Removable Partial Denture Nanjing Medical University Stomatological college Hu jian
Section 1 Introduction w Tooth defect: Fixed denture: inlay, onlay, crowns, post crown w Partially edentulous arch Fixed partial denture w Edentulous jaws Complete denture Removable partial denture
w Prosthesis: the replacement of an absent part of the human body by some artificial part. w Prosthodontics: the branch of dentistry pertaining to the restoration and maintenance of oral functions, comfort, appearance, and health of the patient by the restoration of natural teeth and or contiguous oral and maxillofacial tissues with artificial substitutes.
Removable partial denture w It is designed so that it can be conventional removed from and inserted into the mouth by the patient. w It can restore the oral function of missing teeth and related tissues, such as soft tissue and bone. w It deprive its support, stabilization and retention from the remaining teeth and or mucosa.
Objective of removable partial denture w Prevention: The elimination of oral disease to the greatest extent possible. w Treatment: the restoration of oral function, such as mastication, appearance, speech.
Advantage of removable partial denture w Small reduction of tooth structure w Widely application in the partial edentulous arch w simply fabrication, easily repair, inexpensive cost. w Enable to restore the defect of soft tissue and bone.
Disadvantage of removable partial denture w Taking up much space, it is difficult to tolerate w Poor stability, low effectiveness in mastication w It is difficult to keep oral hygiene
Contraindication of RPD w Psychopath w poor oral hygiene w hypersusceptibility to acrylic w Highly demand in pronunciation
Development of removable partial denture w The development of RPD is keep steps with the dental laboratory technics and oral material science. w In 1930’, acrylic was applied into the field of prosthodontics. w Cast technology w prefabricated attachment
Classifications of RPD (history) w w w w w Cummer(1920) –depending on the position of direct retainers Kennedy(1928) Bailyn (1928)-based on the available support Neurohr(1939)-according to the type of support Mauk(1942) Wild(1949) Godfery(1951) Friedman(1953) Osborne and Lammie (1974)_based on support of a denture as a whole
Classifications of RPD (structure) w Framework type w Cast framework w Base type w Wrought framework
Base type Framework type
Framework type
Classifications of RPD (support) w Tooth-borne RPD ª (tooth-supported RPD) w Mucosa-borne RPD ª (mucosa-supported RPD) w Tooth-mucosa-borne RPD ª (tooth-mucosa-supported RPD)
Mucosa-borne RPD
Tooth-borne RPD Tooth-mucosa-borne RPD
Six phases of partial denture service w Education of patient w Diagnosis, treatment planning, design, treatment sequencing, and mouth preparation w Support for distal extension denture bases w Establishment and verification of occlusal relations and tooth arrangements w Initial placement procedures w Periodic recall
Section 2 Classification of partially edentulous arches
Introduction w It is been estimated that there are over 65000 possible combinations of teeth and edentulous spaces in opposing arches. w Classifications are actually descriptive of the partially edentulous arches, the removable partial denture restoring a particular class of arch is described as a denture of that class.
Requirements of an acceptable method of classification w It should permit immediate visualization of the type of partially edentulous arch that is being considered. w It should permit immediate differentiation between the tooth-supported and tissuesupported removable partial denture. w It should be widely acceptable.
Kennedy classification w Class Ⅰ: bilateral edentulous areas located posterior to the natural teeth w Class Ⅱ: a unilateral edentulous area located posterior the remaining teeth w Class Ⅲ: a unilateral edentulous area with natural teeth remaining both anterior and posterior to it w Class Ⅳ: a single, but bilateral (crossing the medline), edentulous area located anterior to the remaining natural teeth. w Modification: the size of modification depend on the number of additional bounded saddle.
Notice of kennedy classification 1. the most posterior edentulous area determined the class 2. additional edentulous areas were called ‘modifications’ 3. the size of the modification was not important 4. if the third molar was missing, and not to be replaced, it was not considered in classification.
Disadvantage w Kennedy classification is positional or anatomical and conveys a picture of certain tooth/ saddle relationships, but gives little more information than do the teeth present on a cast and their relative position. It can’t indicate the type of supporting and retention properly.
