The indirect retainer
Indirect retainer : A part of an RPD which assists the direct retainers in preventing displacement of distal extension bases .
So it’s a special case for distal extension ,we can apply it on class IV but now we just want to focus on class I & II (distal extension bases).
In last lecture we said that we can use a direct retainer for resist the movement of denture away from the tissue and teeth, sooooo why I need this component to resist displacement??????????????????
If you look to the upper picture you can see that different forces are acting on RPD:
Force act to left it up in lower denture or down in upper denture: forces of gravity, muscles or sticky food.
Force act on another side of RPD to sinking it down in lower denture or up in upper denture
So if RPD move upward from the side of distal extension, rotation around the tips of the clasp on the most posts. Abutment will happen & sinking of ant. Part of RPD downward, so we try to add a different component that aid in resisting this movement cause direct retainer itself will not resist this movement, it will aid in this movement by rotation around the tip of retentive tip and when RPD sinks down this will make irritation to the tissue while the other part will dislodge and this is not acceptable ,sooooo what’s the solution ??????? We can extend the RPD on tooth ant. To the rotational axis which is around the retentive tip on the most distal abutment and put a rest which will resist the movement downward from one side so there is no sinking and this prevent the upward movement from the other side so no rotation happened .
Because it’s a component that aid in retention so we call it retainer
and because it’s not a direct retainer (not a clasp or precision attachment) we call it indirect .
so as a conclusion its indirect retainer for distal extension.
There is a different component that will help in indirect retainer but you have to know that in order to resist the upward movement it should be positioned horizontally and the most efficient is occlusal rest seat.
Effectiveness of indirect retainers:
- Location of the fulcrum line (which is the rotational axis around the retentive tip of the most distal abutment).
- Distance of the indirect retainers from the fulcrum line (the longer the distance the better the indirect retention)
-Rigidity of the connector.
-Effectiveness of the supporting tooth (premolars and canines are better than incisors, occlusal rest give more support than incisal rest so we choose premolar even if it so close to the fulcrum line ).
Types of indirect retainers:
-Auxiliary occlusal rest or canine rest.
-Canine extension of the occlusal rest.
-Continuous bar and lingual plate.
-Indirect retention from the major connector.
Slide 74 :
The doted yellow line is the fulcrum line between the most abutment teeth so if there is lifting on post. Part up that lead to sinking of ant. Part so we extend RPD on premolar as a rest to prevent sinking and rotation.
Slide 65 :
This is a cummor arm which is an auxiliary rest on both side if its class I (the person who discover it called cummer) we put it on a canine which is lesser favorable than premolar but there is no other option.
Slide 76 :
From the rest itself on the most distal abutment we made extension(canine extension of the occlusal rest )it’s not so effective cause its very close to the fulcrum line which is a potential axis of rotation upon movement of the distal extension upward.
In upper denture it can be a bar made between rugae area that will resist sinking , its type of major connecter .
Indirect retention from another type of major connecter can be on horizontal surface ant. To potential fulcrum line or be on lingual plate and to be effective there must be a rest on most distal abutment but in general is a major connecter but can aid in indirect retention only the rest is called indirect retainer cause we put it only for this function .
It can be a continuous bar resting on the teeth which is a major connecter
A Major connector is the unit of the partial denture that connects the parts of the prosthesis located at one side of the arch with those on the opposite side.
Requirements of major connectors:
-Biocompatible and comfortable
-Rigid (proper stress distribution). Flexibility results in tooth mobility, loss and bone resorption.
-Do not interfere with or irritate the tongue.
-Do not impinge on oral tissues during removal or insertion of the RPD
-Do not cause food retention or trapping (self-cleansing)
-Contribute to support of the prostheses and preferably indirect retention (aid in it but not considered as indirect retainers)
-Have as minimum tissue coverage as possible
-Possess smooth rounded edges to avoid discomfort, stress concentration and fatigue
Mandibular Major Connectors:
Types of Mandibular Major Connectors:
-Lingual Bar:the most common used one.
-Lingual Bar with Continuous Bar (Open Kennedy)
Its half- pear shaped.
Its height should be at least 5mm to be rigid which is the most important requirement
The upper border of the major connecter should be away from the lowest part of gingival margin not less than 8 mm.
2. Minimal contact with soft tissues (less plaque, better soft tissue stimulation)
3. It does not contact the teeth.
1. May not be rigid if there is not enough space to place it correctly or if too long.
2. Cannot be used in cases with tori cause if it relived this will make it flexiable.
3. Provides no bracing or indirect retention
-Pear shape lingual bar with thin solid metal piece extended on to the lingual surface of the anterior teeth.
-There must be adequate block out and relief for both soft tissue undercuts, gingival margins and undercuts in the proximal areas of the teeth.
-The superior area of the plate must have intimate contact with the teeth (scalloped) to prevent food impaction
-Must always be supported at each end by an occlusal rest to prevent rotation of the framework and labial displacement of teeth.
-Its biggest disadvantages are poor patients’ tolerance and the need for impeccable oral hygiene (can cause severe soft tissue irritation and caries)
Advantages and indications:
-Rigid (even if relieved around tori).
- Can be used when there is insufficient space for lingual bar (due to gum recession or high frenal attachment).
-Can provide indirect retention and stability against horizontal forces (indirect retention if rests are added at each end).
-Especially useful with distal extensions with excessive bone resorption.
-Can be used to splint periodontally involved teeth and mobile anterior teeth can be added to the old RPD with lingual plate MC
Lingual Bar with cingulum Bar ( Kennedy Bar):
- Rests must be placed at each end of the upper bar mesially at first premolars to prevent the bar from moving inferiorly
- The upper bar should be positioned at the contact points of the teeth, should be half oval in cross-section, 2-3 mm in height (just above the cingula) and 1 mm thick. Its upper border should be scalloped.
