5.1. Application of OLIF51
OLIF51 is considered as minimally invasive ALIF through the oblique corridor in the lateral position.(7) It keeps the advantages of traditional ALIF with direct and extensive exposure of the intervertebral disc and avoidance of neural and muscular injury compared with the posterior approach. OLIF51 is a recently introduced surgical technique, evolved from the ALIF but better than ALIF.(3, 8, 9) First, it can be extended to upper levels in a single position with less mobilization of the great vessels, especially for ALIF at L4-5. Secondly, it can avoid rectus abdominis muscle injury, as well as minimize the mobilization of the peritoneal content.(3) Moreover, OLIF51 is advantageous in obese patients because gravity pulls the visceral fat away from the spine.(10) Even so, OLIF at L5-S1 is still difficult because of the risks associated with mobilization of the vessels and the presence of the iliac wing.
Many literatures have proved the practicability of OLIF51.(3, 7, 11–15) Silvestre et al.(4) first reported OLIF51 through a retroperitoneal approach was performed successfully in 6 patients, but one patient had to be aborted and switched to another operation. Woods et al.(3) retrospectively evaluated 137 patients who underwent OLIF procedure, in which 10 patients who underwent OLIF51 only, and 84 patients who underwent OLIF25 combined with OLIF51, then the author draws a conclusion that OLIF is a safe procedure at L1-5 as well as L5-S1. Mun et al.(16) retrospectively reviewed and compared 74 patients who underwent OLIF51 and 74 who underwent TLIF51 and concluded that OLIF51 was more effective for indirect decompression of foraminal stenosis, providing strong mechanical support. Zairi et al.(11) also showed the feasibility of accessing the L5–S1 level through the mini-open retroperitoneal approach without the need for ALIF.
5.2. OLIF51 surgical window
Similar to ALIF L5-S1, OLIF51 typically involves the corridor under the bifurcation of the great abdominal vessels.(3) Tribus and Belanger(17) performed a cadaveric study in 35 cadavers to examined the size of the central window; The mean distance from the bifurcation to the top of the L5-S1 disc was 18 mm, and the mean width between the left common iliac vein and the right common iliac artery averaged 33.5 mm. In 2014, Davis et al.(18) defined the OLIF51 corridor through 2 measurements. The lateral window was the distance transversely from the midsagittal line of the inferior endplate of L-5 to the medial border of the left common iliac vessel, while the vertical window to the first vascular structure that crosses midline. The author measured the corridor diameters in 20 cadavers in a static state and with mild traction of the psoas and find the L5-S1 corridor mean distance was 14.75 mm transversely and 23.85 mm vertically. Davis et al. concluded that the L5-S1 disc space can be accessed from an oblique angle. Oblique access to L5-S1 has also been studied through radiological measurement. Molinares et al.(19) measured that the L5–S1 corridor mean distance was 10 mm between midline and left common iliac vessel, and 10.13 mm from the first midline vessel to the inferior endplate of L-5. The author found access to the L5–S1 disc established in 69% of the MR images analyzed. However, all these studies were just simple measurements of natural anatomic spaces, without considering the influence of oblique corridor on anatomical parameter. In addition, at the L5-S1 level, the psoas muscle is often lifted off the spine to leave the pelvis, which could also obstruct the surgical corridor. Our aim was to simulate the OLIF operation process at L5-S1 level and then evaluate the feasibility of OLIF51 based on CT images. The similar studies have not been described in the literature.
5.3. Simulation of OLIF51
In our study, the author adopted 18 mm as the width of surgical corridor to simulate the operation process. We had reason to believe that traction-difficultly structure will obstruct the OLIF working corridor and there will be excessive stretch of these structures during the procedure. More than 1/3 of the subjects (33/93, 35.5%) were grouped into traction-difficultly LCIV and nearly three in 10 (28/93, 30.1%) were grouped into traction-difficultly PM. Further analysis revealed that about one-sixth of the subjects (15/93, 16.1%) processed traction-difficultly LCIV and traction-difficultly PM at the same time. Because left common iliac vein and psoas major account for more than half of OLIF working corridor, causing difficulty in operating channel placement and increased injury risk of these structures. Therefore, we consider these subjects not suitable for OLIF51.
The anatomical structure of lumbosacral segment is complex and adjacent to the important structure. Damage to LCIV is the most threatening complication associated with OLIF51 which can be very difficult to control once injured and mobilization of these vascular structures is often a technically demanding procedure.(12, 20) The injury to LCIV is also the most commonly complication observed in clinical practice.(5, 16) This phenomenon is caused by the low iliocava junction positions and the medially located LCIV, reducing the size of the operating field. According to reports in the literature, the vascular injury in the early result of OLIF was 8.6%, and it increases when the L5–S1 segment is involved.(3, 21) Therefore, careful preoperative evaluation of the vascular structure is desperately needed during OLIF at L5–S1.
