J Am Acad Orthop Surg, Vol 16, No suppl_1, July 2008, S94-S100.
© 2008 the American Academy of Orthopaedic Surgeons
How do material properties influence wear and fracture mechanisms?
Clare Rimnac, PhD and
Lisa Pruitt, PhD
Dr. Rimnac is Wilbert J. Austin Professor of Engineering and Chair, Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH. Dr. Pruitt is Lawrence Talbot Professor of Engineering, Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA.
*The Implant Wear Symposium 2007 Engineering Work Group included Donald L. Bartel, PhD, Thomas D. Brown, PhD, Ian C. Clarke, PhD, Roy D. Crowninshield, PhD, Darryl D’Lima, MD, PhD, A. Seth Greenwald, DPhil(Oxon), Steven M. Kurtz, PhD, Jack Lemons, PhD, Michael T. Manley, PhD, Harry A. McKellop, PhD, Orhun K. Muratoglu, PhD, Ebru Oral, PhD, Lisa Pruitt, PhD, Clare Rimnac, PhD, Peter S. Walker, PhD, and Timothy Wright, PhD.
Dr. Rimnac or a member of her immediate family has received research or institutional support from the National Institutes of Health, Sulzer, Stryker Orthopaedics, and Zimmer. Dr. Pruitt or a member of her immediate family has received research or institutional support from the National Science Foundation.
 |
Abstract
|
|---|
The wear and fracture mechanisms of ultra-high–molecular-weight polyethylene (UHMWPE) hip and knee implant components are of great interest. The material properties of UHMWPE are affected by ionizing radiation as used for sterilization and cross-linking. Cross-linking with high-dose irradiation has been shown to improve the wear resistance of UHMWPE. However, cross-linking leads to a loss in properties such as ductility and resistance to fatigue crack propagation. Highly cross-linked UHMWPE may be more susceptible than conventional UHMWPE to fracture under severe clinical conditions (eg, impingement). Contemporary hip and knee simulator studies provide good information with which new UHMWPE formulations can be screened for clinical wear performance. However, comparable methodologies are lacking for screening UHMWPEs for fracture resistance. Mechanical tests as well as computational material and structural models should be developed to evaluate the combined effect of material and geometry (structure) on fracture resistance under clinically relevant loading conditions.
Ultra-high–molecular-weight polyethylene (UHMWPE) is extensively used as a bearing material in hip and knee replacements. Thus, it is of great interest when considering the wear and fracture mechanisms of implant components. UHMWPE is a member of the polyethylene family of polymers. The International Organization for Standardization defines UHMWPE as having a molecular weight 
1 million g/mol; the American Society for Testing and Materials specifies that UHMPWE should have an average molecular weight >3.1 million g/mol.1 UHMWPE formulations used in orthopaedic applications typically have a molecular weight between 2 and 6 million g/mol. UHMWPE is a linear (nonbranching), semicrystalline polymer. It is initially manufactured as powder, with a crystallinity of 60% to 75%, depending on the resin. Orthopaedic components may be made by direct compression molding or by machining from ram-extruded rod or compression-molded sheets.
