Anaes · Applied anatomy

Vertebral column and neuraxial spaces

Also known as Vertebral column anatomy · Neuraxial anatomy · Epidural space · Subarachnoid space · Ligamentum flavum · Meninges

Neuraxial anaesthesia — spinal, epidural and combined spinal-epidural — depends entirely on knowing the layered anatomy from skin to cerebrospinal fluid. The framework rests on six exam-critical ideas. First, the vertebral column is built of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral fused, 4 coccygeal) with four curvatures (cervical and lumbar lordotic, thoracic and sacral kyphotic); a typical vertebra has a body anteriorly, a vertebral arch posteriorly forming the vertebral canal, and seven processes (two transverse, four articular, one spinous). Second, the spinal cord is shorter than the vertebral canal: it ends as the conus medullaris at the L1/L2 disc in the adult (lower, around L3, in the neonate), and below it the lumbar and sacral nerve roots form the cauda equina — which is why a lumbar puncture or spinal needle is inserted at or below L3/L4 to avoid the cord. Third, three meningeal layers invest the cord: the tough dura mater outermost, the delicate arachnoid mater in the middle, and the pia mater adherent to the cord; CSF lies in the subarachnoid space between arachnoid and pia. Fourth, the epidural space lies outside the dura and contains fat, lymphatics, the internal vertebral venous plexus (Batson's valveless plexus) and the spinal nerve roots as they exit; its depth from the skin varies with body habitus and is measurable by ultrasound or MRI. Fifth, the ligamentum flavum (yellow ligament) is the tough elastic ligament joining the laminae of adjacent vertebrae and is the key resistance landmark for loss-of-resistance in epidural placement. Sixth, a midline neuraxial needle passes in order through skin, subcutaneous tissue, the supraspinous ligament, the interspinous ligament, the ligamentum flavum (the epidural endpoint), and then — if a spinal is intended — the dura mater and arachnoid mater into the subarachnoid space. Built on the conus-position MRI study (Saifuddin 1998), the conus fetal-to-adult study (Malas 2001), the intercristal-line ultrasound study (Pysyk 2010), the neuraxial-ultrasound meta-analysis (Perlas 2016), the MRI epidural-space-depth study (Alsaati 2026), the epidural-spread cryomicrotome study (Hogan 2002), the loss-of-resistance air-versus-saline review (Norman 2003), the dura-arachnoid lesion study (Reina 2004), the post-dural-puncture-headache needle meta-analysis (Maranhao 2021), the ultrasound combined-spinal-epidural study (Sethi 2026), the inadvertent-dural-puncture report (Greenspon 2026), and the tethered-spinal-cord anaesthetic-management report (Alessi 2026).