Clinical relevance of endplate morphology Lumbar intervertebral disc herniation is one of the most commonly diseases of low back in clinically, also the most frequent cause of low back pain. Intervertebral disc of L4/5 and L5/S1 are the most frequently herniated discs. At present, intervertebral disc herniation was considered resulted from the degeneration and heredity of discs and rotational and axial flexion compression which act on the anulus fibrous. However, these hypotheses could not satisfactorily interpret the pathological basis of herniation in many non-manual workers and large-body-size people. Previously published research on the disc focused on the nucleus pulposus and the anulus fibrosus, whereas, the studies on the endplate particularly laid emphasis on its biological stress and nutrition. The morphology of disc was rarely investigated. The exact relationship between lumbar disc morphology and intertebral disc herniation is still not clear[4].
Heliovaara[5] suggested that only body height was related to the intervertebral disc herniation. Adams et al[6] found that slight damage to the endplate would result in the progressive destruction of contiguous discs. Lu et al[7] suggested, in a study on spinal loading with 3-d finite element model to separate simulate the disc height of 8 mm, 10 mm and 12 mm, that the height of disc had significant effects on axial shift of verterbral body, intervertebral disc posterior-lateral protrusion and tension of outer anulus fibrosus, but slight on inner pressure of disc and transmission of endplate pressure. However, these studies prophesized that intervertebral disc belong to the linear mechanic model and morphology and area of endplate had not been considered, therefore, were not satisfactory to interpret the full stress-bearing mechanism of lumbar. Zhou et al[8] suggested the width and depth of vertebral Lamina of L3 to L5 increased gradually and the morphology of endplate was direct influential on the height of discs. In the present study, we had measured the circulatory and area of L4 with the result of 14.1 cm and 1 492 mm2, respectively, whereas not measured that of L3, L5 and superior S1 endplate.
The endplate morphology of low lumbar gradually transformed from heart-shaped to ovoid. Whether this kind of obviously morphological changes could affect the biomechanic transmission of disc and whether the endplate morphology is different in intervertebral disc herniation from normal are unclear yet. These problems had been resolved in this study. In addition, the research on the morphology of Lamina of lumbar vertebra is facilitating to rational intervertebral confluence with implanted bone and the design of artificial intervertebral disc prosthesis domestically.
Evaluation of methods for studying endplate morphology
The endplate morphology has been previously investigated by the means of autopsy specimens, X-ray plain film and conventional CT. Nevertheless, the autopsy specimens are hardly available. X-ray plain film merely provided two-dimension images and could not display endplate in sectional plane. The 3-d reconstruction in conventional CT is not satisfactory for the insufficiency of equipment and undesired cooperation of patients. Hall et al[2] tried to establish a geometry model of low lumbar endplate with CT which was difficult to apply in practice. On magnetic resonance imaging, the borderline of endplate is hart to define. Spiral CT has been established is superior to Magnetic resonance imaging in the discrimination of bone and the required imaging time. Therefore, in our opinion, spiral CT is deserved an alternative excellent method to explore the disc[9-10].
Image quality of spiral CT and its clinical significance
Endplate is ovoid shape. It is thicker peripherally and relatively thinner centrally, and the anterior thickness is larger than the posterior thickness, but the bilateral thickness is essentially similar. The 3-d reconstruction in conventional CT is not satisfactory for the insufficiency of equipment and undesired cooperation of patients. With the development of spiral CT and popular application of computer, 3-d technique experienced a marked advance. The application of spiral CT had quickly extended from complex anatomic locations such as craniofacial region, thorax and abdomen to bone joints in extremities and spine. In the 3-d reconstruction of spiral CT, a thinner slice thickness, larger pitcher and lesser reconstruction gap (50 percentage of slice thickness) are appropriate. Generally, a slice thickness no more than 3 mm, a table feed no more than 5mm and a reconstruction thickness no more than 2 mm are recommended for smoothing the cortical bone and without step-like artifacts on MPR images. In addition, image quality is closely related to the window width and widow centre, correctly selection, coordinating and matching are required for the images with high contrast resolution and good quality. In current study, a thickness of 1 mm, a window with of 300 and a window centre of 80 were used for the perfect images. As the disc height is larger in central than that in peripheral. The endplate morphology could not be precisely described on conventional CT due to the parallelized scan image plain to intervertebral space. However, in spiral CT, volume rendering and curve planar cut could be applied, then, make the measurement of morphology more simply and accurately. In addition, another superiority of cure planar reconstruction is that the reconstructed images could be arbitrarily cut, therefore, the scan plane is unnecessary to parallel to the intervertebral space. The scan thus is more easily to perform and the accuracy of examination is higher[11], especially in females with larger lumbar-sacral angle between L5 and S1.
APD and TD of endplate
In our study, the APD and TD were longer in intervertevral disc herniation than that in normal. Especially, there was significantly difference in APD between intervertevral disc herniation and normal control. It suggested that anterior-posterior diameter was the main change in intervertevral disc herniation, consistent with the disc herniation direction seen in clinical.
Circumference and area of endplate
Normally, the endplate and anulus fibrosus are enclosed together by nucleus pulposus, thus, the disc forms similar to a tightly sealed self-limiting container to buffer outer forces. From the research emerged, most scholars conceded that the lumbar endplate morphology transformed from heart-shaped superiorly to ovoid inferiorly and directly affected the height of discs. Harrington et al[12] had found, the multivariates correlation and regression analysis of CT scan that intervertebral disc herniation in L4/5 and L5/S1 is closely related with the morphology of endplate margin, while the area of endplate affected intervertebral disc hernaiton in male. Our data suggested that the endplate of L4/5 and L5/S1 changed from ovoid to round gradually, which indicating increased index of vertebral lamina in intervertebral disc hernaiton. The intervertebral disc hernaiton correlated with endplate circulatory in males, whereas the correlation was found merely in L5/S1 in female intervertebral disc hernaiton, which suspect to result from the larger lumbar-sacral angel in females.
Endplate morphology and inner pressure of anulus fibrosus
Endplate, as a flat disciform structure, is thicker peripherally and relatively thinner centrally. The anterior thickness of endplate is larger than the posterior thickness, while, the bilateral thickness is basically equal. The biomechanics research indicated that increased tension of anulus fibrosus resulted in 1aceration of anulus fibrosus, furthermore, led to intervertbral disc herniation. Water, collagen and proteoglycan are the main components of discs. The water fraction of discs varies with aging from 70% to 90% individually. Whereby, intervertebral disc could be considered as a short cylinder fully filled with liquid. Theoretically, the inner mural pressure of annulus fibrosus could be obtained from Laplace formula as follows: inner mural pressure=transmission pressure×radius/mural thickness. According to Laplace law, inner pressure of lateral intervertebral disc was highest, which suffered most possible to protrude[13]. However, in clinic, the posterior-lateral intervertebral disc herniation is most common. The estimated reasons include as follows: lateral intervertebral disc herniation has not identified for lack of symptom related to neuropathy. Posterior-lateral anulus fibrosus is relative thinner. The posterior-lateral ligament is easily stained for flexion-extension, which is the main physical activity in normal people.
Our results showed that morphological index increased in intervertebral disc herniation, whereby, suggesting endplate morphology was an important factor for intervertebral disc herniation. Nevertheless, posterior-lateral lumbar disc herniation occurred frequently not only for its morphological changes, but the larger stress lumbar endured and the direction of outer forces. Thereby, the stress and vertebral lamina morphology are possible two very important factors for the lumbar intervertebral disc herniation[14].
