The term sciatica refers to pain that is emanating from the sciatic nerve. The sciatic nerve is actually a conglomeration of numerous nerve roots that meet up after coming off from the remnant of the spinal cord. After the first lumbar level, the spinal cord ends and is referred to as the dural sac. It still looks like the spinal cord, but at that point it basically contains nerve filaments and emits a nerve root on each side at each level of the spine.
There are five lumbar vertebrae in a normal spine. At each of these levels a nerve root comes out on each side and is named according to that level. If a nerve root comes out at the fourth lumbar vertebra level, it’s called L4. At the fifth lumbar level, it’s called L5.
Below the five lumbar vertebrae lies the sacrum. The sacrum is one bony anatomic element, however there are holes throughout the sacrum where nerve roots come out as well. The first sacral nerve root is called S1.
The sciatic nerve, as mentioned, is a nerve that combines multiple nerve roots. The roots involved on each side are L4, L5, and S1. The combination into the sciatic nerve makes for a formidable sized structure which runs out the pelvis, behind the hip joint, and down the back side of each leg.
If there is an irritant or pinching to any portion of the sciatic nerve, the resultant pain is called sciatica. The various pain components can be broken down into:
1) Pinched nerves of a nerve root prior to the sciatic nerve formation (L4, L5, or S1 itself) or
2) The sciatic nerve itself is getting pinched with resulting pain after it has been formed.
3) An intrinsic problem of the nerve itself.
In the case of #1, the typical cause is a herniated disc that squeezes out and touches on a nerve root leading to inflammation and pain. It could be simply pain, or the person may also have numbness, tingling, and/or muscle weakness. The symptoms will typically correspond to the particular nerve root that is being compressed by the disk herniation (pinched nerve). For instance, if a disc herniation is pressing on the fifth lumbar nerve root (L5), that patient may end up with a foot drop, which is a weakened ability to lift up the foot (dorsiflex), along with numbness and pain going down the back of the leg into the side of the foot.
Even though the pain is a result of a pinched nerve at L5, the term sciatica is commonly used to describe the painful symptoms since L5 comprises part of the sciatic nerve. It just sounds better than “L5ica”.
Sciatica may also be occurring from a compression of the nerve after the nerve roots combine, as in #2. This can be from a tumor, scar tissue, a tendon, basically anything that is compressing the sciatic nerve enough to cause the symptoms of pain and numbness or the signs of weakness.
An intrinsic problem of the sciatic nerve resulting in sciatica refers to a problem inside the nerve that is not a result of compression. Think of diabetic neuropathy where nerves are being injured on the inside as a result of complications of diabetes. This is a difficult problem since fixing the problem is not as easy as removing a compressive element.