35 Transforaminal Endoscopic Discectomy

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35 Transforaminal Endoscopic Discectomy

J. Krugluger

35.1

Terminology

Minimally invasive surgery has conquered all fields of surgery. In particular, orthopedic surgery has contrib- uted much to the development of instruments and techniques necessary for minimizing surgical approaches.

However, the advances in minimally invasive joint surgery have yet not been seen in spinal surgery.

There are various diagnoses of spinal disorders that would profit from minimally invasive surgery. The most common pathology addressed in spinal surgery is degenerative disc disease. The area of conventional disc surgery has already peaked. Due to patients’ postoperative complaints, generalized under the term “postlami- nectomy syndrome,” the indications for conventional macroscopic disc surgery have been questioned in re- cent years.

Parallel to this increase and decrease of conventional disc surgery, the idea of the minimally invasive approach has grown. The use of the microscope enabled conventional surgeons to minimize their skin incision combined with more accurate surgery in the spinal canal. However, in standard or microdiscectomy, open surgery is carried out and scarring occurs. The role of the scar as a source of postoperative pain is not yet ex- plained. But avoiding the translaminar approach with bone removal, postoperative scarring due to hemato- ma, and all the possible complications is the goal of less invasive approaches.

The posterolateral approach was addressed soon as a possible alternative to conventional surgery. The anatomy of the spine coming from a posterolateral aspect allows very differentiated access to nearly all rele- vant structures that need to be dealt with in disc surgery. Of all the different possible posterolateral approaches, one is called the transforaminal approach.

The name already includes how the access way pro- ceeds. The usual posterolateral approach is directed to the center of the disc. The transforaminal approach has been developed to address the more posterior parts of the disc including to some extent the anterior part of the spinal canal. This is where most of the disc patholo- gies are located. Moreover lateral portions of the disc,

difficult to deal with in standard and microdiscectomy, can easily be reached.

Therefore, the transforaminal approach seems ideal for about 80 % of all disc lesions. Instruments have been developed with a diameter so small that foramina of normal height can easily host the nerve root, the fiberglass optic, and the instruments.

This technique allows access to the epidural space from the lumbar disc as far cephalad as the middle of the vertebral body or approximately 2 – 3 mm caudally.

The foraminal approach is routinely accessible from T10 to L4-5. L5-S1 can be accessed with special techniques that include foraminoplasty of the lateral facet.

35.2

Surgical Principle

The basis of the transforaminal approach is the anatomy of the posterolateral aspect of the spine. The upper border of the foramen is the pedicle of the upper vertebra. The ventral aspect of the foramen includes the posterior wall of the upper vertebra, the disc, and a small part of the posterior wall of the lower vertebra. The lower border is the pedicle of the lower vertebra and the posterior wall is the facet joint. The foramen has an outer plane according to the lateral border of the pedicles, the center of the foramen is the space between the pedicles, and the inner plane is virtually the line con- necting the inner border of the pedicles. The foramen has an upper half, where the nerve root exits the spinal canal, and a lower half where the disc is located.

The way the root leaves the dura is first called the exiting zone, which is still located within the spinal canal. This is followed by the zone where the root pene- trates the foraminal space, the root then forms the gan- glion, and further on joins the lumbar plexus in the lumbar fossa.

The lower half of the foramen is normally filled with fatty tissue. Small veins coming from the paravertebral plexus cross the foramen to join the epidural veins. The lumbar segmental arteries usually do not cross the foraminal working area. In the case of a disc lesion this lower part is filled with disc tissue or protruded annu-

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Triangular Working Zone lus material. The whole space at the foramen is, in nor-

mal anatomy, a non-preformed endoscopic space. Tem- porary virtual space has to be created by fluid to gain visualization. The fluid escapes partially into the spinal canal or into the lumbar fossa along the fascia of the psoas muscle. Sufficient fluid and pressure has to be maintained during surgery to allow permanent good visualization.

