Circumferential Instrumentation

150

• Imaging: Plain radiographs showed diffuse spondylosis throughout the sub- axial cervical spine (Figure 13-1). T2-weighted magnetic resonance imaging (MRI) studies are shown in Figures 13-2 through 13-4. MRI revealed significant cervical stenosis in the setting of degenerative disk disease and broad-based disk bulges from C3 to C7. Cord signal changes were also evident at C3-4 (see Figure 13-3) and C5-6 (see Figure 13-4).

Based on the patient’s history, physical examination findings, and radiographic imaging studies the decision was made to proceed with a three-level anterior cervi- cal corpectomy and instrumented fusion.

SURGICAL OPTIONS

For any case in which a multilevel corpectomy is indicated, the operative surgeon can choose to rely on anterior plate fixation alone or apply circumferential instrumenta- tion with an anterior plate and a posterior fixation construct. The use of anterior plate fixation increases stiffness and reduces cervical motion following corpectomy.¹⁵ Such devices have been shown to improve fusion rates and reduce the incidence of strut graft subsidence.⁵ The addition of posterior spinal instrumentation further enhances construct stability through a tension-band effect.¹⁹ Posterior lateral mass fixation also improves stability due to the fact that it involves multiple points of segmental fixa- tion, lies further from the axis of motion in the cervical spine, and achieves better fixation in the lateral masses than screws placed within the vertebral bodies.¹⁸ In addition, the presence of multiple points of fixation allows forces to be dissipated across several segments rather than concentrated at the ends of a construct.¹⁸

For interventions that may warrant corpectomy at three or more levels, the surgeon can also opt to perform a two-level corpectomy in the area of greatest pathologic change combined with an anterior cervical diskectomy and fusion at the remaining level or levels.^17,20,21 This technique, termed hybrid decompression and fixation by some,²¹ offers the advantage of a single anterior surgery with a theoretically more biomechani- cally stable anterior construct than multilevel corpectomy with end fixation only.

FIGURE 13-1 Lateral radiograph revealing diffuse spondy- losis in the subaxial cervical spine.

FUNDAMENTAL TECHNIQUE

The initial portion of an anterior cervical corpectomy procedure, whether or not hybrid decompression-fixation or circumferential instrumentation will be used, is performed with the patient in a supine position on a radiolucent table. Careful thought should be given preoperatively to whether an awake, fiberoptic-assisted intubation is necessary. Spinal cord monitoring with measurement of somatosensory and motor evoked potentials and electromyography is also recommended during cervical corpectomy procedures.

Once the patient is intubated, an intravenous bag or towels are placed in the interscapular region to hyperextend the cervical spine. The patient’s head can be

FIGURE 13-2 Sagittal T2-weighted MRI scan showing significant cervical degen- erative disk disease with broad-based disk bulging at C3-4, C5-6, and C6-7. Cord signal changes are evident at C3-4 and C5-6 (arrows).

FIGURE 13-3 Axial T2-weighted MRI scan showing significant stenosis at C3-4 with cord signal changes (arrow).

placed in a Mayfield headrest or cushioned head support based on surgeon prefer- ence. Importantly, the neck should be maintained in a neutral position within the patient’s functional range of motion as assessed before surgery. The arms are pad- ded and secured close to the body by means of tucked sheets or arm sleds attached beneath the operating table. The shoulders may be secured to the end of the bed with tape to facilitate exposure (Tips from the Masters 13-1).

Tips from the Masters 13-1 • Shoulder traction should be performed with extreme caution, especially in cases of severe cervical spondylosis, to avoid neurapraxia of the cervical nerve roots.

The approach for a cervical corpectomy can be performed from the left or right side based on the surgeon’s preference. The incision should be plotted using stan- dard anatomic landmarks in the cervical spine and centered over the operative levels. In certain circumstances a two-level corpectomy can be performed using a transverse incision similar to that utilized for an anterior cervical diskectomy. How- ever, in most cases, a longitudinal incision along the anterior border of the sterno- cleidomastoid is recommended for multilevel corpectomies. The cervical levels to be included are exposed in a fashion identical to that used in the standard anterior exposure of the cervical spine. Exposure should extend to the middle of the verte- bral body above and below the fullest extent of the corpectomy. Appropriate surgi- cal levels should be confirmed with intraoperative fluoroscopy.

Once the appropriate surgical levels have been confirmed, Caspar pins can be placed in the middle of the vertebral bodies above and below the corpectomy lev- els. Pins should be carefully placed so as not to violate the end plates. A Caspar dis- traction device can then be used to perform gradual distraction across the operative levels, usually after the corpectomy has been completed, to facilitate cage or strut insertion. Care must be taken to avoid overdistraction, because this can lead to neu- rologic injury. A scalpel with a size 15 blade is then used to perform diskectomies at the disk spaces above, within, and below the corpectomy levels. Disk material is removed with a pituitary rongeur.

