The constituent elements of the metal chassis
Definition
The chassis is defined as being:
“a skeleton which rests on the gingivo-osseous crests, on the one hand supports the replacement teeth, on the other hand clings to and rests on a certain number of the persistent teeth, avoiding subjecting them to actions which could harm their integrity.”
Therefore, the aim of a prosthesis in a partially edentulous person is not only to replace the missing teeth but above all to preserve the remaining teeth. The frame must be sufficiently rigid. If it is not, bone resorption occurs at the support surfaces and the abutment teeth are mobilised.
It presents several elements, each having a well-defined role in the prosthesis, it includes:
Primary connection – secondary connection – saddles – hooks – cingulate bar – coronocingulate bar – occlusal cleat
Alloys indicated on the chassis:
The chassis is most often made from:
- Alloy based on cobalt, chrome, molybdenum or stellite.
The frame must ensure the rigidity of the prosthetic construction. For this, PPAC alloys must have the following qualities:
* high mechanical resistance.
* high resistance to corrosion in oral fluids.
* good flowability.
* absence of toxicity.
* easy handling with usual laboratory techniques.
The constituent elements of the metal chassis
1. The main connection:
- The basic element of the prosthesis.
- Brings together the different components and allows them to work in synergy.
- Covers the gingivo-osseous support surface.
- Must comply with certain requirements:
- Requirements:
- The principle of rigidity: its form
- its thickness
- the nature of the alloy used.
- Patient comfort
-Thickness as reduced as possible
– Do not disturb phonation (if possible, free the retro-incisive area).
–Do not disturb swallowing
-Symmetry of shape. Polished surface.
- Respect for osteo-mucosal tissues: Respect for the principle of decolletage
–Exostoses (torus) are avoided.
-The median suture should not be compressed (spacing of the reinforcement).
-The mandibular retro-incisive region is systematically spaced.
-Any metal part of the prosthesis must remain at a distance from the LRM.
- Main connection to the upper jaw
- Wide plate
- It relies heavily on the palatine vault.
- Ensures equal distribution of occlusal loads
- Indicated for Class: I and II.
- Ends in front of the soft palate or slightly indented
- Anterior palatine foramen region and median papilla exposed.
- Respect for the gingival rings of the remaining teeth.
- Its thickness is reduced (0.6-0.7mm).
- Full Cover Plate
- Anterior part rests on the cingulum of the remaining teeth:
- Preparation of teeth.
- Discharge at the level of the gingival ring.
- Posterior border located at the junction of the hard palate and the soft palate.
- Class I and II of great extent.
- Double spacer plate
- Plate hollowed out in its center.
- Half junction section.
- Thickness of 2mm.
- Width: 6mm ant and 9mm post, 2mm in the center.
- The posterior segment should rest on the hard palate just anterior to PD-PM, in an area inaccessible to the tongue.
- Anterior bar 5 to 6 mm from the abutment teeth.
- Intercalary edentulism with dental support.
- Narrow plate
- Meets the principle of turning.
- It is inscribed between two lines, one anterior and the other posterior, connecting the proximal faces of the teeth bordering the Edentum.
- Thickened in its center.
- Embedded edentulism with dental support only;
- Main connection to the lower jaw
The lingual bar
- Half pear section.
- Flat part facing the mucosa.
- Bulky part towards the floor.
- Width 1.7 to 2mm and its height is 3.5mm.
- Trimming of at least 3.5 mm of the remaining teeth.
- 1mm away from the brake.
- Orientation of the retro-incisive lingual slope
- Its anteroposterior position depends on the orientation of the retro-incisive lingual slope of the alveolar bone.
- If it is vertical 0.2 mm
- If it is oblique 0.3 to 0.4mm (avoid any injury)
- Evaluation of the retro-incisal space.
- Evaluation of the brake collar space.
The lingual strip ; in case of contraindication of the lingual bar.
- Comes into contact with all parts of the lingual surfaces of the teeth located above the cingulum then encroaches without touching it (0.3mm) the fibro-mucosa and ends 1mm from the floor of the mouth.
- Its thickness must be as minimal as possible without affecting its rigidity.
- Indirect support will be necessary.
Double lingual bar: Lingual bar + Continuous cingulate support or cingulate bar.
- It is the metal element placed on the cingulum of the anterior teeth, its section is half-ring;
- HEIGHT:2mm.
- THICKNESS:1mm.
- Connected to the lingual bar by two secondary connections.
- Aesthetic.
AVG:
1. involve a significant number of abutment teeth.
2. contention.
3. Prevents sagittal rotation movements in cl II cases.
The constituent elements of the metal chassis
Secondary connection
- These are metal parts used to connect the hook or indirect supports to the main connection.
- Have a roughly triangular section.
IMPERATIVES
- Rigidity; Diameter: 1.5 to 2mm.
- Respect for osteomucosal tissues Discharge of 0.2 to 0.3 mm.
- Patient comfort located at the level of the interdental spaces of the teeth bordering the Edentulousness.
- roles: Transmit the applied forces to the abutment teeth.
Increase the mechanical resistance of the chassis.
The saddles OR RETENTION GRIDS : aremetal parts located on the toothless ridges.
Spaced 1.5 to 2 mm from the supporting mucosa.
Posteriorly located in front of the tuberosities and retro molar trigones.
- The hooks
It is a metal part developed and cast at the same time as the infrastructure, allowing the prosthesis to be attached to the remaining teeth.
