The ESS surgeons essentially should employ one of the two approaching techniques: UPT or BPT has some differences and similarities. The design of a working-channel endoscope for FESS using a uni-portal method (UPT) changed from a standard long length(170mm) and small diameter (7.3mm) of the working channel into a new shorter length (112 mm) and larger diameter (8.4 to 9.0 mm) one (Techord, Daejon, Korea) within a short time in the comparison between an endoscope for transforaminal route and a new one. One of the critical reasons for the change should be the anatomical (or racial?) differences between Asians and Western. As we know well, most ESS surgical sets are German-made. It should be appropriate that German-made endoscopy is inefficient to was Asian surgeons. As a result, these decreasing changes in size brought about efficiency in a surgeon, predominantly Asian, handling hard an endoscope and keeping the endoscopy vertically on the patient's back during the operation. Meantime, endoscopic-assisted spine surgery (E-ASS) using a bi-portal technique (BPT) did not need a new design for an endoscope or surgical apparatuses because of the size of a skin incision of the 2^nd portal, usually for the conventional surgical tools. It can develop concerning their sizes and the relatively appropriate length and diameter for the slender non-working channel endoscope.

Only a few spine surgeons had tried to use uni- and bi-portal techniques in lumbar spine pathologies. In the middle of the 2010s, several articles concerning both procedures in spinal stenosis came out. And not a few national and international technical endoscopic workshops became popular in various countries, which motivated spinal surgeons to start ESS using uni- or bi-portal techniques in spinal stenosis patients. Some ESS pioneers had introduced these two approaching techniques almost the same time, and the surgeons, who preferred the one technique, competed with one another. BPT had been remarkably familiar to the orthopedic surgeon because the orthopedics had used BPT or multi-portal techniques in the endoscopic treatment for various joint diseases. As a result, the orthopedic surgeon might have given an idea to use an arthroscope for BPT in degenerative lumbar disorders. UPT and BPT can be useful in dorsal as well as transforaminal approaches. UPT can be applied in FESS, while BPT can be applicable in E-ASS.

There are two techniques to approach the primary lesion at the early or final stage of the operation. A surgeon who tries to remove a pathology including HLD and spinal stenosis, particularly the pathology accessible via intervertebral foramen, should decide one of two techniques, inside-out (IOT) or outside-in(OIT). Most spine surgeons prefer OIT to the other [8]. The IOT is Yeung's favorite way of approach to a disc lesion, including HLD [9]. Yeung and some of his followers, who always employ the UPT, still prefer IOT [10], particularly the HLD removal via the intervertebral foramen [11]. Practically speaking, the portal's number has nothing to do with patients' outcomes. This issue is validated as long as the primary lesion is accessible via the intervertebral foramen. We need to understand the principal running place of an ordinary endoscope should be an intervertebral foramen.

In 2020, Hofstetter and 27 endoscopic spine surgeons developed ESS consensus nomenclature under the AOSpine MISS taskforce [10]. They reported a list of terminology related to ESS. They suggested each endoscopic procedure's name by a systematic nomenclature system in regular sequence and consistent combination of words. This consensus nomenclature system may not be an undisputed guideline all ESS surgeons should follow. Some ESS surgeons had gotten a name different from the rule of AOSpine nomenclature [12]. A surgeon should differentiate the UPT from a hybrid operation (mini-open/tubular/bi-portal technique) [10]. These authors also presented that BPT is irrelevant to FESS but relevant to E-ASS. A surgeon can apply the UPT in all the FESS.

It may be a wonder if the approaching technique's difference substantially influences surgical result and outcome. According to the published articles, there were positive results of the clinical variables, which presented postoperatively well decrease of VAS, well increase of the surgical results, good outcome (increased ODI), an increase of satisfaction, disclosed no significant difference between two techniques. In a comparison study to microdecompression in the patients with spinal stenosis, ESS showed better results regardless of uni- or bi-portal technique [10,13]. The extent of collateral damage, including muscle injury, demonstrated the largest microdecompression [14] and least in FESS [6,14]. E-ASS showed an increase of muscle damage instead compared to FESS but statistically not significant. This result seems to be no wonder because a working space would be more significant in BPT, using two apparatuses together, which needs a larger room in the muscle-lamina interface. As a result, more blood loss could be more prominent in BPT compared to UPT. Both techniques have another advantage: facet preservation, so-called facet undercutting, or medial facetectomy, leading to segmental stability [6].

Conclusively, UPT and BPT both techniques demonstrated similar clinical results and outcomes. However, UPT is employed only in FESS, and BPT is only in E-ASS. Each technique has its purpose and specific condition to apply appropriately. Inside-out and outside-in should be involved only in the transforaminal approach [10]. ESS surgeon needs to understand what the unfamiliar terminology stands for.