Ventral versus Dorsal Approach

Sagittal plane deformity in the cervical spine may be corrected ventrally,^8–11 dorsally,^12,13 or ventrally and dorsally (in combination).^12,14–16 The ventral approach is one that is familiar to most spine surgeons and may be performed with minimal morbidity.

Many patients with cervical kyphosis have had a prior cervical operation, often a laminectomy (Fig. 222-1). A dorsal revision strategy is associated with increased morbidity with regard to wound complications, pain, and the risk of neurologic injury. A ventral approach is advantageous in that “virgin” surgical territory is entered. If a prior ventral approach had been performed, the same approach may be used without difficulty, or the opposite side of the neck may be entered for the revision. These factors decrease the morbidity associated with revision cervical surgery.

FIGURE 222-1 Lateral cervical radiograph. The patient has a prior cervical laminectomy and also has a solid fusion mass at C4-7. The radiograph demonstrates significant kyphosis.

In the majority of cases of cervical kyphosis, there exists ventral compression of the spinal cord along with the deformity. This ventral compression must be addressed prior to deformity correction. A ventral decompression may not be adequately performed from a dorsal approach; therefore, a ventral decompression should be included with dorsal instrumentation, which increases the morbidity of the operation. A ventral approach permits adequate spinal cord decompression via multiple intervertebral discectomies or corpectomies. We favor leaving intermediate vertebral bodies and not performing multiple adjacent-level corpectomies (i.e., C5, C6, and C7). A safe intermediate vertebral body is one in which there is cerebrospinal fluid signal dorsal to the body on a T2-weighted MRI. Adequate ventral decompression may be performed with this strategy, and the intervening body is used as an intermediate point of fixation (see subsequent discussion).

Using a dorsal-alone strategy, one is most often unable to correct cervical kyphosis significantly. Only if the deformity is “flexible” and is able to be corrected with cervical traction may a dorsal alone strategy be used. This finding is not at all common. More often, a ventral release procedure is required prior to the dorsal deformity correction procedure. A dorsal deformity correction procedure, with or without a ventral release, may not fully correct the deformity. Even with the use of cervical pedicle screws, Abumi et al. were able to correct cervical deformity only from 28.4 to 5.1 degrees of kyphosis, with all patients achieving a solid arthrodesis.¹² A ventral strategy provides a better surgical leverage for deformity correction while providing very solid fixation points if intermediate points of fixation are used.⁶

As mentioned previously, using multiple points of intermediate fixation is optimal with ventral deformity correction strategies. This is accomplished by leaving intermediate vertebral bodies in place, instead of performing multiple adjacent corpectomies. These intermediate bodies provide solid fixation points for intermediate points of screw fixation. These intermediate points facilitate the “bringing of the spine” to a contoured implant to achieve further lordotic correction. They also provide three- or four-point bending forces to prevent deformity progression and construct failure.⁶ These intermediate fixation points may also be provided with dorsal lateral mass fixation, but entail the addition of a dorsal procedure, in addition to a ventral decompression procedure.

Axially dynamic cervical implants further add to the success of a ventral deformity correction procedure.¹⁷ These constructs are able to provide for the placement of multiple intermediate points of fixation. The dynamic aspect of the implant is able to off-load stresses at the screw/implant interface, which aids in the prevention of nonunion and construct failure, and also provides solid fixation for the prevention of cervical deformity progression.

Clinical Experience

We use a ventral-only approach for the correction of cervical kyphosis in specific clinical scenarios. This technique is optimally used when the kyphosis is fixed (i.e., rigid) and the facet joints are not ankylosed. If the facet joints or other dorsal elements are fused, a dorsal osteotomy is required. Ankylosis may easily be determined by fine-cut CT scanning. The clinical technique has been previously described,^17,18 and surgical steps will be briefly outlined here.

The patient is positioned supine on the operating table. A roll or “bump” is placed under the shoulders, and the head is supported on a “doughnut” built up by enough blankets to keep the cervical spine neutral. Multiple discectomies or corpectomy(ies) combined with discectomies are used for decompression, yet leaving vertebral bodies that may be used for points of intermediate fixation. Distraction points may be placed in a convergent manner, and when distraction is applied, segmental lordosis is attained at each level. Following decompression, a lordotic interbody graft may be placed to maintain segmental lordosis. After complete decompression, the blankets are removed from the “doughnut” and maximal cervical extension is attained; this may be performed prior to interbody graft placement. Lastly, a lordotic plate may be applied to the spine and initially affixed to the proximal and distal vertebral bodies only. Next, screws are placed into the intermediate vertebral bodies. Tightening the screws then brings the “spine to the implant.”

