Preoperative Workups and Surgical Considerations

In Japan, two factors must be taken into consideration that result in the need for a different approach than that used in North America and Europe. These factors significantly affect the surgical strategies for cervical discogenic diseases.

The first factor is the frequent association of cervical spondylosis and disc herniation with OPLL and hypertrophy of the posterior longitudinal ligaments,⁴ causing myelopathy rather than radiculopathy.^5–7 Radiologically, OPLL of the cervical spine has been classified into four types: (1) the local, bridge, or circumscribed type, which is located behind the disc space; (2) the segmental type, which usually is limited to the posterior aspect of one or two vertebral levels; (3) the continuous type, which usually extends continuously over several vertebral bodies; and (4) the mixed type, which is a combination of the continuous and segmental types.¹

In epidemiologic studies, OPLL of the cervical spine is found in 3.2% of those age 50 years and older in Japan and is relatively common in south Asian countries. It is also found not infrequently in New York, Utah, and Hawaii in the United States and in some European countries. Therefore, understanding of this condition (OPLL) is important in determining surgical strategies to treat patients with cervical discogenic disease.⁸

The second factor resulting in the need for a different surgical approach in Japan is unique: allografts are not available in Japan. Therefore, autografts or other alternatives have to be used for anterior cervical fusion.^9,10

In preoperative workups, a routine study with dynamic plain radiographs, thin-slice CT with sagittal reformation, and MRI is mandatory, because less advanced OPLL, such as local or segmental types, may be easily missed with plain radiograph and MRI alone. With the advent of multislice CT with sagittal reformation, conventional and CT myelography may not be necessary; hence, it is not used in our practice. With advanced CT and MRI, the precise diagnosis of spurs, disc protrusion, and OPLL, along with the extent of cord and root compression, can be easily made and surgical strategies properly crafted.

Deciding which surgical strategy—ventral versus dorsal approach—to use can be determined based on the number of spinal levels involved, the extent of OPLL, the presence of canal stenosis, and the alignment of the cervical spine, but more often depends on the surgeon’s experience and philosophy. Generally speaking, the ventral approach is applied to single-level or two-level lesions and the dorsal approach is usually applied to three-level or four-level lesions.^3,5,11,12 The surgical techniques of expansive laminoplasty have been well described.^13–15

Evolution of Surgical Techniques

Over the past three decades, our surgical techniques for cervical spondylosis and OPLL have significantly changed and advanced to a less invasive method with more refined implants and technologies.

Surgical Technique

Under general endotracheal anesthesia, the patient is placed supine with the head slightly extended. The ventral cervical procedure is approached almost always from the right side of the neck, because the right-sided approach is much more comfortable for right-handed surgeons than the left-sided approach. The skin incision is made transversely along the crease for cosmetic reasons even in a two- or three-level approach. The subcutaneous tissue is dissected rostrally and caudally, and the platysma muscle is sectioned obliquely along the ventral border of the sternocleidomastoid muscle.

The ventral aspect of the cervical spine is then approached by dissecting the deeper fascia, usually rostral to the omohyoid muscle, while the right carotid tubercle of C6 is palpated as a landmark with the surgeon’s left index finger. The level of the intervertebral disc space is identified with fluoroscopy with a needle inserted into the disc space at one or two levels, and a small amount of dye, usually indigo carmine, is injected through the needle for further confirmation of level location. The blue coloring of the disc is quite useful in contrasting the bony spur with the disc when drilling the spur. After the introduction of the operating microscope, which enables the surgeon and an assistant to see the operative field at almost the same depth, retractors are placed and the discectomy and osteophytectomy are carried out.

Twin-Cage Method for Cervical Spondylosis and Herniated Disc

In cases of cervical spondylosis and herniated discs, with or without instability, two smaller M-cages of 6, 7, or 8 mm in inner diameter are used side by side in a twin-cage fashion after decompression (Fig. 41-1).

FIGURE 41-1 Twin-cage method of treating cervical spondylosis and herniated discs. A and B, Cervical spondylosis and disc protrusion with cord compression. Striped areas in the vertebrae indicate portions to be removed. C and D, Anterior spurs are removed with a Kerrison rongeur. E and F, Dorsal spurs are eliminated with a high-speed drill and protruded discs removed. G and H, Cages are inserted into the disc spaces in the twin-cage and locking fashion after decompression.

First, after complete discectomy, the ventral spur of the upper vertebra is removed with a Kerrison rongeur; while the disc space is opened using a spreader, the dorsal spur is carefully drilled out using a high-speed drill with a 4- to 5-mm diamond bur and the posterior longitudinal ligament (PLL) is incised with a microknife to expose the decompressed and bulging dura. The fragments of the herniated disc, which are often located between the two layers of the PLL but sometimes are found in the epidural space, are completely removed. It is important to drill out the dorsal spur far laterally enough to decompress the medial portion of the foramen containing the nerve root, especially when the far lateral disc or foraminal stenosis is responsible for the radiculopathy.

