CHAPTER 4: BIOMECHANICS OF REMOVABLE PARTIAL DENTURES

consequence of prosthesis movement under load is due to:

an application of stress to the teeth and tissue that are contacting the prosthesis

the stress should not exceed the level of physiologic tolerance, which is a range of mechanical stimulus that a system can resist without disruption or traumatic consequences

in the terminology of engineering mechanics, prosthesis induces stress in the tissue equal to the force applied across the area of contact with the teeth and/or tissue

removable partial dentures

intended to be placed into and removed from the mouth.

because of this, they cannot be rigidly connected to the teeth or tissue

this makes them subject to movement in response to functional loads, such as those created by mastication

designing this type of denture can be considered similar to the classic, multifaceted design problem in conventional engineering, which is characterized by being open ended and ill structured

open ended

means that problems typically have more than one solution

ill structured

means that solutions are not the result of standard mathematical formulas used in some structured manner

design process for removable partial denture

identifying a need

defining the problem

setting design objectives

searching for background information and data

developing a design rationale

devising and evaluating alternative solutions

providing the solution

rationale for design

should logically develop from analysis of the unique oral condition of each mouth under consideration

abutment teeth

residual ridges

supporting structures for removable partial dentures

are living things that are subjected to forces

supporting structures are capable of resisting the applied forces depends on:

whether resistance changes over time

what typical forces require resistance

what duration and intensity these forces have

how material use and application influence this teeth-tissue resistance

what capacity the teeth, implant(s) and/or mucosae have to resist these forces

forces inherent in the oral cavity

direction

duration

frequency

magnitude of the force

3 desired principles demonstrated by prostheses

support

stability

retention

chewing

the major functional demand is imposed by

2 general categories of machines

simple machines

complex machines

complex machines

are combinations of many simple machines

6 simple machines

lever

wedge

screw

wheel and axle

pulley

inclined plane

lever

wedge

inclined plane

simple machines that should be avoided in the design of removable partial dentures

lever

a rigid bar supported somewhere along its length

it may rest on the support or may be supported from above

fulcrum

the support point of the lever

the lever can move around this

the center of rotation as the distal extension base moves toward the supporting tissue when an occlusal load is applied

the rotation of extension base rpd

rotates in relation to the three cranial planes:

• sagittal

• frontal

• horizontal

this is due to differences in the support characteristics of the abutment teeth and the soft tissue covering the residual ridge

3 types of levers

first class

second class

third class

cantilever

can be described as a rigid beam supported only at one end

a beam supported at one end that can act as a first-class lever

use of a dental implant

one strategy to provide tooth replacement and avoid the cantilever

most efficient means of addressing the potential effects of a lever

to provide a rigid element at the unsupported end to disallow movement

vertical forces

tooth is apparently better in tolerating this directed forces than nonvertical, torquing, or horizontal forces

more periodontal fibers are activated to resist the application of forces to teeth than are activated to resist the application of nonvertical forces

nonvertical forces

a directed force that an abutment tooth will better tolerate if these forces are applied as near as possible to the horizontal axis of rotation of the abutment

fulcrum

classification of lever is based on:

in the tooth / mucosal tissue–supported prosthesis

this is where the greatest movement possible is found because of reliance on the distal extension supporting tissue to share the functional loads with the teeth

first movement of rotation

if direct retainers are functional, rotation should occur

rotation occurs through posterior abutments and occlusal rests

retentive clasp arms and minor connectors act as indirect retainers

fulcrum line shifts towards anteriorly placed components as base moves away from supporting tissue

dislodging forces on partial denture result from vertical pull of food, border tissue movement, and gravity

indirect retainers should be placed far from distal extension base for best leverage against lifting

second movement of rotation

occurs along a longitudinal axis as distal extension base moves in a rotary direction about the residual ridge

prevented by direct retainers' rigid components and major connector's torque resistance

rotation can cause horizontal denture base shifting if connectors aren't rigid or a stress-breaker exists

third movement of rotation

rigid connectors are necessary for this effect

occurs due to diagonal and horizontal occlusal forces

denture rotation about an imaginary vertical axis near the center of the dental arch

stabilizing components like reciprocal clasp arms and minor connectors resist this movement

stabilizing components on one side of the arch stabilize the partial denture against horizontal forces

horizontal forces in dentures

will always exist due to lateral stresses from mastication, bruxism, clenching, and patient habits

influenced by occlusal plane orientation, malpositioned teeth, and abnormal jaw relationships

sagittal plane

rotation around a fulcrum line passing through the most posterior abutments when the denture base moves vertically toward or away from the supporting residual ridges

frontal plane

rotation around a longitudinal axis formed by the crest of the residual ridge

horizontal plane

rotation around a vertical axis located near the center of the arch

movement of the base toward the edentulous ridge is prevented by:

by abutment teeth rests

rigid framework in a tooth-supported partial denture

movement of the base away from the edentulous ridge is prevented by:

by direct retainers on abutments and rigid minor connector stabilizing components

primary aim of a mesial rest

to alter the fulcrum position and resultant clasp movement, disallowing harmful engagement of the abutment tooth

tapered wrought-wire retentive arm

this design would be kinder to the periodontal ligament than would a cast, half-round retentive arm

this design is applicable when the distobuccal undercut cannot be found or created or when the tissue undercut contraindicates placement of a bar-type retentive arm

occlusal / incisal surface

clasps placed in this surface have a greater likelihood of imparting tipping forces to the abutments

occlusal rests

should not have steep vertical walls or locking dovetails

should provide occlusal support only to resist tissue-ward movement

characterized by lack of free movement, which could cause horizontal and torquing forces to be applied intracoronally to the abutment tooth

horizontal direction

movements in tooth-supported denture thus the use of intracoronal rests is permissible

these may be resisted by the stabilizing effects of components placed on the axial surfaces of the abutments

class I & II partial dentures

have one or more distal extension bases

lack tooth support and bounding abutment retention

class III or IV partial dentures

may derive some support from the edentulous ridge

therefore may have composite support from both teeth and ridge tissue

tooth-supported space

with teeth both anterior and posterior to the space

tooth- and tissue- supported space, distal extension

with teeth either anterior or posterior to the space

TYLMAN’S THEORY

“Great caution and reserve are essential whenever an attempt is made to interpret biological phenomena entirely by mathematical computation.”

effort arm

Whenever this arm is longer than resistance arm, mechanical advantage is in favor of this arm, proportional to the difference in length of the two arms

greater movement

when farther from the axis of rotation =

less movement

when near from the axis of rotation =

fulcrum lines

An imaginary line passing through the most posterior abutments as the distal extension base moves towards or away from the ridge