transfer by a maxillary distal-extension partial denture with cap and ring extracoronal Theodore
Berg,
DDS,”
and Angelo
A. Caputo,
removable attachments
PhDb
University of California-Los Angeles,Schoolof Dentistry, Los Angeles,Calif. This study photoelasticaIIy compared the stress distribution characteristics of maxillary, bilateral, distal-extension removable partial dentures retained by light and heavy ERA extracoronal attachments. One prosthesis included supporting rests and the other had no rests. Both designs were tested with and without abutment splinting. The most favorable stress distributions were obtained with light retention elements, supporting rests, and splinting of the abutments. In this configuration the attachment prosthesis compared favorably in stress distribution with the maxillary I-bar retained removable partial denture in a previous study that used a comparable maxillae model. (J PROSTHET DENT 1992;68:784-9.)
T.
his study demonstrates the potential for stress patterns in teeth and bone supporting a bilateral, maxillary, distal-extension removable partial denture retained by the ERA attachment (Stern Metals, Inc., Attleboro, Mass.) (Fig. 1). The ERA attachment is an extracoronal attachment with vertical resiliency and universal stress relief for use where a resilient prosthesis is indicated. The manufacturer does not suggest the use of supporting rests or abutment splinting. The study evaluated the use of the ERA attachment with and without supporting rests and abutment splinting. A prior study by the authors established a basis of comparison with a maxillary, extracoronal I-bar retained removable partial denture (RPD) by use of a standardized maxillary model for observing stress distribution tendencies with removable partial denturesi MATERIAL
AND
METHODS
The technique of quasi-three-dimensional photoelastic stress analysis was used for this investigation. A composite photoelastic maxillae model, with inferior portions of the maxillary sinuses and nasal cavity, was constructed. The model included the right canine, left central incisor, left canine, and left first premolar teeth as described and illustrated in the first maxillary retainer study by the auth0rs.l The photoelastic model was set in a custom-made basewith a surveyor matrix developed during the initial study to standardize orientation of the maxillae. The coronal portions of all teeth were prepared for
Project supportedby BRSGgrant RR05304,awardedby the BiomedicalResearchSupport Grant Program,DRR, National Institutes of Health, and by a researchgift from the Albert and Elaine Borchard Foundation,Inc., of WoodlandHills, Calif. aClinicalProfessor,Sectionof RemovableProsthodontics,UCLA, and Visiting Scholarat the Faculte de ChirurgieDentaire,Universite de Nantes,France. bProfessorand Chairman,Biomaterials ScienceSection. 16/1/4Q362
784
ceramometal crowns. All crowns had positive rest seats. Custom, removable, rigid interdental splints were made so that the abutments could be tested as splinted or not sp1inted.r The right canine and left first premolar crowns included the ERA plastic matrix ring patterns that were oriented by use of the manufacturer’s paralleling mandrel and then cast with the crowns according to the manufacturer’s directions.2 Care was taken in cleaning the rings to assure proper fit of the patrix snap attachments. The crowns were cemented with a zinc phosphate cement (Fig. 2). Two chrome-cobalt alloy RPD frameworks were made with rests for all rest seats on the four crowns and mechanical retention in the base region for the resin that would be placed to house the synthetic patrix retention caps. Both frameworks were fitted to the abutments and adjusted for axis-of-rotation movement with the use of a disclosing medium of chloroform and rouge and a ~10 power binocular microscope.’ Acrylic resin bases were added with a medium-viscosity rubber impression material forming an approximately 2 mm mucosal simulation as done in the first study. Autopolymerizing resin (Duralay, Reliance Dental Mfg. Co., Worth, Ill.) was then used to form the patrix housings for both frameworks with the black fabricating patrices in the kit according to the manufacturer’s directions. By use of kit instrumentation and directions, the black patrices were removed and replaced with the light (white) retention, and later with the heavy (blue) retention synthetic retention caps. The thicker base on the black processing patrix attachment was designed to provide 0.4 mm of vertical resiliency and universal joint hinging for the retention elements. The light and heavy retention caps were selected for study to give reasonable boundaries in retention force. Metal loading cones were then attached to the extension bases in the first molar regions over the ridge crests, with custom surveyor orientation matrices and multiple measurements used to replicate load-cone positions from the
NOVEMBER1992
VOLUME68
NUMBER5
LOAD
TRANSFER
DESIGN
FOR MAXILLARY
RPD
Retention Cap Color Code: White - light Orange - moderate Blue - heavy Grey - very heavy
Retention Ring Pattern - cast with crown
Fig. 3. Attachment-retained prosthesis without supporting rests, shown seated on maxillary model and crowns without splinting, was also tested with abutments splinted.
