Cervical Interbody Strut Techniques

Summary of Key Points

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Corpectomy is favored over multilevel anterior discectomy in cases of long segment ossification of the posterior longitudinal ligament, trauma causing disruption of the vertebral body, osteomyelitis, and neoplasms.
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Fusion rates and clinical outcomes for decompression across greater than two cervical levels may be higher with corpectomy than with discectomy.
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A wide variety of materials have been used and studied as strut grafts including allograft, autograft, titanium, and polyetheretherketone.
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Fundamental principles of grafting include surgical goals balancing decompression and reconstruction depending on the pathology, an understanding of factors affecting cervical spine stability, and a thorough knowledge of the utilized graft materials.
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Greater than two-level corpectomy requires consideration of both anterior and posterior instrumentation for increased biomechanical stability.

Since the 1980s, extensive ventral decompression via corpectomy for cervical spondylotic myelopathy and spinal deformity has become routine. Although neurologic outcomes remain similar between multilevel anterior discectomy and corpectomy, certain clinical scenarios favor corpectomy (e.g., ossification of the posterior longitudinal ligament, trauma, osteomyelitis, and neoplasms). Moreover, fusion rates and clinical outcomes after anterior decompression procedures across more than two disc levels may be higher for corpectomy than discectomy, particularly in uninstrumented cases. Corpectomy for cervical decompression provides improved visualization, more extensive decompression, and fewer graft-host interfaces requiring fusion (compared to multisegmental anterior cervical discectomy and fusion [ACDF]), which theoretically leads to the improved rates of arthrodesis. Therefore, spine surgeons must be comfortable with anterior decompression by corpectomy and also with the subsequent intervertebral strut grafting.

Technologic advances now permit a wide variety of materials to be used as interbody devices, including autograft, allograft, titanium mesh, and expandable cages, as well as polyetheretherketone (PEEK) cages. Furthermore, in the majority of clinical situations today, anterior corpectomy strut grafting is supplemented with anterior spinal plate instrumentation to reduce graft migration and enhance fusion rates. However, certain scenarios, for either clinical or logistical reasons, may dictate uninstrumented strut grafting. The techniques of interlocking bone grafting discussed in this chapter are most germane to the latter category of corpectomy cases. Even in instrumented strut grafts, however, some of the principles delineated here remain important for successful integration of the bone graft.

Fundamentals of Grafting

Three fundamental concepts need to be recognized for successful strut grafting. First is a clear understanding of the surgical objectives of the procedure in general. The primary goal typically is adequate and durable decompression of the neural elements. Although this generally would seem obvious, concerns over reconstruction can alter the operative plan and possibly subvert the primary goals of the surgery ( Fig. 60-1 ). Ideally, the reconstruction must be fit to the decompression, and not vice versa.

The second essential component of strut grafting is an understanding of the factors affecting spinal stability ( Box 60-1 ). An uninstrumented, unstable spine requires prolonged external bracing (e.g., halo brace or Minerva jacket). This is relatively independent of the surgical fusion technique. The stable spine reconstructed with a short-segment strut graft may be managed with a rigid cervical orthosis.

Box 60-1

Factors Influencing Stability

Ventral element integrity
Dorsal element integrity
Dynamic radiographic elements
Sagittal plane translation >3.5 mm
Sagittal plane rotation >20 degrees

The third fundamental concept of strut grafting is knowledge of the material characteristics of the intervertebral graft. Appropriate choices for bone are somewhat limited, and in practice surgeons generally have only iliac crest or fibula, either as autograft or allograft, as options. Autograft calvarium has been used for struts, but it is not surprising that this source has not been embraced widely.

Bone Graft

Both the origin of the graft material and its proper handling are important considerations in bone graft selection. Autogenous iliac crest tends to fuse rapidly, which is a distinct advantage. However, suboptimal harvesting techniques, osteoporosis, and injudicious tailoring can compromise its incorporation. Technical constraints typically limit its use to replacing two or three vertebral segments. In fashioning iliac crest to the bony defect, it is ideal to preserve at least two contiguous cortical surfaces from one end of the graft to the other to optimize axial loading strength. Surgeons must also keep in mind the real complications associated with iliac crest harvest, which fortunately only rarely result in long-term problems.

With a fibular implant, however, there are different characteristics to consider: (1) it is a strong, circumferential cortical strut with a higher modulus of elasticity than mixed cortical-cancellous implants, and as such it must be used with caution in the osteoporotic spine; (2) it can be tailored to any needed length; and (3) it provides a central channel for the packing of autograft cancellous bone to enhance fusion. The disadvantage of fibula is the mismatch of the bone density with that of the vertebral body. As a rule, the receiving vertebra will fail before the fibula graft does. This generally results in “pistoning,” in which the fibula penetrates through the vertebral body and can even enter the next motion segment. Some subsidence may be unavoidable, especially in osteoporosis, but is usually of no significant clinical consequence ( Fig. 60-2 ). Theoretically, subsidence may be limited by using minimal distraction during graft placement and by using an orthotic brace postoperatively to limit flexion. Too much graft loading and excessive neck flexion early in recovery predispose to graft pistoning. Minimal disruption of the vertebral body graft bed site is also important in maintaining the final height of the fusion. A fibula grafted to a partial corpectomy will almost invariably result in substantial subsidence and loss of height. If necessary, an additional vertebral level may need to be resected to preserve the resistance to subsidence at the graft site.