Section 3 Biomechanics of RPDs
Terminology w Retention: the resistance of a denture to removal from the mouth in a direction at right angles to the occlusal surface. w Stability: the quality of a prosthesis to be firm, stable, or constant and to resist displacement by functional, horizontal, or rotational stress. w Support: the foundation on which the denture rests. It comprises the hard and soft tissues that bear the loads of mastication and clenching applied to the denture.
Introduction w The responsibility of dentist is to design a favorable RPD that keep the stability and don’t damage the remaining teeth and residual ridges under functional loads. w It is necessary for dentist to understand the biomechanics of RPD and biological character of oral tissues.
retainer connector rest Artificial tooth base
Machine form w Machines may be classified into two general categories: simple and complex. w The six simple machines are lever, wedge, screw, wheel and axle, pulley, and inclined plane.
There are three types of levers: first, second, and third class (F, fulcrum; R, resistance; E, effort)
The machine form found in RPD w Lever w Inclined plane w Wedge ªOf the simple machines, the lever and the inclined plane should be avoided in designing RPDs
First-class lever (cantilever) should be avoided
Location of stabilizing and retentive components in relation to the horizontal axis of rotation of the abutment becomes extremely important First- class lever
Inclined plane
Biological character of teeth w A tooth is apparently better able to tolerate vertically directed forces than offvertical, torquing, or near horizontal forces. (Box 1935, oral health) w Clayton (1966) has shown that a 90 lb axial load would result in no more displacement than a transverse load of 5 lb – that is, of one-eighteenth of the vertical force.
Biological character of mucosa Because the periodontal afferents appear to take precedence over the mucosal afferents, even the mucosa is damaged for the denture in overload, the teeth isn’t suffered any hurt.
Forces exerted on the RPD w Mastication loads: The loads vary with the muscle activity, occlusal surface area and type of food. it can be divided simply into two kinds: vertical loads and lateral loads w Gravity: the direction of this force is always down- directed. w Adhesion of food: the direction of this force is always occlusal-directed.
Reaction of RPD on forces Evenly Depression support Displacement retention Vertical movement Movement Off-vertical movement Rotation stability
Rotation w Rotation；when the denture suffered lateral loads, the denture will rotate from the fulcrum or fulcrum line. w The greatest movement possible is found in the tooth- tissue-supported prosthesis
The rotational movement of an extension base type of RPD
w One movement is rotation about an axis through the most posterior abutments
w A second movement is rotation about a longitudinal axis as the distal extension base moves in a rotary direction about the residual ridge
w A third movement is rotation about an imaginary vertical axis located near the center of the dental arch
The results of denture applied with improper forces w Rotation w Instability w Displacement w Insufficiently retention w The overload of teeth and mucosa w Poor support
The terminal result w The denture can’t provide sufficiently oral function w The remaining teeth and related tissues will be damaged under the instable situation.
How to solve these problem?
Biomechanical considerations w To a great extent, the forces occurring through a removable prosthesis can be widely distributed, directed, and minimized by the selection, the design，and the location of components of the RPD and by development of harmonious occlusion.
Support w Properly support is deprived from the following factors： 1. Occlusion in harmonious. 2. Properly distribution of force within remaining teeth and residual ridge. 3. Increasing the areas of bases and the number of abutment teeth. 4. Decreasing the occlusal surface of artificial teeth.
Retention w The removal partial denture deprive the retention from these following factors: 1. Mechanical device: These device are called retainer and may be in form of clasps, prefabricated attachments, or extensions of the denture base into undercuts. (retainer) 2. Friction: provided by the intimate relationship of various components of denture with teeth (minor connector) 3. Physical force: provided by denture base. (base)
Stability w Rotation can be produced from fulcrum. So the methods to solve it are to: 1. Add fulcrum 2. Add counterpalance 3. Eliminate fulcrum
Several Examples
Second-class lever
The axis surface contours of abutment teeth must be altered to locate components of direct retainer assemblies more favorably in relation to the abutment’s horizontal axis
Section 4 components of RPD w RPDs are consisted of artificial teeth, retainers, connectors, and bases.
retainer Artificial tooth connector base
Artificial tooth w Artificial tooth is the substitute of natural teeth. It can restore the function and form of natural teeth crown.
