Doctors and Anesiva, a biotech firm focusing on developing novel therapeutic treatments for pain are exploring the use of capsaicin to alleviate post-operative pain.

Nerve cells that sense a type of long-term throbbing pain bear a receptor, or gate, called TRPV1. Capsaicin binds to that receptor and opens it to enter only those pain fibers - and not other nerves responsible for other kinds of pain or other functions such as movement.

These so-called C neurons also sense heat; thus capsaicin's burn. But when TRPV1 opens, it lets extra calcium inside the cells until the nerves become overloaded and shut down. That's the numbness.

Anesiva's drug candidate, Adlea, is purified capsaicin currently under trials involving patients undergoing various surgeries, including knee and hip replacements. Initial results from patients who had open hernia repair and knee replacements indicate that the patients experienced less pain up to two weeks following surgery.
WebMD has an article discussing the mechanism by which capsaicin relieves pain, pointing to a letter published in Nature: Inhibition of nociceptors by TRPV1-mediated entry of impermeant sodium channel blockers.
Hat Tip: PharmaGazette