More than 10 years of research has been carried out since the first clinical application of a surgical device with plasma-generating ablation and catheter navigation technology, designed for minimally-invasive spinal intervention. This device is primarily used for repair of spinal disc herniation and is also applied to chronic lumbar discogenic pain. Clinical efficacy is notable and the device can be used without serious side effects. Several papers have already reported the use of this device, and other studies are underway to develop safer and more effective devices and procedures.

Although clinical efficacy and safety still need to be explored, the focus of the research team involved in the development of this device and the evolutionary aspects of various spine devices for minimally-invasive spinal intervention have important implications from an engineering point of view. In addition, results and accumulated data already reported show advantages and disadvantages of devices and procedures, thus providing a basis for the development of new and innovative methods and devices.

In this manuscript, techniques developed in the field of minimally-invasive spinal intervention are reviewed from a temporal viewpoint, and developers' perspectives and future directions are discussed.

Efforts to simplify traditional open discectomy should recognize the experience of Hijikata when referring to initial work on minimally-invasive spinal intervention. In the mid-1970s, he devised methods and instruments for percutaneous discectomy and was notably successful. After more than 10 years of experience, he documented his procedure and reported his confidence in its efficacy and safety [1]. He accomplished intradiscal decompression with suction using a 30-50 mL syringe and extracted tissue using punch forceps all working through a 4 mm cannula under a fluoroscopic guidance. Local anesthetic was administered by percutaneous infiltration for 8 to 12 cm along the spinous process midline. The procedure took approximately 40-60 minutes, and discography before surgery was used to confirm initial surgical accuracy of the target level. In 136 cohort studies, 72% of patients reported good or excellent results. At least one to three grams of nucleus pulposus could be removed using this procedure. After using the procedure for 10 years, one case of spondylodiscitis and one case of vessel injury were reported as significant sequelae.

In 1987, a specific device, the nucleotome discectomy probe, for percutaneous discectomy was introduced. Using this device, the surgical method, automated percutaneous lumbar discectomy (APLD), used procedural techniques very similar to those used by Hijikata. However, the nucleotome was a guillotine-like automated probe that had a shaft about 2 mm in diameter that could simultaneously perform suction, aspiration and cutting at its distal end. Initially, removal of up to 7 gm of tissue was reported to be possible, and procedure time was suggested to be approximately 45 minutes [2]. In a preliminary clinical study, a success rate of 86.1% (n = 31/36) and a mean hospitalization stay of 1.7 days were reported [3].

Based on the demonstrated clinical efficacy of percutaneous discectomy, minimally-invasive spinal intervention with laser energy has been investigated, initially by Ashcer and Choy [4]. In a clinical study of 164 patients and using a potassium-titanyl-phosphate (KTP) surgical laser system, efficacy of the procedure for subjects with appropriate indications was reported to be 70.7% [5]. In this study, RSD (reflex sympathetic dystrophy), dysesthesia, neurologic deficit, stenotic symptom, and far lateral disc herniation were reported as procedural complications. In a recent systematic review, short-term and long-term clinical outcomes following laser discectomy were assessed and indicated that benefits exceeded risk burden (1C/strong recommendation, low-quality or very lowquality evidence [6]) [7].

The IDET, 1998, presented a somewhat different concept for discectomy for decompression of the nucleus pulposus. The inventor, Saal proposed a new concept of annuloplasty, which hypothesized that pain could be relieved by painful nerve destruction and rupture of the posterior annulus using thermal energy transfer [8]. The design of the device was such that a wire capable of warming to 90°C was inserted into the intervertebral disc and curved to the posterior side to a location inside the posterior fibrous ring. Although significant efficacy in clinical trials of patients with chronic discogenic pain was reported, criticism has been raised concerning insufficient temperature and difficulty of ideal wire placement [9,10].

