INTRODUCTION
Thoracic disc herniations (TDHs) are becoming more frequent due to the increasing abundance of imaging. However, the majority of these patients are asymptomatic [1,2]. Surgically treated TDH comprises only 0.15% to 4% of all cases with disc herniation. The main symptoms of TDH include axial back pain, band-like radiculopathy, bladder and urinary dysfunctions, and different degrees of myelopathy [3]. Rarely, atypical manifestations such as gastrointestinal, cardiopulmonary, and abdominal pain may be present [4,5]. However, the low correlation between radiologic images and clinical findings can pose a challenge for surgical decisions.
Magnetic resonance imaging (MRI) and computed tomography (CT) are equivalent imaging modalities in the diagnosis of TDHs because TDHs are often hard and calcified [6]. Therefore, understanding the natural structure of the herniation (soft vs. hard [calcified]) on CT is of great importance for preoperative planning and preparation [7]. Although TDHs can be central, paracentral, and lateral, they are most often central and paracentral. The exact direction of the herniation is the most important factor in the failure to achieve excision and adequate decompression.
In the thoracic region, the high ratio of the spinal cord to the canal diameter, limitation of its movements by dentate ligaments, kyphotic structure, and relatively poor blood supply make it more sensitive to surgical manipulations [8,9]. For these reasons, it is inevitable that thoracic spine surgery will evolve towards minimally invasive techniques (minimally invasive thoracoscopy) and even full-endoscopic approaches as endoscopy consolidates its place in the field of spinal surgery [10-13]. Although there has been increasing interest in uniportal full-endoscopic thoracic disc surgery in the last 2 decades, the steep learning curve and the unfamiliar anatomy of the thoracic region have caused this evolution to occur slower. In addition, endoscopic approaches have less intraoperative blood loss, fewer postoperative complications, shorter recovery and hospitalization times, and the instability risk of open surgery is relatively eliminated [14].
There are very few studies in the literature reporting transforaminal or posterolateral percutaneous endoscopic approaches in TDHs. Most of the cases in these studies are soft and lateral herniation [10-12,15]. Adequate excision and decompression can be achieved in these cases with simple endoscopic techniques, but for centrally located, calcified, and large herniation, specialized instruments, advanced endoscopic techniques, and surgical experience are required. Furthermore: In the published technical descriptions "outside-in" was always carried out before decompression started [10-13,15-17].
In the “inside-out” technique described by Anthony Yeung on the lumbar spine, the working cannula is first placed just below the herniated part of the disc by passing through the annulus fibrosus, the angle of the endoscope is turned towards the annulus, and then the fragmented part is removed from the inside of the disc and decompression is achieved [18]. This technique requires fluoroscopy to ensure that the tip of the cannula is directly below the fragment. A deeply inserted cannula will make it difficult to access the fragment.
In contrast, the "outside-in" maneuver, requires in every case the use of drills to widen the foramen (foraminoplasty), getting access from the more posterolateral puncture to the spinal canal and the pathology in it [14,19].
In the present study, we demonstrate that in the majority of the analyzed patients the modified inside-out technique alone provides adequate decompression with the advantage of a wide visual angle, and overall good clinical results with lower complication rates compared to the aforementioned technique.
MATERIALS AND METHODS
1. Patient Characteristics
All patients provided informed consent to be included in this case series. This retrospective study included 29 patients (16 females, 13 males; mean age, 54 years; range, 25–85 years) undergoing percutaneous thoracic endoscopic decompression at 31 levels (2 levels in 2 patients, 1 level in 27 patients). All patients were treated by a single surgeon (TL) between February 2016 and March 2022. The patients were categorized based on approach, surgical technique, disc level, and disc calcification. Overall characteristics are summarized in Table 1 and Figure 1. Ethical approval was not required for this retrospective study in accordance with local and national guidelines. This study fully complies with the 1964 Helsinki Declaration and its later amendments. The patients consented to the presented treatment and provided written informed consent.
2. Surgical Technique
Operations were performed under general anesthetic with the patient in the prone position on a radiolucent table to allow radiologic guidance in 2 planes. In every patient, a meticulous individual plan of the surgery was performed preoperatively. Based on the SpineTIP concept (access-optimized system for the interlaminar, transforaminal, and posterolateral approach) provided by Karl Storz Company (Tuttlingen, Germany), we used the color-coded green system with a 15° optic and 2.9-mm working channel and the so-called red optic with 25° orientation and 3.6-mm working channel respectively.
