Exploring cortical trajectory of the lumbar vertebrae: a morphometric study in dry skeletons: a retrospective study in Thailand

Article information

Asian Spine J. 2024;18(5):654-662

Publication date (electronic) : 2024 October 22

doi : https://doi.org/10.31616/asj.2024.0223

Parika Hanarwut¹, Sitthichai Iamsaard¹, Permsak Paholpak², Taweechok Wisanuyotin², Yuichi Kasai², Laphatrada Yurasakpong^3,4, Athikhun Suwannakhan^3,4, Arada Chaiyamoon¹

¹Department of Anatomy, Faculty of Medicine, Khon Kaen University, Khon Kaen, Thailand

²Department of Orthopaedics, Faculty of Medicine, Khon Kaen University, Khon Kaen, Thailand

³Department of Anatomy, Faculty of Science, Mahidol University, Bangkok, Thailand

⁴In Silico and Clinical Anatomy Research Group, Bangkok, Thailand

Corresponding author: Arada Chaiyamoon, Department of Anatomy, Faculty of Medicine, Khon Kaen University, Khon Kaen 40002, Thailand, Tel: +66-4336-3173, Fax: +66-802942459, E-mail: aradch@kku.ac.th

Received 2024 June 11; Revised 2024 July 3; Accepted 2024 July 22.

Abstract

Introduction

The human lumbar spine is important for supporting the body weight, maintaining stability, facilitating trunk movements, and protecting the spinal cord, nerves, and vessels. Lumbar spine abnormalities can cause various spinal problems, such as lumbar stenosis, spondylosis, spondylolisthesis, lordosis, scoliosis, and fractures [1]. Two standard techniques are generally used to fix spinal instability and maintain stability in spinal disorders, namely, pedicle screw (PS) and cortical bone trajectory (CBT) fixations [1–3]. Although PS fixation is widely used for the treatment of spinal disorders, it is associated with screw malpositioning, leading to complications such as spinal dural tearing, cerebrospinal fluid leakage, and neurovascular injury [4]. Moreover, PS fixation results in screw loosening from the lumbar vertebral body, particularly in patients with osteoporosis [5].

Because PS fixation has some weak points and limitations for lumbar spinal stabilization, Santoni et al. [6] proposed a new technique called CBT fixation, which is considered a more rigid fixation. This technique reduces screw loosening, failure in osteoporosis, and stabilize and avoid complications caused by trajectory insertion in PS fixation [6]. CBT fixation increases or enhances the pulling-out strength and insertion torque compared with PS fixation [3,6]. Moreover, CBT fixation could reduce the estimated blood loss, incision length, and surgical dissection compared with the PS method [7].

The lumbar morphological information also refers to preoperative assessment before CBT screw fixation [2,8]. In addition, lumbar morphometry has been documented in various populations [3,9–13]. In the Thai population, only the morphology of dried lumbar vertebrae was used as basic information about PS fixation in North Thai bone samples.

Therefore, this study aimed to investigate all morphometric aspects of the lumbar vertebrae in Northeastern Thais, including pedicle height (PH), pelvic width (PW), length, cortical screw ending zone, and angles for the clinical application of CBT screw fixation. This study might provide basic information about Northeastern Thai lumbar vertebrae when considering the accurate position of the CBT screw fixation and preventing complications after screw fixation.