Classification of artificial tooth (material) w Acrylic resin tooth w Porcelain tooth w Metal tooth
Classification of artificial tooth (anatomy) w Non-anatomic tooth w Semi-anatomic tooth w Anatomic tooth
Principle in selecting artificial tooth Anterior teeth Posterior teeth w Form w Buccolingual width w Color w Occlusal-gingival width w magnitude w Mesiodistal width w rigidity
Denture base
Terminology in denture base w Denture base is the part of a denture that rests on the residual bone covered by soft tissue and to which the teeth are attached. w Saddle area is the area of mucosa in a gap in the dental arch caused by the loss of teeth. w Residual ridge or edentulous ridge is the residual bone with its soft tissue that covers the underlying area of the denture base. w Sometimes saddle can be used as denture base.
Function of denture bases w Transferring and distributing the occlusal forces to the abutment teeth and residual ridge. w Stimulation of the underlying tissues of the residual ridge. w Connecting various components of w Connector RPD. w Restoring the defect of bone and soft w Artificial teeth tissues w Retention and stabilization w retainer
Retention of denture bases It is the result of the following forces: w Adhesion, which is the attraction of the saliva to the denture and tissues w Cohesion, which is the attraction of the molecules of the saliva for each other w Atmospheric pressure, which is depend on a border seal and results in a partial vacuum beneath the denture base when a dislodging force is applied
Type of partial denture base w Acrylic resin base w Metal-acrylic resin base
Advantages of metal bases w Accuracy and permanence of form w Comparative tissue response w Thermal conductivity w Weight and bulk
The extension of base w The maximum extension is similar with the extension of complete denture when applied to mucosa-supported partial denture.
w In maxillary RPD, bases are extended buccaly within physiologic tolerance of border structures and cover tuberosities extending into pterygomaxillary notches. w In mandibular RPD, bases are fully extened, lingual flanges are extended into retromylohyoid fossae.
Thickness of denture base： w 2 mm in the acrylic base w 0. 5 mm in the metal base
The contact between natural teeth and base w The partial denture base should be positioned above the survey line and can’t compress the gingiva.
The relation between base and undercut of bone w It is conventional necessary to avoid that base lie in the undercut of bone. w Sometimes the undercut of bone can be utilized for retention.
The designation of polished surface of base w The form of polished surface of base should be concave that assist the denture in stable.
Distal extension partial denture base w The primary consideration is support. w Of secondary importance are esthetics, stimulation of the underlying tissues, and oral cleanliness.
Retainer
Terminology w Retainer w Direct retainer w Intracoronal retainer w Extracornal retainer
Introduction w A removal partial denture exert the oral function which depend on retention and stabilization. w Primary retention for RPD is accomplished mechanical by placing retaining elements on the abutment teeth. w Secondary retention is provided by the intimate relationship of minor connector contact with the guiding planes, denture bases, and major connectors (maxillary) with the underlying tissues.
Classification of retainer w Direct retainer w Prevention the vertical displacement of denture w Indirect retainer w Enhancing the stability of denture
Direct retainer w Extra- coronal retainer w Telescopic crown retainer w Clasp w Extra-coronal attachment w Intra-coronal retainer ª (Intra attachment)
Extra-coronal retainer w It uses mechanical resistance to displacement by components placed on or attached to the external surfaces of an abutment tooth.
Telescopic crown retainer w Consist of an intra-crown and a extra-crown which connect with framework w Retention depend on friction produced between intra-crown and extra-crown
Clasp w It consist of a flexible arm, reciprocation clasp and occlusal rest. Clasp arm (reciprocation) Clasp arm (retention) Occlusal rest
Extra-coronal attachment w It is a type of manufactured attachment which are placed on the external surface of an abutment tooth and attached within the framework.
Intra-coronal retainer w Consist of a prefabricated machined key and keyway, with opposing vertical parallel walls that serve to limit movement and resist removal of the partial denture through frictional resistance. w This type of retainer is cast or attached totally within the restored natural contours of an abutment tooth.
