- One of the methods of retention of overdenture is bar attachment.
- The typical bar attachment consists of a bar connecting two or more abutments. Joining the two abutments enables splinting
- There are two types of bar attachments.
- Bar joints permit rotational movement.
- They are used as a splint connecting the abutments together
- Bar units (rigid fixation) permit no movement.
- Bar joints permit rotational movement.
- They are placed as a single unit on the abutment teeth like a stud attachment
Category: prosthodontics
REVERSIBLE HYDROCOLLOID = AGAR 2M*
- reversible ( gel can change to paste by heating, then can change back to gel by cooling)
- a major component is seaweed – comes as collapsible tubes
- you boil it at 100° C for 10 mins then store at 65°C for 10 mins then temper at 43-46° C 5-10 mins before you take the impression
- agar can be stored at 65°C for 5 days before it has to be reboiled again
ENAMELOPLASTY 2M**
- Simple enameloplasty to reduce the severe curve of spee and adjust supra erupted teeth
- Enameloplasty is defined as a procedure of recontouring a portion of the enamel to obtain the desired morphology
- A tapered diamond cylinder stone in a high-speed handpiece with air-water spray is used for the procedure.
- After the procedure, fluoride application is done by using plastic mouth guards.
Stress breaker 4m** 2m**
- It is defined as a device, which relieves the abutment tooth of all or part of the occlusal forces (GPT)
- In order to minimize the stress in the case of distal extension partial denture, devices like stress breakers are used.
- Type I = In this type, a movable joint is placed between the direct retainer and denture base.
- Type II = This type consists of a flexible connection between the direct retainer and the denture base.
- Advantages
- Preservation of the alveolar support of abutment
tooth due to the reduction of stress on it. - Balanced stress on residual alveolar ridge and
abutment. - Weak abutment teeth are well splinted even when
the denture base is moved. - Even if relining is not done properly, abutment
teeth are not damaged. - Direct retention is less required.
- A massaging effect is produced on the soft tissues during the movement of the denture base.
- This lessens the need for frequent relining and rebasing.
- Preservation of the alveolar support of abutment
- Disadvantages
- Complicated design and expensive.
- Weak assembly and fractures easily.
- It distorts due to rough handling.
- It is difficult to repair.
- It can counter only the vertical forces on the
denture. - Reduced stability against horizontal forces.
- Inappropriate relining leads to excessive ridge resorption.
- Reduced indirect retention.
- The split major connector tends to collect food
debris at the area of split.
Soldering 2m**
- is defined as joining two components of metal with an intermediate metal whose melting temperature is lower than the parent metal.
- Types of soldering for metal-ceramic restoration
- Oven soldering
- Torch soldering
- Infrared soldering
- Laser welding
Tripoding the cast 2m**
- Tripoding is a procedure where three different widely spaced out points of a single plane are marked on the cast.
- To allow you to reposition the cast according to the selected path of insertion
- These tripod points are used as a reference point and they should not be altered until the treatment is completed.
MINOR CONNECTORS
- Minor connector: connecting link between the major connector and other components of the RPD
- MAXILLA = Must extend up to tuberosity
- MANDIBLE = must extent up to 2/3rd of the edentulous area or cover retromolar pad
- Must be rigid to distribute the stress between the linked components, must not impinge on the mucosa, the mucosal surface must be highly polished
Functions of the minor connector :
- Join the denture parts together
- Transfer functional stresses to abutment teeth through the occlusal rest
- Transfer the effect of the retainer, rests, stabilizing components to the rest of the denture
FOUR TYPES OF MINOR CONNECTORS
- Joins the clasp assembly to major connector
- Broad B-L and Thin M-D = allow easy placement of prosthetic teeth.
