Screw-in Devices

The Bagby and Kuslich cervical cage (BAK/C, Sulzer Spine-Tech, Minneapolis, MN) is the prototypic example of a screw-in design. The BAK/C cage is fundamentally an adaptation of a spinal instrumentation system already validated for disc space arthrodesis in the lumbar spine. After successful completion of a prospective randomized multicenter trial, it received FDA approval for use in the cervical spine.²⁰ Similar to the larger lumbar devices, a BAK/C implant consists of a hollow, threaded cage with multiple side-wall fenestrations. The cages are manufactured from a titanium alloy and are provided in a 12-mm length, with a choice of 6-, 8-, 10-, and 12-mm diameters. Using a modification of the Cloward technique,³⁰ it is usually inserted as a single, midline cage after reaming and tapping of the adjacent central end plates (Fig. 43-1). Alternatively, two smaller (6- or 8-mm) cages may be applied in a side-by-side fashion, although this necessarily limits the height of disc space distraction that can be achieved (Fig. 43-2). The cages are loaded with locally derived bone shavings, sometimes supplemented with allograft, according to volumetric needs. Sofamor Danek (Memphis, TN) has recently released the Affinity cage in North America. This is similar in concept to the BAK/C, but has a conically tapered configuration whose purpose is to help establish cervical lordosis.

FIGURE 43-1 Bagby and Kuslich cervical cage.

FIGURE 43-2 Side-by-side Bagby and Kuslich cervical cages with adjacent single-level cage.

In the BAK/C study a single-level “fusion rate” of nearly 100% was reported. No device-related failures or complications occurred, and other measures of clinical outcome appeared comparable to the noninstrumented ACDF “control” patients.²⁰ Furthermore these outcomes were achieved without the morbidity associated with iliac crest bone graft harvest observed in the control group. However, certain concerns linger about the consequences of violating the central vertebral end plates, as is required for insertion of this device. Hacker observed a 20% incidence of postoperative “sagittal alignment abnormalities” at follow-up.² Lordotic reaming and tapping techniques and a tapered expansile cage (Varilift, Advanced Spine, Irvine, CA) have been developed in an effort to address this issue.

Because screw-in cages have a cylindrical cross section, the contact surface available for load transfer (and for fusion) is a comparatively narrow trough whose width is less than half of the cage diameter. This difference is mechanically disadvantageous in resisting lateral flexion, and the potential for subsidence of the implant with concomitant loss of cervical lordosis may be magnified if two or more adjacent segments are instrumented. Disc space restoration to an average height of 6 to 8 mm requires implantation of 8- to 10-mm diameter cages. When this is attempted at two or more adjacent levels, the resultant encroachment into the intervening body can be substantial and may necessitate offset placement of the devices and result in significant subsidence (Fig. 43-3). When Wang et al.³¹ studied 64 patients who underwent ventral decompression and interbody fusion with BAK/C, they found that the ventral height of intervertebral space decreased significantly after 1 year, when compared with the ventral height immediately after surgery. BAK/C subsidence was observed in nine patients, including five with single-level fusion, one with two-separated-level fusion, and three with double-adjacent-level fusion, according to the standard of loss of intervertebral height of more than 3 mm. BAK/C fusion was generally effective; however, neck pain tended to reoccur in the patients with cage subsidence and two of them needed revision surgery because of the recurrence of myelopathy with progressive neck pain. Although BAK/C was generally effective, the pitfalls in the design of the device have resulted in the observation of clinical subsidence, which, in some cases, required reoperation for the recurrence of symptoms.

FIGURE 43-3 A, Two-level Bagby and Kuslich (BAK) cervical cages, anteroposterior view. B, Two-level BAK cages, lateral view. C, Same case, two-level BAK cages at 1-year follow-up showing subsidence.

Box Cages

In Smith-Robinson–type interbody fusions, the bony end plates are left intact, and a block-shaped structural graft with large, flat, superior and inferior contact surfaces is countersunk into the disc space.³² From a biomechanical standpoint this construct is superior because it retains the integrity of the end plates and provides a large surface area for load transfer and arthrodesis. Several box-type implant designs have been explored as alternatives to the more traditional tricortical autograft or structural allografts for Smith-Robinson fusion. These share a more or less rectangular configuration with a hollow core and fenestrations in their superior and inferior surfaces to allow the through growth of bone. Both titanium and polymer cages have been produced. The Rabea cage (Signus Medizintechnik, Alzenau, Germany), the Syncage (Synthes, Davos, Switzerland), and the Tibon cage (Biomet-Merck, Berlin, Germany) are representative of titanium box designs.

