A variety of spinal fusion techniques have been developed and used to treat degenerative diseases of the lumbar spine. Posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), direct lateral interbody fusion (DLIF), and oblique lateral interbody fusion (OLIF) are surgical techniques aiming at common objectives of improvement of low back pain, radiculopathy, and instability. Numerous studies have been published comparing the radiologic and clinical outcomes of these methods [9,12,13].

TLIF, which was first described by Harms and Rolinger [8], has been widely used for several decades. It is a posterior approach that uses a facetectomy corridor and has benefits of safety, good outcomes, and high fusion rate. Surgeons prefer this approach because they can reduce dural retraction and enable direct neural decompression [4,7,8]. However, characteristic complications include posterior spinal muscle injury and cerebrospinal fluid (CSF) leakage [10,16]. Additionally, TLIF does not provide an adequate sagittal angle at the level of L5–S1 since it is difficult to insert a sufficiently high profile and large angled cage through the small facetectomy space [18]. A wellknown limitation of TLIF is the inability to create the sagittal angle at the L5–S1 level, which is considered the largest contributor to the lumbar lordotic angle.

Davis et al. [5] developed a new fusion technique at the L5–S1 level, i.e., OLIF at L5–S1 (OLIF51), in 2014. OLIF51 approaches the disc space using a left retroperitoneal extrapsoas route in an oblique lateral direction with the patient in right lateral decubitus position [21]. In contrast to the retroperitoneal extrapsoas approach for OLIF at L2–5 (OLIF25), OLIF51 uses the corridor under the bifurcation of the great abdominal vessels [21]. This approach corridor for OLIF51 enables insertion of a cage with a greater height and angle compared with that in TLIF [3,21,22].

We planned a study to evaluate the usefulness of OLIF51 by comparing clinical and radiologic outcomes with those of TLIF at the same L5–S1 level based on our recent experience.

This single-center study enrolled patients who underwent TLIF (TLIF51 group) or OLIF (OLIF51 group) at the L5–S1 level and were followed up for more than 6 months. Two senior surgeons performed TLIF (YBK) or OLIF (SWP). Single-level or multilevel lumbosacral fusion cases including the L5–S1 level for degenerative spine disorders were enrolled. Surgery for fracture, infection, tumor, and congenital anomalies were excluded. Age, bone mineral density (BMD), cages used at the L5–S1 level, and main pathologic conditions were compared between the two groups. In addition, estimated blood loss (EBL) and operative time were compared among patients who underwent single-level fusion at the L5–S1 level. This study was reviewed and approved by the Institutional Review Board (Chung-Ang University Hospital IRB No. 1904-002-16257).

OLIF51 has various advantages by using a lateral retroperitoneal approach [17,19,21,22]. First, the abdominal structures move downward due to gravity in the lateral position, and the access corridor is increased as a result. Therefore, it is possible to approach the disc space with less peritoneal retraction compared to that in ALIF. Second, long level lumbar interbody fusions from L1 to S1 can be performed in a single position, because OLIF25 and OLIF51 use the same lateral position without hip bending. Third, unilateral blunt dissection and retraction of the hypogastric sympathetic plexus in the OLIF51 approach is the same as that used to prevent hypogastric sympathetic plexus injury in the anterior approach to the L5–S1 level, and can reduce the risk of postoperative retrograde ejaculation. Additionally, according to our study, the most frequent pathologic condition was foraminal stenosis at the L5–S1 level in both groups. As foraminal stenosis is a good indication for indirect decompression through an increase in disc height, OLIF51 seems to be quite useful for the L5–S1 level where foraminal stenosis is the main problem.

The OLIF51 group showed a statistically significant increase in MDH and FH compared to the TLIF51 group, and the postoperative DA was significantly greater in the OLIF51 group. These results are predictable because of the differences in surgical technique and cage features. OLIF51 provides easy access and sufficient space for insertion of a high profile and large angle cage in the L5–S1 disc space. On the other hand, it is very hard to increase disc height by inserting a large cage in TLIF due to obstacles such as the facet joints, dural sac, and nerve roots. The cage used for OLIF51 has a high profile (8–16 mm) and angle (6–12º), and a large footprint (30×24 mm), all of which are significantly greater than the cages for TLIF. However, the mean postoperative DA of 22.6±4.7° at L5–S1 level in OLIF51 was greater than the reported normal range for the L5–S1 DA of 14.13° [2]. This may result in an adverse effect, in which foraminal stenosis rather than indirect foraminal decompression is the outcome. The possibility of foraminal stenosis due to increased DA can be overcome by inserting a high-profile cage. The higher postoperative DA at the L5–S1 level may be helpful for prevention of iatrogenic flat back and may provide sagittal correction for degenerative flat back syndrome.

The fusion rate was higher in the TLIF51 group than in the OLIF51 group, which was related to the choice of fusion material. A prior study reported that autologous bone resulted in a higher fusion rate than DBM [6]. However, since OLIF51 has an advantage to insert a larger cage than TLIF, a better fusion rate could be expected through long-term follow-up studies. Subsidence was significantly less in the OLIF51 group than in the TLIF51 group. Le et al. [11] reported a lower rate of subsidence with use of a wider intervertebral cage, and our results showed similar trends. According to our data, use of a wider cage in OLIF51 than that in TLIF at L5–S1 resulted in increased mechanical stability and less cage subsidence.

There were no major complications with OLIF51, but postoperative ileus seemed to increase. The postoperative ileus improved spontaneously after several days. Early ambulation on the first postoperative day and parenteral nutrition for 2 days were recommended to prevent postoperative ileus in our hospital. Previous reports of psoas symptoms, a characteristic feature of the transpsoas approach, were not found in the OLIF51 group [17,20]. In the first three cases, minor injury to the iliac vein was considered related to the introduction of a new technique.

Since our retrospective study was a short-term investigation with a relatively small number of patients, long-term and larger-scale prospective studies are needed for more comprehensive evaluation. Each surgeon performed only TLIF or OLIF51 exclusively, which may result in bias. According to previous studies, percutaneous screw system and open pedicle screw system have not been reported to cause any significant difference to lumbosacral sagittal angle [15]. We preserved the contralateral half of the ALL. However, the outcomes of OLIF51 may have been quite different with total removal of the ALL or use of anterior plate and screw fixation. In addition, the absence of postoperative retrograde ejaculation may be related to the age and sex distribution in our study, which included predominantly elderly male or female patients. Since subsidence and fusion rates were evaluated in a relatively short period with postoperative 6-month radiographs, prospective long-term studies are needed for a more accurate evaluation.

Both OLIF51 and TLIF at the L5–S1 level showed good clinical and radiological outcomes. However, OLIF51 was more effective for indirect decompression of foraminal stenosis, providing strong mechanical support with a larger cage, and making a greater lordotic angle with a high-angle cage than with TLIF.