Prosthodontic Lecture 10\4 Basic Principles of rpd design



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Prosthodontic Lecture

10\4




Basic Principles of RPD Design
The objective of partial denture designing is to control the denture movement without exceeding the physiological tolerance of the oral structure
Response of the Denture to Various Forces Acting on it

Introduction

The forces acting on the artificial teeth of the denture can be transferred through:

1) the metal framework of the RPD (e.g. rest) to remaining teeth

2) the denture base to the supporting soft tissues.




Here the forces acting on the occlusal tables of the artificial teeth are distributed to the supporting soft tissue and the to the abutment tooth

The partial denture can be :
1) Tooth supported partial denture

The denture is supported completely by the remaining teeth, as the soft tissue are covered for aesthetic and hygienic purposes, e.g. class III Kennedy's classification.

Here the soft tissue, that are covered, don’t provide any support; hence the mucosa in this context is not for load bearing, BUT the remaining teeth are, so the forces are transmitted along the long axis of the abutment teeth.
In tooth supported partial denture, the remaining teeth are the ones to be loaded, thus, they act as stoppers preventing denture's tissue-ward movement, eventually they will prevent compression of the soft tissue.

2) Tooth-tissue supported partial denture

The denture is supported by both the soft tissues and the remaining teeth. e.g. in class I, class II Kennedy's classification and extensive Kennedy class IV (long span class IV) cases. The mucosa or the soft tissues under the denture are not designed for load bearing, but in this case the forces acting on the denture are shared between the teeth and the soft tissues because:
First, the number and the location of the remaining teeth only don’t provide sufficient support for the denture, so we depend on the soft tissues to obtain the support the denture needs.
Second, since the teeth are more rigid components than the mucosa, so they are more susceptible to be overloaded when the forces are transferred by the partial denture to the supporting tissue, so this teeth's overloading is prevented via directing the forces more toward the soft tissues.

Hence, the occlusal loads are distributed, on the remaining teeth and on the soft tissues, so the prosthesis design should be wisely established in order to distribute these loads properly.


Note : Forces that act on the artificial teeth of the denture are going to be transferred to the remaining supporting teeth (via the metal framework) and to the soft tissues (via the denture base) in case we have tooth-tissue supported partial denture. In case we have tooth supported partial denture, the forces are dissipated to the remaining teeth via the metal framework.
In tooth-tissue supported partial denture, since the denture is partially supported by the resilient soft tissues, so logically this kind of support allow some movement of the denture, and the amount of movement (especially tissue-ward movement) corresponds to the amount of compression that's happening on the soft tissue. Such movement should not be prevented, because as the previous objective states "control the movement of the denture"; "control" means reducing the denture movement in a way that permits the loads to be distributed.

So, in tooth-tissue supported denture, some movements of the denture are allowed, and as result, the soft tissues are loaded and compressed in order to distribute the loads.

In order to achieve that, we have to provide 3 characteristics:
1- Stability

It's the resistance to horizontal forces and rotational tendencies acting on the RPD, such tendencies would result in unstable denture, in other words, the denture is rocking or moving, so we need to get rid of these tendencies.




These movements in a buccolingual direction (horizontal) affect the stability of the denture, making it to rock.

2- Retention

It's resistance to dislodging forces acting on the RPD, these dislodging forces originating from:

I- Gravity

Affects the upper denture only.

II- The surrounding soft tissues

They provide some push on the denture to get it out from its place.

III- Sticky food

The most important dislodging forces. We should know that these forces act on the occlusal surfaces of the artificial teeth of the RPD, not on the rests, that is due to the small surface area provided by the rest in contrast to the surface area provided by the artificial teeth, making the sticky food dislodging forces acting on the rest neglitiable in comparison to the one acting on the occlusal surface of the artificial teeth.


3- Support

It's the resistance to vertical forces acting on the RPD or on the artificial teeth. It's provided by the occlusal, lingual, and incisal rests.



Support is provided by the rest that act as vertical stop



The support is the resistance to movement of the denture tissue ward


Obtaining the Support for the RPD

The support gained for the RPD should be uniform, it's not a problem for the tooth supported partial denture, since the support is provided by the teeth only, BUT it's a problem for tooth-tissue supported partial denture; since the support provided by the soft tissues is not as the same as the one provided by the remaining teeth.


Thus, since two tissues of different resiliency support the denture, stress is precipitated within the denture due to uneven settlement during occlusal loading.




