Classification of bridges and biomechanical principles

The loss of a tooth causes the integrity of the dental arch to be altered.

There are several prosthetic options to replace missing teeth. The bridge is one of them. The bridge is a plural construction intended to compensate for a tooth loss and restore all functions. This device is composed of at least two prosthetic pieces joined together

There are several types of bridges depending on the location of the supporting teeth, the chosen anchoring method, and the type of indentation.

These multiple restorations are subject to mechanical and biological principles that must be respected in order to preserve their durability and the integrity of the remaining teeth.

/ Classification of bridges:

Depending on the importance of the elements that make up the entire prosthetic restoration, there are several types of bridges varying with the anchoring method used, the nature and number of the abutment teeth.

The shape of the bridge depends on the location and the number of abutment teeth chosen on the dental arch, hence the following classification:

Short span bridge.
Medium span bridge.
Long-span bridge.

A / Short-span bridge

They generally replace a tooth on the arch, easy to design but the disadvantage lies in the mutilation of two teeth to replace a single tooth. We can have:

/ Double recessed BCP (2 pillar teeth)

The bridge has two support pillars anchored to a pulped or depulped tooth; the choice of anchor and span depends on the bridge situation; we can have:

with double rectilinear embedding (lateral sectors of the dental arch).

Figure 1: Straight bridge

with double curvilinear embedding (anterior sector).

/ single-recessed (1 pillar tooth) . There are two types

Single-mount BCP with a supporting inlay

An inlay is less visible than a cap; it will be made on the healthy tooth while the other tooth is used as an abutment. E.g.: replacement of the 1st PM (size of the 2nd PM plus an onlay on the canine).

Figure 2: Short-span bridge

single-recessed (free-extending bridge)

This is a cantilever bridge, the extension part is a traumatic lever arm for the abutment tooth and the mucosa; this bridge can be moved in all directions, and carries a greater risk of twisting and overturning.

Bridges with mesial extension (lateral incisor or premolar) support occlusal forces better than those with distal extension.
Favor the absence of occlusal contacts on extension during lateral or diduction movements.
The extension element is carried by a bridge of at least two pillars is preferable.
Indications more oriented towards the replacement of lateral incisors or premolars, when the implant alternative is impossible

Figure 3: Cantilever bridge

B / Medium-span bridge

They are more difficult to perform than short-span bridges, with replacement of two missing teeth, whether adjacent or not.

/ BMP with double rectilinear embedding : e.g. absence of the 2nd PM and the 1st M; use in this case as a bridge abutment the 1st PM and the 2nd M.
/ BMP with double curvilinear embedding: The anterior bridges take the curvilinear form, the intermediate ones are at a distance from an axis joining the anchoring means. They act as a lever and cause a rotational movement around this axis.
- This movement is all the more important as the curvature is pronounced. In order to limit this rotation, it is sometimes imperative to add one or more latero-posterior pillars (premolars).

– The replacement of the 4 upper incisors will have to use the two canines and the first premolar as bridge abutments. While in the lower jaw 2 canines are sufficient

Figure 4: Rotation of the anterior bridge.

/ Double-recessed BMP with intercalated abutment : e.g. absence of 1st PM and 1st M in this case the execution is more complex and more expensive and more difficult. The intercalated 2nd PM abutment receives impulses from the two bridge abutments which frame it, the parallelism between the abutments is rarely easy.

The movements to which this type of bridge is subjected can cause lysis of the supporting tissues if the retention of the anchor (intercalated pillar) is not carefully studied.

Figure 5: Medium-span bridge (non-precious metal) with two intermediates and a central pillar.

C / Polygonal bridges

Made on several pillars arranged in several planes of the arch, they can be partial or total depending on the number of residual teeth.

Partial polygonal bridges

2 possibilities

Restore the anterior part of the arch from PM to PM: called curvilinear bridge; comes in the form of an arch.
Restore the lateral and rectilinear part of the more or less extended arch of the anterior arch, it then appears in the form of a parabola (parabolic polygonal bridges).
Total polygonal bridge

He restores the entire arcade.

Figure 6: Full metal-ceramic bridge

Depending on the type of junction:

Non-removable (conventional) bridge:

These are sets composed of elements sealed on the teeth (anchors) and which support spans or pontics which reproduce the occlusal form of the missing teeth.

Their advantages include the feeling of comfort, security and stability they provide; however, they require tissue mutilation and are not completely prophylactic.

Removable-non-removable bridge:

They have the same stability and rigidity as fixed bridges, but they are hinged and can be partially dismantled by the practitioner. Because the various elements are held together by nuts or screws, they have a double advantage: their stability and the need to remove them periodically to check the parts of the mucosa underlying the bridge.

3. Removable bridge on non-removable anchor:

They use as retention methods the friction force between the anchors sealed on the abutment teeth and called infrastructure, and a superstructure comprising a span which fits frictionally on the infrastructure.

Figure 7: Removable bridge with an attachment

/ Biomechanical principles

Any joint prosthetic restoration must have 2 essential objectives:

To sustainably restore the various functions more or less disrupted by toothlessness: chewing, aesthetics and phonation.
Respect and preserve the biological structures present in the oral cavity

A / Mechanical principles

Prosthetic construction must meet 3 requirements

Insertion. See the course on parallelism and retention.
Retention.
Mechanical resistance to stresses developed during function.

