Fig. 3

Ultrasound-assisted angle-free paramedian approach in an elderly patient with ankylosing spondylitis (Patient 2). (a) Three-dimensional CT image from the posterior view with right rotation, demonstrating narrowing of the interspaces due to extensive ossification and osteoarthritis with patent left L4/5 interlaminar space (red circle). (b-c) Axial CT demonstrating the potential needle trajectory of the standard paramedian approach with 10° medial angulation. A sagittal oblique reconstruction along the conventional paramedian needle trajectory reveals a needle trajectory through the interlaminar space without craniocaudal angulation. (d) Axial CT demonstrating the design of the horizontal needle trajectory through interlaminar space when the back is lying forward by 10°