Lumbar interbody fusion is a standard technique for treating degenerative lumbar disorders. In 1952, the PLIF procedure was introduced by Cloward as a posterior type of lumbar interbody fusion technique.1 The TLIF procedure was developed as an alternative to PLIF.2 Later, numerous advances in posterior fusion techniques were introduced for clinical use.5-8 The PLIF and TLIF techniques yield stable results. However, the procedures are relatively invasive as posterior interbody fusion techniques involve the need to manipulate the spinal canal, as a result of which dural tears or nerve injury can occur occasionally.
ELIF, a less invasive interbody fusion technique, was introduced by Phillips and Cunnigham3 based on Wiltse’s approach9 in 2002. The approach in ELIF is from the posterolateral direction which is more lateral compared to those in PLIF or TLIF (Fig. 1A). Interbody fusion is performed through Kambin’s safety triangle4 without the need for surgically invading the spinal canal.
In theory, ELIF has many advantages: minimal invasiveness, no canal invasion, and easy revision surgery after the previous decompression of the canal. However, ELIF has not been popular. In our opinion, the reason for this is that ELIF is considered technically demanding because the surgical field is deep, and the illumination is poor. In addition, surgeons were concerned about damaging the dorsal root ganglion of the exiting nerve root. Therefore, Baek et al.10 recommended a wide dissection and meticulous manoeuvre in ELIF.
Based on the same concept that does not involve the surgical invasion of the spinal canal, lumbar lateral interbody fusion (LLIF) was introduced.11-14 LLIF employs a lateral approach, and neither the spinal canal nor the posterior lumbar muscles are surgically invaded. This technique demonstrates that “indirect neural decompression without direct decompression of the spinal canal” can result in good clinical recovery. LLIF procedures, including oblique lateral interbody fusion (OLIF)11,12 and extreme lateral interbody fusion (XLIF),13,14 have become increasingly popular. LLIF has been demonstrated to produce good results in a minimally invasive manner, but the rates of approach-related adverse effects, some of which are very severe, have been reported to be relatively high.15-17 In XLIF which involves a lateral retroperitoneal transpsoas approach, lumbar plexopathy, bowel injuries, ureteral injuries, and vascular injuries have been reported.15,16 OLIF involves the use of a peritoneal approach, and complications, such as lumbar plexopathy, peritoneal lacerations, and ureteral injuries, have been reported.16,17
mELIF is a unique lumbar interbody fusion technique that entails the use of the ELIF approach under spinal microendoscopy. The skin incision is made 6 to 10 cm from the midline in ELIF10,18 and 4 to 6 cm from the midline in mELIF. As stated by Baek et al.,10 a wide dissection is needed in ELIF, and in contrast, minimal invasiveness can be maintained in mELIF by using a 16 or 18mm TR. In mELIF, the approach angle is approximately 45-degrees from the posterolateral direction, which is away from the internal organs and psoas muscles. The procedure is performed under bright and clear microendoscopic vision. Therefore, we believe that mELIF is a safe surgical technique.
Microendoscopic discectomy was invented by Foley and Smith in 1997.19 In this system, surgery is performed using a TR with a diameter of 16 or 18mm and a microendoscope. The indications for this technique have been expanded to include lumbar spinal stenosis,20 lateral disc herniation,21 and extraforaminal stenosis.22,23 We employed the approach used in lateral disc herniation or extraforaminal stenosis in mELIF.
Another endoscopic ELIF technique performed through Kambin’s safety triangle was also reported.24,25 In this technique, the approaches are relatively distant from the midline and are close to those involved in the original ELIF procedure. They used full endoscopy (FES) of which diameter is around 8mm. The advantage of mELIF is that manipulations such as removing bone, securing nerve root and inserting a cage into the disc are clearly seen in 16mm TR. These procedures are difficult to see directly in fusion surgeries using FES. Therefore we believe that mELIF can be performed in a safer manner. Another benefit of mELIF is that bone resection is easier. Therefore complicated cases such as with L5/S1 disorder or with grade II spondylolisthesis can be good candidates for mELIF.
We performed a single-level mELIF procedure in 55 patients, and the results were very favourable. The results in over 90% of the cases were excellent or good. We are certain that indirect decompression is also valid in mELIF as well as in other LLIF techniques. There were neither major clinical complications nor the need to perform revision surgical procedures in this study. Three patients developed asymptomatic cage migration in the disc space. Bone union was obtained in 90% of the cases at 2 years postoperatively. The rate was comparable to that of TLIF studies.5-8
The main index diagnoses in this study were degenerative spondylolisthesis (n = 33), isthmic spondylolisthesis (n = 9), and foraminal stenosis (n = 10). We believe that mELIF can be used for treating most of the single-level lumbar degenerative disorders, including Meyerding grade II spondylolisthesis. One big advantage of mELIF is that with a little additional effort, it can be applied for treating L5/S1 disorders by removing the sacral ala. LLIF techniques, including OLIF and XLIF, are usually not suitable for treating L5/S1 issues for anatomical reasons. There are other advantages of mELIF. The local bone can be kept and used as an autograft. In mELIF performed at L4/5 and L5/S1, if the local graft bone obtained is not enough, additional bone can be easily obtained from the nearby ilium, and because the skin incision is close to the iliac crest, another incision is not necessary. There is no need for changing the patient’s position during mELIF, which is usually needed during XLIF and OLIF procedures. There is no need for changing the patient's position during mELIF, which is usually needed during XLIF and OLIF procedures. We summarize advantage and disadvantage of various fusion techniques in table3.
There were several shortcomings associated with mELIF in this study. This procedure was only applied to treat single-level disorders. Multiple-level mELIF can be performed, but we do not have the experience yet. The operative time was also slightly prolonged, but after our initial experience, we are certain that this can be shortened. We used a single bullet-type cage in most of the cases, and the correction was not as strong as that achieved in the XLIF and OLIF procedures, in which a bigger cage is used. However, recently, we managed to insert two cages in several cases (Fig. 6A, B). In this study, bone union was achieved in 78% of the cases at one-year post-surgery and in 90% at 2 years. This number is comparable to that of TLIF studies5-8.