Biomechanics:
Consequence of prosthesis movement under load is:
Application of stress to the teeth
Tissue that are contacting the prosthesis
Open ended- problems typically have more than one solution
Ill structured- solution are not the result of standard math formulas used in
some structured manner.
It is important that the stress not exceed
the level of physiologic tolerance, which
Design process: is range of mechanical stimulus that a
system can resist without disruption or
-Identifying a need
traumatic consequences
-Defining the problem
- Setting design objectives
-Developing a design rationale
-Devising & evaluating alternative solutions
-Providing the solution
Supporting structures for RPD:
Abutment teeth Living things that are
Residual ridge subjected to forces
Bone- that provides the support for removable prosthesis (i.e alveolar bone by way of
the periodontal ligament and the residual ridge bone through its soft tissue covering)
The longer the handle, the less effort force it takes. This is a simple application of the
mechanics of leverage.
Simple machines - applied to the design of rpd helps to accomplish the objective of
preservation of oral structures.
Machines may be classified into two general categories :
Simple and Complex
Simple machines are:
Lever
Wedge
Screw
Wheel and axle
Pulley
Inclined plane
Lever- is a rigid bar supported somewhere along its length. It may rest on the support or
may be supported from above.
Fulcrum- support point of the lever. And the lever can move around the fulcrum.
Cantilever
- is a beam supported at one end that can act as a first class lever
- Cantilever should be avoided
Most efficient means of addressing the potential effects of lever is
- to provide a rigid element at the unsupported end to disallow movement.
- This is the most beneficial use of dental implants in conjuction with RPDs and
should be considered when support capacity for a distal extension is considered
significantly poor.
Distal extension RPD rotates when forces are applied to the artificial teeth
attached to the extension base.
Tooth tissue supported prosthesis - where greatest movement possible found because
of reliance on the distal extension supporting tissue to share the functional loads with
the teeth.
Movement of a distal extension toward the ridge tissue will be - proportionate to the
quality of that tissue, accuracy and extent of the denture base, and the applied total
functional load.
First movement - is rotation about an axis through the most posterior abutments.
This axis may pass through occlusal rests or any other rigid portion of a direct retainer
assembly located occlusally or incisally to the height of contour of primary abutments.
Fulcrum line - is the center of rotation as the distal extension base moves towards the
supporting tissue when an occlusal load is applied.
- The axis of rotation may shift toward more anteriorly placed components, occlusal
or incisal to the height of contour of the abutment, as the base moves away from
the supporting tissue when vertical dislodging forces act on the partial denture.
Disloding forces – result from the vertical pull of food between opposing tooth surfaces,
the effects of moving border tissue, and the forces gravity against a maxillary partial
denture.
Vertical tissue-ward movement of the denture base - is resisted by the tissue of the
residual ridge in proportion to the supporting quality of that tissue, the accuracy of the fit
of the denture base, and the total amount of occlusal load applied.
Movement of the base in the opposite direction – is resisted by the action of the
retentive clasp arms on terminal abutments and the action of stabilizing minor
connectors in conjunction with seated, vertical support elements of the framework
anterior to the terminal abutments acting as indirect retainers.
Indirect retainers - should be placed as far as possible from the distal extension base,
affording the best possible leverage against lifting of the distal extension base.
Second movement
- is rotation about a longitudinal axis as the distal extension base moves in a rotary
direction about the residual ridge.
- This movement is resisted primarily by the rigidity of the major and minor
connectors and their ability to resist torque.
If the connector are not rigid, or if a stress-breaker exists between the distal
extension base and the major connector, this rotation about a longitudinal axis
applies undue stress to the sides of the supporting ridge or causes horizontal
shifting of the denture base.
Third movement
- Is rotation about an imaginary vertical axis located near the center of the dental
arch.
- This movement occurs under function because diagonal and horizontal occlusal
forces are brought to bear on the partial denture.
- It is resisted by stabilizing components, such as reciprocal clasp arms and
minor connectors that are in contact with vertical tooth surfaces.
Stabilizing components on one side of the arch act to stabilize the partial denture
against horizontal forces applied from the opposite side. It is obvious that rigid
connectors must be used to make this effect possible.
Horizontal forces – always will exist to some degree because of lateral stresses that
occur during mastication, bruxism, clenching, and other patient habits.
- These forces are accentuated by failure to consider the orientation of the occlusal
plane, the influence of malpositioned teeth in arch and the effects of abnormal jaw
relationships
- The amount of horizontal movement occurring in the partial denture therefore
depends on the magnitude of the lateral forces that are applied on the
effectiveness of the stabilizing components.
Tooth-supported partial denture, movement of the base toward the edentulous ridge is
prevented primarily by the rests on the abutment teeth and to some degree by any rigid
portion of the framework located occlusal to the height of contour.
Movement away from the edentulous ridge is prevented by the action of direct
retainers on the abutments that are situated at each edentulous space and by the
rigid minor connector stabiilizng components.
Therefore:
First movement – can be controlled in the tooth-supported denture.
Second possible movement – occurs along a longitudinal axis, is prevented by the
ability of the major connector to resist torque. This movement is much less in the tooth
supported denture because of the presence of posterior abutments.
Third possible movement – occurs in all partial dentures. Therefore stabilizing
components against horizontal movement must be incorporated into any partial denture
design.
For prosthesis capable of movement in three planes:
Occlusal rests - should provide occlusal support only to resist tissue-ward movement.
- Entrance of the occlusal rest into a stabilizing function would result in direct
transfer of torque to the abutment tooth. Because movements around three
different axes are possible in a distal extension partial denture.
- Occlusal rest for such a partial denture should not have steep vertical walls or
locking dovetails.
- This rest design is characterized by lack of free movement, which could cause
horizontal and torqueing forces to be applied intracoronallyto the abutment tooth.
Tooth-supported denture – the only movements of any significance are horizontal,
and these may be resisted by the stabilizing effects of components placed on the axial
surfaces of the abutments.
- Therefore in tooth supported denture, the use of intracoronal rests is permissible.
In these instances,the rests provide not only occlusal support but also notable,
horizontal stabilization.
Impact of implants on movements of partial denture:
Use of implant should be directed toward the most beneficial movement control.
Possible roles for implant use include all three desired principles:
Support
Stability
Retention