The posterolateral access was elected in the cases of lateral and sometimes centrolateral disc herniation in a wide-open foramen. The target area is the center of the disc and the preferred optic was the smaller 15° one (Figure 2). The entry point of the needle was determined by the integrated imaging software. After placement of the needle in the desired position, diskography was conducted and via a guide, the dilator and working channel of the system were placed in the central position. From this desired point the “foraminal retreat maneuver” described by Lübbers et al. [20] was performed. Due to open foramen and no hypertrophy of bony elements in this case partial resection of facet joints was not necessary and only the green system was applied (inside-out, Figure 3). If we are faced with more centrally located disc herniation we used the more lateral transforaminal approach with the target point in the midline and in the posterior fifths of the divided disc sections (Figure 4). The surgical strategy is similar to the posterolateral access bringing the needle into the proper position first, followed by diskography and placement of the beveled tube in the base of the prolapse (Figure 5). No bone removal was necessary either and decompression was sufficient (only inside-out). In case of central calcification of disc herniation, the red system allows the introduction of a straight or flexible high-speed drill via the 3.6-mm working channel with debulking, eggshell release, and detachment of the disc herniation from the dura (Figure 6, only inside-out). In the case of a more narrow foramen with a coronal orientation of the facet joints, we perform a resection of the outer parts of the superior and inferior articular facet (foraminoplasty) to obtain a shallower orientation towards a more central sequestration. In opposite to the technique described by Hoogland we are starting our decompression again in the center of the disc, slight removal to the annulus is conducted, followed by neuro foraminal exploration with the green system (inside-out and foraminal retreat). After completing this maneuver, the green system was substituted by the color-coded red optic and instruments. Hand drills or a high-speed drill were introduced and the lateral part of the facet joints were removed (outside-in). After widening the foramen, a targeted fragmentectomy can be connected (Figure 7).
3. Outcome Evaluation and Statistical analysis
The surgical outcomes were assessed using the modified MacNab outcome criteria and visual analogue scale (VAS) scores preoperatively and postoperatively at the latest follow-up. Patients who had an MRI or CT postoperative were evaluated for sufficient anterior decompression. 58.6% (n=17) of the patients were followed up in the postoperative period and the scores were recorded. The outcome, surgical technique, and radiographic of the following group records were compared using the Wilcoxon test. All of the analyses were performed using IBM SPSS Statistics ver. 22.0 (IBM Co., Armonk, NY, USA), with p<0.05 considered statistically significant.
RESULTS
The treated disc levels included Th5/6 (1), Th6/7(1), Th7/8(6), Th8-9 (1), Th9/10 (2), Th10/11 (4), Th11/12 (7), Th12/L1 (9). Out of the overall 9 cases, calcification of the disc herniation was observed, and in 24 cases, the inside-out maneuver was performed alone. In the remaining 5 cases, an additional outside-in (foraminoplasty) procedure was deemed necessary. According to the MacNab criteria, overall excellent and good outcomes were obtained in 11 patients (64,7 %), fair outcomes in 4 patients (23,5 %), and a poor outcome in 2 patients (11,8 %) at the last follow-up. The mean follow-up was 23.4±18 months.
The preoperative mean VAS was 8.4±2.3, and the postoperative mean VAS was 2.8±3.4. The VAS at the last follow-up had decreased by in average of 66% (5.5±3.4 points) compared to the preoperative VAS, p<0.001. While postoperative MacNab criteria in transforaminal approaches have shown better outcomes compared to the posterolateral approach, there is no significant difference between them (p>0.05). When analyzed the same comparison is based on the presence of calcification, a statistically significant decrease in the VAS is observed in cases where calcification was absent (n=12, p=0.005). However, in situations where calcification was present (including partial calcifications within the presence group), the decrease was not statistically significant (n=5, p>0.05). Statistical analysis showed a significant improvement after surgery in cases with more lateral disc herniation (n=10, p<0.005). However, there was no significant difference among the central disc herniation (n=4, p>0.068) when comparing the pre- and postoperative VAS scores.
