AUTOGRAFT ILIAC CREST

Autograft yields very high rates of fusion, as long as mechanical stability is provided. It shows rapid healing and modulus similar to vertebral body. After fusion has occurred, the construct shows extreme durability (Fig. 37-1). Even though the fusion bed is irradiated, the fusion rate is very good. When survival is expected to be more than 6 months, autograft is the most favored graft material. However, in anterior reconstruction involving at least one vertebral body and its adjacent discs in thoracic and lumbar spine, the size of the defect to be filled necessitates the harvesting of a large amount of autograft, often associated with a significant morbidity at the graft harvesting sites. Furthermore, the mechanical resistance of such a large reconstruction with autograft is usually not sufficient to allow full weight-bearing without external orthosis. Other problems are pseudoarthrosis, necrosis after adjuvant radiation, transplant loosening, and morbidity at the donor site.

Fig. 37-1 After T12 corpectomy, the iliac bone graft is inserted.

VASCULARIZED RIB AUTOGRAFT

The rib is commonly used as a bone graft in surgery of the anterior spine. The advantage of the rib is that it is easily used as a pediculated vascularized graft when it is transferred to the anterior vertebrae. The contraindications of the vascularized rib graft are infection, osteoporosis, and radiation history.

During the thoracic approach, the periosteum of the rib is neither divided nor rasped. Intercostal muscles are divided above and below the selected rib to protect the periosteum and the vessels running below the distal edge of the rib (Fig. 37-2). At the front, vessels are ligated and divided along with the rib. At the back, the periosteum is incised all around the posterior arch of the rib and is rasped with care, pushing away the pedicle in one block. The rib is elevated with its accompanying artery and vein. The intercostal nerve is separated from the vascularized rib. The vascularized rib is isolated. Anteriorly the end is near the costochondral junction, and posteriorly it is near the costal angle. The rib is wrapped up in a wet pad. The mean length of the resected segment is about 10 cm.

Fig. 37-2 Rib resection with underlying vascular pedicle.

The length of the useful segment can vary according to the number of levels to be fused. In thoracic levels, the length of 10–12 cm can cover four or five segments. The average length of the vascular pedicular segment is 5–6 cm. The useful segment usually has a 40-degree curvature (Fig. 37-3).

Fig. 37-3 Usable segment of the rib graft (PL, pedicle length; UL, useful length).

The useful segment of the rib is mobilized around the vascular pedicle. The surgeons must be sure that the pedicle is not twisted, elongated, or tight, and not folded (Fig. 37-4). The whole length of the rib is sectioned into two or three pieces. The length of each fragment adjusts to match the defect in the corpectomy. The rib is folded cautiously to prevent injury to the accompanying vessel (Fig. 37-5). The natural progression of non-vascularized bone graft is a first stage of necrosis, then a second stage of creeping substitution. When the vascularized graft is used, the creeping substitution is avoided, and fusion and bone healing are obtained within 3 months.

Fig. 37-4 Mobilization of the rib graft.

Fig. 37-5 Rib graft positioning in the corpectomy site.

TITANIUM MESH CAGE

Recent studies on the titanium mesh cage (TMC) have reported excellent stability rates compared with the traditional tricortical iliac crest grafts or fibular grafts because they provide immediate immobilization and a larger healing surface when packed with cancellous bone (Fig. 37-6).² The mesh cage allows anchoring by bone because its end tip has spikes. Unlike bone grafts, both ends of a titanium mesh cage have sharp edges that allow it to be anchored into the adjacent vertebral bodies, which results in torsional stability (Fig. 37-7). Early stabilization could be achieved by making the cage ends “bite” into the adjacent vertebral bodies.

Fig. 37-6 Titanium mesh cage filled with autologous bone graft.

Fig. 37-7 After corpectomy, the TMC is inserted into the corpectomy site.

Disadvantages include cost, difficulty in assessing fusion on plain radiographs, difficulty in surgical revision, stress shielding that may affect fusion, and the potential for soft-tissue injury. Subsidence can take place into the adjacent vertebral body, and the mesh’s sharp teeth can penetrate the endplate and accelerate graft subsidence. The mean settling ratio is reported to be 4.35% of cage length at 1 year.

It is postulated that preservation of the endplates may prevent potential telescoping of the graft when placing cages.³ End caps, designed to prevent telescoping, increase the cage’s maximum load-bearing ability but do not affect the construct’s stiffness. They may interfere with the bone-bone interface or the transference of stress lines, which is needed for fusion, through the TMC. The plate/TMC combination results in the strongest, stiffest construct, preventing flexion and extension, rotation, and parallelogram effect. The inability of plain radiography to evaluate bone growth within the TMC can be overcome with computed tomography (CT) scanning. The presence or extent of bone growth within the TMC can be determined from CT scanning. On CT images, hyperdense areas are seen, which suggests bone growth within the TMC, and indirect evidence of fusion is presumed based on radiographically documented stability. The TMC/plate construct has been safe and effective as a vertebral replacement in the spine.

