- Email: [email protected]
Anterior rests for maxillary removable partial dentures
Anterior rests for maxillary removable partial dentures Theodore Berg, Jr., D.D.S.,* and Angelo A. Caputo, Ph.D.** University of California, School of Dentistry, Los Angeles, Calif.
Because of the lingual slope and cingulum morphology of anterior maxillary teeth, conflicting choices are made in the selection and preparation of anterior teeth as rests for removable partial denture abutments. There is considerable controversy regarding what constitutes a secure, positive abutment rest seat, and a clinical consensus for guidelines and a proven rationale do not exist. I m m e d i a t e economic considerations m a y ' d i c t a t e a treatment plan in lieu of known principles. Certain guidelines for posterior rest placement in removable partial denture design have been demonstrated.l. 2 Their application to anterior rest situations leads to the following rationale for the test model design. Anterior maxillary teeth, by virtue of their crown and root orientations, tend to exert a cantilever effect on supporting bone and periodontal m e m b r a n e when receiving occlusal loads. T h e rest-seat form on anterior a b u t m e n t teeth should provide positive support to the rest of the removable partial denture without inducing tipping forces. Forces transmitted to the anterior a b u t m e n t tooth should be received axially. T h e rest for a distal-extension removable partial denture should allow m o v e m e n t of the denture about the a b u t m e n t tooth, transmitting occlusal forces from the rest seat along the axis of the a b u t m e n t tooth. A positive anterior rest seat is also believed to minimize labial m o v e m e n t of the tooth away from a rest when the anterior tooth directly receives an incisal load on the incline of its lingual surface. This report is the first in a series designed to provide definite clinical information for the selecRead before the Pacific Coast Society of Prosthodontists, Portland, Ore. *Associate Clinical Professor. Removable Prosthodontics. **Professor and Chairman. Biomaterials Science.
0022-3913/78/0239-0139500.80/09 1978The G. V. MosbyCo.
tion, preparation, and use of maxillary anterior teeth as abutments for removable partial dentures. T h e study is designed to present comparative data on the effect of anterior root functions on periodontal supporting structures during load stress on the prosthesis and the teeth when different rest forms are used. INVESTIGATIVE DESIGN AND PROCEDURES A photoelastic replica of the maxillae was fabricated with individual simulants for the teeth (A type),* the periodontal m e m b r a n e (B type),t and the bony portions (C type).~ This included the m a l a r struts, inferior orbits, nasal and lacrimal regions, maxillary sinuses, and the a n a t o m i c sutures (D type)w (Figs. 1 and 2). This model permits the photoelastic technique of stress analysis, performed by loading the model and then observing and p h o t o g r a p h i n g the force patterns using polarizing filters. T h e quasi-three-dimensional model has the advantages of a n a t o m i c geometry and repeated loading capability for direct comparisons, but it has the disadvantage of incomplete threedimensional viewing and does not permit minute, detailed stress analysis. T h e maxillary right canine and the left central incisor were prepared to receive full crowns with various cingulum rest formations. T h e remaining three teeth received rest-seat preparations consisting of a cingulum rest seat on the left canine, a distoocclusal rest seat on the first premolar, and a mesioocclusal rest seat on the second molar. This study focuses on the use of the clinically proven cingulum rest. Its seat is designed as a smooth, rounded-out groove that exits in rope-pulley *PLM-1Z. Photolastic, Inc., Malvern, Pa. tSolithane, Thiokol Chemical Corp., Trenton. N. J. ~PL-2, Photolastic. Inc.. Malvern, Pa. w Photolastic, Inc., Malvern. Pa.
THE JOURNALOF PROSTHETICDENTISTRY 139
BERG A N D CAPUTO
Fig. 1. An entire maxillae arrangement was created in sculpture from average-value measurements of available skulls.