In 2017, Chung et al.(15) evaluated the configuration of LCIV and its risk of mobilization during anterior approach at L5-S1 segment. They postulated the presence of perivascular adipose tissue under the LCIV and they categorized the LCIV into three type: type I (no requirement for mobilization), type II (easy mobilization), and type III (potentially difficult mobilization). In their study, type I LCIV configuration was found in 32 (49.2%) patients, type II in 18 (27.7%), and type III in 15 (23.1%). There were 7 (10.8%) patients with LCIV injury (type I, n = 0; type II,n = 2; type III, n = 5) (P = 0.003). The result indicated that their LCIV classification system is valid for the evaluation of LCIV injury during its mobilization. However, we noted that there were still 2 patients of type II had LCIV injury. In addition to improving surgical techniques, how could we further reduce the vascular injury at OLIF51?
5.4. Assessment of V-line
During the surgery, surgeons has to mobilize LCIV for surgical exposure when it obstructs the operative window on the L5–S1 disc. We could mobilize LCIV laterally choosing the central disk space between the bifurcations or stretch it medially choosing lateral disk space external to the left iliac vessels. However, which approach might reduce the risk of mobilization has not been comprehensively studied. Previous work on the morphological characteristics of the LCIV cannot solve the problem properly. In this thesis, a new concept, V-line, is proposed for assessing the mobilization risk of these two approaches. By introducing the concept of V-line, we could qualitatively evaluate the extent of vascular traction of two approaches to guide surgical treatment.
In our work, there were 61.3% (57/93) of the subjects were defined as V-line (-). In this group, a surgical corridor between the bifurcations of the iliac vessels is more favorable which is exactly the mainstream approach in the world. One important concern with this approach is the injury to the SHP (superior hypogastric plexus), which overlies the L5–S1 disk between the bifurcations and supplies the sympathetic function for the urogenital system.(22) Consequently, damage to the SHP could result in retrograde ejaculation in male patients.(23) Careful unilateral blunt dissection of the SHP and avoidance of monopolar coagulation is recommended.(3, 14) Besides, middle sacral vessels are also important structures during this central approach, we could simply divide these vessels by the application of bipolar cautery or vascular clips.
Naturally, the rest of the subjects (36/93, 38.7%) were defined as V-line (+), in which the corridor external to the left iliac vessels is superior for less stretch of the LCIV. On this condition, particular attention should be paid to the identification and handling of the iliolumbar vein (ILV).(10) The ILV travels laterally approximately 3–4 cm below the bifurcation and then traverse medially along the L5 vertebral body, coursing between the obturator nerve and lumbar trunks.(24) In exposing the L5–S1 level external to the LCIV, the ILV is easily avulsed due to medial traction of LCIV. Zairi et al. had gained access to the L5-S1 level by finding and clipping the ILV before retracting the iliac artery and vein anteriorly. This study considered this a safer and feasible approach.(11) Coagulation or ligation is recommended in case the ILV is identified. We could also take gentle dissection of the fat around the L5-S1 level lateral to the iliac vessels in order to obtain better recognition of the ILV.
Recently many studies analyzing the venous anatomy in the lumbosacral area have been reported. Whether they are useful for predicting the surgical approach of OLIF51? Multiple variables regressive analysis demonstrated that gender of male, medial position of LCIV and high iliac crest were predictive factors of V-line, while age of the patient, L5-S1 disk size, sacral slope and iliocava junction position (JP) were not. As to the left iliac vein position (VP), it is found that almost all subjects of the medial group were classified into V-line (+) group and they accounted for nearly 2/3 of V-line (+) group. In addition, height of iliac crest is also crucial for the preoperative evaluation, there were more than half of low iliac crest group could choose external corridor to decrease stretch of the LCIV. However, the iliocava junction position (JP) was not a crucial factor. A possible explanation for this is that although lower junction positions have more medial LCIV, it’s not the only factor, the junction angle will affect distribution pattern of Iliac vein as well. Therefore, in the preoperative planning of OLIF51 especially among males having the LCIV near the midline or the iliac crest relatively low, a surgical corridor external to the LCIV should be taken into consideration to minimize the risk of vascular injury. Finally, it's worth noting that all the subjects in P (+) V (+) group were divided into V-line (-) taking almost a quarter of the V-line (-) group, indicating potential difficulties in operation procedure.