Fatigue by shitingting

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									  Fatigue resistance of two
implant/abutment joint designs
   J Prosthet Dent 2002;88:604-10 Ameen Khraisat et al




          길병원 보철과 김 세 웅
Statement of Problem

 • 여러 가지 mechanical failure가 보고되어 적절한 강도
   를 갖는 implant system components들이 제작되었으
   나 이런 제품들의 endurance를 제대로 조사한 것은
   없다.
Purpose
 • 2 가지 single-tooth implant systems에 있어서
    fatigue strength와 failure mode에 대한
   joint design 영향을 평가

 • Brånemark / ITI
    – Brånemark ; hex mediated-butt joint
    – ITI ; 8-degree internal conical implant/abutment interface
Clinical Implications

 • This in vitro investigation of single-tooth implants
   indicated a significant difference in fatigue
   resistance between 2 commonly used implant
   systems.



 • Failure of the abutment screw in the Brånemark
   system may serve as a safety mechanism in
   securing the implant and the surrounding structure
   from bending overload.
Journal review
 • Osseointegrated dental implants
  ; A successful procedure for the treatment of
   complete, partial edentulism, and single-tooth
   replacements in both the anterior and posterior
   regions.
 • Importance of the integrity of bone/implant
   interface
 • Mechanical problems that affects single-tooth
   implant replacements
    – Screw joint instability
        • Loosening or fracture of the abutment or retaining screws
Journal review
 • Studies of the mechanical properties of implant/
    abutment connections for single-tooth implant
    replacements.
 1. Merz et al.
    – The tensile stresses were higher in the abutment screw
      threads of the butt joint design in contrast to the ITI taper
      connection.
 2. Norton
    – Static bending strength 비교
        • 3.75mm Brånemark implant system – hex mediated butt joint
        • 3.5mm Astra Tech implant system – 11 degree internal
                                            conical interface design
    – Brånemark implant system << Astra Tech implant system
Journal review
 3. MÖllersten et al.
    – Static cantilever bending
    – Implants with a deep implant/abutment joint, such as the
      internal conical connection, favor resistance to bending
      moments in contrast to shallow one like the hex-mediated
      butt joint.
 4. Rangert et al.
    – 39 fractures 0f 10,000 Brånemark implants
       • 3.75mm implants(different types and lengths)
       • No fractures of 4mm implants
       • 36 fractures ; Bone level was at the implant third thread
                                                       or more apical.
    – EsthetiCone and CeraOne gold abutment screws, where
      the upper threads are deleted, should be introduced on
      the standard Brånemark system.
Journal review
    – 4mm implant has 30% higher fatigue resistance than a
      3.75mm implants.
    – 향상시켜야 할 점
       • The bending resistance of the abutment screw by adopting
         new screw geometry and material.
       • The implant strength by increasing the diameter
 5. 107 single-tooth restorations(Brånemark) were
                                 followed (for 5 years).
    – 1 titanium abutment screw fractured(after 3 years)
    – 13 screws were replaced by gold alloy screws
                         (between the third and fifth years)
    – The gold alloy screw proved higher resistance to fatigue
      compared with the titanium screw.
Journal review
 6. Levine et al.
    – The internal conical implant-abutment interface
             (ITI system with an 8 degree tapered connection)
    – 3 fractures among 157 single-tooth implant restorations
    – All fractures
        • Mandibular first molar area
        • 3.5mm hollow implant
    – No fracture
        • 4.1mm solid screw implants
Journal review
                         Fatigue
 • Is defined as the progressive crack propagation
   resulting in a catastrophic fracture under repeated
   loading below the yield stress.
 • The abutment screw might fracture when fatigued
   or overload(Versluis et al)
    – Fatigue was a major possible cause of implant/abutment
      joint instability.
Material and methods
 • Used 7 assemblies of 2 implant systems
    – Brånemark(Mark IV; Nobel Biocare AB)
    – ITI(Solid screw; Institute Straumann AG, Waldenberg)

                                가. 25.4 mm
                                나. 17 mm
                                다. Transparent acrylic-resin blocks

               다                1. Mounted in an engineering lathe
                                2. Drilling for each implant type
                                3. Tapping

가
                                4. The embedded depth was 7 mm
                                  to simulate 3 mm bone resorption
         나
Material and methods