The term "nucleoplasty" has likely been used since the 2000s differentiate it from conventional discectomy Coblation has been a safe and simple procedure to induce histologic changes in fibrous rings and collagenous structures by the transfer of heat energy into the disc [8,11]. In Sharps and Isaac's paper published in 2002, the clinical significance of the device was described. A preliminary clinical study of 49 patients with or without radicular pain and treated via insertion through an intervertebral disc using a 1 mm diameter wand reported a significant decrease in pain intensity over a 12-month period [12]. A recently reported meta-analysis described that this minimally-invasive procedure as having fewer complications in relation to its ability to relieve pain and improve function when applied to lumbar and cervical spines [13].

Hijikata's clinical study reported results from 136 patients with low back pain, of which 106 were diagnosed with disc herniation and 25 were diagnosed with degenerative disc disease. Types of lumbar disc herniation were classified as bulging, protruding, and prolapsed. Patients with protruding discs showed a good prognosis, while prolapse was a poor indicator [1]. Based on his experience, Kambin [14] reported that laterality and size of herniation are not an indicators, and that partial decompression of large contained fragments is possible based on the precise location of the device. However, the preliminary study of APLD did not report the type of disc herniation, but after a large cohort study and many experiences, it was concluded that free fragments that invade the spinal canal, and large extrusions that deviate above or below the disc space level were not indications for the procedure [15,16]. In a study that analyzed 1310 cases, it was concluded that the procedure should not be applied to treat non-contained extruded discs [17].

Laser disc decompression also reported good results in appropriately selected patient groups, but the success rate of the procedure was significantly lower in cases with non-contained discs or extruded fragments [5]. In a recent review article, an acceptable basis exists for the relief of short and long-term symptoms of radicular pain from disc herniation, which reflects efficacy for contained discs containing a large amount of water [7].

Indications for Nucleoplasty using the Perc-D SpineWand include chronic discogenic pain diagnosed by provocation discography; the wand is useful for the types of herniation that cause radicular pain that are also indications for the procedure described above. Early studies classified large extrusion, sequestration, and large herniation involving more than one-third of the sagittal diameter of the spinal canal as contraindications [12]. In a recent systemic review, a selection bias in recent studies was noted perhaps caused by application of the procedure based on initial indications. The review suggests that limitations should not rule out a wider application. As evidence, among five subjects included in a report by Sim, two patients with extruding discs showed a positive response to treatment, and in clinical trials described by Bokov, of 27 patients with extruding discs 44% showed significant symptom relief, and 15% reported complete elimination of symptoms [13].

The indications of the navigable plasma-generating catheter are presented in Table 1.

Table 1 . Indications of focal selective ablation device (L’DSIQ) in patients with low back pain.

Leg pain (radiating pain or radicular pain), dominanatly, relevant with herniated nucleus pulpusus

MRI evidence of contained disc herniation including extrusion

Resistant pain more than 3 months of conservative management

Contraindications

Chronic discogenic pain with positive provocation discography (relatively)*

Severe degenerative disc with less than 5mm intervertebral space or severe deformation of end-plates

Non-contained herniation larger than one third the sagittal diameter of the spinal canal

Disc sequestration

Moderate or severe central spinal stenosis

Unstable hemodynamic status

Spinal malignancy, infection, bony fracture

*More data are needed on efficacy..

Indications for procedures should distinguish between protruding and extruding discs. Commonly used definitions of these terms use radiological nomenclature based on the shape of displaced material. Protrusion is present if the greatest distance, in any plane, between the edges of the disc material beyond the disc space is less than the distance between the edges of the base, in the same plane. The base is defined as the cross-sectional area of disc material at the outer margin of the disc space of origin, where disc material displaced beyond the disc space is continuous with disc material within the disc space. In the cranio-caudal direction, the length of the base cannot exceed, by definition, the height of the intervertebral space. Extrusion is present when, in at least one plane, any one distance between the edges of the disc material beyond the disc space is greater than the distance between the edges of the base, or when no continuity exists between the disc material beyond the disc space and that within the disc space [18].