Tooth-tissue supported dentures are subjected to leverage forces due to the differences in the compressibility of the supporting structures (periodontal ligament of the abutments is less resilient compared to the mucosa overlying the residual ridge hence the denture will settle more in the tissue support areas)

That is, if we exert a force of equal amount on the soft tissues and on the teeth, the response of the soft tissues differs from that of the teeth, e.g. this graph below shows that exerting a force will result in a displacement of the soft tissues (500 micron) that is much more than that of the teeth (25 micron), that means that the soft tissues exhibit different behavior from that exhibited by the teeth; so the support provided by the soft tissues is not similar to the one provided by the teeth.


This difference is not coincide with our goal of support; that is to be uniform, so we try to minimize this difference either by increase the displacement of the tooth, but this will damage the tooth and cause it's mobility, or by decreasing the displacement of the soft tissue; and this the effective maneuver for obtaining the uniform support we seek.



The soft tissues exhibit different behavior than that of the teeth under stress, so the support obtained for the tooth- tissue supported denture is not uniform


Note:

This minimal displacement or compressibility (25 micron) of the teeth is because of the stiffness of the periodontal ligaments surrounding the teeth which has more collagen content than the soft tissues, so these ligaments are the ones that allow such a minimal movement or displacement of the teeth.




The periodontal ligaments allow minimal tooth movement

So, Obtaining the support for the RPD can be gained from:

1) The soft tissues.

2) The remaining teeth.


Obtaining Support from the soft tissues

To control the movements of the denture, first thing we do is to dissipate the forces acting on the denture, including the tissue-ward forces, these harmful forces have to be distributed and resisted in order to get rid of any unwanted movement that may affect the denture. Obtaining the support from the soft tissues is based on:


I- Reducing the occlusal loads acting on the occlusal table of the artificial teeth. This can be achieved via:
Reducing the size of the occlusal tables of the artificial teeth. It will result in:
- If the occlusal table is large and wide, the patient may bites parafunctionally; not on the functional cusps, then these biting forces may act buccally or lingually away from the centre\axis of the artificial teeth causing the RPD to rotate. Thus, if we reduce the occlusal table size, the occlusal forces is then directed at the axis\centre of the artificial tooth and no off-centric forces acting on the occlusal tables anymore [Note: the centre\axis of the artificial teeth resembles the crest of the ridge].





With a wide large occlusal table, there a great possibility that the patient bites parafunctionally away from the center, or the axis, of the artificial teeth causing the rotational movement of the RPD and lack of stability


With a small narrow occlusal table, the possibility that the patient will bite parafunctionally buccally or lingually is limited, the biting forces are now more directed toward the center or the axis of the artificial tooth and no off-centric forces acting on the occlusal tables anymore; the forces are centered by the small size of the occlusal table.

- The food penetration forces or the masticatory functional loads exerted to crush the food per masticatory cycle is going to be reduced when reducing the occlusal table, such as the sharp and the blunt edge of a knife; if the occlusal table is wide, the cusps of the artificial teeth have the effect of a blunt edge of a knife and more forces must be exerted to crush the food, BUT if the occlusal table is small, the cusps have the effect of the sharp edge of the knife, thus the force exerted to crush the food is decreased.


Note: Recalling the equation: pressure = force\area, Someone may say that if we decrease the size of the occlusal table (the area), the pressure will increase, but the increased pressure affects the occlusal surface, not the resilient supporting tissues under the denture.

The increased pressure affect the occlusal surfaces

The increased pressure has no effect on the mucosa underneath

II- Reducing the saddle movement under occlusal loads.

This strategy is implemented during impression making procedure; that is , during impression making procedure, we depend on the "Mucofunctional Concept" in order to compress the soft tissues. Thus, this concept aims to fabricate a denture that fit accurately against functionally displaced mucosa, so the denture, at the insertion and function, will sink less under masticatory loads.
That is, as we mentioned previously the compressibility of the soft tissues equals to 500 microns, at the impression making procedure, and according to the Mucofunctional Concept, we use an non-elastic rigid impression material; impression compound, in order to compress the soft tissues by 300 micron, the remaining 200 micron of the soft tissues thickness is compressed later at the time of insertion and functioning. So, this 200 micron thickness, is of a low magnitude causing minimal movement of the denture under the masticatory loads.
This intended movement of the denture is referred to "Stress Breaking". Stress Breaking is one of the concepts that are used to distribute the forces evenly along the soft tissues and the supporting tooth structure.
Stress Breaking is defined as relieving the abutment teeth of all or part of the occlusal forces.