Fig. 1. ERA attachment parts include plastic ring pattern cast with abutment crown and choices of synthetic retention caps.
Fig. 4. Attachment-retained prosthesis with support rests, shown seated in place and with abutment splinting, was also tested without abutments splinted. Fig. 2. The photoelastic maxillary model has crowns cemented and includes attachment rings. There is provision for removable, rigid abutment splints secured with screws.
first study for comparison (Figs. 3 and 4). Both resin patrix housings were relieved of binding spots against the abutment crowns by loading of the extension bases and use of rouge disclosing medium. The model was bolted into an oil-immersion dish containing mineral oil to improve optical transmission properties. The dish was secured to the center of the straining frame between the light source and camera by use of recorded measurements from the first study to standardize the model during rotation of the straining frame. The light source and camera included the same polarizers and quarter-wave plates used in the first study.l The rests and minor connector struts were removed from one of the frameworks after completion of the total assembly. This resulted in one prosthesis with no supporting rests and the other with rests. Both configurations could be tested with and without abutment splinting (Figs. 3 and 4).
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Loads of 30 lb (13.64 kg) were directed against each load cone1 in the vertical, buccal, lingual, mesial, and distal directions with a load cell monitored with an XY recorder. The model was viewed from four directions for a total of 40 observations for each of the design configurations of (1) light retention, no rests, with and without splints; (2) light retention with rests, with and without splints; (3) heavy retention, no rests, with and without splints; and (4) heavy retention with rests, with and without splints. The results were photographed.
RESULTS Examination of the model before and after placement of the two frameworks confirmed that each framework seated passively with essentially no stresses. Therefore, all isochromatic fringes seen later within the model were the result of applied loads. Schematic representations of stress intensity are included with the selected photographic evidence to facilitate presentation and interpretation of the data. The diagrams do not include actual isochromatic fringe lines.
785
BERG
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CAPUTO
Fig. 5. View of left canine and premolar shows complex, high-level stresses with light retention, no rests, and no abutment splinting. Schematic drawings accompanying the photoelastic pictures represent stress distribution regions: blue for low-level, yellow for moderate, and red for high-level stresses. Black arrow indicates load direction. Fig. 6. Left canine and premolar with prosthesis using light retention under a vertical load show less stress with addition of rests and abutment splinting. White dots in schematics represent occlusal and cingulum rests. Red arrow indicates splinting. Fig. ‘7. View of left canine and premolar shows complex and some high-level stress with light retention attachment prosthesis lacking rests and splints under a buccally directed load. Fig. 8. Stress levels with left canine and premolar were reduced with addition of rests and abutment splinting to prosthesis under a buccally directed load.
Areas represented in blue represent low stress in the order of one fringe. Yellow areas are moderate stress areas in the range of two to three fringes. Red areas indicate the highest order of stress in the range of four or more fringes. A stress distribution that features apical loading with an even distribution of a low order would be most favorable for clinical applications. Higher orders of fringes and closer fringes indicate a tendency for unwanted biologic changes. Different stress effects were observed for the ERA attachment with and without supporting rests, with and without splinting of the abutments, and with light and then heavy retention. Observations also were made with direct loads of 15 lb (6.82 kg) to the right canine and to the lingual cusp of the left first premolar without a prosthesis in
186
place for comparison purposes. Stresses with all loadings again followed the three facial stress trajectories and demonstrated varying degrees of stress in the tuberosity area, consistent with the first study.’
