Tensile Properties of Ultra-high Molecular Weigh Polyethylene Used in Orthopedic Implants

1. Tensile Properties of Ultra-high Molecular Weigh Polyethylene Used in Orthopedic Implants Results Tensile Properties of Ultra-high Molecular Weigh Polyethylene Used in Orthopedic Implants Alden Burnham Manchester-Essex Regional High School, Manchester-by-the-Sea, MA Teacher, Dr. Maria Lonnett Burgess, Manchester-Essex High School Mentor, Dr. Anuj Belare, Brigham & Women’s Hospital, Boston, MA Ultra high molecular weight polyethylene (UHMWPE) is the most common material used for replacements in total hip arthroplasty. Using irradiation to crosslink the molecules in the plastic greatly reduced the wear rate. This work focused on extending the tensile properties of UHMWPE to avoid the brittle effects seen in irradiated samples. We hypothesized that annealing would increase the tensile strength of UHMWPE. Samples were either + or – for vitamin E (to diminish oxidation) and then thermocycled at varying temperatures (120-160oC) to induce crystallization. We found significantly greater tensile strength in the Vitamin E treated group, 53.6 (+/- 10.9) MPavs. of 39.47 in the untreated group. Also, crystallization at 126oC produced the highest ultimate tensile strength of 55.6 (+/-7.9) MPafor the treated group. We conclude that a positive correlation exists between the crystallization temperature and ultimate tensile strength of UHMWPE. This data can be extrapolated for implications for the orthopedic surgical world because it would mean that patients would not have the pressing fear of cracking the plastic used in the replacement. Abstract Introduction and Objectives During this experiment we annealed 1020 Ultra High Molecular Weight Polyethylene (UHMWPE) at varying temperatures to determine the effect of crystallization on the tensile properties. Cut 4 samples from a Vitamin E integrated block of 1020 UHMWPE and 4 samples from untreated 1020 UHMWPE; heated to 160oC Conducted thermocycling to induce crystallization at 120oC, 122oC, 124oC, 126oC, 128oC, 130oC for 48 hr time periods Negative control: crystallization tests using ice water at about 4oC by heating the polyethylene to 160oC then submerging in ice water until completely solid. Re-heated 4 ice water samples to 126oC to observe if we could eliminate the effects of crystallization and return them to the normal control state. Samples were punched out of the untreated UHMWPE and the Vitamin E UHMWPE using a pressure punch with a constant length and width. Used an ADMET tensile testing machine with two vertical clamps that held the sample in place and was calibrated to 7.62. This machine was then set into motion and it slowly pulled the sample at about 10 millimeters every minute until it broke. Completed statistical analysis on the data we received to get the ultimate tensile stress that UHMWPE achieved. • Stress(1.9*force)/(3.18*thickness) • Strainstretch/7.62 • Mod • Yield stress Correlation between the crystallization temperature and ultimate tensile strength of ultra high molecular weight polyethylene. 10 MPa difference between 120oC and 128oC Negative Control: Ice water samples have a lower strength, consistent with the idea that a closer temperature to 130oC shows a higher strength. When we put the samples in ice water and then re-heated them to 160oC, the tensile properties returned. 130oC samples show less change because heating was 48 hr, and we understand that the crystallization rate should be much slower because 130oC is close to the melting point of 133oC and therefore it should take ~2 wk of annealing to determine accurate results. We are also assuming that this data can be extrapolated to the irradiated samples and are currently testing these, and additionally are in the process of testing 1040 UHMWPE in order to verify that the tensile strength is not unique to 1020 or the Vitamin E treating References Methods Substantial results on one slab but less significant on the other, likely due to high standard deviation. Control strength is about 45 MPa; some treated samples got up to 70 MPa, which is extremely significant. The averages are misleading because of standard deviation, but there is an upward climb as it approaches 130oC. The 130oC sample crystallized so slowly as expected. It is closer to the control samples which were the least strong, supporting our hypothesis. Future Work Complete unfinished irradiated samples at 124oC and 126oC Re-test samples because of surprising results Complete an 80oC crystallization sample, and a 122oC and 124oC sample test This data has significant implications for the orthopedic surgical world because patients would not have the pressing fear of cracking the material used in the replacement prosthesis. Bellare A, Turell M, Wang A. Quantification of the effect of cross-path motion on the wear rate of ultra-high molecular weight polyethylene. August-September 2003. Wear, Volume 255, Issues 7-12, 1034-1039. Bellare A, Turell M. A study of the nanostructure and tensile properties of ultra-high molecular weight polyethylene. August 2004. Biomaterials, Volume 25, Issue 17, Pages 3389-3398. Bellare A, Gomoll T, Wanich A. J-integral fracture toughness and tearing modulus measurement of radiation cross-linked UHMWPE. 2002. Journal of Orthopaedic Research 1152–1156. Bellare A, Bistolfi A, Lee Y, Turell M. Tensile and Tribological Properties of High-Crystallinity Radiation Crosslinked UHMWPE. July 2009. Journal of Biomedical Materials Research Part B: Applied Biomaterials Volume 90B, Issue 1, pages 137–144. Acknowledgements Thank you to my amazing mentor Dr. Anuj Bellare, to Brigham and Women’s Hospital for giving me this opportunity, and to Dr. Maria Lonnett Burgess for staying with me for every step of the way. Contact alden.burnham@gmail.com for further information. Ultra high molecular weight polyethylene (UHMWPE) is the most common material used for acetabulum replacements in total hip arthroplasty. The use of polyethylene was recently confronted because of its poorer wear rate with a life expectancy of ~10 to 15 years and introducing immune system problems like osteolysis. A study found that irradiation cross-linking greatly reduced the wear rate, but at a cost. UHMWPE became more brittle after cross linking and shortened in vivo life in response to cracking or chipping of the material. We focused on finding a way to extend the tensile properties of UHMWPE to mitigate the brittleness seen in irradiated samples. We used 1020 grade polyethylene for this experiment because it works well with Vitamin E integration. Our hypothesis was that melting polyethylene and annealing it at temperatures closer to the melting point would increase the tensile strength. Alden Burnham Authentic Science Research Program Manchester Essex Regional High School, Manchester-by-the-Sea, Massachusetts, 01944 Conclusions Fig 3. On right, Admet Force Analyzer used to test the ultimate tensile stress and strain of the prosthetics for this study. Above is a schematic of the set-up, showing the prosthetic (test specimen) in green within the Admet Analyzer. Fig 5. Above is the graph demonstrating the relationship between annealing temperature and ultimate tensile strength for the Vitamin E treated samples and untreated samples. There is a positive trend between temperature and tensile strength iterated through the climb in strength relative to the climb in temperature, but the standard deviation is also significant. Fig 4. Below is a generic strain/stress graph labeled with the results we gathered from the computer-generated graphs. This is a random sample selected for illustration of the parts of tensile strength. Fig 1. above. X-rays of pre-operative (L) and post-operative (R) acetabular and femur ball replacement. Fig 6. Above is a strain/stress curve comparative analysis of different temperatures showing the unique stretching capabilities of some of the samples we annealed at higher temperatures. This figure is a case study and does not account for standard deviation.