Non Parkinson's related pain

I was diagnosed with Parkinson’s in January 2001. In 2006 I was diagnosed with an additional condition (Degenerative Spinal Disease) that had nothing to do with my Parkinson’s. I had surgery in August 2007. This condition deteriorated in 2011 and in September 2011 I had a stress fracture of the lumbar area of my Spine. A MRI scan showed the following.

• Chronic degenerative changes at C5-6 and C6-7 with some narrowing of the intervertebral foraminia on both sides at C5-6.
• Partial bony ankylosis at T2.
• Previous surgery at L4-5 with grade 2 spondylolisthesis and moderate to chronic spinal and intervertebral foraminal stenosis.
• Gross degenerative change at the facet joints. There is a large focal disc protrusion in a left paracentral position but also extending towards the right side.
The pain that I had from these conditions for the last 20 months was extreme. This pain affected my Parkinson’s For the last year and a half I found myself switching off around midday onwards, forcing me to take anything from two to four bolus doses each day of my Apo-Morphine to control my Parkinson’s. I had surgery on 17 April 2013 which included a Fusion (two rods screwed to my spine, a cage to open the spinal column and a bone graft. Three days after surgery I had my epidural removed and discovered that all my pre-operative pain had gone. From that time to date I have not had to take one additional bolus dose of Apo-Morphine
Is there any research or studies into non Parkinson’s related pain and its effect on Parkinson’s?