Footwear plays an important role in the development and/or worsening of osteoarthritis of the knee.  Wearing stiff footwear, like clogs and traditional athletic shoes, has been shown to increase stress on the knees.  This contributes signficantly to arthritis.  On the other hand, when shoes are completely flat and sufficiently wide, the foot can function optimally, and there is minimal stress placed on the knee. So, perhaps surprisingly, flip-flops have proven be better for the knees than clogs or typical athletic shoes.  A study from Rush Medical College analyzed the amount of stress placed on the knee with various shoe types. 

A summary of this research was printed in Health Magazine, by Duke Medical Center:
Think About Your Knees When You're Shopping for Your Feet

You can read the actual research article from Rush Medical College here:
Walking Barefoot Decreases Loading on the Lower Extremity Joints in Knee Osteoarthritis