Figure 13a. Figures 13, 14. (13a) Sequential LFJ arthrograms obtained in a patient with advanced Baastrup disease demonstrate a horizontal communication. Note the progressive opacification of the interspinous process (arrow) followed by that of the contralateral facet joint (arrowhead). (13b) Lateral LFJ arthrogram clearly demonstrates opacification of the interspinous process (arrow). (14) LFJ arthrogram shows a horizontal communication with the classic butterfly appearance.