The intercristal line, also called Tuffier line or the supracristal plane, is an imaginary horizontal line joining the highest points of the two iliac crests. It usually crosses the spine at the L4 spinous process or the L4-L5 interspace. Its main use is finding a safe level for lumbar puncture and spinal anaesthesia, below the end of the spinal cord.
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In adults the spinal cord usually ends at L1-L2 (lower, about L3, in newborns), so a needle placed at L3-L4 or L4-L5 enters the dural sac below the cord. The line is often a level higher or lower than expected, especially in obesity and pregnancy, so ultrasound is used when landmarks are hard to feel. Sitting with the back flexed opens the gaps between the lumbar spinous processes.
Have the person sit and lean forward, or lie on their side with knees drawn up. Place your hands on their waist with the index fingers resting on top of the iliac crests, pressing down through the soft tissue until you are firmly on bone, and move to the highest point on each side. Bring your thumbs horizontally to the midline; where they meet is the L4 spinous process or the L4-L5 gap. Confirm by feeling the spinous process above and below and the soft gaps between them.
Leakage of cerebrospinal fluid through the needle hole causes a headache that is worse on sitting or standing and eases lying flat.
A puncture placed higher than intended can rarely injure the lower end of the spinal cord, causing leg pain, numbness or bladder problems.
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