35.3 History

Invasive approaches to the spinal canal for diagnostic purposes were first described in the early 1930s [15].

The instrumentation used, however, was quite large and there was considerable associated morbidity. Otto- lenghi [14] described both diagnostic and therapeutic techniques. Hult [5] in 1951 described posterolateral fenestration via a small retroperitoneal approach, with good results in 73 % of cases. This is about the result we can reach with current minimally invasive surgery techniques such as chemonucleolysis or percutaneous discectomy. In 1956 Craig [2] described the use of a posterolateral approach with an excellent description of this approach to all levels of the thoracic and lumbar spine, which is still used as the standard uniportal approach for many procedures. In the same year Feffer [3]

described the injection of hydrocortisone into the disc space for the treatment of disc pathology. Smith suggested the enzymatic absorption of the nucleus pulposus by means of chymodiactin in 1963 [16]. It was defi- nitely the first alternative to surgical discectomy. After the Federal Food and Drug Administration (FDA) ap- proved the use of chymopapain it gained widespread use not only in the USA. Several years after chymopapain reached the height of its popularity, another alternative to surgical discectomy emerged [6]. The first report on percutaneous discectomy is credited to Hijikata [4] in 1975. Kambin and Gellmann [7] described simi- lar techniques in 1983. The development of appropriate instrumentation and the description of a safe “triangular working zone” by Kambin were the basis for all further progress (Fig. 35.1). Onik and Helms [13] described in 1985 the use of a small automated aspiration probe for central nucleus decompression. The ease and safety of this procedure was striking and minimally invasive disc surgery was no longer limited to some spe- cially trained surgeons. Early results with up to 90 % reported success rate could not be followed in subsequent reports of randomized controlled studies [6].

The next step was the introduction of discoscopy by a Swiss group of orthopedic surgeons [10]. It was performed as a biportal posterolateral approach to intradiscal pathology. The procedure required triangulation in the disc with direct, simultaneous visual observation

Fig. 35.1. Triangular working zone

during removal of nucleus material. Angled lenses enhanced the range of visualization in the confined spaces. Discoscopy combined with manual decompression was superior to other automated techniques. Visu- al control of instrument placement and decompression enhanced safety and accuracy of the procedure. In 1987 Choy et al. [1] described the use of lasers in percutaneous disc surgery. The first operation was performed in Graz (Austria) by Choy himself. But the initial high in- terest in laser surgery has declined, due to a combina- tion of expensive technical equipment and moderate clinical results.

The description of the transforaminal approach rep- resented a further advance in disc surgery. The aim is to reach the epidural space at the appropriate level of the disc lesion by introducing a rigid endoscope through the small foramen. The triangular working zone de- fined by Kambin is helpful for the orientation but the primary goal of surgery is not the fenestration of the annulus. The first steps of foraminal exploration were done during discoscopy-assisted intradiscal decompression. During removal of the instruments the surgeons had the chance to visualize the foramen. More and more the idea took place not only to inspect the foraminal space while retracting the instruments, but also to enter it for therapeutic reasons. Matthews and Stoll [11] started to use the foraminal access for decompressive disc surgery.

Despite the fact that the technique involves trans- gressing the epidural space, subsequent local fibrosis was rare. Further investigations involved instrument developments. Steerable tips of the rigid endoscopes allowed even better exploration of the spinal canal. The small epidural veins were, in the beginning of endoscopic spine surgery, sometimes the reason to abandon the procedure when bleeding started. Use of bipolar cauterization and radiofrequency techniques are very helpful in coping with these problems nowadays, al- though most of the surgeons use simple fluid pressure to control venous bleeding.

The manual decompression of the disc was done with small forceps that fit into the working channel.

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This limitation and sometimes time consuming procedure if larger herniations had to be removed was overcome by using automated resectors.

The conjoined use of all technical abilities in endoscopic spine surgery allowed the development of a secure and controlled technique of disc surgery.

35.4 Advantages

Minimally invasive transforaminal approach allows very gentle and smooth access in one-level disc surgery.

