CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES.

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES. 

Summary :

1. Introduction.
2. Classification of bridges.
3. Biomechanical principles
4. Conclusion.
5. Introduction 

The construction of dental bridges requires general knowledge in order to better establish, with a diagnosis, a satisfactory treatment. Indeed, a dental surgeon must design oral-dental restorations like an architect.

These fixed prosthetic restorations – bridges – obey well-defined biomechanical principles:

1. Mechanical principles.
2. Principles of balance.
3. Biological and prophylactic principles.
4. 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 number of abutment teeth chosen on the dental arch, hence the following classification: 

1. Short span bridge.
2. Medium span bridge. 
3. Long span bridge. 

A / Short range bridge

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

1 / Double recessed BCP (2 pillar teeth)

The bridge has 2 support pillars with anchoring on pulped or depulped tooth; the choice of anchoring and span is conditioned by the bridge situation, we can have:

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

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

           Figure 1: Straight bridge 

2. BCP with double curvilinear embedding (anterior sector).

2 / Single-recessed BCP (1 pillar tooth)

There are two types:

1. Single recessed BCP with a support inlay 

 An inlay is less visible than a cap and will be made on the healthy tooth while the other tooth is used as an abutment. Ex: replacement of the 1st ^PM (size of the 2nd ^PM   plus an onlay on the Canine). 

Figure 2: Short-span bridge

2. Single-mount BCP (free extension 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 with distal extension.

• Favor the absence of occlusal contacts on the 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.

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

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 contiguous or not. 

1 / BMP with double rectilinear embedding: eg absence of the 2nd ^PM and the ^1st M; use in this case as a bridge pillar the 1st ^PM and the ^2nd M.

2 / 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 like 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 which is harmful to the survival of the pillars and their periodontium, 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 pillars. While in the lower jaw 2 canines are sufficient

Figure 4: Rotation of the anterior bridge. 

3 / Double-recessed BMP with intercalated pillar: eg absence of 1st ^PM and 1st ^M in this case the execution is more complex and more expensive and more difficult. The intercalated pillar 2nd ^PM receives impulses from the two pillars of the bridge which frame it, the parallelism between the pillars 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.

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

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.

1. Partial polygonal bridges 

1. Restore the anterior part of the arch from PM to PM: called curvilinear bridge, is in the form of an arch.
2. 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).
3. Total polygonal bridges

        They restored the entire arcade.   

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

Figure 6: Full metal ceramic bridge

2. According to the type of junction: bridges can also be classified according to the type of junction.

1. Non-removable bridge (conventional ): 

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.

They have the advantages of providing a feeling of comfort, security and stability, but they require tissue mutilation and are not completely prophylactic.

2. Removable-irremovable bridge  :

They have the same fixity and rigidity as the fixed bridges but they are articulated and partly removable by the practitioner. Because the various elements are secured by nuts or screws. They have a double advantage: its fixity and the need to remove it periodically to carry out checks on the parts of the mucosa underlying the bridge.

3-Removable Bridge :

They use as retention methods, the friction force between the anchors sealed on the abutment teeth and called infrastructure, and a supra structure comprising a span which adjusts with more or less tight friction on the infrastructure.

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

        Figure 7: Removable bridge with attachment

3-Biomechanical principles

1. Principles of natural tooth balance: 

    During chewing, the teeth are subjected to forces which tend to move them from their alveoli, these forces are called functional forces . Others

 Forces tend to keep the teeth in place, they are called resisting forces .

The functional forces come from the masticatory muscles. They are of very variable intensity, vary with age, vary according to the nature of the food crushed and its preparation, these forces are variable in the same subject and even more so from one individual to another. The resistance forces are bioreactive and they are the result of the association of two different factors:

– the reaction forces which correspond to the passive eruption of the teeth.

– passive forces represented by the radiculo-alveolar resistance, for each tooth: the root lever arm has a length much greater than that of the crown. Added to this length of the root is the effect of morphological asymmetry of the roots which oppose the rotation of the teeth.

For the anterior group >: the canines and incisors > are subjected to posteroanterior forces directed outwards and upwards. If these teeth were subjected to this type of force without protection, they would be vestibular. The premolar-molar meshing will ensure the protection of the anterior teeth. In edentulous molars,

we see a vestibulization of the incisor-canine block with reduction of the DV

The lower anterior group: the forces will be exerted in the opposite direction of the teeth but will act in the axis of the teeth, generally these are the teeth that the patient loses last outside of periodontal disease.

