INTRODUCTION

Disc herniation is one of the most common causes of radiculopathy and myelopathy, most commonly in the lumbar and cervical spine (^{Ma et al., 2013}). Compared with cervical and lumbar vertebrae, TDH is relatively rare in the clinic, and the incidence of symptomatic TDH is about 0.001‰, accounting for only about 4 % of all cases of disc herniation (^{Hott et al., 2005}; ^{Quint et al., 2012}; ^{Oppenlander et al., 2013}; ^{Yoshihara et al., 2014}; ^{Zhang et al., 2017}). Although the incidence of TDH is very low, it can occur in different segments of the thoracic spine and is more common in the middle and lower thoracic spine (^{Rohde & Kang, 2004}) 75 % of TDH occurs between T8 and T12, and only 4 % of TDH is located above T3 to T4 (^{Gille et al., 2006}).TDH is most common among adults between the ages of 30 and 60, with equal male to female ratios (^{Hurley et al., 2017}; ^{Court et al., 2018}).Compared with patients with cervical and lumbar disc herniation, TDH patients have more postoperative complications, which increases the complexity of TDH treatment (^{Cloney et al., 2018}). The concept of “safety triangle” for lumbar spine surgery was first proposed by ^{Kambin et al. (1998}) and has been recognized by clinicians it is described as a triangular stereoscopic structure surrounded by four boundary lines. The lower end is the upper endplate of the lower vertebral body, the inner boundary is the dural sac/travel nerve root, the posterior margin is the facet joint, and the outer upper boundary is the travel nerve root. This study explores whether there is a safe operating area for the thoracic spine that is similar to the lumbar spine to increase the success rate of thoracic surgery and provide the working cannula parameters for surgery. There have been a large number of anatomical studies on the cervical and lumbar spine, but there is still little literature on the anatomy of the thoracic intervertebral foramen only some case reports and clinical studies were retrieved. In order to reduce iatrogenic injury and improve the success rate of surgery, the study of anatomy knowledge of thoracic intervertebral foramen is essential for successful operation in this area.

MATERIAL AND METHOD

Materials. 10 normal adult fresh cadavers, intact spine (T1 ~ S1), the specific age is unknown, provided by the Human Anatomy Laboratory, School of Basic Medical Sciences, Inner Mongolia Medical University. A total of 20 sides, a total of 340 intervertebral foramen.The morphological structure of the vertebrae has not changed, no bone destruction, no tumors, deformities, fractures, vascular malformations, etc.

Vernier caliper (Japan Mitutoyo, accuracy 0.02 mm), 20 times magnifying glass, bow compass, stainless steel ruler (accuracy 1 mm), medical electric saw drill (Shanghai Bojin Medical Devices Co., Ltd., BYJ-1) surgical instruments: scalpel, surgical scissors , hemostatic forceps, anatomical tweezers, hacksaw.

Preparation of specimen. Dissecting the chest and lumbar segments of 10 adult cadaver specimens, removeing the internal organs, and cut the spine specimens horizontally from the upper end of the hacksaw, Identify the lumbar section, marked by the 12th rib, the lower end is horizontally disconnected from the hip joint, and then the local solution is performed, remove the skin and muscles and surrounding soft tissue structure, carefully remove the psoas muscle, clearly and completely expose the entire thoracic vertebrae, lumbar vertebrae, intervertebral disc, intervertebral foramen nerve root structure, keep the nerve roots in the intervertebral foramen without changes, perform intervertebral foramen, Morphological observation of nerve roots and intervertebral discs. The upper end of the scalpel was disconnected from the C7/T1 segmental disc, and the lower end was self-use medical electric saw to drill the rib and thoracic vertebrae along the rib joint.

Measuring method. The longitudinal diameter of the dorsal root ganglion (D1): Maximum distance between the axial edges of the ganglion; the transverse diameter of the dorsal root ganglion (D2); Maximum distance between the edges of the D1 ganglion; horizontal sagittal diameter of the upper edge of the intervertebral disc (S1). The vertical distance between the upper edge of the disc and the posterior edge of the intervertebral foramen; the high of intervertebral foramen (H1). The distance between the lower edge of the superior pedicle and the upper edge of the lower pedicle; the height of articulationes costovertebrales (H2): Intercortical distance of articulationes costovertebrales; the height of intervertebral disk (H3): left high (h1), left rib cephalic ventral disc height, right high (h2), right rib cephalic ventral disc height; middle heighth3, spinal axis intervertebral disc height, the angle of the dorsal root ganglion (α): the angle between the nerve root and the spinal canal (Fig. 1).