Once the diskectomies are complete, a rongeur is used to create an initial trough in the vertebral bodies to be included in the corpectomy. This bone material should be retained for grafting, especially if a titanium mesh or expandable cage is to be used for reconstruction. It is imperative to clearly establish the cervical midline before initiating the corpectomy to minimize the risk of iatrogenic vertebral artery injury during the procedure. Once an adequate trough has been fashioned in the vertebral bodies with the rongeur, a high-speed cutting bur can be used to complete most of the corpectomy, although it is recommended to switch to a diamond-tip bur once the surgeon visualizes posterior cortical bone or nears the posterior lon- gitudinal ligament (PLL). Remaining cortical bone can be elevated from a nonos- sified PLL using curved curettes. The PLL may then be elevated from its cephalad insertion with a curved curette or nerve hook and removed in piecemeal fashion

FIGURE 13-4 Axial T2-weighted MRI scan demonstrating significant stenosis at C5-6.

with Kerrison rongeurs. At this point the dura is exposed and the decompression is completed.

Vertebral end plates at the cephalad and caudal margins of the corpectomy site, and at the adjacent diskectomy site if a hybrid decompression-fixation is performed, should be prepared with curettes and/or a cutting bur. The surgeon’s graft of choice (autogenous or allogenic; fibula strut, iliac crest, titanium mesh cage, or expandable cage) is then tailored to span the defect made by the corpectomy (Tips from the Masters 13-2). Unless an expandable cage is being used, the strut is inserted into the superior end plate first, followed by tamp impaction into the end plate at the caudal margin. When nonexpandable cages or struts are used, it is useful to slightly overdistract the corpectomy site before insertion. Once the cage is in an ideal posi- tion, the distraction can be relaxed, which will enhance the interference fit at the end plates.

Tips from the Masters 13-2 • Care must be taken to ensure that the graft does not rest too anteriorly on the inferior end plate, because this increases the risk of subsidence and kick-out.

An appropriately sized plate is then placed, spanning the decompression and resting on the vertebral bodies above and below the corpectomy. The plate is pro- visionally secured while screw holes are drilled and locking screws are placed.

In a standard corpectomy four screws are placed to secure the plate. With hybrid decompression-fixation, six screws can be placed: two above and below the cor- pectomy site and two in the vertebral body adjacent to the site where the standard diskectomy was performed (Tips from the Masters 13-3).^16,20,21

Tips from the Masters 13-3 • In situations in which a two- or three-level corpectomy is warranted, a more rigid standalone anterior construct can be fashioned by performing hybrid decompression-fixation.

If circumferential fixation is necessary, the anterior incision is closed in standard fashion and the patient is then transferred to the prone position on a Jackson table.

The patient’s head may be placed in a standard headrest or Mayfield tongs. A mid- line approach to the posterior cervical spine is then performed, extending from one level above to one level below the corpectomy levels. Lateral mass screws are placed using one of the techniques described in the literature. If the corpectomy includes C7 or ends at the C6-7 disk space, it may be advantageous to extend the posterior instrumentation to the level of T2.²²

If this is done, lateral mass screws do not need to be inserted at C7, because they provide little additional biomechanical stability to cervicothoracic constructs (Tips from the Masters 13-4).²² Appropriately sized rods are contoured and secured in place with locknuts. The lateral masses, spinous processes, and laminae can be decorticated, and bone graft, including cancellous bone retained from the corpec- tomy sites, should be placed to potentiate fusion.

Tips from the Masters 13-4 • If the multilevel corpectomy extends to C7 or T1, consideration should be given to extending the posterior instrumentation to the level of T2.

DISCUSSION OF BEST EVIDENCE

The neurologic benefits of a corpectomy procedure are achieved via the extent and quality of decompression performed. Although numerous studies have documented successful outcomes following these procedures,^1-12,23 high failure rates have been reported, particularly in multilevel corpectomies with standalone strut grafting or ante- rior plate fixation.3,5,14,18,24 Pseudarthrosis after corpectomy has been shown to correlate with unsatisfactory outcomes and an increased risk of axial neck pain.⁵ Catastrophic construct failures can result in graft subsidence or kick-out that can lead to esophageal perforation, neurologic injury, and/or life-threatening airway compromise.^3-5