Vertical
Ioads
Vertical loads to both sides resulted in the most uniform and apical stresses with the configuration that included supporting rests and splinted abutments. Stress patterns involving the right canine were all in the low fringe order. Rests more than splinting improved right canine stress patterns. The change from light to heavy retention increased stress for the left premolar in the no-rest, no-splint configuration from a moderate two fringes to an upper moderate range of three fringes and included patterns sug-
NOVEMBER
1992
VOLUME
68
NUMBER
5
LOAD
TRANSFER
DESIGN
FOR MAXILLARY
RPD
Fig. 9. View of right canine shows severe and complex stresses resulting from distally directed load with lreavy retention, no rests, and no abutment splinting. Fig. 10. Stress to the right canine is reduced with the addition of rests and splinting. Moderate stress remains in bone adjacent and distal to abutment. Fig. 11. View of left canine and premolar indicates complex, widespread, and high-level stresses resulting from distally directed loading with light retention, no rests, and no splinting. Fig. 12. Stresses from distally directed loads are significantly reduced overall with addition of rests and abutment splinting even with the presence of heavy retention; however, high stress remains just distal to the left premolar. gestive of root bending. There was less cross-arch transfer of stress with splinting for either loaded side. Either rests or splints provided some improvement to the left premolar side with the most apical, equitdble stress distribution for both sides resulting from a combination of rests and splinting (Figs. 5 and 6). Direct loads without a prosthesis in place to either the incisal edge of the left canine or the lingual cusp tip of the left premolar resulted in low- to moderate-range stress. In comparison to these direct loadings, the attachment with rests, splinting, and vertical loading of the extension bases displayed similar levels of stress, but with the stress distribution more apical and with more equitable internal root involvement. Vertical loads with the attachment both splinted and with supporting rests were favorably comparable to vertical loads of the I-bar retained RPD from the previous study.i The premolar and canine on the left side
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
displayed low to moderate stress patterns more confined within the roots than with the I-bar retained RPD.l
Buccally
directed
loads
Right-side buccally directed loads with light retention showed twisting in the canine abutment without rests or splinting. Less stress in the canine was seen with rests and splinting, confining stress more to the apical area. There was no observable effect from splinting with heavy retention. However, rests did benefit stress distribution for this load direction. Left-side buccally directed loads with neither rests nor splinting resulted in severe stress on the premolar with vertically oriented, multiple fringes and included complex stress on the adjacent canine (Fig. 7). Splinting with no rests reduced stress and showed more cross-arch transfer to the right canine. There was also an improvement in stress
787
BERG
distribution with rests, even with heavy retention. Splinting, rests, and light retention resulted in the most equitable stress distribution (Fig. 8).
Lingually
directed
loads
Right-side loads with light retention resulted in no substantial differences with or without rests. Splinting produced slightly more cross-arch transfer to the left canine and premolar. Stress distributions were improved by splinting for heavy retention, with and without rests. Left-side loads with light retention displayed high stress to the premolar but less with the addition of rests. The stress patterns with the abutments splinted either with or without rests were similar. Therefore, rests or splints, .or both, helped. Heavy retention without rests and splinting resulted in complicated premolar root stress and stress in the mesial region of the adjacent canine root. The addition of rests resulted in substantially less, more uniform premolar stress and no involvement of the mesial area of the canine root. Splinting without rests also improved premolar stress distribution but the mesial area of the canine root remained involved.
Mesially
directed
loads
Splinting was more helpful than rests for the canine abutment under right-side mesially directed loads with light retention. Bending in the canine with heavy retention was much relieved with splinting. Rests adversely affected the canine without splinting for this direction of loading. Splinted versions with or without rests were similar. Left-side loads with light retention and without rests or splinting displayed more axial stresses than with the addition of rests. Rests improved the stress distribution over the premolar abutment and shifted more stress to the adjacent canine, even when the abutments were not splinted. Splinting and light retention resulted in more cross-arch transfer and improved the stress distribution with or without rests. Heavy retention displayed a somewhat different result; the version including rests and splinting did better than rests or splinting alone. Heavy retention resulted in higher and more complex stress patterns compared with light retention, as with loads in other directions.
