Removable partial prosthesis and supporting tissues

Introduction :

A removable partial prosthesis is a foreign body that must be integrated into a complex system which is the masticatory system.

The goal of this prosthetic restoration is the restoration of function and aesthetics while respecting the integrity of the anatomical structures supporting it.

And since the removable prosthesis acts according to its design, it can be:

Either FAVORABLE: if it is well tolerated and plays a therapeutic role and at that point we say that it is BIOPHYSIOLOGICAL
Either UNFAVORABLE : or harmful if it causes pathology as well as alterations and lesions of the surrounding tissues.

Definition

PPAM was defined by Professor ROUOT as being:

“A prosthesis characterized by the existence of a skeleton which on the one hand supports 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.”

Anatomical structures supporting a PPAC:
1. Structures directly related to the a-Email partial denture: is in direct contact with:
  - Retention and stabilization means (hooks, coronary bar, cingulate bar).
  - prosthetic teeth by occlusal contact and/or proximal contact.

Gingiva or fibro mucosa : epithelial-connective periodontal tissue, the gum adheres to the alveolar bone which it protects as well as the desmodontal space. The gingival papilla located in the cervical embrasure is protected by the interdental contact point; it undergoes recession in relation to an edentulous ridge. The prosthesis must avoid irritating or compressing the gum, the inflammation of which can lead to alveolysis.
The oral mucosa

It lines the entire oral cavity, it is in direct contact with the intrados of the PPAM, in particular the edentulous ridges, With the extrados: the peripheral organs, lips, cheeks and tongue and finally with the edges namely the frenulum, the pterygomaxillary ligament, and the soft palate.

Structures indirectly related to the partial denture

a /- At the level of dental organs:

Dentin, pulp and cementum: it is obvious that the removable prosthesis must not come into contact with these tissues
Desmodontium: refers to the fibrous connective tissue located between the dental root and the alveolar bone. It contains ligaments and transmits functional stresses to the bone. Its compressibility allows the tooth to move axially by 0.1 mm. Dento-periodontal-supported prosthetic elements therefore have a significantly smaller range of movement than the latter.
Alveolar bone : essential for maintaining teeth, alveolar bone is constantly being remodeled and is resorbed in cases of hyper or hypofunction. It resists better (thanks to

to the ligaments of the mo dentals) to axial forces as well as to oblique or horizontal forces: the prosthesis must therefore transmit the functional forces along the major axes of the teeth.

Evolution of anatomical structures in the absence of prosthetic restoration

1. Dento-periodontal structures A- The odont:

The tooth next to the extraction, in the absence of a wedge, will move towards the gap formed. This movement of one or more teeth very quickly causes contact breaks between teeth, promoting food jamming.

This leads to the second risk created at the level of the tooth by edentulism, which is caries.

Finally, the most important change is of interest

occlusion : The movement of the teeth will disorganize the

relationships between the opposing occlusal faces. This will lead to premature contacts being formed, the consequences of which can be catastrophic.

The Periodontium:

The gingiva and epithelial attachment

Inflammation of the marginal gingiva is promoted by:

The rupture of the contact points between the remaining teeth resulting in mechanical damage to the epithelial attachment by the compaction of food during chewing
The protective barrier is broken and there will be a deepening of the gingivodental groove, creating conditions favorable to localized inflammation.

The periodontal ligament and the alveolar bone

They reorganize themselves in response to the efforts applied to them. Three situations can be considered:

the remaining teeth having retained their initial position; if the occlusal load is moderate and the marginal periodontium is healthy: the alveolar bone responds favorably thanks to the multiplication of desmodontal fibers
tilted teeth (change in axis) associated with chronically inflamed gums, occlusal pressure

will trigger periodontal disease. Radiological examination highlights the widening of the desmodontal space and angular lysis of the alveolar bone

the extraction of the opposing teeth will cause the absence of functional stress which is objectified on radiological examination by the disorganization of the orientation of the bone trabeculae surrounding the alveolar bone. if the edentation

is old, dental egression is the rule and often the entire periodontium is drawn into this migration, but if it

If a single erupted tooth is involved, the extrusion is accompanied by root denudation and alveolar bone loss.

