BIOMECHANICAL PRINCIPLES OF BRIDGES
1) INTRODUCTION:
Any therapy in joint prosthesis must be guided by the concern to achieve a result with a great potential for success. For this we must approach the realization of the joint prosthesis and in particular the bridges by drawing inspiration from the biomechanical principles which govern them.
Terminology:
Bridge: is a permanent joint prosthesis composed of anchoring means and the intermediate element
Trabecula : suspended element of a joint prosthesis which crosses the edentulous space and replaces the missing teeth. This trabecula is secured by means of anchors which are themselves fixed by sealing or gluing to the abutment teeth.
2) PRINCIPLE OF EQUILIBRIUM
BIOMECHANICAL PRINCIPLES OF BRIDGES
For any force applied to the prosthesis there is a greater or equal resistance of the support structures, to respond to this law of equilibrium it is necessary:
- Know the distribution of forces applied to the teeth
- Taking stock of the forces exerted on reconstruction
- Determine and evaluate the number and nature of the abutment teeth
21) analysis of the forces acting on the teeth:
Each tooth can be subjected to pressures during the different mandibular movements related to the different functions. On a healthy periodontium, the stressed tooth will then stabilize in a balanced position by transmitting the forces to the supporting tissues and adjacent teeth. If this tooth is integrated as a support point for a bridge, it will receive additional constraints. Any forces applied at a point on the dental crown can be broken down into 03 components, namely:
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- An axial component
- A vestibulo-lingual component
- A mesio-distal component
On a physiologically healthy and balanced arch, the movement induced by the mesio-distal component is zero given the wedging provided by the adjacent teeth.
The action of an oblique (transverse) force on a tooth tends to move the tooth in a rotational movement around its hypomochlion located at the apical 1/3 of the alveolar height, this movement is thwarted by the relationship of the periodontal tissues where the traction and compression zones appear, this applied force becomes more harmful when:
- It tends to move closer to the horizontal
- Reduced alveolar height
- Poorly located proximal contact point
- Large occlusal surface
22) Forces applied to the teeth
In position e RC : the resulting force is directed obliquely behind the last teeth of the arch and of progressively increasing intensity from the INC to the molar. This force related to the occlusal surface of each tooth can be broken down into two components:
-a component directed along the axis of the tooth and which is cancelled by the reaction
Periodontal
- a second component parallel to the occlusal plane, in a posterolateral direction
Anterior and participates in maintaining the interdental contact points, hence the notion of physiological mesialization of natural teeth
In PIM : the applied force is oblique to the occlusal plane; it is from bottom to top and from back to front for the mandibular teeth and from top to bottom and from back to front for the maxillary teeth. This force related to the occlusal surface of each tooth is broken down into two components:
- a component directed along the axis of the teeth and it is cancelled by the periodontal reaction
- the other force is parallel to the occlusal plane and in a postero-anterior direction and helps maintain the interdental contact points.
In PROTRUSION : this end-to-end incisor position and a posterior disocclusion, the direction of the forces exerted by the mandibular incisor on the palatal face of the maxillary incisor is close to the axis of the latter, thus reducing the amplitude of the lateral component.
In DIDUCTION :
- on the working side: the dental axes come closer together which has the effect of reducing the horizontal lateral component
- on the non-working side, the axes diverge and in the event of interfering contact, the intensity of the horizontal components increases, which is particularly harmful to the periodontium.
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In conclusion : it would be necessary
- favor axial forces
- decrease horizontal components
- accurately establish interdental contacts
- eliminate non-working contacts (interference)
23) Balance of forces exerted on a bridge:
Several factors determine the intensity of the forces exerted during the function; these are objectified during the clinical examination:
- the patient’s muscular strengths
- periodontal proprioception which regulates occlusal forces
- the precision of occlusal contacts: the occlusal contacts in existing PIM must be established precisely so that the force developed during the function is oriented along the major axis of the bridge-supporting teeth
- eating habits: the harder the food, the greater the crushing force
- dynamic occlusion reports: all muscular activity is regulated by the inputs established between the dental arches, the larger the anterior guide, the weaker the forces developed on the posterior teeth
231) The shape and length of the spans: A prosthesis must last, it must resist all the energies of mastication. It should be remembered that the mechanical resistance of a bridge span is:
• Proportional to its width
• Proportional to the square of its thickness
• Inversely proportional to its length
R= lx e² this implies that the longer a span is, the more rigid it must be
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BIOMECHANICAL PRINCIPLES OF BRIDGES
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In addition to the tensile and compressive forces that are exerted significantly on a single prosthetic construction, other forces are added to the framework of a bridge whose span behaves like a fixed beam:
BIOMECHANICAL PRINCIPLES OF BRIDGES
Bending force : deformation resulting from the application of forces acting in its plane of symmetry or arranged symmetrically two by two with respect to this plane.
Shear force : it is the play of two forces directed towards each other without being on the same straight line.
