What characteristics make a spinal compression fracture unstable?

• loss of > 50% vertebral body height 
• vertebral body body angulation > 20 degrees (measurement "a" in diagram below)

• association with multiple adjacent compression fractures

Unstable fractures may need open reduction and internal fixation, whereas stable compression fractures can be treated with a hyperextension orthosis and early ambulation. 


Source

Koval, K. and Zuckerman, J.  Handbook of Fractures.  2nd ed.  2002. 

Image source: http://www.maitrise-orthop.com/corpusmaitri/orthopaedic/mo68_laporte/index.shtml