Morphometric studies of the ligamentum flavum: a correlative microanatomical and MRI study of the lumbar spine.

Background: Foraminal degenerative lumbar stenosis is traditionally considered a result of bony narrowing due to osteophytic appositions on the superior articular process. Clinical experience reveals that significant additional compression of the neural structures is due to degenerative hypertrophy of the adjacent ligamentum flavum. Therefore, microanatomical and neuroradiological investigations were performed to determine the microtopography of this ligament, especially with respect to its lateral extension.

Methods: Lumbar spine specimens of eight mid-aged human cadavers (mean age 34.5 years) were collected, and MRI studies with T1-weighted images were performed. The specially embedded specimens were sectioned horizontally at the level of the spinal ganglion (slice thickness: 2 mm). Anatomical morphometric data were correlated with identical measurements based on neuroradiological imaging and were analyzed statistically.

Results: The distance between midline and extraforaminal extension of the ligamentum flavum showed a mean value of 17 mm. The distance increased to 19 mm when the lateral insertion was correlated to the origin of the ligamentum flavum at the anterior margin of the lamina. The farthest lateral segment of the ligamentum flavum was determined in each case; it covered the synovial cavity of the lumbar facet joint in the direction of the extraforaminal segment of the intervertebral canal.

Conclusions: Measurements from mid-aged cadavers show the extent of the ligamentum flavum including its intra- and extraforaminal parts. Due to this anatomical situation a hypertrophic ligamentum flavum may contribute significantly to nerve root compression at the level of the lateral spinal recess. This has to be kept in mind during surgical decompression, which might be incomplete unless these hypertrophied parts are completely removed.