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ANTERIOR SURGICAL APPROACHES FOR CERVICAL OPLL(contd …) Krishnamurthy Sridhar

The anterior surface of the vertebral body is approached from the right side (for right handed surgeons) via an incision which curves from the angle of the mandible to a point just off the midline at the level of the thyroid cartilage and then straight down to the Burn’s space in the midline. This incision is preferred by the author to the classical one along the anterior border of the sternomastoid as it facilitates easier dissection. The incision is taken to its entire length ,or is tailored according to the levels to be exposed. It is preferable to have a generous exposure rather than a limited one requiring vigorous retraction in the depth. The anterior surface of the vertebral body is exposed by blunt dissection in the areolar plane between the carotid sheath laterally and the tra¬chea-oesophagus medially. Once the prevertebral fascia is reached, the dissection is continued in the midline. From this point on it is extremely important that the surgeon keep track of the midline so that he /she does not stray to one side. The prevertebral fascia is incised sharply and pushed away laterally to expose the vertebral bodies and the disc spaces with the anterior longitudinal ligament in front. Once this plane is reached, it is very simple to dissect superi¬orly and inferiorly to gain access to the levels required. Occasionally the anterior belly of the omohyoid muscle may pre¬vent adequate exposure especially of the C4 vertebral body. If the muscle is bulky or if it is not possible to dissect and retract the muscle to get enough space, it may be divided and held apart by stay sutures. The muscle should preferably be sutured at the end of surgery . The middle thyroid vein should be cauterised and divided if it crosses the field. Very often it can be pushed up or down and kept away from the field with the dissection being kept to the plane between the two layers of the prevertebral fascia. Surgeons have described the possibility of encountering the superior thyroid artery and the superior laryngeal nerve.

The longus colli muscles lie on the anterolateral surface of the vertebral bodies on either side, and need to be detached from their attachment to the same. Detachment of the muscle is easier if one proceeds from caudal to cranial following the direction of attachment of the fibres. Bleeding at the site of attachment is dealt with by monopolar or bipolar cautery. Retractors are then positioned under the longus colli muscles. In Indian patients and especially in women, the muscle is very thin and therefore sharp pronged self retaining retractors are dangerous to use as they may slip and injure the carotid / oesophagus. The author prefers the use of hand held flat bladed Langenbach retractors.

The disc levels adjacent to the vertebral body levels to be removed are marked by intraoperative x-rays or by the use of an image intensifier. Approximately 1.5 to 1.8cm of the mid portion of the anterior surface of the veretbral bodies selected is cleared of all soft tissue. The anterior longitudinal ligament and the anterior annulus of the disc spaces is incised and a discectomy is performed at these levels. The uncinate processes are defined so as to mark the lateral anatomy of the body. By performing the discectomy before starting the corpectomy, the surgeon can assess the height of the vertebral body and therefore the approximate depth at which the cord lies. The median corpec¬tomy proceeds initially by rongeuring the mid portion of the body. Furthur bone removal is performed with the help of a drill using cutting burrs . The marrow of the body is quite vascular which actively bleed. This is easily tackled using bone wax if profuse, or by contin¬uing the drilling beyond the vascular channel. The cutting burrs are used till the posterior cortex of the body is reached, from which point on diamond burr heads are used. This changeover will safe¬guard against tearing of the dura during drilling. The posterior cortex is thinned out until only a wafer thin layer of bone remains. It is important that this be dissected off the posterior annulus, posterior longitudinal ligament and the dura using microdissectors. During the drilling of the vertebral body one must guard against going to a depth in a small area, going away from the midline and decompressing only a localised area of the dura. As one proceeds towards the posterior cortex , the width of the bone removal increases so that in effect the corpectomy defect is a trapezoid and not a rectangular defect. The width of the corpec¬tomy defect should be approximately 18 mm at the posterior end to effect adequate decompression of the cord. Operating from the right side , drilling the right side of the body is considerably more difficult than the left as it is away from the direct line of vision. Often a post-operative CT scan will show that the decompression more on the left than the right side. Uniform and equal decompres¬sion on both sides can be achieved by tilting the operating table towards the surgeon when decompresssing the right side. Intraoperative ultrasound has been shown to be useful in determining the completeness of decompression and equal decompression on both sides.

