BIOMICHANICS OF REMOVABLE PARTIAL DENTURE

Definition: The relationship between the biologic behavior of oral structures and the physical influence of an R P D.

Bio ------ pertaining to living systems-----inflammation, Caries, b. resorption….etc

Mechanical ----- related to forces and its application to object-----looseness of teeth , bon resorption……etc

Mechanics may be classified into two general categories: Simple & complex.

Complex machines are combination of many simple machines.

There are six simple machines

1 - lever

2 - inclined plane

3 – wedge

4-screw

5 –wheel

6 – axle & pulley

A removable partial denture in the mouth can perform the action of two simple

machines, LEVER & INCLINED PLANE,

LEVER : The lever is a rigid bar supported at some point along it is length.

There are three types of lever:

Classification is based on location of fulcrum (support), resistance, and direction of effort (force).

The first type: the fulcrum (F) is in center of the bar, resistance (R) is at one and the force (E) is at opposite end (called cantilever).

A cantilever: It is a beam supported only at one end, when force is directed against unsupported end of beam cantilever can act as first class lever.

The second-class lever: the fulcrum at one end, the force at opposite end & the resistance in center. This type is seen as indirect retention in R P D.

The third class lever: the fulcrum t one end & the resistance at opposite end & the force in the center. This type is not encountered in R P D. (e.g. tweezers)

Mechanical advantage = Effort arm / Resistance arm

The length of fulcrum to resistance is called Resistance arm, while the length of lever from fulcrum to the point of application of force is called Effort arm.

CLINICAL APPLICATION OF LEVER:

Every effort should be done to avoid class I lever (cantilever). To avoid this cantilever (lever class I) we can made either lever class II or using stress release direct retainer.

a) Lever class II

Where the fulcrum at one end, the force at opposite end & the resistance in center. This type called equipoise force system.{see direct retainer}

In this class, the occlusal rest (F) located mesially, while the retentive tip (R) positioned distally, and the saddle (E) located distal to the retentive tip i.e. the (R) located in between the (F) & (E).

b) Stress release direct retainer

In general, if stress release is desirable, a mesial rest with a mesial undercut or distal rest with distal undercut should be used. A clasp with distal rest and a wrought wire clasp arm engaging the mesial undercut is the exception.

This can explain the difference between location of rest and retentive tip mesially in gingivally approaching clasp as (McCr), and distally location as (Stew). The both authors depending on the concept of stress release.

Inclined plane

Inclined plane is nothing but two inclined surfaces in close alignment to

one another. The direct retainers and the minor connectors slide along the guide

plane of the teeth and can act as inclined planes if no prepared correctly.

When a force is applied against an inclined plane it may produce two

actions:

Deflection of the object, which is applying the force (Denture).

Movement of the inclined plane itself (tooth) .These results should

be prevented to avoid damage to the abutment teeth.

BIOMECHANICAL CLASSIFICATION OF R.P.D. ( Based on the nature of the supporting tissues)

A. TOOTH BORNE (tooth supported or dentoalveolar supported).

1. Abutment teeth border all edentulous areas where tooth replacement is planned.

2. Functional forces are transmitted through abutment teeth to bone.

B. TOOTH - MUCOSA BORNE (tooth and mucosa supported, den to-alveolar and muco-osseous supported or extension base ).

1. Exhibits one or more edentulous areas which are not bordered by abutment teeth (extension base RPDs).

2. forces are transmitted through abutment and mucosa to bone.

3. The majority of these are distal extension RPDs.

4. This category may apply to tooth bordered situations when excessive abutment tooth mobility is present or when long span tooth bordered edentulous areas are present precluding primarily tooth support.

C. MUCOSA BORNE. (muco-osseous supported)

1. Regardless of the natural teeth present, support is derived entirely from the mucoosseous segment.

2. This category includes prostheses fabricated from hard or combinations of resilient and hard denture base materials such as stayplates which function as interim or transitional prostheses.

3. These prostheses usually do not contain a metal framework and usually should not be considered definitive treatment.

CHARACTERISTICS OF FAVORABLE DENTO-ALVEOLAR SUPPORT A.TEETH.

1. Structurally sound. 2. Anatomically favorable.

a. Root surface area. b. Root morphology. c. Presence of multiple

roots.

d. Presence of divergent roots.

e. Crown to root ratio. f. Axial inclination.

B.PERIODONTIUM. 1. Normal (absence of periodontal disease).

a. Gingival indices within normal limits.

b. Absence of increasing mobility or hyper mobility. 2. Anatomically favorable.

a. orrnal epithelial and connective tissue attachment. b.Adequate zone of attached gingiva.

C. ALVEOLAR BONE. 1. Favorable bone index. 2. Anatomically normal.

a. Bone height. b. Degree of

mineralization. c. Presence of lamina dura.

