Microsurgical reconstructive decompression of neural structures in discogenic lesions of lumbar spine.

Kholodov S.A.

Chair of Neurology and Neurosurgery of RSMSU, Municipal Neurosurgical Center, Hospital of War Veterans N 2, Moscow

Introduction

The most spread operation in discogenic lesions of lumbar spine was laminectomy, which ensured a wide approach to intervertebral disks and dural structures. However, this operation often led to undesirable consequences, in particular, instability of the spine and formation of cicatrices and adhesions with involvement of a dural sac and roots. It resulted in relapse of pain syndrome and aggravation of a patient's state. Due to these circumstances, there appeared a tendency to reduction of a traumatic character of surgical interventions with subsequent spread of microsurgical discectomy [4, 5, 7, 8, 10, 12]. But microdiscectomy does not allow to reach maximum decompression of neurovascular formations in case of multi-level lesions and various bone changes within the limits of a spinal lumbar canal, among which one can mention spinal canal stenosis, hypertrophy of articular processes, narrowing of intervertebral foramen, hypertrophy of a vertebral arch, etc. Presence of disk hernia in such patients is not the main factor of compression of dural structures. That is why, surgical intervention, limited to simple removal of a degenerated disk, can fail to give maximum positive clinical results.

It should be noted, that the majority of microdiscectomies is performed, as a rule, only at one level of lesion of a vertebro-locomotive segment. According to Caspar W., microsurgical operation was made at one level only in 94% of cases [7]. The rate of operations in one-level discogenic compression, reported by other authors [10, 13, 16], was equal to 99%. Thus, surgical treatment concerned mainly one-level discogenic compression. Even in case of two or more hernias of intervertebral disks, the authors stick to the opinion, that it is necessary to operate only clinically significant hernias, when compression of neural structures with clinical manifestation in the form of acute root syndrome is watched at a certain level of a vertebro-locomotive segment. Inexpedience of removal of clinically insignificant hernias is explained by possible additional risk of development of postoperative diskitis and spondylitis, probability of epidural fibrosis in a zone of intervention, unnecessary manipulations on a nerve root, possible damage of the anterior longitudinal ligament [7, 8, 9].

Thus, microsurgery of discogenic lesions of lumbar spine lacks clear understanding of problems, concerning determination of clinically significant levels of lesion and a volume of intervention in multi-level lesions. There are no doubts as for surgical tactics in one-level neural compression. But presence of pathology at two or more levels gives rise to the following question: which levels should be subject to surgical intervention. A problem of protection of nervous structures of the spinal canal from a negative effect of epidural fibrosis has not been solved yet.

Material and Methods

Results of microsurgical reconstructive operations, performed in 436 patients with discogenic diseases of the lumbar spine during 1993-2000, were analyzed. There were 230 females and 206 males. A greater part of patients was at the age of 30-49 (301 cases - 69%). Diagnosis was carried out with the help of MRI, CT, CT-myelography, positive myelography, rentgenography. Optic magnification and microsurgical instruments were used during operations.

Average duration of a disease before an operation was 4.4 years. It is generally adopted, that a period of drug treatment is equal to 3 months [7, 10]. Usually, positive dynamics, as a result of conservative treatment, is watched in 90% of cases. The rest 10% of patients need surgical treatment.

Results and Discussion

Lumbar radiculopathies (89.2%) were one of the most frequent clinical forms of compression lesions. A complicated course of the disease in the form of compression radiculomyeloischemias, isolated radiculoischemias and myeloischemias was watched in 1.6%, 6.9% and 2.3% of cases respectively.

Distribution of patients according to a number of operated levels in a single case is given in (fig.1). Operations were performed on one and two levels in the majority of cases; interventions on two levels were more frequent. Distribution of disk hernias according to their localization at various levels of vertebro-locomotive segments is given in (fig.2), L4-L5 and L5-S1 levels were affected more often. Operations on levels with hernia absence were made for subarticular and foramen stenosis, stenosis of the spinal canal, as well as in case of necessary revision of a level.

Absolute indications for an operation were as follows:

  1. progressive neurologic deficit;
  2. syndrome of compression of the horse's tail;
  3. manifestations of myeloradiculoischemia;
  4. persistent pain syndrome.

In case of presence of absolute indications, urgent operations were made.

Relative indications included:

  1. insufficient effect of conservative treatment;
  2. recurrent episodes of ischialgia;
  3. neurologic deficit.

