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8a. Biomechanics and Treatment
             Planning



       John Beumer III DDS, MS
Division of Advanced Prosthodontics, Biomaterials and
             Hospital Dentistry, UCLA
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Implant Biomechanics and
        Treatment Planning
Why should we be concerned with
implant biomechanics when we develop
a plan of treatment?
    Because if we are not, we risk implant
    overload and prosthesis failures such
    as fracture and screw loosening.


Implant overload can lead to bone loss around
   implants and eventually implant failure.
Is it possible to overload the bone anchoring an
             osseointegrated implant?
Bone is a dynamic structure. Excessive loads lead to a
resorptive remodeling response
 !   Hoshaw et al (1994) observed a resorptive remodeling of the
     bone around implants subjected to excessive axial loads
     (300N). Bone loss was observed at the crest around the
     neck of the implant and in the zone of bone adjacent to the
     body of the implant

 !   Brunski et al, 2000 J Oral Maxillofac Implants - Consensus
 !   Isador’s studies (1996, 1997) using a monkey model
     presented data that was consistent with the hypothesis
     proposed by Hoshaw and her colleagues.

 !   Recent studies by Myamoto et al (1998, 2000, 2008) have
     reconfirmed Hoshaw and Brunski’s original hypothesis
Do the new surfaces reduce the risk   Courtesy C Stanford

       of Implant Overload?
v  Excessive   occlusal loads
v  Resulting microdamage
    (fractures, cracks, and
    delaminations)
v  Resorption remodeling
    response of bone
v  Increased porosity of bone in
    the interface zone secondary
    to remodeling
v  Vicious cycle of continued
    loading, more microdamage,
    more porosity until failure
Implant Biomechanics
!   What is the load bearing capacity of
    osseointegrated implant supported restorations?
!   Is the load carrying capacity of implant prostheses
    influenced by the quality of the bone sites?

!   What factors control the magnitude of the loads
    that are delivered through the implant into the
    surrounding bone?
!   What loads should implant borne restorations be
    designed to resist?
Implant Biomechanics




         Karnak      The Great Wall         Pont de Gard


You must over engineer your implant restorations, particularly
when restoring posterior quadrants with linear configurations in
order achieve predictable long term results.
Implant Biomechanics
LOAD BEARING CAPACITY           ANTICIPATED LOAD
1. Quality of bone site                (Affected by)
2. Quality of bone              !   Occlusal factors
                                     Cusp angles
    implant interface
                                     Width of occlusal table
3. Implant microsurfaces             Guidance type
  !   Machined vs                       Anterior guidance
       microrough vs                    Group function
       nano-enhanced            !   Cantilever forces
       surfaces                       Connection to natural
4. Implant                               dentition
  !   Number and                      Size of occlusal table
       Arrangement                    Cantilevered prostheses
        Linear vs Curvilinear   !   Parafunctional habits
  !   Length and diameter            (bruxism)
  !   Angulation                !   Brachycephalics
Load bearing capacity
          Implant number and arrangement
l    Both the number and arrangement
      of implants affect the load carrying
      capacity of any particular implant
      supported restoration.
l    Curvilinear arrangements carry
      withstand more load than linear
      arrangements
Load bearing capacity
            Linear vs Curvilinear




Curvilinear arrangements have the
greatest load bearing capacity.
Load bearing capacity
                  Linear vs Curvilinear
v  Curvilinear arrangements such as seen in this
    patient are very predictable
v  This PFM fixed prosthesis is 8 years post insertion.




           Occlusion: Group function
Load bearing capacity
                        Linear vs Curvilinear
Linear configurations restoring the cuspid region, such as the
patient on the right, are unpredictable, whereas curvilinear implant
arrangements such as shown on the left are very predictable.




          Predictable                        Not predictable
Load bearing capacity
                     Linear vs Curvilinear




v  The
      central incisor sites were the most favorable
  implant sites. Therefore:
   !   They were extracted and implants placed into these sites
v  Result:
   !   More favorable biomechanics and predictability

                                                Courtesy Dr. R. Faulkner
Load bearing capacity
  Linear vs Curvilinear




         v  Centrals extracted
             ! Note the horizontal
               dimension of the central
               incisor sites
         v  Implants   inserted

                          Courtesy Dr. R. Faulkner
Load bearing capacity
  Linear vs Curvilinear




                      Courtesy Dr. R. Faulkner
Load bearing capacity
                Linear vs Curvilinear

v  Completedprosthesis
v  Biomechanics are favorable




                                    Courtesy Dr. R. Faulkner
Load bearing capacity
           Implant number and arrangement
v  Anterior – Posterior Spread




