Dr. Robert T. Romano, D.C. - Magnetic Resonance Imaging of the Sacral Plexus
and Piriformis Muscles
This article describes an experiment whereby researches examined the relationship
between the piriformis muscle and the sacral nerve roots.
Chiropractors are aware that the piriformis, when dysfunctional, can cause symptoms
similar to “sciatica” due to the variability of the nerve roots relationship to the piriformis
muscle. Many chiropractic textbooks say that the sciatic nerve can pass above, through,
or below the piriformis muscle, causing various symptoms of nerve impingement due to
contraction of the piriformis. Piriformis syndrome is a “controversial” diagnosis usually
applied to patients with complaints of sciatic pain in the buttocks and the thigh,
exacerbated by prolonged sitting or walking, and typically without sensory deficit, and
which cannot be distinguished to be from a disc or other issue.
This study involved 100 patients who were to undergo routine MRI examination. While
doing their prescribed MRI, the researchers also took axial and sagittal T1-weighted MRI
of the sacrum to visualize the piriformis and the sacral nerve roots. Each patient was
examined bilaterally (two sets of nerve roots and 2 piriformis muscles), totaling 200
samples to study. The study also aimed to correlate symptoms indicative of piriformis
syndrome with radiological findings. Unfortunately (for us!), none of the 100 patients had
symptoms suggestive of “classic” piriformis syndrome, so that correlation was not able to
be done adequately. This study was primarily undertaken to elucidate the anatomical
variations of the piriformis and its relationship to the sacral plexus roots, with the goal of
perhaps shedding some light on the poorly understood condition called “piriformis
syndrome”.
The results showed that there is variability in the manner with which the sacral roots
relate to the piriformis, and there is also variability of the size of the piriformis muscle
when compared bilaterally in the same individual. Specifically, the S1 nerve roots pass
above the piriformis more than 99% of the time, the S3 roots pass through the piriformis
muscle 97% of the time, and the S2 goes through 75% of the time (other 25% went
above). S4 was almost always below the muscle. It is interesting that S2 is less likely than
S3 to pass through the muscle. Also clinically important is the variation in piriformis size,
with asymmetry common. About 20% of the patients had piriformis asymmetry greater
than 3mm, with differences as big as 8mm between piriformis muscle sizes.
None of the patients with asymmetry had piriformis syndrome. The researchers
concluded that the S2 nerve root passing through the piriformis is a normal variation, and
is not associated with the symptoms of piriformis syndrome in the patients involved in
the study. A limitation of the study was the fact that the symptoms of piriformis
syndrome were not arrived at via examination, but instead with a questionnaire, and
actually physically examining a patient is a more sensitive way to test for piriformis
syndrome.