Microsurgery

A conception of microsurgery of discogenic lesions, developed by us, comprises 4 main principles of surgical treatment:

  1. multi-level surgical approach;
  2. preservation of supporting properties of the spine;
  3. microsurgical reconstruction of the spinal canal;
  4. reconstruction of neural membranes with prevention of epidural fibrosis and its compressing effect on neurovascular structures.

We used surgical approaches, allowing to obtain maximum preservation of anatomic elements of the spinal canal, which participate in providing its stability. Flavotomy and archotomy were the main components of surgical approach to neural structures. Flavotomy consisted in excision of the yellow ligament without resection of margins of vertebral arches and was performed, as a rule, at the level of L5-S1, characterized by a rather broad space between arches. The yellow ligament was incised perpendicularly to its fibers. Oblique incisions could cause damage of the dural sac with subsequent development of liquorrhea. The level of L5-S1, where spina bifida and thinning of the yellow ligament are watched occasionally, demanded special care. Excising the yellow ligament in the area of facets, it was necessary to keep in mind, that the safest place was the lower margin of the inferior facet. It was explained by presence of a fatty layer between the facets. We started exposure of the yellow ligament at this very place and then went along the medial margin of facets in the direction of arches. Usually flavotomy was accompanied by archotomy at other levels. It was characterized by sparing resection of margins of one or both vertebral arches mainly in the lateral direction. Hemilaminectomy was performed in 21% of cases. As a rule, it was used in a narrow vertebral canal, in reoperations and in case of compression of epidural venous plexus by a vertebral arch.

When the spinal canal was reached, a lateral edge of a root was exposed during its dissection. If detection of a root was connected with certain difficulties, it could be indicative of the following phenomena: axillary localization of hernia dislocating a root laterally, insufficient resection of a hypertrophied margin of the superior facet, presence of adhesions or abnormality of root development. In case of axillary localization of a root, the posterior longitudinal ligament was dissected under an arm pit, a part of disk hernia was removed and then final discectomy was made. These approaches were the most adequate means of preserving the spine stability.

Hernia of an intervertebral disk was not considered to be the only and main factor of compression with the exception large sequesters. According to Boden S.D., clinical manifestations were absent in 24% of patients with hernias diagnosed during MRI examination and 4% of cases with canal stenosis [6]. Thus, disk hernia can be the main or additional factor of root compression, as well as it can have no serious effect on neural structures. This was a cause of special attention to estimating a state of bone structures of the spinal canal, which could be an anatomic substrate of root compression, i.e. hypertrophied and deformed articular processes, roots of spinous processes, intervertebral foramens, hypertrophied vertebral arches, etc. Possible bone changes could be a sufficient factor of additional, often tough, compression of roots, arteries and venous plexuses. That was why, decompression of structures of the vertebral canal was carried out with the help of reconstructive methods, aimed at resection of bone formations, being a factor of compression of neural and vascular structures. Reconstruction implied creation of new interrelations inside the vertebral canal (a central part, subarticular zone, intervertebral foramens). It ensured maximum decompression of neural and vascular structures. We used pistol forceps and high-revolution diamond cutters with these purposes. Reconstructive methods included resection of osteophytes, medial facetotomy, sublaminal plasty with resection of an arch anterior margin; total facetectomy was extremely rare (8 patients).

Subarticular Stenosis and Spinal Canal Stenosis

It is generally adopted, that the spinal canal is subdivided into a central canal and a lateral zone. The borderline goes along a medial margin of articular processes. In its turn, the lateral zone is represented by subarticular, foramen and extraforamen zones. Spinal roots are often compressed just in the subarticular zone, which goes from the top to the base of the anterior articular process. Subarticular stenosis is a part of stenosis of the central canal rather often [2, 10, 12]. A clinical picture of subarticular stenosis depends on a type of root compression. There are two types of compression [10].

True double compression. One root is compressed at two different levels. For example, L5 root can be compressed in the subarticular zone by a hypertrophied medial margin of the superior facet at the level of L4-L5 and S1 superior particular process, shifted upwards, at the lower level of L5-S1.

Compressions of two roots at one level. For example, the hypertrophied superior articular process compresses the passing L5 root and the going out L4 root at the level of L4-L5.