We know that the soft tissues are more compressible than the abutment teeth. In a tooth-tissue supported partial denture, when an occlusal load is applied the denture tend to rock due to the difference in the compressibility of the abutment teeth and the soft tissues. As the tissues are more compressible, the amount of stress acting on the abutment increased. This can produce harmful effects on the abutment teeth, so we are breaking the stress, that is concentrated on the on the tooth, in order to distribute the loads to the other areas which are more flexible.


Note:

The anatomy of the ridge is important to be considered when obtaining the support. As the picture below shows; in B , this ridge provide support but no stability, because this ridge posses horizontal surfaces but no vertical surfaces. In C, this ridge provide stability but not support because the ridge owns vertical surfaces but no horizontal surfaces. In D, the ridge is flappy and has neither support nor stability; hence, the soft tissues either they are left or removed surgerically, so that they become thinner to transfer the forces to the underlying bone more efficiently.





B : support but no stability
C : stability but no support
D : flappy soft tissues provide neither support nor stability


Obtaining the Support from the Remaining Teeth

The support for the RPD is obtained from the remaining teeth through using the rests. These primary supportive elements are placed adjacent to the edentulous space on the primary abutment.

A dilemma may arise is to decide where to place the rest; mesially or distally on the abutment tooth, in case of class I Kennedy's classification for example. Thus, if the rest is:
Distally located:

This resembles class I lever system, that is when the load is applied on the artificial teeth in a tissue-ward direction, the rest will act as a fulcrum, and the clasp is going to move upward and engage with the undercut with each masticatory loads, even at the initial closure of the mouth, so the tooth is going to be compromised through this rotational forces acting on it. As a result, the tooth is going to move distally; this has a destructive potential on the tooth and it's eventually going to be destroyed.


Mesially located:

It's advantages: the abutment tooth is going to be move closer to the adjacent tooth, it's more favorable than that if the rest is distally located. The soft tissues are more loaded, since the fulcrum of rotation is far away from the force applied; that is if the distance increase, that will allow more movement of the denture and the soft tissues will be loaded more. In contrast to the distally-located rest in which the soft tissues are loaded less.



Mesially-located rest resembles class II lever system, and the most famous system to be used in the RPD fabrication is the RPI system by which the I-bar clasp won't engage with the undercut with each masticatory force.


RPI System

It consist of a mesial Rest, Proximal plate and an I-bar clasp. The significance of the RPI system is that being a stress breaker; the I-bar is being disengaged with the undercuts with each masticatory cycle, the proximal plate is designed in away that serves as a stress breaker, and although the rest still makes a contact with the abutment, it resembles the hinge of the door that allowing the denture to move, but not out of its place, permitting the soft tissues to be loaded.


The RPI System and Stress Breaking

The Proximal plate design and the Tilt used : In Kennedy's class I the tilt that is made is an anterior tilt, and this tilt aids in the stress breaking. That is, the picture below shows:

In 1: the occlusal forces are seating in nature, and the fulcrum of rotation is the closest rigid contact with the abutment tooth, and that is the long proximal plate. As the force is applied, the cervical part of the proximal plate will still contact the tooth structure at all phases of it's movement. So, there is no stress breaking in here; since it contact the tooth in it's phases of movement.
أي انه طوال فترة الحركة و القوة مطبقة على الذراع , كان الجزء السفلي دائم الاتصال مع سطح السن . أما الجزء العلوي فقد اتجه للأعلى.
So, this long proximal plate has the tendency to transfer the force to the tooth by it's cervical part, and hence, so, there is no stress breaking; since it contact the tooth in it's phases of rotation, where as the occlusal part of the proximal plate goes up away from the tooth
In 2: the occlusal forces are seating in nature, and the fulcrum of rotation is the closest rigid contact with the abutment tooth, and that is the mesial rest. Here, the proximal plate is shortened and makes no contact with the abutment tooth when the denture is fully seated, however, there is a contact, between the guiding plane and the abutment, at the moment that the denture is being inserted guiding the denture to it's place, after that the proximal plate is out off-contact with abutment. So, this kind of stress breaking.
In 3: the occlusal forces are seating in nature, and the fulcrum of rotation is the closest rigid contact with the abutment tooth, and that is the proximal plate. Here, we cut part of the cervical segment of the proximal plate that make the engagement with the tooth structure making no contact between the proximal plate and the abutment cevicallly. When the forces are applied, the occlusal part of the proximal plate goes up and there is no engagement of the cervical part with the tooth surface as well. So, this is kind of stress breaking.
Note: Here, there is space between the proximal plate and the tooth cervically and the I-bar that engages the mid buccal undercut is free and it's not limited to move mesially or distally, so that it is not going to transfer the load to the teeth when the denture is being loaded vertically.

