Provides seal at the posterior border of the denture base. Should be placed just anterior to the vibrating line on the palate, which can be assessed by asking the patient to say Aah. Usually 1 mm deep. It is prepared on the upper cast with a wax knife in the shape of a cuspi’s bow; or by adding low fusing impression compound along the back of the palatal surface of a correctly extended denture.
Cr/Cb is regarded as the standard for RPD. Discuss when other materials are to be considered.
The only satisfactory material for a well designed partial denture is chrome cobalt. Metal bases have greater strength and permits more hygienic design. The only justification for the use of acrylic partial dentures is an economic one. However, need constant repair, cause speech problems due to the thickness of the acrylic, gagging and discomfort.
Acrylic denture may be more appropriate when there may be a need to add teeth to the denture in the future (become an immediate at a later stage) or the chrome cobalt denture could be specially designed to allow for this possibility.
Use circumferential clasps with chrome cobalt on molars (need about 15 mm long), but not on bicuspids or cuspids (because of shortness of the clasp arm). Use stainless steel circumferential clasps for these, only if enough amount of acrylic to embed the clasp in. Gold clasp can be used and can be soldered to the chrome cobalt base, but are more expensive. Roach chrome cobalt can be used in bicuspids.
Double circumferential chrome cobalt clasps may be used in two bicuspids or a bicuspid and a cuspid with the retentive undercut on the tooth further from the denture base.
If there is an anterior undercut labially on the lower ridge, how would you manage that as far as the design of your denture?
What is the best impression material for removable partial dentures if there are periodontally compromised teeth remaining in the arch?
Elastomers are preferable for deep undercuts.
Reasons behind denture design and possible alternatives.
See design specifications.
Questions on other periodontal and prosthodontic principles and treatment option.
Don’t make partial dentures where the oral hygiene and or the periodontal condition of the remaining natural teeth are poor. Sometimes it is better considering extracting these teeth.
Where possible avoid periodontally involved teeth in the design. If a periodontal involved tooth bounds a saddle area and the choices are to put an occlusal rest on this tooth or support the denture on the mucosa, then rest the denture on the tooth with a rest in the long axis of the tooth. Supporting the denture on the mucosa will further damage the periodontal health of this tooth.
Sometimes heavier non retentive clasping above the survey line may make a somewhat mobile tooth considerably firmer.
Reduce periodontal support deserves more flexible clasp to avoid overload.
Purpose of surveying
Determine best path of insertion and withdrawal of the prosthesis, identify proximal surfaces of teeth that can act as guide planes (these are proximal tooth surfaces that are parallel to one another), locate areas of retention, determine undercuts, locate and mark line of maximum convexity.
Indications of RPD
Edentulous areas too long for a fixed prosthesis, need to restore soft and hard tissue contours, absence of adequate periodontal support, structurally or anatomically compromised abutment teeth (lack of clinical crown height, lack of sound tooth structure, unfavourable position, contour or inclination), need for cross arch stabilization, need for an extension base, anterior aesthetics, age, physical and emotional problems precluding fixed partial dentures, attitude and desires of patient, ease of plaque removal.
Requirements of RPD
Retention (resistance to dislodging forces): direct and indirect retainers.
Stability (bracing, resistance to horizontal, lateral, or torsional forces): minor connectors, proximal plates, bracing clasp arms, lingual plates, rest.
Support (resistance to vertical seating forces): rests, major connectors, dentures bases.
Class I: bilateral edentulous areas located posterior to the remaining natural teeth.
Class II: Unilateral edentulous area located posterior to the natural teeth.
Class III: Unilateral edentulous area with natural teeth both anterior and posterior to the area.
Class IV: single, but bilateral edentulous area located anterior to the remaining teeth.
Modification spaces are edentulous areas other than those determining the main classes.
Biomechanical classification of RPD: Tooth borne (dento-alveolar supported): functional forces are transmitted through abutment teeth to bone, Tooth- mucosa borne (dento-alveolar and muco-osseous supported): functional forces are transmitted through abutment teeth and mucosa to bone, Mucosa borne (muco-osseous supported): provisional prostheses, contain no frame work.
Major connector: connects components on one side of the arch with those on the opposite side.
Mandibular major connector
Lingual bar, half pear in cross section 4 mm tall, 2 mm wide, used when functional depth of vestibule is > or = 7 mm, The base is the bottom of the sulcus, simplest with highest patient acceptance.
Lingual plate, when functional depth of lingual vestibule is < than 5 mm, when addition of artificial teeth in future, for splinting teeth, superior border contact teeth above survey line, extend interdentally, should have a rest on each side to prevent labial movement of teeth, lingual portion like a lingual bar. Avoid taking the plate on the lingual of the first lower molar as the lingual bone there is often thick and a plate in this area can intrude on the tongue space.