Subarticular stenosis was diagnosed in 52 out of 436 operated patients. It was associated with central canal stenosis in 31 cases. Surgical treatment of subarticular stenosis lied in adequate decompression of the central canal and lateral zone on one or both sides. Homolateral subarticular decompression with partial medial facetectomy was performed; in case of necessity the intervertebral foramen was widened. As sometimes subarticular stenosis was bilateral and associated with stenosis of the vertebral canal, contralateral decompression of neural structures was made, using the same approach. Use of a high-revolution drill facilitated reconstruction of the contralateral subarticular zone and intervertebral foramen from the technical point of view. Contralateral approach was simplier technically than homolateral reconstruction in the above-mentioned situation, due to direct visualization of a contralateral side. It was obtained by lowering an operating table (by 15-20 degrees) along its axis on a side opposite to a surgeon. Hemilaminectomy was used in combination of subarticular stenosis with spinal canal stenosis.

Spinal canal stenosis causes chronic compression rather than acute one. The horse's tail, which is localized in a narrow space, is incapable of providing itself with good blood supply in physical load (for example, walking). It results in its ischemia and appearance of certain symptoms. Besides, harmful products of metabolism are eliminated from tissues of a zone of the horse's tail with great difficulties due to impaired venous outflow. It causes subsequent formation of arteriovenous shunts. As sensitive nerve fibers are thicker than locomotive ones, they are more vulnerable to compression [10]. That is why, sensitive disorders (paresthesia) dominate over locomotive disturbances. Long-term compression leads to intraneural edema and, as a result, fibrosis. Thus, it is important to perform surgical interventions at early stages.

Multi-level Lesions

When there are two and more levels of pathology of a vertebro-locomotive segment, precise determination of intervention volume is of peculiar importance. Criteria, used with these purposes, are given below:

  1. Intervention is to be performed at all clinically significant levels.
  2. In case of hernia watched at one level and a clinical picture of compression of two roots microdiscectomy is performed at the level of lesion of an intervertebral disk.
  3. Presence of hernia at one level and a clinical picture both of compression of two roots and degeneration of the second disk are indicative of necessary revision at the second level, as subarticular stenosis is watched there rather often.
  4. Clinically insignificant large hernias are removed together with hernias, causing root compression.
  5. A level of operation is conditioned by localization of sequestered fragments of affected disks and direction of their migration.
  6. There can be a combination of large hernia, localized at one level, with a small hernia, watched at the other level against a background of narrowing of a root canal (for example, due to hypertrophy of the superior articular process); then it is necessary to remove both large hernia and small hernia with simultaneous reconstruction of the vertebral canal at this level (resection of a medial margin of the superior articular process).
  7. If there is no hernia, reconstructive stages of an operation, aimed at maximum decompression of dural structures, are performed at the level of compression of neural structures.

There is a problem, which has not been solved yet. It concerns an amount of intradisk tissues, which are to be removed [11, 14]. According to one point of view, it is necessary to perform complete removal of contents of an interbody space up to terminal plates. However, critics of this method consider complete removal of a disk to be far from reality. It increases a risk of damage of internal organs, as well as a risk of aseptic diskitis and instability [3, 12]. We remove only destructed parts of an intervertebral disk.

Disk Hernias of the Upper Lumbar Spine

Hernias of intervertebral disks, localized at high levels (L1-L2, L2-L3, L3-L4) are rare and make about 5% of all lumbar hernias [1, 14]. While planning an intervention, we took into account anatomic peculiarities of the upper lumbar spine. One of them is decrease of a distance between lateral parts of arches in the ascending direction; thus, archotomy was supplemented by partial resection of the inferior articular process. Besides, a vertebral arch becomes wider and an interarch space narrower. Moreover, one should bear in mind, that a lower borderline of an arch exceeds an interbody space and a diameter of the vertebral canal is smaller at the upper lumbar levels. We performed extended archotomy during an approach, the yellow ligament was excised as much as possible in order to ensure a greater field of vision under conditions of the narrowed vertebral canal. Mobility of roots at upper levels is extremely limited and a risk of trauma in mobilization and retraction is higher.

Such hernias were watched 51 times (out of 655 hernias) in 49 patients (7.8%). They were localized at the level of L3-L4 (46 times, 90.2%), L2-L3 (4 times, 7.8%) and L1-L2 (1 time, 2%).