- Lingual surfaces of teeth and interproximal soft tissues are exposed.
- The 2 bars are joined by a minor connector at each end located in interproximal spaces.
3-Stimulation for the uncovered gingival margin
1-More tongue annoyance
2- Entrapment of food debris.
3- Complex design and difficult to make
-Runs at the Mucosa labial to the anterior teeth
- Indicated for malposed or lingually inclined teeth and where there are prominent inoperable tori.
- Not commonly used because of its lack of rigidity (due to its length) and patient irritability. Rigidity can be improved if thickness and height are increased. Also does not provide any bracing or indirect retention.
-A useful modification of the labial bar where the labial component has a hinge on one side and a locking device at the opposite side providing an opening and closing movement similar to a gate.
-The prosthesis is inserted while the lock is opened and this is locked after fully-seating the denture permitting its insertion in inaccessible labial undercuts.
-All teeth are used to retain and stabilize the prostheses as the labial bar has small vertical projections contacting labial surfaces of teeth below height of contour. These projections can be replaced with acrylic base for better aesthetics (especially with short or mobile lip).
-Not preferable sometimes because firmly grasped teeth might be subjected to stresses when distal extensions move towards the tissues, although useful when only few mobile teeth are remaining as it helps
- Used in the presence of anterior lingual undercut
- In case of shallow sulcus depth
-Similar shape to lingual bar but maximum dimension is horizontal
-Needs a special impression technique to record functional depth and width of the sulcus
Indications for Dental bar:
1- Insufficient space for lingual and sublingual bars and poor oral hygiene (lingual plate is contraindicated)
2- Long clinical crowns.
- Good bracing and indirect retention
1-Low patient tolerance
2- Poor aesthetics if spaces are present between teeth.
3- Not rigid enough
Maxillary Major Connectors:
Types of Maxillary Major Connectors:
- Palatal Bar
- Antero-posterior Palatal strap.
-U-shaped Palatal Connector.
-Complete Palatal Plate.
There is more option coz there is more area .
Palatal bar is narrow, gently curved, half oval with its thickest point at the centre.
Used only in interim prostheses as it lacks rigidity. To improve rigidity, should be made bulky
Provides little support because of its limited width.
Used only with very short span bilateral Class III (1 or 2 teeth missing).
Should not be placed anterior to first premolar as its bulk causes discomfort.
Advantages of Palatal Strap:
-Great rigidity with less bulk. Good stress distribution because of its width.
-Offers little interference with the tongue.
-Offers retention through adhesion and cohesion.
-Might give some indirect retention.
Disadvantages of Single Palatal Strap:
The patient may complain of excessive palatal coverage. Also might be associated with papillary hyperplasia with poor oral hygiene.
Antero-posterior palatal strap (Ring connector):
-Each strap is at least 8 mm wide but has thin cross-section.
-Borders should be 6 mm away from gingival margins or covering the lingual surfaces of teeth.
-The two straps joined by a flat longitudinal elements on each side of the lateral slopes of the palate.
Indications for antero-posterior palatal strap:
1-Widely separated abutments (multiple saddles with good abutments).
2-Cases with large inoperable palatal tori.
3-Patients who want to avoid palatal coverage.
4-Long edentulous span in class II modification 1 arch.
5- Class IV.
1-Provides good rigidity for relatively low bulk.
2- Used when the patient objects the large palatal coverage.
3-Used with maxillary Tori.
4- Provides good support
1- The anterior strap might interfere with phonetics if it covers the rugae.
2- Too many borders might be annoying to the patient (the anterior strap at highly innervated area).
In this case we use antro-posterior palatal strap and this is one of the most popular cases for this type .
U-shaped Palatal Connector (Horseshoe-shaped connector):
Thin metal band running along lingual surface of remaining teeth and extends on the palatal tissues for 6-8 mm. Metal borders end at the junction of the vertical and horizontal slopes of the palate but might be extended to increase rigidity. All borders should be gently curved and smooth.
Indications for U-shaped Palatal Connector:
-When many anterior teeth need to be replaced.
-With palatal tori extending to the posterior border of the hard palate.
2-Has moderate indirect retention
3-Has moderate support.
1-When vertical forces are applied on one or both ends it tends to flex and deform.
2-Cannot be used for distal extension RPD.
3-Greater bulk is required to avoid flexing but results in discomfort and interferes with phonetics.
4- Poor cross-arch stabilisation
Complete Palatal Plate:
-It covers the entire palate.
-Its anterior border should be 6mm away from the gingival tissues or extends up to the cingula of anterior teeth.
-The posterior border should extend to the junction of the hard and soft palate.
-A slight seal can be obtained by giving beading posteriorly. This gently rounded elevation prevents collection of debris under the connector. Also intimate contact between the connector and the tissues provides retention by adhesion and cohesion
Indications for Complete Palatal Plate:
-Many posterior teeth are replaced and periodontally compromised abutments.
-Kennedy class I with anterior teeth replacement.
-Patients with developed muscles and natural lower teeth.
-Flat highly resorbed upper ridge and shallow vaults where high stability is required.
1-Best rigidity and support
2-Better transmition of temperature and thus better stimulation for underlying tissues.
3- Little discomfort or effect on speech because of its minimal thickness
1-Soft tissue reaction (hyperplasia with poor oral hygiene).
2- Might interfere with phonetics if thick.
Done by : Sujood Al-Hawamdeh