Thursday, August 8, 2013

BIOMECHANICS OF DENTAL IMPLANTS - SEMINAR



Contents:

þ Introduction.
þ Loads applied to dental implants.
þ Mass, force and weight.
þ Types of forces.
þ Stress, strain relationship.
þ Force delivery and failure mechanisms.
þ Fatigue failure.
þ Scientific rationale for dental implant design.
þ Single tooth implant and biomechanics.
þ Cantilever prosthesis and biomechanics.
þ Biomechanics of frame works and misfit.
þ Treatment planning based on biomechanical risk factors.
þ Conclusion.
þ References.






INTRODUCTION:
          Biomechanics comprises of all kinds of interactions between tissues and organs of the body and forces acting on them. It’s the response of the biologic tissues to the applied loads.
           Dental implants function to transfer load to surrounding biological tissues. Thus the primary functional design objective is to manage (dissipate and distribute) biomechanical loads to optimize the implant supported prosthesis function.
Definition
        Process of analysis and determination of loading and deformation of bone in a biological system.
Natural tooth Vs Implant:

Natural tooth
Implant
1. Natural tooth is anchored in to the bone by flexible periodontal ligament.                                               
2. The periodontal ligament around the natural tooth significantly reduces the amount of stress transmitted to the bone and facilitates even force distribution.
3. The pdl acts as viscoelastic shock absorber serving to decrease the magnitude of stress to the bone.
4. The precursor signs of a premature contact or occlusal trauma on natural teeth are usually reversible and include signs of cold sensitivity, wear facets, pits, drift away and tooth mobility.
5. This condition often helps in the patient seeking professional treatment by occlusal adjustment and a reduction in force magnitude in force magnitude which further reduces the stress magnitude.
6. The elastic modulus of a tooth is closer to the bone than any of the currently available dental implant biomaterial. The greater the flexibility difference between the two materials, the greater the potential relative motion generated between the two surfaces at the endosteal region.
7. The proprioceptive information relayed by teeth and implants also differs in quality. Natural teeth deliver a rapid, sharp, high pressure that triggers proprioceptive mechanism.
8. The surrounding bone of natural teeth is developed slowly and gradually in response to biomechanical loads.
9. A lateral force on natural tooth is dissipated rapidly away from the crest of bone toward the apex of the tooth.
1. Implant is rigidly fixed by functional ankylosis.

2. The concentration of stresses mainly occurs at the crestal region.


    3. The implant is fixed and rigid.


4. These initial reversible signs and symptoms of trauma donot occur with implants.


5. The magnitude of stress may cause bone microfracture, bone loss  which ultimately leads to mechanical failure of implant components.


6. The implant materials differs by 5-10 times from the surrounding  bone structure.



7. Implants deliver a slow dull pain that triggers a delayed reaction if any.

 8. Where as the bone loading around an implant is performed by the dentist in a much more rapid and intense fashion.

9. Lateral forces in implants concentrates at the crestal region.




CHARACTER OF FORCES APPLIED TO DENTAL IMPLANTS:
          Excess loads on an osseointegrated implant may result in mobility of supporting device and excessive loads also may fracture an implant component or body. The internal stresses that develop in an implant system and surrounding biological tissues under imposed load may have a significant influence on the long term longevity of the implants in vivo. A goal of treatment planning should be to minimize and evenly distribute mechanical stress in implant system and contiguous bone.
LOADS APPLIED TO DENTAL IMPLANTS:
o   In function – occlusal loads
o   Absence of function – Perioral forces
§  Horizontal loads  
o   Mechanics help to understand such physiologic and non physiologic loads and can determine which t/t renders more risk.
MASS, FORCE AND WEIGHT:
   Mass – A property of matter, is the degree of gravitational attraction the body of matter experiences.
 Unit – kgs : (lbm)
FORCE (SIR ISAAC NEWTON 1687):
Ø Newton’s II law of motion
                       F = ma
             Where a = 9.8 m/s2
Ø Mass – Determines magnitude of static load
Ø Force – Kilograms of force

 WEIGHT:
     Is simply a term for the gravitational force acting on an object at a specified location.
FORCES AND FORCE COMPONENTS:
Ø Magnitude, duration, direction, type and magnification
Ø ‘Vector quantities’
Ø Direction – dramatic influence
MOMENT / TORQUE:
The force which tends to rotate a body.         Units – N.m; N.cm, lb.ft ; oz.in
     In addition to axial force, there is a moment on the implant which is equal to magnitude of force times (multiplied by) the perpendicular distance (d) between the line of action of the F and center of the implant.
                                