The outcomes of the case series are summarized in Table 2. Postoperative MRIs were obtained for 24% of whole-operated patients, and it was observed that radiological adequate anterior decompression was achieved in all of them. Spondylodiscitis occurred at the operated level in 1 patient and furthermore, 1 patient underwent open surgery 2 months later due to persistent stenosis and instability. In a female patient with paraparesis, a pneumothorax occurred after finishing the endoscopic procedure. One patient, who did not show a decrease in VAS, had a history of chronic pain syndrome after lumbar fusion. Postoperative results of 3 patients could not be obtained due to death. In addition, we did not observe any dural tear, neurological deterioration or recurrent herniation during the follow-up period.
DISCUSSION
The fact that TDHs are not common creates difficulties both in understanding the natural course of the disease and in diagnosis. Wood et al. [3] presented 26-month follow-up results of asymptomatic patients and showed that no patient developed symptoms. However, the majority of the cases consisted of small and medium-sized disc herniation. Broad-based discs may sometimes be asymptomatic but require close clinical and radiologic follow-up. Therefore, complaints guide appropriate treatment in TDHs. Conservative treatment may be the first choice in patients with isolated axial pain, radicular pain only, and no specific complaints. However, surgical intervention should be considered in patients with myelopathy, stubborn pain, and progressive loss of strength [21].
In the thoracic region, the limitation of the spine's movements by the dentate ligaments and its relatively poor blood supply make this region more sensitive to surgical manipulation [22,23]. A spinal cord already compressed by herniation is even more sensitive to retraction. Therefore, a wide range of surgical approaches have been described to minimize cord manipulation. For laterally located herniation, posterolateral approaches such as transpedicular, transfacet pedicle spraring, and costotransversectomy can be used, while anterolateral approaches such as transthoracic (video-assisted or not) and lateral extracavitery approaches can be used for centrally located lesions [24-26]. However, anterolateral transpleural approaches may cause pulmonary complications such as pneumothorax, whereas posterolateral approaches may cause both anterior cord blind spot and impaired spinal stability [27,28]. In addition, these surgical procedures increase morbidity and prolong hospitalization. Complication rates of nonanterior approaches in the literature range from 9.6% to 15% [7,29,30]. Complication rates of uniportal endoscopic cases range from 0% to 5.8% [10,11,17].
Although the number of publications presenting percutaneous uniportal or full-endoscopic transforaminal endoscopic approaches in TDH has increased in recent years, it can be stated that this approach is still not used widely enough [10-13,15-17]. The following reasons can be put for this: Endoscopy in the thoracic region is not as reliable as in the lumbar region, TDHs are often centrally located and calcified, endoscopy has a long learning curve compared to surgery, practitioners are often unfamiliar with the anatomy of the thoracic region, and the relatively narrow intervertebral foramen and the rip head create a narrow block by covering the intervertebral disc surface [6]. However, we believe that these reasons and barriers will disappear as our experience in the use of percutaneous endoscopy increases. We believe that the application of the "inside-out" technique alone or in combination with the "outside-in" technique used in percutaneous posterolateral and transforaminal endoscopy in thoracic disc treatment will provide a reliable surgical option that allows adequate decompression by providing a wider visual angle. In addition, the advantages of percutaneous endoscopic surgery such as short operation time, less blood loss, less soft tissue damage and pain, quick postoperative rehabilitation, and early discharge are satisfactory and meet current patient expectations. The procedure is highly sophisticated and a safe and correct disc puncture in the preplanned area is a must. An individual planning of the surgery is required in every case considering dimensions of the neuroforamen, grade of calcification and migration, orientation of the facet joints, and amount of the disc herniation lateral, centrolateral, or central. Following the SpineTIP concept highly innovative endoscopes and instruments covered in an access-optimized system lead to a highly flexible surgery adjusted to the individual anatomy and pathology. Following this strategy, only 5 patients in our case study requested an outside-in maneuver in addition and sufficient decompression was achieved in the majority of our patients only by applying the modified inside-out technique.
