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High-Cycle-Fatigue Evaluation of Ti-5553 for Orthopedic Applications K. Rivard, T. Lowe, B. Aboud

DePuy Orthopaedics, a Johnson & Johnson Company, Warsaw, Indiana, USA

Abstract Titanium-based alloys are commonly selected for use in orthopedic implant applications, owing to their excellent

mechanical strength, corrosion resistance, and biocompatibility. Ti-6Al-4V in either cast, forged or wrought form is among the most common. A new alloy, Ti-5Al-5V-5Mo-3Cr (Ti-5553), that has shown to exhibit excellent static strength and equivalent castability properties to Ti-6Al-4V, was investigated. The casting alloy Ti-5553, a modified version of the Russian Alloy VT-22, is a near-beta alloy exhibiting a refined alpha/beta Widmanstätten microstructure with no apparent eutectoid formation. Typical room temperature tensile properties are 168 ksi UTS, 153 ksi YS, and 9% Elongation, similar to wrought Ti-6Al-4V properties in the annealed condition.

The first phase of the evaluation of Ti-5553 was to conduct high-cycle rotating beam fatigue (RBF) studies. Implant materials must have a high degree of fatigue resistance, particularly for highly stressed implants such as a femoral hip stem, to perform over the long term. In this study, comparison RBF testing was performed for cast, hot isostatically pressed, and heat treated Ti-5553 against wrought mill annealed Ti-6Al-4V ELI. Both materials were tested in smooth and notched (Kt = 2.9) configurations to assess the endurance limit, or the level of stress below which fatigue failure will not occur, and estimate the notch sensitivity of each alloy. RBF samples were tested to failure, or survival of 10 million cycles, which is estimated to be equivalent to approximately 10 years for a moderately active patient, and considered a “run-out”. The fatigue properties of cast Ti-5553 were found to be comparable to the fatigue properties of wrought mill annealed Ti-6Al-4V when tested in rotating beam.

Introduction Over 300,000 total hip replacements

(THR) take place in the United States every year. The growing number of younger, more active patients and the demands placed on orthopedic devices by today’s aging population require high performance biomedical materials with lower elastic modulus, higher fatigue strength, and superior wear performance. The challenge is being met through partnerships between orthopedic manufacturers, researchers, and material suppliers. Their combined efforts have contributed greatly to long-term clinical performance of surgical implants. The clinical performance of the implants can be dependant upon many factors including: implant design, surgical technique, bone quality, patient weight, patient activity level, sterilization conditions, material quality, material properties, surface finish, and manufacturing quality. Titanium-based alloys are commonly selected for use in orthopedic implant applications, owing to their excellent mechanical strength, corrosion resistance, and biocompatibility. Ti-6Al-4V ELI (Ti-64) in either cast, forged or wrought form is among the most common. However, to meet the increasing demands for orthopedic implants, titanium alloys with lower elastic modulus, higher

fatigue strength, reduced notch sensitivity, and/or improved fretting resistance are continually being sought. A new alloy, Ti-5Al-5V-5Mo-3Cr (Ti-5553), has been shown to exhibit excellent static strength and similar castability properties to Ti-6Al-4V. The casting alloy Ti-5553, a modified version of the Russian Alloy VT-22, is a near-beta alloy exhibiting a refined alpha/beta Widmanstätten microstructure with no apparent eutectoid formation. Typical room temperature tensile properties are 168 ksi UTS, 153 ksi YS, and 9% Elongation, similar to wrought Ti-6Al-4V properties in the annealed condition. The promising static properties of Ti-5553 led to an evaluation of the alloy for orthopedic applications.

Fatigue Evaluation The first phase of the evaluation of Ti-5553 was to conduct high-cycle rotating beam fatigue (RBF) studies. Implant materials must have a high degree of fatigue resistance, particularly for highly stressed implants such as a femoral hip stem, to perform over the long term. In this study, comparison RBF testing was performed for cast, hot isostatically pressed, and heat treated Ti-5553 against wrought mill annealed Ti-6Al-4V ELI. Both materials were tested in smooth and notched (Kt = 2.9) configurations

High-Cycle-Fatigue Evaluation of Ti-5553 for Orthopedic Applications

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with ten specimens in each group to assess the endurance limit, or the level of stress below which fatigue failure will not occur, and estimate the notch sensitivity of each alloy.

