prostho sheet # 9 - Haneen thnebat

بسم الله الرحمن الرحيم
Design principles of RPD

Prostho lec. #9 (#2 for Dr. Ahmad)
This lecture is From PDF file of "Design principles of removable partial denture" (it's not available as a hard copy since most of pages are figures!!)
ة







Revision
Last lecture we was talking about G.P, and the effect of tilting the cast on the ability of G.P to determine the p.o.I and removal of the prostheses .It's very important to consider the way that the denture may move& it's rotation out of place "the denture never removed as one piece, because if dislodgment forces act on one side, the denture will always rotate".
Forces act in denture
1) Vertical forces: if vertical forces act on a denture, it's either seating force or unseating force:
*seating forces: if the forces were seating in nature" directed toward denture" like the forces acting on teeth during biting, the fulcrum(axis) of rotation will be the closes rigid component of the denture that contact the source of the force(force of application),means the most close rest to the side of application.
*unseating forces: if the forces tend to move the prostheses away from tissue, the fulcrum(axis) of rotation will be the most distant contact from the source of force "force of application" means the most distant clasp "retainer"(clasp on the other side or indirect retainer).The whole prosthesis is going to rotate about the fulcrum and if you want to study the movement and how it's going to transfer forces to the surrounding tissue &how we can prevent it, you must imagine it as rotational movement about that distant axis of rotation.
Page 23:
Guiding planes (G.p)
*G.p will determine the path of insertion in translational way, that's mean when the G.P prepared parallel to the p.o.I which is approximately close to the zero tilt, they will force the denture to move only upward& downward in translational way. However, if the denture is going to be affected by dislodgment forces acting on one side, you have to analyze movement as rotational movement, in which the axis of rotation will be the most distant contact.
*G.p have to face different direction in order to be able to: 1) prevent the rotational movement. 2) To determine translational p.o.I .
*The 2 G.p in the following (figure A) will prevent leftward movement of the prostheses but not rightward movement. Rightward movement will prevent by G.P in (figure B)."It's clearer in slide"
Figure A:

Figure B :



*Although we study RPD design in 2-D, you have to consider it as 3-D.
Page 24
The figures where the Dr. was pointing to, located behind the raccoon pic.!! If the G.p prevent the movement anterioposteriorly, the denture will move right& left freely.
*Because the teeth organized on an arch shape ,G.P are parallel to each other vertically but not parallel horizontally. In addition, they're parallel to the analyzing rode of the surveyor. So in horizontal plane, G.P will be in any orientation but not parallel.
*G.P not located only proximally, but also lingually especially if we have metal plates on the lingual surface, it will act as a G.P. Minor connector in lingual embrasure will act as a G.P also, &will help in determining P.O.I.
*As G.P facing different directions in 3-D space, they will help in determining one translational P.O.I, but during functional movement the rotational P.O.I is the important path, not the translational one, because dislodging forces will act either on left or right side of the denture. If it's act on right side, then denture will move outward about the fulcrum of rotation located one the other side (left here)"clasp tips on the other side". Simply because clasp tips on right &left side located nearly on the same vertical location. Ex: that if the clasp "in the following figures" dislodge about the axis (fulcrum) of rotation, it will move vertically upward close to the zero tilt.