Distally
directed
loads
Right-side distally directed loads with light retention displayed high stress and bending in the canine abutment (Fig. 9). This stress intensity was reduced and more apically located with the presence of rests or splinting. Hsavy retention caused canine bending, including cervical bone area involvement, with high stress in the three- and four-fringe range. The stress distributions were improved with the addition of splinting and even more so with rests. The most equitable transfer of load resulted from a combination of rests and splinting (Fig. 10). Although rests and splinting significantly improved the stress condition, the result was not as favorable as that observed in a previous
788
AND
CAPUTO
study with an I-bar retained RPD under the same load conditi0n.l Left-side distally directed loads with light retention resulted in multiple fringes of stress in the premolar that included the periodontal membrane area and stress configurations suggestive of twisting (Fig. 11). The premolar with rests displayed less severe stress with some of the stress shifted to the adjacent canine. Splinting with light retention helped more than rests when loading was on the left side. The presence of splinting was as effective as the combination of rests and splinting. Heavy retention resulted in similar stresses with an increase in severity. The best distribution with left-side loading resulted from a combination of rests and splinting, although high-level stress remained at the distal cervical-bone support region of the premolar abutment (Fig. 12). Rests and splinting significantly improved the stress condition, resulting in a stress state that was somewhat more favorable than that observed in a previous study with an I-bar retained RPD under the same load conditi0n.i
DISCUSSION The use of heavy retention elements in this study created significantly higher stresses than light retention elements for any given configuration of prosthesis, rests, and abutment splinting. Given the often high and severe stress (schematically shown as red areas equal to four or more fringes) observed with heavy retention in this study, it seems clinically preferable to avoid the heavy and very heavy retention elements. Clinically it seems preferable to change the retention elements more often than to risk the high stress levels of heavier, but possibly longer lasting, retention. Another variable would be introduced if the metal ring portion of the attachment was finished with excessive polishing. The combination of supporting rests on the abutment teeth and splinting of the abutments significantly and consistently improved stress distributions. The use of rests or abutment splinting alone did not improve stress distribution for all the combinations of loading directions and abutments. Heavy retention created more stress than light retention for a given configuration. However, heavy retention with the addition of rests and abutment splinting often created less stress than light retention without rests and splinting. The most equitable stress distributions were achieved with light retention and the addition of supporting rests and abutment splinting. The provision of rests in the anterior part of the maxillary arch can be difficult because of occlusal relationships and tooth anatomy. The benefit from supporting rests outweighs the possible difficulty of providing occlusal or cingulum rests on the crown or splinted unit. Vertical loads on the left side produced high stress in the distal region of the premolar root when rests or splinting were not used. This suggests that the 0.4 mm vertical relief space alone may be inadequate for abutment protection. Rests will likely fur-
NOVEMBER
1992
VOLUME
68
NUMBER
5
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FOR MAXILLARY
RPD
ther enhance occlusal stability and masticatory performance. Mushimoto et al3 found significant improvement in masticatory movements by using RPDs with occlusal rests in contrast to RPDs without supporting rests. Rests also permit evaluation of the need for relining and provide a relative position for the attachment elements. The minor connectors associated with the rests allow more precise placement of the retention element, lessening the possibility of damage to the synthetic retention caps. Splinting of abutments adds expense and complexity to an attachment RPD. However, the results of this study suggest that, for a maxillary distal-extension base configuration, splinting may significant1.ycontribute to equitable stress distribution. For example, the right canine with its ample root support benefitted only from splinting when sustaining a mesially directed load. SUMMARY
AND
CONCLUSIONS
A model of a maxillary bilateral distal-extension removable partial denture was used to photoelastically demonstrate and assessthe stress-distributing characteristics of the ERA attachment. The study evaluated the attachment with the addition of supporting rests and abutment splinting. The results indicated the following. 1. Stress distribution was less apically oriented and significantly higher for the heavy retention element than for the light retention element.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
2. The most equitable distributions of stress occurred when the ERA attachment was used with light retention elements, supporting rests, and abutment splinting. 3. The attachment prosthesis with light retention, rests, and abutment splinting compared favorably in stress distribution with the maxillary I-bar retained partial denture in a previous study made on a comparable maxillae mode1.l Appreciationis extendedto Irene Petravicius,UCLA Schoolof Dentistry Illustration Department,for assistancewith preparation of graphicsand diagrams. REFERENCES 1. Berg T, Caputo AA. Comparison of load transfer by maxillary distalextension removable partial dentures with a spring-loaded plunger attachment and I-bar retainer. J Prosthet Dent 1992;68:500-7. 2. Stern ERA extracoronal attachment fabrication instructions. Attleboro, Mass; APM-Sterngold division of Stern Metals, Inc., 1986. 3. Mushimoto E, Kawani A, Ohi H, Tatsuta T, Mitani H. Functional significance of pressuresensation conveyedby removable partial dentures. Int J Prosthodont 1988;1:41-5.