  - The diameters and lengths were those most commonly used
   for posterior implant replacements with the least failures.
Material and methods

 • Brånemark group
   – 7 Mark IV implants
   – 3 mm CeraOne abutments
   – Tightened to 32Ncm(electronic torque control unit)

   – Screw access channels ; obturated with gutta-percha plug
   – CeraOne abutment
      • Titanium abutment로 구성
          – Retentive hexagon of 3.8mm length
          – 2mm gold alloy screw
Material and methods

 • ITI group
    – 7 solid screw implants
    – 4 mm solid abutments
    – Tightened to 35 Ncm(torque wrench)


    – Solid abutment
       • Upper cone(6 degree taper)
                 + lower cone(8 degree taper)
       • The thread part of the abutment has a 2mm diameter
         and emerge from the abutment flat bottom.
Material and methods
 • Fabrication and cementation of the superstructure

                                      Parts assembled for wax-up
                                      A.    Brass base (two)
                                      B. Crown-forming jig
                                      C. Plastic burn-out pattern
                                                covering implant analog


                                           2 side pins ; path controller

  Holed at the center with the same
  diameter as each implant analog
Material and methods
 • Fabrication and cementation of the superstructure
    – Crown-forming jig
        • One center hole of 7 mm diameter to shape a cylindrical
          superstructure.
        • 2 small side holes that passively fit the 2 pins of the corresponding
          base when placed together.
    – The analog was inserted into the hole of the base to the level of
      its coronal shoulder.
    – A separating agent(Wax separator)
    – The plastic burn-out pattern was placed over the implant analog.
    – Molten wax was flowed in the cavity around the plastic pattern.
    – Spruing, investing, burn-out, casting(type IV gold alloy)
    – Cleaned in a pickling agent(Neaacid)
Material and methods
 • Fabrication and cementation of the superstructure
    – No internal relief was provided, whereas castings with poor fit
      and nodules were discarded and remade.
    – The axial wall of each casting was milled and polished with a
      carbide-milling cutter and a parallel polisher.
    – The occlusal surface was finished with caborundum wheels and
      polished with silicone-points.

    – Zinc-phosphate cement( 3.0g/mL ratio)
    – Were seated with finger pressure for 10 seconds followed by a
      sustaining pressure of 6kg for 10 minutes.
    – Excess cement was removed from the margins with a plastic
      instrument.
    – Allowed to set at room temperature for an additional 50 minutes.
Material and methods
 • Loading machine and the loading approach
   – Each specimen was firmly mounted in a brass holder of a lever-
     type fatigue testing machine.
   – Serrate-type cyclic loading between 0 and 100N was applied
     perpendicularly to the flat surface of the underlying abutment.

                                -The loading point was at a distance
                                 of 11.5mm from the block surface
                                 (lever arm length)
                                -100N
                                -Generated bending moment
                                  ; 100 x 11.5 = 1150Nmm
Material and methods
 • Loading machine and the loading approach
   – Before starting each test, a small amount of grease was used to
     reduce friction and wear at the loading point.
   – To simulate 6 years of function, a target of 1,800,000 cycles was
     defined.
   – The loading rate was 75 cycles/min that was similar to the human
     chewing frequency.

   – Every 10,000 cycles the loading machine was stopped to inspect
     all specimens for any deformation, decementation, and abutment
     loosening by a loupe(x8) and operator’s finger.
   – On test completion, fractured surface were examined with a SEM.
   – Specimen preparation and testing were performed by the same
     operator.
Material and methods
 • Statistical analysis
    – Fisher’s exact probability test(both-sided) was performed
      to determine the probability of the association of the joint
      design with the occurrence of fatigue failure.
    – Statistical significance was defined as P <.05.
Results
Results

          The critical zone in the failed
           specimens
          ; at the junction between the unthreaded
           and thread parts of the abutment screw.
           Fractures were located approximately
          3mm from the upper aspect of the Bråne
          -mark implants.
           No signs of decementation or abutment
            loosening were noticed in either implant
            system.
Results


           The coronal surface of the fractured
           abutment screws underwent SEM
           analysis.

           The upper area showed a different
           pattern from the lower one.