A distinction between contained and non-contained discs is a more ideal classification. The test of containment is whether the displaced disc tissues are wholly held within an intact outer annulus. A disc with a “contained” herniation would not leak fluid injected into the disc into the vertebral canal. Although the posterior longitudinal ligament and/or peridural membrane may partially cover extruded disc tissues, such discs are not considered “contained” unless the outer annulus is intact. Strictly speaking, containment refers to the integrity of the outer annulus covering the disc herniation. The technical limitations of currently available noninvasive imaging modalities (CT and MRI) usually preclude distinguishing between contained and uncontained disc herniation. Discography does not allow a capsule consisting of both annular fibers and longitudinal ligament fibers to be differentiated from one consisting only of longitudinal ligament fiber. These methods essentially allow only a distinction between “leaking” and “nonleaking” discs [18].

Nygaard [19] analyzed inflammatory cytokines after removal of contained and non-contained disc tissue classified by inspecting the surgical field through discectomy, and found that a high concentration of Leukotriene B4 and Thromboxane B2 in the noncontained disc and these findings were correlated with a more aggressive clinical presentation. Haro [20] also found a marked infiltration of macrophages and vascular proliferation in granular tissue of herniated nucleus pulposus, and found that inflammatory cells were more abundant in non-contained than in contained herniations. Results from subsequent biochemical studies suggested a poor prognosis for the relief of pain and neurological symptoms for patients with non-contained hernia.

Findings of MRI and CT have been shown to be relatively inaccurate in diagnosing contained and non-contained discs. In Weiner's study [21], in the comparison with analysis of containment determined during surgery, the accuracy of the MRI was 72% sensitive, 68% specific, and 70% accurate.

In conclusion, disc containment is considered to be an appropriate prognostic factor for percutaneous discectomy and neuroplasty in minimally-invasive spinal intervention. Positive clinical outcomes in some cases with extruded discs need validation to confirm the limitation of these procedures to contained herniation.

Schenk [22] described the concept of a closed hydraulic space within discs, explaining the principle of laser disc decompression. In other words, partial decompression of a certain part of the disc may induce decompression of the whole disc or another part of the disc. Thus, decompression of the posterior nucleus in the disc space or center of the disc can lead to decompression of herniated material in the contained disc. In vitro experiments have shown that an increase of intradiscal volume of only 1.0 mL causes the intradiscal pressure to rise by as much 312 kPa or 2340 mmHg, and vice-versa [23].

Pressure transfer from intradiscal space to an extruded disc connected via a full-thickness torn annulus is predictable. However, no direct study exists for extruded regions in non-contained discs that deviate from a closed hydraulic space. Epidural pressure and discography studies can infer possibilities.

Takahashi [24] measured epidural pressure in human subjects using a flexible catheter transducer, and found L4/5 pressures of 18 ± 6.9 mmHg in supine positions at L4/5 level, up to 85.0 ± 39.2 mmHg at maximal lumbar extension in prone positions and 116.5 ± 38.4 mmHg at maximum extension while standing. In addition, He reported that a significant increase in pressure in patients with spinal stenosis or walking posture [25]. Maximum change in pressure was not greater than 160 mmHg in these studies. According to Satós study [26], pressure measured at the center of a non-degenerative disc was 90 kPa (675.1 mmHg) in a prone posture, with greater pressures while standing and sitting. These results suggest that in the region of posterior annulus and posterior longitudinal ligament, a pressure vector almost always acts in the epidural direction. In other words, the pressure to move extruded material to the inside of the intervertebral disc is unlikely at any time, and escape of material from the narrow intervertebral disc to the epidural space that extends caudally and cranially is irreversible. A non-contained disc where the full thickness of the annulus is torn indicates that the formerly closed hydraulic space is open. Under this condition, decompression through intradiscal volume reduction cannot affect extruded materials in the epidural space.

An open hydraulic space combined with intradiscal-epidural pressure differences may be the reason why removal of intradiscal tissue by percutaneous discectomy and nucleoplasty are not effective in cases of non-contained discs.

