Localized Cause of Back Pain – Inter-Spinous Process Bursitis “Baastrup’s Disease”

An MRI found intraspinous processes bursitis in 8.2% of individuals which could be causing segmental back pain.

Spine: 1 April 2008. 33(7), E211-E215.
Lumbar Interspinous Bursitis (Baastrup Disease) in a Symptomatic Population: Prevalence on Magnetic Resonance Imaging
Maes, Robbert MD; Morrison, Willam B. MD; Parker, Laurence PhD; Schweitzer, Mark E. MD; Carrino, John A. MD, MPH abstract here