FORCES ACTING ON THE IMPLANTS:
Three types of forces acting on the dental implants
þ Compressive
þ Tensile
þ shear
Compressive:  
i)       Tend to push masses towards each other.
ii)    Maintains integrity of bone – implant interface.
iii)   Accommodated best. 
iv)   Cortical bone is strongest in compression.
v)     Cements, retention screws, implant components and bone – implant interfaces can accommodate greater compressive forces than tensile or shear forces.
vi)   Hence compressive forces should be Dominant in implant prosthetic occlusion.


                       
TENSILE FORCES                  SHEAR FORCES
          ¯                                          ¯
Pull objects apart                                  Sliding
Ø Distract / disrupt bone implant interface. 
Ø Shear forces are most destructive, cortical bone is weakest to accommodate shear forces.
Ø Cylinder implants –in particular are     highest risk for shear forces at the implant tissue interface unless an occlusal load directed along the long axis of the implant body.
Ø They require a coating to manage the shear forces to manage the shear forces through a more uniform bone attachment.                  
Ø Threaded / finned implants impart a combination of all three types of forces at the interface under the action of single occlusal load. This conversion of a single force in to three types of forces is controlled by the implant geometry.
STRESS:
       The manner in which a force is distributed over a surface is referred as mechanical stress.
    g = F/A
The magnitude of stress depends on two variables:
-         force magnitude.
-         cross sectional area over which the force is dissipated.
Force magnitude may be decreased by reducing magnifiers of force that are:
1.     Cantilever length
2.     Crown height
3.     Night guards
4.     Occlusal material
5.     Over dentures
Functional cross sectional area may be optimized by:
 1.  increased by Number of implants
  2. Selecting an Implant geometry that has been designed carefully to maximize the functional cross sectional area.
DEFORMATION & STRAIN:
Ø A load applied to a dental implant may induce deformation of the implant and surrounding tissues    
Ø Deformation and stiffness of implant material may influence

A.             Implant tissue Interface

B.             Ease of implant manufacture

C.            Clinical longevity




STRESS – STRAIN RELATIONSHIP:
                                       
v A relationship is needed between the applied stress that is imposed on the implant and surrounding tissues and the subsequent deformation.
v The load values by the surface area over which they act and the strain experienced by the object produces a stress strain curve.
v The slope of the linear portion of the curve is referred to as the modulus of elasticity and its value indicates the stiffness of the material.
v The closer the modulus of elasticity of the implant to the biological tissues, the less the relative motion at the implant tissue interface.
              Once a particular implant system is selected the only way for an operator to control the strain experienced by the tissues is to control the applied stress or change the density of bone around the implant.
Ø Greater the strength stiffer the bone
Ø Difference in stiffness is less for CpTi & D1 bone but more for D4 bone
Ø Stress reduction in such softer bone
§  To reduce resultant tissue strain
§  Lower Ultimate strength
Ø Hook’s law
    Stress = Modulus of elasticity x strain
       g = E.e 
BITING FORCES:
Ø Axial component of biting force: (100 – 2500 N) / (27 – 550 lbs)
Ø It tends to increase as one moves distally
Ø Lateral component - 20 N (approx.)
Ø Net chewing time per meal = 450 sec
·        Chewing forces will act on teeth for = 9 min/day
·        If includes swallowing = 17.5 min/day
·        Further be increased by parafunction
FORCE DELIVERY AND FAILURE MECHANISM:
v The manner in which forces are applied to the dental implant restorations within the oral environment dictates the likelihood of system failure.
v An understanding of force delivery and failure mechanisms is critically important to the implant practitioner to avoid costly and painful complications.
v The moment or torque is the product  of the force magnitude multiplied by the perpendicular distance from the point  of interest to the line of the action of the force.
Ø Moment loads are destructive in nature and may result in:
           Interface breakdown
           Bone resorption
           Screw loosening
           Bar / bridge fracture
A total of six moments may develop about the three clinical coordinate axes:
                                 - occlusoapical
                                 - faciolingual
                                 - mesiodistal
 These moment loads induce microrotations and stress concentrations at the crest  of the alveolar ridge at the implant to tissue interface , which lead inevitably to crestal bone loss. Three clinical moment arms in implant dentistry
             - occlusal height
             - cantilever length
             - occlusal width
                                      