                    In the edentulous mandible,
                    curvilinear arrangements such as
                    this one have the greatest load
                    bearing capacity. The cantilever
                    length can be double the A-P
                    spread but not exceeding 20 mm.
Load bearing capacity
             Cantilever length relative to A-P spread
Relatively linear arrangements
combined with excessive
cantilever length such as shown
here are able to withstand less
occlusal load.
v  Result
    •  Mechanical failures
    •  Implant overload

                                                  A-P
   In this patient the result                    Spread
   was recurrent fractures
   of the prosthesis
   retaining screws.
Excessive Cantilever forces
              Implant Overload and Resorptive Remodeling
l    If cantilevers are excessive however, they can lead to implant
      overload and provoke a resorptive remodeling response of
      bone around the distal implants.




       In this patient a fixed edentulous bridge similar to the one
       shown previously, was fabricated for this patient. However,
       the cantilever extensions were in excess of 30 mm. Note the
       bone loss around the distal implants particularly on the
       patient’s left. Eventually this implant fractured.
Maxilla vs Mandible
Courtesy Dr. C. Stanford




                           The size and shape of the
                           trabeculae is different in the
                           mandible as compared to the
                           mandible and may be one of
                           the reasons why the load
                           carrying capacity of implant
                           supported prostheses restoring
                           posterior quadrants in the
                           mandible appears to be
                           superior to those in the maxilla.
Number of Implants per Unit Posterior Maxilla
  When restoring posterior quadrants with implants we
  are forced to use linear arrangements by anatomic
  necessity. Therefore in most instances:
 !  One implant for
 each dental unit.
 !  At least three
 where possible in
 extension areas.

*The third implant
dramatically improves the
biomechanics of the
restoration                  One dental unit = premolar
Number of Implants per Unit Posterior Maxilla
     Curvilinear arrangements are favored over linear arrangements from a
     biomechanical perspective. However, when restoring posterior quadrants
     with implants we are forced to use linear arrangements by anatomic
     necessity. Therefore in most instances:




!  One implant for
each dental unit.
!  At least three

where possible in          *The third implant dramatically improves
extension areas.           the biomechanics of the restoration
Number of Implants per Unit Posterior Maxilla
  The distal implants failed 30 months after loading in
  both these patients because of implant overload.
Number of Implants per Unit Posterior Maxilla
 These implants failed 66 months after
 loading because of implant overload.




Group function was used to restore this patient. Result:
  Another problem:excessive lateral forces
   !   Application of Cusp angles too steep
   !   Implant failure
  and the occlusion was tripodized
Number of Implants per Unit
              Posterior Maxilla
      Space allowed only two implants to be placed in
      this patient. However, note anterior guidance.




Design the occlusion to minimize the delivery of nonaxial forces
Number of Implants per Unit
    Posterior Maxilla
Only two implants were placed.
Note anterior guidance
Bone Augmentation – Horizontal Deficiencies
             !  Grafting bone defects with horizontal deficiencies
             has been relatively predictable, particularly in the
             anterior region.
             !   However, these implants are usually exposed to
             minimal loads. In most patients the graft serves to
             restore bone and soft tissue contours in order to
             enhance the final esthetic result and idealize implant
             position.
             !  Fixation of the graft is easy to accomplish
             !  The blood supply to graft is usually quite good
Bone Augmentation – Vertical Defects




          Grafting vertical defects by adding bone on
          top of the alveolar ridge, as shown here, is
          much less predictable particularly in the
          posterior quadrants.
             Problems:
            !  Tension on the wound secondary to closure of
            tissue flaps
            !  Poor blood supply
            !  Difficulty in achieving fixation
            Result:
            !  Relapse (resorption) rate is 75%
Sinus Lift and Graft
                               Sinus
                              membrane


                                                 Bone graft




                                     Bone of the residual
                                     allveolar ridge
Advantages over only grafts
     Resorption probably less than 25%
Challenge
     Elevate the sinus membrane without perforation
Sinus Lift and Graft
            !  This procedure has been
            reasonably predictable
            although no good long term
            followup studies are
            available.
            !  Sources of graft material
            include chin, ramus, and
            iliac crest sometimes mixed
            with bone substitutes.
            !  Best results with respect
            to implant success rates
            appear to obtained when
            there is at least 4-5 mm of
            residual ridge.
Sinus Lift and Graft
This patient was restored following a sinus lift
and graft. Autogenous chin bone was used.
She is 10 years post treatment and doing well.




