====== McAfee scale ======


Class 0: No HO present.

Class I: HO present in islands of bone within soft tissue but not
influencing the range of motion of the vertebral motion segment.
Bone is not between the planes formed by the two vertebral
endplates.

Class II: HO possibly affecting the vertebral range of motion. HO
present between the two planes formed by the vertebral endplates
but not blocking or articulating between adjacent vertebral endplates
or osteophytes.

Class III: The range of motion of the vertebral endplates is blocked by
the formation of HO and/or postoperative osteophytes on
flexion-extension or lateral bending radiographs.

Class IV: HO causing inadvertent arthrodesis. Bony ankylosis.
Bridging trabecular bone continuous between adjacent endplates and
<3° of motion of lateral flexion-extension radiographs
((McAfee PC, Cunningham BW, Devine J, Williams E, Yu-Yahiro J. Classification of
heterotopic ossification (HO) in artificial disk replacement. J Spinal Disord
Tech. 2003 Aug;16(4):384-9. PubMed PMID: 12902954.
)).