With FDA approval, the Rabea cage is currently available as a “cement restrictor.” It is made of forged titanium to ensure MRI compatibility and is prefabricated in a 12-mm width, 12- and 14-mm depths, and in heights ranging from 4 to 8 mm (Fig. 43-4). The Rabea is offered as a parallel end-plate design and as a “lordosed” version, with 5 degrees of divergence built in. The rostral and caudal surfaces also bear retentive serrations to engage the end plates and resist implant extrusion during neck flexion. Published experience with the Rabea cage is limited, but preliminary biomechanical studies and clinical reports are both favorable.^10,12,15 As with other metallic devices, it is hard to judge fusion according to criteria of radiographically demonstrable bridging bone across the disc space and end plates. Alternative criteria including less than 2 degrees of angulation in dynamic radiographs and absence of any peri-implant bony lucency are substituted.

FIGURE 43-4 Single-level Rabea cage.

Carbon fiber box cages have found favor as intervertebral implants in the lumbar spine.³³ These devices appear to be largely inert biologically; they exhibit good strength in all axes of applied stress, they are impact and fatigue resistant, their elastic modulus is purported to match well with that of the cortical bone of the recipient spine, and because they are radiolucent, they afford an opportunity for direct graft visualization and fusion assessment. Extending these advantages to a cervical interbody application, box-type polymer cages have been under active development abroad and are poised to enter the U.S. market. Most designs have been fabricated from polyetheretherketone (PEEK), some also including a carbon fiber component (Cervical IF Cage, DePuy AcroMed, Raynham, MA). The Rabea device is now offered as a PEEK cage, again with approval in the United States for use as a cement restrictor. Other major vendors, including Stryker Instruments (Kalamazoo, MI [Solis PEEK cage]), Medtronic Sofamor Danek (CornerStone-SR), and Synthes (CR), have polymer cages already in place in overseas markets (Fig. 43-5). Like the Rabea cage, they are offered in a variety of sizes and configurations, including parallel end-plates, lordotic, and superiorly convex designs to conform to individual disc space anatomic requirements. Shared features include a large central opening to contain graft material, ridged end plates to resist implant extrusion, and imbedded radiopaque markers to allow radiographic visualization of the device.

FIGURE 43-5 A, Single-level Synthes CR cage, anteroposterior view. B, Single-level Synthes CR cage, lateral view.

Cho et al.¹⁸ have recently reported on their preliminary experience with the Solis PEEK cage loaded with cancellous autograft in 40 patients undergoing mostly one- and two-level cervical discectomy and fusion.¹⁸ Although the term of follow-up is unclear, the authors reported good functional outcomes, no device-related complications or failures, increased cervical lordosis, enlargement of neural foraminal cross-sectional area, and a 100% fusion rate. The complication rate was 2.5% in PEEK cage patients compared with a rate of 17.5% in a concurrent control group undergoing conventional discectomy and structural autografting in whom problems with graft collapse, dislodgement, and donor site morbidity were encountered. Other reports³⁴ have found a high subsidence tendency of box cages into predominantly C7; this illustrates the need for a modified box cage design that improves and extends contact with the inferior surface. Radiolucent polymer cages are easily inserted; they appear to be biomechanically sound, and they hold much promise as interbody prostheses in the cervical spine, pending the outcome of more comprehensive clinical studies.

Interbody Fusion Cages with Intergrated Ventral Fixation

Recently, newly developed interbody spacers with integrated ventral fixation components have been gaining popularity in clinical use.⁹ The low profile of these devices allows the surgeon to treat ventral pathology without having to disrupt indwelling hardware. An example of the clinical utility of such a device is an adjacent-level disc herniation after an anterior cervical discectomy. These devices can often be inserted at the adjacent site, after the discetomy is performed, without the need to disrupt the indwelling ventral cervical plate at the previously fused adjacent level. The anchored spacer provides similar biomechanical stability to that of the established ventral fusion technique and has potentially lower associated perioperative and postoperative morbidity, such as dysphagia or swallowing difficulties.⁹ Two of these FDA-approved devices are the ROI-C VerteBRIDGE (LDR Inc., Austin, TX) interbody device and the Mosaic cervical implant system (Spinal Elements, Inc., Carlsbad, CA). See Figure 43-6. To date, more than 2500 of these devices have been used in the United States. Studies on the long-term efficacy of these devices are in progress.

FIGURE 43-6 Three-level anterior cervical discectomy and fusion utilizing Mosaic cervical interbody spacers with integrated ventral fixation screws.

