The conus medullaris is the tapered inferior-most part of the spinal cord to which is attached the filum terminale.
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Gross anatomy
The conus medullaris is conical in shape, tapering from the distal spinal cord to a narrow point from which a delicate strand of fibrous tissue called the filum terminale extends down to the sacrum and acts to give longitudinal support to the cord.
The upper border of the conus medullaris is not clearly defined, and thus, neither is its length. It is sometimes defined as the segment of the cord from which the sacral and coccygeal nerve roots arise, in which case it measures approximately 3 cm in length 4.
Clinically, the level at which the conus medullaris terminates is more important. This is most often at the L1/2 intervertebral disc level in children and adults 1-3.
After the cord terminates, the nerve roots descend within the spinal canal as individual rootlets, collectively termed the cauda equina.
Variant anatomy
The termination of the conus medullaris has been reported from mid-T11 to mid-L3 vertebral body levels. The average termination varies from study to study 1-3, but a level at or above the L2/3 intervertebral disc level can be considered normal. A level at L3 vertebral body is equivocal and can be a normal variant or the result of a tethered cord 1.
On fetal MRI, the vertebral bodies can be difficult to count accurately. However, the kidneys may be used as a reference, and a normal conus should not terminate below the lower margin of normally positioned kidneys.
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