Jevan Furmanski

Clinical fracture of cross-linked UHMWPE acetabular liners.

Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is increasingly used as a bearing material in total hip replacements. Cross-linking of UHMWPE has been shown to increase wear resistance but decrease its fracture resistance. We analyzed the clinical fracture failure of four cross-linked UHMWPE total hip replacement components of four different designs via microscopic observation of the fracture surfaces, and found that in all cases fractures initiated at stress concentrations in an unsupported region of the component (termed the elevated rim). Finite element analyses (FEA) of each individual implant design were then conducted. Results from this analysis demonstrated that the predicted magnitude and orientation of maximum principal stress due to mechanical loading of the elevated rim was sufficient to propagate initiated fatigue cracks in each case. FEA also predicted that cracks may arrest after some amount of growth due to a steep stress gradient near the initiation site. Further, while anatomical positioning of the implant and material properties affect the risk of fracture, we examined whether these failures are strongly related to the notched elevated rim design feature that is common to the four failed cases presented here. We believe that cross-linked UHMWPE remains an excellent bearing material for total hip replacements but that designs employing this material should mitigate stress concentrations or other design features that increase the risk of fracture.

Tradeoffs amongst fatigue, wear, and oxidation resistance of cross-linked ultra-high molecular weight polyethylene

This study evaluated the tradeoffs amongst fatigue crack propagation resistance, wear resistance, and oxidative stability in a wide variety of clinically-relevant cross-linked ultra-high molecular weight polyethylene. Highly cross-linked re-melted materials showed good oxidation and wear performance, but diminished fatigue crack propagation resistance. Highly cross-linked annealed materials showed good wear and fatigue performance, but poor oxidation resistance. Moderately cross-linked re-melted materials showed good oxidation resistance, but moderate wear and fatigue resistance. Increasing radiation dose increased wear resistance but decreased fatigue crack propagation resistance. Annealing reduced fatigue resistance less than re-melting, but left materials susceptible to oxidation. This appears to occur because annealing below the melting temperature after cross-linking increased the volume fraction and size of lamellae, but failed to neutralize all free radicals. Alternately, re-melting after cross-linking appeared to eliminate free radicals, but, restricted by the network of cross-links, the re-formed lamellae were fewer and smaller in size which resulted in poor fatigue crack propagation resistance. This is the first study to simultaneously evaluate fatigue crack propagation, wear, oxidation, and microstructure in a wide variety of clinically-relevant ultra-high. The tradeoff we have shown in fatigue, wear, and oxidation performance is critical to the materials long-term success in total joint replacements.

Crack Initiation in Retrieved Cross-Linked Highly Cross-Linked Ultrahigh-Molecular- Weight Polyethylene Acetabular Liners An Investigation of 9 Cases

Nine cross-linked highly cross-linked ultrahigh-molecular weight polyethylene acetabular liners were retrieved at revision surgery. Eight of the liners were fully intact and functional at retrieval. Six cases contained shallow initiated cracks at the root of rim notches; 1 crack had propagated several millimeters. Optical and electron microscopic inspection of the crack surfaces revealed clam shell markings, which are characteristic of fatigue crack initiation. Crack initiation at notches has been identified in reports of catastrophic cross-linked liner failures, with crack initiation sites exhibiting similar morphology and clam shell markings. Thus, we believe that the shallow cracks identified in this case series are precursors to catastrophic rim fracture. The results of this study recommend further investigations to clarify the etiology and prevalence of crack initiation in cross-linked acetabular liners.

Aspherical femoral head with highly cross-linked ultra-high molecular weight polyethylene surface cracking. A case report.