Sublingual bar, used when functional depth of lingual vestibule is 5-7 mm. It is a bar rotated 45 to 90 degrees.
Labial bar, used when mandibular teeth are inclined lingually, when lingual tori, 3mm below gingival margins, lack rigidity, longer.
Design specifications: the superior borders are placed at least 3 mm from the gingival margins, metal should be highly polished on the tissue side to minimize plaque accumulation, relief of tissue surface to prevent tissue impingement at rest or during function.
Maxillary major connectors
Palatal bar (less than 8 mm in width located anteriorly, or posteriorly).
Palatal strap (greater than 8 mm in width located either anteriorly (called horse shoe), mid-palatal or posteriorly). Used in tooth borne partial dentures or short tooth-mucosa borne. Rigidity may be increased by thickening the central portion to 1.5 mm.
Two bar (ant and post called palatal ring), two straps (ant and post). Used when torus, in tooth borne and tooth-mucosa borne.
Modified palatal coverage (called palatal plate, > 12 mm). Used for tooth-mucosa borne.
Complete palatal plate. Used for long span bilateral tooth mucosa borne.
Design specifications: Border at least 6 mm from the gingival margins, metal should be not highly polished on the tissue surface contact, borders tapered, avoid anterior rugae region as covering them can impede the tongue and thus speech and the movable soft palate, not relief required on the tissue surface.
Minor connectors: link major connector to other components. It should be thick enough for rigidity (1.5 mm, 2 mm for gold), minimal gingival coverage, join major connectors in right angle, relief when crossing gingival tissues, 5 mm away from other vertical component.
Clasps: are extracoronal direct retainers that engage an abutment tooth for retention, stability and support of the RPD.
Retentive component: flexible clasp arm whose terminal portion engages an undercut area of an abutment tooth (cast circumferential, cast bar, wrought wire circumferential).
Bracing (stabilizing): rigid component that contacts a non-undercut area of an abutment tooth (cast circumferential bracing arm, cast bar bracing arm, shoulder of circumferential retentive clasp arm, minor connector, proximal plate, lingual plate, rest).
Requirements of clasps: retention, bracing (stability), support, reciprocation, adequate encirclement (more than 180 degrees), passivity.
Types of circumferential (suprabulge) clasps.
Circlet (akers): rest, minor connector, cast circumferential retentive clasp arm and cast circumferential bracing clasp arm. Engages 0.25 mm undercut. Used in tooth borne segments.
Embrasure clasp: two circlet originating from a common minor connector. Used for additional retention, when adjacent teeth are present on one side of the arch.
RPC or RPA clasp (mixed clasp): mesiolingual rest, minor connector, distal proximal plate, buccal cast circumferential arm. Used in tooth-mucosa borne partial dentures where RPI cant be used.
Ring clasp: single cast circumferential clasp arm, rest, minor connector. Used for titled molars, rarely indicated.
Back action clasp: similar to ring clasp.
Circumferential C clasp: rest, minor connector, cast circumferential C retentive clasp arm. Used when distofacial undercut exists.
Cast circumferential and wrought wire circumferential clasp: rest, minor connector, wrought wire circumferential retentive clasp arm, cast circumferential bracing clasp arm, used for tooth-mucosa borne partial denture.
Only the terminal third of the retentive clasp engages an undercut, should finish at the occlusal portion of the gingival 1/3. The proximal 2/3 of the retentive clasp arm and the entire bracing arm should be located at the junction middle and gingival third.
Types of bar (infrabulge) clasps.
R.P.I. bar clasp: mesiolingual rest, distal proximal plate, cast I bar retentive clasp arm. Used for tooth-mucosa borne.
R.I.I. bar clasp: mesioocclusal rest, palatal bracing I bar, buccal retentive I bar. Used for tooth borne segments.
Roach clasp (mixed clasp): disto occlusal rest, lingual supra bulge arm, infrabulge I bar. Used for tooth borne when aesthetics is important or with T bar for distofacial undercut.
Retentive arm should be 3 mm away from free gingival margin, should not impinge in soft tissue, should not be located over a deep tissue undercut, functional vestibular depth of 5 mm, vertical portion of the arm should cross gingival margin at 90 degree angle and 5 mm away from other vertical component. There should be 3 mm parallel gap between clasp and pink acrylic, length from insertion should be 15 mm.
Wrought wire clasp arms, engages 0.5 mm (SS) or 0.75 mm (gold) of the undercut, used for tooth-mucosa borne, provide more flexibility.