Allograft fibula is slower to incorporate than autologous iliac crest. Autograft fibula is less commonly used owing to the increased operative times, blood loss, and significant complications associated with its harvest. One method to enhance fusion but attenuate graft harvest morbidity is to use allograft fibula packed with autograft cancellous bone, taken from the iliac crest or from the resected corpectomy bone itself. Autogenous cancellous bone may be accessed via the superficial surface of the iliac crest through a 3-cm skin incision. The medial and outer surfaces of the iliac crest are not disturbed, as would be needed for the harvest of tricortical grafts. This ideally reduces blood loss and postoperative pain. A 1-cm cortical defect is created in the iliac crest with a high-speed bur, and cancellous bone is taken with a large curette. This, in turn, is packed into the central canal of the allograft fibula with a 3-mm diameter rod. No bone need be placed around the outside of the fibula strut after insertion.

Despite the differences between iliac crest structural autograft and fibular allograft, a significant difference in pseudarthrosis rates has not been consistently demonstrated. Fibular allograft remains the most common choice for reconstruction after cervical corpectomy.

Titanium Graft

The benefit of using titanium is mostly centered on avoiding graft collection complications, and titanium mesh cages (TMCs) have been widely used since their development in the 1980s. Mesh cages provide easy control of cage length, good biocompatibility, and shorter operating times. A brief review of the literature follows, demonstrating titanium as a viable option for grafting.

Jang and colleagues examined 30 patients, with a mean follow-up of 27.6 months, who underwent 1 (24 patients) or 2 level (6 patients) corpectomy reconstructed with TMCs filled with autologous bone chips. They noted a cage subsidence in 93.3% of cases, although this was not associated with negative clinical results during the follow-up period. They noted union between the cage and adjacent end plates, but with the artifact of the cage it was difficult to assess for bridging fusion. There were statistically significant improvements in clinical parameters postoperatively without any cases of hardware failure.

Acosta and coworkers examined TMCs for three or more cervical segments, supplemented with posterior instrumentation. There were 20 patients with an average follow-up of 33 months. They were able to achieve 30.2 degrees of kyphosis correction on average, and all patients had radiographic evidence of fusion without cage subsidence or any instrumentation failure. Clinical pain and functional scores improved in all patients.

Waschke and associates looked at the use of expandable titanium cages in 48 consecutive patients with a mean follow-up of 23 months. About 79% had evidence of fusion or stability of the construct, with a significant ability to restore segmental Cobb angle and cervical lordosis through the expandable nature of the cage.

Polyetheretherketone (PEEK) Graft

The characteristics of PEEK have made it a favorable choice for reconstruction following anterior cervical discectomy and fusion; it is a bioinert semicrystalline polyaromatic liner polymer and has similar elasticity to bone, with a good combination of strength, stiffness, and toughness. PEEK is more elastic than titanium, which possibly decreases the risk of graft subsidence, especially in osteopenic and osteoporotic patients. PEEK also has the benefit of magnetic resonance imaging (MRI) compatibility without causing artifact, and it is radiolucent. Kasliwal and O’Toole published results using PEEK intervertebral cages after anterior corpectomy in 35 patients with at least 6 months of follow-up. The PEEK cage was filled with either autologous bone from the corpectomy or with another graft extender such as demineralized bone matrix or calcium hydroxyapatite. The patients were all treated in a cervical collar postoperatively for 6 weeks. The indications for surgery were degenerative disease in 29 patients, metastases in 3 patients, osteomyelitis in 2 patients, and trauma in 1 patient; 24 of the patients had a single-level corpectomy, and 11 patients had a two-level corpectomy. Clinically, the myelopathy stabilized in 57% of patients and it improved in 43%. All 35 patients had radiographic evidence of fusion on flexion-extension radiographs, and there were no cases of implant failure. Konig and Spetzger compared use of a PEEK cage (19 patients), distractible titanium cage (6 patients), and iliac crest bone graft (7 patients) in a total of 32 patients with single-level cervical corpectomy. They did not find any significant differences among the three groups, but they did find a high rate (32%) of PEEK cage subsidence. This led to their recommendation of iliac crest graft for primary cases and a titanium cage for revision cases.

Currently, expandable cages are available in both titanium and PEEK. The advantages of each material property have been outlined here. The cages can be placed with ease and expanded for optimal “fit” to the end plate. The may be a theoretic advantage during correction of deformity, yet the result may be greater degrees of subsidence.

Strut Graft

Preparation of Vertebral Defect for Strut Grafting

The paramount concern in preparing the vertebral end plates for arthrodesis is the prevention of graft displacement. Although plates and screws prevent graft displacement and improve graft incorporation, even instrumented grafts in rare cases can displace toward the spinal cord. The bed for the graft must be prepared in such a manner that the avenue toward the spinal canal is shorter or narrower than the graft itself. If graft migration were to occur, the direction should be away from the spinal cord. When anterior plating is used, deep slots or mortises in the vertebral body are limited by the need for adequate remaining vertebral body volume for screw purchase. When hardware insertion is not anticipated, spinal canal protection may be attained by one of four strategies ( Figs. 60-3A -C): (1) the keystone mortise and tenon, (2) the dovetail technique, (3) the lateral step method, or (4) the anterior peg method of Niu and colleagues. The primary focus here is on the keystone method.