The insertion of a guide wire and dilatators minimize muscle damage dramatically. Moreover the segmental muscle of the erector spine is not touched at all. Postop- erative muscle strength stabilizing and moving the spine is the same as preoperatively. Immediate trunk- stabilizing muscle training can be performed after the surgery. Pain arising from detached muscle and tendon fibers is not limiting activity of the patient.

The posterolateral approach does not cause fibrosis along the simply muscle-dilating approach. Generally none of the endoscopic surgical procedures are prone to postoperative complications due to fibrosis. This benefit of other endoscopic surgical techniques was re- inforced by the results after transforaminal exploration of the spinal canal. Only very few reports on scar formation are available and scar formation is hardly ever given as a reason for failed surgery in minimally invasive disc decompression.

The facet joints are protected during endoscopic transforaminal surgery. The working cannula is located just beneath the joint and the instruments cannot hit the joint if the cannula is crossing the foramen. But even in those cases where selected foraminal decompression is done by removing the outer parts of the facet first and expanding gently toward the spinal canal, only the bony stenosis due to hypertrophy of the lower zygapophyseal joint is decompressed. The posterior part of the joint where the muscle are inserted is never touched [8].

Posterolateral decompressive disc surgery with all its different techniques of tissue ablation, such as manually, laser, radiofrequency, etc., had as their basic ra- tional the “jelly donut theory”. Removal of a portion of the disc centrally should allow the nuclear jelly-like material of the herniation to ooze back into the newly created space. Many disappointing results defeated the whole theory [6].

The visual control of tissue ablation not only ensures that the right structure is ablated, but also ensures that the amount of ablated tissue is just sufficient to relieve the nerve root. The ability to reach the herniated tissue via the transforaminal approach overcomes the draw- backs emerging from indirect intradiscal decompression.

Last but not least endoscopic stab wound incisions are much less painful than conventional surgical incisions. Postoperative pain management is therefore much easier than in conventional surgery. Whereas conventional disc surgery usually requires general anesthesia, the transforaminal approach can mostly be carried out with local anesthesia, therefore the endoscopic transforaminal approach is a suitable outpa- tients procedure.

35.5

Disadvantages

Limitations of endoscopic disc surgery mainly arise from localization of the herniated nucleus pulposus.

Intraforaminal disc herniations are easily addressed with endoscopic procedures. Some experience is required to treat lateral disc herniations. Orientation in the non-preformed space is more difficult if the standard landmarks such as the facet joint, the exiting nerve root, and the disc are missing.

If self-confident orientation at the triangular working zone its gained, even mediolateral disc herniations are no limitation. The working cannula can be advanced under the facet joint, and the instruments can be advanced to the midline of the spinal canal, sometimes even further. But do not forget, as the foramen is entered, the movement of the instruments becomes more and more limited. Only disc herniations limited to the level of the disc space can be decompressed suffi- ciently. If the herniation is turned cranially or caudally in the spinal canal, the transforaminal approach is not an adequate surgical technique [9].

Extremely large herniations compressing the cauda equina should not be addressed by endoscopic surgery, as one cannot be sure to remove all the tissue.

Technical limitations are obviously the size of the instruments. In the monoportal approach only a working channel of about 3 mm is available if simultaneous visual control of decompression is mandatory. The instruments available nowadays with an automated shav- er are better than manual forceps, but compared to arthroscopic surgery the instruments are very limited.

The quality of fiberglass optical systems lack the quality of instruments known from arthroscopic surgery.

Primary access to place the guide wire and dilatators has to be done under fluoroscopic control. The exposure to X-rays is mandatory to identify landmarks and repeatedly verify optimum needle and/or guide wire placement that distinguishes the appropriate working zone within a spinal segment. To minimize radiation exposure during surgery, principles of radiation safety have to be obeyed. The surgeon and surgical team should wear protective lead shielding and keep maximum possible distance during imaging. A well-colli-

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mated machine with strict quality control monitoring must be used. Image memory mode is superior to live imaging with total exposure time markedly reduced.