So, in 2 and 3 design of the RPI system have stress breaking potential; the denture is allowed to move to distribute the loads to the tooth only by the rests, and such movement will allow the soft tissues to be loaded.


Note:


If the lifting forces are applied, the fulcrum of rotation will be shifted to the proximal plate, (the original fulcrum of rotation was the rest when the denture is fully seated; since it was the proximal rigid contact and no contact was present between the abutment and the proximal plate; this kind of stress breaking prior to the rotational stresses in the denture) and there would be a contact between the abutment and the proximal plate. That is, retention here is provided by the occlusal part of the proximal plate, thus, the cervical part is moving away from the tooth, and the occlusal part is moving into the tooth.

If a posterior tilt is made, thus, more stress breaking potential is provided, since the loads on the abutments are reduced via increasing the retentive potential. [ Note: here the stress breaking does not mean that there is no contact between the abutment and metal framework only, but it also means directing the loads in term of low magnitude retentive forces decreasing the undesirable forces acting on the abutment ].


Transferring the forces in term of retention is more favorable than in term of support that is due to the retentive forces will be in a low magnitude in contrast to the supportive forces that will be in a high magnitude that the tooth can not tolerate, without the soft tissues are being shared in the process of load bearing.
Essentials of design

In this section we will discuss about the key factors to be considered while designing a partial denture for common clinical situations.


Design Consideration for Kennedy's Class III Case

This is a modified class III Kennedy's classification; class III modification 1. it's referred to as Quadrilateral or Trapezoidal Configuration.


As a rule in designing, the design of the RPD should be as simple as possible; extra components means extra cost and this cost can be: coverage of the soft tissue, patient discomfort, and more preparations, that means biological and psychological stresses on the patient.




Quadrilateral Configuration

This design involves the use of 4 clasps and 4 rests. It's used commonly for Kennedy's class III specially when there is a modification space on the opposite side of the arch. The rests and clasps should be positioned on each abutment tooth adjacent to the edentulous area.


Clasp

Four clasps should be placed to obtain Quadrilateral design.


Rest

The rests should be placed adjacent to the edentulous space; on the primary abutments.


Major Connector

It should fulfill the ideal requirements, and they are:

1- Rigid

A major connector should not be flexible. It should be rigid enough to uniformly distribute the occlusal forces acting on any portion of the prosthesis without undergoing distortion.


2- It should be comfortable to the patient.
3- It should not allow any food accumulation and it should be self-cleansing, so it's designed with open angles, where it connect the minor connectors, for hygienic purposes.
4- Provide minimal tissue covering.
5- Does not compress the sensitive tissues underneath.

Note:

If there were more modification areas, the number of the rests would increase But the number of the clasp won't, thus, The maximum number of the clasps in Kennedy class III cases is 4, and they are usually 4 in number at average. Even in an unmodified class III Kennedy's classification; with double Akar's clasps that are made on the dentulous side for cross arch stabilization the clasp number still 4. The minimal number of the rests to be placed is 4, even if there is no modification areas, with double Akar's clasps would double rest on the abutment.




Unmodified class III Kennedy's classification

With the double Akar's clasp for cross arch stabilization, the number of clasps is 4 and the number of the rest is 4 as well; by which the Akar's clasp provides 2 clasps and 2 rests as well

Note:

In class III Kennedy's classification, class III lever system is applied; the most favorable lever system.



Design Considerations for Kennedy's class I and II Cases

First, I should introduce the lever system for better understanding.

Occlusal Forces acting on the RPD (e.g. biting, incising, sticky food dislodging forces) develop internal stresses within the denture, which ultimately lead to lever action; the denture tends to rotate around a Fulcrum Line (axis of rotation).
The Lever is defined as a long bar with a single support around which it rotates when the load is applied to any one of it's ends. The support around which the lever rotates is called as the fulcrum.
The Fulcrum Line is defined as : an imaginary line around which a partial denture tends to rotate. It's formed at the terminal abutment axis (line joining the two posterior-most rests).
levers can be of 3 types:

1st - order lever

2nd-order lever

3rd-order lever


In 1st -order lever:

the fulcrum is in the center, resistance is at one end and effort (force) is at the other end. This type of lever system can occur in patient with distal extension partial denture.



