Partial denture design



Download 99.46 Kb.
Date01.12.2016
Size99.46 Kb.
Rpd design

Partial denture design:

  • The most important thing to consider during designing a partial denture is to control all the movements and prevent them.

  • We have general and specific principles in designing a RPD

General considerations:

  • Only treat if indicated (don't replace teeth unless the patient wants to and do not replace one or two teeth with a partial denture)

  • You should maintain aesthetics

  • The denture has to be "not bulky". It won't be comfortable to the patient and will affect tongue space and speech.

  • Each case has more than one design

  • Try to minimize forces (distribute them on more than one abutment)

  • Try to minimize coverage

  • Try to avoid covering the gingiva and be away from it. If you had to cover it, relief the denture there.

  • Minimize the number of interproximal minor connectors because these areas become more prone to caries. So place them only if needed.

  • Replace missing teeth only if necessary i.e. do not replace a third molar or an area that is not opposed by teeth.

How do we reduce forces?:

  • Increase number of abutments

  • Make the forces as close as possible to the cervical area, in order not to mobilize the tooth (مثال الوتد!) (place the clasp as close as possible to cervical area, but there should be space 0.5-1 mm between them). You can re-contour the tooth to achieve that.

  • Decrease the occlusal table. Decrease number of teeth for e.g. remove a premolar or put 3 premolars and one molar. And use narrower teeth. This way the masticatory function is more efficient and the forces transferred to teeth and tissues are less.

  • Place the teeth in the neutral zone (least forces)

  • Try to use the stress breaking design. In free end saddle you should not use occlusally approaching clasps or distal rests. Placing a distal rest will apply extracting forces on the tooth and leads to its tilting and periodontal damage. So use RPI system( mesial rest. Guiding plate, and I bar) or combination clasp (its tip is made of wrought wire).

  • In RPI system the I bar provides stress breaking as follows: the I bar is place on the maximum contour anterio-posteriorly. If force is applied on the denture posteriorly, so the denture is directed up anteriorly and the I bar will be out of touch. So the I bar is disengaged and does not push the tooth and vice versa. The other stress breaking effect comes from the guiding plates. As we know in the RPI system the guiding plane should be away from the gingiva by 2-3 mm so that when force is applied on the occlusal surface of the denture the guiding plate will be disengaged and be in the undercut area providing the stress breaking effect. The mesial rest provides stress breaking because it tries to tilt the tooth mesially against the adjacent tooth so the tooth won't move and will be stable in its place.

RPD design (specific guidelines)

Surveying and undercut analysis

We have to analyze the undercuts in order to determine which undercuts to be used, where to place the retentive component and which retentive component to use.



removable-partial-denture-survey-lines-path-of-insertion-guide-planes-14-638.jpg

The red and black lines in the picture above represent the zero tilt and alternative tilt; zero tilt is when the cast is parallel to the horizon.

It is very important to bear in mind that by changing the tilt we don’t create undercuts, changing the tilt is done to determine path of insertion. To change or create new undercuts, we either use composite restoration, crowns or do reshaping.

So changing the tilt on the cast simulates the path of insertion in the patient’s mouth.

To distinguish between anterior and posterior tilts follow the following steps:


  1. Hold the cast so that the anterior teeth are facing you (upper or lower cast)

  2. If you tilt the cast toward you, this is anterior tilt, if away from you, this is posterior tilt

Sometimes by changing the tilt we change the path of insertion as well as engaging an existing undercut that we couldn’t engage at zero tilt.

surveying_page_07_image_0004 surveying_page_07_image_0002

In the first picture from the left, the path of insertion is at zero tilt so at that tilt the flange can’t engage the labial undercut, it is away from the ridge pushing the lips out which would result in an ugly appearance. In the second picture we changed the tilt (posterior tilt) which means changing the path of insertion inside the patient’s mouth, so instead of inserting the denture straight it will be inserted with a tilt and by doing this we will be able to engage the labial undercut.

Always achieve limited path of insertion, for a patient who lacks manual dexterity it would be very difficult to insert a denture with multiple paths of insertion, also absence of limited path of insertion would result in the retentive arms hitting everywhere causing their damage and distortion.

Systems available for design

There are two major systems for design; OSCAR and A system of design (British).

A system of design:


  1. First decide the saddle areas

  2. Support

  3. Retentive components

  4. Reciprocation

  5. Bracing

  6. Major and minor connectors

What is the difference between reciprocation and bracing?

Reciprocation prevents horizontal tooth movement of clasped teeth whereas bracing prevents horizontal denture movement. There is no single component in the denture that does bracing; many components can provide bracing such as the clasp

Major connector connects right and left components of the denture whereas minor connector connects each component to the major connector.

Conventional RPD cannot be unilateral, it is always bilateral as the unilateral is dangerous and may be swallowed or inhaled by the patient. Only sectional dentures with attachments can be unilateral.