Studies were performed to determine the feasibility of applying Porocoat® porous coating to Ti-5553, a coating which is metallurgically sintered to the surface of an implant in order to achieve fixation of the implant through bone ingrowth into the porous coating. There were two main objectives for these studies: (1) to determine if typical Porocoat® porous coating properties can be achieved with Ti-5553 as the substrate material; and (2) to determine if the notch sensitivity of Ti-5553 is less than that of Ti-64. The metallurgical bond formed between a bead of the porous coating and the solid substrate acts as a notch. Therefore, for materials with poor notched fatigue strength, porous coating is not applied in the high stress regions of the implant.

Initial studies revealed the sintering cycle currently used for Ti-64 material achieved morphological properties typical of Porocoat® porous coating with Ti-5553 as the solid substrate. Therefore, to evaluate the suitability of Ti-5553 for extensively porous coated high stress applications, smooth and notched configurations were also tested in the cast, hot isostatically pressed, sintered, and heat treated condition. Specimens were prepared by an outside laboratory (Metcut Research Inc., Cincinnati, Ohio) with low-stress grinding into standard smooth RBF specimens, and low-stress grinding into notched RBF specimens (Kt = 2.9), both with gage diameters of 0.250 inch. Testing was performed with a Model RBF-200 Rotating Beam Fatigue Testing Machine (Fatigue Dynamics Inc., Walled Lake, MI). RBF specimens were tested at 5000 rpm to failure, or survival of 10 million cycles, which is estimated to be equivalent to approximately 10 years for a moderately active patient, and considered a “runout”. An extrapolated fatigue limit (EFL) was calculated for specimens that failed prior to the runout value based on the following assumption:

( )σσ ∆+= ip nEFL where pσ is the highest stress passing 107 cycles, in is the fraction of 107 cycles achieved at an incremental stress, and σ∆ is the difference in stress between pσ and the incremental stress. Samples that survived the ten million cycles were assigned an EFL of the value tested. The endurance limit, or the level of stress below which fatigue failure will not occur, for the smooth Ti-64 RBF specimens was 100 ksi and for notched Ti-64 was 28 ksi. The average EFL for the smooth Ti-64 was 90 ksi with a standard deviation of 8 ksi, and for notched Ti-64 was 26 ksi with a standard deviation of 1 ksi. The endurance limit for the smooth Ti-5553 RBF specimens was 96 ksi in the cast, hot isostatically pressed, and heat treated condition, and 94 ksi in the cast, hot isostatically pressed, sintered, and heat treated condition. The endurance limit for notched Ti-5553 was 25 ksi in the cast, hot isostatically pressed, and heat treated condition, and 27 ksi in the cast, hot isostatically pressed, sintered, and heat treated condition. The average EFL for the smooth Ti-5553 specimens was 89 ksi hot isostatically pressed, and heat treated condition, with a standard deviation of 6 ksi, and 90 ksi in the cast, hot isostatically pressed, sintered, and heat treated condition, with a standard deviation of 5 ksi. The average EFL for the notched Ti-5553 specimens was 30 ksi in the cast, hot isostatically pressed, and heat treated condition, with a standard deviation of 8 ksi, and 28 ksi in the cast, hot isostatically pressed, sintered, and heat treated condition, with a standard deviation of 3 ksi.

Conclusions The fatigue properties of cast Ti-5Al-

5V-5Mo-3Cr were found to be comparable to the fatigue properties of wrought mill annealed Ti-6Al-4V ELI when tested in rotating beam. The addition of a sintering cycle necessary to apply porous coating for biological fixation did not adversely affect the fatigue properties of cast Ti-5553. Therefore, cast Ti-5553 would be a viable alternative to current wrought or forged Ti-64 products. Additional studies would be necessary to utilize Ti-5553 in modular applications to evaluate the fretting properties in comparison to Ti-64.