Fig. 2. The composite photoelastic maxillae replica was mounted on a resin base for fixation and uniform support on the straining frame. fashion to the lingual side (Fig. 33. O c c a s i o n a l l y formed in n a t u r a l t o o t h enamel, the c i n g u l u m crescent-groove rest seat is m o r e c o m m o n l y used in a variety of coronal castings. Studies c u r r e n t l y in progress involve o t h e r a n t e r i o r rest formations (Fig. 4). After the c i n g u l u m crescent-groove crowns h a d been cemented, a p a r t i a l d e n t u r e f r a m e w o r k was fabricated. T h e design i n c o r p o r a t e d positive rests on all teeth, vertical p r o x i m a l plates to c o n t a c t g u i d i n g plane p r e p a r a t i o n s on the teeth, a n d 0.010 inch retention c o n t a c t of infrabulge I - b a r retainers on the facial surfaces of the right canine a n d left second molar. T h e retainers were placed so as to be c o m p a t ible with the design's axis of r o t a t i o n for the distalextension base. C h l o r o f o r m a n d gold rouge disclosing m e d i u m was p a i n t e d on all f r a m e w o r k parts that t o u c h e d tooth structure, a n d the f r a m e w o r k was
140
Fig. 3 Crown castings with smooth, positive cingulum rests are in place on the right canine and central mcxsor. then m o v e d to s i m u l a t e posterior occlusal function. T h e i m p i n g i n g areas were disclosed a n d a d j u s t e d until the f r a m e w o r k p i v o t e d freely on the central rest seats of the two most distal rests, w h i c h c o n s t i t u t e d the axis of r o t a t i o n (Fig. 5). Acrylic resin bases were a t t a c h e d to the framework. To s i m u l a t e the s u p p o r t a n d resiliency of mucosa a u n i f o r m 2 m m layer of i n j e c t i o n - t y p e thiokol r u b b e r impression m a t e r i a l was a p p l i e d to the e d e n t u l o u s ridges. A m e t a l cone with sides m a c h i n e d to 45 degree angles was p l a c e d on the distal-extension first m o l a r region to receive both vertical a n d distobuccal loads. A m e t a l wedge with a p l a n e m a c h i n e d to a p p r o x i m a t e the left central incisor and right c a n i n e lingual slopes was p l a c e d in the a n t e r i o r base between these two teeth to receive anterior base incisal-type loads (Fig. 6 ) T h e photoelastic m a x i l l a e replica was m o u n t e d on a p l a t f o r m in the c e n t e r of a straining frame which could rotate 360 degrees. A fiber optic c a b l e delivered the light source to the central p a l a t a l area. A n a d a p t e r was m a d e for the e n d of the light c a b l e to house a diffuser, a p o l a r i z i n g lens, a n d a q u a r t e r wave plate. A n o t h e r q u a r t e r - w a v e p l a t e a n d polarizing filter were l o c a t e d i m m e d i a t e l y in front of the c a m e r a lens. T h e a m o u n t of load delivered was m o n i t o r e d b y a load cell* a n d r e c o r d e d on a n X - Y recorder (Fig. 7). Results were r e c o r d e d using 35 m m color still film in white light. E a c h loading p r o c e d u r e was r e p e a t e d three times. L o a d s of 30 p o u n d s were d e l i v e r e d to the following five locations: (1) vertical to the distal*Universal transducing cell. Statham Instruments. Inc.. Oxnard Calif.
FEBRUARY 1978
VOLUME 39
NUMBER 2
ANTERIOR RESTS FOR MAXILLARY REMOVABLE PARTIAL DENTURES
Fig. 4.Anterior rest design studies in progress include A, the lingual surface shelf rest, B, the lingual circular concave rest, C, the lingual cingulum crescent-groove rest, and D, the attachment rest. extension base, (2) at the mesiolingual incline of the molar cone (to impart a distobuccal force to the distal-extension base), (3) at the lingual incline of the right canine, (4) at the lingual incline of the anterior wedge (to impart a labial-vertical force to the anterior base), and (5) at the lingual incline of the left central incisor. The results were recorded at these four positions: (1) the right canine region, (2) the central incisor and premaxillary regions, (3) the left canine and first premolar regions, and (4) the left second molar region (Fig. 8). The test loadings were instituted at one of the five loading positions and the resulting stresses were recorded at each of the four observation areas. The sequence was repeated for each of the given loading point s. The proximal plates for guiding-plane contacts on the teeth were then removed from the framework, leaving contact only on the rests and the two retainers. The entire test sequence was then repeated.
Fig. 5. The framework is adjusted to remove the small regions of resistance to functional movement of the distalextension base about the axis of rotation.
RESULTS Control photographs were made of the model without and with the unloaded removable partial denture in place. No changes were noted and the removable partial denture was deemed passive while unloaded and in place on the model (Fig. 9). Loads applied vertically t o t h e distal-extension base. Vertical occlusal loading to the distal-extension base resulted in the immediate a b u t m e n t (the maxillary right canine) alone receiving the stress
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 6. The removable partial denture is ready for testing after placement of acrylic resin bases, mucosa simulant, and loading cones.
141
BERG AND CAPUTO
m
Model--
S
Fiber Optic Light Source
Birefringent
Pleetio
d
Oiffueer
Fig. 7. The schematic drawing shows the arrangement of the light, lens, and camera.
Fig. 8A. All regions of the model could be viewed when loading a tooth or the removable partial denture. (Fig. 10). No significant stress was observed at the other teeth or structures, with or without proximal plates, except for slight changes at the second molar on the opposite side. The canine received predominately vertical loading, but this stress appeared to be more intense when the proximal plates were removed. With a positive rest and with physiologic adjustment of the framework, the canine was able to sustain vertical loading of the distal-extension base,
142
showing low-level vertical-type stress with little effect on other teeth. Loads applied distobuccally to the distal-exten, sion base. A distobuccal occlusal force on the distalextension base created moderately high stress (two or three fringes) with a large distal horizontal component on the adjacent right canine abutment. The occlusal half of the canine root was invoived, and radiations formed from the alveolar crests. Low-level
FEBRUARY 1978
VOLUME 39
NUMBER 2
ANTERIOR RESTS FOR MAXILLARY REMOVABLE PARTIAL DENTURES
LOADING
POINTS
A. Posterior ExtensionVertical S. Extension-Oistobuoosl C . R i g h t C u s p i d -- L i n g u a l D. Anterior BaseLingual Incline S. Left Center IncisorLingual VIEWS I Right Cuspid 2. Left Central 3. Left Cuspid 4. Left Second
1
9
"
S P9
w
Incisor Bicuspid Molar
0 3
-
/~/~o~
o~
Oenture
I \ Framework Fig. 8B. Diagrammatic summary of the loading points and the teeth tested for stress.
