In the literature, the incidence of spine operations has increased than before in patients of age 65 and above. Distinctively, in a 2010 report, a 28-fold rise in spine fusion surgeries was observed for elderly patients. Elderly patients with more co-morbid factors, including diabetes mellitus, hypertension, coronary artery disease (prior procedures), depression, and obesity, experience higher postoperative complication rates and expenditure [10]. In literature comparing three operative methods for different age groups with degenerative scoliosis and radiculopathy with minimum 2-year follow-up [11]. In patients with average age of 76.4 who underwent lumbar decompression alone, the complication rate was 10%. In patients with average age of 70.4 years who underwent decompression/limited fusion (1-2 levels), the complication rate was 40%. Nevertheless, for those averaging a lower 62.5 years of age with multilevel full-curve fusions, the highest complication rate of 56% was observed. In conclusion, had the average age for the latter group been higher, the complication rate would have risen even further. Notably, the less extensive procedures (decompression alone or decompression with limited fusion) yielded considerable enhancement on the postoperative Oswestry Disability Index, while the full fusion group did not.

The prime purpose of improving pain and neurological deficit in the practice of spine surgery is shifting to a more ambitious goal, namely to improve the overall quality of life and the future of patients through three key measures (1) preserving the intraspinal anatomical structures; (2) preserving the paraspinal anatomical structures; and (3) preserving the functionality of the spinal segment. Thus, three new concepts have emerged (a) minimal surgery; (b) minimal access surgery; and (c) motion preservation surgery. These concepts are covered in a new term, minimally invasive spine surgery (MISS). The term "MISS" is not about one or several particular surgical techniques, but a new way of thinking, a new viewpoint and a new philosophy [12]. Even if the advancements in minimally invasive spine surgery are recent, its application includes all spine segments and almost all the existing conditions.

Direct access to the disc through transforaminal approach, was advanced and universalized by Prof. Anthony Yeung, and symbolize the first major accomplishment of percutaneous endoscopic discectomy. Transforaminal discectomy through posterolateral approach uses the natural anatomical window of the intervertebral foramen to minimize damage during approach. The working cannula exceeds through muscle planes, passes through the foramen and between the exiting and traversing nerve roots (Kambin’s triangle) to enter directly into the disc [13]. Using this approach neither cutting of muscle nor resection of bone or ligament is required during discectomy.

The transforaminal approach provides direct access to the disc for nuclear decompression and annuloplasty. After withdrawing the working cannula from the disc, structures outside the disc can be easily accessed for foraminoplasty or for removing a sequestered disc fragment. Additionally, by changing the skin entry point and the angle of approach, different disc levels can be accessed using the same incision [14].

Since the conventional inside-out technique approaches only the intervertebral route, it is difficult to remove the disc if it is migrated superiorly or inferiorly. However, there are three different routes in the transforaminal space anatomically, and if used effectively, percutaneous endoscopic spine surgery can be applied to a wider range of lumbar disc herniation.

Central, paracentral, and high canal compromised LDH were approached by the intervertebral route; foraminal, superiorly migrated and far lateral LDH were approached by a foraminal route (Fig. 1) and inferiorly migrated LDH was approached by suprapedicular route [23].

One of the main anatomical barriers of the transforaminal area is the bony structures. In recent years, the developments in endoscopic drill have solved this problem. The use of a high-speed drill under clear endoscopic visualization facilitates safer and more efficient bone removal (Fig. 2). With the articulating bone burr, we can change the drilling direction and cover up more working area. After the undercutting of the hypertrophied facet and part of the pedicle, the remaining bony work and soft tissues can be cleared using endoscopic punches, forceps, and a laser [24].

The conventional percutaneous endoscopic interlaminar approach is similar to microscope-assisted surgery because of removal of ligamentum flavum and cutting of annulus. Therefore, the transforaminal approach has been preferred in terms of minimizing invasive surgery. Although many techniques of PELD (percutaneous endoscopic lumbar discectomy) have been introduced, most studies at the L5-S1 level have preferred the interlaminar approach [25,26]. It is thought that the high iliac crest, narrow foramen, and a large facet joint are a barrier to performing transforaminal PELD. On the other hand, Yeung and Tsou [16] suggested that PELD could access all lumbar levels, even L5-S1. The favorable approach for L5-S1 level by endoscopic route has been a matter of debate for long. However, a surgeons preference for the interlaminar or transforaminal route, in addition to the height of iliac bone continue to remain an important factor in this decision.

Percutaneous endoscopy lumbar discectomy has an anatomical limitation for endoscope insertion, and there are two main surgical approaches: interlaminar and posterolateral. The endoscope insertion for posterolateral approaches is a blind procedure, but it can be safely achieved via the Kambin’s triangle [27]. During the interlaminar approach, structural damage includes not only ligamentum flavum but also paraspinal muscle and bony structures. Through structural preservation PEILD [28], it is possible to avoid this damage (Fig. 3). On the other hand, the insertion of the cannula for the interlaminar approach is performed under endoscopic visualization, but it is impossible to completely avoid direct retraction of the nerve root and/or dural sac by operative instruments. From previous experience with an open, microscopic, or microendoscopic discectomy, we recognize that some extent of transient retraction of the nerve root is acceptable. Nevertheless, we have to minimize the retraction as much as possible. In the meantime, the importance of the interlaminar approach has not been recognized as it is focused on solving some problems that the transforaminal approach cannot solve. However, the importance of the interlaminar approach is reappearing, as it is the basic approach for percutaneous endoscopic decompression.

However, it is important to be aware of the fundamental purpose of percutaneous endoscopic spine surgery. In this respect, the pathophysiology of disc herniation and spinal stenosis and the concerns about the purpose of treatment according to each pathophysiology will provide a basis for this. In other words, the main pathology of lumbar disc herniation is nerve root compression [29] and it is considered that the most effective treatment can be achieved if it is effectively removed without any other peripheral damage. However, the pathologic hypertrophied ligaments, bony structures and vascular condition around the neural structure is the main cause of spinal stenosis [30] so it is necessary to effectively remove the surrounding pathologic anatomical structures while maintaining sufficient spinal stability (Fig. 4).

The first goal to perform percutaneous endoscopic spine surgery is structural preservation procedure, preservation of functional segment being the second goal and rehabilitation for normal return to life is the third goal.

There is a limitation in the application of percutaneous endoscopy in patients with the severe neurological deficit. More surgical expertise and development in armamentarium is required to increase the spectrum of endoscopy in these patients (Figs. 5, 6).

The learning curve of the transforaminal approach is steep and easy to learn, while the learning curve of the interlaminar approach is flat and hard to master [31]. Surgeons willing to spend the time and energy necessary to gain proficiency in endoscopy can expect to be rewarded through the benefits provided to their patients.

In future endoscopic spine surgery is expected to be more widespread. It is planned to execute surgeries with better imaging systems and with 3D images [32]. At the same time, in order to enhance the sensitivity of the hand movements of surgeons and lessen the complication rates, it is expected that robotic surgery will be used in endoscopic spinal surgeries.

Endoscopic spine surgical technique has become more practical and standardized. Therefore, this technique may offer more reliable and reproducible results, especially for elderly or medically compromised patients to maintain healthy spine life (Fig. 7).