MRI has been routinely used in the diagnosis, preoperative planning, and post-operative evaluation of lumbar spine diseases [13]. However, conventional MRI sequences show poor image quality of lumbosacral nerves and can only display lumbosacral nerves with limited range. The anatomical course of the nerve root cannot be observed in multiple directions and continuously. The 3D COSMIC sequence is a GRE sequence based on T2WI multi-echo combining. Compared with the conventional magnetic resonance sequence, the spatial resolution of this approach is improved; the artifacts are reduced, and the nerve root and its anatomical relationship with adjacent tissues can be observed from multiple angles [14]. In this study, a high-field strength 3.0 T MRI scanner was used to scan patients with low back pain by conventional magnetic resonance sequence and 3D COSMIC sequence to measure the anatomical parameters of the OLIF working channel.
The anatomical parameters related to the OLIF working channel and the insertion angle β measured by two radiologists were evaluated using the intra-group correlation coefficient ICC to ensure the consistency of the measurement results, as shown in Table 1. The tested ICC values of all parameters were all greater than 0.75. Therefore, the reproducibility of the data measured by the two investigators was satisfactory, and the relevant data could be used for further analysis. The data obtained by radiologist 1 were selected for the study.
OLIF is a surgical operation performed through the natural gap between the anterior edge of the psoas major muscle and the abdominal great vessels. The thickness of the psoas major is related to the difficulty of stretching the psoas major to extend the surgical field during OLIF surgery. A sufficient width of the natural operating window is a prerequisite for the operation [15]. In the present study, the width of operating window was defined by the shortest distance between the left psoas major muscle and the abdominal aorta. The width of operating window at the level of L2–3, L3–4, and L4–5 intervertebral space is 16.25 ± 4.22 mm, 15.46 ± 4.64 mm and 11.71 ± 6.29 mm, respectively. Davis et al. [16] conducted an autopsy study related to OLIF. The authors measured the access corridor diameters in 20 cadavers in the static state with the following findings: 18.60 mm at L2–3; 19.25 mm at L3–4; and 15.00 mm at L4–5. The results were close to the parameters measured in the present study. The Pearson correlation analysis between the width of surgical window and the intervertebral space showed that r was -0.337 and P < 0.001, which indicated that the surgical window width decreased in lower vertebrae. Women were slightly larger than men at the level of L3–4. For the remaining segments, the average distance between men and women was not statistically significant.
Safe and effective establishment of the surgical channel and adequate exposure of the target operating area are important steps in OLIF surgery. Establishing a surgical channel directly to the center of the target intervertebral space before surgery will make the surgical field more fully exposed, which is conducive to the operation under direct vision [17]. Therefore, it is very important to set up a proper insertion angle of the surgical channel to establish a surgical channel operated under direct vision. Mayer [4] measured the angle formed between the vertebral body and the operating table and made a 4 cm incision in front of the target intervertebral space in the same direction as the external oblique muscle fibers. Silvestre [18] made a 4 cm skin incision in the center of the target intervertebral space parallel to the ventrolateral area of the external oblique muscle fibers. This incision is perpendicular to the line from the anterior superior iliac spine to the umbilicus and is 1/3 away from the anterior superior iliac spine, similar to the McBurney incision.
In the present study, the L3–4 intervertebral level had the largest insertion angle of 49.90 ± 6.53°, while the insertion angle at the L2–3 intervertebral space level was 45.57 ± 6.19°. Notably, the insertion angles of different intervertebral space are different. The insertion angle in the channel in men was larger than that in women, and the results suggested that the incision position in men may be closer to the ventral side. The insertion angle and the thickness of the left psoas major showed a positive correlation. With the increase in the thickness of the psoas muscle, the insertion angle of the channel also increased.
The function of the cage is to accomplish fusion between the vertebral bodies to relieve the symptoms and maintain stability of the vertebrae [19]. The size of the cage used in OLIF surgery is vital for obtaining a long-term and stable curative effect. Improper positioning of the cage may lead to compression of the nerve root, subsidence of the cage or the collapse of the intervertebral space, which greatly impair the prognosis of surgery [20].
Cage subsidence is an important complication of OLIF surgery, which is closely related to the patient’s prognosis [21]. Reasonable selection of the size of the cage can reduce the probability of cage subsidence and achieve better bone graft fusion effect. In this study, the maximum transverse diameter of the endplate was used as the reference for selecting the cage size. In lumbar fusion surgery, the endplate of the surgical segment prevents the cage and bone graft from being embedded in the vertebral body, dispersing the stress, avoids the fusion of the cage, and promotes the fusion of the bone graft [22]. Zhang et al. [23] found that the cage with a length close to the outer region of the condyle ring of the endplate can achieve a larger area of biomechanical support and better prevent collapse of the vertebral body. In this study, the maximum transverse diameter of the upper endplate at L3, L4, and L5 vertebral bodies was measured to provide a reference for the size selection of the cage during OLIF surgery. The transverse diameter of the superior endplate of each vertebral body in men was larger than that in women. Pearson correlation analysis was used to analyze the correlation between the transverse diameter of the upper endplate of the vertebral body and the intervertebral space, showing that r was 0.41 and P < 0.001, which indicated that the transverse diameter of the upper endplate increased in lower levels. According to the results, it is recommended that when performing OLIF surgery, a cage with a length of 50–55 mm should be used for men and a cage with a length of 45–50 mm should be used for women. Chen et al. [24] defined the maximum transverse diameter of the intervertebral disk as the size of the cage. The measurements showed that the most commonly used cage lengths in clinical practice are 50 mm and 55 mm, which are similar to the results of this study.
In clinical practice, the nerve root is easily injured during the implantation of the cage, which may be caused by improper placement of the cage, incorrect size selection, and postoperative cage displacement and compression of the nerve root. In this study, the distance from the left nerve root to the median coronal line of the intervertebral disk was measured to provide a reference for the assessment of the risk of nerve root injury during cage placement. In this study, patients were scanned by 3D COSMIC neuroimaging sequence, and the distance from the left nerve root to the median coronal line of the intervertebral disk (distance D) was measured as the parameter characterizing the risk of nerve root injury during cage placement. According to our measurements, the distance D decreased as the number of intervertebral increased. Pearson correlation analysis was used to analyze the correlation between distance D and intervertebral level, showing that r was -0.57 and P < 0.001, which indicated that distance D decreases as the number of intervertebral increases. Considering the fact that a shorter distance D indicates a higher risk of nerve root injury [25], L4–5 have the higher risk of injury during cage placement. The distance D of L3–4 and L4–5 intervertebral space was greater in men than in women, which suggested that women are at greater risk for nerve root damage in these segments.