
1 13
8 34
6 17
21 64
28
48
23
Premolar
Mandible
patterns
Incisor
Premolar Molar Total
iii
concave U shape was always present in this group. The upward slope of the mandibular ramus beyond the region formerly occupied by natural teeth helped to create the U-shaped angulation of the ridges studied. In the specimens of the second group, the residual ridge was either concave or horizontal between the limiting gingival crests of the adjacent teeth in the toothbounded specimens and between the last adjacent tooth and the tuberosity in the edentulous distal-extension maxillary specimens (Figs. 6 to 9). Buccolingual cross sections. When the casts were sectioned buccolingually at the center of the residual ridge, the cross sections were classified as square, parabolic, or tapered (Figs. 10 and 11). The distribution of these shapes according to jaw regions is shown in Table I. The ridges in the buccolingual sections showed great variation in height as well
Volume 36 Number i!
Residual
ridge
in the partially
edentulous
155
Fig. 12. A periapical radiograph of the specimen in Fig. 3. The crest of the bony ridge is parallel to the corresponding clinical crest of the residual ridge. Note the trabecular nature of the entire bony residual ridge.
Fig. 13. A diagram of properly-shaped pontics in the buccolingual dimension. The form of the pontic base is dictated by the form of the residual ridge it contacts. A broad, square residual ridge requires a broad, square pontic base (left). A thin residual ridge requires a thin pontic base (right). Ideally, the pontic base should be a mirror image of the tissue surface it contacts.
as in width. The height of the ridges in the maxillae ranged from 2.5 to 18 mm., and in the mandible, they ranged from -1.5 to 14.5 mm. The width of the ridges varied in the maxillae from 16 to 32 mm. and in the mandible from 4.5 to 23 mm. Radiographic examination. Radiographs showed that the bone tissue of the residual ridge had the same gross external contour as the clinical contour of the ridge (Fig. 12). The bony architecture of the residual ridge was trabecular. A dense cortical layer was seen at the crest of the residual ridge in only 6 per cent of the cases. DISCUSSION The form and size of the residual ridge are indiiridual for each patient at each region. However, a general pattern of form can be established when the ridge is concave or straight in the mesiodistal plane and parabolic, square, or tapered in the buccolingual plane. In all patients, the crest of the residual ridge is located lingual to the position formerly occupied by the teeth. This variation in the form of the residual ridge challenges the prosthodontist with the problem of the adaptation of the planned prosthesis to the residual ridge. When the planned prosthesis is either a
156
Pietrokouski,
Sorin,
and Hirschfeld
.I. Proathct. Dent. August.
1976
Fig. 14. Proper shaping of the pontic length gingivally. The length of the pontic gingivally should be the same as the adjacent retainers to avoid food impaction: (A) an oversized pontic base; (B) the length of the pontic base is determined by using the gingival height of the abutment teeth as a guide; (C) a properly contoured pontic base where the pontic length is at the same level as the crowns of the abutment teeth.
