Jezzy

Spine ratings themselves are pretty much based solely on Range of Motion measurements.  However, if you have nerve pain (radiculopathy) radiating from your service-connected spinal disability, you may have a strong case to be granted secondary or aggravation service connection for it. 
Have you asked your doctor and/or Physical Therapist their opinions?  Assuming that DDD in your C-spine segment is what you're currently rated for, it would just be a matter of getting medical confirmation, preferably from a doctor, (even better if a specialist), that a) you have radiculopathies and b) they are  "due to", "more likely than not likely", or "at least as likely or not" caused or aggravated by your service-connected disability.
 

acesup is correct on ROM (range of motion) being where its at for DDD or DJD.  10% for arthritis and or pain is pretty much the limit.  However, ROM can get you a higher percentage.  40%-60% if a doctor prescribes bed rest, which they do not do anymore, so you can see the VA regs are a little behind the 8-ball regards bed-rest.
That said, you do not state what joint (area) your service connected for for DDD?
I can tell you that I started out with 10% for arthritis tendinopathy for my right shoulder in 2009. 
Since then, I have gotten an increase for that to 20% for ROM, and as secondaries to that original right shoulder condition, I have received, right elbow 20% ROM, left shoulder 10% x-ray, cervical 20% ROM and lumbar 20% ROM.  
All, degenerative arthritis.  All diagnostic code 5003.
So, that one 10% which was up'd to a 20%, got me three additional 20%'s and a 10% as claimed secondaries.
Arthritis effects adjacent joints and opposite joint when overworked.
My percentage increase were slow in coming, just like the arthritis.  Which makes perfect sense to me.
Keep plugging along,
Hamslice
Here is a good article or ad for that matter regards service connected arthritis;
https://cck-law.com/news/arthritis-va-disability-claim/
Good luck....