Materials and Methods

Measurements of lumbar parameters

The lumbar PH was measured at both sides using a digital Vernier caliper to measure between the narrowest points (or called isthmus) on the lumbar pedicle, as demonstrated in Fig. 1. The distance of PH is located between the superior border (SB) and the inferior border (IB) of the pedicle isthmus on the lumbar pedicle (Fig. 1A). The lumbar PW was also measured at both sides of the narrowest point (isthmus), distancing from the medial border to the lateral border of the pedicle isthmus, as shown in Fig. 1B. The hypothetical starting point (SP) on the lumbar lamina for applying in the CBT was marked as previously described [3,6]. The SP was determined using a drawing line from the vertical midline of the superior articular facet to a point that was measured at 1 mm lower from the IB of the transverse process. The possible endpoint (PE) of the screw tip in CBT was marked with a mechanical pencil (0.5 mm) on the cortical rim of the lumbar body on both sides. To mark this point, a stainless needle was used as a guide by placing it on the superior surface of the vertebrae through the hypothetical SP and middle pedicle. Then, the PE was spotted at the end tip of such needle on the center of the lumbar cortex as shown on the right side. The length for the cortical bone trajectory (LCT) was measured from the SP to the PE using a digital Vernier caliper, as shown in Fig. 1C. After marking the PE points on both sides, each point was further observed for the PE zones using a modified full-circle protractor that was divided into 12 zones (Z1–12) in the clockwise direction (Fig. 1D). Z1–Z6 were located on the right side, whereas Z7–Z12 belonged to the left side. After taking photographs and processing in ImageJ (https://imagej.net/ij/), the axial CBT angle (ACA) of the left and right sides on the superior lumbar view was performed by drawing two lines (Fig. 1E). The ACA of each side was angled between the vertical line dragged from a half of the lumbar body cortex through the spinous process, and the second line dragged from the PE to the midpoint of the spinous process on the axial plane, as shown in Fig. 1E. The cephalad screw angle (CSA) of the left and right sides on the lumbar level was identified by taking photos of the lateral vertebrae. Before loading into the ImageJ program and before taking images, a stainless needle was placed to identify an SP landmark (Fig. 1F). After visualizing under ImageJ, the CSAs of both sides were angled between a transverse line drawn continuously from the SP and in parallel to the superior endplate of the lumbar vertebral body and a line drawn from the SP to the PE, as shown in Fig. 1F. To investigate the PE on the lateral aspect, a mechanical pencil was used to drag down from the PE on the cortical rim.

Fig. 1

All measurements of the lumbar vertebrae. (A) The lumbar pedicle height was measured at the lumbar pedicle between the superior border (SB) and the inferior border (IB). (B) The lumbar pedicle width was measured from the medial border (MB) to the lateral border (LB) of the pedicle isthmus. (C) The length for the cortical bone trajectory was measured from the starting point (SP) to the possible endpoint (PE). (D) The PE zones using a modified full-circle protractor were divided into 12 zones (Z1–Z12) based on the clockwise direction to record the possible safe zone. (E) The axial cortical bone trajectory angle (ACA) was angled between the vertical line drawn from the midpoint of the lumbar body cortex through the spinous process and the second line drawn from the PE to the midpoint of the spinous process (MSP) on the axial plane. (F) The cephalad screw angle (CSA) was measured between a transverse line drawn continuously from the SP and parallel to the superior endplate (SE) of the lumbar vertebral body and a line drawn from the SP to the PE.

Results

Overall morphometry

The highest values of the PH, PW, LCT, CSA, and ACA were found in L1 (15.09±1.44 mm), L5 (16.96±2.42 mm), L3 (35.75±2.61 mm), L1 (20.85°±2.30°), and L5 (21.83°±2.49°), respectively. In contrast, the lowest values of such parameters were registered in L4 (13.38±1.30 mm), L1 (7.09±1.72 mm), L5 (34.71±2.71 mm), L5 (19.43°±2.30°), and L2 (18.47°±1.89°), respectively (Table 1).

Table 1

Ranges and average values of lumbar parameters measured in this study (number of lumbar columns=300)

PH and PW

The morphological data of the PH and PW on L1–L5 were compared between sexes and sides (Table 2). The results showed that the PH and PW of L1–L3 on both sides were significantly shorter in women than in men (p<0.001). In L4, only the right PH and PW on both sides in men were significantly longer than that in women (p<0.001). Although the PH of L5 was significantly shorter in men, the PWs on both sides were not significantly different between sexes. The total PH was significantly shorter in the left L1 but longer in the left L5 (p<0.05). Individually, in women, the PH was significantly longer in the right L1 but shorter in the right L4 (Table 2). For the PW of L1–L5, no differences were found between the left and right sides in each sex (p>0.05). However, the general data of the PW in L5 were significant on both sides (p=0.013).