Note: Best results achieved when there is 4-5 mm
of normal bone over the sinus before the procedure
Sinus Lift and Graft
    This patient was restored following a
    bilateral sinus lift and graft. Freeze
    dried bone was used to graft the left
    maxillary sinus. The implants placed
    in this graft failed 18 months following
    delivery of the implant supported
    fixed partial denture.
Distraction Osteogenesis
This procedure has been used successfully in other sites,
particularly the anterior maxilla and the mandibular body. Its
usefulness in the posterior maxilla is probably limited. The
relapse (resorption) rate is about 25% (Moy et al, 2005)


                           Osteotomy
                           Distracted
                              site
                              bone




                             Distraction
                                       Distraction
                             apparatus apparatus
*Removable Partial Dentures*
Removable partial dentures properly designed and fabricated
provide the patient with masticatory function equivalent to that
obtained with an implant supported fixed partial dentures
(Kapur, et al, 1992) and this service should be offered to the
patient before grafting is considered.
Number of Implants per Unit
      Posterior Mandible
Two is sufficient for most patients
Why? The trabecular bone is more dense
 resulting in better bone anchorage
Number of Implants per Unit
                 Posterior Mandible
         Three are recommended when:
v    There is bone over the nerve for only short implants
v    Bone quality is poor
v    When restoring four dental units
Number of Implants per Unit
   Posterior Mandible
 Three implants were used to
 restore four units in this patient
Posterior Mandible – Limiting Factors
  v    Inferior alveolar nerve(arrow)
  v    Insufficient bone over the nerve to permit
        placement of a 10 mm or longer implant
  v    Uni-cortical anchorage (arrow)
Posterior Mandible – Limiting Factors
Many patients such as this one, present with moderate
to severe resorption precluding placement of implants
unless the inferior alveolar nerve displaced.
Displacement of the Inferior Alveolar Nerve
!  This procedure enables placement of implants of sufficient length with
bicortical anchorage.
!  Although the risk of nerve injury is relatively small the morbidities
associated with injury may be severe.
!   Therefore, these issues must be thoroughly discussed with the patient
before proceeding with the procedure.
Crestal Augmentation
Augmentation of vertical defects in posterior mandibular quadrants with free
autogenous bone grafts (A) has been unpredictable. Following surgery the
relapse rate is about 75% and further bone loss is also seen after loading (B).
Why?
        a) Tension on the wound upon closure
        b) Poor blood supply
        c) Difficulty is achieving proper fixation of the graft




                                 A                                        B
  Presently, distraction osteogenesis is the only reasonably
  predictable method for enhancing this site vertically.
Use of Short Wide Diameter
            Implants in the Posterior Mandible
This practice has not been predictable. The short implants
are particularly prone to occlusal overload and bone loss. This
is a 5 year followup x-ray of two 6 mm diameter implants.
If implants of adequate length cannot be
   used, consider removable partial dentures




Mastication efficiency of distal extension RPD’s is
equivalent to implant supported fixed partial dentures.
Connecting Implants to Natural Dentition
   Semiprecision vs rigid attachments
Linear configurations
Over engineer your cases
      !   When in doubt add the 3rd
      implant in posterior
      quadrant cases.
      !   Minimize the length and
      width of the occlusal table
Over-engineer your linear quadrant cases
                  v When in doubt re: the quality of
                  the implant site bone, history of
                  parafunction etc., add the third
                  implant
                  v Minimize the width of the
                  occlusal table
Over-engineer your linear quadrant cases
However there is a flaw in he design of this
case. What is it?
Note: The buccal-lingual dimension is excessive




v Minimize the width of the occlusal surfaces. They should
be no wider than a premolar
Staggered vs linear configuration in
         posterior quadrants



                            Straight line implant configuration



                                1.5 mm                     1.5 mm
                                         1.5 mm




                             Staggered implant configuration
This has been studied using a photoelastic model
by Itoh, et al, 2003
Staggered vs linear configuration
        Is it biomechanically more favorable?




                                     Straight line implant configuration



                                         1.5 mm                     1.5 mm
                                                  1.5 mm
v Yes, particularly with specific
chewing cycles. Nonlinear
arrangements resist lateral forces    Staggered implant configuration
more effectively
v Is the improvement clinically
significant? This is unknown         Itoh and Caputo, et al 2003
Staggered vs linear configuration
       Is it feasible in the posterior quadrants?