The procedure itself is mainly performed at disc levels L2-5. At level L5-S1 it is often impossible to gain adequate access as to allow controlled disc removal. For this level alternative approaches such as the transiliac approach are suggested. But even at the upper level the procedure is quite demanding. All surgeons report on learning curves [9]. Foraminal stenosis is sometimes limiting the access to the disc spaces and the herniated tissue.

35.6 Indications

The major indication for transforaminal disc surgery are intraforaminal herniated nucleus pulposus. The herniated tissue should not extend markedly caudally or cranially. If the disc herniation is located more medi- ally it can sometimes be reached by endoscopic means too, but it has to be more or less at the level of the disc.

Mediolateral herniations are a second step in training for transforaminal disc surgery. The working cannula has to be passed underneath the facet joint to reach the disc material, but mediolateral herniations quickly join the standard repertoire of the surgeon.

35.7

Contraindications

As in other minimally invasive surgery it is very impor- tant to take into account the patient’s overall conditions if surgery is indicated. Social situation, workers’ com- pensation, and psychological disorders are some of the points that have to be considered.

In addition to the standard contraindications of disc surgery. anatomical aspects must be obeyed. The segment L5-S1 is difficult to access via the lateral approach. The iliac crest may be an obstacle in entering the foramen L5-S1. If in lateral X-ray the iliac crest ex- tends proximally of the disc L4-5 it is difficult to enter the foramen L5-S1 and have sufficient room to move the instruments. Other surgical techniques should be considered. The L5-S1 disc is sometimes protected by an enlarged transverse process. This anatomical varia- tion can easily be detected before surgery and repre- sents an absolute contraindication for this type of surgery. The same is true for a large transverse process with nearthrosis to the sacrum and/or iliac bone.

The height of the foramen should be normal if we decide to use the transforaminal approach. To some extent percutaneous widening of the recessus has been done already. Doing this we have to keep in mind that

the time of the surgery grows rapidly, and this type of surgery is only for experienced surgeons with adequate equipment. Simultaneous treatment of disc herniation and lateral recess stenosis is not a standard indication for the transforaminal approach.

The localization of the herniation can be a contraindication. Cranially or caudally herniated fragments cannot be reached by the transforaminal approach. The more medial the herniation is located, the more the range of movement of the instruments is limited. Her- niations occupying more than 50 % of the spinal canal are a contraindication for this type of surgery.

Experience of the surgeon is necessary to judge from preoperative imaging techniques if the patient’s anatomical condition is suited for endoscopic surgery.

There is no standard answer we can suggest. Every herniation has to be rated individually including the type of herniation, experience of the surgeon, and equipment available.

35.8

Surgical Technique

The surgery needs the standard sterile conditions of an operating theater. Perioperative antibiotic prophylaxis is suggested but not mandatory. The type of anesthesia depends on the preference of the patient, the experience of the surgeon, and the type of hospitalization. Lo- cal anesthesia is suggested in the beginning to identify root contact early and it is preferred for the outpatient’s procedure too. If we do the procedure with local anesthesia, some sedation of the patient is used. Regional and neuroleptanalgesia are suggested as well. General anesthesia is probably more comfortable for the surgeon and the patient too.

We position the patient on a spine frame and watch that the stomach is free. The epidural veins are not filled in this position, and lordosis is reduced during surgery.

The entry point for the skin incision must be more lateral than in standard intradiscal techniques. About 10 – 16 cm lateral to the midline are suggested for the foraminal approach depending on the height of the patient (Fig. 35.2). The approach is above the iliac crest at the level of the disc space. The whole procedure has to be performed under fluoroscopic control to avoid mis- directing the instruments in the retroperitoneal or in- traperitoneal space.

