Research Summary

The Effect of Plasma-Sprayed Coatings on the Fatigue of Titanium Alloy Implants

Todd Smith

The application of titanium plasma-spray coatings to Ti-6AI-4V orthopedic implants results in a dramatic decrease in high-cycle fatigue performance. The better bonding of the plasma sprayed and heat-treated implants results in a lower high-cycle fatigue strength. Therefore, the use of plasma-spray textured coatings on implants must be considered with caution.

Figure 1. An SEM micrograph of the sintered spherical metal powder coatings showing the three-dimensional network of interconnected pore space into which bone tissue can grow.

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INTRODUCTION

Until the late 1970s, polymethylmethacrylate bone cement was the predominant means of fixating a joint replacement implant to the skeletal system. At that time, however, bone cement was implicated as a major contributor to implant loosening, because of its brittle nature and its poor interfacial relationship with the metallic implant and bone tissue. l -4

The use of porous coated implants for long-term biological fixation has been receiving an enthusiastic response, especially when the patients are younger and more active.5- 7 Several types of porous systems have been used in clinical investigations. While all of these systems are porous, each is morphologically unique, as it functions as a dynamic interface between living tissue and the implant. Since the physical characteristics of the porous coating may have significant effects on the long-term performance of the arthroplasty, it becomes necessary to accurately characterize the effects the coating has on mechanical performance of the device. A joint replacement device under normal repetitive physiological loading can incur very high stress levels, so it is imperative to assess the high-cycle fatigue (HCF) performance of the device.

It is an unfortunate reality that the more robust total hip patients often require femoral implants with smaller diameters. Many patients also tend to be overweight. Additionally, the loading that occurs across the joint during normal walking is as high as five to six times the patient's body weight.s The result is that the stresses incurred by the implant may be a significant fraction of the endurance limit of the implant material. High-cycle fatigue is usually taken to be 107 cycles in the orthopedic business, because a patient may take as many as 106 steps per year and the implant should last ten years to ensure patient satisfaction.

Most porous coating systems are produced by sintering either metal powders or chopped metal wires to the implant substrate. They usually produce a network of three-dimensional porosity into which bone tissues can grow, thereby fixating the implantto the bone. Figure 1 shows a low-magnification scanning electron microscopy (SEM) micrograph of a sintered metal powder coating marketed by DePuy (Warsaw, Indiana) under the trade name Porocoat@ porous coating. An alternative coating method has been devised using plasma-spray techniques.9 Under normal circum­stances, plasma spraying is used to produce a fully dense, well-bonded layer of metal or ceramic on a metallic substrate. In this application, the spraying parameters are adjusted such that the metal powder or wire being injected into the plasma is only partially melted as it is being accelerated toward the substrate. The result is that the condensate is somewhat porous, or at least textured at its surface, as shown in Figure 2. Nevertheless, there is some evidence that bone tissue will integrate onto the textured coating, providing some degree of implant fixation.

A consequence of the use of sinter-processed powder or chopped-wire porous coatings on a titanium alloy (Ti-6AI-4V) substrate is a reduction of the HCF strength

Figure 2. An SEM micrograph of the plasma­sprayed coating. Partial melting of the metal being injected causes texturing.

Figure 3. A metallographic cross-sectional view through the sintered metal powder coat­ing interface with the implant substrate.

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from 586-620 MPa to 172-206 MPa, a 70% reduction. lo This dramatic decrease in the endurance limit is the result not of the change in microstructure from the equiaxed alpha-beta to the acicular during the course of sintering attemperatures above the beta transus, but rather the creation of a defect containing a micronotched coating on the surface of the implant. The sinter bonding sites of the wires or powder beads of the coating to the substrate interrupt the otherwise smooth surface and, therefore, act as stress risers, as can be seen on the micrograph presented in Figure 3.11-13

It is well accepted that the Sinter-processed powder and wire coatings have a significant effect on HCF strength. This is especially true for titanium alloys, as most exhibit a high degree of notch sensitivity. The effect of plasma spray coatings on HCF strength is much less well known. In fact, information exists that indicates that the plasma spray-coated implant is immune from any of the deleterious effects experi­enced by the sinter-processed, titanium-coated implants. Theoretically, if the plasma spray coating is well bonded (metallurgically) to the substrate, it will behave in a fashion similar to the conventionally sintered porous coatings and exhibit reduced HCF performance. Therefore, the purpose of the investigation presented here was to determine whether the plasma-sprayed textured coating of commercial purity (CP) titanium onto a Ti-6Al-4V alloy implant substrate adversely affected HCF perfor­mance.

EXPERIMENTAL PROCEDURES

Testing previously conducted in DePuy's biomechanics laboratory indicated the inadequacy of the rotating beam method of HCF testing of porous coated specimens. For this study, actual implants were tested in a fashion similar to that proposed by Semlitschl4 using an MTS 810 closed-loop servo-hydraulic testing machine. Figure 4 is a schematic representation of the test setup.

