微少試験片を用いた 人工関節用uhmwpeの疲労特性評価

微少試験片を用いた 人工関節用uhmwpeの疲労特性評価

Delamination resistance of vitamin E blended and highly cro ss-linked ultra-high molecular weight polyethylene evaluate d by novel accelerated test method Hideyuki SAKODA, Yoshihiro Okamoto and Yuji Haishima Division of Medical Devices, National Institute of Health Sciences Yuta Osaka and Keita Uetsuki Teijin Nakashima Medical Co. Ltd. Delamination Type of fatigue failure Initiates from internal defects beneath the surface Generates millimeter-sized flakes Common destruction mode of ultra-high molecula r weight polyethylene (UHMWPE) used at articula ting surface of artificial joints Found at knee joints

(Medel JBJS-Am 2009, Greulich J arthop 2012, etc) rim of acetabular components of hip joints (Currier JBJS-Am 2007, Sakoda JJCB 2008) shoulder joints (Gunther J Arthop 2002) Complex mechanism induced by stresses bene ath the surface generated by compressive load at low conformity contact (Bartel CORR 1995) Currier JBJS-Am 2007 Previous in vitro tests Joint simulator Advantages Final products are evaluated Design factors

Material factors Most clinically relevant loading and motion Disadvantages Limited acceleration Very complex Not easy to evaluate the effect of component design Component design itself affects joint loading and motion in vivo www.amti.biz * Conforming inserts are more closely associated with delamination than less-conforming inserts in a retrieval study, probably due to the higher in vivo constraint-induced stresses. (Wimmer CORR 2012) Previous in vitro tests Ball-on-flat Distinctive feature Pin-on-plate wear test using simple reciprocation Simple Material test Design factors eliminated

Many experiences (Bell JOA 1998, Reeves IMechE 2000, Popoola JOR 2010, Wannomae JOA 2010, Saikko Tribol Int 2014) Standardized (ASTM F 732-00(2011)) Limitation Delamination is reproduced only in obviously deteriorated UHMWPE Aged historical UHMWPE (gamma air) Aged conventional UHMWPE (gamma inert) Risk of delamination can not be evaluated U-shaped motion First reported in 1999 by Tomita et al. (Tomita JBMR-B 1999) Creates more internal cracks than simple reciprocation (measured by scanning acoustic tomography, in water lubricant) 10 mm Modification (Sakoda JJCB 2012) Serum based lubricant Extended test duration (until delamination or 1 million cycles)

Dramatic acceleration of delamination process Delamination in virgin UHMWPE 1 mm U-shaped motion Gamma air, AA Virgin UHMWPE Virgin UHMWPE U-shaped motion at 100 cycles Simple reciprocation at 1,000,000 cycles U-shaped motion at 160,000 cycles

Objectives To verify the delamination test using U-shaped motion By making direct comparison of delamination resistance of various UHMWPEs using the tests To investigate the relation between the results of delamination tests and those of fatigue crack growth tests To assess the risk of delamination of components made of contemporary UHMWPEs Materials Name Virgin G25 G25air G100-R2 G100-Ax VEPE VE-Ex

Description UHMWPE as received (GUR 1020) Conventional, irradiated at 25 kGy in inert Historical, irradiated at 25 kGy in air Irradiated at 100 kGy in inert, remelted at 160 C for 2 h Irradiated at 100 kGy in inert, annealed at 110 C, x: annealing time in hours 0.3% vitamin E blended UHMWPE (GUR1050) Vitamin E blended HXLPE, x: e-beam irradiation dose in kGy annealed at 110 C for 72 h in vacuum Accelerated aging (AA) 80C in air for 21 days Compulsory aging (CA) Gamma irradiation at 25 kGy in air and AA Used for vitamin E blended materials Methods FTIR Fourier transform infrared microscopy (FTIR) 100200 m thick specimens