Intra-coronal retainer
Advantages of internal attachments w Elimination of visible retentive and support components w Better vertical support through a rest seat located more favorably in relation to the horizontal axis of the abutment tooth.
Indirect retainer w An indirect retainer consists of one or more rests and the supporting minor connectors, the proximal plates, adjacent to the edentulous areas, also provide indirect retention. w The form of indirect retainers: auxiliary occlusal rest, canine extensions from occlusal rest, canine rests, cingulum bars (continuous bars) and linguoplates, modification areas, rugae support
Indirect retainer principle
w Fulcrum lines should be considered the axis about which the denture will rotate when the base move away from the residual ridge.
Factors influencing effectiveness of indirect retainers w Whether the indirect retainer has been reasonably held in their seats by the retentive arms of the direct retainers. w Distance from the fulcrum line. w Rigidity of the connectors supporting the indirect retainer. w Effectiveness of the supporting tooth surface.
Indirect retention and indirect support
Clasp arm (reciprocation) Clasp arm (retention) Occlusal rest Clasp body Minor connector
Clasp w Clasp is consisted of clasp arm (retentive arm and resistant arm), clasp body and occlusal rest.
Clasp body w The central part connect every part of clasp which is rigid. w It can stabilize and support the denture.
Clasp arm (retentive arm) w The clasp tip lie under the survey line w The retention is provided by the distortion of clasp w The retentive arm should be passive until a dislodging force is applied w The clasp arm should taper evenly in breadth and thickness towards their tips. Otherwise concentration of stress would be caused
Clasp arm (retentive arm) w The distance that arm lies below the survey line depends on: w The bulbousness of the teeth. w The rigidity of clasp alloy w The actual position of clasp tip is determined by undercut gauge on the surveyor.
Clasp arm (resistant arm) w A stabilizing (reciprocal) clasp arm should be rigid. w Its average diameter must be greater than that of the opposing retentive arm w It should be tapered in one dimension only
The function of the resistant arm w Stabilization and reciprocation against the action of retentive arm. w Stabilization the denture against the horizontal movement. w Acting to a minor degree as an indirect retainer.
Rest: any unit of a partial denture that rests on a tooth surface to provide vertical support. Rest seat: the prepared surface of an abutment to receive the rest w Rests are designated by the surface of the tooth prepared to receive them. w The topography of any rest should be such that it restores the topography of the tooth existing before the rest seat is prepared.
Primary purpose of the rest 1. Maintains components in their planned position 2. Maintains establishes occlusal relationships by preventing settling of the denture 3. Prevents impingement of soft tissues 4. Directs and distributes occlusal loads to abutment teeth 5. Prevents food trap 6. Restores the occlusion
Position of occlusal rest w Occlusal rest should be placed on the marginal ridge near the saddle. w When have no sufficient space, occlusal rest can be placed on the buccal groove of maxillary molar or lingual groove of mandibular molar.
Form of the occlusal rest w The outline form of rest should be a rounded triangular shape with the apex toward the center of the occlusal surface. w The floor of rest seat should be concave, or spoon shaped, and have lowered, rounded marginal ridge.
Dimension of rest w The mesiodistal length of rest is the third of that in premolar, fourth of that in molar. w The buccolingual length of rest is the second of that in premolar, third of that in molar. w The rest should be 2 mm in length, 1 mm in depth, 3. 5 mm in width. (2 mm in length, 1. 5 mm in depth, 1. 5 mm in width. )
The angle formed by the occlusal rest and the long axis of abutment tooth w Be equal to or larger than 90 degrees, which usually is 110 degrees. 1100
Lingual rest w The form of lingual rest is rounded V shape. w Mesiodistal length of rest should be a minimum of 2. 5 to 3 mm, labiolingual width about 2 mm, and incisal-apical depth a minimum of 1. 5 mm.
Incisal rest
南京医科大学口腔医学院 College of Stomatology, Nanjing Medical University 江苏省口腔医院 Jiangsu Province Stomatological Hospital 地址: 南京市汉中路 136号 邮编: 210029 电话: 86 -2585031858 网址: http: //www. jsdental. cn Address: No. 136, hanzhong Rd, Nanjing, 210029 Phone number: 86 -25 -85031858
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