Direct and indirect retainers & Biomechanics Flashcards
definition of retention
resistance to forces that are putting prostheses away from tissue
aka dislodgement
simple machine is composed of
fulcrum, effort, resistance
3 types of levers and relation to rpd
Class I
-effort on one side, resistance on other side, ideal, ex. biting
Class II
-resistance same side as effort (longer effort), sticky food that can dislodge RPD, most relevant for retention
Class III
-resistance same side as effort (longer resistance), RPD is pressed down to tissue
direct retainers are located on which side of fulcrum line
same side of fulcrum line where dislodgement occurs
-always the clasps
indirect retainers are located on which side of fulcrum line
opposite side of fulcrum line where dislodgement occurs
-rests
three fundamental planes of movement
sagittal plane
-rotate around medio-lateral axis
-denture base moving toward and away supporting ridge
horizontal plane
-rotate around vertical axis
-vertical axis near midline
-stability
frontal plane
-around antero-posterior axis
-fulcrum line extending through rest and along edentulous ridges on ONE SIDE OF ARCH
type of force that can cause damage to tooth structure
non-axial loading
greatest magnitude of rotation around is in this plane
sagittal plane, medio-lateral fulcrum line
-movement produced when biting
-greatest magnitude but not most damaging (force is mesio-apical, greatest vector in apical direction)
movement of free-end is along this plane, what is it called
along horizontal plane (vertical axis), fishtailing
movement along this plane is considered the most damaging, why
frontal plane (antero-posterior), because there is no adjacent tooth and there is thinner support on buccal and lingual
mucosa only offers ___% of support provided by PDL, soft tissues are ____ times more displaceable than adjacent teeth, ____ displacement under 4 newtons of force
0.4%, 250 times, 500um
compression of mucosa is unfavorable because ___ is inhibited and causes ___
blood flow, resorption-remodeling
what is it called when one end of RPD is supported and other is not
support deficit (free end case)
what happens when clasp moves while it is engaged on undercut
rotational movement (torquing, bottle opening effect)
what is more important for retention and stability than number of clasps
spatial distribution (pagkakahiwahiwalay ng clasps)
most stable clasp configuration
quadrilateral (clasp anterior and posterior to each edentulous space)
second most ideal clasp configuration
tripodal (unilateral free end, one clasp placed adjacent to defining distal extension space)
least stable configuration
bilateral (bilateral free end, no other choice, resistance aided by denture base and height of residual ridge)
rationale for clasping both sides of arch
cross-arch stabilization, added stability by engaging teeth on contralateral side
3 stress releasing clasp designs
RPI, RPA, RPH
benefit of placing clasp posterior to fulcrum in RPI
it moves downward and forward (disengages)
meaning of RPA
Rest-Proximal Plate-Aker’s clasp
relief component of RPA
engages undercut on mesial, 2/3 of clasp is not engaged/has space
why was RPI modified and i-bar moved from mesiodistal bulge to far zone
cleansability and stable reciprocation, further retention because clasp hits mesiodistal bulge when it tries to dislodge
indication for RPH
cases where vestibular anatomy cannot support i-bar but want more esthetic design than RPA
methods to reduce loading on mucosa
-full coverage denture base
-functional impression
-mesial rests
-stress-breaking major connector
denture base is extended up to ____ for added stability and retention
buccal vestibule/lingual sulcus
what is functional impression and when is it used
impression taken while mucosa is compressed, used for final impression
-mitigates support deficit and aids in lessening torquing
placing a rest farther mesially from edentulous area results in more (vertical/perpendicular) biting forces which are resisted (worse/better)
vertical, better
wheelbarrow principle in prostho
farther away rest from free-end, lesser load is placed on that abutment