The physical and mechanical properties of UHMWPE are altered by variations in resin and in the process used to consolidate the powder. Native UHMWPE typically has a density of 0.932 to 0.945 g/mL, an elastic modulus of 0.8 to 1.6 GPa, a tensile yield strength of 21 to 28 MPa, an elongation at fracture of 350% to 525%, and an ultimate stress of 30 to 48 MPa.1 As with any polymer, the mechanical properties of UHMWPE are both rate- and temperature-dependent. Thus, the wear damage and surface deformation of total joint components are influenced by plastic flow and yielding.2
|
Effect of Ionizing Radiation on UHMWPE
|
|---|
Oxidation
The physical and mechanical properties of UHMWPE are also altered by ionizing radiation, such as that used for sterilization or for deliberate cross-linking to improve wear resistance.1-3 In addition to the immediate effects of radiation, oxidative degradation of UHMWPE components radiation-sterilized in air-permeable packaging occurs during shelf storage before implantation and continues during in vivo use.1,3,4 Oxidation embrittles UHMWPE, leading to a decrease in the elongation to failure, an increase in elastic modulus, and a decrease in fatigue crack propagation resistance. Oxidative degradation has resulted in the premature failure of UHMWPE components in vivo. Contemporary radiation sterilization practices for UHMWPE have largely eliminated the potential for shelf aging. However, in vivo oxidation remains a concern for gamma-sterilized inert packaged hip and knee components because of the potential for long-term failure resulting from degradation of mechanical properties and embrittlement of the polymer.5,6
Cross-linking
Cross-linking with high-dose irradiation improves the adhesive and abrasive wear resistance of UHMWPE compared with conventional UHMWPE (non–cross-linked, or lightly cross-linked during radiation sterilization). In the United States, the first-generation highly cross-linked UHMWPEs were produced using 50 to 105 kGy of either gamma or electron beam radiation, depending on the manufacturer and the process. In general, cross-linking adversely affects uniaxial ductility, fracture toughness, and fatigue crack propagation resistance.2,7-10 Interestingly, an additional benefit of cross-linking may be reduction in elastic modulus, as this reduction results in lower contact stresses in UHMWPE components.
Radiation-induced cross-linked UHMWPE materials contain free radicals that can lead to oxidative degradation; thus, postprocessing is usually done to reduce or eliminate free radicals (ie, remelting above the melt transition or annealing below the melt transition). Although effective at eliminating entrapped free radicals, remelting leads to a reduction in crystal size, resulting in a reduction in yield stress, ultimate stress, and fatigue crack propagation resistance.1,2 In contrast, annealing better preserves the original crystal structure and better retains mechanical properties than does remelting. However, annealing reduces free radicals less effectively than does remelting; thus, in vivo oxidation is possible. In second-generation cross-linked UHMWPEs, methods to enhance oxidation resistance or to reduce or stabilize free radicals more effectively without having to remelt the material (eg, sequential radiation/annealing cycles, doping with vitamin E) have been introduced as a means to maintain crystallinity and, therefore, retain better mechanical properties for cross-linked UHMWPE.11
A trade-off exists for improved wear via cross-linking of UHMWPE in that there is a loss in strength, ductility, fatigue crack propagation resistance, and toughness.12,13 Figure 1 shows the concomitant fatigue crack propagation resistance of conventional and highly cross-linked UHMWPE formulations listed in Table 1.14 UHMWPE formulations with higher crystallinity and with little or no cross-linking (ie, conventional) provide the greatest resistance to fatigue crack propagation. Highly cross-linked UHMWPE formulations with lower crystallinity offer significantly reduced fatigue crack propagation resistance and easier crack inception. The reduction in fatigue crack propagation resistance in highly cross-linked UHMWPE formulations can be partially mitigated by postprocessing below the melt transition (annealing) rather than above the melt transition (remelting), as the former better preserves crystallinity. However, the relationship between mechanical properties and microstructure (eg, crystallinity, size and distribution of crystalline lamellae, cross-linking network) is complex.14,15 More research is needed to characterize the microstructures and related properties necessary for optimum resistance to crack inception, growth, and fracture.
View larger version (37K):
[in this window]
[in a new window]
|
Figure 1 Fatigue crack propagation rate as a function of cyclic stress intensity for several ultra-high–molecular-weight polyethylene (UHMWPE) material groups. Three distinct groupings of fatigue resistance are demonstrated: (1) a highly fatigue-resistant group composed of conventional (non–cross-linked) UHMWPE (A, B); (2) a moderately fatigue-resistant group composed of UHMWPE with varying cross-link dosages and heat treatments (C, D, E); and (3) the least fatigue-resistant group, composed of highly cross-linked UHMWPE that has been melt-annealed (F, G, H, I).14 CM = compression molded, RE = ram extruded. (Adapted with permission from Atwood SA, Furmanski J, Hoang M, Pruitt L: The relation of lamellar microstructure and mechanical properties to fatigue crack propagation behavior of ultra high molecular weight polyethylene. Presented at the 2nd International Conference on Mechanics of Biomaterials and Tissues, Kauai, Hawaii, December 9-13, 2007.)
