Central Herniated Disc
A central disc herniation is located in the center of the spinal canal and can compress the dura. (sac containing the spinal nerves). Depending on the size of the herniation, the nerves to both legs can be effected and under rare circumstances, the nerves to the bladder and bowel can be compressed.

Posterolateral Herniated Disc
A posterolateral herniation is located to one side of the spinal canal and compresses one side of the dura (sac containing the spinal nerves) and the spinal nerve shown in the diagram to the right. The herniation will effect one leg only because of its location.

Foraminal Herniated Disc
A foraminal disc herniation is located further to the side of the spinal canal underneath the nerve and is in the bony tunnel (foramen) in which the nerve exits the spine. After the nerve leaves the spine, it travels into the leg to perform its intended function. Compression of the nerve in the foramen (bony tunnel) causes symptoms in one leg only.

Extra-Foraminal Herniated Disc
An extra-foraminal herniation is located even further to the side of the spinal canal and is outside the bony tunnel (foramen) where the nerve exits. Compression of the nerve outside the foramen (bony tunnel) causes symptoms in one leg only.

The natural history of the herniated disc is controlled by essentially two factors: space available for the nerve and degree of tension on the nerve. These factors are not predictable from viewing an MRI scan and are three dimensional in nature. The clinical course of the patient reveals the improtance of these two factors. Why do some people improve without surgery (approxiamtely 85%)? The reason is that the space available for the nerve and degree of tenison on the nerve are favorable in most people.