Fifteen femoral hip implants of DePuy's Ti-6Al-4V alloy STD Plus® were obtained. These implants are machined complete from wrought, mill-annealed, extra-low interstitial Ti-6Al-4V alloy bar. Three of the implants were left uncoated to serve as control specimens; the other 12 implants were plasma-spray coated using CP titanium powder by a commercial coater with experience coating dental and orthopedic products (Artech Corporation, Falls Church, Virginia). Six of the coated implants were subsequently subjected to a heat treatment in an attempt to improve the bond between the coating and substrate. It was conducted in a high-vacuum furnace at 732°C for two hours. Only the proximal lateral half of the implant was plasma-spray coated (see Figure 4) to minimize any contributions of the coating to the section properties of the implant.

The implants were mounted with their stem axis parallel to the loading vector of the MTS load frame and were mounted in fixtures such that the proximal 50 mm of the implant was left unsupported, thereby allowing it to flex freely under load. In this position, about 6.4 mm of the stem below the coated region remained above the potting medium. Wood's metal was used as the potting medium to secure the implants to the loading fixture. Because of the proximal-to-distal taper of the implant stem, the section modulus of the implant stem decreased from top to bottom. Therefore, the highest bending moment and stress were in the uncoated region of the lateral stem, as indicated in Figure 4.

The implants were cycled at a frequency of 15 Hz and were continuously bathed in recirculating distilled water at a temperature of 37°C. The initial maximum load was selected to result in a stress of 345 MPa at the highest stress region of the stem. A staircase loading regimen was utilized where the implants were cyclically loaded to 107 cycles or failure. Following 107 cycles, the load was increased by 10% and the sample was retested until failure occurred. If the initial loading conditions caused failure prior to 107 cycles, the load was reduced with speCific regard to the number of cycles achieved. The cyclic load ratio was R = 0.1.

Accurate mass section properties at the location of the fracture were determined for each implant from precision microphotographs of the fracture surface. Knowing the loading moment and section properties, the fracture stress was determined.

An extrapolated fatigue limit (EFL) for 107 cycles was determined from the last stress level at which the implant failure occurred, the number of cycles at that stress level, and the highest stress level at which 107 cycles was achieved. The formula utilized was:

EFL = (fracture stress [in psil- IS) + [(cycles @ failure) x Ratiol

where IS = incremental stress (psi), and Ratio = IS/107•

The EFL is an approximation of the fatigue limit. It assumes a linear relationship between the last stress at which 107 cycles is achieved and the number of cycles achieved at failure. It is assumed that the stress vs. number of cycles curve can be approximated by a straight line for short intervals and the HCF limit is between the highest stress achieved at 107 cycles and the failure stress. Two conditions were imposed on the above method for determining HCF strength. The stress increments are small and each specimen had to attain a minimum of 106 cycles at a single stress level or attain 107 cycles at the previous stress level to have an EFL calculated.

1994 February • JOM

Load

Figure 4. The implant fixturing and setup in the MTS fatigue-testing machine. UHMWPE means ultrahigh molecular-weight polyethyl­ene.

Figure 5. A typical implant after fatigue testing showing the location of the fracture relative to the lower margin of the plasma spray coating.

Figure 6. A metallographic cross-sectional view through the plasma-spray coating in­terface with the implant substrate. Note the low level of porosity in the coating and the intermittent bonding with the substrate. Com­pare to Figure 3.

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Table I. HCF Data for Plasma-Sprayed Implants

Stress Cycles EFL Sample (MPa) (106) (MPa)

1 407 0.2 2 326 0.3 3 469 0.5 425 4 523 0.3 314 5 436 0.5 394 6 343 8.9 339 AverageEFL 368

Table II. HCF Data for Plasma-Sprayed and Heat-Treated Implants

Stress Cycles EFL Sample (MPa) (106) (MPa)

7 306 0.1 8 194 0.5 9 241 3.2 217 10 246 0.5 224 11 406 0.3 343 12 249 6.2 236 Average EFL 256

Table III. HCF Data for the Control Implants (Non-Plasma Sprayed)

Stress Cycles EFL Sample (MPa) (106) (MPa) --A 621 0.2 594 B 621 10.2 621 C 621 10.9 621 AverageEFL 612

ABOUT THE AUTHOR

Todd Smith earned his B.S. in metallurgy and materials science at Lehigh University in 1975. He is currently director of research at DePuy in Warsaw, Indiana. Mr. Smith is also a member of TMS.

For more information, contact Todd Smith, Di­rector of Research, DePuy, P.O. Box 988, War­saw, Indiana 46581-0988.

Metallographic analysis was performed to assess the microstructural condition of the control and processed implants.

Fracture analysis was conducted on the SEM to determine the location of fatigue crack initiation.

