- Ultra-high molecular weight polyethylene (UHMWPE) is commonly used in total hip replacements but has a poor wear rate and becomes brittle after irradiation. - The study aimed to increase the tensile strength of UHMWPE to reduce brittleness by annealing samples at varying temperatures close to the melting point. - Results showed significantly greater tensile strength in vitamin E treated UHMWPE samples annealed at 126°C, supporting the hypothesis that higher annealing temperatures increase tensile strength. This has implications for reducing cracking in orthopedic implants.

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Tensile Properties of Ultra-high Molecular Weigh Polyethylene Used in
Orthopedic Implants
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) MPa
vs. of 39.47 in the untreated group. Also, crystallization at 126oC produced
the highest ultimate tensile strength of 55.6 (+/-7.9) MPa for 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
Methods
• 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
Results Conclusions
• 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.
References
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
Fig 1. above. X-rays of pre-
operative (L) and post-operative
(R) acetabular and femur ball
replacement.
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 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 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 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.