4.1. Review of Anterior Cervical Decompression (ACD) and the Introduction of the TUD Approach
Surgery for cervical spondylosis and discogenic disease is generally divided into two approaches; the posterior and the anterior approach. Robinson and Smith [
12] in 1955 and Cloward [
13] in 1958 pioneered the anterior cervical discectomy with bone fusion to accomplish direct decompression of the compressive spondylotic spur and disc fragment. In 1988, Whitecloud [
14] pointed out the following problems and limitations of the surgical procedures introduced by Cloward, Smith, and Robbinson [
12,
13,
14]: (1) insufficient resection of lateral osteophyte, as it is difficult to open the intervertebral foramen; (2) vertical direction for manipulation of the osteophyte and lesion.
In 1976, Hakuba [
1] introduced the TUD approach, which is a combined anterior and lateral approach to cervical discs. In addition to resecting the uncovertebral joint, the entire disc is removed including the ipsilateral posterior osteophyte and the contralateral uncinate process. Snyder and Bernhardt [
3], in 1989, reported an anterior cervical fractional interspace decompression for the treatment of cervical radiculopathy. Decompression is for the vertebral disc and is limited to the lateral one-third of the intervertebral disc and is limited to a radius around the nerve root. Joh [
4] in 1996 described a modified approach that completely exposes the vertebral artery when the entire uncinate process is removed. Lee [
5] in 2006 described a small keyhole transuncal foraminotomy for unilateral cervical radiculopathy with preservation of the intervertebral disc. The TUD described in the present study is greatly different from their previous methods. In the TUD approach under direct vision using an operative microscope, we can safely and completely remove extensive lateral spurs without exposing the vertebral artery in its canal. Extensive removal of the posterolateral corner and transverse ridge of the vertebral body allows sufficient space for the nerve root and spinal cord, there is little danger of encroachment of the intervertebral foramen and spinal canal.
4.2. Application of TUD Approach
We divided the TUD approach into three procedures: (1) unco-foraminotmy; (2) removal of the lesion and resection of the dorsal osteophyte of the vertebral body; (3) unco-foraminotomy at the contralateral side. Furthermore, we classified the cases into four types based on the location of compression of the nerve roots and the spinal cord.
Type I: compression of the nerve roots in the intervertebral foramen. This type is indicated above (1) unco-foraminotomy.
Type II: compression of the spinal cord in the posterior direction due to disc hernia, osteophytes, and ossification of the posterior longitudinal ligament. This type is indicated above as (2) removal of the lesion and resection of the dorsal osteophyte of the vertebral body.
Type III: compression of nerve roots and the spinal cord occurs posteriorly from the vertebral foramen. This type is indicated above (1) and (2).
Type IV: in addition to Type III, compression of the spinal cord and nerve roots on the opposite side of the intervertebral foramen. This type is indicated above (1), (2), and (3) unco-foraminotomy at the contralateral side.
By applying the three procedures in the TUD approach appropriately to these four types, it is possible to remove lesions effectively. The surgery can be performed with minimal invasiveness tailored to the specific pathology, allowing for optimal outcomes and reducing the occurrence of complications.
4.3. Surgical Outcome, Factors Leading Good Outcome Sand Comparison with Other Reports
We applied the TUD approach based on the location of the compression, demonstrating its utility, our modifications, and considerations. The TUD approach allows for direct visualization and safe, reliable lesion removal of lesions in a less invasive manner than the previous approach. It is highly effective and safe, resulting in a high improvement rate of neurological symptoms and activity of daily living, and minimal complication (
Table 3). By opening the intervertebral foramen and Luschka’s joint, and setting of cages, the alignment can be reduced and good alignment can be achieved and maintained while avoiding subsidence. Anterior cervical decompression (ACD) by TUD achieved and maintained good neuroradiological alignment and sagittal balance (
Table 4 and
Table 5).
In this study, the outcome was better than those in previous reports of ACD [
15,
16,
17,
18,
19,
20,
21,
22]. Several authors reported the efficacy of the various anterior foraminotomy. Matz [
16] reviewed the indications and utility of anterior cervical nerve root decompression. In summary, the success rates were 52–99% but recurrent symptoms were as high as 30% [
15,
16,
17,
18,
19,
20,
21,
22]. Conversely, the efficacy of posterior cervical foraminotomy for cervical radiculopathy has been reported, and in a review [
23,
24,
25], McAnany et al. [
26] stated that there was no significant difference in the pooled outcome between a traditional open or minimally invasive foraminotomy using a tubular retractor. The pooled clinical success rate was 92.7% for open foraminotomy and 94.9% for minimally invasive foraminotomy. But the improvement rate in these reports was not better than that in this report. We guess that the foraminotomy in these reports is not enough and not appropriate [
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26].
