As a key component of spine structure, endplate plays an important role in nourishing intervertebral disc and conducting stress, and has a significant impact on physiological state and pathological changes of intervertebral disc. In the treatment of IDD, motion preservation techniques are increasingly used. However, these techniques have certain requirements for endplate shape and height of intervertebral space. Therefore, it is helpful to study the correlation between morphology of endplate and IDD in clinical work. Previous studies on the morphology of the endplate mostly focused on the anatomical structure or the use of imaging to observe the transverse and sagittal diameter, circumference and area of endplates, as well as the measurement of their concave angle and relative curvature.[13] Although measuring the concave angle can objectively and quantitatively reflect the depression degree of endplate, it is so complex, and the uncertainty of the arc vertex positioning makes larger deviations. By contrast, the method used in this study can intuitively define the shape of endplate, which is simple to operate.
Harrington et al.[14] observed the influence of axial shape of the endplates on herniated discs, and pointed out that the endplate morphology with high curvature is an independent risk factor for LDH. Pappou et al.[7] analyzed the correlation between endplate morphology and IDD in patients with low back pain. Though their results were similar to ours, the study subjects were different, and they didn’t take Modic changes and disc herniation into consideration.
According to our results, we found that concave endplate is the main type in intervertebral discs with lower grade of degeneration on MRI. The biomechanical studies[15,16] show that the stress conducted by normal disc is mainly concentrated in the center of the vertebral endplate. The stress not only affects the volume and shape of the disc, but also has an effect on the endplate. The load of axial stress, especially, can lead to the deformation of endplate and trabecula under it. That may explain why concave endplate is commonly seen in discs with milder IDD. In addition, when the intervertebral disc degenerates, the hydrodynamic characteristics of the nucleus pulposus gradually disappear, and the stress shifts from the center of the endplate to the periphery, resulting in relatively concentrated stress on the peripheral endplate, which not only increases the shear force and makes it prone to microfracture,[15] but also activate the bone reconstruction process of the endplate and vertebral body,[3] which will eventually lead to the gradual loss of the peripheral height of vertebral body and the flattening of endplate.[17] Meanwhile, the stress load of the peripheral part of lumbar intervertebral disc exceeds the normal range, which will lead to the injury of annulus fibrosus and accelerate the occurrence of LDH. Moreover, studies have shown that the pressure of lumbar endplate increases gradually from top to bottom.[18] In our study, with the descent of lumbar segments, the concave endplates gradually decreased, and the flat endplates were increasingly common. The endplates of L5/S1 segment were mainly flat type, which may be related to the higher stress load of L4/5 and L5/S1 segments. The proportion of irregular type was the least and mainly concentrated in L5/S1 segment. We believed that the long-term effect of large stress load may easily lead to the irregular shape of endplate. Since L5/S1 segment is located at the lumbosacral junction and has high shear force, irregular endplates are more likely to appear in this segment than in others. In the process of disc herniation, the endplate morphology changes from concave to flat and then irregular, and the degeneration degree of corresponding disc gets increasingly severe. Meanwhile the herniated discs are mainly accompanied with flat and irregular endplates. The result that the proportion of Grade V IDD in irregular endplate is far greater than that of the other two types can also explain this.
Modic changes in MRI signal of endplate are highly correlated with IDD[19] account for approximately 19%-59% of disc degenerative diseases.[20,21] At present, it is generally believed that abnormal stress load after IDD,[4] lumbar instability[22] and release of inflammatory factors[23] will cause vertebral microfracture and affect local microenvironment of marrow, leading to histological changes which ultimately manifest as changes in MRI signal of endplate. Based on our results, Modic changes accounted for 30% in patients with LDH, and irregular endplates had more Modic changes than the ones of other two types. It is reasonable for us to speculate that the Modic change may be the result of the endplate subjected to long-term abnormal stress and degeneration to a certain extent.
The current study has several limitations. Firstly, limited by the hardware conditions of our radiology department, we didn’t use computed tomography (CT) reconstruction which was better for evaluating the sagittal morphology of endplate than MRI. Studies have shown that the bony edge of endplate is more easily identified on CT images.[24] Secondly, the relatively few evaluators. Considering the cognitive differences among specialties, we selected physicians from orthopedics and radiology department to assess the data. Nevertheless, increasing the number of evaluators will eliminate coincidence as much as possible, and improve the reliability of the results. Finally, we didn’t analyze the coronal morphology of endplate, and its effect on LDH or other lumbar degeneration disease needs further research. Therefore, high-quality, large sample, and multicenter studies should be performed in our future clinical work to provide spine surgeons with the best evidence-based information.