RESULTS AND DISCUSSION

Each of the 12 coated implants fractured through the coating near its bottom margin, indicating a strong preference for crack initiation to occur in the coated region of the stem (Figure 5). Each of the three uncoated control implants failed at the level of the potting medium, as expected. The application of a titanium plasma-spray coating to the surface of Ti-6AI-4V alloy implant resulted in a reduction of the fatigue limit by 40%; comparing the heat-treated implants with the uncoated control implants re­vealed a 58% reduction (Tables I-III). Microstructural analysis verified that neither the plasma spraying process nor the sub-beta heat treatment altered the microstructure from the equiaxed alpha-beta microstructure of the original implant. SEM examina­tion of the fracture surfaces indicated that the fatigue crack initiation site for all implants tested was atthe outermost surface on the lateral side of the implant stem. For the plasma-sprayed implants, this site was at the interface between the coating and substrate, at a point where the coating appeared to be metallurgically bonded to the substrate.

As with conventional sintered porous coatings, the application of a coating that contains "defects," regardless of the substrate microstructure and even with limited metallurgical bonding to the substrate, effectively bypasses the crack initiation stage of the HCF mechanism, allowing crack propagation and eventual failure to occur at much lower load (stress) levels. The addition of the post-coating heat treatment to improve coating bond strength resulted in a further reduction in HCF strength, most likely due to a higher frequency of bonding sites between the coating and substrate and a more intimate metallurgical bond at those sites. It may be that the more intimate bonding permits easier propagation of cracks originating in the highly convoluted coating into the substrate. Figure 6 shows a metallographic cross section through the porous coating into the substrate. The intermittent metallurgical bonding of the coating to the substrate is very apparent. It was also learned that in preparation for plasma spraying the substrate is grit blasted in an attempt to improve the bond with the coating. Previous studies in DePuy's laboratory have indicated a 10% reduction in HCF strength by grit blasting alone. This may also have affected this study.

References 1 H.C. Amstutz, K.L. Markol!. G.M McNeice, and T.A. Gruen, "Loosening of Total Hlp Components Cause and Prevention," The HIp: Proceedings of the Fourth Open Scientzftc Meeting of the HIp Society, ed. C V. Mosby (St Louis, MO The Hip SoCiety, 1976), pp 102-116. 2. T.A. Gruen, G.M McNeice, and H D Amstutz, "Modes of Fauure' Cemented Stem-Type Femoral Components-A Radio­graphic AnalYSIS of Loosenmg," ClImcal OrthopaediCS, 141 (1979), pp 17-27. 3 H.G Willert, J Ludwig, and M Senllitsch, "Reaction of Bone to Methacrylate After Hlp Arthroplasty' A Long-Term Gross, Light M,croscop'c and Scanning Electron MICroscopic Study," J. Bone and Joint Surgery, 56A (1974), p. 1368 4 M.D. Willert, "Tissue Reactions Around Joint Implants and Bone Cement," Symposium on Arthroplasty of the HIp, ed. G. Chapcal (Stuttgart, Germany: Thieme, 1973), pp. 11-21 5 T.5. Smith, "Rationale for BIOlOgical Fixation of ProsthebGDevlces," SAMPE Journal, 21 (3) (May/June 1985) 6 C A. EnghandJ D. Bobyn, "Biologic F,xahon of Hlp ProsthesIs' A ReviewoftheClmlcaIStatusandCurrentConcepts," Advances In OrthopaediC Surgery, 18 (1984), pp 136-149 7 J P. Colher, FE Kennedy, M B Mayor, and C a Townley, "Stress Dtstnbuhon in the Human Femur: The Role of Femoral ProstheSIS Geometry and the Mechamcs ofFlxabon," TransactlOns,30th Annual Meetmg of the Orthopaedic Research Society (Atlanta, GA: Orthopaedic Research Society, 1984). 8 Dana C Mears, Materials and OrthopaediC Surgery, p 508 9. Hahn, U.S. patent 3,665,123. . 10. Zimmer Techrucal Documentabon, "Fabgue and Porous Coated Implants" (Warsaw, IN. Zuruner, 1984). 11 S Yue, RM. Pillar, and G.C Weatherly, "The Fabgue Strength of Porous Coated-Coated Ti-6AI-4V Implant Alloy," J. BlOmed Mater. Res, 18 (1984), pp 1043-1058. 12 D. Wolfarth, M. Flliaggi, and P. Ducheyne, "Parametnc Analysis of InterfaCIal Stress Concentrahons in Porous Coated Implants," J Appl. BlOmaterlals, 1 (1990), pp. 3-12. 13 D Wolfarth and P Ducheyne, "Effect 01 Porous Coating Geometry on InterfaCIal Stress Under a Shear Load," J. BlOmed. Mater Res, 27 (1993), pp 1585-1590 14. M Senllitsch and B Pamc, Ten Years of Expenence With Test Cnterla for Fracture-Proof Anchorage Stems of ArtlftclOl HIp Jomts (Medizmaltechnik, 1983)

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