CHARACTERISTICS OF FAVORABLE MUCO-OSSEOUS SUPPORT A.MUCOSA.

1. Normal. 2. Keratinized. 3. Firmly bound.

B. SUBMUCOSA.

1. Normal sub mucosa serves as an "hydraulic cushion". 2. Firmly bound and dense.

C. BONE. 1. Cortical bone. 2. Favorable bone index.

3. Presence of muscle attachments which direct tension to bone (or the equivalent in terms of resistance to pressure induced resorption).

OPTIMAL FORCE BEARING MUCOOSSEOUS ANATOMIC REGIONS

A.MAXILLARY. 1. Horizontal hard palate.

a. Keratinized mucosa.

b. Presence of fatty (anterior) and glandular (posterior) submucosa (excluding midline suture).

c. Cortical bone. 2. Posterior ridge crest.

a. Keratinized mucosa. a. Presence of dense firmly bound submucosal connective tissue

which may contribute to clinically observed resistance to pressure induced resorption.

Maxillary primary (10) supporting areas are the horizontal hard palate

and posterior ridge crest.

The periphery of the denture bearing area is non-contributory (N/C).

The midline suture often requires relief (R)

and the anterior ridge crest serves as a secondary (2°) supporting area.

B. MANDIBULAR. 1. Buccal shelf. A primary force bearing area which is comprised of

cortical bone. It extends from the base of residual ridge in the poste-rior part of the mandible to the external oblique ridge. a. Presence of submucosa. b. Cortical bone. a. Buccinator muscle attachment. The longitudinally directed fibers

apply tension to the underlying bone but do not dislodge the denture base during contraction.

2. Pear-shaped pad. The most distal extension of keratinized tissue covering the ridge crest. It is formed by the scarring pattern following the extraction of the most distal mandibular molar. It should be differentiated from the m~e posterior retromolar pad during clinical examination. a. Keratinized mucosa. a. Presence of dense firmly bound submucosa. a. Medial tendon of the temporalis muscle inserts lingually in the

area of the apices of the mandibular third molars and applies tension to the underlying bone.

Mandibular primary (10) supporting areas are the buccal shelf and pear-shaped pad. The anterior facial incline of the ridge is non-contributory (N/C). The lingual ridge inclines may require relief (R)and the genial tubercle area

and ridge crest serve as secondary (2") supporting areas.

Stresses acting on a partial denture are transmitted to the teeth, and

tissues of the residual ridges. The stresses, which tend to move the denture in

different directions, may be summarized as follows:

1- Masticatory stresses.

2- Gravity tends to displace a maxillary denture downwards.

3- Sticky food tends to pull the denture occlusally away from the tissues.

4- Muscle pull and tongue action tend to displace a denture from its

position.

5- Intercuspation of teeth may tend to produce horizontal and rotational

stresses unless the occlusion is balanced.

FORCES ACTING ON REMOVABLE PARTIAL DENTURES

The Supporting structures for removable partial are structurally adapted to receive and absorb forces within their physiological tolerance. The ability of these structures to tolerate forces is largely dependent upon the magnitude, the duration and the direction of these forces in addition to the frequency of force application.

The magnitude of forces acting on partial dentures depends on age and sex of the patient, the power of the muscles of mastication and the type of opposing occlusion.

Natural teeth are better able to tolerate vertical directing forces acting on them. This is because more periodontal fibers are activated to resist the application of vertical forces. On the other hand, lateral forces are potentially destructive to both teeth and bone. Lateral forces should be minimized in order to be within the physiologic tolerance of the supporting structures.

TYPE OF FORCES ACTING ON RPD

I- Vertical forces

A) Tissue-ward movements B) Tissue-away movements

II- Horizontal forces:

A) Lateral movements B) Antero-posterior movements.

III- Rotational forces:

They are due to the variation in compressibility of supporting structures, absence of distal abutment at one end or more ends of denture bases, and /or absence of occlusal rests or clasps at any end of the bases.

1-Rotation of the anterior and posterior extension denture base around coronal (transverse) fulcrum axis:

A) Rotation of the denture base towards the ridge around the fulcrum axis joining the two main occlusal rests:

B) Rotation of the denture base away from the ridge around the fulcrum axis joining the retentive tips of the clasps.

2-Rotation of all bases around a longitudinal axis parallel to the crest of the residual ridge (Buccolingual or labiolingual).

3-Rotation about an imaginary perpendicular axis, this axis either near the center of the dental arch in class I, or is the long axis of abutment tooth in class II partial denture.

I- Tissue-ward movements

a) Tissue-ward forces are, “Vertical forces acting in gingival direction tending to move the denture towards the tissues”.

They occur during mastication, swallowing and aimless tooth contact. Biting forces falling on artificial teeth are transmitted to the soft tissues and bone underlying the denture base.

b) The partial denture should be designed to resist this movement byproviding adequate supporting components. This function of the partial denture is called “Support”.