In a retrospective review of data for patients treated with multilevel corpec- tomy, Vaccaro and colleagues reported a 9% early failure rate for two-level corpec- tomies reconstructed with strut graft and anterior plate fixation, as opposed to a 50% failure rate for three-level corpectomies treated in the same manner.¹⁸ Similar findings were reported by Kristof and colleagues¹³ and Sasso and co-workers.¹⁴ In a retrospective review published by Sasso and co-workers, failure rates were reported for 40 patients undergoing multilevel anterior cervical corpectomy with iliac crest autograft and locked plate fixation. A 6% failure rate was documented in the two-level corpectomy group, whereas 71% of patients undergoing three-level corpectomies in this series experienced catastrophic failure, including subsidence and anterior dislodgment.¹⁴

In a prospective study conducted by ElSaghir and Bohm,²⁴ anterior corpec- tomy with titanium cage reconstruction and anterior plate fixation was found to be associated with a 23% incidence of screw breakage and implant subsidence. In this investigation, the authors reported a significantly higher risk of hardware failure after this anterior procedure than after posteriorly based reconstruction. Findings in these clinical reports are substantiated by numerous biomechanical investiga- tions.15-17,22,25,26 DiAngelo and associates¹⁵ reported that anterior plating following multilevel corpectomy resulted in a reversal of loading patterns in the cervical spine.

Under flexion, the strut graft is shielded from loading, but excessive forces are applied to the graft during extension. DiAngelo and colleagues¹⁵ proposed that this biomechanical phenomenon could be responsible for inferior end-plate cavitation and anterior strut extrusion.

Investigators have hypothesized that longer corpectomies, reconstructed with strut grafts and anterior plate fixation, have concomitantly increased rates of failure owing to longer lever arms and cantilever forces acting on the construct.5,16,17,27,28

In a cadaveric biomechanical study, Isomi and colleagues found a significantly increased range of motion, and consequent failure risk, in three-level corpectomies reconstructed with anterior plates compared with single-level corpectomies.²⁷ Simi- lar biomechanical findings were also reported by Singh and co-workers.¹⁷

Many surgeons maintained that the risk of failure following multilevel anterior corpectomy could be mitigated by the application of posterior instrumentation. The use of posterior fixation, particularly lateral mass screw-rod constructs, would cre- ate a posterior tension band in the cervical spine, limiting the forces felt to be responsible for graft failures.4,5,16,19,26 Schmidt and co-workers²⁶ compared anterior plate fixation with circumferential instrumentation in a C4 to C6 corpectomy model.

A statistically significant reduction in range of motion was found for circumferential instrumentation compared with anterior plate fixation, which led these authors to conclude that circumferential procedures were preferential to standalone anterior plating.²⁶

In a biomechanical model simulating corpectomy following cervical flexion- extension injury, Do Koh and associates²⁵ reported a structural advantage for circum- ferential fusion over anterior fixation in terms of limiting flexion, extension, rotation, and lateral bending across involved motion segments. Ames and colleagues²² reported like findings in a similar model representing severe flexion-compression injury at C7. In this biomechanical investigation, circumferential instrumentation was found to significantly limit cervical flexion, extension, lateral bending, and rota- tion compared with anterior fixation alone. These authors also recommended that, when the C7 vertebral body is compromised by trauma or involved in a corpectomy, consideration be given to extending the posterior fusion to T2.²²

Adams and colleagues²⁸. characterized limitations in range of motion for anterior cervical plating and circumferential instrumentation in a C5 corpectomy model. The addition of anterior plate fixation produced significant range of motion limitations compared with a normal spine, but only in flexion and extension. Circumferential instrumentation led to a significant reduction in range of motion compared with a normal spine in flexion, extension, rotation, and lateral bending. Compared with anterior plate fixation, circumferential instrumentation was associated with a sig- nificant reduction in range of motion for all tested parameters except axial rotation.

Adams and co-workers²⁸ maintained that circumferential fixation was not indicated for every patient undergoing multilevel corpectomy, but recommended that strong consideration be given to this procedure in individuals with osteoporosis, those with significant deformity, and those requiring early mobilization after surgery.

Singh and co-workers¹⁶ examined the biomechanical performance of anterior cervical locking plates with circumferential instrumentation in a two-level (C4 and C5) corpectomy model. Results of this study indicated that anterior plate fixation was significantly less rigid than circumferential instrumentation in flexion, extension, lateral bending, and axial rotation. These findings led the authors to conclude that circumferential instrumentation was justified in instances in which patient factors might compromise the integrity of a standalone anterior plate construct.¹⁶

Porter and co-workers²⁰ compared the rigidity of three-level corpectomy (C5 through C7) with anterior plate fixation, and two-level corpectomy (C5 and C6) plus anterior cervical diskectomy (C7 through T1) with anterior plate fixation (hybrid decompression-fixation). The hybrid decompression-fixation construct provided more rigidity in flexion, extension, and lateral bending, although only limitations in flexion-extension were considered significant. Singh and associates¹⁷ have also shown that a hybrid decompression-fixation construct performs better under bio- mechanical testing than a two-level corpectomy and anterior plate fixation. The rationale for hybrid decompression-fixation includes the fact that additional fixation can dissipate forces that otherwise would be borne by the inferior end plate in a multilevel corpectomy.^17,20,21 In a cohort of 25 patients treated with hybrid decom- pression-fixation, Ashkenazi and co-workers²¹ reported a 96% fusion rate and no instances of construct failure.