Hip simulator and early clinical studies of highly cross-linked ultra-high molecular weight polyethylene have demonstrated less wear and less femoral head penetration when compared with conventional ultra-high molecular weight polyethylene1-6. However, cross-linking also alters some of the mechanical properties of ultra-high molecular weight polyethylene, including its ultimate tensile strength, strain to failure, fracture toughness, and fatigue crack propagation resistance7-9. Analyses of early retrieved highly cross-linked components have shown initiation of surface cracking, which is possibly related to the decrease in ductility caused by cross-linking10. A reduction in ductility, fracture, and fatigue properties is the hallmark of material embrittlement. Accordingly, newer so-called second-generation highly cross-linked polyethylenes have been developed in an effort to better retain the desirable mechanical properties of conventional ultra-high molecular weight polyethylene as well as the benefits of cross-linking11. Wear behavior of conventional and highly cross-linked ultra-high molecular weight polyethylene acetabular liners can be affected by the geometry of the femoral head. Geometrical factors include not only roughness associated with asperities and machining marks but also asphericity of the implant. Roughening of cobalt-chromium femoral heads occurs in vivo and may increase wear of the liner12-14. Root-mean-square roughness values of clinically retrieved cobalt chromium components heads have been reported to be between 0.15 and 0.20 μm13,14. Asphericity, or out-of-roundness, of the femoral head influences the bearing wear of the implant. It is possible to have an extremely smooth, polished femoral head surface that has relatively large-size scale deviations from the optimal implant shape. Asphericity can manifest as a region of relative flatness or as a protrusion15. Substantial asphericity of the head has been associated with increased acetabular wear16. This case report focuses on a retrieved …

Peak stress intensity factor governs crack propagation velocity in crosslinked ultrahigh-molecular-weight polyethylene†

Ultrahigh-molecular-weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint replacement components. However, these bearing materials can fail as a result of in vivo static and cyclic loads. Crack propagation behavior in this material has been considered using the Paris relationship which relates fatigue crack growth rate, da/dN (mm/cycle) versus the stress intensity factor range, ΔK (Kmax - Kmin , MPa√m). However, recent work suggests that the crack propagation velocity of conventional UHMWPE is driven by the peak stress intensity (Kmax ), not ΔK. The hypothesis of this study is that the crack propagation velocity of highly crosslinked and remelted UHMWPE is also driven by the peak stress intensity, Kmax , during cyclic loading. To test this hypothesis, two highly crosslinked (65 kGy and 100 kGy) and remelted UHMWPE materials were examined. Frequency, waveform, and R-ratio were varied between test conditions to determine the governing factor for fatigue crack propagation. It was found that the crack propagation velocity in crosslinked UHMWPE is also driven by Kmax and not ΔK, and is dependent on loading waveform and frequency in a predictable quasistatic manner. This study supports that crack growth in crosslinked UHMWPE materials, even under cyclic loading conditions, can be described by a relationship between the velocity of crack growth, da/dt and the peak stress intensity, Kmax . The findings suggest that stable crack propagation can occur as a result of static loading only and this should be taken into consideration in design of UHMWPE total joint replacement components.

Crack Initiation in Retrieved Cross-Linked Highly Cross-Linked Ultrahigh-Molecular-Weight Polyethylene Acetabular Liners

Nine cross-linked highly cross-linked ultrahigh-molecular weight polyethylene acetabular liners were retrieved at revision surgery. Eight of the liners were fully intact and functional at retrieval. Six cases contained shallow initiated cracks at the root of rim notches; 1 crack had propagated several millimeters. Optical and electron microscopic inspection of the crack surfaces revealed clam shell markings, which are characteristic of fatigue crack initiation. Crack initiation at notches has been identified in reports of catastrophic cross-linked liner failures, with crack initiation sites exhibiting similar morphology and clam shell markings. Thus, we believe that the shallow cracks identified in this case series are precursors to catastrophic rim fracture. The results of this study recommend further investigations to clarify the etiology and prevalence of crack initiation in cross-linked acetabular liners.

Dynamic-tensile-extrusion of polyurea

Polyurea was investigated under Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) loading where spherical projectiles were propelled at 440 to 509 ms-1 through a conical extrusion die with an area reduction of 87%. Momentum of the leading edge imposes a rapid tensile deformation on the extruded jet of material. Polyurea is an elastomer with outstanding high-rate tensile performance of interest in the shock regime. Previous Dyn-Ten-Ext work on semi-crystalline fluoropolymers (PTFE, PCTFE) elucidated irregular deformation and profuse stochastic-based damage and failure mechanisms, but with limited insight into damage inception or progression in those polymers. The polyurea behaved very differently; the polymer first extruded a jet of apparently intact material, which then broke down via void coalescence, followed by fibrillation and tearing of the material. Most of the material in the jet elastically retracted back into the die, and only a few unique fragments were formed. The surface texture of all failed surfaces was found to be tortuous and covered with drawn hair-like filaments, implying a considerable amount of energy was absorbed during damage progression.