Support is the function of partial denture which prevents movement of the denture towards the tissues.

Support is mainly provided by:

a) Properly designed supporting rests placed in rest seats, which are prepared on the abutment teeth,

b) Broad accurately fitting denture bases in distal extension partial dentures. Therefore, the entire available ridge posterior to the abutment teeth must be covered with the denture.

c) Rigid major connectors that are neither relieved from the tissues nor placed on inclined planes also provide support.

d) Rigid portion of clasps placed over the survey line

II- Tissue-away movements

a) Tissue-away dislodging forces are, "Vertical forces acting in an occlusal direction tending to displace and lift the denture from its position”.

Tissue-away forces occur due to: The action of muscles acting along the periphery of the denture, gravity acting on upper dentures or by sticky foodadhering to the artificial teeth or to the denture base.

b) The partial denture should be designed to resist this movement byproviding adequate Retention.

Retention is “The function of partial denture which prevents the denture from being displaced in an occlusal direction (away from the tissues)".

Retention in partial dentures is mainly provided by: {see direct retainer for detail}

a- physical forces which arise from coverage of the mucosa by the denture.

b- Physiologic factors: Patient’s muscular control acting through the polished surface of the denture.

c- Mechanical means such as clasps which engage undercuts on the tooth surface.

In order to retain the denture, the anticipated intensity of occlusally displacing force exerted during function should be less than the force required for retaining the denture.

3) Horizontal movements:

A) Lateral movements

a) Lateral forces are “Horizontal forces developed when the mandible moves from side to side during function while the teeth are in contact”.

Lateral movements have a destructive effect on teeth leading to tilting, breakdown of the periodontal ligament and looseness of abutment teeth. The application of lateral forces causes areas of compression of the periodontal membrane, which leads to bone resorption. Hence lateral forces play a major role in bone resorption,

b) Partial dentures should be designed to prevent the deleterious effects of lateral forces by using stabilizing or bracing components.

Bracing is "The function of partial denture which resists lateral movement of the appliance".

Stabilizing components are "Rigid components of the partial denture that assist in resisting horizontal movement of the denture". They help in distributing lateral stresses to all supporting teeth:

1. Bracing clasp arms placed at or above the survey line of the tooth.

2. Minor connectors in contact with axial (vertical) surfaces of abutment teeth

3. Proximal plates.

4. Adequate extension of the flanges of the denture helps to stabilize the prosthesis against horizontal forces.

5. Rigid portions of clasps.

6. Lingual plates.

7. Rests - When the walls of the rest seat are relatively parallel to the path

of placement (e.g. channel rests).

The magnitude of lateral forces could also be minimized by:

1. Reducing cusp angles of artificial teeth.

2. Providing balanced occlusal contacts free of lateral interference.

The removable partial denture being anchored to both sides of one arch and joined by a rigid major connector can provide cross arch stabilization to forces acting in bucco-lingual direction.

B) Antero-posterior movements

a) Antero-posterior forces are "Horizontal forces which occur during forward and-backward movement of the mandible while the teeth are in contact". This may result in movement of the denture.

There is natural tendency for the upper denture to move forward and for the lower to move backward.

b) Partial dentures should be designed to prevent the deleterious effects of antero-posterior forces by

Forward movement of the upper denture could be resisted by:

1. Anterior natural teeth.

2. Palatal slope.

3. Maxillary tuberosity.

4. The natural teeth bounding the edentulous space.

The backward movement of the lower denture could be resisted by:

1. The slope of the retromolar pad.

2. The natural teeth bounding the saddle area.

3. Proximal plates.

VI- Rotational movements:

a)Rotational forces are “Forces acting on the partial denture either in vertical or horizontal direction causing rotation (torque) of the denture base around an axis.

In tooth supported removable partial dentures, the abutment teeth on both sides of the edentulous area provide adequate support and resistance to rotational forces through supporting rests and clasps placed on them.

In distal extension partial denture when vertical forces are applied the difference in displaceability of the supporting structures often results in rotation of the partial denture around a fulcrum axis and application of torque on abutment teeth.

Rotational movements must be counteracted in the partial denture design to minimize their destructive effect on both, teeth and the residual ridge.

Rotational forces acting on distal extension partial denture may result in three possible rotational movements these are

I- Rotation of the denture base around the fulcrum axis (Torque).

II- Rotation about a longitudinal axis formed by the crest of the residual ridge (Tipping movement).

III- Rotation about an imaginary perpendicular axis near the center of the dental arch (Fish tail movement).

I-Rotation of the denture base around fulcrum axis joining the principal abutments:

Movement of the component parts of the denture lying on the opposite side of the fulcrum axis occur in a direction opposite to that of the applied force. This leads to rotation of the denture:

The fulcrum axis is an “imaginary line passing through teeth and component parts of the partial denture around which the distal extension partial denture rotates when a vertical force is applied”.

