The patient is prepared and positioned as described previously. An appropriate vertebral interspace is chosen following careful palpation. Spinal anaesthesia is not performed above L2, as this is the lower limit in the normal adult population of the conus medullaris (where the solid spinal cord terminates and continues as the cauda equina). Inserting a spinal needle into the subarachnoid space above this carries a risk of damage to the spinal cord. For safety, the lowest suitable space should be used.

The L4/5 interspace can be estimated by drawing an imaginary line (Tuffier's line) between the superior aspects of the iliac crests (see yellow line on Fig 1), although there is considerable anatomical variability in the population.

More recently, the level of the tenth rib has been described as a useful landmark (see blue line on Fig 1). On flexion, it corresponds to a mean level of L1/2. It is at least as reliable as Tuffier’s line, and may be more easily palpable in larger subjects. (Ref: Jung et al. Anaesthesia 2004;59:359-363).

In 2008, NICE issued guidance that ultrasound can be useful in choosing the level of entry and in determining the depth of the epidural space.