Parallelism and retention in joint prosthesis
Introduction :
A joint prosthetic construction only makes sense if it is immobilized on the underlying supporting tooth, no sealing cement offers the adhesion properties necessary for the fixity of the prosthetic element, it is necessary to ensure retention and stabilization of the assembly, to use particular geometries
I/ Retention:
A-Definition:
Retention is the force which opposes the disinsertion of the reconstruction along its axis of insertion, the result obtained by all the means of connection of the prosthetic element.
It can be:
1-Primary retention: depends on:
- Height of preparation
- Total surface area of the preparation
- Approximate parallelism of the walls or preparations between them
2-Secondary retention: corresponds to all preparation devices allowing primary retention (grooves; boxes; wells)
3-Joint retention: ensured by hermetic joining materials (sealing cement, bonding composite)
B- the different movements which tend to detect a bridge : in plural prosthesis, due to the existence of a span; the detachment forces exerted on the anchors are increased:
B1: Tilting movement: if pressure is exerted on a posterior element, this causes the anterior pillar to rise, and if the pressure is anterior, the tilt is reversed.
And in the case of 3 anchors, pressure at the posterior level causes an elevation of the anterior pillar and a rotation around the intermediate pillar
B2-Overturning movement: it is caused by tangential forces in protrusion or lateral movements when there are interferences.
B3-Torsional movement: Torsional movements are complex, combining rotation and noticeable reversal in the canine and premolar region; they are often due to a non-rigid framework or a poor quality weld connection.
B4-Bending movement: Caused by vertical forces; and are always due to poor design of the ultra metallic structure: an alloy that is too soft or too elastic.
C- Factors involved in retention:
1-The preparation strip: to have a retentive preparation a slight convergence of the walls of the preparations is required (3° per wall in relation to the insertion axis and 6° ……)
2- the extent of the preparation: the more the surface of the preparation is stretched, the better the retention
a-Height of the preparation : for two crowns of equal diameter, the longest preparation ensures the best retention
b – Preparation width: for two crowns of the same height, the widest preparation ensures better retention
Parallelism and retention in joint prosthesis
c-Relative height between support tooth and reconstruction: if two crowns of different height on two preparations of the same length and width are subjected to identical forces, the highest crown is the most fragile because the lever arm of the force exerted on it is greater
The importance of coverage by the prosthetic element : a preparation which comprises 4 opposing walls 2 by 2 (total coverage anchoring) has a single axis of insertion and is more retentive than a preparation with dilapidated walls; when a wall is no longer involved (partial coverage anchoring), numerous axes of detachment appear.
3-The surface condition of the preparation:
The reciprocal roughness of the surfaces increases the contact surface, however significant roughness harms wettability by trapping organic debris, so it is necessary to obtain a clean preparation surface without seeking real polishing.
4- The collage: made with:
-Resin cements
-Bonding composites
-The adhesive properties of these materials are very favorable to the increase of prosthetic retention. However, the imperative absence of humidity requires supra or juxta gingival prosthetic limits and therefore the bonding responds to precise indications and requires a rigor of implementation guaranteeing its reliability.
-The intimacy of contact prosthesis preparation which depends on the precision of the casting allowing to increase the coefficient of friction promoting retention
-The reduction of the occlusal face follows the direction of the inclined planes of the tooth if the occlusal surface of the tooth is flat the thickness of the construction materials may prove insufficient in relation to what were the grooves and pits of the prepared tooth and decreases retention.
D- Role of the clinical examination in overcoming the problem of retention:
Any prosthetic element must be able to withstand the constant occlusal forces to which it is subjected; this takes on particular significance when it comes to making a bridge; because the constraints
Exercising at the level of the missing teeth are transmitted to the support points via the connections and the anchoring means consequently the supporting teeth are called upon to receive additional constraints. Hence the need for a thorough clinical examination for a good choice of supporting teeth of their nature and anchors.
II/Insertion axis
1-Definition: it is the imaginary line along which the prosthesis must be put in place or removed.
A limitation on the number of insertions is always less retentive.
Parallelism and retention in joint prosthesis
Insertion is done either by:
-A translational movement
-A complex movement combining translation and rotation
2-Factors determining the choice of an insertion axis:
-In the presence of a pulped tooth and a depulped tooth, it is the pulped tooth which will provide the axis of insertion
-In the presence of several pulpless teeth, it is the least voluminous tooth in slight malposition which will cause the insertion axis to deviate.
-If the bridge has root tenon anchors, it is the longitudinal axis of the weakest root which will give the insertion axis.
-In the case of extreme malposition that cannot be corrected of the abutment tooth, this will require a significant coronal resection and the creation of a core inlay with a root post.
Parallelism and retention in joint prosthesis
3-Insertion according to the type of prosthetic construction
a-Case of single prosthesis:
The insertion axis coincides with the major axis of the tooth.
b-Case of multiple prosthesis:
– by a translational movement: the bridge support preparations presenting an approximate parallelism
– By complex movement: which combines translation and rotation indicated in the case where the divergence of the dental axes is significant
Biomechanically, a divergence of 20° to 30° of the axes of the supporting teeth is recognized as acceptable at the level of the posterior sectors.
Note: only insertion by translation is acceptable in the case of a prosthesis on osteointegrated implants due to the rigidity of the pillar, otherwise there will be significant internal constraints.
III/Parallelism:
In practice, perfect parallelism cannot be achieved, it is almost parallelism due to the convergence of the preparations which facilitates the insertion and removal of the anchors.
1-Principle of parallelism: parallelism is obtained either by:
a-Preparations: convergence of 6° between the opposite faces is desirable
b-Inlay cores which will be made using a parallelizer in the laboratory
c-An orthodontic treatment
d-If parallelism cannot be obtained, we will resort to either:
-precision attachment
– removable fixed prosthesis
Other principles:
The root tenons control the parallelism when they are attached to the metal infrastructure
The dentinal posts follow the general parallelism, they must be placed following the pre-established insertion axis.
2-Parallelism in the mouth
Once the insertion axis is chosen; the cutter is placed stationary on a first prepared wall then it is brought back into a horizontal translation movement without deviating its direction on the second wall finally it is put back into motion
Probe number 6, moved from preparation to preparation under the same conditions, allows parallelism to be checked.
In the first root tenon housing or the first well a sufficiently long calibrated guide rod can be placed, the drill will then work parallel to this guide
There is a mechanical device which is the paralleloguide which keeps the abrasive instrument in the mouth in the previously chosen insertion axis.
3_Parallelism in the laboratory:
Once the preparations in the mouth are done, an alginate impression will be taken which will be poured in hard plaster and the parallelism will be checked on the model using the parallelizer, the faces to be retouched are marked, then this operation is repeated as many times as necessary.
Good oral hygiene Regular scaling at the dentist Dental implant placement Dental x-rays Teeth whitening A visit to the dentist The dentist uses local anesthesia to minimize pain