Human factors engineering is a discipline that has always fascinated me. It can be loosely described as engineering while keeping the end user squarely in focus. The dentist in the above story has experience and knows what makes a good dental implant; and so, basically, human factors engineers will try to design around that. Ergonomics is a good example of human factors design in the workplace; the urinal fly is another, which enables better use of existing materials. Kim Vicente has written a great, recent book on the subject, The Human Factor , and is definitely worth a read.
The dentist story above also provides an example combining human factors and materials science: good implants require certain properties of friction, wear, stiffness, elasticity, etc., for successful handling, installation, and end use. In many ways, engineers consider materials as black boxes: they have their respective inputs and outputs that can be summarized using material property space maps. But also often encountered are less immediately-considered characteristics. For example, a visual preference of one metal finishing over another, despite the two possessing essentially equivalent properties. Or, the user's response to particular materials: for example, sports equipment can lead to a measurable improvement in the user's performance, even if the product is only a Placebo effect . Or, the ease with which users will adapt to the new technology.
One of the most interesting cases (to me, anyway) where human factors and materials science interact is the design of prosthetic limbs. Many of you have by now seen a range of types. The ideal design and applicability of these devices requires a lot of input from the end user, such as: desired ranges of motion and control, force levels, dexterity, aesthetics, and symmetry. The latter of these is particularly important in lower-limb prosthetics, e.g. for mechanical property and weight balance between limb pairs, and for loads imparted on the remaining parts of the body. However, in cases where both limbs in a pair are prosthetic, the design space can be expanded as symmetry becomes less constrained. One well-known case is the Paralympic athlete Oscar Pistorius, dubbed the "Blade Runner" (Figure 1).
|Figure 1: Oscar Pistorius, the "Blade Runner" (image from Engadget). His prosthetic legs are constructed using carbon fiber, one of the lightest and stiffest materials currently available.|
 K. Vicente. The Human Factor. Vintage Canada, Toronto (2003).
 C.J. Beedie. Placebo Effects in Competitive Sport: Qualitative Data. Journal of Sports Science and Medicine, Vol. 6 (2007) pp. 21-28. (Available On-Line.)
 G. Lippi and C. Mattiuzzi. (Editorial) Pistorius ineligible for the Olympic Games: The Right Decision. British Journal of Sports Medicine, Vol. 42 (2008) 160-161.
 S. Camporesi. (Editorial) Oscar Pistorius, Enhancement and Post-Humans. Journal of Medical Ethics, Vol. 34 (2008) 639.