Who invented the first crutches and when?
How many pairs of crutches are manufactured and used annually?

The following does not answer your question but may be of interest

http://www.emedicinehealth.com/articles/9039-1.asp

History of Crutches

Since antiquity humans have fashioned support devices to hold
themselves up when they became sick or injured. Evidence of such
devices dates to 2830 B.C. A carving on the entrance of an Egyptian
tomb depicts a figure leaning on a crutch like staff.

Kind regards
Ross (Australia)