- Mosbyã¢â‚¬â„¢s Anatomy & Physiology 2nd E Study and Review Flashcards – Isb# 978-0-323-18725-1
Dentistry
Yr : 2011 | Month : Oct | Volume : 5 | Result : v | Page : 1128 - 1133 
Full Version
Retention in Conventional Fixed Partial Dentures: A Review
Correspondence Address :
Siddharth Narula
Department of Prosthodontics, Darshan Dental College &
Hospital, Loyara, Udaipur – 313011 Rajasthan, India.
Phone : +91-9694355041
Electronic mail : drvikas81@gmail.com
Abstract 
The long-term clinical event of fixed prosthodontic handling depends on guidelines that promote the cosmos of mechanically, biologically, and aesthetically sound tooth preparations. Successful tooth grooming and success of subsequent restoration depend on important factors like retention and resistance form. The quality of a preparation that prevents the restoration from becoming dislodged past such forces parallel to the path of withdrawal is known as retentivity. For good retentiveness in fixed prosthesis, there are various factors starting from the size of the teeth, magnitude of dislodging forces, geometry of tooth preparation, roughness of fitting surface, cement to be used and the film thickness of luting amanuensis. The purpose of this article is to review and enumerate all the retention factors, which are necessary to increase the clinical longevity of the restoration that could exist considered permanent in the traditional sense.
Keywords
Retention form, Tooth grooming, All Ceramic Restoration, Luting Cement
INTRODUCTION
Teeth do not possess the regenerative power establish in most other tissues. Therefore, in one case enamel or dentin is lost equally a issue of caries, trauma, or vesture, restorative materials must be used to reestablish form and function. Teeth require training to receive restorations, and these preparations must be based on fundamental principles from which bones criteria can be developed to help in predicting the success of prosthodontics handling (ane),(2),(iii). Through a review of the dental literature, several disquisitional aspects of retention class in tooth preparation have been identified. This article presents various key principle considerations, factors and guidelines for the role retention in stock-still partial dentures, based on current scientific evidence and literatures.
"Retention is defined as the quality inherent in a prosthesis acting to resist the forces of dislodgement forth the path on insertion". Thus retentivity is a resistance to removal in a direction opposite to that on insertion. Teeth crave grooming to receive restorations and the preparations must exist based on primal principles from which basic criteria tin can east developed that held predict the success of prosthodontic treatment. A good training will ensure that subsequent techniques e.chiliad. provisionalization, impression making, pouring of dies and casts, waxing etc. can be readily achieved.
MECHANICAL PRINCIPLES OF Retention IN TOOTH PREPARATION (2)
For the restoration to be retentive, acceptable and long lasting at that place are certain principles of molar preparation which should be taken into consideration (iv).
Among these principles include:
1. Biologic considerations: These affect the health of the oral tissues which includes conservation of tooth structure, avoidance of overcontouring, supragingival margins, harmonious occlusion, and protection against molar fracture.
2. Mechanical consideration: These touch on the integrity and immovability of the restoration.
3. Esthetic consideration: These affect the advent of a patient.
Mechanical Considerations
Mechanical considerations can be divided into, providing retention form, resistance form and preventing deformation of the restoration.
Retentivity course
Forces develop on teeth from a myriad of angles. A force placed on a retainer can result from mastication, bruxism, dietary intake and also a log of unpredictable stresses. And so this element of the Stock-still Partial denture must not be compromised otherwise it can lead to failure and the restoration. The following factors must be considered in deciding whether retention is acceptable for a given fixed restoration. These include:
one. Magnitude of dislodging strength 2. Geometry of the tooth grooming 3. Taper four. Area 5. Stress concentrations. 6. Type of preparation 7. Roughness of the fitting surfaces of the restorations 8. Materials being cemented.
1. Magnitude of Dislodging Forces
Forces that tend to remove a cemented restoration along path of withdrawal are small-scale as compared to those that tend to unseat information technology or tilt it due east.g. pulling with floss under the connectors. Generally the greatest removal forces arise when exceptionally viscid food, due east.g. bubble gum is eaten or chewed. The magnitude of the dislodging forces depends on the stickiness of the food and the surface area and texture of the restoration being pulled.
