UC Berkeley

There are currently over 2 million total joint replacement procedures completed every

year worldwide. The number of total joint replacement procedures is predicted to grow. At

the same time, the percentage of younger, more active patients undergoing joint replacement

procedures is also predicted to increase. Such predictions motivate a need for joint replacements to last longer and perform better in more active patients. The greatest challenge for current joint replacement device designs is wear-related failures. One potential solution to

improve wear performance is to use alternative materials with superior wear performance.

Polycarbonate polyurethane (PCU) has been proposed as an alternative material to improve

the performance of joint replacements. It is softer and more elastomeric than the current

standard polymer, ultra-high molecular weight polyethylene. It has been hypothesized that,

due to its lower elastic modulus, PCU will have improved lubrication performance, reducing

wear. Historically, improvement to ultra-high molecular weight polyethylene wear performance

has come at the expense of fracture resistance. Therefore, in the highly cyclic loading

environment of a total joint replacement, the fatigue and fracture properties are also important

to consider.

This thesis evaluated the long-term mechanical performance of PCU in orthopedic implant

applications. First, we characterized the fatigue crack growth mechanisms in PCU

as a function of loading, frequency, hydration, and thermal annealing treatment. We found

highly time-dependent behavior as mechanisms of crack growth and failure changed with

loading regime. Second, we looked at changes in the structural organization of PCU as a

function of thermal treatments and strain. We found trends of increasing ductility with

increasing annealing temperature and increasing hydrogen bonding in the ordered hard domain. The amount of hydrogen bonding decreased with increasing strain. Finally, we used an 3D-transient elastohydrodynamic lubrication model to characterize the lubrication regimes in PCU during a simulated gait cycle. Our model contradicts the optimistic predictions

of 1D-steady state modeling and may explain the disconnect between early modeling and

the experimental wear results for PCU. Moving forward, this work serves as a foundation

for many questions that still need to be answered to understand the fatigue, fracture, and

tribological performance of this complex material in long-term clinical implant applications.