Downward migrated LDH is sometimes discontinuous and sequestrated from the origin intervertebral disc, and it is generally fragmented into small pieces [2, 5, 14]. The posterior longitudinal ligament can be breached by the disc migrated sequestrations, which can drift cranially or caudally into the anterior epidural space, with an estimated prevalence of 35 to 70% [15, 16]. Conventionally, the removal of these highly down-migrated sequestrations via the traditional surgical approaches requires extensive soft tissue dissection, even at the cost of sacrificing part of the lamina and facet joints, which may result in potential lumbar instability and associated complications [17, 18]. Thus, in order to minimize the possibility of iatrogenic lumbar instability, an experienced surgeon may create a bony opening through the lamina [18, 19]. Nevertheless, it is difficult to reach and remove thoroughly these migrated fragments, since some sequestrations around the corner cannot completely be examined.
Specifically, several surgical approaches have been developed to easily get through the sequestrated LDH. In 2008, Choi et al.  conducted a retrospective analysis of 59 patients with highly migrated intracanal disc herniations who underwent percutaneous endoscopic approach using foraminoplastic technique, and found that the outcomes of foraminoplastic-PELD were comparable to those of conventional open procedures. Also, the author described patients with highly downward migrated LDH had superior outcomes compared with cases with highly downward migrated LDH. Kim et al.  in 2012 investigated surgical outcomes of interlaminar PELD for highly migrated LDH in 18 patients. The disc sequestrations were migrated inferiorly in 11 cases and superiorly in 7 cases. Of them, 12 patients had excellent outcomes in 66.67% of cases, good in 3 patients. But in one case, dural tear without cerebrospinal fluid leakage was suspected. Lin et al.  described their experience with accessing a highly migrated LDH using full endoscopic technique via a translaminar keyhole approach in 13 patients. Although these patients presented with highly upward disc migration and only two cases located at L5/S1, the satisfactory outcomes were observed in 92.3% of cases, which was consistent with our results. The difference was that the patients in our series were with a younger average age.
Nonetheless, the full-endoscopic foraminoplasty has not yet been well clarified as an effective removal for high-grade downward migrated LDH. Our experience in the present series demonstrates an effective and safe procedure for achieving majority removal of a highly downward migrated LDH in our patients. The clinical findings reveal that the effectiveness with respect to pain subsides, perioperative parameters, and neurological functional recovery was similar to those achieved with aforementioned approaches, which efficiently avoid an open limited incision, significant soft tissue dissection, and irreversible bony destruction. In terms of safety, the entire surgical procedure was performed under direct vision, which has the advantages of smaller wounds and less bleeding, and has the potential to decrease the likelihood of nerve damage and maximize rehabilitation potential with faster resumption of activities while minimizing hospitalization time. Ruetten et al. analyzed a series of cases who underwent interlaminar PELD, and found that 2% of the patients had nerve damage . Fortunately, the retraction injury that can happen in posterior surgery was not observed in our series.
Technically, the full-endoscopic foraminoplasty can directly access initial site of a herniated disc and reach highly down-migrated fragments through adjusting the working cannula, possibly indicating it can be used as an alternative to traditional open discectomy since it is not restricted by the high iliac crest. Moreover, the area of operative exploration is very bright illumination and fully visualized, since the endoscope provides clearly high-resolution image under direct vision. It creates favorable conditions to avoid nerve damage and remove the migrated disc as completely as possible. Importantly, the radiation exposure and fluoroscopy time were significantly reduced because the establishment of working cannula was completed under continuous visualization, as well as foraminoplasty.
Importantly, several negative aspects of the full-endoscopic foraminoplasty should also be emphasized. First, complete removal of the migrated sequestrations that were separated into multiple pieces may not be attainable [2, 5]. However, the existence of the hidden fragment can be acceptable if there were no clinical symptoms after operation. With the time extension, some residual disc material can even be resorbed [5, 20]. Secondly, despite the skillful full-endoscopic transforaminal technique, traction or exfoliation between the nerve root and the migrated disc may cause additional dural sac or nerve root damage, especially in case with tight adhesion. Therefore, careful attentions should be taken to protect the dural sac or nerve root during the operation. Thirdly, partial surgical resection of the ventral superior articular process is a time-consuming procedure when foraminoplasty using the endoscopic trephine, and it could result in bleeding on the bone surface, which is hardly controlled with an aid of bipolar radiofrequency coagulators [23, 24]. Additionally, a large proportion of lumbar spine diseases may not be suitable for this technique, such as vertebral endplate (Modic) change, metastatic vertebral lesions, and central lumbar spinal stenosis.
Several shortcomings of the present study should be mentioned. First, as this is non randomized and not matched to control/cohort study, solid conclusion may not be drawn about the efficacy and advantages of the technique. Secondly, although all included patients’ radiating pain were relieved immediately after operation, a minority of patients were not reviewed by MRI during the follow up. There may be the existence of the hidden fragment. Thirdly, a limited number of cases with short-term follow-up may decrease the credibility of the findings that drawn from this series. Therefore, we believe that a larger number of cases is necessary to investigate the long-term efficacy of transforaminal full-endoscopic discectomy for highly down-migrated LDH.