There have been some studies in literature evaluating the sciatic nerve with elastography.In 2013, Santos claimed that sciatic nerve can be visualized and evaluated by US elastography[9].After that,Andrade assessed the elasticity of sciatic nerve during passive movements, using SWE. It was demonstrated that the sciatic nerve stiffness can be assessed accurately using elastography,and the stiffness of the sciatic nerve can be affected by the lower limb movements[10].
LDH,which is manifested by low back pain or sciatica, is one of the common causes of disability in the community[11]. In LDH, nucleus pulposus(NP) material exits the disk space area through the annulus fibrosus(AF), causing mild compression on the spinal nerves and resulting in neurological dysfunction including pain, sensory deficits, and weakness in the low back and leg[12].There are several changes in the biology of the intervertebral disc which is thought to contribute to LDH. These include increased percent of type I collagen within the NP and inner AF [13] , reduced water retention in the NP [14, 15, 16],degradation of collagen and extracellular matrix materials (ECM)[17], and upregulation of systems of degradation such as apoptosis, matrix metalloproteinase (MMP) expression,and inflammatory pathways [18].Increased expression of chemokines are associated to clinical severity of sciatic pain in lumbar disk herniation patients[19] .Mechanical compression and accompanying chemical stimulation may cause severe nerve root damage, which in turn affects the axonal transport, affecting the circulation of nerve roots and the metabolism of neurotransmitters[21]. Thereby causes neurilemma edema, fibroblast infiltration, and nerve fiber deformation.These changes can lead to demyelination of the nerve and can alter the nerve connective tissue [20], causing the proliferation of scar tissue and the acceleration of the transverse wave in the nerve.SWE is an imaging technique that emits transverse waves to tissue through a transducer.According to E=3ρc∧2 (E is the Young's modulus, c is the shear wave propagation velocity, and ρ is the tissue density), the machine displays the E value,The faster the shear wave propagation speed, the higher the E value.SWE can quantitatively calculate the E value according to the shear wave propagation velocity in the region of interest, and evaluate the elastic properties of tissues qualitatively by the E value.The tissue stiffness is displayed by color coding.The harder the tissue, the faster the shear wave travels, the higher the E value, and the image appears red. Conversely, the smaller the E value, the softer the tissue and the blue color of the image.In this study, the sciatic nerve of affected appeared blue-green, while the normal one appeared blue. In addition, axonal injury and demyelination occur in parallel with the severity of the injury. Increased expression of chemokines is closely related to the clinical severity of sciatica in patients with LDH[19].
Limitations
There are a couple of tissues that have to be mentioned and discussed in this study.First,the sample size is small,and the method is not suitable for thick gluteal subcutaneous fat tissue. Second,demographic data such as body mass index、walking habit should be considered.Third,a study ,which evaluated the cross-sectional area of the sciatic nerves with tracing method, observed the sciatic nerve enlargement in patients with unilateral sciatica[3].In contrast,this study showed no significant difference between the affected and unaffected sides in respect of the sciatic nerve diameter. The difference in this measurement method can be attributed ed as a limitation. The tracing method is more sensitive and accurate than other methods to measure nerve size .Fourth,Andrade demonstrated that the stiffness of the sciatic nerve can be affected by the lower limb movements[10].This study observed the sciatic nerve in prone position during plantar flexion for the knee 180° condition.Therefore,other position and lower limb movements should be considered.