That's why it's important to engage undercut according to zero tilt, to achieve retention for any clasp assembly. Undercut may engage by minor connector or by the retentive clasp tip in case that the minor connector wouldn't engage the undercut according to zero tilt.
Rule: For G.P to determining P.O.I properly, they shouldn't face the same direction, so raccoon alignment is wrong!!
Page 25
In case of Kennedy class Ⅰ &Ⅱ, the path of insertion &removal isn't well determinant, why? Because we design the (guiding planes/proximal plates) contact not to be intimate, with G.P preparation just 1-2mm in height. The Lack of intimate contact between (G.p& proximal plate) aim to create what's called "Stress Breaking Effect". So because we need stress breaking effect, we will lose the "intimate contact between "G.P&P.P" criteria, which will compromise the ability to determine one path of insertion &removal!!
That's why we will deal with Kennedy class Ⅰ &Ⅱ as cases that don't have a well path of insertion &removal. And as a result, clasp tip on right &left side second premolar in p.25 will enter the undercut according to zero tilt because the other components most likely to engage undercut not @zero tilt.
To have a clasp tip engaging the undercut according to zero tilt &to have an undercut in zero tilt with an amount=amount of undercut in the alternative tilt, place the clasp tip under the intersection of the 2 survey lines. Because if we move denture vertically with the same distant from each survey line, the amount of undercut approximately will be the same in the 2 tilt (zero and alternative).
Page 26
In this case class Ⅰ &Ⅱ, what determine the P.O.I is the proximal plate that located distally "distal side of second premolar in the figure" and the minor connector which located mesially " the minor connector for the mesial rest that extend lingually in second premolar in the figure". So minor connector &proximal plate will determine path of insertion &removal. This what we do in Kennedy class Ⅰ &Ⅱ to make RPI system to affect stress breaking effect.
Unfortunately ,if the contact between proximal plate &G.P is intimate, any downward movement of the saddle area" in the figure" will be accompany by movement of tooth itself (red line) because the force is acting through unfavorable lever system "mean lever system of class Ⅰ &Ⅱ" that oppose class Ⅲ lever system which is the most favorable one).
As a result, any force that moves the denture , will move the tooth unless the force is designed to stay away from the denture during movement.
Page 29
In case of lonely standing tooth "as upper right second premolar in the figure", and I prepare long G.P and apply proximal plate, unfortunately tooth has to move with the prosthesis! Why?? "The answer is in the following question& explanation"
*Stress breaking effect, what dose it mean???
To simplify the concept, you can consider the hinge of the door as a stress breaker .Once you open the door, the door only will move while the wall will not! But if the door &the wall are connected rigidly, either the door will not move or the wall will move with the door. So stress breaking simply mean that the prosthesis is connecting to the remaining tooth with points of contact that allow some movement ,by allowing some movement ,denture can move and can load soft tissue .
Loading soft tissue mean, movement of denture without the need of the teeth to move with it "teeth will not be overloaded".
You know that compressibility of soft tissue is much grater than compressibility of the periodontal ligaments of teeth, so if the tooth have to move with each movement of the denture, the tooth will be overloaded and this what we mean by stress breaking effect.RPI system is an example of stress breaking design.