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Metallurgy

High-Cycle-Fatigue Evaluation of Ti-5553 for Orthopedic Applications

Kori RivardDePuy Orthopaedics

a Johnson & Johnson Company

Titanium 2006San Diego, CA

October 2, 2006

Metallurgy Research

Metallurgy

Established 1895Established 1895•• Worldwide Franchise Headquarters Worldwide Franchise Headquarters -- Warsaw, Indiana, USWarsaw, Indiana, US•• International Headquarters International Headquarters -- Leeds EnglandLeeds England• Employees Employees -- 6,000 Worldwide6,000 Worldwide•• Sales Revenue Sales Revenue -- $3.8 Billion Worldwide (2005)$3.8 Billion Worldwide (2005)•• Main Facilities :Main Facilities :

Warsaw, INWarsaw, IN Leeds, EnglandLeeds, EnglandRaynham, MARaynham, MA Cork, IrelandCork, IrelandNew Bedford, MANew Bedford, MA LeLe LocleLocle, Switzerland , Switzerland

Chaumont, FranceChaumont, France

• Sales offices in over 40 countriesSales offices in over 40 countries• Johnson & Johnson acquired DePuy in November 1998.Johnson & Johnson acquired DePuy in November 1998.

DePuy Overview

Metallurgy Research

Metallurgy

$24.8 Billion

12% Growth

Hips$4.6, 7%

Knees $4.9, 14%

Extremities$0.5, 17%

Bone Cement$0.5, 9%

Ancillary$2.8, 4%

Arthroscopy & Sports Medicine

$2.3, 10% Trauma $2.6, 11%

Spine & Spinal Stimulation $4.4, 19%

Bone Stimulation $0.2, 5%

Bone Grafts & Orthobiologics

$2.1, 18%

Source: DePuy Market Research

2005 Worldwide Orthopaedic Market$ in billions, growth %

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Metallurgy

Charnley Hip

Hips

C-Stem

Pinnacle Cup

Summit Hip

S-ROM Hip System

AMLHip System

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Metallurgy

LCS Total Knee LCS Total Knee SystemSystem

Knees

P.F.C. MobileP.F.C. MobileBearing KneeBearing Knee

PFC SigmaPFC Sigma

Preservation UNI

Metallurgy Research

Metallurgy

Global Total Global Total Shoulder SystemShoulder System

Extremities

Agility AnkleAgility Ankle NeuFlex Finger JointNeuFlex Finger Joint

Biax Biax WristWrist

Acclaim ElbowAcclaim Elbow

Metallurgy Research

Metallurgy

• Implant design• Surgical technique• Bone quality• Patient weight• Patient activity level• Sterilization

conditions

Clinical performance is a multifactorial process

• Material quality • Material properties • Surface finish• Reproducibility/

Quality

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Metallurgy

Porocoat® Porous Coating

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Metallurgy

Scope of Alternative Titanium Alloys

Desired properties (compared to Ti-6Al-4V ELI):• Lower elastic modulus• Higher Strength (particularly fatigue strength)• Reduced fatigue notch sensitivity• Improved fretting resistance• Similar manufacturing costs to current

technologies

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Metallurgy

Cast Ti-5Al-5V-5Mo-3Cr (Ti-5553)• Near Beta Alloy, excellent static properties compared to

Ti-6Al-4V ELI (wrought, mill annealed)

15.2-16.8 x 10317.1 x 103Elastic Modulus (ksi)

10090Fracture Toughness (ksi√in)

90100Shear Strength (ksi)

125165Compressive YS (ksi)

159Elongation (%)

120153Yield Strength (ksi)

130168Ultimate Strength (ksi)

Ti-64Ti-5553Property

Source: Howmet Castings, an Alcoa business

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Ti-5553 Porocoat Sintering Trials

• Initial trials indicated Ti-5553 could be sintered at a reduced temperature and yield acceptable results.

• For compatibility with current operations, current sintering cycles also evaluated.