Table 2

Comparisons of the pedicle height and width from different lumbar levels (L1–5) between females (n=150) and males (n=150)

LCT

The LCTs on L1–L5 on both sides were significantly shorter in women than in men (p<0.001) (Table 3). Both sexes demonstrated significantly longer left LCT (L1–L5) than the right LCT (p<0.001). Individually, in women, only the LCTs on all lumbar levels on the left side were significantly longer than those on the right side (p<0.001). In contrast, in men, the LCT was not significantly different between sides in all lumbar levels (p>0.05), as shown in Table 3.

Table 3

Comparisons of the length for cortical bone trajectory from different lumbar levels (L1–5) between females (n=150) and males (n=150)

CSA and ACA

Table 4 shows the CSA and ACA, which were compared between sexes and sides. The average right CSA in L1–L5 was significantly narrower than that of the left side (p<0.05). Particularly, at L2 and L5, the CSA and ACA in women were significantly wider in the right side (p<0.05). In L4, in women, only the CSA in the right side was significantly wider than that on the left side (p=0.020). Although the CSA of the L3 and L4 on left side in men were significantly narrower, the ACA was wider than the left side (p<0.05). In men, the CSA of L5 was significantly wider on the right side (p<0.001). In addition, the left CSA in L3–L5 was significantly wider; however, its ACA was narrower in women than in men (p<0.05). Individually, the CSA of L3 in men was significantly wider on the right side but was shorter on the left L5 (Table 4). Moreover, the ACAs of L1 and L2 in men were significantly wider on the left side (p<0.05). In contrast, the ACA of L3 in women was significantly wider on the right side (p=0.025). At L2, the ACA in men was significantly wider on the left side but was narrower in women on the right side (p<0.05), as shown in Table 4.

Table 4

Comparisons of the CSA and the ACA from different lumbar levels (L1–5) between sexes (total=300: females=150 and males=150)

Incidences of the cortical zones for the cortical screw trajectory

Overall data of the cortical zones (Z1–12) on L1–L5 for the cortical screw trajectory are presented in Fig. 2. Z3, Z4, Z9, and Z10 are nearly the most endings of the screw tip at in all lumbar levels. Notably, right and left zones of L1–L5 were with the highest incidences were Z3 (L1 [77%], L2 [81%], L3 [69%], L4 [95%], and L5 [95%]) and Z10 (L1 [81%], L2 [86%], L3 [89%], L4 [94%], and L5 [97%]), respectively (Fig. 2). Z3 and Z10 were more prevalent on the right and left cortical zones in both men and women. L1–L3 was noted in approximately 80%–90% (Table 5). Interestingly, in both sexes, approximately 95% of the ending zones were found on L4 and L5 (Table 5).

Fig. 2

Possible safe zone for the cortical bone trajectory and surrounding structures at L1–L5. (A) The psoas major muscle and crus of the diaphragm are monitored when using an unsuitable screw length at L1. (B–D) The psoas major muscle and sympathetic trunk are monitored when using an unsuitable screw length at L2–L4. (E) The psoas major muscle, inferior vena cava, and ureters are monitored when using unsuitable screw lengths at L5. (F) The percentage of the ending points at the cortical zones observed in the lumbar vertebrae (L1–L5). A, abdominal aorta; IVC, inferior vena cava; SC, sympathetic chain; LSN, lumbar spinal nerve; QL, quadratus lumborum; MF, multifidus; LG, longissimus; IC, iliocostalis; SN, spinal nerve; PM, psoas major; LP, lumbar plexus; ES, erector spinae; CIA, common iliac artery; U, ureter; CD, crus of diaphragm; CG, celiac ganglion.

Table 5

Incidences of cortical ending zones for cortical screw trajectory in different lumbar levels compared between sexes (N=300 lumbar columns: 150 females and 150 males)