Measurement through thickness Oxidation index A1720 / A1370 (ASTM F2102-01) Maximum value (OIMAX) was used for further analysis Crystallinity index (A1896 / A1305) / (A1896 / A1305 + 0.25) x 100 (Costa Poym Test 2001) Average value (CI) was used for further analysis Methods Delamination test Two dimensional pin-on-plate wear test machine was used Co-Cr alloy pin (SR = 3 mm) (Ra < 0.02) Vertical load (196 N) Plate specimen (40 x 40 x 10 mm)

Bovine serum lubricant 20g/L protein EDTA, NaN2 Sliding motion (10 x 1 mm) (Top view) Methods Delamination score Periodical stops to visually inspect for occurrence of delamination Test duration: Up to 1 million cycles Delamination score based on cycles to delamination Cycles to delamination is exponential Need to differentiate delamination at 1 million cycles and no delamination at 1 million cycles

Cycles Delamination score No delamination 5 1,000,000 300,000 100,000 4 3 2 10,000

1 1,000 0 Methods Fatigue crack growth test Based on ASTM E647 W = 30, B = 10, 1 Hz Stress intensity factor range threshold KKth was used for further analysis KKth da/dN C(KK)m KK Dimensions of the compact tension specimens used for fatigue crack growth tests Schema of tensional loading

and fatigue crack growth Schema of Paris low and parameters obtained from fatigue crack growth tests Results Oxidative stability G25air-AA (aged historical UHMWPE) had the highest OIMAX and CI indicating oxidative degradation There were some materials with inferior oxidative stability indicated by increase in both OIMAX and CI after artificial aging G100-A2 G25 Virgin * *

* * * * * *: Significant difference between non-aged and aged specimens. Without aging After aging * *

* Results Delamination score # Without aging After aging * # # # # # * #

*: Significant difference between non-aged and aged specimens. #: Significant difference against the virgin UHMWPE. G100-A2-AA at 10,000 cycles VE-E300-CA at 2,040,000 cycles (Additional cycles tested) Results Stress intensity factor threshold ns Without aging After aging ns ns: No significant difference between non-aged and aged specimens.

Discussion in vivo vs in vitro Results of delamination test agrees with results of retrieval studies (and other in vitro delamination studies) in vitro (Delamination test) VE-E300, VEPE, G100-A G100-R, G25 Virgin G100-A2-AA G25-AA G25air-AA* in vivo (Retrieval studies) No delamination on ethylene oxide gas sterilized UHMWPE (Greulich JOA 2012, MacDonald CORR 2012) Rim delamination on annealed HXLPE (Currier JBJS-Am 2007)

Delamination reported by retrieval studies (and other in vitro studies) *: Results for delamination test of G25air-AA are previously reported that delamination was reproduced on all three specimens within 100 cycles, corresponding to delamination score of 0 (Sakoda JJCB 2012) Discussion Fatigue crack growth Results of delamination test was NOT consistent to results of fatigue crack tests Fatigue crack growth tests only evaluate the final step of the complex mechanism of delamination Delamination test Fatigue crack propagation test Initial internal defects formed at consolidation Additional defect formation by loading

Crack growth Delamination Discussion Why U-shape motion? With simple reciprocation, conformity improves due to plastic deformation U-shape motion prevents full contact At Start Simple reciprocation Conformity improves to reach full contact U-shape motion Two tracks formed and prevents full contact Conclusion Accelerated delamination test using U-shape motion was developed Material test (design factor eliminated)

Comparative test (needs control) Accelerated test (not quantitively mimic in vivo condition) Consistent to retrieval studies (and other in vitro studies) Results were NOT consistent to those of fatigue crack growth test Delamination has complex mechanism Fatigue crack growth test evaluates only final step of delamination Delamination test can not be substituted by fatigue crack growth test Conclusion and significance Contemporary UHMWPEs have little risk of delamination (Unless there is no oxidation) Crosslinking improves delamination resistance Vitamin E blending improves delamination resistance Note: the test does not quantitatively predict durability in vivo How utilize the delamination test To avoid inappropriate material to be used at articulating surface with l ow conformity in the future Many new materials are under development

Carbon-PEEK (Brockett Wear 2017) CNT-UHMWPE Cellulose nanofibers Thank you for your attention

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