More than one fulcrum lines may identified for the same removable partial denture depending on the direction and location for force application.

(a) Rotation of the denture base towards the ridge:

This movement results from occlusal stresses occurring during mastication and occlusion of teeth. The free extension denture base moves tissue-ward while other components on the opposite side of the fulcrum line moves away from the tissues. This result in rotation of the denture about a diagonal supportive fulcrum line joining two occlusal rests on the most posterior abutments on either side of the dental arch

Tissue ward movement of the base could be limited by supporting structures, which are:

1. Supportive form of the residual ridge,

2. Accurate and properly extended bases.

3. Artificial teeth set on the anterior two third of the base

Flexible clasps are preferred over rigid clasping to reduce stresses and torque applied on abutments. If the clasps are rigid, the abutments tend to rotate distally during tissue ward movement of the denture base resulting in periodontal breakdown and looseness of teeth.

(B) Rotation of the denture base away from the ridge.

This movement occurs due to the pulling effect of forces applied by sticky food, gravity on upper dentures and the elastic rebound of soft tissues covering the edentulous areas.

Tissue-away rotation of denture base is counteracted by:

1- Indirect Retainers: which are the components of partial denture located on the side of the fulcrum axis opposite to the distal extension base.

2- The retentive tip of the clasp arm.

3- Adequate coverage and extension of the base (direct indirect retention )

4- Effect of gravity on mandibular bases.

II-Rotation around a longitudinal axis formed by the crest of the residual ridge (Tipping movement)

This rotation occurs due to application of vertical forces on one side of the arch only. It causes twisting of the denture base.

This movement is counteracted by:

1- Cross arch stabilization (The action of clasps on the opposite side of the arch).

2- Broad base coverage.

3- Proper placement of artificial teeth (teeth on the ridge or lingualized occlusion).

4- Narrow teeth bucco-lingually.

5- The effect of rigid major connectors.

III- Rotation around an imaginary perpendicular axis near the center of the dental arch

Application of horizontal or off-vertical force results in rotation around an imaginary vertical axis located either about the axis of abutment in class II or near the center of the dental arch, lingual to anterior teeth in class I.

It results due to the application of masticatory forces falling on distal extension bases causing buccolingual movement of the base. This rotation is called fishtail movement.

This movement is counteracted by :

1- Providing adequate bracing components in the partial denture.

2- A rigid major connector.

3- Broad base coverage.

4- Balanced contact between upper and lower teeth.

Forces accruing through a removable restoration can be widely distributed, directed, and minimized by the selection, the design, and the location of components of removable partial dentures and by developing a harmonious occlusion.

Force Cause of the Force Counteraction of the force Function

I- Vertical Forces :

1- Tissue-ward displacing

forces.

Functional movements

during mastication,

swallowing and

occlusion of upper and

lower teeth.

- Rests placed on abutments in

bounded saddles.

- Rests & proper base coverage in

free end bases.

- Maxillary connectors

- Support

2- Occlusally displacing

forces.

Pulling effect of sticky

food Gravity on upper

dentures. Muscles acting

on periphery of denture

- Retainers.

- Adhesion & cohesion between

denture base & tissues

- Retention

II- Horizontal Forces

1- Lateral forces.

Side to side movement of

the mandible while teeth

are in contact.

- Rigid bracing clasp arms.

- Major connectors.

- Balanced occlusion.

- Maximum extension of the flanges

- Bracing

(Stabilization

2- Antero-posterior forces Forward and backward

movement of mandible

while teeth are in contact

- Abutments adjacent to the denture.

- Guiding planes.

-

Stabilization

III- Rotational forces :

1- Vertical forces in gingival

direction in free-end saddles.

- Functional movements

while teeth are in

occlusion.

- Supporting rests.

- Properly adapted bases.

.

- Support

2- Vertical forces in occlusal

direction in free-end saddles.

- Sticky food, gravity on

upper dentures, elastic

rebound of tissues under

the base.

- Indirect retainers.

- Direct retainers.

-Indirect

retention.

Factors affecting stress generation and transfer

1- Length of span: - the longer edentulous span, the greater force will be transmitted to the abutment. so the Posterior teeth should be preserved as far as possible to reduce the length of the edentulous span

2- Quality of the supporting tissues:

Form of the residual ridges: large well developed ridges, absorb more amount of force than small, thin ridge.

Type of mucosal covering : atrophic and flabby mucosa are not preferred.

3- Quality of clasp: - the more flexible clasp arm, the less force transmitted

to the abutment.

4- Clasp design: - a passive clasp when it is completely seated on the

abutment teeth will exert less stress on the tooth than the non passive.

A clasp should be designed so that the reciprocal arm contacts the

tooth before the retentive tip passes over the greatest bulge of the tooth

during insertion and it should be the last component to lose tooth

contact during removal of the prosthesis.