Discussion

In this study of the Northeast Thais population, the PH on all lumbar levels (L1–L5) was between 9.53 mm and 27.34 mm. Compared with other populations, the height of L1 (15.09 mm) was close to that of New Zealander (15.10 mm) [14], North Indian (15.11 mm) [15], Nepalese (15.00 mm) [16], and Israeli (15.10 mm) populations [17]. L2 height (14.45 mm) was similar to that in North Thais (14.35 mm) [18], and Turkish (14.35 mm) [19]. In addition, L3 height (14.16 mm) was similar to that in North Thais (14.25 mm) [18]. The PH of L4 (13.38 mm) was also close to that in Nepalese (13.44 mm) [16]. Moreover, the PH in L5 in the examined Thai population (14.10 mm) was similar to that in Chinese (14.17 mm) [20], Turkish (14.20 mm) [21], Korean (14.20 mm) [22], and American (14.00 mm) populations [23]. However, the PH was shorter than those reported in previous studies. In the present study, the PH at all levels was longer than that in Central Indian, North Indian, Nepalese, Pakistani, Taiwanese, Turkish, Chinese, and Chinese Singaporean populations [9,12,13,19,24,25].

In the examined Northeast Thai population, the PW of the lumbar vertebrae was between 3.20 mm and 25.62 mm. The PW of L1 (7.09 mm) was similar to those in other ethnicities including Nepalese (7.02 mm and 7.17 mm) [16], Chinese (7.00 mm) [20], and Korean (7.10 mm) [22]. The PW of L2 (7.34 mm) of Thais was close to the central Indian (7.37 mm) [9] and Chinese (7.40 mm) [20]. For L3 (9.06 mm), the PW was similar to that in North Thais (9.10 mm) reported by the Chiang Mai University research group [18] and Taiwanese (9.15 mm) [12]. The PW of L4 (11.39 mm) was comparable to that of Japanese (11.30 mm) [26]. The highest PW at the L5 level (16.96 mm) in this study was similar to that of Chinese (17.00 mm) population [20]. Interestingly, our PW measurements were narrower than that of various populations including Indian [9,15], Chinese [20], Japanese [5], Korean [22], Taiwanese [12], Turkish [19], Bangladeshi [19], American [23], and North Thais [18]. Despite some differences among populations, these findings indicate that the optimal screw diameter in designing the CBT is commonly similar among populations, and the screw diameter appears to be still suitable under the commercially available sizes, i.e., 4.5–5.5 mm and 25–35 mm of screw lengths [27].

The LCT on all lumbar levels of the Northeast Thais population examined in this study was between 26.18 mm and 44.80 mm. Given the importance of the LCT, surgeons must consider this parameter when screwing because it can overprotrude into other soft tissues surrounding the vertebral body at each level. The CSA on lumbar levels (L1–L5, 13.30°–31.42°) of the Northeast Thais population examined has been demonstrated for the first time in this study, which were narrower than that of Chinese [10,20] and Japanese [3,26].

For the ACA on L1–L5, which was also first reported in the Northeast Thais population (12.48°–30.87°). Compared with the other population, the ACA of Thais was wider than that of Chinese [20] and Japanese [3]. The CSA and ACA are also very important to consider before the surgeons insert the screw to the lumbar vertebral cortex.

For the first time, this study attempted to investigate the possible safe zone for CBT in the Northeast Thais population. Z3 and Z10 on right and left cortical zones are the relatively correct safe zone for the CBT, as shown in Fig. 2. The cortical ending zones observed in this study on each lumbar level can be used in surgical mapping and awareness of injuries in surrounding structures from unsuitable screws.

For L1, an unsuitable screw length can damage the psoas major muscle or crus of the diaphragm as shown in Fig. 2A. Fig. 2B–D demonstrates that the psoas major muscle and sympathetic trunk should be monitored for possible damage if surgeons use unsuitable screw lengths for the CBT. For L5, not only the psoas major muscle but also the inferior vena cava and ureters must be monitored when using unsuitable screw lengths for the CBT (Fig. 2E).

However, this study has a few limitations. The ages of the donors were unknown, which made it impossible to establish age-related correlations. In addition, the pars interarticularis, another important area that could fracture if large screws are used, were not examined.

Conclusions

This study provides the optimal screw size for CBT screws in Northeastern Thais, which is 7.09–16.96 mm in diameter and 34.71–35.75 mm in length. In addition, the optimal CBT angle is approximately 19.43°–20.85°. Hopefully, these basic data of lumbar morphometric anatomy are beneficial for orthopedic and spine surgeons when performing cortical screw insertion for the CBT. For Thai populations, data obtained from this study can be used as basic data for research and development in screw production.

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