removable partial denture or a complete denture, the problem is simple as far as adaptation of the prosthesis to the mucosa is concerned. The denture base must cover and be closely adapted to the entire basal seat of the residual ridge regardless of the ridge size and shape, except in spaced areas over sharp edges or in pathologic conditions. However, a different problem confronts the dentist when a fixed partial denture is to be made. The form of the pontic and the manner of contact between the base of the pontic and the crest of the residual ridge may be important factors in determining the success or failure of the prosthesis. Most dentists tend to make the pontic in a stereotyped form (bullet-shaped, ridge-lap type, modified ridge-lap type) regardless of the individual form of the residual ridge that the pontic base is to contact. The form of the pontic base should be shaped according to the specific position and form of the contacting residual ridge to create a physiologic climate for the tissues in the buccolingual dimension (Fig. 13) . The length of the clinical crown of the adjacent abutment teeth should be used
ylhre;
&U
“2”
Residual ridge in the partially
edentulous
157
to determine the length of the pontic at the buccal and lingual surfaces, It should he of the same length as the adjacent teeth. The pontic should be provided with sufficient clearance at its mesial and distal embrasures, resulting in a pontic surface which makes minimal contact with the tissue surface of the residual ridge. This permits self-cleansing and hygienic activities at the intersurface between pontic and tissues (Fig. 14). An exception to this design concept is when upper anterior teeth are mis:sing and the loss of the labial alveolar plate together with the lingual shift of the crest of the residual ridge necessitates a thicker and larger pontic surface in order to achieve an adequate esthetic result. SUMMARY The residual ridge which forms after the extraction of teeth is characterized by a loss of substance when compared with the dentulous ridge. The residual ridge shrinks in an apical direction and shifts lingually, This remodeling pattern takes place mainly at the expense of the bony crest, the buccal plate of the alveolar process, and the gingival tissues covering it. The lingual positioning of the residual ridge crest was observed in maxillary and mandibular specimens in all the regions of the jaws and regardless of the number of teeth missing. Cross sections of the residual ridge were square, parabolic, or tapered. The base of the pontic for a fixed partial denture should be made buccolingually as the mirror image of the crest of the residual ridge it is to contact, and it should follow mesiodistally the contour and length of the rlinical crowns of the adjacent abutment teeth. References 1. Harmon, C. B.: Pontic Design, J. PROSTHET. DENT. 8: 496-503, 1958. Into the Changes Occurring in the Oral Mucosa Beneath Bridge Pontics, Thesis, University of Indiana School of Dentistry, 1961. 3. Stein, S. R.: Pontic-Residual Ridge Relationship. A Research Report, J. PROSTHET. 2. Henry, P. J.: An Investigation
DENT. 16: 251-285,
1966.
4. Podshadley, A. G.: Gingival Response to Pontics, J. PROSTHET. DENT. 19: 51-57, 1968. ri. Johnston, J., Phillips, R. W., and Dykema, R. W.: Modern Practice in Crown and Bridge Prosthodontics, Philadelphia, 1973, W. B. Saunders Company, pp. 281, 335. 6. Crown
ridge
in partially
edentulous
patients
Jaime Pietrokovski, Dr.C.D., MS.,* Saul Sorin, Dr.Odont.,** and Zvia Hirschfeld, D.M.D.*** The Hebrew University, Hadassah School of Dental Medicine, Jerusalem, Israel, and Tufts University, School of Dental Medicine, Boston, Mass.
I
n fixed partial denture construction, most considerations on the relation between the residual ridge and the pontic are directed to the form of the contacting surface of the pontic and/or the material to be used for the ponticl+ The form of the residual ridge is an important factor in determining the contour of the contacting pontic surface. The formation and structure of the residual ridge have been studied mainly in experimental animals,‘-” but not enough information is available as to its anatomic and topographic configurations in man.12-20 Elbrecht’” described four types of residual ridges in Kennedy’s Class I or Class II partially edentulous arches according to their shape and the angle the ridge forms with the distal surface of the tooth adjacent to the edentulous ridge. They were: Group A, a horizontal ridge; Group B, a descendent distal ridge; Group C, an ascendent distal ridge; and Group D, a concave configuration. Pendleton’” and Rebosio’” made individual descriptions of residual ridges with their main emphasis on the tissue components of the ridges of the maxillae and mandible. Pietrokovski and Masslerlz found that from the occlusal aspect, the center of the residual ridge shifts palatally in the maxillae and lingually in the mandible as related to the original position of the tooth before its extraction. The forms of residual ridges and dental arches were studied in completely edentulous patients.‘?, Is Pietrokovski and Sorin17 found that parabolic, tapered, and square-shaped ridges and high, medium, and low ridges were present at the different regions of the maxillae and mandible. Thus, the size and the shape of the residual ridge vary considerably according to the specific region measured, *Senior Lecturer, Department of Oral Rehabilitation, School of Dental Medicine; Visiting Associate Professor, Tufts University, School of Dental Medicine. **Clinical Lecturer, Department School of Dental Medicine. ***Lecturer, Department Jf Dental Medicine.
