THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
Introduction
For many years, the partial denture was considered a step before the total denture. It is not only about filling the gaps, but also about protecting and preserving the alveolar ridges, mucosa, bones, teeth and TMJ against harmful actions, in order to maintain long-term functional and aesthetic integrity.
It is for these many reasons that the removable partial prosthesis with cast frame holds a place of choice in complex rehabilitation therapies of the oral cavity, and therefore a rigorous knowledge of its different constituents is essential for the smooth running of prosthetic therapy.
Definition
The partial prosthesis with metal frame was defined by Professor ROUOT as being:
“A prosthesis characterized by the existence of a skeleton which on the one hand carries the replacement teeth and on the other hand attaches and rests on a certain number of persistent teeth, avoiding subjecting them to actions which could harm their integrity”
The different elements of the chassis
It consists of a metal frame made of chrome-cobalt or titanium called a chassis, fitted with an acrylic resin saddle supporting the replacement teeth,
The retention of this prosthesis is ensured by hooks/attachments which are inserted on/into the natural or possibly crowned teeth,
on/in natural or possibly crowned teeth,
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
A metal frame prosthesis has dental supports that prevent it from gradually sinking into the underlying mucosa, which considerably limits the risk of gingival inflammation around the remaining teeth .
- Main connection (armature)
This is the soul of the removable partial prosthesis,
In the maxilla
- Narrow palatine plate:
- Indication: Single or bilateral intercalated edentulism of small extent for which dental support is required: class III, V, VI.
- Medium Palatal Plate: Wide Palatal Plate: Full Palatal Plates
- Indications
- It is indicated in CL I ka, CL II ka edentulous cases when more than six anterior teeth remain present and the alveolar edges provide valid support; therefore when dento-osteo-mucosal support is sought;
- It is also indicated in CL IV ka edentulousness of medium and large extent and CL V ka.
- Full or sub-full coverage plate
Plate covering the entire palatal surface with a thickness equivalent to the other plates
- Indications
- Edentulousness in which only a few teeth remain on the arch, i.e. large-scale edentulousness justifying extensive osteomucosal support.
- Edentulism of cl I and II in which the abutment teeth are weak (less than 6 teeth) and/or significant alveolar resorption seeks indirect retention.
- U-shaped or horseshoe plate:
- Indications: It is indicated in the presence of
- Extremely deep palatal vault or
- Inoperable torus extending to the hard palate-soft palate boundary.
- In distal edentulousness and in the case where the patient presents a gag reflex
- As well as when casting techniques and dynamic behavior contraindicate palatal plates.
.
- Inconvenience
- Due to its lack of rigidity, this form of reinforcement should be used as little as possible.
- Double palatal bar
(Double palatal spacer)
- Indications
- In classes I and II, when the pillars and supporting tissues are of excellent quality and no indirect retention is required.
- In class III, in which the posterior pillars are positioned distal enough that the posterior strut is not perceived by the tongue.
- In intercalated edentulousness: classes III, VI, IV of small extent for prostheses seeking Dento-periodontal support.
- Advantage :
- Rigidity.
- Lightness.
- Low tissue coverage.
- Disadvantages
- They have a less marked supporting role than that of the full palatal plates .
- The thickness of the anterior bar can cause discomfort for patients (unpleasant tactile sensation on the tongue).
- If the central recess is not large enough: risk of diapneusia.
- Simple spacer
Directly inspired by the lines of
the skeleton prosthesis
- In the mandible
- The lingual bar
According to classical standards:
- Its width is 1.7 to 2.5 mm
- Its height is 3.5 to 4 mm.
- Its upper edge of the bar must be placed at a minimum distance of 4mm from the top of the marginal gingiva,
- Its lower edge should be located as low as possible without constituting a trap for food debris or interfering with lingual movements, i.e. 1 mm from the lingual frenulum.
The lingual bar should be spaced from the internal table of the mandible by 0.2 to 0.4 mm depending on whether it is vertical or oblique.
The spacing is all the more important as the orientation of the internal table is oblique, this is to avoid any contact of the bar with the mucosa.
