PARALLELISM AND INSERTION AXIS

PARALLELISM AND INSERTION AXIS

1. Introduction  : 

In fixed prosthesis, the notion of parallelism between the walls of the same preparation or of several preparations, coronal or corono-radicular, cannot be dissociated from the notion of ‘ ‘Axis of insertion” , and the same goes for retention.

Approximate parallelism is never absolute, we must obtain, according to BELIARD ‘s expression , ” an almost parallelism “.

In current practice, the walls of the various parts of the preparation converge slightly in the direction of disinsertion; the preparation is said to be tapered . This geometric shape facilitates the insertion and disinsertion of the anchor and should be less marked as the teeth are short.

2. Insertion axis
   1. Definitions: This is the direction in which a prosthesis is placed or removed; for some it is confused with the major axis of the tooth.

 ”It is the virtual straight line that connects the occlusal face or the incisal edge to the extremity 

apical of the tooth”. It must be studied according to anatomical and mechanical factors,

 using a parallelizer.

It can also be defined as:

• An imaginary line along which the prosthesis is to be inserted or removed, which must be determined before decortication of the tooth.

3. Parallelism: 
   1. Definition: Defined by the dictionary ”la rousse” as Property of straight lines, planes, curves, which are parallel; The parallelism of the wheels.

• Parallelism can also be considered as a particular geometric state of systems considered in association.

In joint prosthesis  : it is the relationship between the different walls of the same preparation or of several preparations.

• The insertion surfaces of our bridge abutments can never be parallel, they are tapered to each other; in the direction of the insertion of the bridge, therefore it is almost parallel .

2. Parallelizers: term designating any instrument allowing to define, trace or measure parallelism, we can distinguish two types:
   1. Intraoral parallelizer:

1. The parallelometer: used for diagnostic , tracing or measurement purposes comparable to a compass, they allow the study of teeth between them and the measurement of the distance of the teeth, the tracings and the monitoring of parallelism during preparation. Ex: DENTOPLAN parallelometer by FISHER IN 1930.
2. The paralleloguide: used to guide the rotating instrument, it allows preparations to be made parallel to the same axis by horizontal or vertical translation movements, and guides the rotating instruments in the mouth. Ex: P by CHARLES MERTENS which attaches to the head of the contra angle.
3. Laboratory parallelizers: Simple and passive devices used to search for insertion axes and perform tracings and measurements.

PARALLELISM AND INSERTION AXIS

3. Goals of parallelism:
   1. Complete insertion of anchors on or in the preparations.
   2. Easy and passive insertion of anchors on or in preparations.
   3. Perfect adaptation of the prosthesis to the cervical edges of the preparations.
   4. Stabilization of the prosthesis by the self-locking phenomenon” by the walls and the preparation accessories without constraint during insertion.
4. Principles of parallelism according to L’EXBRAYAT :
   1. Parallelism is an ideal towards which we must strive.
   2. The axes of the various anchor preparations must be perpendicular to a single plane.
   3. Since parallelism is only approximate for multi-face preparations, three opposing faces converge, while the two closest faces diverge.
   4. The choice of the insertion axis is made by taking into account the greatest number of elements, among the strongest and safest.” Exception: If a weak root constitutes an essential pillar, it directs the parallelism”
   5. The root tenons control the parallelism.
   6. Dental posts follow parallelism. Their position may vary.

NB  : Absolute parallelism is an ideal towards which we must strive. However, a slight tolerance of 3 ^o to 5 ^o of convergence of the unit preparation walls is desirable.

For a plural prosthesis the tolerance can go up to 6 ^or even 8 ^o especially if the preparation height is significant.

5. How to obtain parallelism: Near parallelism can be obtained by several processes such as:

• the preparations.
• reconstruction by inlaycore which will be carried out in the laboratory using a parallelizer.
• orthodontic treatment.
• If parallelism cannot be obtained, we will resort to:
  • Either to precision attachments.
  • Either to the exceptionally removable fixed prostheses.
• The root tenons control parallelism when they are attached to the metal infrastructure.
• The dentinal posts follow the general parallelism; they must be placed along the pre-established insertion axis; it is their topographical situation on the preparation which varies.

