I am not entirely sure here because epidural injections are into the spine and facet joint injections are into the joint. Although both are used to treat various kinds of back pain, they are not the same procedure.
Having difficulty finding an English to Hebrew translation for either facet joint or Zygapophysial joint, I am thinking the writer of the passage you are translating may have decided to use a term which sounds similar. (The official name of the facet joint is the Zygapophysial joint). You will find more information about the two procedures here:
http://www.rad.washington.edu/clinical/radiology-clinics/uwm...
and there are more if you google facet v epidural injection
I cannot think what else it could be in your context.