What are the risks of spinal decompression surgery?

A 1995 study called "Spinal Cord Stimulation for Chronic Low Back pain: A Systematic Literature Synthesis" compiled the previous research and had some interesting findings. They followed up with patients of the surgery 1-45 months after the surgery with a mean of 16 months:

• an average of 59% of patients had ≥50% pain relief (range, 15-100% of patients).
• complications occurred in 42% of patients, but were generally minor.
• approximately 50 to 60% of patients with failed back surgery syndrome report >50% pain relief with the use of spinal cord stimulation at follow-up
• however, the study was not conclusive in terms of the effectiveness of the treatment:

  the lack of randomized trials precludes conclusions concerning the effectiveness of spinal cord stimulation relative to other treatments, placebo, or no treatment

Another page that I found useful lists contraindications, warnings, precautions, and adverse effects. I think the information you are looking for would be under the adverse effects.