Reprintrequests to: DR. THEODORE BERG REMOVABLE PROSTHODONTICSSECTION SCHOOL OF DENTISTRY UNIVERSITY OF CALIFORNIA Los ANGELES, CA 90024-1668
789
Berg,
DDS,”
and Angelo
A. Caputo,
removable attachments
PhDb
University of California-Los Angeles,Schoolof Dentistry, Los Angeles,Calif. This study photoelasticaIIy compared the stress distribution characteristics of maxillary, bilateral, distal-extension removable partial dentures retained by light and heavy ERA extracoronal attachments. One prosthesis included supporting rests and the other had no rests. Both designs were tested with and without abutment splinting. The most favorable stress distributions were obtained with light retention elements, supporting rests, and splinting of the abutments. In this configuration the attachment prosthesis compared favorably in stress distribution with the maxillary I-bar retained removable partial denture in a previous study that used a comparable maxillae model. (J PROSTHET DENT 1992;68:784-9.)
T.
his study demonstrates the potential for stress patterns in teeth and bone supporting a bilateral, maxillary, distal-extension removable partial denture retained by the ERA attachment (Stern Metals, Inc., Attleboro, Mass.) (Fig. 1). The ERA attachment is an extracoronal attachment with vertical resiliency and universal stress relief for use where a resilient prosthesis is indicated. The manufacturer does not suggest the use of supporting rests or abutment splinting. The study evaluated the use of the ERA attachment with and without supporting rests and abutment splinting. A prior study by the authors established a basis of comparison with a maxillary, extracoronal I-bar retained removable partial denture (RPD) by use of a standardized maxillary model for observing stress distribution tendencies with removable partial denturesi MATERIAL
AND
METHODS
The technique of quasi-three-dimensional photoelastic stress analysis was used for this investigation. A composite photoelastic maxillae model, with inferior portions of the maxillary sinuses and nasal cavity, was constructed. The model included the right canine, left central incisor, left canine, and left first premolar teeth as described and illustrated in the first maxillary retainer study by the auth0rs.l The photoelastic model was set in a custom-made basewith a surveyor matrix developed during the initial study to standardize orientation of the maxillae. The coronal portions of all teeth were prepared for
Project supportedby BRSGgrant RR05304,awardedby the BiomedicalResearchSupport Grant Program,DRR, National Institutes of Health, and by a researchgift from the Albert and Elaine Borchard Foundation,Inc., of WoodlandHills, Calif. aClinicalProfessor,Sectionof RemovableProsthodontics,UCLA, and Visiting Scholarat the Faculte de ChirurgieDentaire,Universite de Nantes,France. bProfessorand Chairman,Biomaterials ScienceSection. 16/1/4Q362
784
ceramometal crowns. All crowns had positive rest seats. Custom, removable, rigid interdental splints were made so that the abutments could be tested as splinted or not sp1inted.r The right canine and left first premolar crowns included the ERA plastic matrix ring patterns that were oriented by use of the manufacturer’s paralleling mandrel and then cast with the crowns according to the manufacturer’s directions.2 Care was taken in cleaning the rings to assure proper fit of the patrix snap attachments. The crowns were cemented with a zinc phosphate cement (Fig. 2). Two chrome-cobalt alloy RPD frameworks were made with rests for all rest seats on the four crowns and mechanical retention in the base region for the resin that would be placed to house the synthetic patrix retention caps. Both frameworks were fitted to the abutments and adjusted for axis-of-rotation movement with the use of a disclosing medium of chloroform and rouge and a ~10 power binocular microscope.’ Acrylic resin bases were added with a medium-viscosity rubber impression material forming an approximately 2 mm mucosal simulation as done in the first study. Autopolymerizing resin (Duralay, Reliance Dental Mfg. Co., Worth, Ill.) was then used to form the patrix housings for both frameworks with the black fabricating patrices in the kit according to the manufacturer’s directions. By use of kit instrumentation and directions, the black patrices were removed and replaced with the light (white) retention, and later with the heavy (blue) retention synthetic retention caps. The thicker base on the black processing patrix attachment was designed to provide 0.4 mm of vertical resiliency and universal joint hinging for the retention elements. The light and heavy retention caps were selected for study to give reasonable boundaries in retention force. Metal loading cones were then attached to the extension bases in the first molar regions over the ridge crests, with custom surveyor orientation matrices and multiple measurements used to replicate load-cone positions from the
NOVEMBER1992
VOLUME68
NUMBER5
LOAD
TRANSFER
DESIGN
FOR MAXILLARY
RPD
Retention Cap Color Code: White - light Orange - moderate Blue - heavy Grey - very heavy
Retention Ring Pattern - cast with crown
Fig. 3. Attachment-retained prosthesis without supporting rests, shown seated on maxillary model and crowns without splinting, was also tested with abutments splinted.