Pathologically the ossification process in OPLL starts in the middle lamellae of the posterior longitudinal ligament. This means that in cases where there is extensive and massive ossification, the ossifica¬tion is continuous from the posterior surface of the vertebral body till the dura. The disease is also notorious for the involve¬ment of the dura. A double layer OPLL seen on CT or MR scans is more likely to involve the dura . It is important in these cases that the plane of the posterior surface of the vertebral body and the posterior longitudinal ligament be well defined in the disc space, before progressing with the corpectomy. This will ensure that the surgeon does not lose orientation of the anatomy. It is more important in these cases than in any other that on reaching the posterior cortex of the vertebral body, the drill head be changed to a diamond head. It is also important that the decompression proceed in a uniform manner over the entire surface of the cord. This is because very often the OPLL is eccentric and invaginates deep into the cord on one side. A preoperative assessment of whether the OPLL is eccentric or central helps in planning. If one side is decompressed before the other, the edge of the cord is likely to get hitched against the ossification which is still present, leading to a deterioration in the neurological level. A safe strategy would be to make the bone over the entire surface of the cord paper thin before attempting to dissect it off the cord surface. The surgeon should therefore start drilling the OPLL where it is thickest. If there is dural involvement, one must be extra careful when dissecting the bone fragments off the cord. No rough ron¬geuring of the fragments should be done as this may cause trac¬tion on the microvasculature of the cord leading to disaster. Microdissectors should be used and the fragments should be gently lifted off the arachnoid/dura. If there is any difficulty releasing a fragment it may be left lying as a free floating piece on the surface of the cord

Repair of any dural defect is done with fascia and free fat and post-operative continuous lumbar drainage may be instituted to prevent a troublesome CSF fistula.(24) Suturing the graft in place with special sutues has also been reported. It is however difficult to perform through the depth of the corpectomy defect.


STABILISATION

Following multi-level orpectomy and fusion, it is advisable to follow the procedure with stabilisation using an anterior plating system. This will prevent the possible complications of graft migration, will prevent micromotions between the graft and the host bone surfaces and therefore promoting better fusion rates, and will provide an immediate stability allowing early mobilisation of the patient. There are many who do not feel that a stabilisation procedure is essential. Patients in whom a stabilisation has not been performed require cumbersome external orthotic devices for a prolonged period .


ANTERIOR SEGMENTAL DECOMPRESSION

TAnterior segmental decompression (ASD) is useful in those cases where the compression is opposite the disc space or extends just beyond. Patients with OPLL are prone to develop disc herniations and when this is the cause of cord compression, an ASD is indicated. Epstein (3) found these patients to have an intermediate outcome compared to patients with a corpectomy or a posterior decompression.

In essence an ASD is a cervical discectomy with furthur drilling of the adjacent vertebral bodies to gain access to the OPLL. The procedure can be performed through a classical skin crease neck incision, similar to that made for cervical disc disease. Magnification used during the final stages of disc removal help in delineating the OPLL and in its excision either by the use of a drill or microcurettes. A fusion procedure is generally necessary following an ASD.


OUTCOME

OPLL is a progressive disorder and surgery is offered not as a cure, but for relief of symptoms from cord and root compression at a specific time in the natural histo¬ry of the disease. The outcome of surgical decompression in patients with OPLL is good.


AUTHOR’S SERIES

Sixty three patients with cervical OPLL were operated by the author between 1992 and January 2000. Forty- four of these patients underwent an anterior decompressive procedure. Median corpectomy and excision of the OPLL was performed in 33 patients, while an ASD was performed in 11. Nine patients underwent a single level corpectomy , 22 patients two levels and two patients underwent a three level corpectomy. There were no intraoperative complications, or post-operative mortality. One patient developed a C5 monoradiculopathy following a C4 and C5 corpectomy. Improvement in neurological grade was seen in 31 of the 33 patients (93.3%) who underwent a corpectomy and in 8 of the 11 (90.9%) of those who underwent an ASD. Of the twenty-nine patients of corpectomy available for followup of between 8 years and 8 months , only one patient remained neurologically stationary. All nine patients of ASD available for followup had improved .


CONCLUSIONS

Surgical approaches for cervical OPLL have to be individualised to each patient, depending on the clinical and radiological data available. Anterior procedures provide the ideal decompression, as they deal with the compressive element directly. It is essential to use magnification and microsurgical techniques when dealing with the OPLL as dural involvement is common and to prevent major neurological disaster. Results of anterior decompressive procedures are excellent , in both the immediate post-operative period and in the long term.