We have reported our experience with this technique with 12 patients. A dynamic implant was us in most patients.¹⁷ The majority of patients presented with mechanical neck pain as part of their symptom complex. The average magnitude of deformity correction (preoperative to postoperative) was 20 degrees of lordosis. The average postoperative sagittal angle was 6 degrees of lordosis. The average change in the sagittal angle during the follow-up period was 2.2 degrees of lordosis.

Using this ventral technique, lordosis was attained in all but one patient (Fig. 222-2). This posture was effectively maintained during the follow-up period. All patients demonstrated improvement postoperatively, and three had complete resolution of their preoperative symptoms.

FIGURE 222-2 Lateral cervical radiograph following surgery (same patient as in Fig. 222-1). A ventral-alone strategy using multiple intermediate points of fixation. The kyphosis is effectively reduced.

Summary

It is rare that a dorsal strategy may be used alone for correction of cervical kyphosis. A combined ventral/dorsal approach is used for adequate decompression and deformity correction. This strategy is associated with an increased morbidity compared with a ventral-alone strategy. A ventral approach is familiar to spine surgeons. It permits decompression and correction via a single surgical approach. The ventral correction provides a better surgical leverage compared with a dorsal procedure. The correction attained is effectively maintained with strut grafting and ventral instrumentation, especially if an axially dynamic ventral construct, using intermediate points of fixation, is used.

Cervical Laminectomy and Lateral Mass Fusion

Cervical laminectomy for the treatment of CSM has been shown to be safe and effective. As a stand-alone treatment for CSM, cervical laminectomy has recently fallen out of favor with many surgeons because of the risk of development of postlaminecomy kyphosis. Ryken et al.¹³ performed a systematic review of the National Library of Medicine and the Cochrane database to examine the efficacy of cervical laminectomy for the treatment of CSM and concluded that it remains a viable consideration for treatment of CSM. They found that the risk of developing postlaminectomy kyphosis in patients undergoing laminectomy for CSM ranges from 14% to 47%. It is unclear how this risk relates to clinical outcome; however, a straight or kyphotic cervical spine alignment is associated with an augmented risk of developing a postlaminectomy kyphosis.

In an attempt to avoid the development of late kyphosis after laminectomy, many surgeons opt to perform a lateral mass fusion at the time of laminectomy. Gok et al.¹⁴ retrospectively reviewed 54 patients who underwent cervical laminectomy and fusion for CSM. Patients were selected if they had clinical signs of myelopathy and cervical lordosis or straight spine with advanced age (>65 years) and significant medical comorbidities. In this study, 81% of patients improved in Nurick grade, and the remaining 19% remained the same. Ten percent required revision surgery, and only 4% (2 patients) had lateral mass screw pull-out. Anderson et al.¹⁵ performed a systematic review of the Cochrane database and the National Libary of Medicine to examine the efficacy of cervical laminectomy and fusion for CSM. Although the evidence is largely class III, the investigators concluded that 70% to 95% of patients show neurologic improvement with this procedure, and overall recovery is approximately 50% of the initial Japanese Orthopaedic Association (JOA) score deficit. Several other studies^16–18 reported good success with cervical laminectomy and fusion. However, patients with preexisting cervical kyphosis were universally excluded from all of these studies.

Although cervical laminectomy with or without lateral mass fusion may be successful in the treatment of CSM in patients with lordotic or slightly straight cervical alignment, these procedures are contraindicated in patients with a kyphotic spine. Because the spinal cord is draped over a kyphotic deformity, it does not shift dorsally after laminectomy. This often results in suboptimal surgical results and progression of neurologic decline. Furthermore, a laminectomy in the presence of kyphosis may worsen the deformity.^13,14,17 Therefore, kyphosis in the setting of CSM is considered to be a contraindication to laminectomy with or without lateral mass fusion.

Facet Osteotomy

Mason and Urist popularized techniques for cervical osteotomy in the 1950s for management of cervical kyphotic deformity in patients with ankylosing spondylitis. Variations of these techniques are also applicable to the management of patients with degenerative fixed cervical kyphotic deformity. A multilevel extended foraminotomy, or facet osteotomy, provides lateral release at multiple levels, thus allowing for correction of the kyphotic deformity and creation of lordosis while sparing an adequate amount of lateral mass for screw placement and fixation.