The premolar-molar group: The forces transmitted to these teeth are distributed along their major axis during opening and closing movements in centric occlusion. And even in the diduction movements, the tangential lateral forces are compensated by the cusp balance and the root morphology.

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 

1. Insertion. parallelism.
2. Retention. 
3. Mechanical resistance to stresses developed during function. 

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

3. 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: 

1. The mechanical qualities of the alloy: two cases;

1. For small constructions, precious or non-precious alloys based on NI-Cr will be chosen, which give satisfactory results. 
2. For very large constructions with long spans, non-precious alloys based on Ni-Cr will be chosen. 

which guarantee increased rigidity. 

2. The metal structure 

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

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

1. Increase the thickness. 
2. Establish a sufficiently wide contact while clearing the embrasure to make it accessible to interdental brushes. 
3. Ensure that a sufficient section of the span is achieved 
4. 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.

/ Biological principles 

They apply to both anchors, intermediate elements and occlusal surfaces 

1. Morphology of anchors: 

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

1. The cervical limit of the anchors, through its correct location, 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.
2. Safety for deep periodontium. 

2. Morphology of intermediate elements: 

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

1. Supra mucosal intermediate: designed by different authors so as to be located at a distance from the crest, perfectly tolerated but unsightly.

1. Arch shape according to PERL. 
2. The pentagonal section according to POGGIOLI.
3. The ovoid section according to STEIN. 

Figure 10 : Vestibular view of a supramucosal intermediate

1. Intermediate contra mucosal:
2. 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  : Selleovoid

2. 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  : involve corrective surgery of both gingival and bone tissues, their aim is to create sufficient space in the vertical direction and a harmonious outline of the crest. 

Gingivectomy will remove the hyperplastic tissue while additive crestal surgery

will give a favorable convex profile to the bridge intermediate. 

c / Surface condition of the intrados:

The surface condition in contact with the crest will be as smooth, polished and regular as possible. The preferred areas for dental plaque retention are the areas where the cosmetic material/metallic material meets, which will therefore be located away from the crest and in an area accessible to maintenance instruments.

   Figure 12  : Ceramic-metal pontic with irregular and cracked ceramic/metal junction zone opposite the gingival crest: to be avoided because it is a source of inflammation.

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

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

3. Morphology of occlusal surfaces and occlusal equilibration:

The shape of the occlusal surface is essentially linked: 

1. To the functional movements of the mandible. 
2. The morphology of the antagonist teeth aims to direct and distribute the efforts on the deep periodontal tissues at the level of the pillar teeth. 

Carrying out this step must take into consideration 3 factors:

1. The cuspal angulation must be less marked to avoid bridge loosening during diduction movements.
2. The width of the occlusal tables at the pontics should be slightly reduced to reduce the working forces. 
3. 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 equilibrium conditions by limiting the number of rotation axes. 

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

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

SADRIN’S LAW 

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

DUCHANGE’s law 

It takes into consideration the coronal morphology, the surface of the occlusal table and the position of the tooth on the arch. A replacement tooth provides the same work in 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).

Upper teeth	IC	HE	CANINE	1st PM​	2nd PM​	1st M​	2nd M​	DDS
Masticatory coefficient	2	1	4 or 5	3	3	6	6	2 to 5

Lower teeth	IC	HE	CANINE	1st PM​	2nd PM​	1st M​	2nd M​	DDS
Masticatory coefficient	1	1	4	3	3	6	6	4 to 6

ROY’s law

He divided the dental arch into 5 planes: 

1. An incisive plane that is subjected to post-anterior forces. 
2. A plane for each canine. This plane is subjected to lateral forces.
3. A plan for premolar-molar. Which is subjected to horizontal forces.

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

4-Conclusion

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

Multiple prosthetic restorations must be designed so that they do not cause occlusal imbalances or significant overloads on the abutment teeth.

Bibliography:

1-herbert shillingburg: the fundamental bases in fixed prosthesis.

2-shillingburg-jacobi-brackett: fixed prosthesis preparations principles and clinical applications.

3-L.Roucoules: general principles of dental prosthesis with irremovable elements.

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES

  Wisdom teeth can be painful if they are misplaced.
Composite fillings are aesthetic and durable.
Bleeding gums can be a sign of gingivitis.
Orthodontic treatments correct misaligned teeth.
Dental implants provide a permanent solution for missing teeth.
Scaling removes tartar and prevents gum disease.
Good dental hygiene starts with brushing twice a day.
 

CLASSIFICATION OF BRIDGES AND BIOMECHANICAL PRINCIPLES