|
|
View this table:
[in this window]
[in a new window]
|
Table 1 Material Properties of Conventional (Non–cross-linked) and Cross-linked UHMPWE for a Variety of Doses and Heat Treatments*
|
|
|
Fracture of UHMWPE Components
|
|---|
In vivo fracture of conventional and highly cross-linked UHMWPE hip components has been reported.16-20 Hip simulator21 and fatigue crack propagation studies suggest that highly cross-linked UHMWPE may be more susceptible than conventional UHMWPE to fracture under severe clinical conditions (eg, impingement), although the mechanical properties of conventional UHMWPE can be expected to deteriorate with time in vivo as a result of oxidative degradation. Several instances of fracture-related early failures of highly cross-linked UHMWPE have recently been reported.17-20 Retrieval analysis of cross-linked UHMWPE acetabular total hip arthroplasty components has shown evidence of fracture mechanisms and fatigue fractures under impingement conditions that support the hypothesis that fatigue fracture can occur in both tibial total knee arthroplasty components and acetabular components.
A recent case series by Furmanski et al20 examined four rim fractures in moderately (5 mrad) and highly (10 mrad) cross-linked remelted or annealed UHMWPE acetabular cups. The fracture surfaces of these retrievals were analyzed with scanning electron microscopy to determine the failure mechanisms and identify common fracture patterns. A three-dimensional finite element analysis was developed for each design to determine whether forces associated with cup impingement could produce the observed failures. The retrieved acetabular cups had fractures along the rim with crescent-shaped fragments separated from the remaining interior of the component. The fractures initiated on the outer surface of the liner at a stress concentration or material discontinuity. Scanning electron microscopy at the site of crack of initiation revealed clamshell markings. Figure 2 shows the typical fractography of one of the four fractured liners, with gross fracture initiating at the sharp corner of a notch in the component. Characteristically brittle fracture processes and crack growth are also evident in the river markings observed in the micrograph. This fractography, which is representative of the four fractured components of the series, suggests that cross-linked UHMWPE may be susceptible to fracture in the vicinity of stress concentrations, discontinuities, or flaws. Further, the cracks appear to have been initiated by the action of normal tensile stresses acting in the liner. Cracks preferentially propagated on the inner (bearing) surface of the components, suggesting that the interior surface may be in a more tensile state of stress than the exterior of the liner in that same region. This hypothesized mechanism was supported by the finite element analysis, which revealed that maximum principal stresses due to impingement were on the order of 15 to 40 MPa and were greatest in the observed locations of crack initiation and propagation.
View larger version (94K):
[in this window]
[in a new window]
|
Figure 2 Scanning electron micrograph of the fracture surface of a fractured cross-linked acetabular liner. The fracture initiated at the outer perimeter of the metal shell and liner. Clamshell markings (bottom) are evidence of fatigue crack propagation from this location, proceeding to form the crescent-shaped primary fracture surface and leading to river markings (top), which are indicative of a brittle fracture process.20
|
|
For interpretation of these stress magnitudes, an approximated value of stress required for initial crack propagation was derived from fatigue crack propagation data and component geometry. For a value of stress intensity, K=Y
a, where K is the stress intensity, Y is a dimensionless geometry factor, is the far-field (well away from the crack tip) tensile stress, and a is the effective crack length required for the inception of crack propagation of 0.7 MPam, and a crack the effective length of which is the full depth of the notch (2 mm in this case) with Y = 1, the stress required for crack propagation is approximately 8.8 MPa. Thus, the presence of a stress concentration, coupled with the tensile stresses resulting from impingement loads in the cross-linked liners, was sufficient to propagate fatigue cracks. Overhanging UHMWPE, thin UHMWPE at the rim, sharp corners or discontinuities along the metal shell, and rim notches all contributed to fracture failure in the cross-linked liners when subjected to impingement.