2. Geometry of the Tooth Preparation
Fixed prosthesis depend on the geometric grade of the preparation rather than on adhesion for retention. The cement is effective merely if the restoration has a unmarried path of withdrawal i.e. the tooth is shaped in a manner to restrain the complimentary movement of the restoration. A preparation is cylindrical but if the ii horizontal cantankerous sections of the prepared axial tooth surfaces are coincident. A partial denture will be retentive if the sections are coincident and perpendicular motion is prevented by grooves (5). Even so, if 1 wall of the complete crown training is over tapered, it will no longer be cylindrical and the cemented restoration will not exist constrained past the preparation considering the restoration then has multiple paths of withdrawal. Nether these circumstances, the particles of the cement will tend lift abroad from rather than slide along the preparation and the only memory will be a result of the express adhesion of the cement (4).
3. Taper
Selection of the appropriate caste of taper for tooth training
is very important. Too modest taper may lead to unwanted undercuts
and too large will no longer exist retentive. The recommended
convergence between opposing wall is 6 degrees. The molar
should be prepared with a instrument of the desired taper that is
held at a constant angulation (1).
4. Surface Expanse
Provided the restoration has a limited path of withdrawal, its retention is dependent on the length of this path or more than precisely on the surface area in sliding contact. Therefore crowns with long axial walls are more retentive than those with short axial walls and molar crown of same taper are more than retentive than premolar crown of the same taper.
5. Stress Concentration
When a retentive failure occurs, cement is ofttimes constitute adhering to both the tooth preparation and the plumbing fixtures surface of the restoration. In these cases, cohesive failure has occurred through the cement layer because the forcefulness of the cement was less than the induced stress. It has been proved that changes in the geometry of the preparation (east.m. rounding of the internal line angles) reduces stress concentrations and hence increases the retentivity of the restoration.
half-dozen. Blazon of Preparation
Different types of the preparations accept different retentive values and these correspond to the surface area of the axial walls, provided other factors (east.thou. taper are kept abiding). Thus the retention of a complete crown is near double of partial coverage restoration.
7. Roughness of the Surface Existence Cemented
When the internal surface of a restoration is very smooth, retentive failure occurs non through the cement but rather at the cement restoration interface. Air abrading has been shown to increase the retentiveness of the castings past 64%.
eight. Materials Being Cemented
Retention will be affected by both the casting alloy and the core or buildup material. It is said that more retentive the alloy, the more adhesion there will be with the luting agents. Therefore the base of operations metallic alloys i.e. nickel, cobalt and chromium are more than retentive and improve retained than less reactive high gold content metals.
Additional METHODS OF GAINING Retentivity (6)
One method of increasing retention without lengthening axial surfaces is with grooves or boxes. Pins are likewise used to increase retention. 4 ways to resist displacing forces and increase retention are:
1. Preparinga Suitable Gingival Cease Line
Whenever possible, the finish line should be placed in an area where the margins of the restorations tin be finished by the dentist and kept clean by the patient. Placement of the finishing lines creates a bulwark by preventing the cement to come in contact with the oral fluids and thus these finishing lines help in preventing microleakage and ultimately the retention and longevity of the restoration is increased. They likewise provide support to the metal and porcelain or acrylic used in restoration. In that location are four basic types of finishing lines shoulder, bevel shoulder, chamfer and pocketknife-edge.
two. Contouring and Placing Suitable Contact Areas
3. Incorporating Occlusal Locks i.e. Dovetail, Boxes and Grooves
four. Adding Tapered or Parallel Pins
FACTORS AFFECTING RETENTION IN Fixed PART IAL DENTURES
1. Length of Span
In addition to the increased load placed on the periodontal ligament by long span bridge, the longer spans are less rigid and and then less retentive.
2. Curvature of Curvation
Arch curvature has its effect on stresses occurring in a fixed span. When pontics lie exterior the inter abutment axis line, the pontics act as a lever arm which will produce a torquing move which leads to loss of retention of bridge.
three. Type of Bridge
There are two types of bridges made according to the prevalent condition and position of abutments in the arch. a. Rigid connector b. Non-rigid connector. A completely rigid restoration is not indicated for all situations requiring a fixed prosthesis. In many instances, an edentulous span volition occur on both sides of a tooth creating a solitary free standing pier abutment. The use of a form of non-rigid connector can lessen these hazards. The non-rigid connector is a broken stressmechanical union of the retainer and pontic instead of usual rigid solder joint.
four. Apoplexy
Interference with undesirable occlusal contacts produce deviation during closure of maximum intercuspation, hinder smooth passage to and from the intercuspation position and atomic number 82 to deflective occlusal strength on the bridges which may pb to damaging furnishings on abutment and also on the retention of the casting. At that place are four types of occlusal interferences, centric, working, non-working and protrusive. All these interferences should be removed on suitable articulator and a harmonious occlusion should be achieved in the final casting.