Fig. 9. The control photographs with and without the partial denture in place indicate the passivity of the casting and the appearance of a given region with no force applied.
Fig. 10. A vertical load applied to the distal-extension bases affected mostly the root periphery of the adjacent right canine. A N S is the anterior nasal spine; S I N U S is the right maxillary sinus; and 1 is the area of low stress.
stress was noted at all of the other tooth regions. T h e stress greatly intensified in the right canine when the proximal plates were removed (Fig. 11). Alveolar crest patterns increased to indicate high stress (three fringes) and very high stress (five or more fringes) surrounding the entire root. M o d e r a t e stress of a distal pulling nature was seen on the central incisor. The stress in the central incisor region was reduced when the proximal plates were removed. Distobuccally directed loads on the extension base induced heavy stress on the immediate a b u t m e n t , and the stress markedly increased when proximal plates were not present. Loads applied directly to the a b u t m e n t for the distal-extension base. W h e n loading was applied
directly on the lingual surface of the right canine, it sustained moderate stress and fringes were noted as far as the adjacent inferior nasal rim (Fig. 12). Lowto-moderate stress (one or two fringes) was evident on all other a b u t m e n t teeth regions a n d remained constant when the proximal plates were removed (Fig. 13). T h e positive rests and molar retainer contributed to restraining the canine. Loads applied to the anterior d e n t u r e base. Loading of the anterior denture base induced stress in all a b u t m e n t teeth whether or not proximal plates were used. W i t h proximal plates present the framework tended to lift off the molar a b u t m e n t slightly as the anterior base was increasingly loaded. M o d e r a t e stress (one or two fringes) radiated high above the
THE JOURNAL OF PROSTHETIC DENTISTRY
143
BERG AND CAPUTO
Fig. 11. Distobuccal loads applied to the extension base induced high stress in the adjacent canine region which intensified greatly when the proximal plates were removed. A N S is the anterior nasal spine, S I N U S is the right maxillary sinus, 1 is low stress, 2 is moderate stress, 3 is moderately high stress, and 4 is high stress. Fig. 13. Loads applied to the right call ne resulted in lowto-moderate stress in all other arch positions as typified by the distal region of the left first premolar. A N S is the anterior nasal spine, S I N U S is the left maxillary sinus, 1 is low stress, and 2 is moderate stress.
Fig. 12. Occlusai lingual loads applied directly to the right canine induced moderate stress within the root periphery and adjacent supporting region. A N S is the anterior nasal spine, S I N U S is the right maxillary sinus, 1 is low stress, 2 is moderate stress, and 3 is moderately high stress. left canine a n d p r e m o l a r a n t e r i o r to the m a x i l l a r y sinus. W i t h o u t p r o x i m a l plates present the removable p a r t i a l d e n t u r e was d e c i d e d l y less stable, lifting far off the m o l a r a b u t m e n t . T h e stress increased significantly in the central incisor b u t only w i t h i n the root p e r i p h e r y , e m p h a s i z i n g the f u n c t i o n a l significance of the positive c i n g u l u m rests (Figs. 14 a n d i5). Loads
applied
directly
to t h e
central
incisor.
W h e n the central incisor was l o a d e d d i r e c t l y on the
144
Fig. 14. A view of the central incisor with loading applied to the adjacent base shows moderate stresses which concentrate in and about the root. A N S is the anterior nasal spine, 1 is low stress, and 2 is moderate stress. lingual surface, m o d e r a t e - t o - h i g h stress was genera t e d t h r o u g h o u t the root a n d into the s u r r o u n d i n g regions. T h e r e was a n o t i c e a b l e c o n c e n t r a t i o n of stress above the a p e x at the inferior nasal r i m (Fig. 16). M o d e r a t e - t o - h i g h stress was e v i d e n t on all o t h e r a b u t m e n t regions. T h e most r e m o t e r a d i a t i o n of
FEBRUARY 19"/8
VOLUME39
NUMBER 2
ANTERIOR RESTS FOR MAXILLARY REMOVABLE PARTIAL DENTURES
Fig. 16. Lingual loading applied directly to the central incisor resulted in stress at the inferior nasal rim. A N S is the anterior nasal rim, I is low stress, 2 is moderate stress, and 3 is moderately high stress.