5- Length of the clasp.

Doubling the length increases the flexibility by five times. This

decreases the stress on the abutment tooth. Using a curved rather than a

straight clasp on an abutment tooth will aid to increase the clasp length

6- Material used in clasp construction A clasp constructed of chrome alloy will exert more stress on

the abutment tooth than a gold clasp because of its greater rigidity. To decrease the stress, the chrome alloy clasps are constructed with a smaller diameter.

7- Abutment tooth surface: - the surface of a gold crown or restorationoffers more functional resistance to clasp arm movement than does of enamel surface of a tooth therefore greater stress is exerted on the abutment.

8- Occlusal relationship of the remaining teeth and orientation of the

occlusal plane.

Type of the opposing occlusion

Harmony of the occlusion should be present.

Improper occlusal relationship and a steep occlusal plane tend to

increase the amount of force acting on the denture. The force

applied on natural teeth is 300 pounds and the force acting on

artificial teeth is about 30 pounds. Poor occlusal relationship can

lead to supra-eruption of the opposing natural teeth.

9- Musculature of the patient.

10- Response of oral structures to previous stress. The periodontal

condition of the remaining teeth, need for splinting and the amount of

abutment support remaining are all a result of the previous stress

subjected on the oral tissues.

RESPONSE OF FORCE BEARING TISSUES TO MECHANICAL LOADING

The forces directed to the supporting tissues will be partially absorbed and partially transmitted to adjacent tissues.

The percentage of force absorbed or transmitted will vary depending upon which tissue is involved.

Bone is the tissue which ultimately absorbs the greatest amount of the force applied to both the muco-osseous and dento-alveolar segments.

A.DENTO-ALVEOLAR SEGMENT.

1.Tooth.

a. Structurally sound vital teeth are capable of withstanding normal functional forces.

b. Excessive forces may result in adverse effects.

Structural failure (tooth fracture).

Tooth movement.

Pulpal irritation. Reversible pulpitis (hyperemia) or irreversible pulpitis,

c. Structurally compromised teeth may fail in response to normal functional forces.

Teeth with large intracoronal restorations.

Endodontically treated teeth.

2.Periodontium

including gingiva, crevicular epithelium, junctional epithelium, connective tissue attachment, cementum, periodontal ligament and alveolar bone.

a. A normal periodontium permits some force absorption without damaging effects.

b. Excessive forces may increase the width of the periodontal ligament and result in increased tooth mobility.

c. Plaque induced inflammation may compromise the periodontium. It can lead to apical migration of the crevicular epithelial attachment (functional epithelium) and destruction of the fibroblasts and connective tissue of the connective tissue attachment. In the presence of inflammation normal functional forces may accelerate the rate of periodontal attachment loss.

3.Alveolar bone.

a. Pressure - tension theory. Bone tends to resorb in response to compressive force and to be stimulated by tensional force. In order to preserve remaining alveolar bone, it is important that functional forces be transmitted to bone primarily as tension rather than pressure whenever possible.

In tooth borne situations the majority of functional forces are transmitted as tension to bone through proper rest design and rest seat preparation. In tooth-mucosa borne situations some of the vertical seating forces are transmitted as tension to the bone through the rests. Horizontal forces are transmitted as a combination of compressive and tensional forces to the alveolar bone (e.g. those forces directed through bracing clasps, proximal plates and minor connectors contacting proximal tooth surfaces and guiding planes). Vertical displacing forces are transmitted to the bone as both compressive and tensional forces (e.g. sticky foods or retentive clasps engaging undercuts).

b.Bone index or Bone factor. The response of bone to pressure varies in terms of the rate of resorption depending on genetic, nutritional, hormonal and biochemical and other intrinsic factors. The bone index is determined by analyzing the previous response of bone to force.

c. Cortical vs. cancellous bone. Cortical bone is more dense, more highly mineralized, less cellular, and less metabolically active. It tends to be more resistant to pressure induced resorption than cancellous bone. Lamina dura is cortical bone.

d. Excessive forces which increase compressive components of forces transmitted to bone may increase the rate of bone resorption.

e. Periodontal disease. The presence of plaque induced periodontal disease is associated with a loss of bone height. Moderate forces may accelerate the disease process resulting in further bone loss, less bone support, and increased mobility of the teeth.

B. MUCO-OSSEOUS SEGMENT.

1.Mucosa.

a.Normal. firmly bound, keratinized tissues withstand mechanical forces within physiologic limits.

b. Excessive mechanical forces may cause mucosal ulceration (e.g. denture sore spots).

2.Submucosa

a. Provides an "hydraulic cushion" effect.

b. Increased thickness of the submucosa improves tolerance of the residual ridge to applied forces.