5. Periodontal Condition
The abutment tooth must be able to provide good back up for the bridge. This support is related to both the amount of root and the amount of bail nowadays.
6. Tooth or Teeth Existence Replaced
A span replacing a maxillary canine is subjected to more stresses than the mandibular since forces are transmitted outward (labially) on the maxillary arch against the inside of the curve (its weakest point).
When a cantilever pontic is employed to supervene upon a missing tooth, the forces practical to the pontic have an entirely dissimilar event on the abutment tooth. The pontic acts as a lever which tend to be depressed under forces with a strong occlusal vector.
7. Blazon of Retainer Used
In that location are two types of retainers which are mostly used • Intra coronal • Extra coronal In the intra coronal retainers, the retention is obtained betwixt the inner wall of the molar preparation i.e. the internal wall of the prepared cavity and the casting. On the other manus, in extra coronal retainers, the retentivity is obtained between the outer wall of the tooth preparation and the inner wall of the servant.
seven. Materials Employed in the Construction of Retainers
The material used in the construction of the fixed partial dentures calls for certain requirements which help to increase the longevity of the restoration.
Cobalt chromium or nickel chromium alloys mostly used for making stock-still bridges fulfill majority of these ideal requirements. On the other manus acrylic is generally weak, is not rigid and cannot provide strong connectors. Information technology also has lower compressive and tensile force compared to other alloys and is thus easily subjected to fracture. Hence acrylic is used for interim on temporary restorations in the mouth.
8. Arch Position of the Abutment Teeth and Retention
When the abutment teeth are more or less parallel to each other, consummate or partial crown retainers can exist made. If the abutment teeth are non parallel, consummate crown retainers with a common path of insertion are non feasible.
9. Spring Cantilever Bridges and Retention
This bridge provides a method of supporting a pontic at some distance from the retainers. This type of bridge is both tooth and tissue supported. A gold bar which fits in contact with the palatal mucosa connects the pontic to the retainers.
Dissimilar Tooth PREPARATION AND WAYS OF ACHEiVING Retentivity IN EACH
Consummate Cast Crown Preparation
Ways of Gaining Retention While Tooth Preparation:
Later on the occlusal reduction is completed, the guiding grooves are placed on the centric walls. When these guiding grooves are placed, the dentist should be sure that the shank of the diamond is parallel to the proposed path of withdrawal of restoration. A diamond taper bur with a taper of iii-vi° should exist used and thus an identical taper on the preparation wall will result. Identify the cervical chamfer meantime with axial reduction. Width of the chamfer should exist approximately 0.5mm which volition allow adequate bulk of metal at the margin (7), (8).
The Metallic Ceramic Crown Preparation
Factors affecting retention that should be taken into consideration while grooming.
ane. The completed reduction of the incisal edge on an anterior tooth should allow 2mm of adequate material thickness to permit translucency in the completed restoration. Caution must exist used here to preclude over reduction because excessive occlusal reduction shortens the axial wall and thus is a common crusade of inadequate retentiveness and resistance form of completed restoration (3). 2. Labial reduction of 1.5mm should be washed for the adequate memory of metallic and porcelain and the shoulder training should have a 90° butt articulation. 3. Reduction of the proximal and linguo-centric surfaces should exist done with a diamond held parallel to the path of withdrawal of the restoration giving an guess taper of 6o. If this is not followed, a slightly more taper or discrepancy in taper of 2 walls volition result thus affecting retentiveness. 4. In a completed restoration, all the line angles and point angles should be rounded. This will assist in reducing the stress concentration and thus will heighten retentiveness.
THE PARTIAL VENEER CROWN PREPARATION
Posterior Teeth Three Quarter Crown
1. During axial reduction place grooves for axial alignment in the eye of the lingual surface and in the mesiolingual and distolingual transitional line angles. These grooves should be made parallel to the long axis of tooth. 2. During proximal reduction the proximal grooves are placed parallel to the path of withdrawal. The groove should not be deeper than 1mm and is best washed with a tapered carbide bur. The grooves prepared should resist lingual displacement of the periodontal probe. iii. If additional bulk is needed to ensure rigidity of the restoration information technology can be provided with an occlusal kickoff. This V-shaped groove extends from the proximal grooves along the buccal cusp.