Due to the nature of the surrounding organs, the transforaminal access has several limitations in terms of the lateralization of the entry point. For that reason, puncture trajectory has a more posterolateral orientation and an opening of the neuroforamen is required to get access to the more central disc herniation. Based on the aforementioned concept we turned over from the thinner green posterolateral system (working channel: 2,9 mm) with 15° optic control, to the wider red scope with 25° optic orientation and a working channel providing 3,6 mm for instruments and corresponding burrs. The latter we used for the opening of the foramen from outside to get into the spinal canal as described [14,19]. With this technique, the hypertrophic facet is shaved under endoscopic vision or radiologic control, and safe surgery is performed. In the present study including 29 patients, we combined these 2 techniques in only 5 patients with large centrolateral or central herniation and facet joint hypertrophy. In the remaining 24 patients, no bone resection on the facet was requested and the green system was introduced first in the center of the disc followed by disc removal in the intradiscal pathway of the herniation. If necessary, we changed the green to the red optic with the beveled tube still inside the disc to introduce drills and to resect calcified disc prolapse from the base to the tip (eggshell technique). At this stage, the appearance of free ripples and pulsations of the dura will indicate that adequate decompression has been achieved. For centrally located herniation, this combined use allows the targeted point in the disc to be reached without too high lateral angling, and the angled view of the endoscopes used provides a wider field of view on the ventral aspect of the dura. With this technique, fragmented herniated tissue can be removed from the disc without the need for transthoracic transpleural approaches in appropriate cases [10].
When comparing the strategies behind “inside-out” and “outside-in” the most critical difference is the initial location of the tip of the working cannula. With the “inside-out” technique in transforaminal approaches, the tip of the working cannula rests anterior and directly underneath the dural sac. The only structure separating the 2 is the posterior annulus whose degenerative pathologies are often painful. With the “outside-in” maneuver the tip of the working cannula is placed directly posterolateral to the dural sac in the neuroforamen after enlarging the foramen, which is often not required, even on the thoracic spine, following the “inside-out” technique. It enables the surgeon to visualize the anterior aspect of the dural sac and provides better visualization of intradiscal conditions that can cause pain [31].
It is well known that thoracic-level surgical interventions are accompanied by a lot of technical difficulties, starting with limited access to the canal due to anatomical constraints of the rib attachments, natural thoracic kyphosis flattens the dural sheath against the posterior margin of the disc, limited mobility by the denticulate ligaments, the ratio of cord diameter to canal leaves little space around the cord and, at some levels, the medullary vascularization is limited. Dural injury by instruments or radiofrequency, spinal canal adhesions, large disc fragments, and a loose dura are risk factors for dural tears. The incidence of dural rupture increased to 1.1% when percutaneous endoscopic lumbar discectomy was switched from an “inside-out” to an “outside-in” technique [32].
Furthermore, continuous irrigation in the spinal canal can raise the intraspinal pressure with the consequence of increased neck pain and even provoked seizures, while the fluid is running back through the outside irrigation channel when “inside-out” is carried out [33].
Thanks to the use of percutaneous endoscopic spinal surgery in the lumbar region becoming more widespread, postoperative outcome scales yield similar results when compared with conventional surgery. Percutaneous endoscopic treatment of TDHs has significant postoperative scale and patient satisfaction levels [10,13,16,17]. In our series, a poor outcome was noticed in 2 patients according to MacNab criteria with documentation of a sufficient removal of the disc herniation in one of them and the other patient had a history of chronic back pain. In the 2 patients requiring a second open surgery, an MRI after the endoscopic procedure reveals adequate anterior decompression in both of them. In our case series VAS for back and radicular pain improved significantly and these rates are consistent with the literature [10,17].
There are certain limitations to our study. The small number of patients and the fact that it is a retrospective study are the most important ones. Knowledge and experience in endoscopic spinal surgery is increasing daily and the articles published are enriched. With this, multicentric studies in the future could strengthen the evidence base for our conclusions.
CONCLUSION
There is still no gold standard method in the surgical treatment of TDH. Despite the shortcomings of traditional surgical methods, it is generally the most preferred. In our case series, more than 80% of hard or soft TDH could be treated in the modified inside-out maneuver alone with percutaneous endoscopic transforaminal discectomy. This is a result of careful preoperative individual planning combined with the use of special scopes and instruments in a highly sophisticated surgical procedure. However, the close relationship between the low complication rates of full-endoscopic spinal cases and surgical experience should always be kept in mind. The rarity of this pathology hinders the accumulation of large case series and homogeneous cohorts in clinical studies.
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