3.Bone

a. Pressure-tension theory. The functional loading of a tooth-mucosa borne denture base transmits force to the bone of the rnuco-oss ous segment almost exclusively as pressure which tends to cause resorptive changes. Resorption occurs in proportion to the intensity, duration, and direction of the applied force and as influenced by the bone factor. With some longer span tooth borne partial dentures or when excessive mobility of abutment teeth is present some force may also be delivered through the mucosa to the underlying bone as pressure.

b. Bone index. The bone index of the alveolar bone surrounding natural teeth may differ from that of the bone comprising the residual ridges. (Fig. 3-6)

c. Cortical vs. cancellous bone. The residual ridge crest is comprised mainly of cancellous bone and is less resistant to resorption. The facial and lingual inclines of the residual ridges are comprised of cortical bone and are more resistant to remodelling. The rate of cancellous bone resorption has been described as being approximately three times that of cortical bone.

d.Excessive forces may increase the rate of bone resorption.

e. Moderate forces may result in accelerated bone resorption when intrinsic factors, local abnormalities or systemic disorders compromise the bone index of the individual.

REACTION OF TISSUE TO METALLIC COVERAGE

The reaction of tissue to coverage by the metallic components of a removable partial denture has been the subject of significant controversy, particularly in regions of marginal gingiva and broad areas of tissue contact.

These tissue reactions can result from

1) Pressure from lack of support,

2) lack of adequate hygiene measures,

3) prolonged contact through continual use of a prosthesis.

Pressure occurs at regions where relief over gingival crossings and other areas of contact with tissue that are incapable of supporting the prosthesis is inadequate. Impingement will likewise occur if the denture settles because of loss of tooth and/or tissue support. This may be caused by failure of the restareas as a result of improper design, caries involvement, fracture of the rest itself, or intrusion of abutment teeth under occlusal loading. It is important tomaintain adequate relief and support from both teeth and tissue. Settling of the denture because of loss of tissue support may also produce pressure elsewhere in the arch, such as beneath major connectors. Settling of a prosthesis must be prevented or corrected if it has occurred. Excessive pressure must be avoided whenever oral tissue must be covered or crossed by elements of the partial denture.

Lack of adequate hygiene measures can result in tissue reactions because of an accumulation of food debris and bacteria. Coverage of oral tissue with partial dentures that are not kept clean irritates those tissue because of an accumulation of irritating factors. This has led to a misinterpretation of the effect of tissue coverage by prosthetic restorations. An additional hygiene concern relates to the problem of maintaining cleanliness of the tissue surface of the prosthesis.

The first two causes of untoward tissue reaction can be accentuated the longer a prosthesis is worn. It is apparent that mucous membranes cannot tolerate this constant contact with a prosthesis without resulting in inflammation and breakdown of the epithelial barrier. Some patients become so accustomed to wearing a removable restoration that they neglect to remove it often enough to give the tissue any respite from constant contact. This is frequently true when anterior teeth are replaced by the partial denture and the individual does not allow the prosthesis to be out of the mouth at any time except in the privacy of the bathroom during tooth brushing. Living tissue

should not be covered all the time or changes in those tissue will occur. Partial dentures should be removed for several hours each day so that the effects of tissue contact can subside and the tissue can return to a normal state.

Clinical experience with the use of linguoplates and complete metallic palatal coverage has shown conclusively that when factors of pressure, cleanliness, and time are controlled, tissue coverage is not in itself detrimental to the health of oral tissue.

Controlling Stress by Design Considerations

1- Direct Retention :

1. Clasp The retentive clasp arm is the element of RPD that is responsible for

transmitting most of destructive forces to the abutment teeth. A RPD should always be designed to keep clasp retention to a minimum yet provide adequate retention to prevent dislodgment of the denture by unseating forces. It should also be remembered that the retentive clasp should be designed such that it is active only during insertion and removal.

2. Forces of adhesion and cohesion To secure the maximum possible retention through the use of forces of

adhesion, the denture base should cover the maximum area of availablesupport and must be accurately adapted to the underlying mucosa.

3. Frictional control The RPD should be designed so that guide planes are created on as many

teeth as possible. Guide planes are areas on teeth that are parallel to the path of insertion and removal of the denture. The plane may be created on the enamel surfaces of the teeth or restorations placed on teeth. The friction of RPD against parallel surfaces can contribute significantly to retention of the denture.

4. Neuro-muscular control The design and contour of the denture base can greatly affect the ability of

lips, checks and tongue to retain the prosthesis. Any over-extension of the denture base either facially, lingually in the mandible or posteriorly onto the soft palate will contribute to the loss of retention and the abutment teeth bearing the direct retainers will be over stressed.