Cylindrical Mesh Cages

Vertically oriented cylindrical cages fabricated of titanium mesh are now gaining use in the cervical spine, both for disc-space arthrodesis and for segmental reconstruction after corpectomy. The Harms cage (DePuy AcroMed, Warsaw, IN) is the primary example of this type of device; the SynMesh cage (Synthes) and Pyramesh cage (Sofamor Danek) are fundamentally similar designs. The FDA has approved use of this instrumentation in the thoracic and lumbar spine. Published biomechanical data and clinical studies describing its use in the cervical spine are sparse.

Similar to the box cage designs, mesh cages are intended to interface with intact end plates at the rostral and caudal ends of the construct. This increases the strength of the construct and reduces the risk for subsidence. Manufacturers provide stock material as sleeves with both circular and oval cross section and in a variety of lengths and diameters. Cage diameters between 10 and 16 mm are most suited for use in the cervical spine. In general the largest diameter that can be safely accommodated within the bed of the decompression is used because the cage will then bear on the end-plate perimeter, the apophyseal ring, which is its strongest part. The mesh can be trimmed to any desired length and tailored to the particular reconstructive need if a stock length is not an exact match. All three systems can fit end caps to the top and bottom of the cylinder, thereby enlarging the metal-bone interface and lessening the risk of end-plate perforation. Large central apertures within the end caps permit through growth of bone. The palisading teeth of the cage ends project slightly above the plane of the end caps, or the end caps themselves are serrated to grip the adjoining vertebral end plates and resist ventral cage extrusion. The Harms instrumentation provides a large selection of precut straight and custom-lordosed cages suitable as “off the shelf” implants for cervical disc space arthrodesis. Cylindrical cages are usually loaded with densely packed autograft. As with other metallic devices, the radiographic evaluation of fusion status within mesh implants is indirect.

There is a paucity of literature pertaining to the use of mesh cages for cervical disc space arthrodesis.^19,23 Our own experience with these devices in single- and multilevel discectomy operations suggests a significant potential for subsidence when these devices are used in a stand-alone mode. We attribute this to the relatively small contact area at the bone-implant interface in small-diameter disc space cages, despite the use of end caps.²³ To retard subsidence, we came to augment almost all ventral mesh cage reconstructions for degenerative disc disease with a dynamic ventral cervical plate. Although the addition of a ventral plate contributes only minimally to the duration and risk of operation, it does constitute a significant added expense. It is hoped that some of the newer box cage designs may yield satisfactory clinical results without a need for supplemental ventral fixation.

Mesh Cage Reconstruction of Corpectomy Defects

A more distinct advantage of using cylindrical mesh cage constructs is in the reconstruction of corpectomy defects. Indications for cervical corpectomy can include spondylosis, ossification of posterior longitudinal ligament, trauma, tumor, deformity correction, and occasional cases of infection. We and others have come to favor it in treatment of adjacent segment cervical disc disease as well.^21,24 Removal of a cervical corpus and the adjoining discs, and any associated osteophytes, can frequently be accomplished more expeditiously than two separate microdiscectomies (Fig. 43-7). The quality of the decompression is unsurpassed, and ample bone is acquired from the corpectomy material to satisfy any need for graft.

FIGURE 43-7 Harms cage, one-level corpectomy.

Except in cases of total spondylectomy for tumor, the central corpectomy bed is flanked on either side by a shell of lateral cortex and disc material. The method of decompression is addressed elsewhere in this volume. Ventral reconstruction seeks to restore the normal height of the vertebral column, which is frequently foreshortened with loss of cervical lordosis by the underlying conditions dictating the need for decompression. In the course of decompression, axial distraction is routinely applied via halter or cranial tong traction, and a thin interscapular roll is placed to help establish slight neck extension. Screw-in posts are inserted into the ventral midbodies of the rostral and caudal vertebrae adjacent to the decompression. Powerful forces can then be applied using a rack and pinion distraction arm, which has the combined effect of lengthening the ventral column of the spine and restoring lordosis. In significantly kyphotic deformities, divergent distraction techniques may be advantageous. As a consequence of this maneuver, the planes of the recipient rostral and caudal end plates are brought more nearly to a parallel alignment. Some divergence usually persists, however, and this needs to be accounted for in the subsequent fashioning of the construct. Often a small ledge of osteophyte projects caudally from the ventral caudal edge of the rostral vertebral body. This should be removed to create a relatively planar end-plate surface that will allow unimpeded insertion of the construct. The cartilaginous end plates are scraped down with curettes, but every effort is made to leave the bony end plates inviolate.

