Lumbar radiculopathy is characterized by chronic pain that occurs in the lower back and legs. Early and correct diagnosis is essential. Magnetic resonance imaging is currently the most effective method for diagnosing lumbar radiculopathy[6–8]. However, some patients who have large intervertebral disc herniation, according to imaging, have very mild symptoms of pain in the lower back leg. Therefore, González Espinosa de Los Monteros et al. proposed to check the patient's nerve root status by measuring their neurodynamic or orthopedic tension. The results of the slump test and Dejerine's triad test combined with straight leg test and Bragard test in multiple parallel manners showed the highest diagnostic accuracy and validity. The essence of the aforementioned study is to induce pain or paresthesia, by increasing tension in the nerve root, and determine the incidence of lumbar radiculopathy. This concept is consistent with the theoretical basis of our study: The diagnosis of lumbar radiculopathy should be based on the tension of the nerve root, rather than completely based on imaging technology.
Professor Shi proposed that the pathological basis of lumbosacral nerve bowstring disease is nerve root or axial traction injury. However, most cases of this disease have no positive imaging findings. Furthermore, the results were consistent with those of our study that lumbar radiculopathy is caused by increased tension in the nerve root.
Some studies had demonstrated that the pathogenesis of lumbar radiculopathy is the nerve root axial traction injury. The spinal cord and its nerve roots showed rubber-like elasticity[11–13]. Singh et al. found that the nerve root is elongated and the cross-sectional area is reduced when a nerve root is statically or dynamically pulled.
Electrophysiological monitoring found that the nerve conduction velocity and potential amplitude gradually decreased and the comprehensive action potential area became smaller or even disappeared, which eventually led to complete blockade of nerve root conduction.
Along with the increase in nerve root tension, stretching tension can not only directly lead to abnormal nerve conduction but also reduce blood flow in nerve roots and cause nerve ischemia-reperfusion (I/R) injury. Lin et al.13 found that when radiation pain occurred in a straight leg elevation test, the blood flow was reduced by more than 70% in the L5/S1 nerve roots.
Because there is only a thin outer membrane around the nerve root, the nerve root is prone to mechanical injuries and its degree of damage is also high. The resistance to traction only accounts for 10% of the peripheral nerve[14–15]. In addition, the dorsal root ganglion cells, located on the dorsal side of the nerve root, are not only vulnerable to various inflammatory factors, which causes a series of hypoxia-ischemia reactions, but are also more vulnerable to mechanical injury[16–18]. Our results also confirmed that after decompression, the nerve root tension of the patient was significantly reduced, while the VAS score was also significantly improved; these changes were positively correlated with one another.
Methods that protect the nerve roots and avoid excessive traction during surgery have been the main concerns for many surgeons. To date, several studies have reported on nerve root blood flow[13, 19, 20], but only a few studies have reported on the mechanical tension in nerve roots because, from an ethical point of view, it is almost impossible to perform such invasive measurements in patients. Only human specimens or in vivo animal models can be used to measure the tension in the spinal nerve roots.
Recently, a surgical simulation system was developed, which involves a virtual reality simulator combined with real anatomical structures composed of synthetic materials. An inexperienced doctor can simulate the surgical operation through this system and record the nerve root damage during surgery. However, the tension value of the nerve root cannot be immediately obtained during surgery, in order to guide the surgeon how to decompress properly and expand the intervertebral space, of appropriate height, to achieve the best results.
Our device for measuring nerve root tension is modified from a transverse gauge. A nerve hook is installed at the end of the force rod and the tension value is obtained by effective displacement of the nerve root when it is pulled by the nerve hook. The process is the same as with the normal intraoperative traction of the nerve root to expose the intervertebral space, but the degree of traction is much smaller than that of the normal operation. Therefore, it is safer for the nerve root and will not cause additional injury. The surgeon can judge whether the decompression is complete by comparing changes in nerve root tension before and after decompression. When the surgeon installs the intervertebral fusion cage and measures the tension again, a significantly increased tension will indicate that the cage height is too large and needs adjustment.
We consider that the loss of nerve root contractures and elasticity is in parallel with spinal degeneration and loss of intervertebral space height in elderly patients. With traditional lumbar spine surgery, the risk of iatrogenic nerve root injury is very high if the nerve root was in the axial direction.
Previous studies have shown that the nerve root traction injury caused by excessive expansion of the intervertebral space during surgery and the excessive use of the intervertebral fusion cage will cause nerve root I/R injury, postoperative pain and numbness in the lower limb, and relatively severe complications of foot drop[4, 22]. Professor Shi Jiangang proposed the three heights (anatomical, natural, and pathological) idea of intervertebral disc. The size of the intervertebral fusion cage is appropriate to restore the natural height of the intervertebral space to maintain the optimal tension of the spinal nerve root, which is the “nerve standard” for spinal decompression. Our study on measuring intraoperative nerve root tension aimed to provide an accurate basis for this standard. The hand-held nerve root tension meter used in this study belongs to the first-generation product, which still has some shortcomings, including measurement error caused by unstable control of the tester during the measurement process. Nevertheless, to the best of our knowledge, no similar study has been reported to date. The method described in our study enables, for the first time, the immediate and non-invasive measurement of nerve root tension during surgery, which provides a new clinical direction.
In future studies, we will focus on improving the accuracy of the measuring instrument. Meanwhile, this study highlighted that more spine surgeons should pay attention to the nerve root tension and not be satisfied with physical compression of nerve roots and excessively pursue the restoration of the anatomical height of intervertebral space.