Extruded discs can be improved clinically by direct decompression at the site of escape. However, this solution has obstacles. First, accurate, safe, and effective access is difficult considering the complex of neurovascular and skeletal structures. Second, a close approach to the nerve tissue, such as the dura and DRG, is necessary, which increases risks of damage and irritation. Plasmagenerating tissue ablation and proportional catheter navigation techniques are key to overcoming these problems. (Fig. 1 and 2)

Plasma-generating tissue ablation technique is very stable and quantitatively adjustable. The authors developed optimal conditions for effective plasma generation by using a discharge tip made of stainless steel and samples of porcine nuclei. The correlation between removal rate of porcine nuclei and corrosion rate of tip was analyzed for commercialization of the final product. In this study, a continuous discharge for about 300 seconds was established as a point at which efficiency decreased. The amount of removable nuclei during this period was 1.35 gm [27]. Also, plasma discharge and tissue ablation occurred at the surface of the bubble around the tip during discharge. After a time, carbonization started due to focal accumulation at the heated tip. To effect tissue removal and prevent carbonization, contact between tip and tissue must be constantly changed. Contact is altered clinically by rotating, drawing, and vibrating the catheter handle. A study of the optimal speed of this motion was also carried out, and it confirmed that a speed of 2.5 mm/s at an output voltage of 280 V is optimally efficient [28].

Figure 1. Plasma-generating tissue ablation and proportional catheter navigation devices.

Figure 2. Composition of a wand for discoplasty (L’DISQ, U & I Corporation, Uijeongbu-si, Gyeonggi-do, Korea)

A proportionally controlled navigation technique was applied to the distal end of the catheter to enable manipulation from the handle. The anatomical range and the length of the distal end of the catheter, the radius and angle of rotation were determined in studies with human subjects.

The device is designed to facilitate its operation and approach to the disc during the procedure. Particularly because the approach is based on an initial concept of device development, it was also necessary to establish a procedural technique. The initial approach was to penetrate the annular fissure. This approach did not allow access to all extruded regions. Experience shows that if the fissure is narrow, or the inlet of the fissure is not aligned with the extruded center, or the fissure is complex with radial and tears around the circumference, the tip may not penetrate the fissure to the extruded region. Although this result was not common, prognosis in such cases was poor. In the past, in passing the fissure, the catheter was not able to enter and bend at the same time, but was bent after entering. It did not pass through only one intradiscal pathway, but rather bent toward and hit the nucleus.

Currently, the guide-needle approach does not pass through the fissure; instead, a direct approach to the base of the extruded portion is used. This method can penetrate the posterior annulus horizontally since the needle enters to the opposite side of the lesion (trans-annulus approach). Anatomical characteristics of L5/S1 limit the angle of entry of the catheter. In this case, catheter approach is steeper, often failing to locate the catheter on the opposite side or becoming located too far forward. Thus, the approach to Kambin's triangle at L5/S1 is oblique from just above the iliac crest to reduce the entry angle as much as possible. In this case, the tip of the guide needle is slightly bent, to prevent the guide-needle from touching the lower endplate. More detailed information on this clinical procedure is available [29].

Clinical efficacy of this device was reported in a preliminary study in 2011. A cohort study of 27 patients with disc herniation who had experienced radicular pain for 6 months showed significant improvement in both pain intensity and function. Of these subjects, 20 had extruded discs on MRI, with this condition confirmed with radiculopathy on electromyography [30].

A two-year follow-up study of 170 patients with low back and extremity pain caused by lumbar intervertebral disc herniation indicated that, despite L5/S1 involvement in 42% of patients (n = 72) and a large proportion of extruded discs (86%, n = 146), pain intensity and functional improvement were significant, as was normalization of passive straight leg raise tests. After 2 years, the percentage of patients whose pain intensity improved more that 50% was 78.3%, and recurrence rate among subjects was 4.7% [31]. These studies are observational studies through tracking participants after the procedure, and have a number of limitations; no randomization, no control group, small size of subjects, and no long-period follow-up data.

A preliminary study included patients with chronic lumbar discogenic pain confirmed by provocation discography. Twenty patients underwent the procedure with the catheter tip placed in the prominent tear site identified by discographic analysis. Plasma was generated to ablate, and heat energy was transferred to surrounding tissues. Clinical results were statistically significant at 48 weeks, but some aggravation of symptoms was observed between 4 and 12 weeks after the procedure [32]. Results of the study based on unpublished data showed that subjects’ pain intensity was highly variable before the procedure, and differences between groups with and without pain improvement was large. In some cases, an intensity of 10 (NRS (numeric rating scale)) improved to zero after 12 weeks of treatment; in other cases, an intensity of 3 before the procedure aggravated to 6 after 12 weeks post-operation. A reliable prediction of prognosis is difficult when assessing use of the procedure in a clinical setting when outcomes are so variable (Fig. 3).