150
of Oral Rehabilitation,
of Oral Rehabilitation,
The Hebrew University, Department of Restorative The Hebrew
The Hebrew
University,
University,
Hadassah Dentistry, Hadassah
Hadassah School
$0’~~~ u
36
Residual
2
ridge
in the partially
edentulous
151
Fig. 1. The contour of the natural adjacent teeth (broken line) is superimposed on the edentulous residual ridge to determine the position of the crest of the residual ridge (solid line) in relation t’o the position formerly occupied by the natural teeth. Fig. 2. The contour of the adjacent clinical crowns (broken lines) is superimposed on the edentulous space to determine the amount of tissue resorption in the gingival direction.
Since a systematic classification of residual ridges in partially edentulous patients is not available, the purpose of this work was to study the morphology of the residual ridge clinically and radiographically. From this information, the form of the residual ridge and, by inference, the contour that should be given to the pontic surface according to each residual ridge can be determined.
MATERIALS AND METHODS Tlvo-hundred adult patients were selected with one or several adjacent missing teeth in the incisor, premolar, or molar regions of either jaw. The teeth had been missing for a minimum of three months, and the patients had not worn prostheses on the residual ridge to be studied. Occlusal and lateral clinical intraoral photographs were made of the residual ridge and the adjacent teeth to determine the shape of the residual ridge as related to the adjacent teeth. Periapical radiographs of the residual ridge were made to determine the nature and morphology of the bony residual ridge. The bases of dental casts made from alginate (irreversible hydrocolloid) impressions were ground parallel to the occlusal plane of the remaining teeth. The occlusal aspect of the cast was projected on a sheet of translucent tracing paper with an opaque object projector.* The crest of the edentulous ridge and the adjacent teeth were traced with a pencil on the translucent paper so that the amount of resorption of the edentulous ridge was evaluated as measured between the projection of the alveolar process of the adjacent teeth and the actual crest of the residual ridge (Fig. 1) Tissue remodeling from the lateral aspect vvas evaluated similarly by tracing the gingival contour of the adjacent teeth and the contour of the residual ridge (Fig. 2). The form and size of the residual ridge from the buccolingual aspect were de*Comparator,
Bausch & Lomb, Inc., Chicago,
III.
152
Pietrokovski,
Sorin,
and Hirschfeld
Fig. 3. The crest of the residual ridge is palatal to the original position of the missing left and right prernolars and molars. Fig. 4. The crest of the residual ridges is placed lingual to the place formerly occllpied by the nattlral teeth.
termined by sectioning the casts through the center of the residual ridge in a plane perpendicular to the occlusal plane of the remaining teeth. Using an opaque object projector, the contour of each section was traced on calibrated paper and its shape and size were recorded as in a previous work.‘7 FINDINGS Occlusal view. From the occlusal aspect, the crest of the residual ridge was found to be lingual to the position previously occupied by the natural teeth. This finding was common for both maxillary and mandibular specimens regardless of the number of missing teeth or their location in the dental arch (Figs. 1, 3, and 4). The lingual position of the crest of the residual ridge was especially noticeable when one or a small number of teeth were missing, as the adjacent natural teeth with their periodontal tissues made this position apparent (Figs. 1 and 3). However, when all the molars were missing, the lingual position of the crest of the residual ridge was more pronounced (Fig. 4). The remodeling of the supporting tissues after the extraction of the molars MYIS evaluated by comparing it with the homologous structures on the opposite side of the jaw region where the molars were still present using a previously described method.” Lateral view. From the lateral aspect, the reduction in height of the alveolar tissues in an apical direction \vas evident in all the specimens studied (Figs. 2 and 5 tog). After screening, the specimens were separated into two groups. One group was the mandibular edentulous distal-extension ridge specimens (58 patients), and the second group was the maxillary edentulous distal-extension ridge specimens (32 patients) and the maxillary and mandibular tooth-bounded ridge specimens (110 patients). In the mandibular edentulous distal-extension specimens with all molars missing (first group), the residual ridge was concave, running down from the distal end of the marginal gingivae of the last natural tooth, toward the center of the residual ridge., and upward again in the direction of the ascending mandibular ramus (Fig. 5). The concavity varied in its sharpness and angulation in each specimen, but the
Residual
ridge
in the partially
edentulous
153
Fig. 5. A marked concavity is formed by the residual ridge and the ascending mandibular ramus. A large amount of tissue is missing after the extraction of the molars and subsequent tissue remodeling.