The skin is infiltrated with local anesthetic, the sup- posed working canal is infiltrated, and we may put some local anesthetic around the facet joint and the outer annulus. The infiltration of the root has to be avoided, as controlled surgery relying on root reaction in case of contact would otherwise not be possible. A stab wound incision is usually sufficient. The guide

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Fig. 35.2. Position of the entry point: above the iliac crest, 10 – 16 cm lateral the midline

Fig. 35.3. Instrument position in the lower foramen

wire is directed toward the caudal part of the foramen under lateral fluoroscopic control (Fig. 35.3). Elastic re- sistance indicates the ideal position entering the outer annulus. We suggest checking the position of the guide wire with anterior-posterior (AP) fluoroscopy too. The wire should reach the medial interpedicular line in AP view.

Fig. 35.4. “Bullet sign”

The dilatators are entered with slight rotating move- ments watching not to protrude the guide wire further into the disc. If the position of the dilatators is just at the tip of the guide wire the working cannula is advanced over the dilatators with gentle rotating move- ments. The ideal instrument position is described as the bullet sign: the guide wire tip, the dilatators just be- hind it, and the working cannula at the end of the shaft of the dilatators (Fig. 35.4). The position of the instruments is documented fluoroscopically, and then the di- lator is removed. The guide wire may be left as guid- ance for the endoscope. As the endoscope is inserted, the wire is removed [17]. Cold Ringer’s solution is used for irrigation.

First of all the visible structures have to be identified. The endoscope is partially advanced and retracted until the triangular working zone is clearly identified with all its surrounding elements: facet joint, exiting root, and disc. The orientation is sometimes difficult if the intraforaminal herniation prohibits the visualization of the foramen. Nevertheless the facet joint and the localization of the root should be identified first before decompression of the disc material is started. If the anatomical structures are identified, secure ablation of the protruded disc material can be started by the method of choice: manually, laser, radiofrequency, or automated instrumentation. Interfering bleedings from epidural veins are rare if suprarenin is used with the irrigation. If it happens, bipolar cautery or radiofrequency is helpful.

The decompression of herniated material has to lib- erate the exiting nerve root as well as the traversing root. Sufficient decompression of the traversing root

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Fig. 35.5. Treatment of medial herniations

sometimes demands entering the spinal canal via the foraminal approach (Fig. 35.5). The working cannula is passed underneath the facet joint, but it should never be advanced more than the medial interpedicular line to avoid dura lesions with the sharp rim of the cannula.

One has to keep in mind that the range of movement of the instruments is quite limited once the foramen is entered with the working cannula. The direction of the cannula for reaching dislocated fragments must be ex- plored meticulously before advancing the working cannula.

After removal of disc material the exiting root and traversing root are checked with a hook for residual disc material.

35.9

Postoperative Treatment

After surgery the patient is discharged either the same day in the evening or the day following surgery. Bed rest is suggested for a maximum of 24 hours. Muscle- balancing physiotherapy and medical training therapy is started immediately. Neural mobilization exercises, if available, can reduce postoperative root irritation pain.

Residual pain is treated with standard painkillers. No permanent pain medication is suggested.

Return to work depends on social factors, working situation, and type of work. Sitting occupations and work without lifting will be possible within the first 2 weeks. Heavy lifting will need at least 6 weeks rehabil- itation. Sport activity may be started according to progress in medical training therapy.

35.10

Complications

The complication rate of transforaminal surgery is low in general. The main reported complications are transient. The approach itself has some inherent dangers.

Lesions of the retroperitoneal structures and even lesions of the bowels have been reported. Careful preoperative evaluation of MRI or CT scan may indicate far posteriorly dislocated parts of bowels. The positioning of the patient on a frame to avoid pressure on the abdo- men minimizes the risk of bowel injury.

The facet joint, its capsule, and the posterior branch of the nerve root are the posterior border of the working triangle. According to their position they are en- dangered by instrument manipulation. Definite complications of these structures are very seldom reported.

The cranial border of the working triangle is the exiting nerve root. Lesions of the exiting root or its gangli- on are usually transient. Paraesthesia or some motor weakness may occur. They subside spontaneously within the first weeks postoperatively. Neural mobilization techniques may support full recovery.