Mechanical resistance:

It is the rigidity of the prosthetic construction which determines the mechanical resistance; it depends on the mechanical qualities of the alloy and the morphology of the bridge:

The mechanical qualities of the alloy: two cases;

For smaller constructions, precious or non-precious alloys based on NI-Cr will be chosen, which give satisfactory results.
For very large constructions with long spans, non-precious alloys based on Ni-Cr should be chosen, which guarantee increased rigidity.
The metal structure

Whether or not it is covered with a cosmetic layer, it must have sufficient thickness, taking into account the nature of the alloy used.

To avoid deformation or fracture under chewing forces it is necessary to:

Increase the thickness.
Establish a sufficiently wide contact while clearing the embrasure to make it accessible to the interdental bosses.
Ensure that a sufficient section of the span is created
The lingual part can be reinforced by a thickening of the metal which will effectively resist the different movements

Figure 8: The bending of the bridge span is 8 times greater if the length is doubled.

Figure 9: The bending (X) of a bridge span of thickness (t) is eight times less than that of a span half as thick.

B / Biological principles

They apply to both anchors, intermediate elements and occlusal surfaces

Morphology of anchors:

Takes into account respect for tissues and preservation of pulp vitality.

The cervical limit of the anchors, by its correct position, must ensure a dento-prosthetic joint and an axial morphology which allows the anchors to be located in the continuity of the roots of the supporting teeth.
Safety for deep periodontium.

Morphology of intermediate elements:

a / Relations with the edentulous crest : their purpose is to protect the fibro-mucosa from any irritation.

Supramucous intermediate: designed by different authors to be located at a distance from the crest, perfectly tolerated but unsightly

Figure 10: Vestibular view of a supramucosal intermediate

Intermediate contra mucosal:
Ovoid type (the general shape of the bridge span is egg- or shell-shaped, its intrados is convex and in contact with the top of the ridge over a small surface, the embrasures are largely clear.

Figure 11: Ovoid saddle

The type of saddle modified by STEIN: the author makes a modification: he only keeps the vestibular part of the intermediate element and gives it a convex intrados which comes into contact with the top of the crest; the gingival embrasure is widely open on the lingual side.

b / Tissue adjustments : these are corrective surgery for both gingival and bone tissues, and aim to create sufficient space in the vertical direction and a harmonious contour of the ridge.

Gingivectomy will remove hyperplastic tissue while additive crestal surgery for a convex profile favorable to the bridge intermediary.

c / Surface condition of the intrados:

The surface condition in contact with the ridge will be as smooth, polished and regular as possible.

The preferred areas for dental plaque retention are the cosmetic material/metallic material junction areas, which will therefore be located away from the crest and in an area accessible to maintenance instruments.

FIGURE 12: Ceramic-metal pontic with irregular ceramic/metal junction zone and facing the gingival crest: to be avoided.

d / Axial morphology faces V and L have a convex profile.

The palatine faces delimit wide embrasures easily accessible by passing a boss between the different elements of the bridge.

3. Morphology of occlusal surfaces and occlusal equilibration:

The shape of the occlusal surface is essentially linked to:

To the functional movements of the mandible.
The aim of the morphology of the antagonist teeth is to direct and distribute the forces on the deep periodontal tissues at the level of the abutment teeth.
Carrying out this step must take into consideration 3 factors:

The cusp angulation must be less marked to avoid loosening of the bridge during reduction movements.
The width of the occlusal tables at the pontics should be slightly reduced to reduce the working forces.
The exhaust grooves will be created.

Figure 13: Reduction of the occlusal faces of the pontics

4 Choice of support teeth:

In the case of a plural joint construction, the occlusal forces resulting from the function are fully transmitted to the abutment teeth, which must satisfy intrinsic resistance conditions to sustainably oppose the forces thus applied.

Various authors have stated laws aimed at determining the choice of support teeth:

BELIARD’s Law

Increasing the number of unaligned abutment teeth improves balance conditions by limiting the number of rotation axes.

Figure 14: Beliard’s law increasing the number of pillars improves the equilibrium conditions.

SADRIN’s Law

A pronounced curvature determines a tilting movement which must be balanced by the use of additional supports.

Duchange’s Law

It takes into consideration the crown morphology, the surface of the occlusal table and the position of the tooth on the arch. A replacement tooth provides the same work in a fixed prosthesis as a natural tooth. An abutment tooth has a resistance force at least equal to or double the masticatory forces usually applied.

DUCHANGE attributes to each tooth an intrinsic value coefficient; under these conditions the sum of the coefficients of the pillar teeth (resistance force) must be greater than or equal to the sum of the coefficients of the absent teeth (working force).

	1	2	3	4	5	6	7	8
CM	2	1	4/5	3	3	5/6	5/6	2/5
Mandibular dentition	1	2	3	4	5	6	7	8
CM	1	1	4	3	3	5/6	5/6	4/6

ROY’s Law

He divided the dental arch into 5 planes:

An incisal plane that is subjected to post-anterior forces.
A plan for each canine.

This plane is subject to lateral forces.

A plan for premolar.

Which is subject to horizontal forces.

ROY’s theory is interesting for retention bridges; the abutment teeth must be chosen in several planes to ensure the immobilization of the bridge.

– If the 2 teeth to be replaced are located in two different ROY planes, it is necessary to take 4 pillars at a rate of 2 for each side of the gap.

Conclusion

The construction of bridges depends on a large number of parameters which lead to a real set of specifications. It is the result of comparing clinical observation data , various elements leading to the prognosis, and compliance with the general rules for the design of fixed prostheses .