L4/5 accessibility for extreme lateral interbody fusion (XLIF): a radiological study

Precise knowledge of the anatomic relationship between the lumbar plexus and intervertebral disc is a prerequisite for the safe performance of interbody fusion through a minimally invasive retroperitoneal transpsoas approach. Venous anatomic variants and teardrop-shaped psoas with an anteriorly located plexus may preclude safe access to L4/5 [14]. However, the anatomical location of the L4/5 disc makes access to the disc space difficult and technically challenging [5]. Since the disc space between L4/5 is below the iliac crest in some patients, in these cases angled instruments have to be used.

We investigated the feasibility of minimally invasive XLIF surgery at the L4/L5 level considering the iliac crest, retroperitoneal vessels, and lumbosacral plexus nerves using coronal, sagittal, and axial CT images.

We confirmed our hypothesis that the L4/5 segment is a particular challenge for surgeons through a lateral transpsoas approach. In our study, considering the safe zone described, the access angle, and the adequate reaching of the opposite side, XLIF was only feasible in 76% of patients from the right side and 70% from the left side.

The XLIF technique has been introduced to restore and maintain disc height, restore lordosis, and enlarge the neuroforamen [15,16,17]. Using XLIF results in indirect decompression of the neural structures and improved intervertebral stability can be achieved through ligamentotaxis [2, 3, 18], avoiding the great vessels and bowel [17, 19, 20].

Given its minimally invasive, XLIF further reduces access morbidity, postoperative pain, and hospital stay and allows rapid return to daily living activities [21,22,23]. Furthermore, excellent fusion rates have been described [19, 20].

However, a remarkable increase in neurological complications was recorded using XLIF compared to anterior or posterior fusion procedures. Plexus injuries (13–28%), sensory deficits (0–75%), motor deficits (0.7–33.6%), anterior thigh pain (12.5–25%), and sympathectomy (4–8%) have been described [7, 8, 6, 24]. Because of this, anatomical studies have attempted to define safe working zones for the XLIF approach, thereby reducing the risk of neurological complications.

Guerin et al. defined a safe zone ventral to the nerves and dorsal to the great vessels. They were able to show that the area of this zone decreases steadily from L1/2 to L4/5 and is about half as large in L4/5 compared to L1/2 [10, 25].

In an anatomical cadaver study, they further demonstrated that the safe zone shifts ventrally from L1/2 to L4/5. For this purpose, they divided the disc into four zones (1: anterior quarter; 2: middle anterior quarter; 3: middle posterior quarter; 4: posterior quarter). A safe working zone was defined by the absence of crossing off a lumbar plexus branch. The safe working zone includes zones 2 and 3 for L1/2, zone 3 for level L2/3, zone 3 for level L3/4, and zone 2 for level L4/5. They concluded that the transpsoas approach L4/5 is particularly challenging and risky given the anatomical relationships between the lumbar plexus and disc. Alternative approaches such as TLIF, PLIF, or ALIF should be used [10, 25].

Uribe et al. define the safe working zone in terms of the lumbar plexus branches. They investigated this in five cadaveric specimens. In their study, the safe zones at the disc from L1/2 to L3/4 were in the middle of zone 3, and the safe zone at L4/L5 was at the boundary between zone 2 and 3 [12].

Some authors pointed out that there is no absolute safe zone, and they would recommend either direct visualization of the nerve and/or the use of neuromonitoring [26]. Also, the L4/5 level presents an increased risk of intraoperative nerve and vessel injury because the nerve roots of the lumbosacral plexus run more anteriorly than in the cranially located intervertebral disc spaces and the retroperitoneal vessels run directly in front of the vertebra [27].