                                          
        Minimization of each of these moment arms is necessary to prevent unretained restorations, fracture of components, crestal bone loss or complete implant system failure.
1) Occlusal height:
- Occlusal height serves as the moment arm for force components directed along the faciolingual axis:
- working or balancing occlusal contacts, tongue thrusts or peri oral musculature, and the force components directed along the mesiodistal axis.
- force components along the vertical axis is not affected by the occlusal height because there is no effective moment arm.
- in division A bone initial moment load at the crest is less than in division C or D bone because the crown height is greater in Cand D.
2) Cantilever length:
§  Large moments may develop from vertical axis force components in prosthetic environments designed with cantilever extensions or offset loads from rigidly fixed implants.
§  A Lingual force component may also induce a twisting moment about the implant neck axis if applied through a cantilever length.
§  Force applied directly over the implant does not induce a moment load or torque because no rotational forces are applied through an offset distance.
§  Antero posterior spread is the distance to the center of the most anterior implant and the most distal aspect of the posterior implants.
§  The greater the A-P spread the smaller the resultant loads on the implant system from cantilevered forced because of the stabilizing effect of the antero-posterior distance.
According to MISCH  
§  Cantilever length is determined by the amount of stress applied to system
§  Generally –Distal cantilever – not be > 2.5 times of A-P spread
§  Patients with parafunction – not to be restored by cantilever.
§  Square arch form involves smaller A-P spreads between splited implants and should have smaller length cantilever.
§  Tapered arch form – largest A-P spread – larger cantilever design. 
3). Occlusal width:
         Wide occlusal tables increase the moment arm for any offset occlusal loads. Faciolingual tipping (rotation) can be reduced significantly by narrowing the occlusal tables or adjusting the occlusion to provide more centric contacts.
      A vicious destructive cycle can develop with moment loads and result in crestal bone loss.
FATIGUE FAILURE:
      Fatigue failure is characterized by Dynamic cyclic loading conditions, four factors significantly influence the fatigue failure.

