Design principles for class I and II - PowerPoint Presentation

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Design principles for class I and II

Dr.Nesreen

Salim

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O : Outline the saddle areas :

The

kennedy

classification of the case.

The primary abutment teeth are determined.

The number of guiding planes are determined.

The most appropriate AP tilt is determined

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Class I Kennedy

Primary abutments : LR 5 & LL 5

Potential guiding planes

Posterior saddle : Anterior tilt !

GP 1

GP 2

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Anterior tilt

exposure of

lingual

sulcus

(elimination of undercut) for placement of major connector.

exposure of the

distal surface

of the primary (terminal) abutment : minimum preparation for guiding plane achievement.

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S: Support

Tooth-tissue support

It is dependent on

the

residual ridge for a portion of its support.

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Factors influencing support:

1.

Contour

and

quality

of the residual ridge

2. Extent of residual ridge

coverage by thedenture base3. Type and accuracy of the impression registration4. Accuracy of the fit of the denture base5. Design of the partial denture framework6. Total

occlusal load applied

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Type and accuracy of impression registration

The residual ridge has two forms: the

anatomic form

and the

functional form

. The anatomic form is the surface contour of the ridge when it is

not supporting

an occlusal load. The functional form of the residual ridge is the surface contour of the ridge when it is supporting a functional load.The anatomic form is recorded by a soft impression material, such as a metallic oxide impression paste, if the entire impression tray is uniformly relieved. Depending on the viscosity of the particular impression material used and the rigidity of the tray.

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many dentists believe that certain regions of the residual ridge(s) are more capable of supporting dentures than other regions. Their impression methods are directed to place

more stress

on primary stress-bearing regions with

specially constructed individual trays

and at the same time record the anatomic form of other basal seat tissues, which cannot assume a stress-bearing role. The form of the residual ridge recorded under some

loading

, whether by occlusal loading, finger loading, specially

designed individual trays, or the consistency of the recording medium, is called the functional form. This is the surface contour of the ridge when it is supporting a functional load.

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Occlusal load

Reducing occlusal loads

Reducing the saddle movement under occlusal loads

How??

Decreasing the size of occlusal table :

a-

buccolingually

: - using narrow teeth

- using canines and premolars instead of premolars and molars b- antero-posteriorly : - leaving a tooth off a saddle

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2)

Wide coverage of the residual ridge area by the base of the saddle as is compatible with function.

Maximum coverage within the physiologic tolerance of the limiting border structure.

Reduce the load per unit area.

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Reducing the saddle movement under occlusal loads

Methods :

Mucofunctional

concept

Sufficient

retetion

(indirect support)

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Mesio-occlusal rest !

Disto-occlusal rest !

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movement of a distal extension base around posteriorly placed direct retainers is inevitable under functional loading. The extent of

rotational forces

to which abutment teeth are subjected is directly related to

the position and resistance

of the food bolus. The

greatest movement

takes place at

the most posterior extent of the denture base. The retromolar pad region and the tuberosity region therefore are subjected to the greatest movement of the denture base. As the rotational axis (fulcrum line) of the denture is moved anteriorly, more of the residual ridge is used to support the denture base, thereby distributing stresses over a proportionally greater area. Occlusal rests may be moved anteriorly to better use the residual ridge for support.

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Disto

-occlusal rest:

Simplicity

proximal plate

Disto

-occlusal rest

But???1- Creation of class I lever

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2. Distal displacement

of the abutment tooth

Distal drifting of the terminal abutment opening of the mesial contact point = food impaction = caries = periodontal disease

3. Less alveolar bone for support

The alveolar bone utilized for support lies distal to the primary supporting element…

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Mesio

-occlusal rest:

Additional minor connector

But???

1. Creation of class 2 lever

Occlusal forces will deactivate the lever action

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2. Mesial displacement of the terminal abutment

causing enhancement of the mesial contact point

3. More alveolar bone utilization for support

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4. Stress breaking effect :

Disengagement of primary abutment tooth upon occlusal load application (short guiding planes)

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Full tooth length plane ?

* Interferes with the physiological movements of the abutment tooth.

* Requires excessive preparation.

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Support access

2 rests = fulcrum

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Clasp access

Indirect retainers

Indirect retention is required

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Indirect retention

Indirect retention, to prevent the denture

from lifting away

from the residual ridge, should be incorporated in the design. The tooth-supported base is secured at either end by the action

of a direct retainer and supported at either end by a rest

, whereas this degree of support and direct retention is lacking in the distal extension prosthesis

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The posterior part of the distal extension saddle is capable of

rotating in

the horizontal plane. If a long saddle is clasped rigidly to a single

abutment tooth the rotatory movement can transmit

considerable force

to that tooth.

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Lever Classification

Resistance

Force

Fulcrum

Class I (seesaw)

Effort arm

Resistance arm

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Lever Classification

Class II

Fulcrum

Resistance

Force

Resistance arm

Effort arm

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Indirect retainers

do not prevent

displacement

towards

the

ridge. This

movement is resisted by the occlusal rest on the abutment tooth and by full extension of the saddle to gain maximum support from the residual ridge.

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