With the growing favorable evidence of lateral lumbar interbody fusion (LLIF), the use of LLIF is increasing [9,12,15]. Ureter injuries are a rare but serious complication of LLIF [5], and some related case reports have been published [1,6,7,10,16]. To prevent ureter injury, using preoperative computed tomography (CT) to evaluate the location of the ureter has also been recently reported [3]. Huang et al. [4] recently published a study with a morphometric analysis of the ureter in Journal of Korean Neurosurgical Society, providing information on ureter position according to each level in a relatively large number of patients. Ouchida et al. [11] demonstrated the ureter location in the lateral decubitus position, and they proved that the ureter moves anteriorly compared to the supine position.

However, we should note that the LLIF procedure starts with retroperitoneal dissection to the psoas muscle. We then insert an electrode (extreme lumbar interbody fusion, XLIF) or dissect more anterior of the psoas muscle (oblique lateral interbody fusion, OLIF). This means that preoperative CT imaging alone cannot predict the real anatomy we find in LLIF surgery. Previous studies also suggested that ureteral injury may be related to the limited exposure of the retroperitoneal space and blind maneuvering [2,6,7,10,13]. In this letter, we aim to describe the three positions of the ureter that we can encounter after retroperitoneal dissection (Fig. 1A) [14] : 1) above the psoas muscle (Fig. 1B) : the ureter is located inside the retroperitoneal fat, anterior to the psoas muscle, does not contact with the psoas muscle. It is easily retracted medially during retroperitoneal dissection. This location of ureter has a low risk of injury during XLIF and OLIF. The ureter is not visible unless the retroperitoneal fat is disrupted. 2) On the psoas muscle (Fig. 1C) : the ureter is in contact with the anterior border of psoas muscle, and it is easily visible and retracted medially during retroperitoneal dissection. If the retroperitoneal fat above the psoas muscle is not clearly swiped off, the XLIF procedure can be related to ureter injury. For the OLIF procedure, it has low risk of ureter injury because the retroperitoneal fat should be swiped off to the anterior border of psoas muscle. And 3) beside the psoas muscle (Fig. 1D) : the ureter is located at anterior margin of the psoas muscle, adjacent to disc space (or vertebral body) and, in some cases, under the psoas muscle. The ureter is attached to or slightly floats on the annulus. This location corresponds to II-v and I-p. In this case, XLIF could be performed safely but we should be concerned about the possibility that the ureter could be under the psoas (a rare location). However, for OLIF procedures, we should pay special attention to ureter injury. Since the ureter is not identified during retroperitoneal dissection to the anterior border of the psoas muscle, the surgeon might assume that the ureter is retracted medially. However, the ureter is close to the operative corridor, and this location has high risk of ureter injury. We retrospectively found that two ureter injuries occurred in this area from our more than 700 OLIF cases. We recently reported these cases, but also briefly introduce them here [8].

In the first case, the ureter was kinked or penetrated by retractor pin insertion; in particular, the pinning was performed over the psoas muscle where the retroperitoneal fat had not been clearly dissected (Fig. 1E). In the second case, the ureter was retracted laterally rather than in the medial direction, and the ureter was damaged when the trials were pulled out orthogonally (Fig. 1F).

We also introduce a recent interesting case where the ureter was under the psoas muscle. In this case, both XLIF and OLIF were risky. However, since we were aware of the location of the ureter before surgery, we were able to safely perform OLIF after medial retraction of the ureter, which was hidden below the psoas muscle (Fig. 2).

We would like to convey the message that it is important to predict the position of the ureter after retroperitoneal dissection, rather than simply evaluating the position of the ureter on preoperative CT before performing LLIF. Ureters located above or on the psoas muscle are relatively safe, but the locations of II-v and I-p should be regarded as a dangerous zone. For this danger zone, we need to perform clear dissection to the anterior border of psoas muscle and check the ureter location through direct visualization.