Fig. 1. ERA attachment parts include plastic ring pattern cast with abutment crown and choices of synthetic retention caps.
Fig. 4. Attachment-retained prosthesis with support rests, shown seated in place and with abutment splinting, was also tested without abutments splinted. Fig. 2. The photoelastic maxillary model has crowns cemented and includes attachment rings. There is provision for removable, rigid abutment splints secured with screws.
first study for comparison (Figs. 3 and 4). Both resin patrix housings were relieved of binding spots against the abutment crowns by loading of the extension bases and use of rouge disclosing medium. The model was bolted into an oil-immersion dish containing mineral oil to improve optical transmission properties. The dish was secured to the center of the straining frame between the light source and camera by use of recorded measurements from the first study to standardize the model during rotation of the straining frame. The light source and camera included the same polarizers and quarter-wave plates used in the first study.l The rests and minor connector struts were removed from one of the frameworks after completion of the total assembly. This resulted in one prosthesis with no supporting rests and the other with rests. Both configurations could be tested with and without abutment splinting (Figs. 3 and 4).
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Loads of 30 lb (13.64 kg) were directed against each load cone1 in the vertical, buccal, lingual, mesial, and distal directions with a load cell monitored with an XY recorder. The model was viewed from four directions for a total of 40 observations for each of the design configurations of (1) light retention, no rests, with and without splints; (2) light retention with rests, with and without splints; (3) heavy retention, no rests, with and without splints; and (4) heavy retention with rests, with and without splints. The results were photographed.
RESULTS Examination of the model before and after placement of the two frameworks confirmed that each framework seated passively with essentially no stresses. Therefore, all isochromatic fringes seen later within the model were the result of applied loads. Schematic representations of stress intensity are included with the selected photographic evidence to facilitate presentation and interpretation of the data. The diagrams do not include actual isochromatic fringe lines.
785
BERG
AND
CAPUTO
Fig. 5. View of left canine and premolar shows complex, high-level stresses with light retention, no rests, and no abutment splinting. Schematic drawings accompanying the photoelastic pictures represent stress distribution regions: blue for low-level, yellow for moderate, and red for high-level stresses. Black arrow indicates load direction. Fig. 6. Left canine and premolar with prosthesis using light retention under a vertical load show less stress with addition of rests and abutment splinting. White dots in schematics represent occlusal and cingulum rests. Red arrow indicates splinting. Fig. ‘7. View of left canine and premolar shows complex and some high-level stress with light retention attachment prosthesis lacking rests and splints under a buccally directed load. Fig. 8. Stress levels with left canine and premolar were reduced with addition of rests and abutment splinting to prosthesis under a buccally directed load.
Areas represented in blue represent low stress in the order of one fringe. Yellow areas are moderate stress areas in the range of two to three fringes. Red areas indicate the highest order of stress in the range of four or more fringes. A stress distribution that features apical loading with an even distribution of a low order would be most favorable for clinical applications. Higher orders of fringes and closer fringes indicate a tendency for unwanted biologic changes. Different stress effects were observed for the ERA attachment with and without supporting rests, with and without splinting of the abutments, and with light and then heavy retention. Observations also were made with direct loads of 15 lb (6.82 kg) to the right canine and to the lingual cusp of the left first premolar without a prosthesis in
186
place for comparison purposes. Stresses with all loadings again followed the three facial stress trajectories and demonstrated varying degrees of stress in the tuberosity area, consistent with the first study.’