Page 30 Reciprocation
Page 31,32,33
Back to the figure plz:
-Dark circle tooth surface
-Left red half circle reciprocal clasp arm
-Right red half circle retentive clasp arm
The most common reciprocal component is the reciprocal clasp arm which will give a kind of reciprocation called delay reciprocation. Why??
The figure show movement of retentive clasp arm(right)& reciprocal clasp arm(left) upon insertion of the denture, "as it's PDF, no animation is available so try to imagine as a stages!!!"
Retentive Clasp arm move downward to reach the final location in tooth. 1) The contact between tip of the retentive clasp arm& tooth surface will start while the reciprocal clasp arm is still faraway! 2) Retentive clasp tip start to deflect above the survey line or above suprabulge of the tooth, and the reciprocal clasp arm still away.
3) Reciprocal clasp arm will come in contact with tooth surface only when the denture is fully seated.
So the whole time that the Retentive clasp tip was pressing on the tooth lingually, no component was available to prevent the pressing lingually or to oppose the effect of clasp tip. The root of the tooth which fixed to bone was the only structure that prevented this lingually movement by holding the whole force.
What is the objective of the reciprocal clasp arm??
1) Encirclement to prevent orthodontic movement of the tooth "in this case lingually".
2) To prevent horizontal movement of denture that transfer horizontal force to the teeth.
3)To allow tooth move back to it's original location if retentive clasp tip manage to till the tooth lingually.
Although reciprocal clasp arm give delayed reciprocation, it's still acceptable in healthy tooth.
Page 34
In case of periodontally compromise tooth Bone support is lost recession of gingival mobility of tooth in this case instead of deflection of retentive clasp arm tooth will move and go backward so clasp will cause 1)tooth overloading because it's already compromised periodontally.
2) No retention will be gained because no deflection of the retentive clasp arm occurs, so it's not retentive now.
So to gain support to the tooth/proper retention, I need instantaneous (immediate) reciprocation by apply reciprocation lingual plate (left red line in figure p.34) instead of reciprocal arm ,this lingual plate will make contact earlier before the denture fully seated and tooth is supported lingually.
But there is a part of the vertical movement, the tooth wasn't supported!(mean there is movement which have contact between retentive tip& tooth without any contact lingually. the tooth in that movement will deflect lingually, until denture move downward the instantaneous reciprocation will start.
Page 35
Action distant: is the vertical distant from first point of contact of retentive tip until denture is fully seated. Right vertical line in the figure Action distant of clasp arm.
Left vertical line in the figure Action distant of reciprocal component.
To have 100% reciprocation, then the action distant of retentive clasp arm must equal action distant of reciprocal arm.
*How can we increase the action distant of reciprocal component??
By make a preparation lingually. The most important thing is to have reciprocation@ the time that the clasp approach the survey line because in that moment, the amount of deflection will be the maximum.
As a revise: healthy sound teeth delay reciprocation by reciprocal arm is enough.
In compromise tooth instantaneous reciprocation by lingually plate is a must.

