retention Flashcards
what are the 3 factors contributing to retention
1 PHYSICAL FORCE
- surface forces associated with adhesion of saliva to denture and mucosa
- atmospheric pressure
- gravity (for mandibular)
- fluid forces
2 MUSCULAR FORCE
- provided by tongue, cheek, lips
- polished surface does not encorach on muscle/ frenal attachment
3 MECHANICAL FORCE
- direct retention (clasps etc)
- indirect retention
- friction (GP, bracing elements)
how must the reciprocal clasp contact the tooth
contact the abutment slightly before the retentive element contacts tooth, must maintain contact until prosthesis is fully seated to protect abutment from potentially destructive lateral forces
features of a retentive clasp assembly
1) RETENTION
- resists dislodgement in an occlusal direction (by clasp being in undercut)
2) RECIPROCATION
- resists force exerted on tooth by clasp arm
3) SUPPORT
- resists displacement in gingival direction (by rests)
4) BRACING
- resists horizontal movement of prosthesis (reciprocal arms, minor connector)
5) ENCIRCLEMENT (aka engagement >180)
prevents prosthesis from moving away from tooth
6) PASSIVITY
at rest and fully seated, direct retainer should not exert force against tooth. the retentive arm should be activated only when dislodging forces are applied to removable partial denture
principles of retention planning
1) retainers should be positioned such that no axis of rotation exists
2) retention should be as close and adjacent to saddle area as far as possible
3) try to balance retention area on both sides of the arch
4) try to distrubute clasps around the arch and avoid cluster of clasps close to an axis for maximum effect
compare features of cocr and wrought wires
- cocr has higher modulus of elasticity (more rgid) and lower proportional limit (less likely to deform)
- hence cocr retentive clasp arms must be thinner and engage less undercuts
- amount of undercut requried for cocr is 0.25 and wrought wire is 0.5mm
factors affecting choice of retentive clasp
1) location of undercut
2) health of PDL
- if tooth has reduced Periodontal support, we should choose a more flexible clasp
3) shape of sulcus
- if there is a deep soft tissue undercut, and we give a gingivally approaching clasp, the prominence of the clasp arm is likely to irritate buccal mucosa and trap food debri, becoming intolerable to patient
4) length of clasp
- longer the clasp, more flexible and hence a c clasp on the molar is more flexible than premolar, can consider that
5) appearance - gingival approaching clasp can be hidden better
6) occlusion
- for suprabulge clasps, need to provide adequate space for clasp
what is the concern with distal end saddles such that RPI was introduced
the concern is that the distal end saddle acts as an effort arm across the distal rest which is a fulcrum, and causes the clasp tip which is the resistance arm, to engage the tooth’s undercut, resulting in harmful torquing of the tooth
and hence RPI system was designed to address the concern of a class i lever bc RPI is designed to allow the vertical rotation of a distal extension saddle into the denture bearing mucosa under occlusal loading without damaging the supporting structures of the abutment tooth by disengagement of the retentive clasp
advantages of RPI system
1 less torqueing on abutment
2 less tooth contact
- I bar itself makes very little contact with the tooth
- mesial minor connector together with the proximal plate provides the necessary reciprocation and eliminates the need for a lingual arm
3 esthetics
contraindications for RPI
1) insufficient depth of vestibule
- inferior border of I bar must be at least 4mm from gingival margin
2) undercuts
- if there is svere soft tissue undercut (meaning I bar is too far away from tissues) then will give food trap
- if no labial/ buccal undercut then gg
- contra also if only the disto buccal undercut is available because then its less than 180 encirclement
3) cases with high FOM suhc that lingual plate is used on the abutment, but we cant be having any lingual thing there to avoid torqueing lor
what is an alternative to RPI & RPA
CAST COMBINATION CLASP
it is a cast reciprocal element and a wrought wire retentive clasp which is more flexible (so half cast and half wrought wire)
+ distal rest
benefits:
- stress breaking action
- dissiptes stress of torque and leverage because the retentive arm is round and hence flexible in any direction
- clasp itself is wrought so can be used in smaller gauges, further increases flexibility and permits clasp to function as stressbreaker
- then the reciprocal arm is cast and hence rigid, so able to resist lateral forces effectively + reciprocate force exerted by retentive wrought wire as the partial denture is placed or removed
where shhould the indirect retainer be positioned
positioned in a properly prepared rest seat on an abutment tooth with good perio support
should be perpendicular to and as far from the primary fulcrum line as is practical
as far as possible from the distal extension base so as to gain best leverage against lifting of the DEB
advantages and disadvantages of C clasp
adv:
- does not impinge on vestibular tissue
- more rigid than I bar
- can be used in cases where undercuts are severe
- good retention
disadv:
- half round cross section so can only adjust in BL direction
- increased circumference of clinical crown can interfere with elimination of food from occlusal table
- more tooth coverage than roaches which promotes decalcification
- poor aesthetics
- may need to trim more coronal structure if occlusion is tight (less space for c clasp to go thorugh)
adv and disadv of I bar
adv:
- less distrotion of coronal contours
- less tooth contact
- cleaner
- less prone to caries
- aesthetically superior bc less conspicuous
- greater adjustability
disadv:
- more bothersome to vestibular tissues
- too flexible for effective bracing
- aesthetically objectionable in patients with high lip line
- not suitable where abutment teeth have severe buccal/lingual tilt
(tys shen ti)
points to mention when askd about retainer designs conducive to health of abutment teeth
health can cover
- caries
- perio
- mobility
features of retentive terminal:
- should be opposed by a reciprocal component capable of resisting any transient pressures exerted by the retentive arm during placement and removal
- reciprocal arm should be in contact with the tooth during the entire period of retentive clasp deformation to stabilise horizontal movement
to avoid excessive torque on abutment:
- clasp retainers on abutment teeth adjacent to DEB should be designed through RPI system
for health of soft tissue
- clasps must not impinge on soft tissue
- tissue surface of clasps should be smooth, polished
- superior border of body of clasp should be located at least 3mm from FGM
- minimum vestibular depth of 5mm during function required so that body of bar clasp should not interfere with movable tissue
features of RPI
- mesial rest to avoid distal tipping of abutment tooth
- proximal plate of sufficient faciolingual dimension to prevent lingual movement of tooth
- I bar to provide retention
I bar and proximal plate move in a mesiogingival direction and disengage from tooth during function, thus acts as a stress releasing directing assembly
