Spinal needle access into the subarachnoid space by the median or lateral approach

In the lateral approach, the needle is inserted lateral to the upper border of the L3 or L4 spinous process. The needle is then directed slightly medially and cephalad. The lateral approach bypasses the supraspinal ligament (which can be calcified in older patients, causing deflection of the needle) and penetrates the ligamentum flavum, passing through the dura into the subarachnoid space.