1st – order lever

E=effort , F=fulcrum, L=load


A distal extensions denture base is an example for a 1st-order lever where in the masticatory force (E) lift the anterior part of the denture (L) using a direct retainer as a fulcrum (F)






Adding an auxiliary rest anterior to the fulcrum prevents the lever action by acting as an indirect retainer. When an indirect retainer is given, the 1st lever system is converted to a line order lever which is more beneficial for the prosthesis





2nd-order lever system

In this lever the fulcrum is at one end, effort is at the opposite end and the resistance or the load is at the center.



2nd – order lever

E=effort , F=fulcrum, L=load

3rd-order lever

In this lever, the fulcrum is at one end, resistance is at the opposite end and the effort is at the center.



3rd – order lever

E=effort , F=fulcrum, L=load


a tweezer is a typical example of the 3rdorder lever


Lever action in a Kennedy's class I prosthesis:

In Kennedy class I, there is no posterior abutments, so the support is shared between the soft tissues and the remaining teeth as mentioned previously.


In a distal extension partial denture, rotation occurs around 3 principles fulcrums. They are:

1- Saggital fulcrum line.

2- Vertical fulcrum line.

3- Horizontal fulcrum line.



The vertical fulcrum line



The saggital or the Anteroposterior fulcrum line

The Horizontal fulcrum line passing between 2 principal abutment teeth.

This controls the rotational movement of the denture, toward or away from the supporting soft tissues.


Horizontal fulcrum line passing through the terminal abutment axis in a Kennedy's class I partial denture


- If the forces were unseating in nature (dislodging forces, e.g. sticky food dislodging forces) , the rest would resemble the fulcrum of rotation; it's the fulcrum about which the clasp is going to exert their retentive function.
That is, if consider a long bar which has a single support. When the bar is pulled up on one end, the other end goes down. Now, if the same bar has another support on the other end, then the bar will not go down on that end.



It is easy to lift a bar with a single support as it will act like a fulcrum and allow free rotation of the bar







The same bar supported by more than one support cannot be easily lifted at one end because the support away from the effort (E) will prevent downward movement of the bar. If additional force is applied to lift the bar, the support away from the effort will act as a fulcrum of rotation. Since the fulcrum of rotation is away from the effort, additional force is required to destabilize the bar.

This is the same mechanism present in an indirect retainer. When the denture tends to rotate along the fulcrum line, the denture rotates around the single support i.e. the direct retainer. providing an additional support away from the fulcrum line in the form of occlusal rest can prevent the rotation of the denture and function as an indirect retainer.




When the denture is lifted away from the tissues, it tends to rotate around the direct retainer. Adding an auxiliary rest anterior to the point of rotation of the denture will function as an indirect retainer and prevent the rotation of the denture




- If the forces acting on the artificial teeth, were seating in nature (tissue-ward forces), the fulcrum of rotation would be the closest rigid contact; that is, either they would be the rest or the proximal guiding plates, if they are the one that make the closest rigid contact.


Note: If the fulcrum of rotation is far away from the force applied, then the clasp would exert more retentive force (R) exerted by the indirect retainers that are placed anterior to the fulcrum line, that is, D1*force =D2*R. Thus, supposing that the force and the D2 is constant, when D1 increases; D1: the distance between the fulcrum of rotation and the force applied, R would increase; that means that the clasp exerts more retentive force on the abutment tooth (in the picture below, the abutment tooth that the indirect retainers (clasp) exert it's retentive force is the canine).


These indirect retainers that are placed anterior to the fulcrum line have destructive potential, because with each masticatory cycle, loading of the artificial teeth on downward direction, will cause the anterior clasp to move upward; if it goes upward that means, the tooth is going to be loaded upward, so the tooth will be compromised by this unfavorable loading, thus, any clasp anterior to the fulcrum line is going to be contraindicated in Kennedy's class I and II , and that's due to:


  1. The unfavorable loads that are exerted by the retentive claps, as mentioned previously

  2. Not acceptable aesthetically; since they are placed on the anterior teeth.

BUT if we consider class II modification 1, in which the fulcrum line runs from the left second premolar to the right second molar. Here the supportive element of the direct retainer (rest) on the right fist premolar can act as an indirect retainer because it is far enough from the axis of rotation.


That is, if we imagine here that we have two fulcrum lines; one that runs from the from the left second premolar to the right second molar, and such a rotation occurs a round this fulcrum line is controlled via the direct retainer (rest) that is located on the right first premolar, that is also act as an indirect retainer. The other fulcrum line runs from the left second premolar to the right first premolar, and such a rotation tendency around this fulcrum line is controlled via the direct retainer that is located on the right second molar, that is also acts as indirect retainer.