Fig. 15. Moderate stress is seen as far posteriorly as the second molar when the anterior base is loaded. S I N U S is the left maxillary sinus and I is low stress. stress reached a b o v e a n d in front of the m a x i l l a r y sinus (Fig. 17). R e m o v a l of p r o x i m a l plates m a y have resulted in some stress r e d u c t i o n or at least greater stress u n i f o r m i t y in the central incisor itself, b u t r e m o v a l h a d no a p p r e c i a b l e effect elsewhere. Loads a p p l i e d to the central incisor were t r a n s m i t t e d to other regions of the arch as h i g h e r orders of observed stress t h a n the loads a p p l i e d to the right canine. DISCUSSION AND CONCLUSIONS This initial test series used positive c i n g u l u m crescent-shaped rests on the a n t e r i o r teeth. T h e horizontal force c o m p o n e n t s , p a r t i c u l a r l y in anteroposterior directions, p r o d u c e d the most widely dispersed a n d most intense stress patterns. L o a d s to the a n t e r i o r base a n d the c e n t r a l incisor p r o d u c e d the greatest transmission of stress to o t h e r c o m p o nents in the arch. D i s t o b u c c a l l y d i r e c t e d loads on the extension base affected the i m m e d i a t e c a n i n e a b u t m e n t most severely. These findings w o u l d seem to s u p p o r t the clinical practice of l i m i t i n g extension-base occlusion to the centric position, where a n t e r i o r tooth g u i d a n c e is available. P r o x i m a l plates give gross s t a b i l i t y to the r e m o v a b l e p a r t i a l d e n t u r e a n d in some loadings c o n t r i b u t e to transmission of stress a r o u n d or across the arch, t h e r e b y lessening stress to a n y one region.
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 17. Stress fringes are seen throughout the lateral nasal region forward of the maxillary sinus and above the left canine and premolar with loads applied to the central incisor. A N S is the anterior nasal spine, O R B I T is the infraorbital rim of the zygomatic bone, I is low stress, 2 is moderate stress, and 3 is moderately high stress. This was p a r t i c u l a r l y true for the isolated canine a b u t m e n t w h e n it was affected b y l o a d i n g s on the distal-extension base. Positive c i n g u l u m rests of the t y p e t h a t were tested function as direct t r a n s m i t t e r s of force to the lingual inclines of the a n t e r i o r teeth a n d to a n t e r i o r toothborne bases. Positive rests t h r o u g h o u t the arch are significant in restraining the a n t e r i o r teeth. T h e central incisor benefits s o m e w h a t m o r e t h a n the canine b y this stabilization. Stress in all a b u t m e n t teeth indicates a s u p p o r t i v e
145
BERG AND CAPUTO
function for a n a n t e r i o r base from o t h e r parts o f the r e m o v a b l e p a r t i a l denture. T h i s a n d the value of the a d j a c e n t rests could a t t a i n m a j o r significance w h e n the clinical p r o b l e m of h a v i n g to i n c l u d e unfavorable a n t e r i o r a b u t m e n t s in a r e m o v a b l e p a r t i a l d e n t u r e design is faced. These factors suggest the m a j o r i m p o r t a n c e o f good positive rests a n d a n extensive f r a m e w o r k s u p p o r t w h e n d e a l i n g with the clinical p r o b l e m of stabilizing a n d controlling the position of m o b i l e or p e r i o d o n t a l l y w e a k e n e d a n t e r i o r teeth.
a n d p l a c e m e n t . Guidelines for using the n a t u r a l crown as o p p o s e d to a positive rest seat in a restoration are needed. C o n t i n u i n g studies of o t h e r a n t e r i o r rest forms should increasingly clarify this phase o f treatment planning. T h e dentist should recognize the need for structural integrity in the a n t e r i o r m a x i l l a r y s e g m e n t when designing r e m o v a b l e p a r t i a l dentures. A practical, defensible r a t i o n a l e for the r o u t i n e use of m a x i l l a r y a n t e r i o r teeth as r e m o v a b l e p a r t i a l d e n t u r e a b u t m e n t s m u s t be established.
SUMMARY
Acknowledgment is made to Mark Koenig, who fabricated the test apparatus. We would like to thank Dr. K. Kapur, Chief of Dental Service, Veterans Administration Hospital, Sepulveda, Calif., for allowing us the use of laboratory facilities.