Anterior Fractional Veneer Three Quarter Crown Training
With the advent of metal ceramic restorations the use of partial veneers on inductive teeth has lessened somewhat during recent years. However ii types of partial veneer anterior crown preparations are even so washed. ane. Maxillary canine three quarter crown. two. Pin ledge preparations. To enhance the retention and resistance grade of the preparation a slightly exaggerated chamfer on the lingual aspect of the tooth should be placed and a guiding groove in the middle of the cingulum wall. The mesial and the distal proximal grooves provide about of the retention form for the anterior fractional veneer crowns. They are made with a 170L carbide bur and converage at an angle of three-5o degree.
Pin Ledge Preparation and Retention
A pin ledge is occasionally used equally a unmarried restoration generally to re-institute inductive guidance, in that example only the lingual surface is prepared. More than commonly, however, it is used as a retainer for an fixed fractional denture or to splint periodontally compromised teeth (9).
RETENTIVE REATURES FOR ALL CERAMIC RESTORATION
An all ceramic restoration remains the most aesthetic restoration for duplicating individual anterior teeth. Acceptable molar reduction is created to achieve space for the porcelain bulk required for the force of the restoration.
Retentive features to be taken into consideration during each pace of the prepa ration
Incisal Reduction
There should be an adequate incisal reduction of 2mm otherwise breakable failure of the material occurs.
Facial Reduction
The facial reduction is performed with a coarse flat end diamond to remove the labial surface while establishing a preliminary shoulder. The incisal 2/3rd of the facial surface should exist inclined lingually to provide uniform porcelain and ensure suitable aesthetics. Insufficient molar reduction on the facial surface can atomic number 82 to either a tooth thin coverage contoured restoration. This can also lead to the failure of the restoration.
Proximal Reduction
Excessive taper of the proximal surface should exist avoided which can likewise lead to loss of retention by decreasing the surface surface area and also the parallelism of walls.
Lingual Reduction
Proper lingual reduction is very important for the strength and retention of the restoration. The lingual surface of the tooth is generally reduced in ii planes. First cingulum shoulder is placed with a apartment ended tapered diamond to crest a 0.75mm shoulder in the cingulum with a two-5o taper. The cingulum reduction is at present completed.
A flame shaped or wheel shaped diamond is used to form the lingual concavity of the anterior teeth.
Inadequate tooth reduction of the lingual surface tin lead to loss of clearance and also diminished strength for the porcelain which can over all lead to loss of retentiveness of the restoration.
Proper Cease Line
A proper marginal finish line is very important for the retention. Inadequate finish line in some areas of the preparation can lead to microleakage thus leading to the loss of retentivity (10).
Precipitous Points and Undercuts
All the sharp points and undercuts should exist removed or rounded off to prevent the aggregating of the stresses and thus prevent the subsequent failure of the restoration.
Memory IN ENDODONTICALLY TREATED TEETH
It has been demonstrated experimentally that endodontically treated teeth are weaker and more brittle than vital teeth. So for this reason attempts have been made to strengthen the teeth by removing role of the root canal filling and replacing it with a metal mail (11).
Also when the teeth will be serving every bit an FPD abutment, a complete crown becomes mandatory. Under these circumstances, the retention and support about be derived from within the root culvert (12).
Canal Retention
It is recommended that the root canal should be enlarged merely to corporeality necessary to enable the post to fit snugly for strength and retention.
Retentivity IN PORCELAIN LAMINATE VENEERS
To ensure a uniform thickness and the retention of the laminate veneer, the post-obit criteria must be met: (13) a. There should exist a uniform reduction on the labial surface of the tooth and the preparation should remain within the enamel whenever possible. b. The margin of the porcelain laminate veneer should generally exist hidden within the embrasure area.
A modified chamfer finish line ensures right enamel grooming exposing correctly aligned enamel rods for increased bond strength at the cervical margin thus increased retentiveness.
Information technology also ensure an adequate bulk at the margins and hence it increases the force (fourteen).
Carving the porcelain is too said to be a predominated factor in producing the retention.
Retentivity FOR CERAMIC INLAYS AND ONLAYS
Ceramic inlays and onlays provide a durable alternative to posterior composite resins for patients demanding aesthetic restoration (fifteen).
For maximum retention post-obit points should be taken cared of: • The outline and the reduction of the tooth is governed by the existing restorations and caries. At present here is resin bonding, the centric wall undercuts tin can be blocked out with GI cement preserving additional enamel for adhesion and thus the increased retention of the restoration. However undermined and weakened enamel should always be removed. • The outline should avoid occlusal contacts. Areas to exist onlayed demand 1.5mm of clearance in all excursions to foreclose ceramic fracture and thus increase the longevity of the restoration.