Vertical
Ioads
Vertical loads to both sides resulted in the most uniform and apical stresses with the configuration that included supporting rests and splinted abutments. Stress patterns involving the right canine were all in the low fringe order. Rests more than splinting improved right canine stress patterns. The change from light to heavy retention increased stress for the left premolar in the no-rest, no-splint configuration from a moderate two fringes to an upper moderate range of three fringes and included patterns sug-
NOVEMBER
1992
VOLUME
68
NUMBER
5
LOAD
TRANSFER
DESIGN
FOR MAXILLARY
RPD
Fig. 9. View of right canine shows severe and complex stresses resulting from distally directed load with lreavy retention, no rests, and no abutment splinting. Fig. 10. Stress to the right canine is reduced with the addition of rests and splinting. Moderate stress remains in bone adjacent and distal to abutment. Fig. 11. View of left canine and premolar indicates complex, widespread, and high-level stresses resulting from distally directed loading with light retention, no rests, and no splinting. Fig. 12. Stresses from distally directed loads are significantly reduced overall with addition of rests and abutment splinting even with the presence of heavy retention; however, high stress remains just distal to the left premolar. gestive of root bending. There was less cross-arch transfer of stress with splinting for either loaded side. Either rests or splints provided some improvement to the left premolar side with the most apical, equitdble stress distribution for both sides resulting from a combination of rests and splinting (Figs. 5 and 6). Direct loads without a prosthesis in place to either the incisal edge of the left canine or the lingual cusp tip of the left premolar resulted in low- to moderate-range stress. In comparison to these direct loadings, the attachment with rests, splinting, and vertical loading of the extension bases displayed similar levels of stress, but with the stress distribution more apical and with more equitable internal root involvement. Vertical loads with the attachment both splinted and with supporting rests were favorably comparable to vertical loads of the I-bar retained RPD from the previous study.i The premolar and canine on the left side
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
displayed low to moderate stress patterns more confined within the roots than with the I-bar retained RPD.l
Buccally
directed
loads
Right-side buccally directed loads with light retention showed twisting in the canine abutment without rests or splinting. Less stress in the canine was seen with rests and splinting, confining stress more to the apical area. There was no observable effect from splinting with heavy retention. However, rests did benefit stress distribution for this load direction. Left-side buccally directed loads with neither rests nor splinting resulted in severe stress on the premolar with vertically oriented, multiple fringes and included complex stress on the adjacent canine (Fig. 7). Splinting with no rests reduced stress and showed more cross-arch transfer to the right canine. There was also an improvement in stress
787
BERG
distribution with rests, even with heavy retention. Splinting, rests, and light retention resulted in the most equitable stress distribution (Fig. 8).
Lingually
directed
loads
Right-side loads with light retention resulted in no substantial differences with or without rests. Splinting produced slightly more cross-arch transfer to the left canine and premolar. Stress distributions were improved by splinting for heavy retention, with and without rests. Left-side loads with light retention displayed high stress to the premolar but less with the addition of rests. The stress patterns with the abutments splinted either with or without rests were similar. Therefore, rests or splints, .or both, helped. Heavy retention without rests and splinting resulted in complicated premolar root stress and stress in the mesial region of the adjacent canine root. The addition of rests resulted in substantially less, more uniform premolar stress and no involvement of the mesial area of the canine root. Splinting without rests also improved premolar stress distribution but the mesial area of the canine root remained involved.
Mesially
directed
loads
Splinting was more helpful than rests for the canine abutment under right-side mesially directed loads with light retention. Bending in the canine with heavy retention was much relieved with splinting. Rests adversely affected the canine without splinting for this direction of loading. Splinted versions with or without rests were similar. Left-side loads with light retention and without rests or splinting displayed more axial stresses than with the addition of rests. Rests improved the stress distribution over the premolar abutment and shifted more stress to the adjacent canine, even when the abutments were not splinted. Splinting and light retention resulted in more cross-arch transfer and improved the stress distribution with or without rests. Heavy retention displayed a somewhat different result; the version including rests and splinting did better than rests or splinting alone. Heavy retention resulted in higher and more complex stress patterns compared with light retention, as with loads in other directions.