Originally the disc space was opened with a reamer for cage insertion³; however, this reaming was soon abandoned to avoid subsidence of the cages into the vertebrae. Now, by using the spreader efficiently and by drilling the medial portion of the uncinate process after fine adjustment, cylindrical cages, most frequently 7-mm cages, are snugly inserted side by side in a locking fashion. Cages are usually packed with small bone chips of the vertebrae and hydroxyapatite granules (Apaceram, Hoya Corp., Tokyo). The ventral surface of the cages is leveled to the ventral cortex of the vertebral body to avoid subsidence of the cages.

Single-Cage Method for Ossification of the Posterior Longitudinal Ligament

In cases of OPLL, usually of the local or segmental type, some degree of corpectomy is often necessary to remove the ossified ligament that extends behind the vertebrae (Fig. 41-2). For this reason, a larger M-cage of 10, 12, or 14 mm in inner diameter (most frequently 12 mm) is usually used in the single-cage method. After complete discectomy, a Williams microsurgical saw or an ultrasonic bone scalpel (Sonopet, Stryker, Kalamazoo, MI) is used to perform an 8- to 9-mm square corpectomy followed by 10-mm reaming to make a round hole; usually a 12-mm cage packed with bone chips and hydroxyapatite granules is snugly inserted after decompression.^3,29

FIGURE 41-2 Single-cage and combined methods in cervical ossification of the posterior longitudinal ligament (OPLL). A and B, OPLL of segmental type with cord compression. Striped areas in the vertebrae indicate portions to be removed. C and D, Drilling of OPLL through a keyhole with a high-speed drill. E and F, Separation of paper-thin OPLL from the dura and excision for decompression and drilling of OPLL through the adjacent disc space. G and H, Removal of residual OPLL behind the vertebra through the keyhole and disc space for decompression. I–L, Solid cage fixation in the single-cage and twin-cage methods.

If a Williams saw or Sonopet is not available, a round hole can be made stepwise by using progressively larger reamers. Through this keyhole, the remaining vertebra, spur, and ossified ligament are drilled out with great care, making the OPLL paper-thin by using a high-speed drill with a diamond bur and an ultrasonic bone curette (Sonopet, Stryker). The thinned-out OPLL and hypertrophied ligament, as well as associated disc fragments, are then carefully separated from the dura and excised by using a microdissector, microknife, curettes, and Kerrison rongeurs.

Most of the ossified ligament behind the vertebrae can be removed through this keyhole and through the adjacent disc space above or below the keyhole (see Fig. 41-2). After good hemostasis a cage is inserted with its ventral surface leveled to the ventral cortex of the vertebrae to minimize subsidence. For the past decade, dural ossification has not been removed; instead, it is left alone after good decompression of the dura to avoid leakage of cerebrospinal fluid (CSF). Dural ossification is found in 15.3% of all cases of OPLL, in 10.5% of segmental types, and in 41% of nonsegmental types.³⁰

Surgical Cases

Between August 1997 and December 2007, a series of 449 cases was operated on with cervical interbody cage fixation. There were 312 (69.5%) males and 137 females, with the average age being 57.2 years. The main symptom was myelopathy in 81% and radiculopathy in 19%. Among these 449 cases, 314 (69.9%) had cervical spondylosis and herniated discs; 135 (30.4%) had OPLL. Seven cases received a second cage fixation at an adjacent level within these years.

In 319 operations of cases with cervical spondylosis and herniated discs, one-level cage fixation was done in 186 (58.3%), two-level in 126 (39.5%), and three-level in 7 (2.2%); in 137 operations of cases with OPLL, one-level cage fixation was performed in 36 (26.3%), two-level in 89 (64.9%), and three-level in 12 (8.8%).

Surgical results were satisfactory (excellent and good) in 88% judging by Odom’s criteria and the Neurosurgical Cervical Spine Scale (NCSS)³¹; 56% returned to their previous work and 32% to lighter work. No significant difference in surgical results was observed between patients with cervical spondylosis and those with OPLL. The group aged 70 years and older had less favorable results but showed significant neurologic improvement—as much as 72%. The younger group showed satisfactory results in 91%. The most influential factor for poor prognosis was the severity of preoperative neurologic status. Cases with severe myelopathy disclosed satisfactory results in only 65%, and cases with mild to moderate myelopathy disclosed satisfactory results in 93%.^27,28

Major complications in cervical cage fixation are relatively rare. CSF leakage resulting from dural tear or defect occurred in several cases with prominent OPLL but was well managed with local repair and lumbar drainage when necessary. By not removing dural ossification, the frequency of CSF leakage was reduced significantly. Subsidence is a problem in ventral cervical fusion with any kind of graft or cage, particularly in heavy smokers and elderly patients with osteoporosis, but it has not been the major problem after reaming of the disc space was abandoned to minimize subsidence of the cages, especially in use of the twin-cage method in cervical spondylosis and disc herniation. Two elderly female patients with two-level cage fixation developed kyphotic deformity with compression fracture of the vertebra after a fall, causing deterioration of myelopathy; both were successfully treated by performing multilevel corpectomy with an elongated cage and plate fixation, one with and the other without additional posterior fixation.

