How a herniated disc causes pain
As a disc degenerates, it can herniate (the inner core extrudes) back into the
spinal canal, which is known as a disc herniation (or a herniated disc). The
weak spot in a disc is directly under the nerve root, and a herniated disc in
this area puts direct pressure on the nerve, which in turn can cause pain to
radiate all the way down the patient’s leg to the foot (see Figure
1).
Approximately 90% of disc herniations will occur at L4- L5 (lumbar segments 4 and 5) or L5- S1 (lumbar segment 5 and sacral segment1), which causes pain in the L5 nerve or S1 nerve, respectively.
L5 nerve impingement from a herniated disc can cause weakness
in extension of the big toe and potentially in the ankle (foot drop). Numbness
and pain can be felt on top of the foot, and the pain may also radiate into
the rear.
S1 nerve impingement from a herniated disc may cause loss of
the ankle reflex and/or weakness in ankle push off (e.g. patients cannot do
toe rises). Numbness and pain can radiate down to the sole or outside of the
foot (see Figure 2).