Sagittal T2-weighted fast spin-echo sequence; 59 y/o female; congenital block vertebra C6/7 with degenerative changes in segments C3/4, C4/5, and C5/6 (disk prolapse); however, the segment C7/Th1 is not affected by degeneration, representing a combination of ASDI in one segment and natural degenerative changes. The degeneration score in this patient, who has degeneration in segments C3/4 and C4/5 but not in segments C1/2 and C2/3, is 2 (1 point per segment for loss of height of the intervertebral disk, disk bulging over the dorsal level, and evidence of retrospondylophytes). The respective degeneration ratio is 0.167 (2/12). The degeneration score for the adjacent segments in this patient, who has degeneration in segment C5/6 but not in segment C7/Th1, 1 (loss of height of the intervertebral disk, disk bulging over the dorsal level, and evidence of retrospondylophytes). The adjacent segment ratio is 0.167 (1/6). Consecutively, the instability ratio is 0 (0.167–0.167), which means that the natural degeneration and the degeneration caused by the block vertebra are equal.