Extreme Tensile Damage and Failure in Glassy Polymers via Dynamic-Tensile-Extrusion

Dynamic-tensile-extrusion (DTE) is an integrated test technique that allows the study of material deformation at high strain-rates (>10,000 s−1) and large strains (>1), under hydrostatic tension. This is an important compliment to the more traditional Taylor cylinder impact test, which achieves large strain and high strain-rate deformation, but under hydrostatic compression. Hydrostatic compression is known to suppress many forms of damage in materials. DTE has been previously employed on a number of metal and polymer systems that manifested tensile instabilities. More recently, this technique has explored stable tensile damage in high-density polyethylene (HDPE), which pointed to a pressure-mediated shear damage phenomenon. The current work extends the technique to the behavior of the glassy polymers poly-methylmethacrylate (PMMA) and polycarbonate (PC). PMMA was found to undergo unstable brittle fracture at nearly all conditions, and therefore did not yield interpretable experimental results. PC (discussed herein) necked and either failed in a brittle fashion or the neck was arrested prior to failure. In the arrested condition, the neck was seen to become opaque from an apparent accumulation of small-scale damage, and a void nucleated at the centerline. A corkscrew fracture process was observed in PC, though its mechanics are not yet understood. It is worth noting that simulations of pressure-hardening PC indicate that it will not extrude or even neck during DTE without the action of a damage process reducing the flow strength of the material.

Application of viscoelastic fracture model and non-uniform crack initiation at clinically relevant notches in crosslinked UHMWPE.

The mechanism of crack initiation from a clinically relevant notch is not well-understood for crosslinked ultra high molecular weight polyethylene (UHMWPE) used in total joint replacement components. Static mode driving forces, rather than the cyclic mode conditions typically associated with fatigue processes, have been shown to drive crack propagation in this material. Thus, in this study, crack initiation in a notched specimen under a static load was investigated. A video microscope was used to monitor the notch surface of the specimen and crack initiation time was measured from the video by identifying the onset of crack initiation at the notch. Crack initiation was considered using a viscoelastic fracture theory. It was found that the mechanism of crack initiation involved both single layer and a distributed multi-layer phenomenon and that multi-layer crack initiation delayed the crack initiation time for all loading conditions examined. The findings of this study support that the viscoelastic fracture theory governs fracture mechanics in crosslinked UHMWPE. The findings also support that crack initiation from a notch in UHMWPE is a more complex phenomenon than treated by traditional fracture theories for polymers.

Incipient and Progressive Damage in Polyethylene Under Extreme Tensile Conditions

The Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) test was developed at LANL by Gray and coworkers to probe the tensile response of materials at large strains (>1) and high strain-rates (>1,000/s) by firing projectiles through a conical die at 300–700m/s. This technique has recently been applied to various polymers, such as the fluoropolymers PTFE (Teflon) and the chemically similar PCTFE, which respectively exhibited catastrophic fragmentation and distributed dynamic necking. This work details investigations of the Dyn-Ten-Ext response of high density polyethylene, both to failure and sub-critical conditions. At large extrusion ratios (~7.4) and high velocities, such as those previously employed, HDPE catastrophically fragmented in a craze-like manner in the extruded jet. At more modest extrusion ratios and high velocities the specimen extruded a stable jet that ruptured cleanly, and at lower velocities was recovered intact after sustaining substantial internal damage. Thermomechanical finite element simulations showed that the damage corresponded to a locus of shear stress in the presence of hydrostatic tension. X-ray computed tomography corroborated the prediction of a shear damage mechanism by finding the region of partially damaged material to consist of macroscopic shear-mode cracks nearly aligned with the extrusion axis, originating from the location of damage inception.