Posterior Subaxial and Cervicothoracic Instrumentation

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CHAPTER 299 Posterior Subaxial and Cervicothoracic Instrumentation

The posterior approach to cervical spine instrumentation was pioneered by B. E. Hadra in 1891 when he wrapped loops of silver wire around the spinous processes of the cervical spine for stabilization.¹ Hadra treated chronic fractures, kyphotic deformities, and Pott’s disease with this technique. Fusion of the cervical spine was first described in 1911 by Hibbs and Albee in independent publications.^2,³ Hibbs’ method of stripping the periosteum from spinous processes, detaching them from the laminae, and placing them in direct contact with a decorticated surface established the ultimate goal of spinal fusion surgery—solid bony arthrodesis. Today, techniques of posterior cervical instrumentation have advanced considerably from simple wiring of the spinous processes to constructs that impart immediate stability to the cervical spine. Despite technologic advances, bony fusion is still the end goal of instrumentation in the majority of cases, with the hardware serving to stabilize the spine and promote fusion.

Indications

The goal of posterior cervical spine instrumentation is to provide immediate stability, promote fusion, prevent neurological compromise, and allow early mobilization of the patient. Cervical instability is the primary indication for posterior instrumentation of the subaxial cervical spine. Instability has been defined as loss of the ability of the spine, under physiologic loading, to maintain its displacement pattern and prevent increased deformity or neurological deficit (or both).⁴ Therefore, injury to or destruction of the bony structures or ligaments may introduce instability. This description includes anatomic alterations that occur as a result of operative decompression. Such alterations and their resultant effects on stability must be taken into account when considering the placement of instrumentation. Frequently, fusion and instrumentation are indicated preemptively to treat anticipated postoperative instability or to prevent anticipated postoperative deformity. Fusion with concurrent instrumentation may be required as part of the treatment of a variety of disease processes, including trauma, degenerative disease, congenital anomalies, neoplasm, infection, or inflammatory conditions such as rheumatoid arthritis or ankylosing spondylitis.

The choice of anterior, posterior, or combined anteroposterior instrumentation is based on the clinical scenario and surgeon’s experience. Generally, the anterior and middle columns must be capable of weight bearing for posterior instrumentation to be used alone; a combined anterior-posterior approach may be needed if such is not the case. Likewise, posterior instrumentation alone is generally less desirable in the presence of a significant kyphotic deformity of the cervical spine. Although restoration of sagittal alignment can occasionally be accomplished from a posterior approach, consideration should be given to the use of an anterior approach or a combined approach in such instances.

Anatomy/Exposure

Dissection should begin in the midline over the level of intended fusion and continue through the nuchal fascia while being careful to stay in the relatively avascular midline raphe. Subperiosteal dissection of the spinous processes, laminae, facets, and lateral masses to their lateral edges should be performed. Interspinous ligaments should be left intact when possible, as well as the muscular attachments and nuchal ligament to C2. It has been suggested that exposure of the C7 transverse processes aids in identification of the entry point for placement of pedicle screws.⁵

The nerve root exits the neural foramen at the anterolateral aspect of the superior facet.⁶ Viewed from a posterior exposure, the nerve root is located at the level of the articular line.⁷ The vertebral arteries are directly anterior to the depression formed between the posterior surface of the lateral mass and the lamina.⁷ The pedicle forms a bridge from the posterolateral portion of the vertebral body to the anteromedial aspect of the lateral mass, midway between the superior and inferior articular surfaces. The mean mediolateral pedicle angle, measured from the midline sagittal plane, ranges from 39 degrees at C2 to a maximum of 48 degrees at C4 and C5. The mean width of the pedicle measured from the outer cortices ranges from 4.8 mm at C3 to 6.9 mm at C7.⁸

Fluoroscopy or plain radiography is vital to identify the correct level of instrumentation. The use of intraoperative aids, including fluoroscopy or neuronavigation, although not essential, may greatly facilitate the safe placement of posterior instrumentation.

Techniques of Instrumentation

The bony posterior elements that are available to anchor instrumentation include the spinous processes, laminae, facets, and lateral masses, depending on the patient’s anatomy and pathology. The pedicles are also available for instrumentation, although the small diameter of the C3, C4, and C5 pedicles frequently precludes safe screw placement.⁸ The individual patient’s pathology, the suitability of the bony structures to accept hardware, the biomechanics of each construct, and the surgeon’s experience should be considered when selecting the method of instrumentation.

Interspinous Wiring

Interspinous wiring is technically simple to perform and generally safe, with minimal risk to the neural elements. Limitations include the necessity of having intact posterior elements for fixation and the occasional necessity of incorporation of uninjured segments into the construct for adequate stabilization. Osteopenic bone is not well suited to this technique secondary to focal forces exerted by wire or cables under tension. Multistranded cables made of stainless steel, titanium, or polyethylene are biomechanically superior to monofilament stainless steel in their ability to resist fatigue.⁹ Monofilament wire is tightened and secured by twisting, whereas cable is tensioned and crimped according to the manufacturer’s recommendations. Numerous interspinous wiring techniques have been described.