5. Clasp Position a- Quadrilateral configuration

Four abutments are utilized for clasping. Quadrilateral configuration is indicated in Class III particularly when there is a modification space on the opposite side of the arch. A retentive clasp should be positioned on each abutment adjacent the edentulous space. This result in denture being confined within the outline of four clasps

b- Tripod ConfigurationTripod clasping is used primarily for class II arches. If there is a modification

space on the edentulous side the teeth anterior and posterior to the space are clasped. If a modification space is not present. One clasp on the edentulous side of the arch should be positioned as far posterior as possible and the other, as far anterior as factors such as interocclusal space, retentive undercut, and esthetic considerations will permit. By separating the two abutments on the tooth-supported sides as far as possible, the largest possible area of the denture will be enclosed in the triangles formed by the clasps.

c- Bilateral configuration

Most RPD with bilateral distal extension group in class I fall into bilateral configuration. In the bilateral configuration the clasp exert little neutralizing effect on the leverage induced stresses generated by the denture base. These stresses must be controlled by other means.

6. Clasp design : a- Circumferential clasp : The conventional circumferential cast clasp originating from a distal occlusal

rest on the terminal abutment tooth and engaging a mesio-buccal retentive undercut should not be used on a distal extension RPD. The terminal of this clasp reacts to movement of the denture base toward the tissue by placing a distal tipping, or torquing, force on the abutment teeth. This particular force is the most destructive force a retentive clasp can exert. This clasping concept must be avoided.

On the other hand if the circumferential clasp with mesial occlusal rest approaches a disto-buccal undercut form the mesial surface of the abutment, is acceptable. The effect on the abutment is reversed from that of the conventional clasp. As the occlusal load is applied to the denture base, the retentive terminal moves further gingivally into the undercut area and loses

contact with the abutment teeth. In this manner torque is not transmitted to the abutment tooth.

b- Vertical projection or Bar clasp :

The vertical projection clasp, or bar clasp is used on the terminal abutment tooth on a distal extension RPD when the retentive undercut is located on the disto-buccal surface. As the denture base is loaded toward the tissue, the retentive tip of the clasp rotates gingivally to release the stress being transmitted to the abutment tooth.

c- Combination clasp :

When a mesio-buccal undercut exist on abutment tooth adjacent to a distal extension edentulous ridge, the combination clasp can be employed to reduce the stress transmitted to the abutment tooth. wrought alloy wire, by virtue of its internal structure, is more flexible than a cast clasp. It can flex in any plane, whereas a cast clasp flexes in the horizontal plane only. The wrought wire retentive arm has a stress-breaking action that can absorb torsional stress in both vertical and horizontal planes.

Flexible clasps produce the least stress and rigid cast circumferential clasps produce the maximum stress in an abutment.

2- Indirect Retention

The indirect retention is essential in the design of class I and II RPD, byusing the mechanical advantages of leverage; it counteracts the forces attempting to move the denture base away from the residual ridge by moving the fulcrum farther from the force.

In class I prosthesis, the fulcrum line would be moved from the tips of the retentive clasp to the most anteriorly located component, the indirect retainer. Because the indirect retainer resists lifting forces at the end of a long lever arm, it must positioned in a definite rest seat so that the transmitted forces are diverted apically through the long axis of abutment tooth. Theindirect retainer also contributes to a lesser degree, to the support and stability of the denture.

Class I : indirect retainer must always used.

Class II: it is not as critical as in class I but still required. Modification space can provide indirect retention. A definitive occlusal rest seat anterior may increase the effectiveness of indirect retention.

Class III : indirect retention is not ordinarily required except in :

- long lingual bar major connector to provide additional vertical support. - Lingual plate major connector.

Class IV : is considered reverse of class I and II. The lever arm is anterior to the fulcrum line, so the indirect retainer must be located as far posterior as possible. Occlusal rests and clasp assemblies are placed on the most posterior teeth for both direct retention and support.

3- Occlusion

The occlusal surfaces, or food table, of artificial teeth can transmit various amounts of stress to the supporting structures. A large or broad occlusal surface deliver more stress than does one that has been reduced in bucco-lingual width. The number of teeth replaced may also be reduced to decrease stress. Harmonious occlusion should be developed.

4- Denture Base

The denture base should be designed to cover as extensive an area of supporting tissue as possible. The stress created by the partial denture in function will thus be distributed over a large area, so no single area will be subjected to stress beyond its physiologic limit. The denture base flange should be made as long as possible to help stabilize the denture against horizontal movements.

The distal extension denture base must always extend onto the retromolar pad area in the mandible and cover the entire tuberosity of the maxilla. Both structures are capable of absorbing more stress than alveolar ridge anterior to them.

The type of impression used to record the residual ridge will influence the amount of stress the residual ridge can effectively absorb. Several techniques are used to make functional impression of the residual ridge. Each technique is based on the theory that if the ridge were recorded in its

functional state rather than its resting form, when the denture base is actually subjected to occlusal loading, the tissue would not displaced to any great stint. The magnitude of stress transmitted to the abutment teeth, therefore, would be minimal.

Denture base should be accurate and stable. The polished surfaceshould have the proper form and contour.