4.4. Comparison between Advantages of the TUD Approach and Disadvantages of the Other Procedures
- (1)
We use a vertebral spreader and manually operate a retractor. By doing so, tools such as a drill, curette, Kerrison rongeur, and bipolar forceps can be inserted diagonally into the surgical field, utilizing the space between the legs of the spreader. This allows the surgical field and manipulation site to be operated on while they are visualized directly, as the line of sight and these tools do not overlap [
1,
2,
6,
7]. Consequently, surgery can be performed safely without damaging the dura mater, venous plexus, vertebral artery, and nerve roots, enabling the reliable removal of lesions [
1,
2,
6,
7].
- (1’)
Disadvantages of the other procedures:
Expanding up to the vertebral body, many surgical techniques are used to secure the surgical field, with surgeons utilizing fixed retractors [
12,
13,
14,
15,
16]. However, only the surgical field within the range of the retractor can be obtained and it is not possible to secure adequately the surgical field outside the retractor. Furthermore, tools such as forceps, suction, bipolar forceps, and drills cannot be applied outside of this surgical field. During surgery, procedures are limited to the area under direct vision. Therefore, there are limitations regarding the removal of the outer uncinate process, decompression of the intervertebral foramen, and release of the Luschka’s joint [
12,
13,
14,
15,
16].
- (2)
Unco-foraminotomy allows for the removal of the uncinate process, securing a working space for surgical operations in that area. By opening the intervertebral foramen and Luschka’s joint and setting of cages, the alignment can be reduced and good alignment can be achieved.
- (2’)
Disadvatage of the other procedures:
In conventional surgical methods, it is not possible to open the outer intervertebral foramen. Attempting to open the intervertebral foramen with the same approach would involve obliquely transversing the intervertebral space and necessitate the complete removal of the intervertebral disc, which results in significant invasiveness in certain cases such as Type I [
12,
13,
14,
15,
16].
While the transcorporeal approach offers the advantage of direct access to a compressive lesion from the anterior direction, this increases the use of the vertebral body as supporting tissue, thereby increasing the long-term risk of the vertebral body compression [
27,
28].
While the posterior approach removes compressive lesions such as intervertebral disc herniation and osteophytes located anteriorly to the nerve root, it becomes necessary to operate beyond the nerve root, inevitably leading to some degree of nerve root retraction. This increases the risk of nerve root damage [
29,
30,
31].
- (3)
Advantages of the TUD approach
By taking an oblique view, traction on the pharynx, larynx, esophagus, and nerves surrounding the vertebral and carotid arteries can be minimized. Additionally, we alternate between a vertebral spreader and a manual retractor every 15–20 min instead of using a fixed retractor. This prevents prolonged retraction of the pharynx, larynx, esophagus, and nerves in the surrounding area. As a result, complications such as C5 palsy, swallowing disturbance, and hoarseness due to soft tissue damage and swelling, including the pharynx, larynx, and esophagus, and manifestations of Horner’s syndrome due to disorders of the sympathetic nerve trunk are believed to be less likely to occur (
Table 6) [
15,
16,
17,
18,
19,
20,
21,
22].
- (3’)
Disadvantages of the other procedures:
In other surgical methods, fixed retractors are used to secure and maintain the surgical field, but this can exert strong retraction on the esophagus and pharynx, potentially leading to the aforementioned complications.
4.5. Caring Points in Technical Aspects and Limitations in the Application of the TUD Approach
This ensures that disc removal/excision is limited to approximately the outer one-third, eliminating the need for fixation in Type I and Type IV cases and preventing post-operating instability [
1].
Since the surgical field is observed from an oblique angle, it is crucial to proceed with the operation while confirming the exact center. The manual retractor is manipulated and held by an assistant. However, it is necessary to accurately pull and protect the esophagus, pharynx, larynx, and carotid sheath.
It is important to minimize traction in the esophagus, pharynx, and larynx during cage insertion, as is required for inserting cages straight from the midline, similar to artificial intervertebral discs, aiming to minimize traction as much as possible and for a short duration during the insertion procedure (
Figure 7).
The TUD approach can be applied between C2/3 and C7/Th1 according to anatomical limitations. In the upper limitation, the mandibular bone hinders its use, while in the caudal limitation, the subauricular bone prevents its use. It is difficult to apply the TUD approach to lesions over the spinal cord and nerve roots.
4.6. Modified and Advanced Procedures
We have extended the application of the TUD approach to include vertebral body resection, addressing lesions not only in the posterior aspect of the vertebral body such as ossification of the posterior longitudinal ligament, but also lesions inside and outside the dura mater on the ventral side of the spinal cord. We have applied the TUD approach to lesions both inside and outside the spinal cord [
32,
33,
34,
35].
We have also applied the TUD approach to antero-lateral vertebrectomy [
35]. Various methods of anterior foraminotomy and transcorporeal anterior cervical foraminotomy have been proposed [
36,
37,
38,
39,
40]. More recently anterior transcorporeal procedure using computed tomography-based intraoperative spinal navigation and percutaneous endoscopy was reported [
36,
39]. In addition, their combination has been described [
38,
40]. As a new modality, endoscopy has been applied [
41]. The TUD approach can be combined with these new procedures.