2-Osteomucosal structures a-The mucosa:

Thickening of the mucosa at the top of the ridges resulting in a pyramidal morphology.
Mucogingival line migration

Bone tissue:

The insufficient stress on the bone tissue at the level of the edentulous ridges thus causes its resorption:

Centripetal to the maxilla and the anterior mandibular sector
Centrifugal to the rest of the mandible in the transverse direction

3. Peripheral structures

Hypertrophy of the tongue, floor of the mouth raised by the sublingual glands overflowing onto the atrophied bony ridges, proliferation of diapneusis of the inner face of the cheek which is sucked in

Behavior of the periodontium in the presence of prosthetic restoration

Bacterial plaque buildup Bacterial plaque accumulated under the prosthesis can cause denture stomatitis. The contour of the prosthesis influences plaque buildup, which is more likely to accumulate under a lingual strip than under a lingual bar.
Trauma related to prosthetic components

A lingual bar too close to the marginal gingiva or a continuous hook providing only limited dental support causes the prosthesis to sink at the expense of the tissues, thus decapitating the gingiva.

Transmission of excessive forces When the prosthesis is only supported by mucosa or the dental support is poorly designed, bone resorption is the rule.
Balancing errors
1. premature contact on natural teeth will cause inflammation.
2. premature contact with stool will cause resorption of the ridge.
3. If the patient tries to relieve himself by avoiding premature contact, he will cause craniomandibular dysfunction with muscular symptoms.
Design requirements for a PPAC

According to Le Joyeux: “During the main functions of chewing and swallowing, all removable prostheses, however well designed, move.”

Therefore, PPAs are considered potentially pathogenic with respect to periodontal tissues due to:

their removable nature and the frequency of insertion and removal movements which expose the supporting teeth to harmful forces
The difference in tissue compressibility between the desmodontium (0.1 mm) and the supporting fibromucosa (0.4-2 mm) means that the ground on which the stool rests, particularly in distal edentulous areas, is unstable.
Extension movements of the stool
Divergent orientation of the longitudinal axes of the support teeth.

PPA is therefore one of the most complex and difficult prosthetic restorations to carry out, especially if it is dento-mucco supported, and requires perfect mastery of the techniques specific to its production.

To be biofunctional, the PPA must be designed according to certain imperatives:

A- The Balance of the Prosthesis The success of a prosthetic treatment implies

an integration from an aesthetic and functional point of view
respect for the integrity of: dento-periodontal, osteomucosal, neuromuscular and articular structures.
if the prosthesis/structure relationships are permanently ensured during the function This is only possible if Housset’s balance triad is satisfied:

Sustenance, stabilization and retention.

Sustenance

“Reaction which opposes the axial forces tending to push the prosthesis into its supporting tissues” E.Batarec

It is necessary to oppose the forces developed during chewing with a greater or equal resistance and this depends on:

Anatomical factors

Very favorable supports	Insufficient support
Wide and high edentulous ridgesDense fibromucosa well attached to the periosteumMuscular insertion far from the ridge lineTeeth with healthy periodontium and sufficient root height	Severely resorbed edentulous ridgesHyperplastic mobile mucosa poorly inserted into the underlying boneMuscular insertion close to the ridge lineRemaining teeth with weakened periodontium and/or low root height

Factors related to prosthetic design

-Occlusal stops

They are essential. The more they number, the better the distribution of occlusal loads. This precaution is especially necessary when the periodontal structures are weakened.

They transmit the forces exerted on the saddles to the supporting teeth. They must be designed to transmit occlusal forces in an axial direction – the long axis of the tooth – because oblique forces cause bone resorption.

They require an adequate size of the lodge – without occlusal interferences

-The frame

The main connection to the maxilla contributes to support (the wide plate, the full-coverage plate, the U-shaped plate)

A wide covering on the palate is sought whenever there is a significant gap. On the other hand, in the mandible: the lingual bar, which is always distant from the tissues, offers no support. The lingual band and the cingulate spacer are additional support elements, so it is always appropriate for mandibular gaps to calculate the number of occlusal stops broadly.