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
- Indication
The lingual bar is used systematically if the anatomical conditions allow it, i.e. each time that the height of the internal table between the moving floor of the mouth and the lower limit of the marginal gingiva allows it, approximately 8 mm.
- Contraindication
- In case the depth of the floor is less than 8mm;
- In the case of a lingual frenulum that is too high;
- Or if the morphology of the retro-incisal table presents an undercut;
- Or in the presence of gingival recession.
- Inconvenience
- It does not participate in support or stabilization because it is never in contact with the dento-mucosal structures.
- Lingual strip (apron, blade, classic lingual plate)
its height is delimited by two lines:
- One drawn above the cingulum of the anterior teeth
- The other located 1mm above the floor of the mouth and the lingual frenulum
The lingual strip must necessarily be associated with dental supports because in the absence of these elements the prosthesis can move on the inclined plane formed by the lingual face of the teeth and then risks exerting:
- An orthodontic effect on the teeth, a pressure in a vestibular direction
- A traumatic action on the marginal gingiva by sliding and sinking into the soft tissues.
- Indication: Its use results from the contraindications of the lingual bar
- Contraindication: In case of poor hygiene (causes tooth decay)
- Disadvantages:
- food retention
- Lack of comfort (impedes speech)
- Cingulate spacer: single lingual plate
This framework has the form of a metal bar resting on the enamel of the lingual surfaces of the teeth of the incisor-canine group .
Its upper part is very close to the free edge, its lower edge does not reach the enamel-cement limit from which it remains 0.5 mm away.
The intrados in intimate contact with the dental surface penetrates into the embrasures without blocking their cervical portion
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
- Indication: Its use is justified when:
- An available coronal height of the anterior teeth;
- Resorption of the alveolar bone resulting in a reduction in the distance between the marginal gingiva and the alveololingual sulcus (recession);
- A contraindication for the lingual bar and headband.
- Secondary connections
These are metal parts which connect the main connection to the other elements of the chassis and more particularly to the direct and indirect supports, they can be called brackets or secondary junctions.
Their role is to:
- Transmit the forces developed during chewing to the supporting teeth
- increase the mechanical resistance of the entire chassis
- Retention grids and saddles
- Retention grids:
Rigid grid with more or less wide mesh covering the edentulous ridges and serving as an anchor for the resin of the saddles, a discharge of 0.5 mm is provided at the level of the ridges in order to receive the resin of the saddles and will be 5 mm apart in front of the molar tuberosities and trigones
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
- Saddles
They cover and encompass the edentulous ridges up to the vestibular and lingual limits determined by the anatomo-functional imprint depending on the case they are:
- Either entirely metallic (tight occlusion): preferred for the functional stresses that they transmit to the osteo-mucosal support.
- Either partially made of acrylic resin directly in contact with the mucosa (allowing easier assembly of the prosthetic teeth but also possible relining of these) but mixed use is ideal
- Lugs
These are individual anchors in the form of a retentive metal rod.
Indication:
- Single intercalated edentulism
- Anterior edentulism without false gum
- the counter plates
These are cast metal elements integral with the main connection reproducing most of the lingual or palatal faces of the teeth to be replaced.
Their vestibular face is provided with mechanical retentions for anchoring the cosmetic material.
- Indication
- Significant supraclusion
- Short teeth to be replaced => tight occlusion.
- Means of retention
The bonding of the prosthesis with the remaining teeth is carried out either by:
- Hooks.
- Attachments (secondarily aggregated prefabricated elements).
- The hooks
The different parts of the hook
These hooks systematically include 3 elements:
- Supports.
- A wedging arm
- A retention arm.
The hook is in direct relation with the abutment tooth, it transmits part of the loads exerted on the prosthesis and partially ensures retention.
The different types of hooks
- Proximal junction hooks
Hooks connected to the frame directly at the saddles with a proximal secondary connection (proximal contact).
- ACKERS hook or n 1 DE NEY: KENNEDY hook:
- Indication:
Gives excellent results during tooth extraction
recessed on premolars and molars
- RING HOOK
Derives directly from the rear action hook, described in the Ney system
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
- Indication
It surrounds the tooth almost entirely, so it is stabilizing and finds its main indication at the level of isolated molars.