1) Biomaterials
2) Geometry
3) Force magnitude
4) Loading cycles
1) Bio materials:
v Fatigue behaviour of biomaterials is characterized to a plot of applied stress vs no. of loading cycles
v High stress – few loading cycles
v Low stress – infinite loading cycles
v Ti alloys exhibits a higher endurance limit compared with commercially pure titanium (Cp Ti)
2) Macro geometry:
§  The geometry of an implant influences the degree to which it can Resists bending and torque
§  Lateral loads also causes fatigue fracture
§  The fatigue failure is related as 4th power of the thickness difference
§   Also affected by the difference in Inner and outer diameter of screw and abutment screw space
3) Force magnitude:
     The magnitude of loads on dental implants reduced by careful consideration of arch position
§   Higher loads on posteriors
§   Limitation of Moment loads
§   Geometry for functional area
§   Increasing the No. of implants
 4) Loading cycles
Ø Reducing the No. of loading cycles
Ø Elimination of parafunction
Ø Reducing the occlusal contacts
SCIENTIFIC RATIONALE FOR DENTAL IMPLANT DESIGN
v Dental implants function to transfer of load to surrounding biologic tissues.
v Thus the primary functional design objective is to manage (dissipate and distribute) biomechanical loads to optimize the implant supported prosthesis function.
v Biomechanical load management depends on two factors that are
            1) Character of applied load.    2) Functional surface area
v Forces applied to dental implant characterized in terms of Magnitude, duration, type, direction and magnification.
FORCE MAGNITUDE:
§  The magnitude of biting force varies as a function of anatomic region and state of dentition. The magnitude of force is greater in molar region and lesser in canine region.
§  Higher magnitude demands increased bone density and Influence the selection of biomaterials.
§  Materials such as silicon hydroxyapatite and carbon are characterized by lesser ultimate strengths even though they are highly compatible with the biological tissues.
§  In contemporary applications, these materials are considered for use as coatings applied to stronger substrate materials.
§  Silicone, HA, carbon has- High biocompatibility
                                          - Low ultimate strength
§  Titanium and its alloy – Excellent biocompatibility
                                             - Corrosion resistance
                                             - Good ultimate strength
                                             - Closest approx. to stiffness of bone
FORCE DURATION:
§  The duration of bite forces on dentition has a wide range under ideal conditions; the total time of those brief episodes is less than 30 minutes per day.
§  Patients who exhibit bruxism, clenching or other parafunctional habits may have their teeth in contact several hours each day.
§  The endurance limit or fatigue strength is the level of highest stress through whish a material may be cycled repetitively without failure. The endurance limit of a material is often less than one half its ultimate tensile strength.
§  The ability of implants and abutment screws to resist fracture from bending loads is related directly to the moment of inertia of the component.
§  This parameter is a function of the cross sectional geometry of the component.
§  Implant bodies are particularly susceptible to fatigue fracture at the apical extension of the abutment screw within the implant body or at the crest module around abutment (eg: with an internal hexagon)
§  The formula for the bending fracture resistance in these conditions is related to the outer diameter radius to the fourth power minus the inner diameter radius to the fourth power.
§  The wall thickness of the implant body in this region controls the resistance to fatigue failure. Even a small increase in wall thickness results in a significant increase in bending fracture resistance because the dimension is multiplied to a power of four.
TYPE OF FORCE:      
§  Three types of forces  may be imposed on dental implants within the oral environment
                -Compression
                -Tension
                -Shear
§  Bone is strongest when loaded in compression. 30% weaker when subjected to tensile forces and 65% weaker when loaded in shear
§  A smooth sided implant may be called a cylinder design, and this cylinder implant body result in essentially a shear type of force at the implant to bone interface. Thus this body geometry must use a microscopic retention system by coating the implant with titanium plasma spray or hydroxyl apatite
§  If the hydroxyapatite resorbs from infection or bone remodeling, the remaining smooth sided cylinder is severely compromised for healthy load transfer to the surrounding tissues
§  A threaded implant may use microscopic and macroscopic design features to load the bone in compression and tensile loads
§  Threaded implants have the ability to transform the type of force imposed at the bone interface through careful control of the thread geometry. Thread shape is particularly important in changing force type at the bone interface
§  Thread shapes in dental implant design include square, v shape and buttress
§  Under axial loads to a dental implant a v thread face (typical of paragon, 3i and Nobel Biocana) is comparable to the buttress thread and has a 10 times greater shear component of force than a square or a power thread
§  A reduction in shear load at the thread to bone interface reduces the risk of overload; which is particularly important in compromised D3 and D4 bone. A threaded implant also may have a surface condition such as hydroxyapatite, TPS or other roughed surface.
FORCE DIRECTION:
§  The anatomy of the mandible and maxilla places significant constraints on the ability to surgically place root form implant suitable for loading along their long axis.
§  Bony undercuts further constrain implant placement thus force direction. Most of all undercuts occur on the facial aspects of the bone, with the exception of the submandibular fossa in posteroior mandible. Hence implant bodies often are angled to the lingual to avoid penetrating the facial undercut during insertion.
§  As the angle of the load increases, the stresses around the implant increases, particularly in the vulnerable crestal bone region. As a result all implants are designed for placement perpendicular to the occlusal plane. This placement allows a more axial load to the implant body and reduces the amount of crestal loss.
FORCE MAGNIFICATION:
There are various factors which can magnifies the forces on dental implants
§  Surgical placement resulting in extreme angulation of the implant
§  Para functional habits
§  Cantilever and crown height
§  Increase in functional area
§  Increased density of the bone
§  Increase in implant number decreases cantilever length and limits the force magnifier.
FUNCTIONAL SURFACE AREA:
§  Functional surface area is defined as the area that actively serves to dissipate compressive and tensile non shear bonds through the implant to bone interface and provides initial stability of the implant following surgical placement.
§  The total surface area may include a passive area that does not participate in load transfer.
§  Functional surface area also plays a major role in addressing the variable implant to bone contact zones related to bone density.
§  D1 bone, is the densest bone found in the jaws is also the strongest bone and provides an intimate contact with a threaded root form implant at initial implant loading.
§  D4 bone has the weakest biomechanical strength and the lowest contact area to dissipate the load at the implant to bone interface.
§  Thus an improved functional surface area per unit length of the implant is needed to reduce the mechanical stress to this weak bone.
§  Implant macrogeometry and implant width are two important design variables for optimizing surface area.
IMPLANT MACROGEOMETRY:
v The macro design or shape of an implant has an important bearing on the bone response.
v Growing bone concentrates preferentially on protruding elements of the implant surface, such as ridges, crests, teeth, ribs or the edge of threaded surface.
v The shape of the implant determines the surface area available for stress transfer and governs the initial stability of the implant.
v Smooth sided cylindrical implants provide ease in surgical placement, however the bone to implant interface is subjected to significantly larger shear conditions.
v A smooth sided tapered implant allows for a component load to be delivered to the bone implant interface, depending on the degree of taper, however the greater the taper of smooth sided implant the less the overall surface area of the implant body.
v Threaded implants with circular cross sections provide for ease of surgical placement and allow for greater functional surface area optimization to transmit compressive loads to bone implant interface.
v A smooth surface cylinder depends on a coating or microstructure for load transfer to bone.
IMPLANT WIDTH:
v An increase in implant width adequately increases the area over which occlusal forces may be dissipated.
v Wider root form designs exhibit a greater area of bone contact than narrow implants of similar design because of an increase in circumferential bone contact.
v The larger the width of the implant the more it resembles the emergence profile of the natural tooth.
v The increased width of implants 6-12 mm also enhances the bending fracture resistance. But the crestal bone anatomy most often constrains implant width to less than 5.5mm.
THREAD GEOMETRY
Threads are designed to maximize initial contact enhance surface area and facilitate dissipation of stresses at the bone- implant interface.
Functional surface area per unit length of the implant may be modified by varying three thread geometry parameters
-         thread pitch
-         thread shape
-         thread depth




