What causes sciatica?

Sciatica is defined as pain in the lower limb that has its source in the spine. 
The sciatic nerve is a large nerve, which supplies the back of the thigh, the whole leg and foot. 
Sometimes the patient doesn't actually have any back pain, but they have symptoms down the lower limb. 
Many things can cause sciatica, but it is most often as a result of some sort of pressure on the nerves as they exit the spine, or the nerves as they run through structures in the buttock. 
The 2 most common causes of sciatica that I see in my clinic are piriformis syndrome (which is a muscle in the bottock which can compress the sciatic nerve) and degeneration of the spine causing the disc to push on the nerve or the spinal joints to grow spurs and compress/iritate the nerves. 

In a nutshell, all sciatica should be investigated ASAP by a suitably trained musculoskeletal physician. 

Sciatica is a syndrome that refers to pain along the path of the sciatic nerve - the largest nerve in the body. Pain is normally felt in the buttock and down the leg, and can be associated with pins and needles, numbness and in long term cases muscle weakness.

Causes:

1. Bulging / herniated disc
   
2. Bony spurs (osteopahytes)
   
3. Pregnancy
   
4. Piriformis syndrome
   

Sciatica is the term commonly given when pain is referred down your buttock or leg. It may involve burning, tingling, pins and needles, aching, sensory changes (hot and cold sensations) and sharp shooting pain.It is most commonly caused by a disc bulge in your lower back, where the disc is bulging far enough to touch the nerve (the lowest three nerve roots, located behind the 3 lowest discs in your lower back, join together to form the Sciatic nerve, hence the term “Sciatica”.

Sciatica can also occur by chemical irritation (inflammation released from the tear inside the disc), and sometimes this requires anti-inflammatory medication/nerve specific medication to address this component of your problem. This is called Chemical Radiculopathy ;(Radiculopathy is just a scientific term that means nerve pain).  The more common cause is certainly mechanical compression of the nerve via the disc bulging, but your McKenzie Physiotherapist is well skilled at identifying your particular cause and therefore treatment required.

The main reason people develop sciatica:
The short answer is lifestyle!

The spine has normal curves, including a Lumbar LORDOSIS (natural curve in the lower back). Loss of the lordosis occurs with many lifestyle positions on a frequent basis.

Loss of the lumbar lordosis whether prolonged (eg. Slouched sitting or sleeping curled up) or repeated (eg. This occurs with many daily activities such as vacuuming, emptying the dishwasher, ironing, putting make-up on or shaving)starts to put excessive loading on the discs and ligaments.

Initially this increase in loading on the spine caused by regular loss of the lordosis puts strain on the soft tissues such as the ligaments in the spine. At this stage pain is felt while sitting slouched or bending, but quickly disappears once you get out of the offending position.

As the process continues with SLOUCHED SITTING and repeated bending, overstretching of the ligaments occurs, and as they weaken over time this then starts to cause loading on the disc leading. This excessive loading on the disc causes weakening of the cartilage inside it. If not corrected this then leads to bulging of the disc.If this overloading from regular loss of the lordosis is not corrected (posture is the key!!!) the disc can bulge far enough to touch the nerve behind it, giving rise to radicular symptoms in the leg called (Sciatica).

While some people lift something heavy to trigger their back pain, more often the thing that starts the pain is something trivial like a bend forward (especially in the early hours of the day after waking), getting up from sitting or a sneeze. This is as poor lifestyle habits have begun weakening and damaging the disc long before pain arises, and this trivial movement is the last straw.

Joel has answered this question very comphrensively.  Your local McKenzie physiotherapist will be able to help you get on the road to recovery.