- Triangular cross-section
- Lingual embrasure = bulk is not evident
- Joins indirect retainer to major connector
- 90 degrees to the major connector but slightly curved as it will decrease the concentration of stress
- Should always fit into embrasure areas
- Joins denture base to major connector (draw)
- Open lattice
- Mesh or ladder pattern = Require acrylic attachments
- Bead, wires, and nail heads = capture acrylic material
- Class III is completely tooth supported = do not reline procedure
- Class I and II = are distal extension cases = Relining or rebasing procedure, therefore metal parts should not be in direct contact with tissue = Hence, METAL nail heads and beads come in direct contact with underline tissue.
- External and internal finish lines are necessary for class I and Class II situations
- Serves as an approach arm for the vertical projection or bar clasp.
REST AND REST SEATS
- Rest = part of RPD = rigid extension of the RPD – provides support.
- Rest Seat = Prepared on abutment teeth
- Functions:
- Directs and distributes occlusal force apically to abutment teeth
- Acts as vertical stop = doesn’t allow RPD to move in the tissue-ward direction
- Maintains retentive clasp in position = SUPPORT in RPD
- Rest do allow slight movement = dissipates horizontal forces
- TYPES
- PRIMARY = part of the clasp assembly
- SECONDARY/AUXILLARY = indirect retention
OCCLUSAL
- Shape = ROUNDED TRIANGULAR
- Base = Margin of tooth
- Apex = center of tooth
- Floor = spoon-shaped
- Dimensions = 1/3rd to half of M-D diameter of tooth and ½ of B-L diameter of the entire tooth
- Measured from cusp tip of buccal side to cusp tip of lingual side. (draw)
- ACUTE angle = floor is inclined to towards center of tooth and angle is less than 90* = hence, Increases the support (draw)
- If an obtuse angle is present, it causes Slippage of the prosthesis = therefore, avoiding the inclined plane effect
- Occlusal rest = 0.5mm thick at the thinnest point
- 1-1.5 mm = when it crosses the marginal ridge
- We use diamond or carbide round burs with water and light pressure
- Preparation is done entirely on enamel only
- If the rest seat needs to be placed away from the edentulous area→it is not flared to the facial line angle and should be more flared lingually ( to provide space for minor connector)
- Interproximally extended occlusal rest seat:
- Flared more buccally and lingually without affecting the contact area
- U shaped trough that is 1mm wide and 1 mm deep
- Without breaking the contact = Use a shoulder bur to create a 1 mm Buccolingual clearance from the proximal side then use a round bur to create the seat
- In RPA with intracoronal occlusal rest seat
- A partial denture that is totally tooth supported by means of cast retainers on all abutment teeth may use intracoronal rests for both occlusal support and horizontal stabilization
- Open the margins more and do slight lingual flaring for the minor connector
- Extended occlusal seat rest
- In Kennedy Class II, modification 1, and Class III situations in which the most posterior abutment is a mesially tipped molar
- an extended occlusal rest should be designed and prepared to minimize further tipping of the abutment and to ensure that the forces are directed down the long axis of the abutment.
- This rest should extend more than ½ the MD width of the tooth and 1/3 rd the B L width of the tooth
- 1mm thickness of metal = the preparation should be rounded with no undercuts or sharp angles
LINGUAL REST/CINGULUM REST
- Only for maxillary canine
- The mandibular canine is avoided as it lacks the bulk of enamel.
- V-shaped = boomerang shape – Cross section view
- Cresant/Half moon shape – Lingual view
- Dimensions = 2.5mm M-D to 2mm B-L
- Depth = 1.5mm = as average thickness of enamel is 2 mm
INCISAL REST
- Mandibular canines and rarely incisors
- V-shaped notch = 1.5 – 2mm away from the proximal incisal angle
- Purpose = functioning as secondary rest for indirect retention
- Dimensions = 1.5mm wide, 1.5 mm deep, and 1.5mm away from the proximal incisal angle
- If multiple incisal rests are needed = rests are connected with linguloplate i.e Choice of major connector
- Lingual rest are more commonly preferred than incisal= as lingual rest will be very close to the rotational center of tooth = Bear loads in a much better manner than incisal rest
- Major problem = Anterior zone are aesthetic zones. Hence, lingual is chosen over incisal.