Rogers’ Technique

The Rogers technique may be used for injuries to the posterior ligamentous complex or facet capsule, or both, in the absence of bony injury.^10,¹¹ At surgery, a transverse hole is made with a small bur at the base of the spinous processes to be instrumented. Wire or cable is passed through the hole of the superior spinous process, looped around the spinous process superiorly, and then passed back through the hole. The wire is next passed through the hole of the inferior spinous process, looped around the spinous process inferiorly, and then passed back through the hole. The wire or cable is secured under tension. Fusion is achieved with bone graft placed between decorticated laminar surfaces. The procedure is relatively fast and simple to perform and provides a posterior tension band, if needed, to resist flexion forces and to supplement the strength of anterior cervical instrumentation.¹²

Bohlman’s Triple-Wiring Technique

The Bohlman triple-wiring technique was developed as an evolution of the Rogers interspinous wiring technique to impart greater biomechanical stability.¹³ First, Rogers’ interspinous wiring is performed. Then, two additional wires are passed through the spinous processes and looped around each respective spinous process, if space permits. Each cable is next passed through corresponding holes in two corticocancellous autologous bone grafts placed on either side of the spinous processes.¹⁴ The ends of each wire or cable are then secured under tension (Fig. 299-1).

FIGURE 299-1 The Bohlman triple-wiring technique. After Rogers’ interspinous wiring is performed, two additional wires are used to affix corticocancellous bone grafts firmly along the spinous processes.

Dewar’s Technique

Another variation of interspinous wiring is the Dewar procedure (or tension band configuration).¹⁵ Two corticocancellous strips of bone are placed on the lateral surfaces of the spinous processes and medial laminae of the vertebrae to be fused. Threaded Kirschner wires (K-wires) are introduced percutaneously to affix the bone grafts to the spinous processes and cut with 1 cm of overhang laterally. Wire is threaded around the K-wires in a Gallie-type fashion. Cervical flexion therefore causes medially directed pressure on the bone graft. The posterior elements must be intact to use this technique.

Facet Wiring

Facet wiring, originally described by Robinson and Southwick,^16,¹⁷ may be used for unilateral or bilateral facet dislocations or in instances in which the posterior neural arch has been damaged or surgically removed. Holes are drilled in the inferior facet processes at a 90-degree angle relative to the articular surface while protecting the superior facet processes with a Penfield dissector. Wire or cable is then passed through each hole and tightened around longitudinal strut grafts for fusion (Fig. 299-2).

FIGURE 299-2 Facet wiring technique. After holes are drilled in the inferior facet processes, wires are used to affix longitudinal strut grafts to the dorsal surface of the lateral masses.

For improved stiffness in axial rotation, Cahill and colleagues introduced a technique wherein the facets are secured to the spinous processes.¹⁸ The inferior facet processes are drilled in a fashion similar to that described by Robinson and Southwick, and wire or cable is passed from the facet to the spinous process of the level below (Fig. 299-3). Wire or cable is then wrapped around the spinous process or looped through a hole drilled at the base of the spinous process of the vertebra below. This technique affords improved stiffness in axial rotation over interspinous wiring or the Robinson and Southwick facet wiring technique.

FIGURE 299-3 Cahill’s oblique wiring technique. Wires or cables are passed through holes in the inferior facet processes and secured to the spinous process of the level below. The wire or cable may be looped around or through the base of the spinous process.

Sublaminar Wiring (Cabling) Techniques

Sublaminar wires have been used extensively for instrumentation of the subaxial cervical spine. Braided cable is the preferred material to use for passing wire into the neural canal because of its increased flexibility and lesser likelihood of being passed anteriorly into the spinal canal.¹⁹ Braided cable may be doubled over on itself and the blunt end passed more safely beneath the laminae. After bilateral cable placement, a bone graft is placed in the interspinous space or along the laminar surface and the cable is tightly secured by crimping. Sublaminar wiring has demonstrated stiffness characteristics in flexion and extension and in flexion with fatigue testing similar to Rogers’ and Bohlman’s triple-wiring techniques in a bovine model.²⁰ Placement of sublaminar wires or cables is associated with a 7% risk for neural injury.^13,^21,²²

Sublaminar cables may be used as fixation points for segmental instrumentation. The prototypical device is the Luque rectangle.²³ This is a variant of Robinson and Southwick’s facet wiring and consists of a metal rod in the shape of a rectangle that is affixed to the facets in a manner similar to the Robinson and Southwick facet wiring technique. Sublaminar wires are then placed one level cephalad and one level caudal to the levels of fusion and tightened to the horizontal portion of the metal rod. When compared with Robinson and Southwick’s facet wiring technique, this method has improved biomechanical stiffness and decreased range of motion.^23,²⁴ It may be used after surgical decompression with laminectomies that span multiple levels.

Lateral Mass Screw Fixation

Lateral mass screw placement was first described by Roy-Camille in 1964 and has undergone numerous refinements since that time.^6,^7,²⁵^–²⁸ Placement of screws in the lateral mass is technically simple, although a potential for neurovascular injury exists because of the proximity of the vertebral artery and cervical nerve root. Careful preoperative consideration of patient anatomy is warranted, especially in those with severe degenerative disease, in whom erosive arthropathy may reduce the size and distort the shape of the lateral masses considerably.

Lateral mass screws were first used with plates for fixation as early as 1979.²⁹ These constructs, however, were constrained by variable patient anatomy and the inability to easily extend existing constructs cranially or caudally.³⁰ Screw and rod–based systems were subsequently developed and allowed easier adaptation to patient anatomy and extension of existing constructs (Fig. 299-4). Although the biomechanics of individual screws is similar to that of screws used in lateral mass plating techniques, the flexibility of screw and rod–based systems allows each to be placed in an optimal location at an optimal trajectory. In addition, many systems permit individual screws to be locked to the rod-based longitudinal element to prevent screw backout. Lateral mass screw-based methods of fixation offer general applicability but are particularly useful in cases in which the spinous processes and laminae are compromised or absent and fixation of the posterior neural arch is not possible with interspinous wiring or other techniques.