PARALLELISM AND INSERTION AXIS

4. Insertion axis according to anchoring: 
   1. Single anchorage: In the case of a single prosthesis ”CRT” 

generally we have no problems, the insertion is done according to a 

vertical translational movement along an axis called the insertion axis

This axis is among other things only the large one of the tooth.

• So for a unit preparation the insertion axis 

of the anchor is confused with the major axis of the tooth. 

When carrying out. It is appropriate to converge

slightly the walls towards the long axis of the tooth.

PARALLELISM AND INSERTION AXIS

• The obliquity of the lateral faces of the preparation

determined with the insertion axis ” or the major axis

of the tooth” a clearance angle of approximately 3 ^{o .}

Increasing convergence harms retention

and stability by reducing the contact surface

and the multitude of insertion directions .  

2. Multiple anchoring: Insertion can be considered: 
3. Either by a translational movement as for 

a single prosthesis and this requires parallelism 

approached the walls of the different preparations.

• Insertion by translation is essential whenever 

dentine tenon anchors are used 

attached to the metal structure, or

when dental preparations present 

vestibular or proximal ‘a’ grooves.

2. Either by a complex movement combining vertical translation and rotation ” 1.2.3 ”: 

PARALLELISM AND INSERTION AXIS

PARALLELISM AND INSERTION AXIS

• If we have the expression the control of this axis will be done 

visually in the mouth or on a model. 

The parallelizer can also be used.

• The degree of convergence towards the occlusal face 

varies from 0 ^to 10 ^depending on the clinical cases encountered, 

the ideal angle always remains 6 ^o .

• However, the direct vision of a preparation in 

mouth is rarely achievable and is therefore done 

generally using a mirror which will be held at 1.5 cm 

above the latter. Here too the observation

must be monocular and the axis will need to be checked 

insertion common to all preparations.

5. Factors determining the axis of insertion:

1. In the presence of a pulped tooth  ; it is the pulped tooth which will provide the axis of insertion
2. In the presence of several pulped teeth, it is the least voluminous and slightly malpositioned tooth which will cause the insertion axis to deviate.
3. If the bridge has root tenon anchors, it is the longitudinal axis of the thinnest root which will give the insertion axis.
4. The insertion axis of full or partial coverage crowns for cuspidate teeth is generally parallel to the long axis of the teeth.
5. If the tooth is poured ”B” the proximal faces of the teeth adjacent to the preparation

               prevent the insertion of the reconstruction ”C”, The choice 

                            then moves to a direction of the insertion axis 

                                         perpendicular to the occlusal plane ”D”.

6. In the extreme case ‘ A ‘ (in the absence of orthodontic therapy) this will require a significant coronal resection and the creation of an inlaycore ‘ B ‘, to straighten the axis.
7. Cases of large reconstructions with very divergent dental axes ; several possibilities can be considered:

• Correction of axes by coronal-radicular reconstructions ‘RCR’
• Sectorization and reunion of the different fragments by precision attachments ”A”.
• Use of orthodontics

8. Bridge on pulped and pulpless teeth  : in this case it is the preparation on pulped teeth which will give the I axis, the walls of the tooth will be more cut to get closer to the I axis of the bridge.
9. Conclusion :

• Good parallelism along a well-chosen insertion axis supports the success of fixed prosthetic treatment by satisfying the various biomechanical and preparation principles .

PARALLELISM AND INSERTION AXIS

  Sensitive teeth react to hot, cold or sweet.
Sensitive teeth react to hot, cold or sweet.
Ceramic crowns perfectly imitate the appearance of natural teeth.
Regular dental care reduces the risk of serious problems.
Impacted teeth can cause pain and require intervention.
Antiseptic mouthwashes help reduce plaque.
Fractured teeth can be repaired with modern techniques.
A balanced diet promotes healthy teeth and gums.
 

PARALLELISM AND INSERTION AXIS