Preoperatively, CT and/or MRI should be carefully scrutinized to identify aberrant vertebral artery anatomy. After a standard dorsal cervical approach and multilevel laminectomy is performed, foraminotomies are performed bilaterally, exposing the nerve roots from the spinal canal to their entry into the lateral soft tissues. The lateral masses are only partially resected, leaving an adequate amount intact laterally to allow for screw placement. The lateral masses are undercut to avoid compression of the nerve roots by bony remnants during deformity correction. Once the decompression is carried out, lateral mass screws are placed. An assistant then loosens the Mayfield head holder from its attachment to the bed and places the patient’s head in true neck extension and no capital extension. After this maneuver, the Mayfield is reattached to the bed and the screws are secured to the rods. Autograft morselized bone is then placed at the osteotomy sites.

The number and location of levels to be osteotomized varies depending on the patient’s pathology. It is generally inadvisable to end a long construct at the cervicothoracic junction; long constructs that incorporate C7 should be extended to T2 or T3 (Video 222-1).

Cervical Laminoplasty

Cervical laminoplasty is effective for the treatment of CSM. “Open-door” or “canal expansive” cervical laminoplasty was originally developed in Japan for the treatment of multilevel CSM, especially for treatment of ossification of the posterior longitudinal ligament (OPLL).¹⁹ Laminoplasty was initially used as a strategy to avoid the common complication of postlaminectomy kyphosis, the rationale being that the laminae that are left behind will form points of attachment for the cervical musculature during healing, thus retaining the dorsal tension-band and preventing the development of postsurgical kyphotic deformity. The technique relies on two mechanisms for success. The first is a direct dorsal decompressive effect from removal of the laminae and expansion of the cervical spinal canal. The second is an indirect decompressive effect from migration of the cervical spinal cord dorsally away from the ventral compressive structures.²⁰ There are several variations of the laminoplasty (French door, Z-plasty, to name a few).

This technique proves most effective in patients with CSM and intact cervical lordosis or mild spinal straightening, because the cervical cord is able to shift dorsally after laminoplasty in these patients. Several studies have demonstrated the efficacy of this procedure in preventing postsurgical kyphosis and improving neurologic status in properly selected patients.^21–24 However, the presence of cervical kyphosis in conjunction with CSM is widely considered an absolute contraindication to laminoplasty. Nearly all of the major prospective studies examining the efficacy of laminoplasty in the management of CSM excluded patients with presurgical kyphotic deformity. The vast majority of patients with CSM and a kyphotic deformity who undergo laminoplasty develop worsening of their kyphosis and further neurologic deterioration, necessitating a second corrective surgery. Kimura et al.²⁵ examined 29 patients treated with laminoplasty for CSM and found that patients with preexisting kyphotic or S-shaped swan-neck deformity did significantly worse than those with preexisting lordotic alignment. Suda et al.²⁰ determined that the maximum preoperative local kyphosis angle for successful expansive laminoplasty is less than 13 degrees (approximately).

In summary, laminoplasty is associated with a high risk of failure when preoperative cervical kyphosis is present. Hence, the presence of kyphosis should be considered a contraindication to laminoplasty.

Cervical Pedicle-Screw Fixation

The major shortcoming of most dorsal cervical spine procedures in the setting of CSM with kyphosis is the failure to allow the spinal cord to shift dorsally after the procedure, which occurs because most dorsal procedures do not restore lordosis to the spine, and most actually worsen preexisting kyphosis or reduce lordosis. For a dorsal procedure to adequately treat CSM with kyphosis, the procedure must not only decompress the spinal cord but also correct the kyphotic deformity and restore lordosis. Furthermore, such a procedure must provide enough structural integrity to withstand the considerable biomechanical forces in play as the surgeon attempts to reduce cervical kyphosis. Dorsal cervical decompression and pedicle-screw instrumentation is a technique that has been used with success in this situation. It provides a biomechanical advantage over lateral mass screw fixation, in that several biomechanical and clinical studies demonstrate the increased stability of pedicle-screw fixation over lateral mass fixation.^26,27 Although it provides superior biomechanical strength and greater potential for kyphosis reduction, when compared with lateral mass instrumentation, cervical pedicle-screw fixation is also associated with a higher complication rate and is a much more technically difficult procedure.