Statistical Analysis. All statistical analysis was performed using SPSS 21.0 (IBM Corporation, Armonk, NY, USA): The measurement data were expressed as mean ± standard deviation (X ± s): paired t test was used between the left and right sides; the same index was analyzed by variance for different segments; p<0.05 was considered significant.

Fig. 1 A. The high of intervertebral foramen (H1); B. The height of articulationes costovertebrales (H2); C. The height of intervertebral disk (H3); horizontal sagittal diameter of the upper edge of the intervertebral disc (S1); D. The angel of the dorsal root ganglion (a); E. The longitudinal diameter of the dorsal root ganglion (D1). The transverse diameter of the dorsal root ganglion (D2)2. 

RESULTS AND DISCUSSION

The longitudinal diameter of the dorsal root ganglion (D1): the maximum is T10~T11~T7~T8, smaller in T12~L1, increasing trend after increasing. The transverse diameter of the dorsal root ganglion (D2): no obvious change trend, the maximum is T12~L1, smaller in T7~T8; Horizontal sagittal diameter of the upper edge of the intervertebral disc (S1); the maximum is T11~T12 and the minimum is T7~T8, trend of increasing first and then decreasing; The high of intervertebral foramen (H1): larger in T8~T9, T10~T11, T11~T12, smaller in T7~T8, fluctuating trend; the height of articulationes costovertebrales (H2): no obvious change trend, larger in T9~T10, T10~T11,T12~L1, smaller in T7T8; The height of intervertebral disk (H3): the maximum is T12~L1 and the minimum is T7~T8, increasing trend; The angel of the dorsal root ganglion (a): T7~T8 is greater than 90°, the angle of the segment below T10 is obviously reduced to an acute angle, and the middle and lower thoracic segments are decreasing (Tables I to VII).

The most common first symptom of TDH is pain, including chest pain, back pain, intercostal neuralgia, another common symptom is numbness or paresthesia. Clinical examination often shows symptoms of central or mixed nerve injury such as sensory disturbance and muscle weakness below the affected plane. The thoracoscopic discectomy surgery is less traumatic, the incidence of intercostal neuralgia is reduced, is the preferred treatment for thoracic intervertebral discs at the center of T4T11 (^{Elhadi et al., 2015}). Posterior thoracic laminectomy with or without discectomy is associated with uniformly poor results and severe neurological complications and has been abandoned .Currently accepted approaches for thoracic disc herniation include (1) posterolateral approaches (transpedicular, costotransversectomy, lateral extracavitary), (2) transthoracic approaches (transpleural, retropleural, thoracoscopic), and (3) ventral approaches (transsternal and manubrial window approach for T2~T5 discs) (^{Mehdian & Nasto, 2016}). Study pointed out that the overall neurological improvement rate of the anterior and posterior approaches is similar, and similarly, the degree of pain improvement is similar (Hurley et al.). Study considered that posterior unilateral modified transpedicular pedicled in-segment fixation and interbody fusion is a safe and reproducible method, which can achieve circumferential decompression and segmental stabilization (^{Carr et al., 2017}). The purpose of fusion with the segment.

Table I The longitudinal diameter of the dorsal root ganglion (D1) (unit: mm). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.05.

Table II The transverse diameter of the dorsal root ganglion (D2) (unit: mm). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.05.

Table III The horizontal sagittal diameter of the upper edge of the intervertebral disc (S1) (unit: mm). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.05.

Table IV The high of intervertebral foramen (H1) (unit: mm). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.005, ##p<0.001.

Table V Theight of articulationes costovertebrales (unit: mm). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.05.

Table VI The height of intervertebral disk (H3) (unit: mm). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.05.

Table VII The angle of the dorsal root ganglion (α) (unit:°). 

*Comparison between left and right sides *p<0.05. #Comparison between different segments. #p<0.05.