THREAD PITCH:
                                 
v Thread pitch is defined as the distance measured parallel with its axis between adjacent thread forms or the number of threads per unit length in the same axial plane or on the same side of the axis.
v The smaller the pitch (finer) the more threads on the implant body for a given unit length, and thus the greater surface area per unit length of the implant body.
v If force magnitude increase or bone density decreases one may decrease the thread pitch to increase the functional surface area.
v Some of the current popular designs which have different pitches.
v The distance between pitches:
                  ITI Implant – 1.5mm
                  Sterioss      - 0.8mm
                  Nobel biocare,zimmer, 3i & life core – 0.6mm
                  Biohorizons  - 0.4mm
-the fewer the threads , the easier to bond or insert the implant.
THREAD SHAPE:
v Thread shapes in implant geometry (dental implant designs include square, Vshape and buttress.
v The V shape thread design is called a fixture and is primarily used for fixating metal parts together not load transfer.
v The buttress thread shape was designed initially for and is optimized for pullout loads.
v The square or power threaded provides an optimized surface area for intrusive, compressive load transmission.
v The shear force on a V threaded face (typical of Zimmer, 3i and Nobel biocare) is about 10 time greater than the shear force on a square thread.
THREAD DEPTH:
v The threaded depth refers to the distance between the major and minor diameter of the thread.
v the greater the thread depth, the grater the surface area of the implant if all the other factors are equal.
IMPLANT LENGTH:
v As the length of an implant increases so does the overall total surface area.
v D1 bone is the strongest and densest bone of the oral environment. The strength of the bone and the intimate contact between the bone and implant provide resistance to lateral loading. Bicortical stabilization is not needed in D1 bone because it is already a homogenous cortical bone.
v A long implant in D2 or D3 bone in the anterior mandible may cause increased surgical risk, since attempting to engage the opposing cortical plate and preparing a longer osteotomy may result in overloading of the bone.
v In poor quality D3 and D4 bone functional surface area must be maximized to distribute occlusal loads optimally, the placement of longer implants in posterior regions require surgical modifications like nerve repositioning, placement of sinus grafts in maxillary posterior regions.
v The shorter and smaller diameter implants had lower survival rates than their longer or wider counter parts.
CREST MODULE CONSIDERATIONS:
Ø Crest module of an implant body is the transosteal region from the implant body and characterized as a region of highly concentrated mechanical stress.
Ø Slightly larger than outer diameter, thus the crest module seats fully over the implant body osteotomy, providing a deterrent for the ingress of bacteria or fibrous tissue.
Ø The seal created by the larger crest module also provides for greater initial stability of the implant following placement.
Ø Polished collar (0.5 mm) – perigingival area, provides for a desirable smooth surface close to the perigingival area.
Ø Longer polished collar – shear loading – crestal bone loss
Ø Bone is often lost to first thread, because the first thread changes the shear force of the crest module to a component of compressive force in which bone is strongest.
APICAL DESIGN CONSIDERATIONS:
               Round cross sectional implants do not resist torsional shear forces when abutment screws are tightened hence anti rotational feature is incorporated usually in the apical region of the implant body, with a hole or vent. Bone can grow through the apical hole and resist torsional loads applied to the implant. The apical hole region may increase the surface area available to transmit compressive loads on the bone.  
                     The disadvantage of the apical hole occurs when the implant is placed through the sinus floor or becomes exposed through a cortical plate. The apical hole may fill with mucous and become a source of retrograde contamination. Another anti rotational feature of implant body may be flat sides or grooves along the body or apical region of the implant body.
                       The apical end of each implant should be flat rather than pointed, this allows for the entire length of the implant to incorporate design features that maximize desired strain profiles.
Progressive Loading
Misch (1980) proposed that         
    Gradual increase in occlusal load separated by a time interval to allow bone to accommodate.
    Softer the bone à increase in progressive loading period.
Protocol  Includes, 
Ø     Time
Ø     Diet
Ø     Occlusal Contacts and occlusal material
Ø     Prosthesis Design