EMBRASSURE REST
- B-L = 3-3.5mm wide
- Depth = 1.5mm – 2mm deep
- If not met = definite chances of fracture of RPD components. It is very difficult to fix it with the same strength of RPD design
Reciprocation 4m**
- Each retentive arm should be opposed by a reciprocal component capable of resisting any orthodontic pressure exerted by the retentive arm
- Functions of reciprocal arm 4m*
- It provides stabilization/reciprocation against the action of the retentive arm.
- It also stabilizes the denture against horizontal movements.
- The reciprocal arm may act as an indirect retainer to a certain extent, i.e. when it rests on a supra-bulge surface of an abutment tooth lying anterior to the fulcrum line.
- Design specifications for a reciprocal arm
- It is positioned on the side of the tooth opposite to the retentive arm.
- The reciprocal arm must be rigid and should not be tapered.
- It is positioned on a tooth surface that is reasonably parallel to the denture’s path of insertion and withdrawal.
- It is ideally located at the junction of the gingival and middle third of the abutment tooth.
Some special reciprocal elements
- At times, linguoplate can be used to provide reciprocation.
- Sometimes, an occlusal rest positioned on the opposite side of the tooth will provide reciprocation.
Swing-lock dentures 4m***
- Swing-lock RPDs are used, when a number of remaining teeth is too few to support a conventional design.
- This denture has a labial bar extending labially all along the arch like a major connector.
- The labial bar is connected to the remaining parts of the denture by a hinge on one side and a lock on the other.
- The labial bar can be unlocked during insertion and locked after insertion.
- Indications
- Missing key abutments
- When the number of remaining natural teeth is less
- When the remaining teeth are too mobile to serve as abutments
- When the soft tissue and the tooth are in unfavorable contours
- Provide retention and stability for maxillofacial prostheses, such as obturators
- For retention of the prosthesis in case of loss of large segments of teeth and alveolar ridge, due to trauma or infection.
- CONTRAINDICATIONS
- When the vestibule is shallow
- High labial frenal attachment
- Patient with poor oral hygiene
- Advantages
- The construction is relatively simple and inexpensive.
- Disadvantages
- It is unaesthetic in patients with short lips.
- Tipping is seen in the remaining anterior teeth in the distal extension case.
Fabrication
- Selection of metal for swing-lock framework
- The metal of choice is chrome alloy.
- Gold alloys wear off due to constant movement
- Surveying and designing
- The cast is mounted on the surveyor.
- The path of insertion is from a lingual direction with the labial arm open.
- Lingual plate
- The lingual plate should be designed to end above the survey line.
- It prevents the tissueward displacement.
- Occlusal rest
- helps to prevent tissueward displacement of the denture.
- Major connector
- The mandibular major connector extends above the survey line with scallops extending up to contact points.
- The maxillary major connector is the complete palatal or closed horseshoe with borders extending up to or above the survey line.
- Labial arm design
- The vertical projection of the labial bar should be designed to touch the teeth below the height of contour.
- Conventional design
- This consists of a labial bar with metallic vertical I-bar or T-bars attached to it.
- Using acrylic resin retention loop
- This is indicated for patients with short lips and where aesthetics is of concern.
- Selection of impression material = Heavy-bodied alginate is best preferred.
- Tray selection = A custom tray should be used for recording maximum labial and buccal vestibular depths.
- For the secondary impression, 5–6 mm space should be given between the teeth and the tray when placed in the mouth.
- Making the impression.
- It is similar to conventional dentures.
- Dual impressions may be required for distal extension.
- Framework fabrication.
- Fitting the framework.
- Arranging artificial teeth to occlusion
- Insertion = Lingual path of insertion is done.
- Post insertional care = Oral hygiene should be maintained by the patient. Distal extension RPD has to be frequently relined.