Fig. 6. Lateral view of a maxillary edentulous distal-extension ridge. The loss of substance created a concave ridge between the gingival margin of the second premolar and the maxillary tuberosity. Fig. 7. L,ateral view of a maxillary residual ridge. The ridge has a concave shape running tween the gingival crests of teeth adjacent to the edentulous space.
Fig. 8. Note the U-shaped form of the residual residual ridge. Fig. 9. The ridge forms a straight the ridge.
line between
ridge on a tooth-bounded the gingival
margins
anterior
be-
mandibular
of the teeth adjacent
to
154
Pietrokovski,
Sorin,
and Hirschfeld
Fig. 10. Diagrams sections.
of the three
Fig. 11. Diagrams section.
of the three typical
Table
I. Distribution
Casts Maxilla
typical
patterns
of maxillary
residual
ridges in buccolingual
of mandibular
residual
ridges in buccolingual
of cross sections of the residual Region of crosssection Incisor
ridge according
to shape
No. of specimens Square
Parabolic
Tapered
Total
5 7
4 5 2
18 21 57
16
96
Molar
10
Total
22
9 9 40 58
8
6
5
19
1 13
8 34
6 17
21 64
28
48
23
Premolar
Mandible
patterns
Incisor
Premolar Molar Total
iii
concave U shape was always present in this group. The upward slope of the mandibular ramus beyond the region formerly occupied by natural teeth helped to create the U-shaped angulation of the ridges studied. In the specimens of the second group, the residual ridge was either concave or horizontal between the limiting gingival crests of the adjacent teeth in the toothbounded specimens and between the last adjacent tooth and the tuberosity in the edentulous distal-extension maxillary specimens (Figs. 6 to 9). Buccolingual cross sections. When the casts were sectioned buccolingually at the center of the residual ridge, the cross sections were classified as square, parabolic, or tapered (Figs. 10 and 11). The distribution of these shapes according to jaw regions is shown in Table I. The ridges in the buccolingual sections showed great variation in height as well
Volume 36 Number i!
Residual
ridge
in the partially
edentulous
155
Fig. 12. A periapical radiograph of the specimen in Fig. 3. The crest of the bony ridge is parallel to the corresponding clinical crest of the residual ridge. Note the trabecular nature of the entire bony residual ridge.
Fig. 13. A diagram of properly-shaped pontics in the buccolingual dimension. The form of the pontic base is dictated by the form of the residual ridge it contacts. A broad, square residual ridge requires a broad, square pontic base (left). A thin residual ridge requires a thin pontic base (right). Ideally, the pontic base should be a mirror image of the tissue surface it contacts.
as in width. The height of the ridges in the maxillae ranged from 2.5 to 18 mm., and in the mandible, they ranged from -1.5 to 14.5 mm. The width of the ridges varied in the maxillae from 16 to 32 mm. and in the mandible from 4.5 to 23 mm. Radiographic examination. Radiographs showed that the bone tissue of the residual ridge had the same gross external contour as the clinical contour of the ridge (Fig. 12). The bony architecture of the residual ridge was trabecular. A dense cortical layer was seen at the crest of the residual ridge in only 6 per cent of the cases. DISCUSSION The form and size of the residual ridge are indiiridual for each patient at each region. However, a general pattern of form can be established when the ridge is concave or straight in the mesiodistal plane and parabolic, square, or tapered in the buccolingual plane. In all patients, the crest of the residual ridge is located lingual to the position formerly occupied by the teeth. This variation in the form of the residual ridge challenges the prosthodontist with the problem of the adaptation of the planned prosthesis to the residual ridge. When the planned prosthesis is either a
156
Pietrokouski,
Sorin,
and Hirschfeld
.I. Proathct. Dent. August.