Furmanski et al20 hypothesized that implants of similar design to these that remain in functional clinical use could have nascent or noncritical cracks initiated near the rim and that the incidence of fracture could increase with time in vivo. The findings of early fractures indicate that cross-linked UHMWPE components should be employed in a way that mitigates the ability of cracks to initiate or propagate. Designs using cross-linked UHMWPE should aim to reduce stress concentrations, unsupported rims, and likelihood for impingement.
Loss of ductility and static fracture and fatigue crack propagation resistance is a primary challenge in designing cross-linked UHMWPE for total joint arthroplasties. Furmanski and Pruitt22 have shown that, in the presence of a stress concentration, conventional UHMWPE fractures in a characteristically brittle manner, with the rate of crack advance having both a cyclic and a static component:
da = [
a/N] t dN + [a/t] N dt
The first term denotes the cyclic component (where N is a cycle of load), and the second term denotes the time-dependent component of crack growth (where t is time). Moreover, the fatigue crack propagation behavior is dominated by the peak stress intensity
(Kmax): da/dN 
CKmaxq
This dependence on Kmax indicates that cracks can propagate in the absence of significant cyclic loading; that is, conventional and cross-linked UHMWPE can potentially fail under long-term static loading conditions. Such findings indicate that 
Kincep (the stress intensity required for crack inception) by itself is not a sufficient design criteria and that more research is needed to characterize the fatigue and fracture resistance of cross-linked UHMWPE as relevant to both normal and adverse in vivo loading conditions.
Cross-linked UHMWPE may be susceptible to fatigue fracture as its inception stress intensity (the stress intensity range, K, needed to propagate a crack at a rate of 10–6 mm/cycle) is reduced in comparison with conventional unaged UHMWPE.23 In fact, the inception stress intensity of UHMWPE cross-linked with 10 mrad of gamma radiation is comparable to that of conventional UHMWPE that has been gamma-sterilized in air and shelf-aged for 5 years.24 Thus, the immediate loss of fatigue resistance is a limitation in the mechanical properties of cross-linked UHMWPE.
|
Predicting Performance of UHMWPE Components
|
|---|
To predict UHMWPE component behavior, accurate predictions must be made of the multiaxial loading response of UHMWPE materials. A constitutive model framework that incorporates the characteristic deformation behavior of UHMWPE (eg, initial linear elastic response, distributed yielding, large-scale yielding, orientation hardening at large strains) has been developed.25,26 The new constitutive hybrid model is micromechanism-inspired and builds on prior models for glassy, semicrystalline polymers. The model accurately predicts the deformation behavior of conventional and highly cross-linked UHMWPE materials under complex loading conditions, including the triaxial stress-state condition that occurs in the vicinity of a component-type notch.27 The model also accurately predicts fracture under monotonic and equibiaxial (small-punch test) loading conditions.28 Fracture appears to be dependent on a critical chain stretch-failure criterion (Figure 3). This material model should allow for better finite element analysis predictions of static and cyclic fracture of UHMWPE components.
View larger version (20K):
[in this window]
[in a new window]
|
Figure 3 Comparison of monotonic failure criteria demonstrating that a chain-stretch failure criterion predicts uniaxial monotonic fracture better than do other failure criteria. (The quality of the predictions was based on the coefficient of determination [r2] between experimental failure values and the predicted failure value; bars represent the average r2 for two conventional and two highly cross-linked UHMWPE materials.) (Reproduced with permission from Bergström JS, Rimnac CM, Kurtz SM: Molecular chain stretch is a multiaxial failure criterion for conventional and highly crosslinked UHMWPE. J Orthop Res 2005;23:367-375.)
|
|
|
Future Directions for Research
|
|---|
Contemporary hip and knee simulator studies provide good information with which new UHMWPE formulations can be screened for clinical wear performance. However, comparable methodologies are lacking for screening UHMWPEs for fracture resistance, particularly with respect to structural performance. Better predictions are needed as to which UHMWPE formulation and component geometry combinations lead to increased fracture risk in vivo.