- The imperatives
- Must surround more than half the circumference of the tooth.
- Any retention portion of the hook must correspond to a rigid portion in order to eliminate any scoliodontic risk: reciprocity of action.
- He must stay in touch with the email and must be perfectly polite.
- The retentive end of the hook must be passive as soon as the prosthesis is in place.
- The different parts of the hook should be located as close as possible to the middle of the cervical 1/3 to avoid versions.
- The resonant parts of the hooks of each arch must be symmetrically arranged so that a vestibular resonant head corresponds to a vestibular head of the hook of the other hemi-arch.
- The location of the occlusal supports is imperative.
- The two arms of the hooks should be as close to each other as possible.
- It must contribute to retention, stability, and sustenance.
- Hook functions
SUPPORT:
Reaction which opposes the axial forces of depression of the prosthesis under the influence of the occlusals.
STABILITY:
It is the reaction opposing the forces which tend to make the prosthesis undergo horizontal translation or rotation movements.
RETENTION:
This is the reaction which opposes the disinsertion of the prosthesis in the axial direction.
Different parts of hook: The cast hook is made up of 3 parts:
RETENTION ARM : The active tip, thin and flexible, represents the retention function by attachment, it is generally vestibular and ensures retention by its terminal part which is flexible and which is located under the guide line.
WEDGE ARM (stabilization arm) : The body of the hook, thicker and less flexible, has a function of stabilizing the prosthesis; it allows the tooth to be wedged and supported before the retentive arm passes the vestibular bulge; it is located above the guide line.
DIRECT OCCLUSAL BRACKET (SUPPORT): Must be rigid, it is located on the occlusal surface, it occlusally extends the stem or secondary connection and participates in the support of the prosthesis. The occlusal support must not create interference or occlusal prematurity.
Occlusal supports
These are metal elements resting on the occlusal surface of the teeth.
Occlusal stops (PM and M) and cingulate supports (INC and Can).
- The cubicle should be spoon-shaped (avoid any sharp angles).
- Preparation with diamond ball cutter.
- Its size is proportional to that of the tooth concerned: 1/3 of the mesio/distal dimension for a PM, 1/4 for an M.
- Its depth: between 1 and 2 mm.
- Its buccal-lingual width: 3 mm.
- ROLES OF OCCLUSAL SUPPORTS:
- Participates in the distribution and transmission of occlusal forces along the major axis of the tooth.
- Prevents the prosthesis from sinking.
- Maintains constant tooth to hook ratios.
- Helps to restore a failed point of contact.
- They provide protection to the papilla.
The different types of cleats
- direct dental supports : located on the teeth limiting the edentulous segments
- part of a hook
- indirect dental supports ; located at a distance from the edentulous segments
- not part of a retentive complex
- additional function of opposition to rotational movements
The constituent elements of the metal chassis
- Classification of hooks
Proximal junction hooks:
- ACKERS hook.
- RING hook.
Lingual junction hooks:
- Rear action hook.
- BONWIL hook.
- NALLYT MARTINET hook.
Vestibular junction hook: ROACH system
- T hook
- Y hook
- Hook I
- RPI Hook
- ACKERS hook: it is a hook with secondary connection and occlusal support located on the side of the edentulism, the rigid arm is located above the guide line, on the buccal face and going up to half of the vestibular face, a flexible arm representing the end of the vestibular arm located in the zone of resonant undercut, it is used in the case of intercalated edentulism of classes III and IV of Kennedy-applegate
The constituent elements of the metal chassis
- RING hook; :occlusalsupportsincluding:
• one is mesial, it extends the secondary connection connected to the saddle,
• the other is distal located between the rigid part and the flexible part of the hook
• A rigid arm connected to the secondary connection
• A flexible retention arm.
It is indicated for isolated molars (last molar) on the upper or lower arch.
- NALLYT MARTINET hook: this is a hook with secondary connection andocclusallocated on the side opposite the edentulism, a vestibular retention arm and a buccal wedging arm.
This hook is systematically indicated in Kennedy-Applegate classes I and II.
Posterior action hook: It consists of 3 parts:
– 1 a rigid arm attached to thesecondarywhich connects it to the main connection
– 2 An occlusal support located on the edentulous side
– 3 A flexible arm Constituting the extension of the rigid arm and located in the withdrawal zone on the vestibular face of thetooth.
Indication: posterior tooth.
- BONWILL hook :is a double Ackers hook, composed of:
• 2 occlusal supports, one mesial and one distal
• 2 retentive vestibular arms
• 2 wedging arms, one mesial and one distal (on the buccal side).
It is indicated in Kennedy-Applegate classes IV or II
- ROACH System Hooks:
Unlike the hook system proposed by Ney, that of Roach allows to solve the problems posed by obtaining the retention, very difficult to solve in certain cases by the circumferential hooks. ROACH proposed 07 variants which are the letters of the alphabet: C, L, U, S, T, I, R and which is adapted to the teeth supporting the hook.
The special feature of these hooks is:
The wedging arm is not part of the hook as well as the occlusal support
The position of the occlusal support, whether mesial or distal to the edentulous area, creates a rigid or semi-rigid connection.
C hook L hook U hook
Conclusion
The production of the removable partial prosthesis frame has undergone many improvements; it will only fulfill its therapeutic role with a good knowledge of the indication and the role of each element of the frame.