OSCAR system of design

O: outline

S: support

C: connection

And

R: retention

Outline includes:

  • -Case classification

  • -Abutment teeth

  • -guiding planes (path of insertion)

  • -tilt determination (Surveying)

  • -Saddle area

Support

  • Type of the rest (occlusal, cingulum or incisal). We avoid incisal rests as they are aesthetically unpleasing.

  • Position of the rest (mesial or distal). In free end saddle cases we use mesial rests

  • Using components other than rests to provide support such as major connector, minor connector

  • Rest seats, we prepare them to achieve: 1- favorable tooth surface for support
    2- reduces the prominence of a rest (less bulky) 3- prevent interferences with occlusion.
    The apex of the seat should be directed to the centre to direct the forces along the long axis of the tooth and the highest margin of the seat is at the marginal ridge and it shouldn’t be sharp to avoid stress concentration. In cases of tight occlusion it is important to prepare a tunnel at the proximal areas to allow a space for the minor connector holding the rest



Connection

Major connector must be:

1-rigid to have effective force distribution and avoid destructive torque and flexing forces

2-compatible:



  • Does not impinge on oral tissues (especially the tongue). Mid palatal bar major connector is no longer used because it is objectionable to the tongue. Does not impinge on the gingva; lower major connector must be 3-4 mm away from the gingival and upper major connector 6 mm away from the gingva, why?? Because the space is limited in the lower arch (only 6-7 mm) therefore the space needed is 3 mm, whereas in the upper we have enough space

  • Does not trap food

  • Covers no more tissues than is absolutely necessary

  • Avoid bony and soft tissues prominences

Minor connectors shouldn’t meet other denture components at 90 degrees, the angle should be rounded, any sharp edges in co-cr will eventually break due to stress concentration.

Types of maxillary major connectors:

the standard (ideal) upper major connector is the palatal strap and its modifications, for example in cases of free end saddle we use full palatal coverage – a modification of the palatal strap-, bounded saddle use the palatal strap, in cases of torus palatinus we use ring design (anterior-posterior palatal strap) … So full palatal coverage and ring connector are modifications of the palatal strap.

U shaped connector (horse shoe) is the worst design, it is weak, undergoes flexion and distortion and is used only when the patient has a huge torus palatinus extending all the way posterior to the vibrating line (into the mobile soft palate which precludes the placement of the connector). Always try to avoid using the U shaped connector and replace it with the ring design when possible. In gagger patients we prefer using ring design instead of U shaped.

In full coverage palatal connector it is preferable to make the post dam area from acrylic as it can be adjusted (addition/trimming) whereas metal can’t be adjusted. Metal can be made smooth enough to be used in the post dam area without injuring soft tissues so this is not a contraindication for using metal in the post dam rather the inability to adjust it.

Types of mandibular major connectors:

The connector of choice is lingual bar, if the 7-8mm space from the gingival margin to the depth of the Sulcus is not available we use the lingual plate. Sublingual bar is not preferable as it resembles the U shaped connector in its weakness and susceptibility to flex, sublingual bar is thick horizontally (4mm) but thin vertically (2mm) that’s why it distorts easily unlike the lingual bar which is 4mm thick vertically and 2 mm horizontally.

Lingual bar is rigid and away from the gingiva

The 7-8 mm space needed for the lingual bar is divided into 4 mm the thickness of the connector and 3 mm space from the gingiva.

If anterior teeth were questionable, use lingual plate because it is easier to add teeth to the lingual plate in the future.

If multiple diastemas are present between teeth with insufficient space to place a lingual bar, in this case use modified lingual plate (interrupted lingual plate) (cut the visible metal parts between teeth).

Labial connector is the worst to use for esthetic reasons and difficulty of fabrication; lower teeth have lingual undercuts so using labial bar will complicate the design by using metal inserts on teeth (swinglock).

Crowded teeth use lingual bar, if not indicated then use lingual plate.

If neither lingual plate nor lingual bar can be used because of insufficient space, this indicates using the dental bar (Kennedy bar) it must be free from the incisal area by 2 mm and from the gingiva by 2-3 mm. using dental bar needs long clinical crowns, it can’t be used on crowded or spaced teeth.

If lingual plate can’t be used, lingual bar can’t be used because of insufficient space and dental bar can’t be used because of short clinical crowns, in this case we can combine the sublingual bar with the dental bar (thinner than the dental bar alone) in this case they would strengthen each other and become rigid enough to provide connection.

In some patients the lingual frenum attachment is very high (into the ridge) which precludes using lingual plate (no space for the plate). Also in some patient with periodontally compromised teeth that we want to stabilize, lingual plate worsens the problem although it allows easier teeth addition, so in patients with severe periodontal disease we prefer to avoid the lingual plate.

Labial bar is used when lower teeth (anterior and posterior) are collapsed lingually.

Swinglock is one type of labial bar



Minor connectors

  • Of sufficient bulk to be rigid but unobjectionable as possible

  • Located in an embrasure not on a convex surface

  • Passes vertically from major connector

  • Tapers toward contact area

Minor connectors includes the ones connecting denture components to the major connector, internal and external finish lines, stoppers, ladder and lettuce saddle areas, all of these are considered minor connectors.

