The compression of the sciatic nerve by the Piriformis muscle is a mechanical action.
The exact mechanism by which the Piriformis muscle compresses the nerve is not fully understood, and there may be multiple contributing factors. However, there are a few proposed mechanisms that could explain how the compression occurs;
Muscle Tightness; When the muscle tightens, it can put pressure on the sciatic nerve, which runs very close to or in some cases, through the muscle.
Muscle Spasm; In some cases, the Piriformis muscle may go into spasm, causing it to contract forcefully and compress the nearby sciatic nerve.
Anatomical Variation; The sciatic nerve and the Piriformis muscle have a close anatomical relationship in some individuals. As described above, in some people, the sciatic nerve may pass through or underneath the Piriformis muscle rather than beside it. If the Piriformis muscle is tight or spasms, it can put direct pressure on the nerve.
The sciatic and peroneal nerve is the thickest and longest nerve in the body. Sometimes both nerves pass the underside the Piriformis muscle before dividing and sometimes they divide and only the sciatic nerve passes through the Piriformis muscle. Other times they both pass through the Piriformis muscle before travelling down the back of the leg, and eventually branching off and ending in the top and the soles of the feet.
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