The direct retainers of the modification space may also act as an indirect retainers provided they are far enough from the fulcrum line


Summary of Design Considerations for Kennedy's class I and II Cases
Clasp

For class I case, two clasps on each terminal abutment are needed (the maximum number of clasps needed in class I is 2). For a class II case three retentive clasps are required; one clasp is placed on the edentulous side and the two clasps are placed on the dentulous side.

The indirect support is contraindicated in class I and II Kennedy's classification, but it's indicated in cases of Kennedy's class IV cases. The Indirect Support means: placement of indirect retainers far enough from the fulcrum line to prevent the tissue-ward movement of the part of the denture with the artificial teeth.




Class IV Kennedy's classification

The primary supporting elements (rest) are placed on the first premolars. When there is incising force acting anteriorly, the components of the denture anterior to the fulcrum line will move tissue-ward, and the other components that lies posterior to the fulcrum line will move away from the tissues. Such rotational tendency is controlled by placing indirect retainers on the second molars (stronger teeth) far enough from the fulcrum line, to prevent the tissue-ward movement of the part of the denture with the artificial teeth; this is referred to as "Indirect Support".


Rest

The rests should be placed adjacent to the edentulous space.


Major and Minor connector

It should fulfill the ideal requirements.


In Kennedy's class II, I and II lever system are applied, which are less favorable lever systems.
The Anticipated Movement Concept

Generally, In partial denture designing, we have to apply what is called the "Anticipated Movement Concept", that is we have to imagine how the denture will move under a specific kind of loading, and we will see if that movement is prevented by components, and if the movement controlled without exceeding the physiological tolerance of the tissues. It is also involves the studying of the stress breaking that is If we want to make one, we have to imagine how the movement of the components that are going to transfer the stress to the teeth, in order to correct the design so that the stress is going to be broken, and as we stated earlier that such a movement is going to be allowed, so that the soft tissue are be loaded later on.


Case Study using Anticipated Movement Concept

This case is class II modification 1 case. In this case , the I-bar clasp have some specification in the design.




Retention That is, if sticky food dislodging forces acting on the occlusal surface of the artificial teeth of the bounded saddle area, then the denture tend to get out of it's place from that area. Thus, the I-bar clasp is going to provide retention; this clasp will dislodge around the fulcrum line; the fulcrum line would be the most distant retentive elements; the clasps on the other side. So, this clasp will move vertically upward, this for
Support That is if a vertical force acting on the bounded saddle, the I-bar clasp is not affected because the rests on the primary abutments will have the load.
Stability The horizontal movement of the denture, either Anteroposterior movement or the lateral movement , are prevented by the proximal plates and clasps assembly respectively, regardless the undercut the clasp engages,
- If the occlusal force are acting as in the picture above, the denture tends to rotate, because the loading is buccal to the access of resistance, the clasp arm is going to be recruited for cross arch stabilization.

All of these function whether it's an I-bar or c-clasp , is going to fulfill this function. Using such a clasps anteriorly arises an aesthetic problem, and this may be solved if the lip covers the tooth, then we can use a c-clasp. BUT in this case, the c-clasp is contraindicated, due to the clasp's movement upward within each masticatory load acting on the distal extension.



Such a movement will result in either:


  1. The abutment tooth will move mesially making more contact with the adjacent tooth. This mesial displacement of the tooth is because of the placement of the I-bar to engage the distobuccal undercut when it move upward.

  2. Compromised tooth. This is because the placement of c-clasp, thus this c-clasp would goes upward and tooth might be pushed distally, away from the adjacent tooth, and because that the c-clasp is rigid, upward downward direction due to the half circle cross section, it is flexible in outward inward direction, so with each movement the tooth is doing to be compromised, the transfer of this load to the tooth, we call it as indirect support, and the indirect support in this case is contraindicated, so we use an I-bar engaging the distobuccal undercut, unless it's contraindicated, and these contraindicators are:

    1. Shallow sulcus, if it was less than 4 mm; 4mm is what you need so that you can place an I-bar.

    2. Deep soft tissues undercuts.

    3. Deep tooth undercuts.

So if the I-bar is contraindicated, in such a case we use route wire clasp; the route wire clasp is similar to the c-clasps but it's circular in cross section providing more flexibility.

Done by : Rawan Hamdan


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