Time:
    Two surgical appointments between initial implant placement and stage II uncovery may vary on density.
Ø     D1     -        3 Months
Ø     D2     -        4 Months
Ø     D3     -        5 Months
Ø     D4     -        6 Months
Diet:  
Ø     Limited to soft diet – 10 pounds
Ø          Initial delivery of final prosthesis-21 pounds
Occlusal Material:
    Initial step – no occlusal material placed over implant
    Provisional – Acrylic – lower impact force
    Final -  Metal / Porcelain
Occlusion:
Ø     Initial                    -        No occlusal contact
Ø     Provisional           -        Out of occlusion
Ø     Final           -        At occlusion
Prosthesis Design: 
First transititional – No occlusal contact
                                        No cantilever
    Second transititional -      Occlusal contact
                                            With no cantilever
    Final restoration   -   narrow occlusal table and cantilever with implant protective occlusion guidelines.


SINGLE TOOTH IMPLANTS:
v Single tooth implants require good bone support and control of harmful effects of occlusal levers that are not parallel to the long axis of the implant.
v The prosthesis must be designed to allow good oral hygiene, with easy access to inter proximal surfaces and the retaining screw.
v A molar can be replaced with two standard diameter implants or one wide implant.
v This type implant is contraindicated for larger spaces because the masticatory and occlusal forces to the most distal or mesial portions will be harmful.
v To avoid excessive loads, the implant must be centered in the edentulous space during placement.



ANTERIOR SINGLE TOOTH RESTORATIONS: 
v The anterior  single tooth restoration is achieved using a standard diameter implant, which is preferred over a narrow implant because it provides a larger surface for osseo integration
v Generally the use of wide implants in this area is not advocated because it may compromise good esthetic results.
v To avoid levers that may be produced during parafunction in centric and eccentric positions, its recommended that the implant supported restoration be left out of occlusion.
SHORT SPAN FIXED PARTIAL DENTURE:
                           The construction of a 3 unit particularly cantilever fixed partial dentures require a posterior triangular zone of occlusal surface between the supporting implants.
                           The chances of overloading the implants are far less  and this provides a better long term  prognosis, because it offers a wider  active zone while also achieving good occlusal load in relationship to the axes of the implants. the use of wide implants to support cantilever fixed partial dentures improves the prognosis further, especially in those cases where only two wide implants  are needed compared to three of standard diameter. wide implants allow for an increased occlusal surfaces in  these circumstances.
                           The proximity of anatomical features such as the mandibular canal or the maxillary sinus limit the use of long implants. In the presence of adequate bucco lingual bone width these limitations ca be managed with the use of wide implants.
CANTILEVER FIXED PARTIAL DENTURE:
Ø It results in greater torque with distal abutment as fulcrum.
Ø     May be compared with Class I lever arm.
Ø     May extend anterior than posterior to reduce the amount of force
It depends on stress factors  
Ø     Parafunction
Ø     Crown height
Ø     Impact width
Ø     Implant Number
The design of cantilever fixed partial dentures is dependent on the occlusal forces that can be elicited at the free end of the denture and the length and width of the implants selected.
CASE 1:
v A case with two implants placed for the lateral incisor and the canine with a free end central incisor.
v Two implants of adequate length are required.
v The cantilever tooth should avoid contacts on the central incisors during protrusion, lateral excursions and maximum intercuspation.
         



