What does it mean when mri report say c4c5 facet arthrosis and posterior disc osteophyte complex resulting in mild effacement of the thecal sac?

Back trouble!: Depends on your symptoms whether just a finding or a cause of trouble. C- is cervical-neck 4-5 of the usual C1-7! Osteophytes are bony prominences from arthritis and changes. Thecal is sac around nerves and cord.

Degenerative change: Basically in the spectrum of degenerative disc disease/degenerative arthritis.. Potential source of pain, but usually "mild effacement"(i.e. mild compression of the sac that encloses the spinal cord) by disc/osteophyte(osteophyte is a bony overgrowth that buttresses the protruding or bulging disc) does not cause neurological symptoms. However, spine symptoms may not obey the rules, see your doc