Once the desired degree of distraction in the corpectomy bed has been obtained, calipers are used to measure the desired construct length at both its ventral and dorsal limits. Because of the divergence of the end plates, the ventral height is often 1 to 2 mm longer than the dorsal measure. We are most experienced with the Harms system, which provides cages suitable for use in the cervical spine as straight cylindrical stock ranging from 12 to 16 mm in diameter and from 30 to 50 mm in length. Oval stock 15 mm × 12 mm is also available, as well as prelordosed tubular stock. We advise selecting the largest cage that can be safely accommodated within the corpectomy bed, without intruding on the ventral spinal canal. The desired ventral and dorsal heights of the cage are marked according to caliper measurements, and the cage is cut accordingly. For straight mesh stock spanning three or more levels, it may be advantageous to cut the lower end of the cage on a slight bias to optimally accommodate the divergence of the end plates to which they will be paired. Other systems offer end-fitted, lordotic end caps as an alternative to direct cage modification. Intraoperative lateral fluoroscopy showing the rostral and caudal recipient end plates, with distraction applied, is helpful in forming an image of the desired cage geometry. Ideally, full cross-sectional contact will be achieved between each end of the cage and the adjoining end plate.

An appropriately sized end cap is fitted to one end of the cage. The bias cut is never so severe as to preclude a snug fit. The cage can then be loaded with morselized autologous bone obtained from the corpectomy bed itself or elsewhere. This bone is repeatedly tamped down to create dense packing within the construct without any internal voids. Good packing of the construct is achieved when graft material is seen extruding centrifugally through the interstices of the mesh. Only in cases of tumor and infection is locally derived graft material quantitatively or qualitatively insufficient. If necessary, it is easily supplemented by cancellous autograft obtained from the iliac crest or combined with allograft. The second end cap is seated, and a final bit of additional graft is packed into the central orifice of each end cap. Just before the cage is to be inserted, the distractor mechanism is opened to lengthen the corpectomy trough an additional 1 to 2 mm. The cage is then seated in the corpectomy bed until a good friction fit is achieved. Small impactors are applied alternately over the rostral and caudal ends of the cage, tapping the cage deeper into the corpectomy trough until its presenting edge is recessed just below the ventralmost cortices of the adjacent vertebrae. At this point, axial traction is removed to allow elastic recoil of the soft tissues to lock the construct in place. A nerve hook can be inserted behind the cage to assess the tightness of the fit and confirm the patency of the ventral spinal canal. Anteroposterior and lateral fluoroscopy are then used to control proper placement in the coronal and sagittal planes, and any indicated adjustments are performed.

Reconstruction of one- and two-level corpectomies is generally straightforward. For degenerative conditions, we secure the construct with a dynamic plate and screw system, which provides additional rigidity during neck extension and helps limit subsidence during healing. If bone quality and screw purchase are good, use of an external orthosis is optional in single-level corpectomy. In weakened osteoporotic bone, bicortical screw fixation of the ventral plate and an orthosis are used. Especially precarious constructs or trauma cases with dorsal ligamentous disruption are supplemented with dorsal instrumentation and fusion. For acute spinal traumas, a rigid plate and screw system is preferred because of the frequently associated soft tissue injury and the heightened risk of instability. A minor degree of subsidence is routinely observed during healing, but graft or plate dislodgement or significant cervical straightening is most unusual.

As an alternative to long-segment corpectomy or multilevel discectomy in patients who need decompression/fusion at three or more motion segments, we have performed corpectomy plus adjacent-level discectomy (three motion segments), or “discontinuous corpectomies” with retention of an intervening body (four motion segments). A ventral plate is thereby afforded at least two extra intermediary points of fixation (Figs. 43-8 and 43-9). The advantage of this technique over multilevel discectomy is the speed and completeness of the decompression and the provision of locally derived autograft. We have experienced no instances of plate loosening or graft migration in this group of patients.

FIGURE 43-8 Harms cages, one-level corpectomy and adjacent disc space arthrodesis.

FIGURE 43-9 Harms cages, two-level discontinuous corpectomies.

Reconstruction of three- and four-level corpectomies is more problematic. The potential for graft migration, plate loosening, and end-plate fracture is considerably greater. Frequently, such patients are being operated on for correction of a kyphotic deformity, and the bone quality can be suboptimal. Long-segment ventral plates fixated only at their rostral and caudal ends are very prone to failure. Failure typically occurs at the inferior end of the construct where the hardware either fractures or levers off of the caudal body (Fig. 43-10). Simple “kick plates” fitted to the vertebral bodies at the rostral and caudal ends of the construct are worthy of consideration in this circumstance. They do not contribute to the inherent stability of the construct other than to prevent ventral extrusion of the implant. Immediate stability is secured by supplemental, instrumented dorsal spinal fusion and/or halo immobilization (Fig. 43-11).

FIGURE 43-10 Long-segment cage failure at caudal end of construct.

FIGURE 43-11 Long-segment reconstruction with kick plate and dorsal fixation.