Magnetic resonance imaging of the sacral plexus and piriformis muscles.
OBJECTIVE
The objective was to evaluate the piriformis muscles and their relationship to the sacral nerve roots on T1-weighted MRI in patients with no history or clinical suspicion of piriformis syndrome.


METHODS
Axial oblique and sagittal T1-weighted images of the sacrum were obtained in 100 sequential patients (200 pairs of sacral roots) undergoing routine MRI examinations. The relationship of the sacral nerve roots to the piriformis muscles and piriformis muscle size were evaluated, as were clinical symptoms via a questionnaire.


RESULTS
The S1 nerve roots were located above the piriformis muscle in 99.5% of cases (n=199). The S2 nerve roots were located above the piriformis muscle in 25% of cases (n=50), and traversed the muscle in 75% (n=150). The S3 nerve roots were located above the piriformis muscle in 0.5% of cases (n=1), below the muscle in 2.5% (n=5), and traversed the muscle in 97% (n=194). The S4 nerve roots were located below the muscle in 95% (n=190). The piriformis muscles ranged in size from 0.8-3.2 cm, with an average size of 1.9 cm. Nineteen percent of patients had greater than 3 mm of asymmetry in the size of the piriformis muscle, with a maximum asymmetry of 8 mm noted.


CONCLUSIONS
The S1 nerve roots course above the piriformis muscle in more than 99% of patients. The S2 roots traverse the piriformis muscle in 75% of patients. The S3 nerve roots traverse the piriformis muscle in 97% of patients. Piriformis muscle size asymmetry is common, with muscle asymmetry of up to 8 mm identified.
Department of Radiology, St. Vincent's Medical Center, 1800 Barrs Street, Jacksonville, FL, 32204, USA.
J Matthew
JM
Russell
Mark J
MJ
Kransdorf
Laura W
LW
Bancroft
Jeffrey J
JJ
Peterson
Thomas H
TH
Berquist
Mellena D
MD
Bridges
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002081:Buttocks / Q000294:innervation*; D005260:Female; D006801:Humans; D008160:Lumbosacral Plexus / Q000033:anatomy & histology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009132:Muscles / Q000294:innervation*; D011446:Prospective Studies; D012584:Sciatic Nerve / Q000033:anatomy & histology*; D011795:Surveys and Questionnaires