Have excruciating pain on my sciatica radiating to my hip then down to right above my knee on the back side of leg. do have psoriatic arthritis.

No fun!: Sounds like you need to see a doctor for possible prescription of steroids.

Osteoarthritis: You most likely have osteoarthritis, or OA, of the knee or hip. Unless you already have a diagnosis of psoriasis, OA is far more common. OA & vertebral disc disease can cause sciatica, but then the pain usually radiates all the way to the foot. See your clinician, who can also determine whether you need to see a specialist like a rheumatologist, an orthopedist or a neurosurgeon. Hope that helps.