Positive a n t e r i o r rests were shown to be a n essential e l e m e n t of design w h e n considering stress distrib u t i o n with a r e m o v a b l e p a r t i a l d e n t u r e involving a n t e r i o r teeth. P r o x i m a l plates often a d d e d s t a b i l i t y a n d r e d u c e d stress a c c u m u l a t i o n in a n y one region. Distobuccal forces on the extension base adversely affected the p r i m a r y c a n i n e a b u t m e n t . Forces in the regions of the a n t e r i o r base a n d c e n t r a l incisor a b u t m e n t p r o d u c e d the most w i d e s p r e a d transmission of stress t h r o u g h o u t the arch. G r e a t e r stress t h r o u g h o u t the a r c h was observed w h e n forces were a p p l i e d to t h e central incisor r a t h e r t h a n to the right canine a b u t m e n t . T h e r e is a need for established criteria of rest form
146
REFERENCES 1. Kratochvil, F. J., and Caputo, A. A.: Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J PROSTHETD~NT 32:52, 1974. 2. Kratochvil, F. J.: Maintaining supporting structures with a removable partial prosthesis. J PROSTH~T DRNT 25:167, 1971. Reprint requests to:
DR. THEODOREBERt UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY
Los A.~GELES,CALIF.90024
FEBRUARY1978 VOLUME39
NUMBER2
Because of the lingual slope and cingulum morphology of anterior maxillary teeth, conflicting choices are made in the selection and preparation of anterior teeth as rests for removable partial denture abutments. There is considerable controversy regarding what constitutes a secure, positive abutment rest seat, and a clinical consensus for guidelines and a proven rationale do not exist. I m m e d i a t e economic considerations m a y ' d i c t a t e a treatment plan in lieu of known principles. Certain guidelines for posterior rest placement in removable partial denture design have been demonstrated.l. 2 Their application to anterior rest situations leads to the following rationale for the test model design. Anterior maxillary teeth, by virtue of their crown and root orientations, tend to exert a cantilever effect on supporting bone and periodontal m e m b r a n e when receiving occlusal loads. T h e rest-seat form on anterior a b u t m e n t teeth should provide positive support to the rest of the removable partial denture without inducing tipping forces. Forces transmitted to the anterior a b u t m e n t tooth should be received axially. T h e rest for a distal-extension removable partial denture should allow m o v e m e n t of the denture about the a b u t m e n t tooth, transmitting occlusal forces from the rest seat along the axis of the a b u t m e n t tooth. A positive anterior rest seat is also believed to minimize labial m o v e m e n t of the tooth away from a rest when the anterior tooth directly receives an incisal load on the incline of its lingual surface. This report is the first in a series designed to provide definite clinical information for the selecRead before the Pacific Coast Society of Prosthodontists, Portland, Ore. *Associate Clinical Professor. Removable Prosthodontics. **Professor and Chairman. Biomaterials Science.
0022-3913/78/0239-0139500.80/09 1978The G. V. MosbyCo.
tion, preparation, and use of maxillary anterior teeth as abutments for removable partial dentures. T h e study is designed to present comparative data on the effect of anterior root functions on periodontal supporting structures during load stress on the prosthesis and the teeth when different rest forms are used. INVESTIGATIVE DESIGN AND PROCEDURES A photoelastic replica of the maxillae was fabricated with individual simulants for the teeth (A type),* the periodontal m e m b r a n e (B type),t and the bony portions (C type).~ This included the m a l a r struts, inferior orbits, nasal and lacrimal regions, maxillary sinuses, and the a n a t o m i c sutures (D type)w (Figs. 1 and 2). This model permits the photoelastic technique of stress analysis, performed by loading the model and then observing and p h o t o g r a p h i n g the force patterns using polarizing filters. T h e quasi-three-dimensional model has the advantages of a n a t o m i c geometry and repeated loading capability for direct comparisons, but it has the disadvantage of incomplete threedimensional viewing and does not permit minute, detailed stress analysis. T h e maxillary right canine and the left central incisor were prepared to receive full crowns with various cingulum rest formations. T h e remaining three teeth received rest-seat preparations consisting of a cingulum rest seat on the left canine, a distoocclusal rest seat on the first premolar, and a mesioocclusal rest seat on the second molar. This study focuses on the use of the clinically proven cingulum rest. Its seat is designed as a smooth, rounded-out groove that exits in rope-pulley *PLM-1Z. Photolastic, Inc., Malvern, Pa. tSolithane, Thiokol Chemical Corp., Trenton. N. J. ~PL-2, Photolastic. Inc.. Malvern, Pa. w Photolastic, Inc., Malvern. Pa.
THE JOURNALOF PROSTHETICDENTISTRY 139
BERG A N D CAPUTO
Fig. 1. An entire maxillae arrangement was created in sculpture from average-value measurements of available skulls.
