OCCULT LUMBAR LATERAL SPINAL STENOSIS IN NEURAL FORAMINA SUBJECTED TOPHYSIOLOGICAL LOADING

PURPOSE: To measure the effect of extension, flexion, lateral bending, and axial rotation loads applied to the spine on the anatomic relatio nship of the spinal nerves in the neural foramen to the ligamentum fla vum and the intervertebral disk; and to determine the effect of disk d egeneration on the response to loading. METHODS: Cadaveric lumbar moti on segments were examined with CT and MR imaging, loaded with pure mom ent forces, frozen in situ, reexamined with CT, and sectioned with a c ryomicrotome. The morphology of the intervertebral disks was classifie d on the basis of the appearance of the cryomicrotome sections. The ne ural foramina were classified as having no evident stenosis, as being stenotic, as having occult stenosis, or as showing resolved stenosis o n the basis of the images and sections before and after loading, The s tenotic and nonstenotic foramina were stratified by disk level, interv ertebral disk classification, and type of loading applied. The effect of spinal level, disk type, and load type on the prevalence of stenosi s was studied. RESULTS: On average, extension, flexion, lateral bendin g, and axial rotation resulted in the ligamentum flavum or interverteb ral disk contacting or compressing the spinal nerve in 18% of the neur al foramina. Extension loading produced the most cases of nerve root c ontact, and lateral bending produced the fewest cases. Each of the loa ding types resulted also in diminished contact between the spinal nerv e and the intervertebral disk or ligamentum flavum in some cases. Disk degeneration significantly increased the prevalence of spinal stenosi s. All foramina associated with advanced disk degeneration and half of the foramina associated with disks having radial tears of the annulus fibrosus either developed occult stenosis or were stenotic before loa ding. CONCLUSIONS: The study supports the concept of dynamic spinal st enosis; that is, intermittent stenosis of the neural foramina. Flexion , extension, lateral bending, and axial rotation significantly changed the anatomic relationships of the ligamentum flavum and intervertebra l disk to the spinal nerve roots.