According to the literature, there are many studies on the anatomy of the lumbar intervertebral foramen, and there are few studies on the intervertebral foramen of the thoracic spine. ^{Vialle et al. (2015}) performed a lumbar study on 8 adult corpses (average 54 years). The H1 averaged 15.25 mm, the D1 averaged 13.25 mm, and D2 averaged 7.05 mm, the measured value is significantly larger than the measured value of the thoracic vertebra (^{Leng et al., 2018}). Studied the thoracolumbar segment of 24 adult male corpses (average 47 years), the D1 T1 ~ L5 gradually increased, the most obvious increase in the lumbar segment, the D2 T1~T9 did not change much, the segment below T9 gradually increased, and the nerve root and spinal canal axis angles T1~L5 gradually decreased (Leng et al.) obtained that the D1 value is larger than the measured value of this study, which is different from the trend of this study. The D2 result is consistent with this study and the measured value is similar. The angle between the nerve root and the spinal canal axis is smaller than the angle a in this study, the reason is that Leng et al. measured one side of the angle as the direction from which the nerve root is emitted from the spinalcanal, while the side of the a value in this study is the direction in which the nerve root leaves the intervertebral foramen. Studied 16 corpses, 10 females and 6 males (68~106 years old) lumbar vertebrae: At L5, D2 is at most 6.5 mm; the value of α is the smallest on the left side of L5 (50.50~58.80)°, and the right side of L4 (50.50~57.20)°, and measure the distance between adjacent pedicles (similar to H1 in this study) L1~L5 (7.2~10.3) mm. Maximum distance from the disc to the posterior margin of the intervertebral foramen (similar to S1 in this study) L1~L5 (6.6~8.7mm) (^{Silverstein et al., 2015}). The ganglion measurements are significantly larger than this study. ^{Silav et al. (2016}) measured the waist D1 average L1 5.39 mm; L2 5.83 mm; L3 7.24 mm; L4 7.97 mm and L5 was 10.83 mm. D2 average L1 4.36mm; L2 4.56 mm; L3 4.99 mm; L4 5.22 mm and L5 5.82 mm. D1 and D2 gradually increased from L1 to L5 and the lumbar nerve roots were significantly larger than the thoracic spine measurements in this study. ^{Arslan et al. (2012}) studied and measured 14 formalinfixed corpses, and obtained lumbar H1 L1~L5 (17.3~24.3) mm, which was significantly larger than the measured value of this study. For the lumbar vertebrae, the measured values are larger than the thoracic vertebrae, which makes the surgical safety area through the lumbar intervertebral foramen large, and the difficulty coefficient is lower than that of the thoracic vertebra. However, the study of the fine anatomical structure of the thoracic intervertebral foramen is of great significance and value.

Most of TDH occurs in the middle and lower thoracic vertebrae (between T8 and T12) (Gille et al.; ^{Angevine & McCormick, 2012}), causing the anterior or anterolateral optic nerve (^{Arce & Dohrmann, 1985}; ^{Cornips et al., 2011}). Although the mechanism of TDH is not fully understood, some researchers believe that TDH may be caused by the relative weakness of the posterior longitudinal ligament and the high mobility of the lower thoracic spine (^{Dickman et al., 1999}; ^{Melissano et al., 2009}). The potentially serious consequence of TDH is myelopathy, which is characterized by severe lower limb sensation and movement disorders. Thoracic spinal cord disease may have catastrophic and irreversible neurological consequences if not diagnosed and treated promptly and appropriately (^{Brown et al., 1992}; ^{Hou et al., 2016}). Because the clinical symptoms caused by cervical and lumbar lesions are similar, and often misdiagnosed, making it difficult to diagnose, which may delay surgical intervention and lead to permanent disability (^{Smith et al., 2013}; ^{Takenaka et al., 2014}). Thoracoscopic discectomy has been developed in recent years. This is a new technique for T4~T12 discectomy, which gradually replaces traditional thoracotomy. However, this method requires advanced technology and equipment and is difficult to promote and popularize (^{Yamasaki et al., 2013}; ^{Kerezoudis et al., 2018}). Thoracic anatomy research is essential for exploring safe and reliable surgical approaches.

In this study, the ribs of the T7~T9 segments blocked the intervertebral foramen, while the T10~T12 had no ribs and the field of view was wider. The size of the cannula depends on S1, because H1 is larger than S1, so the middle and lower thoracic segments allow a 6 mm cannula to pass. The posterior edge of the intervertebral disc constitutes the lower part of the leading edge of the intervertebral foramen, and the nerve roots travel from the upper part of the intervertebral foramen because the vertebrae grow faster than the spinal nerve. If the central disc of the disc will compress the spinal cord and the posterior lateral projection, it will compress the nerve or / and the spinal cord. For the angle a, throughout the entire spine, from the cervical vertebrae to the lumbar vertebrae, the angle experiences an obtuse angle, a right angle, and an acute angle. Lumbar vertebrae can find an obvious "safety triangle", but in this study, the angle of the thoracic vertebrae is more than a right angle, and it is impossible to determine the obvious "safety triangle", which theoretically expands the operable surgical area, the area under the spinal nerves is allowed, so the angle and position of the surgical approach become the focus and key.

CONCLUSION

Thoracic disc herniation remains a challenging procedure?key factors for preoperative decision making include the anatomical location of the herniated disc and the functional status of the patient and the surgeon's experience. Among them, the doctor's understanding of the anatomy of the thoracic spine is the key to surgery. Although the anatomical measurement of the parameters of 10 adult intact spine specimens for intervertebral foramen surgery is carried out, the sample size is less in this study. The sample size will be expanded in the next step, in order to summarize the more valuable parameters and provide reference data for clinical treatment.