Fig. 2. The composite photoelastic maxillae replica was mounted on a resin base for fixation and uniform support on the straining frame. fashion to the lingual side (Fig. 33. O c c a s i o n a l l y formed in n a t u r a l t o o t h enamel, the c i n g u l u m crescent-groove rest seat is m o r e c o m m o n l y used in a variety of coronal castings. Studies c u r r e n t l y in progress involve o t h e r a n t e r i o r rest formations (Fig. 4). After the c i n g u l u m crescent-groove crowns h a d been cemented, a p a r t i a l d e n t u r e f r a m e w o r k was fabricated. T h e design i n c o r p o r a t e d positive rests on all teeth, vertical p r o x i m a l plates to c o n t a c t g u i d i n g plane p r e p a r a t i o n s on the teeth, a n d 0.010 inch retention c o n t a c t of infrabulge I - b a r retainers on the facial surfaces of the right canine a n d left second molar. T h e retainers were placed so as to be c o m p a t ible with the design's axis of r o t a t i o n for the distalextension base. C h l o r o f o r m a n d gold rouge disclosing m e d i u m was p a i n t e d on all f r a m e w o r k parts that t o u c h e d tooth structure, a n d the f r a m e w o r k was
140
Fig. 3 Crown castings with smooth, positive cingulum rests are in place on the right canine and central mcxsor. then m o v e d to s i m u l a t e posterior occlusal function. T h e i m p i n g i n g areas were disclosed a n d a d j u s t e d until the f r a m e w o r k p i v o t e d freely on the central rest seats of the two most distal rests, w h i c h c o n s t i t u t e d the axis of r o t a t i o n (Fig. 5). Acrylic resin bases were a t t a c h e d to the framework. To s i m u l a t e the s u p p o r t a n d resiliency of mucosa a u n i f o r m 2 m m layer of i n j e c t i o n - t y p e thiokol r u b b e r impression m a t e r i a l was a p p l i e d to the e d e n t u l o u s ridges. A m e t a l cone with sides m a c h i n e d to 45 degree angles was p l a c e d on the distal-extension first m o l a r region to receive both vertical a n d distobuccal loads. A m e t a l wedge with a p l a n e m a c h i n e d to a p p r o x i m a t e the left central incisor and right c a n i n e lingual slopes was p l a c e d in the a n t e r i o r base between these two teeth to receive anterior base incisal-type loads (Fig. 6 ) T h e photoelastic m a x i l l a e replica was m o u n t e d on a p l a t f o r m in the c e n t e r of a straining frame which could rotate 360 degrees. A fiber optic c a b l e delivered the light source to the central p a l a t a l area. A n a d a p t e r was m a d e for the e n d of the light c a b l e to house a diffuser, a p o l a r i z i n g lens, a n d a q u a r t e r wave plate. A n o t h e r q u a r t e r - w a v e p l a t e a n d polarizing filter were l o c a t e d i m m e d i a t e l y in front of the c a m e r a lens. T h e a m o u n t of load delivered was m o n i t o r e d b y a load cell* a n d r e c o r d e d on a n X - Y recorder (Fig. 7). Results were r e c o r d e d using 35 m m color still film in white light. E a c h loading p r o c e d u r e was r e p e a t e d three times. L o a d s of 30 p o u n d s were d e l i v e r e d to the following five locations: (1) vertical to the distal*Universal transducing cell. Statham Instruments. Inc.. Oxnard Calif.
FEBRUARY 1978
VOLUME 39
NUMBER 2
ANTERIOR RESTS FOR MAXILLARY REMOVABLE PARTIAL DENTURES
Fig. 4.Anterior rest design studies in progress include A, the lingual surface shelf rest, B, the lingual circular concave rest, C, the lingual cingulum crescent-groove rest, and D, the attachment rest. extension base, (2) at the mesiolingual incline of the molar cone (to impart a distobuccal force to the distal-extension base), (3) at the lingual incline of the right canine, (4) at the lingual incline of the anterior wedge (to impart a labial-vertical force to the anterior base), and (5) at the lingual incline of the left central incisor. The results were recorded at these four positions: (1) the right canine region, (2) the central incisor and premaxillary regions, (3) the left canine and first premolar regions, and (4) the left second molar region (Fig. 8). The test loadings were instituted at one of the five loading positions and the resulting stresses were recorded at each of the four observation areas. The sequence was repeated for each of the given loading point s. The proximal plates for guiding-plane contacts on the teeth were then removed from the framework, leaving contact only on the rests and the two retainers. The entire test sequence was then repeated.
Fig. 5. The framework is adjusted to remove the small regions of resistance to functional movement of the distalextension base about the axis of rotation.
RESULTS Control photographs were made of the model without and with the unloaded removable partial denture in place. No changes were noted and the removable partial denture was deemed passive while unloaded and in place on the model (Fig. 9). Loads applied vertically t o t h e distal-extension base. Vertical occlusal loading to the distal-extension base resulted in the immediate a b u t m e n t (the maxillary right canine) alone receiving the stress
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 6. The removable partial denture is ready for testing after placement of acrylic resin bases, mucosa simulant, and loading cones.
141
BERG AND CAPUTO
m
Model--
S
Fiber Optic Light Source
Birefringent
Pleetio
d
Oiffueer
Fig. 7. The schematic drawing shows the arrangement of the light, lens, and camera.
Fig. 8A. All regions of the model could be viewed when loading a tooth or the removable partial denture. (Fig. 10). No significant stress was observed at the other teeth or structures, with or without proximal plates, except for slight changes at the second molar on the opposite side. The canine received predominately vertical loading, but this stress appeared to be more intense when the proximal plates were removed. With a positive rest and with physiologic adjustment of the framework, the canine was able to sustain vertical loading of the distal-extension base,
142
showing low-level vertical-type stress with little effect on other teeth. Loads applied distobuccally to the distal-exten, sion base. A distobuccal occlusal force on the distalextension base created moderately high stress (two or three fringes) with a large distal horizontal component on the adjacent right canine abutment. The occlusal half of the canine root was invoived, and radiations formed from the alveolar crests. Low-level
FEBRUARY 1978
VOLUME 39
NUMBER 2
ANTERIOR RESTS FOR MAXILLARY REMOVABLE PARTIAL DENTURES
LOADING
POINTS
A. Posterior ExtensionVertical S. Extension-Oistobuoosl C . R i g h t C u s p i d -- L i n g u a l D. Anterior BaseLingual Incline S. Left Center IncisorLingual VIEWS I Right Cuspid 2. Left Central 3. Left Cuspid 4. Left Second
1
9
"
S P9
w
Incisor Bicuspid Molar
0 3
-
/~/~o~
o~
Oenture
I \ Framework Fig. 8B. Diagrammatic summary of the loading points and the teeth tested for stress.