Cast + HIP + HT Cast + HIP + Sinter + HT

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Smooth Rotating Beam Fatigue

83.4Fracture36,200100.0

St Dev = 8.6

Average = 90.4

75.1Runout13,557,45075.1

85.0Runout14,630,20085.0

90.0Runout13,353,00090.0

95.0Runout11,121,60095.0

100.0Runout104,910,400100.0

100.3Runout34,002,000100.3

101.2Runout10,207,600101.2

86.1Fracture110,250103.1

87.6Fracture64,700104.9

EFL (ksi)ResultsCyclesStress

(ksi)

Ti-6Al-4V ELI (Wrought, Mill Anneal)

St Dev = 5.7

Average = 89.4

90.0Runout14,270,00090.0

95.0Runout10,700,00095.0

96.0Runout10,755,70096.0

96.0Runout10,132,90096.0

81.8Fracture74,50098.0

87.2Fracture3,370,80098.0

83.7Fracture242,400100.0

83.4Fracture32,100100.0

91.7Fracture36,200110.0

EFL (ksi)ResultsCyclesStress

(ksi)

Ti-5553 (Cast+HIP+HT)

92.4Runout13,981,40092.4

St Dev = 5.2

Average = 90.1

90.2Runout16,909,30090.2

93.5Runout16,620,80093.5

93.9Runout15,113,60093.9

79.1Fracture490,90094.0

94.1Runout11,343,70094.1

94.1Runout11,639,90094.1

82.3Fracture2,422,10094.2

94.4Runout12,337,60094.4

86.6Fracture4,697,50095.0

90.5Fracture6,989,80095.2

EFL (ksi)ResultsCyclesStress

(ksi)

Ti-5553 (Cast+HIP+Sinter+HT)

Endurance Limit ≅ 100 ksi Endurance Limit ≅ 96 ksiEndurance Limit ≅ 94 ksi

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Metallurgy

Notched Rotating Beam Fatigue

27.9Runout16,715,70027.9

St Dev = 1.4

Average = 26.2

25.3Runout47,066,70025.3

27.2Runout50,360,20027.2

27.4Runout15,613,60027.4

27.4Runout19,529,50027.4

23.7Fracture333,20028.2

25.5Fracture4,006,60028.3

25.6Fracture3,857,40028.5

26.1Fracture1,640,20030.4

EFL (ksi)ResultsCyclesStress

(ksi)

Ti-6Al-4V ELI (Wrought, Mill Anneal)

23.0Runout15,589,30023.0

St Dev = 8.6

Average = 29.6

20.0Runout10,079,90020.0

25.0Runout35,571,80025.0

25.0Runout13,130,50025.0

21.9Fracture2,562,30025.0

30.0Runout11,427,20030.0

30.0Runout11,100,50030.0

33.4Fracture85,30040.0

41.7Fracture25,60050.0

45.8Fracture17,20055.0

EFL (ksi)ResultsCyclesStress

(ksi)

Ti-5553 (Cast+HIP+HT)

26.1Runout11,895,55026.1

St Dev = 2.6

Average = 27.5

25.1Runout33,321,30025.1

27.0Runout13,757,60027.0

23.1Fracture671,00027.3

27.7Runout40,295,30027.7

27.7Runout12,116,70027.7

29.2Runout13,837,40029.2

25.1Fracture396,70029.9

30.0Runout51,636,50030.0

30.1Runout13,030,40030.1

31.9Runout10,393,50031.9

EFL (ksi)ResultsCyclesStress

(ksi)

Ti-5553 (Cast+HIP+Sinter+HT)

Endurance Limit ≅ 28 ksiEndurance Limit ≅ 30 ksi

Endurance Limit ≅ 30 ksi

Kt = 2.9

Metallurgy Research

Metallurgy

Rotating Beam Fatigue

15

20

25

30

35

40

45

1.E+04 1.E+05 1.E+06 1.E+07 1.E+

Ti-6Al-4V ELI (Wrought, Mill Anneal)Ti-5553 (Cast+HIP+HT)Ti-5553 (Cast+HIP+Sinter+HT)

70

80

90

100

110

120

1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

Ti-6Al-4V ELI (Wrought, Mill Anneal)Ti-5553 (Cast+HIP+HT)Ti-5553 (Cast+HIP+Sinter+HT)

Smooth RBF Notched RBF (Kt=2.9)

104 105 106 107 108 104 105 106 107 108

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Metallurgy

Conclusions

• Fatigue properties of cast Ti-5Al-5V-5Mo-3Cr comparable to wrought mill annealed Ti-6Al-4V ELI

• Sintering cycle did not adversely affect the fatigue properties of cast Ti-5553.

• Cast Ti-5553 viable alternative to current wrought or forged Ti-64 products.

• Future studies: modular applications, evaluate the fretting properties