ALLOGRAFT

Allografts include femoral cortical rings, tibial rings, fibular rings, and humeral cortical rings (Figs. 37-8 and 37-9). Some surgeons advocate the use of an allograft-autograft composite by packing the medullary canal of the allograft with autografts. Deep-frozen cortical rings and freeze-dried cortical rings are available. Both deep-frozen and freeze-dried cortical allografts have been strong enough to withstand loads taken by the reconstructed spine. The results of the fusion operation have been good. Infection and implant failure are rare.

Fig. 37-8 Allograft—femoral ring.

Fig. 37-9 After thoracic corpectomy, a femoral ring allograft is inserted.

Allografts are of multiple sizes and provide reliable fusion with the host bone with partial bone remodeling, preventing fatigue fracture. The major disadvantage is the risk of transmission of the disease. The risk can be reduced with several physical and chemical treatments, including washing, lipid extraction, prion inactivation, freeze drying, packing, and sterilization by 2.5 Gy gamma irradiation.⁴

Two-dimensional (2D) CT evidence of bony ingrowth into the central canal of a fibula strut allograft provides an adequate means of quantifying the extent of fusion (Fig. 37-10).

Fig. 37-10 Femoral ring allograft provides a strong cortical construct with a large contact surface.

Immediate postoperative baseline 2D CT studies reveal no bone within the central canal. Within 6 postoperative months, 2D CT studies demonstrate 3.5–5 mm of bony ingrowth at the superior and inferior ends (confirmed by measuring 500–900 Hounsfield units) into the central fibula canal.

CARBON FIBER STACKABLE CAGE

A carbon fiber stackable cage (CFSC) system is presented to promote the reconstruction of the anterior column after vertebrectomy or corpectomy in tumor surgery.^5,6

Modularity, immediate stability, early fusion of the graft, radiolucency, and no risk of disease transmission are the main advantages of this system (Fig. 37-11).

Fig. 37-11 Postoperative anteroposterior (AP) view shows a radiolucent modular carbon cage in the corpectomy site.

The CFSC is octagonal, has a generous open chamber to accept bone chips, and can be connected to an anterior plate or posterior rod system. Carbon fiber reinforced polymer is produced by a manufacturing process in which carbon fibers are embedded in a polymeric matrix. It is biologically inert, noncarcinogenic, and strongly resistant to vertical and shear forces. The carbon fiber modulus of elasticity closely approximates cortical bone, thus facilitating load shearing and subsequent hypertrophy of bone graft and fusion between the host vertebral bodies.

A modular prosthesis in carbonium and titanium with pedicles may be connected to the posterior implant (plates and bars) (Fig. 37-12).

Fig. 37-12 After total spondylectomy, the CFSC was inserted. Additional posterior fixation was performed with pedicle screws. A posterior rod was connected to the CFSC.

Modularity means that surgeons have the ability to intraoperatively reconstruct any loss of spinal substance resulting from unexpected extension of the resection (Fig. 37-13). Radiolucency enables surgeons to recognize whether the bony fusion is successful, and tumor recurrence also can be detected earlier.

Fig. 37-13 Modularity of the CFSC. The height of the cage can be adjusted to unexpected extension of the vertebral body resection.

POLYMETHYLMETHACRYLATE RECONSTRUCTION IN METASTATIC TUMOR SURGERY

PMMA-assisted reconstruction is a reasonable alternative to bone grafting for cancer patients with a limited life expectancy because it achieves immediate stabilization after radical tumor resection. PMMA is unaffected by tumor invasion and seems to be safe in patients with postoperative radiation therapy.

For the anchorage of the PMMA, several methods are used.

Partial Vertebral Body Defect

For fixation of the bone cement graft, a partial bone defect is created in the endplate of the adjacent vertebral body. The defect site is filled with bone cement to form the anchorage (Fig. 37-14).

Fig. 37-14 After making the holes in the endplate, the PMMA is filled.