5- Major Connector

In the mandibular arch the lingual plate major connector that is properly supported by rests can aid in the distribution of functional stresses to the remaining teeth. It is particularly effective in supporting periodontally weakened anterior teeth. The lingual plate also adds rigidity to the major connector. The added rigidity contributes to the effectiveness of cross-arch stabilization.

In the maxillary arch the use of a broad palatal major connector that connects several of the remaining natural teeth through lingual plating can distribute stress over a large area. The major connector must be rigid and must receive vertical support through rests from several teeth.

It should distribute the occlusal load over a wide area and at the same time produce the least amount of stress. There are three important principles for design exclusively used for a major connector. They are:

L-bar or L-beam principle.

Circularconfiguration.

Strut configuration.

L-bar or L-beam principle

The L-beam or L-bar or Linear beam theory states that the flexibility of a bar is directly proportional to the length of the bar and inversely proportional to its thickness.

When a load is placed on the bar or beam supported at its ends, maximum stress is present in the centre and zero stress at the supported ends.

A bar supported at both its ends can be divided into two parts namely the parabolic and quartic parts. The parabolic part forms the middle2/4th of the distance between the supports and the remaining l/4th on either sides of the bar form the quartic part.

The parabolic part shows maximum stress concentration and thequartic part shows minimum or zero stress concentration. Hence, if we design a bar such that it has a smaller parabolic part and a larger quartic part it will be less flexible. The material becomes more rigid (less flexible) without adding bulk to the bar.

The next question is how do we do this? The answer is very simple. IT we bend the bar on either side, the length of the bar lying in the quartic part will increase.

Now apply this concept in the design of a major connector. The palate has a flat vault and two lateral slopes.

If the slopes are shallow, the quartic part of the major connector also decreases leading to increased flexibility of the prosthesis under occlusal load. The major connector should be located and designed such that it lies over thesteeper slopes in the palate.

Hence, broad palatal major connectors, palatal strap major connectors can be fabricated with lesser bulk of material (but with adequate rigidity)because it extends in three planes (one central vault and two lateral slopes) with the length of the quartic part (the two lateral slopes) being greater than the parabolic part.

Circular configuration

The advantage of a circle is that it is a continuous unit without an end. Any force acting on a circular bar can be easily distributed all along the circumference. Hence, a circular bar is more rigid than a linear bar with the same area of cross section. This concept can be used to reduce the bulk of themajor connector with a circular configuration anteroposterior double palatal bar and closed horseshoe.

Strut configuration

According to this configuration, a straight bar bent at its ends near thesupport is more rigid because, the bent slopes of the bar aid to transfer the load acting on the horizontal portion.

This is similar to the linear bar theory (L-beam discusses stressconcentration but struts discuss stress distribution).

The major connector on a narrow vault is more rigid than a major connector extending over a shallow vault. In other words, the major connector extending in two different planes has more rigidity.

This concept is seen in the anterior plate of the double palatal bar, where the slope of the rugae area acts as an additional strut.

6- Minor Connector

The most intimate tooth-to-partial denture contact takes place between the minor connector joining the clasp assembly to the major connector and the guiding planes on the abutment tooth surfaces. This close metal-to-enamel contact serves two purposes:

1- It offers horizontal stability of RPD against lateral forces. 2- Through the contact of the minor connector and the abutment teeth, the

teeth receive stabilization against lateral stresses. 7- Rests

One of the most critical points of the rest seat is that the floor of the preparation must form an angle of less than 90 degrees with the long axis of the tooth. This permits the rest, whether occlusal, incisal or lingual, to grasp

the tooth securely and prevent its migra on. If more than 90 degrees, an inclined plane action is set up and stress against the abutment tooth is magnified.

In class I and II RPD the rest seat preparation must be saucer-shaped, completely devoid of any sharp angles or ledges. As the forces are applied to the partial denture, the rest must be free to move within the rest seat to release stresses that would otherwise be transferred to the tooth. The more teeth bear rest seats, the less will be the stress placed on each individual tooth.

8- Splinting of abutment teeth :

Adjacent teeth may be splinted by means of crowns to control stress transmitted to a week abutment tooth. splinting two or more teeth actually increases the periodontal ligament attachment area and distributes the stress over a large area of support. It also stabilizes the abutment teeth in a mesio-distal or antro-posterior direction.

Splinting could be achieved by clasping more than one tooth on each side of the arch using a number of rests for additional support and stabilization and preparing guiding planes on as many teeth as possible to contribute to horizontal stabilization of the teeth and the prosthesis. The multiple clasps should not all be retentive.

Splinting is indicated for the following clinical conditions.

Abutments with a tapered or short root.

Terminal abutments located on the edentulous side of a distal extension denture base.

Fixed splinting is given if there is some loss of periodontal attachment, after a periodontal disease and therapy.