          A ; second stage(crack propagation)
          B ; third stage(catastrophic fracture)
          Vertical arrow indicates load direction
Results
         Upper area                Lower area




   Lines of slender grooves
                               A fine equi-axis dimple
   perpendicular to the load
                               pattern
   direction
Discussion
 • Gold alloy abutment screw(Brånemark system)
     – Designed for the stabilization of the butt joint.
     – Allowed a higher torque application that elastically deformed the
       screw to work as a spring at the joint interface.
      The axial preload of the screw was a determining factor for the
       joint stability.


 • Under lateral loading, tensile stresses in the abutment screw
   threads in the butt joint design were higher in contrast to the
   ITI taper connection
     – The butt joint opens on the tension side under lateral loading
       and the small screw, instead of the joint interface, is compelled
       to take this tension transfer.
Discussion
 • All failures occurred at the junction between the unthreaded
   (1.5mm diameter) and threaded parts(2mm diameter) of the
   screw.
    – The diameter difference caused stress concentration at the
      junction and resulted in fatigue crack initiation, which propagates
      until complete catastrophic fracture.


 • In a taper connection, the friction locking of the abutment to
   the implant with a less than 10um gap eliminated vibration and
   micromovement of the abutment screw.
    – As a result, lateral loading was resisted by the taper interface,
      which prevented the abutment from tilting off and protected the
      abutment screw from excessive stress.
Discussion
 • Basten et al.
    – CeraOne abutment의 fatigue performance를 조사
    – The weak component in the implant assembly was the 4mm
      diameter implant, which fractured in 10 of 15 specimens.
    – The gold alloy abutment screw fractured in a wide range of
      169,000 to 1,492,000 cycles in 4 specimens, of which 3
      occurred below the screw head and 1 in the threaded part.
 • This study
    – Lateral load를 이전의 연구 보다
      3 배정도 강하게 가하였으나 fatigue
      life가 이전의 연구보다 더 길었다.
    – All failures occurred at the junction
       between the unthreaded and threaded
       parts of the screw.
        • Neither below the screw head nor in the threaded part
Discussion

 • 두 연구의 차이점
   – Some factors
      • Loading pattern
      • The distance between the loading and supporting points,
        which was about 1mm shorter in this study than in the other
        test.

   – The supporting material was acrylic resin
      • Has a smaller Young’s modules compared with epoxy resin
        used in the other test.
Discussion
 • Brånemark system에서 fatigue failure가 발생했을 경우 고려해야
   할 점.
   – The implants and abutments could be brought to function by
     replacing the fractured abutment screws and applying technical
     skills to diagnose and eliminate the reasons for overload.

   – The fail-safe mechanism of sacrificing the abutment screw could
     be applied to protect the other components and the supporting
     tissues.
       • The stronger implants will not solve the problem of overload
         but may lead to bone damage.
       • For implant systems with abutment screws, the screws should
         be designed as the weak link because it is the most easily
         replaced component.
Discussion

 • These 2 important features contrast with those of the ITI
   system, where the design aims to have a strong assembly
   that transfers the functional load through the taper interface
   to the implant and secures the abutment threads.



 • 이번 연구의 기간은 6개월에서 5년 정도로 다양한 형태의
   implant/abutment connection과 연관된 문제점들 및 장기적인
   성공을 평가하고 확신하기 위해서는 좀더 긴 연구가 필요하다.
Conclusions
 • 이번 연구의 제한성 내에서 다음과 같은 결론을 내릴 수 있다.


 • For the Brånemark system
   – Even though the geometry and composition of the
     abutment screw were modified, the screw was the weak
     link in the implant assemblies.
   – The junction between the unthreaded and threaded parts
     of the abutment screw was the crucial point for all tested
     specimens in the presence of simulated bone resorption.
Conclusions
 • For the ITI system
    – The internal conical implant/abutment joint showed fatigue
      resistance superior to the hex mediated butt joint.
    – The stress dispersion over the joint interface may be the
      reason for the high resistance to repeated lateral loading.


 • For both implant system
    – Although the intraoral environment was not completely
      simulated in this study, the absence of cement failure
      under repeated lateral loading may add to the advantages
      of the use of cement-retained crowns for single tooth
      implant applications.
경청해 주셔서 감사합니다!!!




    길병원 보철과 김 세 웅

								
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