Figure 3. Catheter applied to the painful disc provoked by discography, and discogram at the level.

The underlying cause of the observed variability may be corresponding variability in pathology. Pathophysiology of chronic discogenic pain may be associated with neurogenic, mechanical, and chemical factors. However, within the disc, only the vulnerability of the posterior annulus is reported and additional pathology may be present. Further, discographic findings are not highly correlated with clinical symptoms. Thus, discography, which reports only the most prominent tear site, does not guarantee that the source of discogenic pain has been located. In a preliminary study, local treatment of only a specific area of the disc was believed to be the reason for high individual variation in the clinical results because variation remained after randomized treatment of the lesion site.

Major complications of the procedure are nerve stimulation and damage. After the procedure, some patients reported discomfort and sensory abnormalities consistent with a radicular pattern for a period of time. They improved within 1 ~ 3 months. However, some developed foot drop. In one case, marked axonal injury of an L5 root was associated with muscle weakness over 1 year [31]. The distance between nerve tissue and the tip may be reduced during tissue removal even if the tip position is correct, especially where extruded materials are closely attached to the root or nerve without even an outer thin annulus. If the distance between the bipolar system and nerve tissue is reduced, the current can induce nerve stimulation symptoms. If contact is substantial, electrical stimulation resulting in axonal injury due to heat injury is possible.

A report of local osteonecrosis and inflammatory reactions due to thermal injury caused by discharge from the catheter tip touching the endplate was found. Ten patients (three with cervical spine and seven with lumbar spine hernia) were evaluated with MRI after the procedure. Post-operative conditions were classified as aseptic spondylodiscitis, which could be a procedure-related complication. These complications seem to be preventable if caution is taken during the procedure [33].

There was a case of taking antibiotics due to local infection of the skin puncture site, and there was a case of pain in the needle entry point and surrounding stiffness, but symptoms improved within four weeks without any special treatment. There was an unpublished report in which a blood patch was applied to headache complaints due to dural injury after the procedure. At the early period of device development a case occurred where the needle sheath was severed at the subcutaneous level. Also, in one case of percutaneous removal the polymer material was changed to increase strength. One case where the covering of the end was peeled off after the tip discharge occurred.

Although decompression by the procedure may be successful, there are negative evidences for the disc degeneration at the level that continues to progress over a long period of time. Long-term follow-up studies have been reported that interventions that cause direct damage to the intervertebral disc (especially discography) accelerate degeneration [34,35]. Both studies, as the authors mentioned, have limitations in that they were subjects with a high risk of degeneration and that they did not closely reflect the results of provocative discography. In addition, based on the clinical experience of not reversing the disc degeneration, there is also no evidence for what clinical advantages preventing degeneration to control pain. Nevertheless, since there is always a concern that such discoplasty may accelerate degeneration, it is thought that it is necessary to narrow the indication to disc herniation accompanied by end-grade degeneration. Accumulated experiences for a long period are needed.

Based on the accumulated experiences that intradisc procedures can be performed safely under local anesthesia, there have been concerns for more non-invasive and more effective spine intervention. Based on the control technology of medical radiofrequency and the development of an insertion catheter into the human body, more precise discoplasty was attempted, and the effect of improving symptom was reported in extrusion as well as protrusion in disc herniation. The evidence is weak in chronic discogenic pain. Although there is a risk of neural damage, it can be expected to secure safety through improvement of procedural techniques.

Like many of the devices introduced throughout this manuscript, additional medical devices for improved minimally-invasive spinal intervention are likely to be developed in the future. Introducing new technology that expands the utility of focal selective ablation devices and reduces their drawbacks is likely, and the new device described above may be one discussed when ablation therapy is reviewed at some time in the future.

This research was supported by a grant from Korea University Ansan Hospital (O1903511) in 2019.

No potential conflict of interest relevant to this article was reported.