Bleeding from epidural veins may be disturbing during the operation but usually stop and do not represent a postoperative problem. Lesions of the dura may cause leakage of some CSF, which usually subsides soon after surgery but may cause a temporary headache postoperatively.

The infection rate was 0 % in primary endoscopic operations. There was one postoperative wound infection following open revision surgery. Mechanical complications such as fracture of instruments sometimes occur. No severe complications due to instrument fail- ure have been reported. All instrumentations available are very safe and easy to use nowadays.

35.11

Critical Evaluation

The results of the transforaminal approach for disc surgery have to be rated very carefully. Early reports were very enthusiastic. The concept of transforaminal discectomy has now been expanded to utilize a variety of instruments to ablate the disc material in a more effi- cient manner than the early instruments could. The clinical data, however, vary a lot. The clinical efficacy of these methods is not yet established, and there is a lack of validation compared with other techniques. There is a remarkable learning curve for each surgeon even with sound experience in spine surgery and/or arthroscopic surgery. Orientation in the non-preformed paravertebral space is challenging. A minimum of 20 operations has to be done together with an experienced surgeon till secure approach and instrument handling is

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reached. That is the reason why the results reported needs differential analysis. Reports of small series are prone to having learning curve bias and are not repre- sentative for the overall success of the method. In large series usually the learning curve for indication has already been overcome and the learning curve of the surgical technique has also been surmounted. Generally in publications of larger series an overall of rate of 80 – 90 % good or excellent results of surgery are reported [11, 17]. This compares favorably with the results of microdiscectomy [12]. Nevertheless we have to keep in mind that this surgical technique is suitable for only a very limited type of disc herniation.

When using it in the narrow indication range, it yields good pain relief and functional recovery for the patients. When expanding to other indications, the results deteriorate dramatically. The transforaminal approach can be used successfully in disc surgery if anatomical considerations and indications are obeyed. The major advantage of the transforaminal approach is the minimal tissue trama necessary to remove herniated disc fragments. Indications are disc herniations located in the foramen, lateral to the foramen, or just medial of the foramen.

Sparing the nucleus pulposus by just removing the herniated part of the disc is controversial. On the one hand younger patients benefit from nucleus pulposus residuals in the disc, and not loosing disc height postoperatively. On the other hand reherniation rate is reported to be higher in endoscopic surgery than in microdiscectomy.

Postoperative recovery and the option to perform the procedure as an outpatient procedure are additional advantages of this type of minimally invasive disc surgery. Cost effectiveness is a major point to consider in surgery nowadays. It is probably one of the strongest arguments for transforaminal disc surgery.

Limitations concern the anatomical situation. The foramen height as limiting factor may be overcome by foraminoplasty, but foraminoplasty is a surgical technique in itself; it is not a procedure for routine transforaminal disc surgery and additional equipment is needed. The localization of the herniation limits the surgical indication too. Free fragments, sequestered caudally or cranially are absolute contraindications as well as herniations filling more than 50 % of the epidural space. Depending on the experience of the surgeon the indications may be extended. But we have to know before surgery very exactly the position of the herniation and the estimated range of movement of the instruments. The overall exclusion rate for anatomical reasons is about 30 % of patients with disc herniations.

When starting to do transforaminal disc surgery the learning curve must be considered as well as the neces- sity of keeping up the standard of surgical skill. This

means that a certain number of procedures have to be done. Therefore it is obvious that the procedure is limited to surgeons regularly performing disc surgery.

Some of all the herniations done during a period of time may be ablated by the transforaminal approach.

The procedure is not suitable for occasional disc surgery.

The limitations of instruments and visualization will be overcome soon. Any instrumentation we use now is somewhat old-fashioned after 6 month.

Transforaminal disc surgery is a fascinating procedure. It is an additional tool for spine surgeons with very good results and great benefit for the patients, but sound training of the surgeon and observance of indications and limitations are advisable.

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