Table 1

Fusion criteria on 3-mm high-resolution CT scan reconstructions in coronal and sagittal planes (with permission from Mayfield Clinic)

Grade	Time period	Definition
I	Applicable at any follow-up	Bridging bone, advanced fusion. Presence of continuous bridging bone extending
		from the L5 to the S1 endplate on the reconstructed images, occupying >50%
		of available space for fusion.
II	Applicable at any follow-up	Bridging bone, fusion. Bridging bone filling <50% of available space between
		vertebral bodies.
III	Early: 6 months	Early: Presence of developing bone connected to either end plate without bridging.
	Late: 12,18, 24 months	Late: Presence of developing bone connected to either endplate without bridging
		with fusion mass greater than previous scan.
IV	6 months	No bridging or developing bone connected to either endplate.
V	Applicable at 12, 18, 24 months	No bridging or developing bone. Decrease or nonprogression of developing bone since previous visit.