1976
Fig. 14. Proper shaping of the pontic length gingivally. The length of the pontic gingivally should be the same as the adjacent retainers to avoid food impaction: (A) an oversized pontic base; (B) the length of the pontic base is determined by using the gingival height of the abutment teeth as a guide; (C) a properly contoured pontic base where the pontic length is at the same level as the crowns of the abutment teeth.
removable partial denture or a complete denture, the problem is simple as far as adaptation of the prosthesis to the mucosa is concerned. The denture base must cover and be closely adapted to the entire basal seat of the residual ridge regardless of the ridge size and shape, except in spaced areas over sharp edges or in pathologic conditions. However, a different problem confronts the dentist when a fixed partial denture is to be made. The form of the pontic and the manner of contact between the base of the pontic and the crest of the residual ridge may be important factors in determining the success or failure of the prosthesis. Most dentists tend to make the pontic in a stereotyped form (bullet-shaped, ridge-lap type, modified ridge-lap type) regardless of the individual form of the residual ridge that the pontic base is to contact. The form of the pontic base should be shaped according to the specific position and form of the contacting residual ridge to create a physiologic climate for the tissues in the buccolingual dimension (Fig. 13) . The length of the clinical crown of the adjacent abutment teeth should be used
ylhre;
&U
“2”
Residual ridge in the partially
edentulous
157
to determine the length of the pontic at the buccal and lingual surfaces, It should he of the same length as the adjacent teeth. The pontic should be provided with sufficient clearance at its mesial and distal embrasures, resulting in a pontic surface which makes minimal contact with the tissue surface of the residual ridge. This permits self-cleansing and hygienic activities at the intersurface between pontic and tissues (Fig. 14). An exception to this design concept is when upper anterior teeth are mis:sing and the loss of the labial alveolar plate together with the lingual shift of the crest of the residual ridge necessitates a thicker and larger pontic surface in order to achieve an adequate esthetic result. SUMMARY The residual ridge which forms after the extraction of teeth is characterized by a loss of substance when compared with the dentulous ridge. The residual ridge shrinks in an apical direction and shifts lingually, This remodeling pattern takes place mainly at the expense of the bony crest, the buccal plate of the alveolar process, and the gingival tissues covering it. The lingual positioning of the residual ridge crest was observed in maxillary and mandibular specimens in all the regions of the jaws and regardless of the number of teeth missing. Cross sections of the residual ridge were square, parabolic, or tapered. The base of the pontic for a fixed partial denture should be made buccolingually as the mirror image of the crest of the residual ridge it is to contact, and it should follow mesiodistally the contour and length of the rlinical crowns of the adjacent abutment teeth. References 1. Harmon, C. B.: Pontic Design, J. PROSTHET. DENT. 8: 496-503, 1958. Into the Changes Occurring in the Oral Mucosa Beneath Bridge Pontics, Thesis, University of Indiana School of Dentistry, 1961. 3. Stein, S. R.: Pontic-Residual Ridge Relationship. A Research Report, J. PROSTHET. 2. Henry, P. J.: An Investigation
DENT. 16: 251-285,
1966.
4. Podshadley, A. G.: Gingival Response to Pontics, J. PROSTHET. DENT. 19: 51-57, 1968. ri. Johnston, J., Phillips, R. W., and Dykema, R. W.: Modern Practice in Crown and Bridge Prosthodontics, Philadelphia, 1973, W. B. Saunders Company, pp. 281, 335. 6. Crown
