Threaded Cylindrical Interbody Cage Fixation for Cervical Spondylosis and Ossification of the Posterior Longitudinal Ligament

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Chapter 41 Threaded Cylindrical Interbody Cage Fixation for Cervical Spondylosis and Ossification of the Posterior Longitudinal Ligament

Hiroshi Nakagawa, Yasunobu Itoh, Junichi Mizuno, Hidenori Matsuoka, Yoshitaka Hirano

With advances in neuroimaging using CT and MRI, the diagnosis of cervical disc herniation, spondylosis, and ossification of the posterior longitudinal ligament (OPLL) has become more precise and less invasive in recent years.¹ In addition, routine microsurgery with refined drills and implants such as interbody cages has facilitated less invasive and more efficient ventral cervical spine procedures.^2,³

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.

Corpectomy with Iliac Bone Graft

From 1980 to 1991, multilevel corpectomy with iliac crest interbody graft was carried out for multilevel OPLL and spondylosis, but graft problems, donor site discomfort, and the necessity of postoperative application of a halo brace were drawbacks of this method. Development of ventral plate fixation dramatically reduced the usage of halo brace application.^5,^7,^11,¹⁶

Corpectomy with Vertebral Graft

From 1992 to 1997, limited or keyhole corpectomy with vertebral graft using a Williams microsurgical saw (Ace Medical Co., Los Angeles) was carried out with reasonable results in 60 patients with cervical spondylosis with segmental OPLL. One of the pitfalls of this method is that bone grafts taken from the cervical spine are often more fragile than iliac grafts, especially in heavy smokers and elderly women with osteoporosis.¹⁷^–¹⁹

Microdiscectomy without Grafting

Microdiscectomy for central and paramedian discs and spurs without grafting has been done with reasonable results, but the small opening is often not adequate to decompress lateral spurs or OPLL.²⁰^–²² Ventral transuncal foraminotomy was also added to lateral or foraminal discs and spurs with satisfactory results.^23,²⁴

Threaded Cylindrical Interbody Cage Fixation

Threaded cylindrical titanium cages were first introduced for posterior lumbar interbody fusion (PLIF) of lumbar spine instability in the early 1990s.²⁵ In 1997, the cylindrical Bagby and Kuslich cervical interbody cage (BAK/C; Spine-Tech, Minneapolis, MN) became available in Japan for ventral cervical fusion.²⁶ However, these instruments were made for macrosurgery and were too large and difficult to use under the operating microscope. Therefore, we developed smaller and more slender instruments, so that the entire procedure of decompression and cage fixation could be done under microsurgical control as a less invasive procedure (M-cage, Ammtec Inc., Tokyo).^2,^3,²⁷

Modified Keyhole Microsurgical Approach

By using the advantages of keyhole discectomy and limited corpectomy and at the same time avoiding the pitfalls of the aforementioned procedures, a modified keyhole microsurgical technique with interbody cage fixation for cervical spondylosis and OPLL was developed.^3,²⁸

In this chapter, the surgical indications and techniques for both the twin-cage method and single-cage method are presented, as well as a combined method.

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 insertion ³

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