MRI-based evaluation of lower back pain with reference to ligamentum flavum hypertrophy

Introduction

One of the most common clinical symptoms in day-to-day clinical practice is lower back pain. It affects around five percent of the adult population each year and the lifetime incidence of lower back pain in adults is around seventy percent to eighty percent. Lower back ache is foremost among disorders attributed to degenerative disease.¹

The vertebrae in the vertebral column are aligned cranio-caudally to help in the motion and stability of the body. An intervertebral disc is present between two vertebrae which provides balance to the vertebral column. The intervertebral disc with the two vertebrae forms the vertebra-discal complex. Each disc is made up of an outer annulus fibrosis and the nucleus pulposus forms the inner core of disc. NP is formed from the notochord. It is a gel-like material formed predominantly from proteoglycans with some chondrocytes and is composed of collagen fibers (type II) that traverse through hydrophilic glycosaminoglycans (GAGs).

Ligamenta flava are paired ligaments that pass between the laminae of adjacent vertebral bodies. They are present from C2/C3 to the sacrum and form part of the posterior ligamentous complex. Its attachment is from the front of the upper lamina to the back of the lower lamina.

Degeneration starts in one joint, spreads to the other two, and in due course affects all three joints and then the complex (the disc, the two posterior joints). The mechanical alterations arise later and have similar effects on the intervertebral levels above and below. As a result, there are several levels of lumbar spinal spondylosis and stenosis.²

The degenerative changes can involve the ligamentum flavum, intervertebral disc space, or adjacent soft tissue structures. Stenosis of the lumbar spinal canal is the most common spinal disorder in the elderly, which causes leg pain, backache, and paresis. The ligamentum flavum hypertrophy results in canal narrowing and eventually canal stenosis.³

Disc structure and function undergo progressive degeneration, beginning in the young adult, and is predominantly evident in the Nucleus Pulposus (NP). Changes associated with early degeneration occur in the disc’s nucleus pulposus. Loss of water, increase in denatured type II collagen and depletion of proteoglycans are observed in the Nucleus Pulposus which occur with aging and degeneration undergoing a transition from “fluid-like” behavior to more “solid-like” behavior leading to reduction of the capacity of the nucleus pulposus to attract and bind water, causing disc dehydration and decreased hydrostatic pressure. Similar to this, type II collagen levels rise in the annulus fibrosus, which also lowers disc hydration. The disc becomes friable as a result, developing fissures and gradually losing structural integrity.

The usual thickness of LF is less than 4 mm above which is considered hypertrophy. The ligamentum flavum thickens with aging and this increase is most marked at the lower lumbar vertebral levels. Hypertrophy is mostly from fibrosis caused by the build-up of mechanical strain with the aging process, predominantly along the dorsal aspect of the ligamentum flavum.⁴

Mechanical stress is the most important factor that influences LF hypertrophy. In particular, from L2/3 to L5/S1 in males, and L5/S1 in both females and males, the effect of mechanical stress seems to be dominant because there were no statistical correlations between age and LF thickness. Reduced estradiol levels brought on by menopause and aging may contribute to LF hypertrophy in females. No other sex hormones have been reported to have the effects. For LF hypertrophy in males, increased mechanical stress on the LF due to IVD degeneration seems to be a more dominant factor than age. Therefore, the onset of instability including spondylolisthesis and lumbar canal stenosis may be related to gender differences.

Given that the ADC can estimate the diffusion of unbound water molecules and that DW MRI is a non-invasive method for measuring the diffusion of water within the tissue, it may be used as a quantifiable method to evaluate the pathological changes in water diffusion brought on by disc disease. Former studies exhibited that the decrease in nutrient supply and the integrity of the disc matrix causes a diminished diffusion, and the degenerative changes could be reflected through the measurement of the value of ADC in the nucleus pulposus (NP).⁵ ADC depicts microscopic structure, the organization of tissue, and its specific diffusion capacity. The restriction of the molecule movements leads to a loss of signals in the diffusion sensitivity sequences. By calculating the ADC, this loss of signal can be measured. The areas of restricted diffusion will appear as areas of low signal (contrary to Diffuse Weighted images) on the ADC map and suggest high cellularity. The lesser the viscosity of the fluid or cellularity of the tissue, the higher the Apparent Diffusion Coefficient value (no restriction).⁶ An estimation of the free diffusion of unbound water is provided by the Apparent Diffusion Coefficient and the ADC could be used to evaluate deteriorating changes. The difference in ADC value between visually normal and moderately degenerated discs was estimated to be 4%. Severely degenerated discs have exhibited five percent higher ADC values compared to the normal discs, likely because of the free water in the fissures and cracks formed in those degenerated discs.⁷

As morbidity increases due to spinal canal degeneration and leads to disability, the need for early intervention and prompt treatment is needed. Therefore, through this study, we aim to compare the ADC value of the nucleus pulposus to the size of ligamentum flavum in patients with a degenerated spine. This study also intends to help clinicians in understanding the changes in intervertebral discs analyzed via MRI. The role of ADC values of nucleus pulposus and its changes in degenerative disc disease would further help in understanding the nature of the progression of the disease towards spinal canal stenosis. Therefore, this study plays a key role in early detection of degeneration and helps stop the evolution of the disease via early intervention.

Methods

Discussion

Daily, a lot of patients present with complaints of back pain. MR Imaging is a standard imaging modality for detection of the disc disease which is ideally appropriate for defining the presence, extent, and spine degeneration complications due to its advantage of the multiplanar imaging, capability of precise localization of changes in intervertebral discs, and excellent spinal soft-tissue contrast.⁸ It can readily determine pathology in the disc, degenerative variations in endplate, facet, and ligamentum flavum changes, and the sequelae of instability.

Mechanical stress is the most important factor that influences LF hypertrophy. In particular, from L2/3 to L5/S1 in males, and L5/S1 in both females and males, the effect of mechanical stress seems to be dominant because there were no statistical correlations between age and LF thickness. The process of aging and menopause which lowers the estradiol levels may contribute to the hypertrophy of ligamentum flavum in females. No other sex hormones have been reported to have the effects. The increased mechanical stress on the LF due to Inter vertebral disc degeneration seems to be a more dominant factor than age in males. Therefore, the onset of instability including spondylolisthesis and lumbar canal stenosis may be related to gender differences. However, the difference in the thickness of ligamentum flavum between the genders is not very significant.

Through the use of MRI, the ADC values of the measured degenerated discs can be compared to those of the measured discs without ligamentum flavum hypertrophy and correlated, allowing for the possibility of earlier degeneration detection and stopping further progression. Since it is non-invasive, it reduces the burden of a patient going through invasive procedures.