Steinmann Pin–Assisted Polymethylmethacrylate Graft

After the corpectomy, Steinmann pins are placed into the vertebral bodies above and below the level of the resection, and PMMA is poured into the resection cavity (Fig. 37-15).⁷ Two Steinmann pins are cut to appropriate lengths (approximately one full vertebral body height longer than the total vertebrectomy defect) and curved slightly to facilitate placement (Fig. 37-16). The direction of passage will be rostral first, and caudal second. The first pin is passed, with the aid of large needle holders, into the center of the rostral vertebral body through the bony endplate of the disc space. The concave curve is facing away from the vertebral bodies until the other tip is passed into the bottom of the vertebrectomy defect. That caudal tip is then passed through the center of the lower endplate until both rostral and caudal tips are well buried in bone (Fig. 37-17). The second pin is inserted in a similar fashion, next to the first. The placement of two pins allows for stability of the resected segment in translational as well as angular movements. Pins must be carefully placed to prevent injury to the adjacent pleural, neural, and vascular structures.

Fig. 37-15 Steinmann pins are placed into the vertebral bodies above and below the level of the resection, and PMMA is poured into the resection cavity.

Fig. 37-16 A Steinmann pin is cut to the appropriate length and curved to facilitate placement. The direction of passage will be rostral first, and caudad second.

Fig. 37-17 Postoperative x-ray, lateral view. The tip of the Steinmann pin is passed through the endplate of the vertebral body.

When the PMMA is poured, Gelfoam or fat is placed over the dura to protect against thermal injury from the polymerization reaction. Saline irrigation is used to dissipate the heat of polymerization.

EXPANDABLE CAGE

Vertebral Body Replacement Cage

The use of cages for spinal surgery is shown to be effective in providing anterior column support and long-term stability. For non-expandable cages, adjusting the cage size to the endplates and height of the corpectomy defect is limited by predefined cage endplate angle and height. Direct correction of internal kyphosis of the anterior column is difficult with currently available non-expandable cages. With expandable cages, these difficulties can be resolved (Fig. 37-18). No biomechanical differences can be observed between the expandable and non-expandable cages. However, the clinical use of these implants as stand-alone cages cannot be advocated.

Fig. 37-18 VBR cage insertion after thoracic corpectomy.

TECHNIQUE FOR VERTEBRAL BODY REPLACEMENT CAGE

The expandable VBR cage is a commercially available, expandable titanium cage consisting of a round, flat endplate surface with small spikes (Fig. 37-19). Rotational stability is enhanced when the spikes are firmly purchased into the endplates of the corpectomy defect.⁸ After corpectomy, the appropriate VBR implant size is determined by using a caliper to measure the height and width of the corpectomy defect. Autograft bone from the corpectomy and iliac crest are used to fill the cage. The cage can be expanded with an expansion wrench. Once the cage is distracted to its final length, additional graft material can be added to fill the end pieces completely. As expansion of the cage occurs, increasing stiffness of endplate purchase can be felt and seen.

Fig. 37-19 VBR cages.

ADVANTAGES

• It can be expanded to provide the best fit into the corpectomy defect.

• The correction of kyphotic deformity is possible because of its in-vivo expansion properties.

• Its porous nature allows for ingrowth of bony elements. The expandable cage endings with spikes and relatively large implant-bone interface allow for excellent purchase into the vertebral endplates, minimizing cage migration and sinking-in.

DISADVANTAGES

• Cost.

• Difficulty in assessing fusion on plain radiographs.

• Potential difficulty in cases requiring surgical revision.

• Potential of stress shielding may affect fusion.

• Small fusion area: The bone fusion area is similar or smaller at the elongating portion than in non-expandable mesh cages.

• Bone particle impaction into the elongated VBR cage is somewhat difficult and may not provide adequate filling.

These cages are highly versatile and easily expandable and can be used to reconstruct the vertebral column defect and to correct the sagittal alignment of the spine.^9,10 They can easily distract across the resected vertebral defect for correction of the deformity, allow immediate load bearing after corpectomy, and provide a satisfying long-term functional result. Very minimal subsidence occurs with expandable cages because of their broader surface and duller edges at each end.

In the cervical area, the VBR cage can be of use with or without plate application in anterior reconstruction. Because the cage is not designed to be a stand-alone device, additional anterior plating is placed in the lower cervical levels (Fig. 37-20). However, in the upper cervical spine, the plating is very difficult. In cases in which anterior plating is not available, the VBR cage is favored because its spine can purchase the endplate firmly, which provides the stronger construct compared with other devices (Fig. 37-21).

Fig. 37-20 The VBR cage in the lower cervical level with anterior plating. After a C5–6 corpectomy, a VBR cage was inserted, and anterior plating was performed from C4 to C7.

Fig. 37-21 VBR cage insertion in the C2 body site. The C2 body was resected, and the VBR cage was inserted. Posteriorly occiptito-C5 fixation was added.

Synex

This is another type of expandable titanium cage.¹¹ This cage is distracted in situ via a ratchet mechanism. This provides more comfort and also press-fit positioning with pretensioning of the spinal ligaments (Fig. 37-22).

Fig. 37-22 Synex cage.