The RPI clasping system (rest, proximal plate, I bar) and distal extension base partial dentures: designed as stress releasing clasping unit. Stress release is the redirection of the forces of occlusion that are translated to the abutment teeth by the lever of the distal extension. Permits movement of the denture base without being transferred to the abutment tooth. The I bar extends from the framework so as to remain at least 3 mm from the gingival margin and then cross the gingival margin at right angles. 2 mm of the I bar contact the tooth, taper from base to tip, place towards mesial to permit movement of the I bar away from the entire buccal surface in function, bringing the proximal plate into tight contact with the distal surface. If the abutment tooth is a canine, mesiolingual rest preparation deep enough to prevent slipping gingivally
Stress breaker is distinct from this, reserved for mechanical devices placed between the denture base and the abutment tooth preventing undesirable forces being transmitted to the abutment tooth.
Indirect retainers: contacts an abutment tooth on the opposite side of the fulcrum line, assisting the direct retainers in preventing displacement of an extension base through mechanical leverage. Indirect retention only acts with correctly functioning clasps. Indirect retention is worthwhile if it can be achieved without unduly complicating a design. It works best on the perpendicular bisector of the fulcrum line, it works better as the distance form the fulcrum line increase, it only works while the direct retainers function
Types of indirect retainers: rests (extending from a clasp), minor connector and proximal plates. An indirect retainer which moves away from its rest seat during seating forces may indicate a need for relining of the extension base.
Extended rest eliminates need for an additional minor connector, minimizing tissue coverage.
Fulcrum line: passes through the two rests in closest proximity to the tooth-mucosa borne edentulous area.
Major connector: promotes cross-arch force transmission.
Minor connectors: transfer forces to and from abutment teeth.
Direct retainers: control forces, minimize lateral and torsional forces to abutment teeth.
Rests: provide dento alveolar support, efficient force transmission to abutment teeth.
Factors influencing design: preservation of teeth and peridental structures (the natural teeth remaining are more important than those already lost), minimal tooth and minimal gingival coverage, nature of PD- tooth borne or tooth-mucosa borne, anatomic limitations, tooth inclination, position or contour, contingency planning (future modifications), potential magnitude of applied forces (parafunction), ease of placement and removal, aesthetics, desires and previous experience of the patient.
Sequence: after surveying, locate undercut area for retentive elements with a red line 1-2 mm, outline rests, outline minor connectors and proximal plates, indicating exact width, height, and relationship to prepared guiding planes, outline major connector incorporating maximum gingival tissue exposure in the area joining the minor connectors and proximal plates, outline clasp arms indicating precise width, taper, relation to survey line and the free gingival margins, outline the retentive mesh of the resin base areas, posterior extension to ½ to 2/3 the length of the edentulous span. Internal finish lines. Highlight any additional features such as posts, loops, or beads. Measured depth of the undercut may be written on the cast together with other special instructions, such as relief or specific material.
Guiding planes should be confined to occlusal 1/3, 2-3 mm in length, flat occlusogingivally, confined to proximal surface, 3-4 mm in width, rounded faciolingually in harmony with existing tooth contour. Prepared with cylindrical diamonds or burs. Not more than 0.5 mm in depth.
Thickness of metal alloy of 1.5 mm at the junction of rest and minor connector.
Occlusal rest seat preparations: apically inclined (spoon-shaped), rounded triangle whose base is at the marginal ridge, 1/3 of the faciolingual width of tooth, ½ the width between the facial and lingual cusp tips. 1 to 1.5 mm deep. Prepared with # 6 or #8 round bur.
Mesiolingual rest seats on premolars(RPI, RPA, or indirect retainers): same as occlusal rest seat, but its location is in the mesiolingual fossa of the teeth.
Incisal rest: faciolingually convex, mesiodistally concave. Horizontal gingival floor 1.5 mm long, 1 mm height. Cylindrical diamond bur.
Lingual rests for anterior teeth: cingulum rest, faciolingually concave v or u, mesiodistally inverted v or u. Lingual ball rest, bur #4 or #6.
The placement of a mesial rest would tend to tip the abutment tooth mesilally, but it would be resisted by contact with the adjacent tooth.
Checking occlusion it is better with red articulating paper as blue does not imprint very well.
Draw the outline, so that the denture margins finish in natural grooves or hollow. Avoid margins of teeth if possible. Base approaching tooth should form an obtuse angle to avoid potential food trap areas. Make sure metal don’t show from the front. Join metal to acrylic on curved line joining gingival of proximal teeth. Keep outlines curvaceous, avoid sharp angles. Previous palatal denture experience is important, ask patient preferences. Aim for triangular clasping whenever possible. Reciprocate with blunt thicker Chrome cobalt arm. It is preferred to vary the position of the undercut and/or the type of clasping used, instead of using composites to provide undercuts.