Page 28
You can find instantaneous reciprocation in RPI system in figure
(Proximal plate "distally"&minor connector "mesially") will cause instantaneous reciprocation.
So, one of the advantages of RPI system to achieve instantaneous reciprocation.
RPI:
-provide stress breaking effect as the rest is located mesially.
-More soft tissue loading.
-The clasp arm doesn't extend above survey line but extend below the survey line.
-once clasp arm is fully seated, there is no contact here" the dr. was pointed in mesial part of the tooth" because of the undercut presence, in the other area "distally" the survey line will be in contact all through the seating process.
Page 37/38
Our objective is to control denture movement in our RPD designing .We must know the forces acting on the denture, how to transfer forces to the supporting tissue without exceeding their physiological limit.


Page 39
Some maneuvers to reduce amount of forces acting on denture?
By reducing occlusal table as we've already applied in complete denture. By decrease the occlusal table (narrow buccolingually), the amount of food crush per masticatory cycle is less which mean less forces are applied to the denture(less forces are going to be transfer to the supporting tissue).
Page 40
To control denture movement, we need to provide proper support, stability, retention.
Page 41
Support in RPD is classified to:
1)Tooth support(Kennedy class Ⅲ& limited Kennedy class Ⅳ cases) .
2) Tooth &tissue support (Kennedy class Ⅰ &Ⅱ)& extensive Kennedy class Ⅲ &Ⅳ "but class Ⅳ mainly consider as tooth supported".
Upper right figure designing of tooth supported RPD which is very simple! 4 abutments, from each we gain support( by rest),stability( by vertical component, clasp arm, minor connector ,proximal plate),retention(clasp arm).If you apply these components on each abutment, then you're done!
In case of rotational movement, there is a cross-arch stabilizing component in the other side.
In unmodified Kennedy class Ⅲ (missing tooth in one side),we apply cross arch stabilizing double Aker(clasp on the other side).Some people apply 2 clasp arms "ant. & post.", however, double Aker is enough in most of cases.
*Maximum number of clasps needed for retention in RPD is 4, the fifth one indicates incorrect design!!
*Sometime we may apply extra clasps for stability, like in case that half of arch is lost, which's more probably in patients who suffer from tumor in which soft tissue support is lost due to the absent of bone. In this case we may place more than Y clasp in the other side, but they're needed here for stability not for support!!! "not as usual , as we used to consider clasp for retention or indirect support". Mean that there is a cross arch stabilization but no cross arm support).
Lower left figure designing of tooth-tissue support
Due to the absence of primary abutment posteriorly, so we will depend on other source for support, stability, retention.
Support it can't be transfer away from the side of force application easily, the only way to gain support is soft tissue loading.
Retention cohesion& adhesion may help but peripheral seal is very weak because denture isn't closing the entire boarder so the lost retention will gain by indirect retainer.
Stability gained by minor connector &soft tissue.
*As we loading soft tissue in this case, the denture design must have stress breaking mechanism to allow movement of denture into the tissue without the need of the teeth to move with it.
Page 45 Anticipated movement concept
Usually applied in Kennedy class Ⅰ &Ⅱ in which we need stress breaking effect. As we said stress breaking effect mean once the denture is fully seated, there is a contact point between RPI component &primary abutment. To be sure that this contact point will disengage when the denture is vertically loaded, you must understand how this point will move. During movement of RPI component, make sure that components move out of contact with the tooth instead of move into more contact because more contact means the components will engage the tooth which will move with the denture and become overloaded.
Moving out of contact means that, the RPI components move in undercut area (away from point of contact) so denture will move without need the tooth movement.
In any horizontal movement, the teeth will move with the denture because teeth can manage, tolerate, and resist the horizontal movement, but it can't resist vertical forces of mastication since it very strong forces.
The story of truck &boy "if you're standing at inclined street, you can catch &save a boy "similar that the teeth can resist horizontal forces", but you can't catch a truck!! Sure you'll allow it to move "teeth can't resist vertical forces". So if forces acting on prosthesis are much bigger than the teeth resistance, you allow this engagement to happen "truck is going to move" and eventually, forces will be resisted by something else like soft tissue.
Page 47
Soft Tissue ability to give support& stability.
Figure B If mucosa is horizontally oriented, it will provide good support but no stability.
In Kennedy class Ⅰ in which mucosa& ridge is flat which mean ridge isn't going to give any stability! So what's the ideal major connector here??Definitely it's lingual plate to distribute the forces to all remaining teeth, never lingual bar although it's less coverage &more comfort.