Fig. 9. The control photographs with and without the partial denture in place indicate the passivity of the casting and the appearance of a given region with no force applied.
Fig. 10. A vertical load applied to the distal-extension bases affected mostly the root periphery of the adjacent right canine. A N S is the anterior nasal spine; S I N U S is the right maxillary sinus; and 1 is the area of low stress.
stress was noted at all of the other tooth regions. T h e stress greatly intensified in the right canine when the proximal plates were removed (Fig. 11). Alveolar crest patterns increased to indicate high stress (three fringes) and very high stress (five or more fringes) surrounding the entire root. M o d e r a t e stress of a distal pulling nature was seen on the central incisor. The stress in the central incisor region was reduced when the proximal plates were removed. Distobuccally directed loads on the extension base induced heavy stress on the immediate a b u t m e n t , and the stress markedly increased when proximal plates were not present. Loads applied directly to the a b u t m e n t for the distal-extension base. W h e n loading was applied
directly on the lingual surface of the right canine, it sustained moderate stress and fringes were noted as far as the adjacent inferior nasal rim (Fig. 12). Lowto-moderate stress (one or two fringes) was evident on all other a b u t m e n t teeth regions a n d remained constant when the proximal plates were removed (Fig. 13). T h e positive rests and molar retainer contributed to restraining the canine. Loads applied to the anterior d e n t u r e base. Loading of the anterior denture base induced stress in all a b u t m e n t teeth whether or not proximal plates were used. W i t h proximal plates present the framework tended to lift off the molar a b u t m e n t slightly as the anterior base was increasingly loaded. M o d e r a t e stress (one or two fringes) radiated high above the
THE JOURNAL OF PROSTHETIC DENTISTRY
143
BERG AND CAPUTO
Fig. 11. Distobuccal loads applied to the extension base induced high stress in the adjacent canine region which intensified greatly when the proximal plates were removed. A N S is the anterior nasal spine, S I N U S is the right maxillary sinus, 1 is low stress, 2 is moderate stress, 3 is moderately high stress, and 4 is high stress. Fig. 13. Loads applied to the right call ne resulted in lowto-moderate stress in all other arch positions as typified by the distal region of the left first premolar. A N S is the anterior nasal spine, S I N U S is the left maxillary sinus, 1 is low stress, and 2 is moderate stress.
Fig. 12. Occlusai lingual loads applied directly to the right canine induced moderate stress within the root periphery and adjacent supporting region. A N S is the anterior nasal spine, S I N U S is the right maxillary sinus, 1 is low stress, 2 is moderate stress, and 3 is moderately high stress. left canine a n d p r e m o l a r a n t e r i o r to the m a x i l l a r y sinus. W i t h o u t p r o x i m a l plates present the removable p a r t i a l d e n t u r e was d e c i d e d l y less stable, lifting far off the m o l a r a b u t m e n t . T h e stress increased significantly in the central incisor b u t only w i t h i n the root p e r i p h e r y , e m p h a s i z i n g the f u n c t i o n a l significance of the positive c i n g u l u m rests (Figs. 14 a n d i5). Loads
applied
directly
to t h e
central
incisor.
W h e n the central incisor was l o a d e d d i r e c t l y on the
144
Fig. 14. A view of the central incisor with loading applied to the adjacent base shows moderate stresses which concentrate in and about the root. A N S is the anterior nasal spine, 1 is low stress, and 2 is moderate stress. lingual surface, m o d e r a t e - t o - h i g h stress was genera t e d t h r o u g h o u t the root a n d into the s u r r o u n d i n g regions. T h e r e was a n o t i c e a b l e c o n c e n t r a t i o n of stress above the a p e x at the inferior nasal r i m (Fig. 16). M o d e r a t e - t o - h i g h stress was e v i d e n t on all o t h e r a b u t m e n t regions. T h e most r e m o t e r a d i a t i o n of
FEBRUARY 19"/8
VOLUME39
NUMBER 2
ANTERIOR RESTS FOR MAXILLARY REMOVABLE PARTIAL DENTURES
Fig. 16. Lingual loading applied directly to the central incisor resulted in stress at the inferior nasal rim. A N S is the anterior nasal rim, I is low stress, 2 is moderate stress, and 3 is moderately high stress.