Torsional force : play of two forces directed towards each other by torsion
All these new constraints seem to be preponderant and depend on the shape, span and thickness of the span and none of them is exerted independently of the others. A force applied at the center of the span exerts forces of opposite directions at the level
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anchors (reaction) under the action of this force, the span bends and its bending is proportional to the cube of its length and inversely proportional to the cube of its thickness this implies that the torsion is proportional to the curvature of the span
BIOMECHANICAL PRINCIPLES OF BRIDGES
232) Choice of support teeth:
In multiple reconstructions, the occlusal forces resulting from the function are fully transmitted to the support teeth which must meet intrinsic resistance conditions. For this, there are laws which dictate this choice, namely:
- BELIARD’s law : increasing the number of misaligned abutment teeth improves balance conditions
- A single-pillar (cantilever) bridge can be moved in all directions
- a two-pillar bridge can be moved around its own axis
- A bridge with three unaligned pillars has no axis of rotation
- SADRIN’s law : for this law, the value of a pillar is a function of its place on the arch:
- “U” shaped arch: the force is exerted slightly to the outside of the canine (main axis)
- “V” arch: the force is exerted on the same main axis (most favorable case)
- “Square” arcade the force is exerted completely outside the main axis “(unfavorable case)
A pronounced curvature determines an overturning moment which must be balanced by the use of additional supports.
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BIOMECHANICAL PRINCIPLES OF BRIDGES
- DUCHANGE’s law : it takes into consideration
- coronary morphology
- the surface of the occlusal table
- the position of the tooth on the arch
This author attributes to each tooth an intrinsic value coefficient
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The sum of the coefficients of the abutment teeth must be greater than or equal to that of the teeth to be replaced; an exception can be made if the opposing arch carries a removable prosthesis 50 to 150 times less load than a natural tooth.
- ANTE’s law : it assigns to each tooth a coefficient in mm² of root surface, the sum of the root surfaces of the supporting teeth must be greater than or equal to that of the teeth to be replaced
Clinical C/R ratio : a crown/root ratio equal to 2/3. Clinically a ratio of 1/1 may sometimes be acceptable (in the absence of occlusal parafunction).
- ideally between 2/3 and 1
BIOMECHANICAL PRINCIPLES OF BRIDGES
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- The shape of the roots :
Roots with a diameter VL > MD are preferable to those with a circular section.
- For a single root: oval section is favorable to a circular root section
- For a multi-rooted plant, diverging roots provide a better foundation than converging ones.
BIOMECHANICAL PRINCIPLES OF BRIDGES
3) THE MECHANICAL PRINCIPLES OF BRIDGES:
The prosthetic construction must meet 03 requirements
31) insertion :
In the case of a single reconstruction, the insertion generally causes few problems as it is carried out according to a vertical translational movement along an axis called the insertion axis which coincides with the major axis of the tooth.
In the case of plural reconstruction, the problem is completely different, the insertion of anchors on the preparations can be considered:
- itself by vertical translation movement
- itself by complex movement combining rotation and translation in the three planes of space
Which leads to a judicious choice of the type of anchoring (total or partial)
32) retention : it is conditioned essentially by
- the height of the preparation which must be greater than its width
- the extent of the contact surfaces to increase the frictional forces and the sealing surface
- the weak convergence of the axial walls = 30°
- the mechanical resistance of the bridge : it is ensured by the rigidity of the prosthetic construction conditioned by:
- the quality of the alloy used
- the morphology of the metal structure of the bridge which must have sufficient thickness always modulated according to the alloy used, in this sense three essential precautions must be respected:
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- increase the thickness of the anchor opposite the span
- establish a wide span-anchor contact while clearing the embrasures for hygienic purposes
4) BIOLOGICAL AND PROPHYLACTIC PRINCIPLE
These principles simultaneously concern morphology
- anchors
- bridge span
- occlusal surfaces
- precision and permanence of occlusal contacts
41) morphology of anchors : the anchor must respect
– dental tissues
– guarantee safety with regard to the marginal periodontium ensured by:
- an optimal cervical limit situation
- a sufficiently watertight dento-prosthetic joint not exceeding 70u
- deflecting axial morphology
- morphology of the bridge span:
The creation of an intermediary in harmony with the edentulous crest: for this it would be necessary:
- Sufficient space in the vertical direction,
- Harmonious parabolic contour of the crest
421) relation to the edentulous crest : so that the fibromucosa of the edentulous crest is preserved from any irritation, 2 possibilities are offered to us:
- supra mucosal trabeculae
- contra mucosal span
422) choice of material in contact with the crestal mucosa : this contact must be made either with the metal or with the cosmetic material but never in contact with the junction zone of the two materials under risk of bacterial plaque retention
423) axial morphology:
The vestibular and lingual faces will have a convex profile without creating areas of significant withdrawal promoting the formation of bacterial plaque and non-stimulation of the gingival marginal contour.
The proximal faces describe wide, easily accessible and hygienic embrasures; the trabecula-anchor connections must be located within and without exceeding the occlusal third.