Figure C If mucosa was sharp with vertical walls, it will not give any support because all of the inclined surfaces but it will provide a good stability.
Figure D If tissue was floppy , it will act as a buffer between the bone & the denture base, tissue will prevent any forces transfer into the bone, so denture neither going to be probably support nor probably stabilized. Note: "in some cases, there is no sound enough regions to provide proper stability & support to the denture, surgical correction of this floppy area is the best solution".
Page 48
It's a Kennedy class Ⅰ &Ⅱ ,if we have a vertical loads fulcrum of rotation will locate in the closes point of contact "rest in distooclussal surface of second premolar in figure" .Anything located anteriorly to this point of contact "rest", will going to move about this fulcrum "move in direction of left single black arrow in the figure". Near undercut, clasp arm will move outside the undercut and apply a force on the tooth" in the direction of left vertical upward red arrow" ,while rest will apply downward movement "right red vertical arrow". Due to these opposite forces , tooth will move distally "going to be rotated, overloaded…finally we may lose the tooth!"
Page 49
In this case we apply rest mesially "mesioocclusal surface of second premolar". If the denture will move downward, the axis of rotation will be in the area of rest.
Placing rest mesially isn't enough! Because there is another source of forces transfer to the tooth which is the proximal plate.
In the upper region (White upper line), proximal plate will move 90⁰ away from the tooth.
In the lower region (White lower line), proximal plate will move 90⁰ into the tooth(move along the white line).So in this case in which proximal plate contact extend much cervically(long) as it’s in the figure (which is wrong!),more forces will directed toward the tooth and the center of rotation( area where the mesiooclusal rest of second premolar located) is going to move from the rest into the proximal plate distally. Any downward vertical movement will be accompanying by push on the tooth by proximal plate, as a result, tooth will move with denture distally.
The doctor repeated the above point, it's may be clearer here:
-rest located mesially indicates the center of rotation.
-upper point of proximal plate(final point of upper white line that contact the proximal plate)will move away from teeth by 90⁰ without any contact.
-If proximal plate extend cervically as in figure (which is wrong), the lower point (final point of lower white line that contact the proximal plate) of proximal plate will move 90⁰ into the tooth.
-The more cervically you go, the more the tendency to have engagement between proximal plate &guiding plane surface.
Engagement means no stress breaking effect!!And contact is present anywhere.
To have stress breaking effect you have 2 options:
1 Restrict the length of proximal plate into the occlusal part (reduce length of proximal plate because occlusal part is less engaging. The more cervically you move , the more the proximal plate is going to move into the tooth Breaking the stress breaking effect.
2-The Dr. will mention it in the following figures.
Page 50
Figure#1( upper one):1)long contact of proximal plate,2) most cervical part is going to engage the tooth,3) lose of the stress breaking effect.
Figure#3(upper one): In this case we confined the contact between P.P & G.P into the occlusal part, where it's parallel to the tooth and no contact present. As a result, whenever there is a vertical loading (red arrow), proximal plate will disengage by moving into the undercut (Dr. consider it as the second option for providing stress breaking effect!!! ).
Third option: Figure #2(upper):P.P don't have any contact once the denture is fully seated, contact between p.p &G.p is only during seating(during insertion) to determine the translational path of insertion, and again, no contact once it's fully seated. Any movement about the rest (MO rest of second premolar) ,p.p will move into the undercut(in this case between tooth&p.p) and distal base will move freely.
Figure 2/3 (upper): represent an applicable choices in Kennedy class Ⅰ &Ⅱ, that’s why we don't have a well determinant p.o.I in distal extension RPD .
Figure 1(upper): is an applicable choice in Kennedy class Ⅲ ,we will have well determinant p.o.I, but @ the extent of overloading the tooth.
Figure#1(lower):
A case of lifting force (denture tends to move upward). This figure from our textbook! Unfortunately it’s wrong! Center (fulcrum) of dislodging movement is the indirect retainer (the most distant rest).
*We can accept that the p.p transfer dislodging forces to the tooth because amount of force is minimum .But it's not acceptable at all for the p.p to transfer vertical forces of application, as the amount of force is high.
*To decide that the p.p will provide resistant against dislodging movement (retention) or not, draw a straight line from indirect retainer and draw another straight line by 90⁰ in the first one, the most cervical part is going to separate, the most occlusal part has more tendency to give retention. In distal extension RPD, the part that causes violation to stress breaking effect is the most cervical part, so it's wise to keep the contact occlusally as it will give retention.
Figure#1(Lower): In case of Kennedy class Ⅰ &Ⅱ, if we apply ant. tilt, we will have:
* More chance to get retention stress breaking effect.
*less preparation needed because chance of contact between p.p& occlusal part of G.P will increase.
* Once the denture is fully seated, chance of contact between p.p & most cervical part of G.P will decrease. And all these three point is preferable.
*If we apply post. tilt: with each vertical force ,p .p will hit the tooth.
Note: Dr. doesn't have to discuss any case in the lec .as we've already discussed it in the lab. In the exam, there is a q. about design (application about the principles that explained in the lec).


Page 51
About Biting, if you've vertical forces, Mesial part will move downward less than the distal part due to differences in compressibility between teeth &mucosa. At the time of bite registration step, if patient bite on wax(not very soft), the denture will move distally downward. In this case, Applying the upper model on cast "which isn't compressed as in patient mouth" will cause the cast to elevate distally! Again, at the time of bite registration, patient bite on wax , the whole denture will move downward. When you apply it on the cast model ,denture will return to its natural condition (means will elevate & will not move downward (upper model elevate posteriorly ,so space on model is larger than space inside patient mouth& when I apply it inside patient mouth and asked him to bite, the first contact will occur distally.
*If the patient bites by an amount=amount of biting forces during bite registration, an even contact will be gained.
*If he bites with higher forces, forces will move mesially.