                                 Straight line implant configuration



                                     1.5 mm                     1.5 mm
                                              1.5 mm


Probably not. Inthe posterior
quadrants you can’t get enough    Staggered implant configuration
stagger to make much of a
difference biomechanically.      Itoh and Caputo, et al 2003
Implants in Compromised Sites
Can we use shorter implants?
  !   Posterior maxilla
  !   Posterior mandible over the
      inferior alveolar nerve in partially
      edentulous patients
  !   Craniofacial application
                                 Theoretically perhaps.
                                 However we need well
                                 designed clinical
                                 outcome studies to
                                 determine predictability
Length and diameter of Implants
Avoid the use of implants less than 10 mm in length and
4mm in diameter when restoring posterior quadrants.

                  v Short implants, such as this 7 mm
                  screw shaped implant, demonstrate
                  unfavorable stress distribution
                  patterns as seen in this study
                  performed with finite element
                  analysis. Longer implants distribute
                  stresses more favorably.
                  v Given the bone anchorage
                  achieved with modern surfaces,
                  failures are most likely to occur in the
Cho et al, 1993   trabecular bone
Length and diameter of Implants




•  Two year followup data from Moy and Sze,’93
•  Note the high failure rates with the 7 mm and
       10 mm implants in the posterior maxilla.
Implant length vs diameter
              Does increasing the
              diameter compensate for
              the lack of sufficient
              length?
              Using a photoelastic model,
              Caputo et al, 2002 attempted
              to determine whether
              increasing the diameter of the
              implant or increasing the length
              of the implant had a significant
              impact on stress distribution.
              They concluded that:
Implant length vs diameter
                                     !  Most equitable load transfer
                                     with axially directed loads.
                                     !  Under comparable loading
                                     conditions, the stresses
                                     transferred by the wide
                                     diameter implant were only
                                     slightly lower than the same
                                     length narrow implant.
                                     !  For implants tested,
                                     increased length was more
                                     important than diameter in
                    Axial   Buccal
    Lingual
                    load     load
                                     stress reduction.
      load
Caputo et al,2002
Implant length vs width
These data appear to have clinical significance. In our clinical
experience length is more important than width. Short wide
diameter implants appear to be susceptible to overload when used
in linear configurations such as shown here.



                                                        2 years




                                                        5 years



Cho,In Ho et
  al, 1992
Ideal Implant Diameter
             4-5 mm in diameter
!    Less than 4 mm the rate of implant
     fracture is unacceptably high
     ! Implants
                3.75 mm in diameter have a 5-7%
     fracture rate


!    More than 5 mm the higher the
     failure rate.
     ! Implants
                  6 mm in diameter have a 20%
             failure rate
     !  Implants 4-5 mm in diameter have a less than
             5% failure rate
Implant Angulation – Posterior vs Anterior
v  Implants in the posterior
    quadrants should be placed
    so that occlusal loads can be
    directed axially in the
    posterior quadrants.

v  In the anterior region, anatomic
    necessity precludes implant
    placement perpendicular to the
    occlusal plane. However, the
    forces used to incise the bolus are
    only about ¼ of those used
    posteriorly to masticate the bolus.
    For this and other reasons implant
    overload is rarely seen in the
    anterior regions.
Implant angulation
v    Nonaxial loads result in load magnification. Kinni et al
      (1987), using photoelastic analysis and Cho et al (1993),
      using finite element analysis, demonstrated that nonaxial
      loads concentrated potentially clinically significant stresses
      around the neck and at the tip of the implant.




                                               Cho,In Ho et al, 1992
Biomechanics – Partially Edentulous Patients
          Nonaxial loads and implant overload in posterior
                            quadrants




v    Because of the curve of Spee and the distal angulation of the implants, the
      occlusal loads (arrow) are nonaxial. Note the bone loss around the implants.
      Linear configurations in the posterior region, such as in this patient, are
      particularly vulnerable to the effects of nonaxial loading, particularly
      brachycephalic individuals.
Cantilever forces
Cantilever forces are potentially detrimental particularly when
applied to implants with a linear configuration and single implants
placed in posterior quadrants.
!  The longer the
cantilever the greater the
load magnification and
the more stress
concentrated in the bone
anchoring neck of the
distal implant.

!  Note the dramatic
increase in stresses
associated with the 20
mm cantilever as
opposed to the 5 mm
one.
Cantilever forces


                                        Cantilever
                                        section
They are well tolerated when
implant supported
restorations are used to
restore the edentulous
mandible, so long as:
l    The cantilevered section is
      within a reasonable limit
l    The implants are arranged in a
      reasonable arc of curvature.
l    Rigid frameworks with cross
      arch stabilization are used
Excessive Cantilever forces
                  Implant Overload and Resorptive Remodeling

l    If they are excessive however, they can lead to
      implant overload and provoke a resorptive remodeling
      response of bone around the distal implants.




