Anatomy of soft tissues of the spinal canal

Background and Objectives. Important issues regarding the spread of solutio ns in the epidural space and the anatomy of the site of action of spinal an d epidural injections are unresolved. However, the detailed anatomy of the spinal canal has been incompletely determined. We therefore examined the mi croscopic anatomy of the spinal canal soft tissues, including relationships to the canal walls. Methods. Whole mounts were prepared of decalcified ver tebral columns with undisturbed contents from three adult humans. Similar m aterial was prepared from a macaque and baboon immediately on death to cont rol for artifact of tissue change after death. Other tissues examined inclu ded nerve root and proximal spinal nerve complex and dorsal epidural fat ob tained during surgery. Slides were examined by light microscopy at magnific ations of 10-40x. Results. There is no fibrous tissue in the epidural space . The epidural fat is composed of uniform cells enclosed in a fine membrane . The dorsal fat is only attached to the canal wall in the dorsal midline a nd is often tenuously attached to the dura. The dura is joined to the canal wall only ventrally at the discs. Veins are evident predominantly in the v entral epidural space. Nerve roots are composed of multiple fascicles which disperse as they approach the dorsal root ganglion. An envelope of arachno id encloses the roots near the site of exit from the dura. Conclusions. The se features of the fat explain its semifluid consistency. Lack of substanti al attachments to the dura facilitate movement of the dura relative to the canal wall and allow distribution of injected solution. Fibrous barriers ar e an unlikely explanation for asymmetric epidural anesthesia, but the midli ne fat could impede solution spread. Details of nerve-root structure and th eir envelope of pia-arachnoid membrane may be relevant to anesthetic action .