Distally
directed
loads
Right-side distally directed loads with light retention displayed high stress and bending in the canine abutment (Fig. 9). This stress intensity was reduced and more apically located with the presence of rests or splinting. Hsavy retention caused canine bending, including cervical bone area involvement, with high stress in the three- and four-fringe range. The stress distributions were improved with the addition of splinting and even more so with rests. The most equitable transfer of load resulted from a combination of rests and splinting (Fig. 10). Although rests and splinting significantly improved the stress condition, the result was not as favorable as that observed in a previous
788
AND
CAPUTO
study with an I-bar retained RPD under the same load conditi0n.l Left-side distally directed loads with light retention resulted in multiple fringes of stress in the premolar that included the periodontal membrane area and stress configurations suggestive of twisting (Fig. 11). The premolar with rests displayed less severe stress with some of the stress shifted to the adjacent canine. Splinting with light retention helped more than rests when loading was on the left side. The presence of splinting was as effective as the combination of rests and splinting. Heavy retention resulted in similar stresses with an increase in severity. The best distribution with left-side loading resulted from a combination of rests and splinting, although high-level stress remained at the distal cervical-bone support region of the premolar abutment (Fig. 12). Rests and splinting significantly improved the stress condition, resulting in a stress state that was somewhat more favorable than that observed in a previous study with an I-bar retained RPD under the same load conditi0n.i
DISCUSSION The use of heavy retention elements in this study created significantly higher stresses than light retention elements for any given configuration of prosthesis, rests, and abutment splinting. Given the often high and severe stress (schematically shown as red areas equal to four or more fringes) observed with heavy retention in this study, it seems clinically preferable to avoid the heavy and very heavy retention elements. Clinically it seems preferable to change the retention elements more often than to risk the high stress levels of heavier, but possibly longer lasting, retention. Another variable would be introduced if the metal ring portion of the attachment was finished with excessive polishing. The combination of supporting rests on the abutment teeth and splinting of the abutments significantly and consistently improved stress distributions. The use of rests or abutment splinting alone did not improve stress distribution for all the combinations of loading directions and abutments. Heavy retention created more stress than light retention for a given configuration. However, heavy retention with the addition of rests and abutment splinting often created less stress than light retention without rests and splinting. The most equitable stress distributions were achieved with light retention and the addition of supporting rests and abutment splinting. The provision of rests in the anterior part of the maxillary arch can be difficult because of occlusal relationships and tooth anatomy. The benefit from supporting rests outweighs the possible difficulty of providing occlusal or cingulum rests on the crown or splinted unit. Vertical loads on the left side produced high stress in the distal region of the premolar root when rests or splinting were not used. This suggests that the 0.4 mm vertical relief space alone may be inadequate for abutment protection. Rests will likely fur-
NOVEMBER
1992
VOLUME
68
NUMBER
5
LOAD
TRANSFER
DESIGN
FOR MAXILLARY
RPD
ther enhance occlusal stability and masticatory performance. Mushimoto et al3 found significant improvement in masticatory movements by using RPDs with occlusal rests in contrast to RPDs without supporting rests. Rests also permit evaluation of the need for relining and provide a relative position for the attachment elements. The minor connectors associated with the rests allow more precise placement of the retention element, lessening the possibility of damage to the synthetic retention caps. Splinting of abutments adds expense and complexity to an attachment RPD. However, the results of this study suggest that, for a maxillary distal-extension base configuration, splinting may significant1.ycontribute to equitable stress distribution. For example, the right canine with its ample root support benefitted only from splinting when sustaining a mesially directed load. SUMMARY
AND
CONCLUSIONS
A model of a maxillary bilateral distal-extension removable partial denture was used to photoelastically demonstrate and assessthe stress-distributing characteristics of the ERA attachment. The study evaluated the attachment with the addition of supporting rests and abutment splinting. The results indicated the following. 1. Stress distribution was less apically oriented and significantly higher for the heavy retention element than for the light retention element.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
2. The most equitable distributions of stress occurred when the ERA attachment was used with light retention elements, supporting rests, and abutment splinting. 3. The attachment prosthesis with light retention, rests, and abutment splinting compared favorably in stress distribution with the maxillary I-bar retained partial denture in a previous study made on a comparable maxillae mode1.l Appreciationis extendedto Irene Petravicius,UCLA Schoolof Dentistry Illustration Department,for assistancewith preparation of graphicsand diagrams. REFERENCES 1. Berg T, Caputo AA. Comparison of load transfer by maxillary distalextension removable partial dentures with a spring-loaded plunger attachment and I-bar retainer. J Prosthet Dent 1992;68:500-7. 2. Stern ERA extracoronal attachment fabrication instructions. Attleboro, Mass; APM-Sterngold division of Stern Metals, Inc., 1986. 3. Mushimoto E, Kawani A, Ohi H, Tatsuta T, Mitani H. Functional significance of pressuresensation conveyedby removable partial dentures. Int J Prosthodont 1988;1:41-5.
Reprintrequests to: DR. THEODORE BERG REMOVABLE PROSTHODONTICSSECTION SCHOOL OF DENTISTRY UNIVERSITY OF CALIFORNIA Los ANGELES, CA 90024-1668
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