Specifically, standardized mechanical tests (ie, uniaxial tension, fracture toughness, cyclic stress versus life [S/N] fatigue, fatigue crack propagation) should be established to evaluate fracture resistance under different conditions, then used to rank fracture resistance of new UHMWPE formulations. Mechanical tests should be developed to evaluate the combined effect of material and geometry (structure) on fracture resistance; these could include notched tensile and fatigue tests27,29 and tests of standardized component geometries. Finally, further development should be pursued of computational material and structural models that are predictive of fracture behavior of material and geometry combinations under clinically relevant loading conditions.
|
Figures
|
|---|
|
Tables
|
|---|
|
References
|
|---|
- Kurtz SM: The UHMWPE Handbook: Ultra-High Molecular Weight Polyethylene in Total Joint Replacement. New York, NY: Elsevier Academic Press, 2005.
- Pruitt LA: Deformation, yielding, fracture and fatigue behavior of conventional and highly cross-linked ultra high molecular weight polyethylene. Biomaterials 2005; 26:905-915. [ISI][Medline]
- Rimnac CM, Kurtz SM: Ionizing radiation and orthopaedic prostheses. Nucl Instrum Methods Phys Res B 2005; 236:30.
- Kurtz SM, Rimnac CM, Hozack W, et al: In vivo degradation of polyethylene liners after gamma sterilization in air. J Bone Joint Surg Am 2005; 87:815-823. [Abstract/Free Full Text]
- Kurtz SM, Hozack WJ, Purtill JJ, et al: 2006 Otto Aufranc Award Paper: Significance of in vivo degradation for polyethylene in total hip arthroplasty. Clin Orthop Relat Res 2006; 453:47-57. [Medline]
- Currier BH, Currier JH, Mayor MB, Lyford KA, Van Citters DW, Collier JP: In vivo oxidation of gamma-barrier–sterilized ultra–high-molecular-weight polyethylene bearings. J Arthroplasty 2007; 22:721-731. [ISI][Medline]
- Kurtz SM, Villarraga ML, Herr MP, Bergström JS, Rimnac CM, Edidin AA: Thermomechanical behavior of virgin and highly crosslinked ultra-high molecular weight polyethylene used in total joint replacements. Biomaterials 2002; 23:3681-3697. [ISI][Medline]
- Gencur SJ, Rimnac CM, Kurtz SM: Failure micromechanisms during uniaxial tensile fracture of conventional and highly crosslinked ultra-high molecular weight polyethylenes used in total joint replacements. Biomaterials 2003; 24:3947-3954. [ISI][Medline]
- Goldman M, Pruitt L: Comparison of the effects of gamma radiation and low temperature hydrogen peroxide gas plasma sterilization on the molecular structure, fatigue resistance, and wear behavior of UHMWPE. J Biomed Mater Res 1998; 40:378-384. [ISI][Medline]
- Simis KS, Bistolfi A, Bellare A, Pruitt LA : The combined effects of crosslinking and high crystallinity on the microstructural and mechanical properties of ultra high molecular weight polyethylene. Biomaterials 2006; 27:1688-1694. [ISI][Medline]
- Oral E, Christensen SD, Malhi AS, Wannomae KK, Muratoglu OK: Wear resistance and mechanical properties of highly cross-linked, ultra-high– molecular weight polyethylene doped with vitamin E. J Arthroplasty 2006; 21:580-591. [ISI][Medline]
- Baker DA, Hastings RS, Pruitt L: Study of fatigue resistance of chemical and radiation cross-linked medical grade ultrahigh molecular weight polyethylene. J Biomed Mater Res 1999; 46:573-581. [ISI][Medline]
- Bradford L, Baker D, Ries MD, Pruitt LA: Fatigue crack propagation resistance of highly cross-linked polyethylene. Clin Orthop Relat Res 2004; 429:68-72. [Medline]
- Atwood SA, Furmanski J, Hoang M, Pruitt L: The relation of lamellar microstructure and mechanical properties to fatigue crack propagation behavior of ultra high molecular weight polyethylene. Presented at the 2nd International Conference on Mechanics of Biomaterials and Tissues, Kauai, Hawaii, December 9-13, 2007.