Furthermore, the anatomical location of the L4/5 disc space to the iliac crest presents a challenge to the surgeon. Based on direct operative experience in XLIF access surgery, Pimenta et al. defined a maximum access angle of 15° for a lateral retroperitoneal transpsoas approach at the L4/L5 level. This angle can be accommodated by angled instruments, which can be used when removing the disc at the L4/5 level [28]. In the present study, an access angle α smaller than 15° was measured in the CT images from both the right and left sides in 97 patients. Among the 97 patients, an angle α equal to 0° was measured in 67 patients when accessed from the right side and in only 60 patients when accessed from the left side. In contrast, an angle α less than 15° was determined in 30 patients on the right and 37 patients on the left. This difference between an approach from the right and left side possibly arises from the fact that the patients did not lie in the computed tomography scanner with an upright spine.

In 2013, Fontes et al. studied the lateral retroperitoneal transpsoas approach at the L4/5 level whishing to ascertain how often the iliac crest prevents an XLIF using 20 cadavers [29]. In 13 of 20 candidates (65%), the disc space at the L4/5 level could be completely instrumented through a lateral transpsoas approach [29]. In the present imaging study, accessibility of the disc space with angled instruments could be demonstrated in 97% of patients. However, Fontes et al. did not use angled instruments [29]. Therefore, the disc level L4/5 had to align perfectly with the instruments. In our study, this would be the case in patients with an angle α equal to 0°. Thus, the results are comparable [1].

The transpsoas access is not safe at every location. In our study, based on its diameter, the psoas muscle was divided into four equal zones (Fig. 4a). The genitofemoral nerve is located in the anterior part (zone I), and other nerves of the lumbosacral plexus (iliohypogastric nerve, ilioinguinal nerve) are located in the dorsal part (Zone IV) of the psoas [11]. Therefore, splitting of the muscle in zone I or IV may result in injury to the nerves. Furthermore, the retroperitoneal vessels are also located in zone I and there is a high risk of injury to the vessels splitting the psoas in zone I [10, 25, 12]. The blue-shaded box in Fig. 4b represents the "Safe Zone".

We also evaluated how the “safe zone” of the psoas muscle can be reached in axial sections.

Taking into account all three conditions, our study suggests that the XLIF is only feasible in 76% of patients from the right and 70% from the left side.

Therefore, precise preoperative planning is essential to avoid complications in XLIF. However, intraoperative neuromonitoring is mandatory to avoid neurological complications [17, 26]. In patients in whom the L4/5 space cannot be safely reached using XLIF based on preoperative planning, another lumbar interbody fusion (LIF) option should be used. In a cadaveric study, Fontes et al. performed an iliac crest osteotomy when needed and were able to complete all XLIFs (13/20 without and 7/20 with osteotomy) [29]. Whether this should be routinely performed is questionable. Oblique lumbar interbody fusion (OLIF) is a possible alternative for the ventral operative treatment of the L4/5 level.

To eliminate negative influencing factors on the radiographic measurements as far as possible, only the appropriate section was included. This positively changed the type of patient that fulfilled the defined imaging prerequisites for performing an XLIF in this study. Patients who were not selected for the study because of the exclusion criteria could negatively influence the results of the study. The measurements of the present study were taken on CT scans. The lack of a control group is another limitation of the study.

The accessibility of the intervertebral disc depends on the anatomy of the iliac crest and the anatomy of the psoas major muscle. The influence of criteria such as the height of the disc, the width of the cover plate, and the diameter of the psoas major muscle were not analysed in greater detail.

The CT scans of the patients were not performed in an extreme lateral position. Accuracy in measuring access angles could be increased if the patient's CT scans were performed in an extreme lateral position.

The direct transfer of the acquired information to clinical use is not fully possible. The position in which the CT scans were performed is different from positions in activities of daily living and real-life situations, respectively. The information could nevertheless be used for preoperative planning of the intervention because the surgical reconstruction often takes place in a similar position and orientation as the imaging. Furthermore, the images give a good overview of the pathologies and the current (anatomical) variations so that the surgeon could include this in his (surgical) decisions.

Furthermore, the demographic data of the patients should be considered. Equal numbers of men and women were not selected and 34 women and 66 men were included in the study. The male iliac crest is anatomically different from the female iliac crest. We do not know to what extent our results would have changed if the genders had been equally represented.

留言 (0)

沒有登入
gif