Fatigue resistance of two implant/abutment joint designs

1. Fatigue resistance of two implant/abutment joint designs 길병원 보철과 김 세 웅 J Prosthet Dent 2002;88:604-10 Ameen Khraisat et al

2. Statement of Problem • 여러 가지 mechanical failure가 보고되어 적절한 강도를 갖는 implant system components들이 제작되었으나 이런 제품들의 endurance를 제대로 조사한 것은 없다.

3. Purpose • 2 가지 single-tooth implant systems에 있어서 fatigue strength와 failure mode에 대한 joint design영향을 평가 • Brånemark / ITI • Brånemark ; hex mediated-butt joint • ITI ; 8-degree internal conical implant/abutment interface

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. 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

10. 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.

11. Fig. 1. Assembly components of Brånemark and ITI systems. 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 1. Mounted in an engineering lathe 2. Drilling for each implant type 3. Tapping 4. The embedded depth was 7 mm to simulate 3 mm bone resorption 다 가 나

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

13. 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

14. 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.

15. Material and methods • Fabrication and cementation of the superstructure • Parts assembled for wax-up • Brass base (two) • Crown-forming jig • Plastic burn-out pattern • covering implant analog 2 side pins ; path controller Holed at the center with the same diameter as each implant analog

16. 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)

17. 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.

18. 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

19. 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.

20. 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.

21. Results

22. 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.

23. 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

24. Results Upper area Lower area Lines of slender grooves perpendicular to the load direction A fine equi-axis dimple pattern

25. 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.

26. 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.

27. 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

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. 경청해 주셔서 감사합니다!!! 길병원 보철과 김 세 웅