The relationship of lumbosacral plexus to the sacrum and the sacroiliac joint.

The lumbosacral plexus was dissected bilaterally in 20 adult cadavers to define the anatomic relationship of the lumbosacral plexus to the sacrum and the sacroiliac joint. All results are mean values +/- standard deviation. The length of the nerve roots of the lumbosacral plexus gradually decreased from L-4 to S-3 (from 93.8 +/- 6.9 mm in males and 108.7 +/- 7.7 mm in females at L-4 to 43.7 +/- 4.3 mm in males and 49.0 +/- 7.6 mm in females at S-3). The angle projected by the nerve roots of the lumbosacral plexus with respect to the sagittal plane gradually increased from L-4 to S-3 (from 14.3 degrees +/- 3.4 degrees in males and 16.7 degrees +/- 4.8 degrees in females at L-4 to 51.8 degrees +/- 9.0 degrees in males and 57.8 degrees +/- 9.1 degrees in females at S-3). The width of the nerve roots of the lumbosacral plexus was greatest at S-1 (9.8 +/- 1.8 mm in males, 8.6 +/- 1.5 mm in females). The L-5 nerve root was the thickest in males (4.4 +/- 0.5 mm), and the S-1 nerve root was thickest in females (4.3 +/- 0.4 mm). The lumbosacral trunk was 30.0 +/- 9.0 mm in length in males and 32.0 +/- 6.0 mm in females; 11.4 +/- 1.8 mm wide in males and 11.2 +/- 1.5 mm in females; and 4.4 +/- 0.5 mm thick in males and 4.0 +/- 0.6 mm in females. The fifth lumbar nerve root and lumbosacral trunk coursed across the sacroiliac at a level 2.0 +/- 0.2 cm below the pelvic brim and were relatively fixed to the sacral ala with fibrous connective tissue.