Fig. 15. Moderate stress is seen as far posteriorly as the second molar when the anterior base is loaded. S I N U S is the left maxillary sinus and I is low stress. stress reached a b o v e a n d in front of the m a x i l l a r y sinus (Fig. 17). R e m o v a l of p r o x i m a l plates m a y have resulted in some stress r e d u c t i o n or at least greater stress u n i f o r m i t y in the central incisor itself, b u t r e m o v a l h a d no a p p r e c i a b l e effect elsewhere. Loads a p p l i e d to the central incisor were t r a n s m i t t e d to other regions of the arch as h i g h e r orders of observed stress t h a n the loads a p p l i e d to the right canine. DISCUSSION AND CONCLUSIONS This initial test series used positive c i n g u l u m crescent-shaped rests on the a n t e r i o r teeth. T h e horizontal force c o m p o n e n t s , p a r t i c u l a r l y in anteroposterior directions, p r o d u c e d the most widely dispersed a n d most intense stress patterns. L o a d s to the a n t e r i o r base a n d the c e n t r a l incisor p r o d u c e d the greatest transmission of stress to o t h e r c o m p o nents in the arch. D i s t o b u c c a l l y d i r e c t e d loads on the extension base affected the i m m e d i a t e c a n i n e a b u t m e n t most severely. These findings w o u l d seem to s u p p o r t the clinical practice of l i m i t i n g extension-base occlusion to the centric position, where a n t e r i o r tooth g u i d a n c e is available. P r o x i m a l plates give gross s t a b i l i t y to the r e m o v a b l e p a r t i a l d e n t u r e a n d in some loadings c o n t r i b u t e to transmission of stress a r o u n d or across the arch, t h e r e b y lessening stress to a n y one region.
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 17. Stress fringes are seen throughout the lateral nasal region forward of the maxillary sinus and above the left canine and premolar with loads applied to the central incisor. A N S is the anterior nasal spine, O R B I T is the infraorbital rim of the zygomatic bone, I is low stress, 2 is moderate stress, and 3 is moderately high stress. This was p a r t i c u l a r l y true for the isolated canine a b u t m e n t w h e n it was affected b y l o a d i n g s on the distal-extension base. Positive c i n g u l u m rests of the t y p e t h a t were tested function as direct t r a n s m i t t e r s of force to the lingual inclines of the a n t e r i o r teeth a n d to a n t e r i o r toothborne bases. Positive rests t h r o u g h o u t the arch are significant in restraining the a n t e r i o r teeth. T h e central incisor benefits s o m e w h a t m o r e t h a n the canine b y this stabilization. Stress in all a b u t m e n t teeth indicates a s u p p o r t i v e
145
BERG AND CAPUTO
function for a n a n t e r i o r base from o t h e r parts o f the r e m o v a b l e p a r t i a l denture. T h i s a n d the value of the a d j a c e n t rests could a t t a i n m a j o r significance w h e n the clinical p r o b l e m of h a v i n g to i n c l u d e unfavorable a n t e r i o r a b u t m e n t s in a r e m o v a b l e p a r t i a l d e n t u r e design is faced. These factors suggest the m a j o r i m p o r t a n c e o f good positive rests a n d a n extensive f r a m e w o r k s u p p o r t w h e n d e a l i n g with the clinical p r o b l e m of stabilizing a n d controlling the position of m o b i l e or p e r i o d o n t a l l y w e a k e n e d a n t e r i o r teeth.
a n d p l a c e m e n t . Guidelines for using the n a t u r a l crown as o p p o s e d to a positive rest seat in a restoration are needed. C o n t i n u i n g studies of o t h e r a n t e r i o r rest forms should increasingly clarify this phase o f treatment planning. T h e dentist should recognize the need for structural integrity in the a n t e r i o r m a x i l l a r y s e g m e n t when designing r e m o v a b l e p a r t i a l dentures. A practical, defensible r a t i o n a l e for the r o u t i n e use of m a x i l l a r y a n t e r i o r teeth as r e m o v a b l e p a r t i a l d e n t u r e a b u t m e n t s m u s t be established.
SUMMARY
Acknowledgment is made to Mark Koenig, who fabricated the test apparatus. We would like to thank Dr. K. Kapur, Chief of Dental Service, Veterans Administration Hospital, Sepulveda, Calif., for allowing us the use of laboratory facilities.
Positive a n t e r i o r rests were shown to be a n essential e l e m e n t of design w h e n considering stress distrib u t i o n with a r e m o v a b l e p a r t i a l d e n t u r e involving a n t e r i o r teeth. P r o x i m a l plates often a d d e d s t a b i l i t y a n d r e d u c e d stress a c c u m u l a t i o n in a n y one region. Distobuccal forces on the extension base adversely affected the p r i m a r y c a n i n e a b u t m e n t . Forces in the regions of the a n t e r i o r base a n d c e n t r a l incisor a b u t m e n t p r o d u c e d the most w i d e s p r e a d transmission of stress t h r o u g h o u t the arch. G r e a t e r stress t h r o u g h o u t the a r c h was observed w h e n forces were a p p l i e d to t h e central incisor r a t h e r t h a n to the right canine a b u t m e n t . T h e r e is a need for established criteria of rest form
146
REFERENCES 1. Kratochvil, F. J., and Caputo, A. A.: Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J PROSTHETD~NT 32:52, 1974. 2. Kratochvil, F. J.: Maintaining supporting structures with a removable partial prosthesis. J PROSTH~T DRNT 25:167, 1971. Reprint requests to:
DR. THEODOREBERt UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY
Los A.~GELES,CALIF.90024
FEBRUARY1978 VOLUME39
NUMBER2