43) morphology of occlusal surfaces :
Two situations may arise:
- even though the bridge completely restores both dental arches in this case the occlusal morphology can be completely recreated according to the occlusal scheme relative to the clinical situation
- even if the bridge partially restores the arch in this case the occlusal morphology will be conditioned by
- functional movements of the mandible guided by the occlusal schema
- the morphology of antagonist teeth
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In this perspective it is appropriate to successively analyze
- cuspal angulation of prosthetic restorations
- the width of the occlusal tables
- the exhaust grooves
- Cuspid angulation: it is dictated mainly by the anterior guide which conditions
Actually the height and orientation of the cusps which implies that:
a1) a strong immediate and rapid disocclusion of the sector cusped by the canine allows the construction of a marked cuspid relief without risk of conflict between the antagonist teeth
a2) conversely, a weak anterior guide induces contacts in working laterality on the cuspid teeth which will then present a flattened occlusal surface
- the width of the occlusal tables:
It is recommended according to ACKERMANN’s 3H law (hetromorphy) to reduce the width of the occlusal tables to limit the importance of the forces applied and control their direction without however making too great a narrowing favorable to the reduction of the overjet source of jugal bite or the creation of bacterial plaque retention.
- exhaust grooves:
They must be reproduced in order to delimit the cuspid volumes and facilitate food crushing.
44) Precision and permanence of occlusal relationships :
This is only possible if a precise PIM is stable, an occlusal morphology with adequate interporoximal contact points is achieved
5) PSYCHOLOGICAL AND SOCIOLOGICAL PRINCIPLE:
Like any prosthetic therapy, the joint prosthesis is never completely accepted by our patients who are less concerned with mechanical resistance than with aesthetic results. The practitioner must ensure a progressive adaptation of the prosthesis by explaining the priorities to the patient. For this, an aesthetic success and the feeling of comfort must be our objective in the face of the patient’s complaints.
6) CLASSIFICATION OF BRIDGES
Bridges come in various shapes, from the simplest to the most complex, and a classification has been established taking into account
- abutment teeth
- of the type of span
- materials used
- of the type of span-anchoring means junction (fixity)
61) classification according to abutment teeth
611) depending on the nature of the abutment teeth
6111) bridge on pulped teeth
6112) bridge on pulpless teeth
62) according to the number of pillar teeth:
621) single-pillar bridge
622) two-pillar bridge
623) bridge with three or more pillars
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63) classification according to the material used
631) metal bridge
632) metal-resin bridge
633) ceramic-metal bridge
64) classification according to the type of span
641) depending on the shape of the span
6411) supragingival span bridge
6412) contra-gingival span bridge
6413) intragingival span bridge (to be avoided)
BIOMECHANICAL PRINCIPLES OF BRIDGES
642) depending on the span length
6421) short span bridge: 1 to 2 single or double recessed teeth
6422) medium span bridge: 2 to 3 teeth with double embedding or with intercalated intercalated abutments
6423) long span or polygonal bridge: more than 3 teeth with double embedding or intercalated pillars occupying the different planes
by ROY who divided the arcade into 5 plans:
• 1 incisal plane;
• 2 canine plans;
• 2 premolar-molar planes.
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BIOMECHANICAL PRINCIPLES OF BRIDGES
Polygonal bridges fit into several Roy planes (3 or more).
• When they are included in 3 or 4 plans, they are called partial;
• When they fit into the 5 planes, they are called total polygonal bridges.
These bridges can be made on pulped or depulped teeth.
They are very stable and their stability increases with the number of abutment teeth.
These bridges allow a harmonious and balanced distribution of chewing forces which are transmitted to the level of the pillar teeth and the periodontium.
65) classification according to the type of span- anchoring means junction (connection type)
651) fixed or fixed-fixed bridge
652) removable-irremovable or mobile-fixed bridge
653) removable or mobile-mobile bridge
In summary:
| Bridge range | Embedding | Span shape | Type of fixing | Nature of abutment teeth | |
Short range | Double | Straight | ImmovableAmovo-imovibleremovable | Pulped or depulped | |
Curvilinear | |||||
Simple | Straight | with or without Inlay | |||
Curvilinear | |||||
Average range | 2 or more teeth | Straight | |||
Curvilinear | |||||
Large spanPolygonal bridges | Multiple abutment teeth | Partial | |||
Total | |||||
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7) CONCLUSION : the success of any prosthesis and particularly the joint prosthesis depends on a perfect knowledge of the biomechanics which govern it, the competence of the practitioner and the smooth integration of the definitive prosthesis into the patient’s stomatognathic apparatus via the temporary prosthesis.
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BIOMECHANICAL PRINCIPLES OF BRIDGES
Deep cavities may require root canal treatment.
Interdental brushes effectively clean between teeth.
Misaligned teeth can cause chewing problems.
Untreated dental infections can spread to other parts of the body.
Whitening trays are used for gradual results.
Cracked teeth can be repaired with composite resins.
Proper hydration helps maintain a healthy mouth.