Postoperative wound hematoma and infection were present in less than 1% of patients. Postoperative temporary dysphagia was seen in 2%. No neurologic deterioration was seen, except for one patient with temporary root sign and another with temporary worsening of a long tract sign.

Fusion in cervical cage fixation has been evaluated by dynamic radiographs in flexion and extension and multislice CT in sagittal and coronal reformation. Fusion rate in 1-year follow-ups was 90% and bony formation encasing the cages was easily observed. However, even in cases with some motion at operated sites, it did not affect surgical results.

Discussion

Since the ventral approach with interbody fusion for cervical discs was introduced by Cloward and Smith and Robinson in 1958, ventral discectomy with iliac bone graft, with or without the help of an operating microscope, has been the standard procedure with reasonably satisfactory results.^32–34

However, postoperative kyphotic deformity, graft collapse, and donor site discomfort cannot be totally disregarded. To avoid these disadvantages, microdiscectomy without bone graft has been advocated by many authors with excellent results.^20–22 This method, however, is often not appropriate for spondylosis with prominent bilateral spurs and OPLL. Allografting is one solution to the problems of the autograft, but is reported to have a lower fusion rate compared with autograft. Therefore, ventral plate fixation seems to be necessary for allografting.¹⁰ Because allografting is not available in some countries, including Japan, hydroxyapatite, coralline, and titanium threaded cages have been introduced as a substitute for autogenous grafts for cervical interbody fusion.^9,26,33,35 Threaded cylindrical cages, which were initially introduced for lumbar interbody fusion,²⁵ have been used for a cervical ventral approach for cervical spondylosis and local and segmental OPLL for the past decade.^3,26–28

OPLL of the cervical spine, which was first reported in an autopsy study by Tsukimoto in 1960,³⁶ has been extensively studied for its pathophysiology and surgical management over the past 30 years, especially in Japan.^6,13,37–39 OPLL is often associated with cervical spondylosis and disc herniation and is one of the major causes of cervical compressive myelopathy in Japan, but it is also found in other countries to a lesser degree.¹⁸ Therefore, it is absolutely vital to establish the precise diagnosis in patients with cervical myelopathy or radiculopathy by using routine radiographs, CT with sagittal reformation, and MRI to make the right and appropriate decision for surgical treatment.

If segmental or local type OPLL is present with cord compression at one or two levels (sometimes at three levels in cases with kyphotic spines), the ventral approach is often selected by surgeons with expertise in microsurgery.^12,28 In extensive OPLL of continuous or mixed type with multilevel cord compression over three to four levels, expansive laminoplasty (open-door or double-door) is commonly used because multilevel decompression is readily obtained and the procedure is less risky compared with the ventral approach.^13–15

Since 1997, ventral interbody cage fixation with autologous vertebral graft and hydroxyapatite granules became our standard surgical technique for cervical spondylosis and OPLL of local and segmental types, because this method provides immediate stabilization with rare cage-related complications and no donor site problems in addition to sufficient space for microsurgical decompression. However, the surgeon’s microsurgical technique must be further refined and adjusted to the relatively smaller keyhole corpectomy, compared with wide corpectomy, to perform safe decompression.

The twin-cage method is more commonly used than the two-levels method to treat single-level cervical spondylosis and herniated discs. In cases associated with OPLL, keyhole corpectomy with the single-cage method with a larger cage is the procedure of choice to access OPLL behind the vertebral body, but more frequently the combined approach, in which the single-cage and twin-cage methods are performed at adjacent levels, is carried out in two-level operations (65%). The three-level cage fixation is relatively rare in cervical spondylosis (2.2%) but is sometimes indicated in cervical OPLL (8.9%). Surgical results are generally satisfactory with rare complications and early ambulation, short hospital stays, and early return to work. Biomechanical testing of cervical interbody cages has indicated better stability with the twin-cage method compared with the single-cage method, and no significant difference in design variations between the cages.^40,41 Good bony fusion was observed in 90% at 1-year follow-up.

Conclusion

Threaded cylindrical interbody cage fixation after microsurgical decompression is a safe, effective, and less invasive method for a ventral approach to treat cervical spondylosis, herniated discs, and OPLL with few complications. The single-cage and combined methods are commonly used for OPLL; the twin-cage method is normally used for cervical spondylosis and herniated discs. The surgical method is tailored to the pathology of each level after precise diagnosis from preoperative neuroimaging. This procedure facilitates early ambulation, short hospital stays, and early return to work.

Acknowledgement

The authors wish to thank Mr. Shunji Ono for preparing the figures and Ms. Emiko Nagase for her editorial assistance.