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
- Variant: Modified NEY ring (NEY No. 5):
Its advantage is to prevent mesioversion of the teeth.
- Lingual junction hook
- NALLY MARTINET hook or force breaker:
This hook includes:
- A mesial support,
- A secondary mesial connection
- A single arm: a wedging arm that starts from the bracket and surrounds ¾ of the tooth to end with a retentive end in the mesiovestibular angle
- Indication
- Indicated on premolars in posterior edentulous areas in extension
- Can be used at the canine level but it is unsightly and not very effective at this level.
- Advantage :
- The use of a hook with a mesial secondary connection and mesial occlusal support aims to distribute the risks more equitably between the frame with mucosal support and the hook with dental support.
- Avoid distal version of the abutment tooth
- It is the least traumatic hook for the tooth; its elasticity relieves the supporting tooth, and it also causes less pinching of the marginal gum adjoining the distal face of the abutment tooth.
- Very retentive, effective belting
- Long retentive arm therefore very flexible
- Allows free play of the saddles thanks to its mesial secondary connection
It effectively ensures the functions of guidance, stabilization, support and retention; it contributes to the favorable orientation of forces on the supporting teeth.
- Variants: rear action hook :
Indication:
- identical to that of NALLY-MARTINET
- Isolated molar
- but its main indication remains in the case of tight occlusion because of its distal support
- The BONWILL hook
Also called “rider hook” because it crosses the arch, it is the fusion of two ACKERS hooks at the level of their occlusal supports two occlusal supports
- Indications: cl VI KA, cl II KA (toothed side) cl IV KA (large area)
- The EQUIPOISE hook
- design :
Its design is close to that of the rear action hook.
It is performed on a sealed restoration
- Indications:
- Due to the rigidity of the system and the connection method similar to a precision slide, any rotational movement of the saddle in the sagittal plane generates harmful forces on the supporting tooth.
- Consequently, the indication of this hook should be limited to restorations of embedded edentulousness.
- This hook, which is very inconspicuous, is aesthetic.
- Contraindications: short clinical crown
- Advantages: aesthetic (not visible because it does not surround the vestibular surface)
- Disadvantages: dental mutilation, sometimes leading to devitalization.
- Variation: equipoise on natural teeth:
- Indications:
- This hook is used on PM and C even abraded,
- In case of recession
- Or in case of short teeth (tight occlusion).
- Vestibular junction hook
- RPI system
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
R: Rest (connection and mesial occlusal support)
P: Proximal plate (distal plate)
I: I-shaped retentive arm
THE DIFFERENT ELEMENTS OF THE CHASSIS (PPAC)
- Indication: CL I KA and CL IIKA
- Contraindication
- External vestibule not deep enough for the retentive arm
- Pillar not showing vestibular withdrawal zone
- Significant mucosal undercut => distant retentive arm => irritating
- For the cheek, lip, plus great food retention.
- Gummy smile.
- ROACH SYSTEM:
The basic principle of this system developed by ROACH lies in the use of mesial and distal withdrawal zones (flexible part)
- Indication: CL I ka and CL II ka
- Advantage: aesthetics
- Disadvantages:
- lip and gum irritation
- lack of belting
The choice of different hooks is also guided by the so-called rigid and semi-rigid designs.
The rigid design uses the proximal junction hook, the bracket (secondary connection) is directly linked to the saddle. The advantages are multiple, the transmission of forces is made directly to the teeth which improves proprioception, limits the sinking of the saddle and guarantees a stable, precise occlusion. This concept should be favored as often as possible. Indicated for embedded edentulism and has disadvantages: for terminal edentulism Cl I II KA,
The semi-rigid design is based on the principle of moving the stem away from the edentulous area to gain elasticity, which would avoid too much stress on the supporting teeth. The stem connects the tooth to the main connection by a lingual junction, it is longer than in the rigid system allowing a very slight possibility of deformation. The disadvantages are the difficulty of maintaining a healthy periodontium under the stem, a less precise occlusion.