• In this preparation, it is preferred that the margin is kept supragingival, if this is not possible, crown lengthening is advisable. • All the internal line angles should be rounded to preclude stress concentration and to thus enhance retention. • A 90o butt articulation should be given for ceramic inlay margin. Bevels are contraindicated because bulk is needed to prevent fracture and thus increase the longevity. • Final retention is accomplished during the bonding of the inlay as it is washed with a resin luting cement. In this procedure acrid carving is done which creates micro tags and help in mechanical retentivity.
RETENTION IN RESIN BONDED BRIDGES
The retention of this prosthesis depends on the adhesive bonding between the etched enamel and the metal casting. To enhance retentivity in these restorations, significant clinical crown length should exist nowadays. If in that location is bereft wet control, memory is minimized. Short clinical crown and narrow embrasures are also a contra indication for resin retained FPD considering in these type of teeth, surface area is reduced and thus the retentiveness. If a patient has parafunctional habits, this restoration should not be given because they lead to early on failure of the restoration (five),(sixteen).
Discussion
ROLE OF LUTING CEMENTS IN Retention
The type of luting agent chosen affects the retention of cemented restoration.
Five kinds of luting agents are almost commonly used: i. Zinc Phosphate ii. Zinc Polycarboxylate 3. Drinking glass ionomer 4. Zinc oxide eugenol 5. Resin bonded cement. The Retentiveness of restorations has been achieved primarily past mechanical interlocking of the cement into irregularities on the internal surface of the fabricated restoration and the molar preparation.
Polycarboxylate and glass ionomer cements adhere directly to calcified tissues by chemical attraction to calcium ions in addition to mechanical interlocking (17).
Truthful adhesion betwixt cement and tooth is desirable because of potential to reduce microleakage between the tooth and the restoration (xviii).
Retention failure has been shown if the internal surface of the surface of the restoration is very polish (19). So it is recommended to air abrade the internal surface of the casting with 50μm alumina. Retention has been seen to be more with more reactive alloys i.e. nickel, cobalt and chromium are more retentive and better retained than less reactive high gold content metals.
Film Thickness of the Luting Cement
At that place is a conflicting prove on the event of increased thickness of cement pic on retention of the restorations. But its proved there,a uniform thickness of cement between restoration and tooth provides more memory than a non-uniform thickness.
A film thickness of 2.5μm or less has been preferred for successful restoration.
Conclusion
Retentivity in fixed partial denture, is one of the important cistron in the success of fixed partial dentures. There is no single cistron on which retentiveness is totally dependent. In fact retention comprises of a listing of factors, all of which have to be taken into consideration during all the stages starting from molar training to the concluding cementation. Even if a single factor is neglected it tin can affect the retentivity of the casting which further has a directly influence on the longevity of the restoration.
Cardinal Message
The principles, factors and guidelines identified in this article tin aid dentists to meliorate understand in club to pattern, appraise, and modify the modes of retentivity in fixed fractional denture prosthesis to ensure clinical success for the treatment of a variety of fixed prosthsis/restorations.
1.
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12.Rivasina. Clinical procedure for fractional crown, inlays, onlays and pontics. Quintessence Publishing Co., 1991
xiii.Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. second ed. St. Louis, MO: Mosby–Year Book; 1995. pp. 137-8, 170-73, 184- 85, 229.
14.Shillingburg HT, Hobo S, Whitsett LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. third ed. Chicago, IL: Quintessence Publishing Co; 1997. pp. 120, 139-42, 151-52.
fifteen.Simonsen R. and Van Thomson. Etched cast restoration, clinical and laboratory techniques, 1983.
16.Standlee and Caputo. Effect of surface design on retention of dowels cemented with various luting cements. JPD 1993; 70, 403-5.
17.Walls AW. Cantilever FPDs take lower success rates than cease abutted FPDs later on 10-years of follow-up. J Evid Based Dent Pract. 2010;10(ane):41-43.
18.Piovesan, Edno Moacir; Demarco, Flávio Fernando and Piva, Evandro Fiber-reinforced stock-still partial dentures: a preliminary retrospective clinical study. J. Appl. Oral Sci. [online]. 2006; .fourteen, 2, 100-4.
19.G Zalkind, P Ever-Hadani and Northward Hochman. Resin-bonded stock-still partial denture retention: a retrospective 13-year follow-up. J Oral Rehabil. 2003; xxx(10):971-77.
DOI and Others
JCDR/2011/1582
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