Step ladder saddle is used in the lower, meshwork saddle is used in the upper, because in the mandible the ridge is narrow so we use the ladder (which is thicker) whereas in the maxilla the ridge is wider. Don’t use the ladder in the upper arch because it is narrow compared to the ridge so it will fracture with time (the acrylic separates from the metal), the meshwork design extends to cover the whole ridge.

So ladder doesn’t work in the upper.

The mesh can be used in the lower but the ladder is more preferable, because the mesh in the lower would be narrower than it usually is hence weaker. (Mesh design is thinner than the ladder, so when used in the lower it would be narrower as the lower ridge is narrower than the upper and this will weaken the mesh)



Retention

  1. Direct retention

  2. Indirect retention

To have effective indirect retention you have to have effective direct retention.

Retention is usually gained by using clasps



For adequate retention:

  • For molar teeth use occlusally approaching clasps

  • For premolars and anterior teeth use gingivally approaching clasps

  • For isolated molars (especially in the maxilla) use ring clasp if possible

  • In premolars, if using gingivally approaching clasps is not possible, use wrought wire clasps

  • If the survey line is high, use wrought wire clasps, or you can recontour the crown and lower the survey line. When the survey line is high using occlusally approaching claps is contraindicated as there is no enough space to accommodate the clap which would result in occlusal interferences, gingivally approaching clasps (I bar) are also contraindicated as they would be esthetically unpleasing (extending into the occlusal plane). Wrought wire in this case are indicated as they can engage deeper undercuts (you don’t extend the wrought wire clasp into the occlusal plane, it engages the deep part of the undercut only as it is flexible, engages 0.75 inch undercut)I bar (cor-cr) cannot engage deep undercuts, it will fracture, it engages 0.25 undercuts only

  • Polyoxymethylene clasps are flexible white resin materials used when aesthetics are of major concern.

  • Changing the tilt will allow adequate retention without clasps

  • The retentive arm of the clasp is always placed away from the saddle (use the undercut away from the saddle), the longest the clasp the better the retention. In premolars we don’t use occlusally approaching clasps because the premolar s narrow (about 7mm) so the clasp would be too rigid or I’ll have to make it thin resulting in its fracture.

  • If there is no undercut away from the saddle, use fish hook design clasp or change the tooth contour. The problem with fish hook clasp is plaque retention and it may be objectionable to the patient (ydalo ykser)

  • Use reciprocal component whenever using a clasp.

  • A guide surface allows a reciprocating component to maintain continuous contact with a tooth as the denture is displaced occlusally. The retentive arm of the clasp is thus forced to flex as it moves up the tooth. It is this elastic deformation of the clasp that creates the retentive force

  • A clasp is effective in retention from its position when the denture is fully seated to where it escapes over the bulbosity of the tooth. This vertical measurement may be termed the 'retention distance'. It will be appreciated that the reciprocal element on the other side of the tooth should be in continuous contact with the tooth surface as the retentive arm traverses the 'retention distance'. Effective reciprocation can be achieved either by a clasp arm contacting a guide surface of similar height to the 'retention distance', or by a plate making continuous contact with the tooth surface as the retentive arm moves through its 'retention distance’. If the reciprocating clasp is placed on a tooth without an adequate guide surface, it will lose contact with the tooth before the retentive arm has passed over the maximum bulbosity of the.

  • In RPI system the reciprocation is provided by the rest and minor connector as well as the plate

The following concept “cross arch reciprocation” is not important as the Dr said. Cross arch reciprocation, where a retentive clasp on one side of the arch opposes a similar component on the other side. The retentive clasps can be placed either buccal/buccal or lingual/lingual. Disadvantage: as bracing arms leave teeth surfaces, teeth will move in their sockets, the jiggling action is potentially damaging to the supporting tissues and reduces effectiveness of retention. This concept is more than an idea rather than being applicable clinically.

Diametrically provided reciprocation: clasps should encircle more than 180 degrees of tooth circumference. Reciprocation is not only buccal or lingual but also diametrically (through the shoulders of the clasp)

De van and buccal undercut, some patients have high frenum opposing the abutment teeth on which gingival bars are to be placed, in this case the bar would be too short/rigid, DeVan came up with a clasp design to solve this problem, the clasp extends along the denture border from the saddle area until it reaches the undercut, the clasp now is long enough to be flexible.

If the patient doesn’t have any undercuts (neither buccally nor lingually) and we don’t want to place crowns/ restorations, the solution in this case is “dimpling” (t provides less retention than conventional clasps)



Indirect retention, indirect retainer must be as far as possible from the saddle. Support axis (fulcrum line) is a line passing between the last rests, once the rests are away from the teeth the denture will no longer rotates around this axis, the last component to leave the teeth is the retentive tip so the denture will rotate around this fulcrum line after the rests move away from the teeth (clasp axis or retentive axis is a line passing between the last retentive tips).

Salma Khayyat


Share with your friends:


The database is protected by copyright ©dentisty.org 2019
send message

    Main page