-Stools:

Must largely cover the ridges to request osteo-mucosal support.

The larger the developed surface, the less the prosthesis tends to sink under the pressure induced by chewing, therefore an anatomical-functional impression is necessary to accurately determine the extension of the prosthetic edges in KA class I and II. 2-Stabilization

“Reaction opposing the forces tending to make the prosthesis undergo translational or rotational movements” E.Batarec

Apart from the insertion-disinsertion movement, displacements are always possible in relation to the teeth and ridges.

An extended saddle can move and generate six movements according to TABET; the design of the prosthesis must help to counteract them.

3-Retention

“reaction refers to the set of forces that oppose the separation between the prosthesis and the support surface” E.Batarec ; it depends on:

a – Anatomical and physiological factors

✠ The morphology of the ridges can hinder disinsertion

✠ The peripheral organs, by pressing on the external slopes of the stools, participate in retention

✠ Neuromuscular control contributes to retention

✠ Patient satisfaction with their prosthesis even if retention factors are deficient

Physical factors

Attraction by adhesion: developed during contact of the prosthetic base with the mucosa STANIZ’s law Force of attraction F= 2C XA/a

So in PPA you have to opt for a wide overlapping frame

C: surface tension of saliva
A: extent of the surface in contact
a: salivary film thickness

C- Mechanical factors

These are the devices by which the prosthesis is connected to the teeth. Two families can be described: hooks which exploit the undercut areas and prefabricated attachments which act by friction.

The hooks can be aggressive towards the enamel and the periodontium:

1 – poorly designed hooks exerting uncompensated oblique and horizontal forces at each insertion and removal.

Located too close to the gum, they continually attack the superficial periodontium.

3 – Poorly adapted, they constitute a real trap for bacterial plaque. To avoid all this, the design of the hooks must comply with the following requirements:

It must surround more than half the circumference of the supporting tooth.
It must remain in contact with the enamel and must be perfectly polished so as not to cause damage to the enamel.
The elastic retentive end must be passive as soon as the prosthesis is in place; it only becomes active to oppose the movement of the prosthesis.
In order to avoid any direct aggression, the hook must be at a distance from the marginal gingiva and supported by an occlusal stop.
the use of an alloy with a lower hardness than enamel in addition to careful polishing prevents any wear of the enamel.
Every flexible portion must correspond to a rigid portion in order to eliminate any scoliodontic risk.
Attachments are precision retention devices with two separable parts:
- One integrated into the removable prosthesis
- The other has a joint element.

Retention is achieved by friction between these two male and female parts. The advantage of these attachments is that they do not exert horizontal forces during insertion.

B-Choosing an insertion axis

Definition: Trajectory along which the prosthesis will be INSERTED AND DISINSERTED without constraints for the periodontal and osteomucosal tissues.

The search for an insertion axis is done by means of a parallelizer

In order to preserve periodontal health, the prosthesis should be inserted along a trajectory parallel to the axes of the supporting teeth, this not being possible in practice because the edentulations are not

compensated are accompanied by numerous disorders: the dental axes are rarely parallel to each other and therefore the insertion axis can only be a compromise between these axes, as far as possible we will prefer to choose an axis approaching the perpendicular to the occlusal plane,

If not properly positioned, the hooks exert dangerous twisting forces on the abutment teeth.

To ensure that this axis does not generate these harmful forces for the periodontium, its choice must be judicious and based on the following factors:

Retraction zones: the position of LG is a function of the inclination of the model in the frontal plane.
the guide surfaces : allow the rigid elements of the prosthesis to slide until it is correctly positioned on its supports),
aesthetics : which involves the assembly of the teeth , the positioning of the hooks and the creation of the false gum
interferences: must be removed so as not to create an obstacle to the insertion of the prosthesis (badly positioned teeth, exostoses, etc.)

NB: Osteomucosal undercuts constitute obstacles to the insertion of the prosthesis. If they are not surgically corrected during the pre-prosthetic phase, these interferences become a priority when choosing the insertion axis in relation to the withdrawal zones and the guide surfaces.