CASE II:
v When the implants serve as support for the central and lateral incisors with a free end canine, the occlusal configuration should provide group function during lateral movements and avoids loading of canine.
v If it’s not possible lateral guidance may be provided by the central and lateral incisors avoiding any contact with the canine.
                       
   CASE III:
v When two implants are placed unilaterally at the site of two maxillary premolars, the free end canine must be left out of occlusion.
CASE IV:
v Molar replacements achieve best results with a three Implant supported fixed prosthesis providing premolar morphology to the restorations.
v The length of the implants influences the outcome of treatment
v Due to the enormous occlusal loads in the second molar area the use of a free end fixed prosthesis is contra indicated.
                  

BIOMECHANICS OF FRAMEWORKS AND MISFIT
Frameworks:
Ø Metal framework for full arch prosthesis can fracture
Ø More towards the cantilever section
Reasons:
1) Overload of cantilever
    Unlikely to occur – typical prosthetic alloy.
2) Metallurgic fatigue under cyclic loads
    Prevention – substantial cross sectional area 
                      – 3-6 mm

TREATMENT PLANNING BASED ON BIOMECHANICAL RISK FACTORS
Ø Design of final prosthetic reconstruction
Ø Anatomical limitation
Geometric risk factor
1) No. of implants less than no. of root support
Ø One implant replacing a molar – risk.
§  1 wide – plat form implant / 2 regular implants
Ø Two implants supporting 3 roots or more – risk
§  2 wide – platform implants
2) Wide – platform implants
Ø Risk – if used in very dense bone
3) Implant connected to natural teeth 
4) Implants placed in a tripod configuration
Ø Desired à counteract lateral loads
5) Presence of prosthetic extension
 6) Implants placed offset to the center of the prosthesis à in tripod arrangement, offset is favorable.
  7) Excessive height of the restoration

OCCLUSAL RISK FACTORS:
Ø Force intensity and parafunctional habit
Ø Presence of lateral occlusal contact
§  Centric contact in light occlusion
§  Lateral contact in heavy occlusion
§  Contact at central fossa
§  Low inclination of cusp
§  Reduced size of occlusal table
BONE IMPLANT RISK FACTORS
Ø Dependence on newly formed bone
Ø Absence of good initial stability
Ø Smaller implant diameter
§  Proper healing time before loading
§  4 mm diameter minimum – posteriors
Technological risk factors
Ø Lack of prosthetic fit and cemented prostheses
§  Proven and standardized protocols
§  Premachined components
§  Instrument with stable and predefined tightening torque
WARNING SIGNS:
       Repeated loosening of prosthetic / abutment screw
       Repeated fracture of veneering material
       Fracture of prosthetic / abutment screws
       Bone resorption below the first thread 

CONCLUSION:
                                 Biomechanics is one of the most important consideration affecting the design of the frame work for an implant bone prosthesis. It must be analyzed during diagnosis and treatment planning as it may influence the decision making process which ultimately reflect on the implant supported prosthesis.

REFERENCES

1.      Dental implant prosthetics – Carl E. Misch
2.     Principles and practice of implant dentistry – Charles Weiss, Adam Weiss.
3.     Tissue – integrated prosthesis. Osseointegration in clinical dentistry – Branemark, zarb, Albrektsson
4.     Oral rehabilitation with implant supported prosthesis -Vincente
5.     ITI dental implants- Thomas G.Wilson