Foramen and Extraforamen Hernias

Lindblom (1944) and Harris and Macnab (1954) were the first to describe foramen hernias [10]. Though it happened long ago, this pathology was diagnosed very seldom. Macnab introduced the term "hidden zone". It is a zone, lying laterally from an interarticular part, where foramen hernias can be localized. However, it is difficult to detect them even during an operation. MRI, CT and CT-myelography are of great importance in diagnosis of foramen hernias. They were watched by us in 18 (4.1%) out 436 patients with discogenic lesions of the lumbar spine. Conventional interlaminal approach was used in 14 cases. It was replaced by paraspinal lateral access in foramen hernias with extraforamen components (4 patients). The purpose of this approach was preservation of articular processes. Surgical stages, offered by McCulloch J.A. [10], were used:

Advantages of the paraspinal approach lie in ensuring a direct view of a pathologic zone and preservation of articular processes. However, the paraspinal approach may be performed only in case of absence of pathologic changes in the spinal canal, which would demand use of the interlaminal approach and reconstructive methods (subarticular stenosis, a sequestered fragment, hypertrophy of a root of the spinous process, etc.).

Epidural Fibrosis

As a rule, surgical treatment of patients is carried out against a background of existing adhesive changes, being the result of frequent exacerbations of inflammatory processes in a zone of dural structures. Intervention proper creates conditions for appearance of adhesions due presence of blood clots in an operation wound, inflammation, etc. Literature contains description of numerous methods of protection from sequelae of epidural fibrosis: fatty tissue, amniotic membrane, silicon membranes and plates made of silastic, irrigation of nervous structures by steroids and non-steroid anti-inflammatory drugs. However, their efficacy is rather low [10]. It has been proved, that in case of repeated operations exposure of neural structures against a background of fat transplantation is not easier than in patients without implanted fat flaps. Besides, there appears a syndrome of the horse's tail in some cases, resulting from saturation of fatty areas with blood and subsequent compression of a dural sac [10, 13].

A positive clinical effect can be achieved by carrying out measures, which provide maximum prevention of cicatricial-adhesive changes. We use original microneuroprotectors made of porous silicon , which are placed on a dural arm of the root at a final stage of operation (fig.3). It is important to place it just on the root arm , i.e. in projection of an intervertebral disk. It is that very place, where marked fibrosis, spreading over nervous structures with a passage of time, is formed in an interbody space after removal of disk tissues.

We began to use GORE-TEX spinal membrane as a permanent substitute of dura mater with the purpose of prevention of sequelae, caused by an effect of fibrous tissue on dural structures. It is made of polytetrafluoethylene with a thickness of 1 mm, which is biologically compatible with human tissues. Of course, the spinal membrane proper cannot prevent development of fibrosis, but it protects tissues of the spinal canal from a harmful effect of cicatrices, which can grow into them.

Initially the spinal membrane was used in patients with three-level discogenic pathology and in repeated operations. Today indications for its use are much wider due to economic availability of the membrane.

Technique of the membrane implantation lies in its placing on a dural sac without additional suturing. Dural plasty is performed so that a smooth surface of the membrane would be adjacent to dura mater and a textural surface would ensure its growing into surrounding tissues and fixation.

Long-term resultsof microsurgical treatment were studied in 1-8 years after operation. Results of catamnesis were estimated on the basis of clinical criteria: dynamics of root and low back pain, locomotive disorders, sensibility disturbances, impaired functions of pelvic organs. Evaluation of total results was based on the whole complex of clinical signs (fig.4).

Fatal outcome was watched in 1 case (0.2%). It was caused by thromboembolism of the pulmonary artery. Other complications were represented by wound infection, damage of dura mater, spondylitis, diskitis (less than 1 % of observations). Repeated operations were performed in 8 patients (1.9%) (table 1).

Table 1 Causes of Repeated Operations

Causes A number of patients
Absolute
A number of patients
%
Hernia relapse 2 0.5
Disk hernia at another level 2 0.5
Disk hernia on the other side 1 0.2
Adhesive radiculopathy 3 0.7
Total 8 1.9

Thus, microsurgery of hernias of intervertebral disks, combined with multi-level approach to vertebro-locomotive segments and reconstructive methods, promotes achieving good positive results of treatment of discogenic lesions of the lumbar spine and reduces a number of repeated interventions. Surgical manipulations, which preserve supporting function of vertebro-locomotive segments, make it possible to avoid instability of the spine and to provide maximum decompression of neural and vascular structures of the vertebral canal. A complex of intraoperative measures, including use of silicon microprotectors and GORE-TEX membranes, placed on nerve roots and a dural sac respecitvely, allows to reduce severity of a cicatricial-adhesive process in an intervention zone and protects from a negative effect of fibrous tissue on neurovascular structures of the spinal canal.

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