      In this patient a fixed edentulous bridge similar to the one
      shown previously, was fabricated for this patient. However,
      the cantilever extensions were in excess of 30 mm. Note the
      bone loss around the distal implants particularly on the
      patient’s left. Eventually this implant fractured.
Excessive Cantilever forces
  Implant Overload and Resorptive Remodeling
                       Case Report




This tissue bar uses nonresilient attachments in the distal with a
long cantilever anteriorly and is therefore an implant supported
design. The implants were exposed to tipping forces magnifying
the occlusal loads, in turn leading to a resorptive remodeling
response of the bone around the implants and eventually loss of
the implants.
Excessive Cantilever forces
   Implant Overload and Resorptive Remodeling



                                                           Cantilever
  Cantilever


          Overlay Dentures in Edentulous Maxilla
!  During the eighties, tissue bar designs using four implants, such as the
one above, were commonly used at UCLA to retain overlay dentures. Hader
bar attachments were used anteriorly and in the extension areas.
!   Such designs result in most of the posterior occlusal forces borne by the
implants and therefore are implant supported.
!  The followup data (collected by the author from his private patients)
indicated significant bone loss and implant failures of the distal implants as
shown in the following table.
Excessive Cantilever forces
        Implant Overload and Resorptive Remodeling



 Cantilever                                       Cantilever

             Overlay Dentures in Edentulous Maxilla
Four implanted supported overlay dentures with nonresilient
(Hader) attachments (arrows) and distal cantilevers
 Patients # Implants Followup Failures Position Time of
                                       of failed failure
                                        implants

   10         40      5-12 yrs.   4       all distal 39-73 mths.
 ***Failures were attributed to implant overload, with its
 resultant loss of bone around the implants
Cantilever forces
    Implant Overload and Resorptive Remodeling

       l  Implant   Assisted Design – 4 implants




When implant tissue bars with resilient attachments
connected to the distal portion of the bar (ERA type in
this patient) were used in the maxilla the failures after
loading were completely eliminated.
Cantilevers and Linear Configurations in
                Posterior Quadrants
                 Mesial and distal cantilevers
l    They are particularly detrimental and are therefore
      contraindicated when using linear configurations to restore
      posterior quadrants. They cause load magnification and
      overload the bone around the implant adjacent to the
      cantilever.
Cantilevers – Implant Overload
l    Note the bone loss around the dental implants
      adjacent to the cantilever.




        Restorations designed in this
        fashion have a poor prognosis.
Cantilevers – Implant Overload
Avoid buccal, lingual and cantilevers
         The occlusal tables are
         excessively wide in this
         case. Buccal and lingual
         cantilever forces may
         lead in selected patients
         to:


         Prosthesis failures
           • Porcelain fractures
           • Screw fractures
         Implant overload and
              bone loss
Occlusal Anatomy and Biomechanics
            v  Narrow   occlusal table




Goal: Reduce the buccal - lingual cantilever effect
Avoid buccal and lingual cantilevers




The occlusal table must be narrowed
to avoid buccal and lingual cantilevers.
Molars should be no wider than
premolars as shown in these two
examples.
Solitary implants restoring single molars –
             Cantilever effect
                     A                           B




When the food bolus is applied to the marginal ridge (B), the
restoration is easily tipped because the crown is supported by
such a narrow platform.
Result: Cantilever forces lead to screw loosening, implant
fracture and overload the bone anchoring the implant.
Solitary implants restoring single molars
              Cantilever effect




Fracture
    Implant fractured after 30 months of function
Single tooth restorations in the molar
        region – Cantilever effect
                           Mesial cantilever

   4 mm
   diameter
   implant




This implant was too short and too narrow to
withstand occlusal loads and bone loss caused by
the resorptive remodeling response led to its loss.
Single Tooth Restorations Distal
       Extension Defects
Distal Extension Defects
Restoration of single molar sites - Solutions
    Eliminate the cantilever by using
   !   Wide  diameter
   !   Multiple implants




 In this patient a wide diameter implant was used to
 restore the first molar.
Restoration of single molar sites
In this patient, two 4 mm diameter implant were used to
restore the first molar. The width of the occlusal table was
limited to the width of the
natural premolar,
thereby elimating any
possible buccal or
lingual cantilevers.



 Custom abutment Lingual set screw
Restoration of single molar sites
Note:
!   Hygiene access for proxy brush
!   Note width of occlusal table
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