Although no comparative studies presently exist investigating outcomes in patients with multilevel corpectomies treated with anterior plate fixation or circumferential instrumentation, several case series report improved outcomes in patients undergo- ing stabilization with anterior-posterior fixation after multilevel corpectomies.^1,2,8,10 In a prospective study by McAfee and colleagues⁸ investigating outcomes after ante- rior decompression and circumferential instrumentation for myelopathy, 15 of 100 patients underwent multilevel corpectomies. In this subset of patients, no instances of construct failure or graft dislodgment were reported.

In a series of 26 myelopathic patients treated with multilevel anterior cervical decompression, Sevki and co-workers¹⁰ included 12 cases in which circumferential instrumentation was used for stabilization. Eight patients treated with anterior-posterior fixation in this study had undergone three-level corpectomies, whereas the remaining four had undergone two-level corpectomies. No instances of hardware failure, graft migration, or nonunion were reported for this cohort.¹⁰ These authors advocated rou- tine circumferential instrumentation for all corpectomies involving three or more levels.

Acosta and co-workers¹ published one of the few reports to document outcomes solely for individuals undergoing multilevel corpectomies and circumferential instru- mentation. This retrospective review included 20 patients who underwent cor- pectomy at three or more levels with a mean follow-up of 33 months. Neurologic improvement was documented in all patients, and there were no instances of hard- ware or graft failure. The fusion rate for individuals in this series was 100%.¹ Acosta and colleagues echoed the sentiments of Sevki and associates in recommending that anterior-posterior fixation be utilized on a routine basis for stabilization following anterior cervical corpectomy exceeding two levels.^1,10

At present, the threat of anterior construct failure after multilevel cervical corpec- tomy must be weighed against the added surgical risks and morbidity of circum- ferential instrumentation. Unfortunately, no prospective or comparative studies are available to contrast the performance of these reconstructive options, and guidance must be derived from biomechanical investigations and small case series. Nonethe- less, based on the best available evidence it is possible to make a strong recom- mendation for circumferential fixation in multilevel corpectomies involving three or more levels.1,10,16,18,26,28 If anterior-posterior instrumentation is not desirable, it may be prudent to perform hybrid decompression-fixation with a two-level corpectomy and adjacent-level diskectomy.^17,20,21

For multilevel corpectomies involving two vertebral bodies it could be possi- ble to reconstruct the spine with a strut graft and anterior plate fixation. How- ever, in patients with significant kyphosis, osteoporosis, or compromised posterior osseoligamentous structures the optimal construct may still be a circumferential instrumentation.^1,10,16,28

Based on the evidence reviewed in this chapter, the patient was treated with an anterior cervical corpectomy of C4 through C6 and circumferential instrumenta- tion (Figure 13-5). Lateral mass screws were placed posteriorly at C3 through C7.

Because good lateral mass fixation was achieved at C7 intraoperatively, the con- struct was not extended across the cervicothoracic junction.

COMMENTARY

A successful outcome after multilevel anterior corpectomy depends on an adequate decompression and stable reconstruction. Pseudarthrosis, hardware failure, and graft subsidence lead to unacceptable failure rates following surgery. Based on the best available evidence in the literature, it is clear that circumferential instrumentation provides more significant rigidity to the cervical spine than standalone anterior plate fixation, even following one-level corpectomy. The questions of how much rigid- ity is necessary to facilitate fusion and when the critical threshold of risk-benefit is reached for additional posterior fixation cannot be definitively answered based on the current research data.

Nonetheless, it would appear that in light of the significant risk of failure for corpectomies involving three or more levels reconstructed using standalone ante- rior plate fixation,3,5,14,18,24 routine circumferential instrumentation for these cases can be justified. Another alternative would be to perform a hybrid decompression- fixation with anterior plate fixation, a procedure that has been found to perform well in both clinical and biomechanical investigations.^17,20,21 In cases that warrant

A B

FIGURE 13-5 Postoperative anteroposterior (A) and lateral (B) plain radiographs. The patient was treated with a C4 through C6 anterior corpectomy and reconstruction with circumferential instrumentation. Lat- eral mass screws were placed posteriorly from C3 to C7. Because the patient did not have osseoliga- mentous abnormalities or preoperative deformity, the posterior construct was not extended across the cervicothoracic junction.