Anatomical study on the relationship between the dorsal root ganglion and the intervertebral disc in the lumbar spine.

OBJECTIVE: To describe the location of the dorsal root ganglion in relation to the intervertebral disc, including the "triangular" safety zone for minimally invasive surgery in the lumbar spine.
METHODS: Eight adult cadavers were dissected bilaterally in the lumbar region, using a posterolateral approach, so as to expose the L3L4 and L4L5 spaces, thereby obtaining measurements relating to the space between the intervertebral disc, pedicles cranial and caudal to the disc, path of the nerve root, dorsal ganglion and safety triangle.
RESULTS: The measurements obtained were constant, without significant differences between levels or any laterality. The dorsal ganglion occupied the lateral border of the triangular safety zone in all the specimens analyzed.
CONCLUSION: Precise localization of the ganglion shows that the safety margin for minimally invasive procedures is less than what is presented in studies that only involve measurements of the nerve root, thus perhaps explaining the presence of neuropathic pain after some of these procedures.

Introduction

Advances in surgical treatments for degenerative pathological conditions, and specifically in relation to minimally invasive surgery for the lumbar spine, have given rise to the need to reassess previous concepts of surgical anatomy, given that new accesses that are less invasive have been developed. Minimally invasive approaches have the advantage of tissue preservation, reduction of unnecessary damage to healthy areas, acceleration of healing and reduction of the duration of treatment.1, 2, 3 However, these approaches do not always enable direct viewing of the neural structures and it is at this point that reviewing anatomical knowledge becomes important.

The dorsal nerve roots are composed only of sensory fibers coming from the spinal nerves that head toward the spinal cord. The ventral nerve roots are mostly composed of motor fibers, but they may transport a small quantity of sensory fibers. Close to the junction between the dorsal and ventral roots, the dorsal root presents a dilatation known as the dorsal root ganglion (DRG), which consists of a grouping of cell bodies of sensory fibers. The DRGs are located in the intervertebral foramina, except for the sacral DRGs, which are located inside the vertebral canal, and the coccygeal DRGs, which are intradural.4, 5, 6

The DRG differs from the nerve roots in that it is sensitive to mechanical pressure even in the absence of chemical irritation, since it contains a large number of nociceptors that are mechanically more sensitive. Moreover, it may have greater chance of causing neuropathic pain if it is injured.4, 5, 7

Neuropathic pain is a state of neural dysfunction provoked by functional and structural alterations to the central and peripheral sensory pathways, which produce modifications to the processing of the nociceptive information. It can be caused by injuries to the roots, DRGs, spinal cord or encephalon.8, 9

Transforaminal intersomatic arthrodesis or transforaminal lumbar interbody fusion (TLIF) was developed to enable access to lumbar intervertebral discs by means of a unilateral extracanal route, thereby avoiding the problems and limitations of the original technique (posterior lumbar interbody fusion, PLIF). One of the complications relating to using TLIF that has been reported in the literature is the presence of neuropathic pain subsequent to the surgery. This has been attributed to excessive manipulation of the dorsal ganglion of the emerging root.10, 11, 12

In view of the sparseness of the literature on the anatomy of the dorsal ganglion, as applied to the TLIF technique, we conducted a study on cadavers, with the aim of determining the safest area for undertaking a transforaminal approach for treating lumbar intervertebral discs, with emphasis on their anatomical relationship with the dorsal ganglion.

The “triangular safety zone” is considered to be a safe area for accessing the intervertebral discs of the lumbar spine. This zone was first defined by Mirkovic et al., with its lateral limit as the dura mater, its lower limit the upper vertebral plateau and its hypotenuse the lumbar root. In this zone, the intervertebral disc can be accessed without putting the surrounding neural structures at risk. The position of the DRG is not mentioned in this triangle and, because of its oval shape, it may reduce the lateral limits of the area of safety for minimally invasive approaches.12, 13

The present study had the aim of describing the location of the DRG in relation to the intervertebral disc, including the “triangular” safety zone for minimally invasive surgery on the lumbar spine.

Methods

This study was conducted in our institution's Department of Human Anatomy. Eight cadavers without any previous scarring on the lumbar spine were studied. They were all male, ranging in age from 45 to 62 years (mean: 54). The number of cadavers was defined by the institution by calculating the variability of the sample.

The cadavers were positioned in ventral decubitus and a posterior access route in a single layer was constructed. Extensive laminectomy was then performed on the lumbar spine, to expose the dural sac, the lumbar roots with their respective DRGs, the pedicles and the intervertebral discs (Fig. 1). The segment L5-S1 was not studied because of the need for more extensive dissection and possible damage to the anatomical specimen. Using a straight osteotome, osteotomy was performed on the joint facets and lateral recess, and the anatomical structures of the intervertebral foramen were viewed.

Fig. 1

Anatomical exposure of the nerve root, intervertebral disc (ID), dorsal root ganglion (DRG), pedicle and dural sac.

Using a digital pachymeter (Mistainless®), the following measurements in millimeters were made (Table 1, Fig. 2):

Table 1

Description of the measurements made.

Measurement	Description	Color in Fig. 2
1	Upper limit of the intervertebral disc to the axilla of the emerging root
2	Axilla of the root to the start of the dorsal ganglion
3	Lower portion of the pedicle to the dorsal ganglion
4	Triangular safety zone
5	Vertical distance between pedicles
6	Greatest diameter of the dorsal ganglion
7	Smallest diameter of the dorsal ganglion
8	Axilla of the root to the lower limit of the cranial intervertebral disc

Fig. 2

Measurements made during the study. 1, upper limit of the intervertebral disc to the axilla of the emerging root; 2, axilla of the root to the start of the dorsal ganglion; 3, lower portion of the pedicle to the dorsal ganglion; 4, green triangular area: triangular safety zone; 5, vertical distance between pedicles; 6 and 7, diameter of the DRG; 8, axilla of the root to the lower limit of the cranial intervertebral disc.