- Oral E, Malhi AS, Muratoglu OK: Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE. Biomaterials 2006; 27:917-925. [ISI][Medline]
- Garcia-Rey E, Garcia-Cimbrelo E: Long-term results of uncemented acetabular cups with an ACS polyethylene liner: A 14-16-year follow-up study. Int Orthop 2007; 31:205-210. [ISI][Medline]
- Bradford L, Kurland M, Sankaran H, Kim H, Pruitt L, Ries M: Early failure due to osteolysis in highly cross-linked ultra-high molecular weight polyethylene: A case report. J Bone Joint Surg Am 2004; 86:1051-1056. [Free Full Text]
- Halley D, Glassman A, Crowninshield RD: Recurrent dislocation after revision total hip replacement with a large prosthetic femoral head: A case report. J Bone Joint Surg Am 2004; 86:827-830. [Free Full Text]
- Tower SS, Currier JH, Currier BH, Lyford KA, Van Citters DW, Mayor MB: Rim cracking of the cross-linked longevity polyethylene acetabular liner after total hip arthroplasty. J Bone Joint Surg Am 2007; 89:2212-2217. [Abstract/Free Full Text]
- Furmanski J, Atwood S, Bal B, et al: Fractures of highly cross-linked UHMWPE acetabular liners. Proceedings of the 75th Annual Meeting of the American Academy of Orthopaedic Surgeons. Rosemont, IL: American Academy of Orthopaedic Surgeons, 2008, SE18.
- Holley KG, Furman BD, Babalola OM, Lipman JD, Padgett DE, Wright TM: Impingement of acetabular cups in a hip simulator: Comparison of highly cross-linked and conventional polyethylene. J Arthroplasty 2005; 20( 7 suppl 3 ):77-86. [ISI][Medline]
- Furmanski J, Pruitt L: Peak stress intensity dictates fatigue crack propagation in UHMWPE. Polymer 2007; 48:3512-3519.[ISI]
- Baker DA, Bellare A, Pruitt L: The effects of degree of crosslinking on the fatigue crack and propagation resistance of orthopedic-grade polyethylene. J Biomed Mater Res A 2003; 66:146-154.[Medline]
- Pruitt L, Ranganathan R: Effect of sterilization on the structure and fatigue resistance of medical grade UHMWPE. Materials Science & Engineering 1995; C3:91-93.
- Bergström JS, Rimnac CM, Kurtz SM: An augmented hybrid constitutive model for simulation of unloading and cyclic loading behavior of conventional and highly cross-linked UHMWPE. Biomaterials 2004; 25:2171-2178. [ISI][Medline]
- Kurtz SM, Bergström JS, Bowden AE, et al: Validation of hybrid model and ultimate chain stretch criterion for a second-generation highly crosslinked UHMWPE. Presented at the 6th Combined Meeting of the Orthopaedic Research Societies, Honolulu, Hawaii, October 20-24, 2007. Available at: http://www.ors.org/web/Transactions/6thCombinedMeeting/0498.PDF. Accessed March 26, 2008.
- Sobieraj MC, Kurtz SM, Rimnac CM: Notch strengthening and hardening behavior of conventional and highly crosslinked UHMWPE under applied tensile loading. Biomaterials 2005; 26:3411-3426. [ISI][Medline]
- Bergström JS, Rimnac CM, Kurtz SM: Molecular chain stretch is a multiaxial failure criterion for conventional and highly cross-linked UHMWPE. J Orthop Res 2005; 23:367-375. [ISI][Medline]
- Sobieraj MC, Kurtz SM, Manley M, Wang A, Rimnac CM: Multi-axial fatigue of a conventional and sequentially annealed highly crosslinked UHMWPE. Presented at the 6th Combined Meeting of the Orthopaedic Research Societies, Honolulu, Hawaii, October 20-24, 2007. Available at: http://www.ors.org/web/Transactions/6thCombinedMeeting/0024.PDF. Accessed March 26, 2008.
Top
Abstract
Effect of Ionizing Radiation...