*If I took the bite with a flowy material ,No sinking downward occur, apply denture on upper model, set the teeth, when applied it inside patient mouth &asked him to bite lightly, an even contact everywhere will be achieved, but if he bites with a force(not lightly) ,biting forces will be much less distally than mesially .
*All above explanation, to know what happens in bite registration, it’s not our concern now to decide which is the right, wrong cases!!




Page 53
Advantages of Mesial rest:
1) provide stress breaking effect,
2) increase the amount of soft tissue loading(As we move the center of rotation(rest) more anteriorly, amount of soft tissue loading /degrees of rotation of the denture is going to be more).
*As center of rotation is located more anteriorly, direction of movement of denture base will be more vertical than if the center of rotation is located distally.
Page 54
Here, to prevent contact, we limit the p.p into the occlusal part & apply an anterior tilt .suppose the denture rotated about 2 degree. Now if the center of rotation located distally, 1) amount of movement into the tissue will be less,2) direction of movement will be less vertical ,which means less favorable direction of loading on the soft tissue.


Page 55
p.p in figure will not provide stress breaking effect due to intimate contact with G.P especially in cervical part, they extend the p.p without any advantage, except closing the space to prevent food impaction there. However, a plenty space between denture &teeth usually exist!
Page 56
Another case of limit p.p to occlusal part so more chance to have “stress breaking effect”, and no contact when denture is fully seated.
Page 58
Modified Kennedy class Ⅰ, consideration about denture movement. In case of vertical force act on distal extension region, fulcrum of rotation will be the closes rigid contact (mesial rest in left second premolar& mesial rest in left Right second molar). Clasp arm on the right first premolar (ant. abutment on modification area) is going to move upward about the axis of rotation (black line), any upward movement mean increase in tendency to extract the tooth with each masticatory cycle on the other side which is highly unfavorable.
So that as a rule, any clasp in right second premolar, will be apply in distobuccal undercut. Why? because during upward movement ,instead of extract tooth from its place, it(clasp) will push the tooth into the adjacent tooth .What's more, this clasp also as a rule need to be flexible in any upward ,downward direction. As a result, we apply either "I-bar clasp" engaging distobuccal undercut or wrought wire clasp. If we apply C-clasp, it might be rejected from an esthetics and biomechanical points of view.
*Mechanism of engagement here is what we called indirect support (gained by clasp).
*Indirect support in case of Kennedy class Ⅰ &Ⅱ is contraindicated.
*In Kennedy class Ⅲ: Nothing is indirect! (Neither indirect support, nor indirect retention) everything is direct
*In Kennedy class Ⅳ, indirect support is indicated especially in upper arch, because ant. Teeth located labial to the axis line, so any downward movement means the denture posteriorly is going to move upward. If we've a clasp arm engaging molar posteriorly, it will act as indirect support (prevent movement upward toward tissue). So indirect support is acceptable, actually it's our goal in class Ⅳ, because origin of forces is minimal, as it acting on ant. teeth, here the indirect support provide by post. teeth which are multirooted ,stronger teeth. In addition to that, the resistance arm is much longer than force arm, especially in Kennedy class Ⅳ.
If the four anterior teeth were missing (class Ⅳ), and the rest applied on ant. Abutment(suppose on distal side of both canines R,L), the fulcrum of rotation will be a line extend between distal side of right & distal side left canine this line much shorter than the line(axis) indicated when the rest was located on post. Abutment as in previous case (axis represent by black line in the figure). So in case of class Ⅳ& rest applied on ant. Teeth, force arm is much shorter than resistance arm, in addition to that, the indirect supporting teeth are ant. teeth which are week, single rooted teeth.







Note: In class Ⅰ with ant. modification , never apply clasps anteriotly!! Any more clasp anteriorly to get more retention lead to indirect support& overloading the ant. teeth ,as a result, the whole design will disturbed !!! Keep in mind that usually # of clasps in class Ⅰ = 2, but never anteriorly.

The End

Done by: Haneen Thnebat
Date of lecture:12/4/2011
Lecture # 9(#2 for Dr.Ahmad)/week#10