C-Respect for mucous tissues

PPAC must respect the tissue integrity of the stomatognathic apparatus
It must cover and exploit the anatomical and physiological elements favorable to support “Positive Indices” and stabilization and avoid the unfavorable elements “Negative Indices”.

The gingival ring :

Must be avoided because compression at its level by the edge of the framework results in gingival hyperemia, which is why the HOUSSET decolletage principle must be used

HOUSSET decolletage : the gingival ring is systematically exposed to avoid lesions or irritations around the necks of the teeth. The line begins at a perpendicular to the dental surface halfway between the ridge line and the most lingual point of the palatal surface. It then describes a curve

regular, approximately parallel to the gingival neck, leaving a distance of 5 to 6 mm at the level of the anterior teeth and 7 to 8 mm at the level of the posterior teeth.

If the detachment is insufficient, the gum is compressed, sucked into the hiatus, becomes hyperplastic and bleeds at the slightest contact.
If the neckline is poorly done and has sharp angles, it will promote food stagnation and consequently inflammation sets in.
Too much undercutting risks reducing the support surface too much, leading to overloads on the crests

At the level of the mandible:

The lingual bar should be spaced from the inner table of the mandible by 0.2 mm to 0.4 mm. The spacing is greater the more oblique the orientation of the inner table is.
Secondary connections: Make contact with the teeth in their upper part but are 0.2 mm away from the marginal gum, avoiding compressing the interdental papilla. They must not be too far from the lingual surface of the tooth so as not to hinder the tongue.
The saddles: Must largely cover the ridges in order to transmit the occlusal forces beneficial to their maintenance. The connection of the saddles to the abutment teeth must be done without promoting the accumulation of bacterial plaque.

All chassis elements must be well polished to ensure patient comfort, not to damage the mucous membranes and not to promote food retention.

D- Rigidity

This is an essential mechanical characteristic for the frame except the flexible end of the retentive arm, all the elements of the frame must be rigid.

Insufficient rigidity creates dangerous torsional forces for the hook support teeth. This is directly linked to the thickness and nature of the alloy used:

cobalt chrome being the material of choice, because at low thickness it fully ensures rigidity and mechanical resistance.

E-Occlusal balance

The assembly of artificial teeth must ensure a good distribution of occlusal loads between natural teeth and prosthetic teeth according to the chosen occluso-prosthetic concept
Occlusal balance is crucial in the stability of the prosthesis.

Maintaining maximum intercuspation occlusion is indicated:

In the absence of any craniomandibular dysfunction
When the vertical dimension is not changed
When anterior guidance is provided by natural teeth, the use of centric relation occlusion is required:
When PPA is opposed to a total prosthesis
When a pathology requires balancing grinding
When the DV is modified

F-PATIENT COMFORT

The PPA must provide a certain comfort to the patient by achieving:

A symmetrical layout so that it does not remain poorly perceived
Phonetic areas released when possible
Intimate contact of the frame with the fibromucosa (Prevents food infiltration)
Secondary connections should be placed in the interdental space so that they are not perceived by the tongue.
Smooth edges and a well-polished prosthesis.
Aesthetics must be restored.

Interest in post-prosthetic treatment

A- Rigorous hygiene:

It should be noted that the presence of PPA in the mouth promotes the accumulation of bacterial plaque, therefore the instruction and motivation of the patient in hygiene constitutes an essential step in the treatment.

B-Periodic checks:

In order to maintain a balanced and perfectly integrated prosthesis over time, regular visits are essential to detect any changes that could cause damage to the supporting tissues or the masticatory system.

Conclusion

The preservation of remaining teeth and their periodontal health is closely linked to the replacement of missing teeth with a well-designed prosthesis and rigorous hygiene as well as regular check-ups.

Bibliography

Batarec E. – Buch D- Summary of partial removable prosthesis – 1989 Edition
Lejoyeux J- Removable prosthetic restoration of partial edentulism -Maloine 1980.
JC BOREL,j- SCHITTLY,j- EXBRAYT . PPA Manual 2nd edition Masson 1994.
J-Champion, S-Soumeillan, JJ-Guyonnet and R-Esclassan . Removable partial prosthesis: Design and production of a cast removable partial prosthesis. Encycl Méd Chir