Upper limit of the intervertebral disc to the axilla of the emerging root;

Axilla of the root to the start of the dorsal ganglion;

Lower portion of the pedicle to the dorsal ganglion;

Triangular safety zone (height, base and hypotenuse):

Height of the triangle: lateral border of the dura mater;

Base: upper vertebral plateau of the lower vertebra;

Hypotenuse: spinal nerve;

Vertical distance between pedicles;

Greatest diameter of the dorsal ganglion;

Smallest diameter of the dorsal ganglion;

Axilla of the root to the lower limit of the cranial intervertebral disc.

Results

The data and the results obtained are summarized in Table 2, Table 3.

Table 2

Means of the values obtained in the study.

Measurement	Description	Mean (mm)
1	Upper limit of the intervertebral disc to the axilla of the emerging root	14.65
2	Axilla of the root to the start of the dorsal ganglion	7.95
3	Lower portion of the pedicle to the dorsal ganglion	5.45
4	Triangular safety zone	Table 3
5	Vertical distance between pedicles	15.25
6	Greatest diameter of the dorsal ganglion	13.25
7	Smallest diameter of the dorsal ganglion	7.05
8	Axilla of the root to the lower limit of the cranial intervertebral disc	8.0

Table 3

Measurement of the triangular safety zone.

Measurement	L3 left	L3 right	L4 left	L4 right
Base (mm)	14.25	13.75	14.55	14.17
Height (mm)	14	16.55	17.52	16.4
Hypotenuse (mm)	18.98	21.53	23.03	21.72

In all the spines evaluated, the DRG touched or entered the limits of the triangular safety zone and thus was shown to be a risk factor for procedures that come close to this region. Another factor that emphasizes the close relationship of the DRG with the triangular safety zone is the ratio of its height (around 15 mm) to the greatest length of the DRG (7.5 mm). If the portion of the triangular zone relating to the cranial pedicle and the vertebral body (around 8 mm) is excluded, it can be seen that the remainder of the external border of the triangular zone necessarily ends up being occupied by the DRG (Fig. 2).

Discussion

One of the most unpredictable complications of minimally invasive approaches to the spine is the presence of postsurgical neuropathic pain. This is difficult to manage and its duration may range from days to years. Manipulation of the DRG and possible variations in its anatomy have been attributed as causal factors.8, 9, 10, 11

Most studies have described the DRG as an oval structure that is almost entirely located within the foramina at lumbar levels,10, 11 but they do not describe the safety limits for it. This knowledge is of great importance for minimally invasive procedures on the spine.

The triangular safety zone was first described by Mirkovic et al., who defined it as a space in the foraminal region that would make it possible to introduce cannulae for posterolateral percutaneous procedures in the lumbar region, without putting the surrounding neural structures at risk. There was no mention of the position of the DRG in that initial report.13, 14, 15

The triangle constructed from the data of Choi et al. presented anatomical characteristics that differed from those described by Mirkovik et al. in relation to height and width. However, the general mean of the hypotenuse of the triangular safety zone was concordant. They obtained a mean of 23 mm versus 25.49 mm in the study by Mirkovik, but neither of these studies made any mention of the position of the DRG.

Kambin et al. described the safety limits of percutaneous procedures for accessing intervertebral discs, with introduction of instruments at a distance of 10 cm from the midline and in parallel with the vertebral plateau, with penetration of the annulus at a ten o’clock or two o’clock position. They also reported the limitations of these procedures, such as extruded and sequestered hernias.

In the present study, attention was paid to the safety limits of the DRG, which was located 7 mm from the exit point of the nerve root, usually in a location within the foramina. The distance from the intervertebral disc to the lower part of the nerve root was sufficient for a safe approach. The lower pedicle, which marks the lower limit of the safety zone, was located 5 mm from the DRG, i.e. differing from the findings of Mirkovic et al. and Choi et al. (Fig. 3).

Fig. 3

Comparative schematic appearance of the safety zones for the spinal nerve. (A) Choy et al.; (B) Mirkovic et al.; (C) Vialle et al. HDM, height of the dura mater; LN, length of the spinal nerve; DDMN, distance from the dura mater to the spinal nerve; HID, height of the intervertebral disc; P, pedicle of the vertebral arch; G, dorsal root ganglion (DRG); rectangular area in red represents the safety zone of the DRG.

The limitation of this study was the small number of specimens for analysis, which may have affected the values of the data presented. However, similar studies in the literature that presented larger numbers of levels analyzed included the entire lumbar spine, which generated wide variation of root angles, especially at L2-L3 and L5-S1. Furthermore, those studies did not place any emphasis on the dorsal ganglion and its location, perhaps for introduction points for endoscopy. In the present study, we sought to find spaces for accessing the disc in minimally invasive procedures.

Conclusion

The DRG infringes the lateral limits of the triangular safety zone, in the intervertebral foramen, and may be injured in foraminal procedures such as minimally invasive TLIF. We propose that a rectangular safety zone should be used, with the aim of reducing the nerve injury and the presence of neuropathic pain in percutaneous procedures.

Conflicts of interest

The authors declare no conflicts of interest.