Cervical pedicle screws provide greater stability and increased pull-out strength when compared with lateral mass screws.^28,29 Hasegawa et al.³⁰ reviewed their series of 58 patients with and without destructive lesions of the cervical spine treated with dorsal decompression and pedicle-screw fixation. They found that postoperative kyphosis was reduced and maintained when compared with laminoplasty alone for management of CSM with kyphotic deformity. However, there was no difference in JOA score improvement between the two groups, and 17.2% of patients experienced serious complications from pedicle-screw placement (including two vertebral artery injuries). Hasegawa concluded that pedicle-screw fixation has a definite application to cervical diseases with kyphosis and loss of stability but no role in the management of “typical” CSM and OPLL (i.e., without kyphosis) or in postlaminectomy kyphosis (for which they recommend ventral decompression and/or a safer dorsal instrumentation).

Abumi et al.³¹ reported their results in 30 patients with cervical kyphosis with a variety of etiologies (trauma, degenerative, rheumatoid, infectious, and postlaminectomy) treated with cervical pedicle-screw fixation. Of the 30 patients, 17 were managed with a dorsal procedure alone, and 13 were managed with a combined ventral and dorsal approach. In the case of rigid kyphosis, a ventral procedure was necessary to provide release for dorsal kyphosis reduction. In both groups, kyphosis improved from an average of 28.4 degrees to 4.4 degrees postoperatively and 5.1 degrees at an average of 42 months follow-up. The combined approach resulted in outright reversal of kyphosis to 0.3 degrees of lordosis postoperatively and 0.5 degrees of kyphosis at an average of 42 months follow-up. Of the 24 patients who had CSM preoperatively, 14 saw an improvement in Frankel grade and 10 remained at the same grade. No patients deteriorated neurologically, although 6.3% of the screws placed were in malposition and two patients required screw revision. Abumi has demonstrated that in patients with kyphosis associated with some degree of segmental motion, pedicle-screw fixation is a reliable and stable construct for kyphosis reduction and stabilization in conjunction with a dorsal decompressive procedure. Abumi also has concluded that cervical kyphosis in the setting of CSM with bony union mandates a combined dorsal-ventral approach.

A major drawback of the cervical pedicle-screw fixation technique is its technical difficulty and associated potentially disastrous complication rate. It is recommended that this procedure should be attempted by surgeons who have a great deal of experience with this technique and only in patients with proper indications. In another study, Abumi et al.³² reported a series of 180 patients who underwent cervical pedicle-screw fixation. A total of 712 screws were placed in these patients, and 669 of these were radiographically evaluated postoperatively. One patient suffered a vertebral artery injury but no neurologic complication from this. Forty-five (6.7%) of the screws placed were found to violate the pedicle; two of these screws caused radiculopathy, and nine of these perforated laterally into the foramen transversarium.

Summary

The pathophysiologic mechanism of CSM with kyphotic deformity is compression of the spinal cord due to a reduced diameter of the spinal canal and draping of the spinal cord over the ventral spinal elements at the apex of the deformity. Progressive myelopathy and cord ischemia results, and early surgery is favorable. The goals of a strict dorsal approach for this disease are twofold: (1) adequate decompression of the spinal cord by widening the spinal canal and (2) correction/stabilization of the kyphotic deformity to allow the spinal cord to settle dorsally, away from the ventral compressive elements. Very few dorsal procedures can accomplish these goals, and therefore cervical kyphosis in the setting of CSM is a contraindication to laminoplasty and laminectomy with or without lateral mass fusion. The only strictly dorsal procedure capable of decompression and reduction of kyphotic deformity is dorsal decompression and pedicle-screw fixation. This procedure relies on retained segmental motion, because ventral bony fusion requires a combined ventral-dorsal approach to provide adequate ventral release for kyphosis reduction. Pedicle screws provide a significant biomechanical advantage over lateral mass screws in reducing and maintaining reduction of a kyphotic deformity. However, the complications from pedicle-screw placement in the cervical spine are potentially disastrous. In summary, dorsal decompression and pedicle-screw fixation is a viable option for management of CSM with kyphotic deformity in a limited number of patients and should be attempted by surgeons who are well familiar with the procedure.

Dorsal versus Ventral versus Combined Approaches

Once a cervical deformity is present, and the decision is made for surgical intervention, three fundamental approaches are possible: dorsal,^6,11 ventral,^12–16 or a combination.^1,6,17,18 A number of surgical procedures using the dorsal approach have been described in the literature, including simple laminectomy, various derivations of laminoplasty, and laminectomy augmented with dorsal fusion (with or without instrumentation).^6,11 The advantages of the dorsal approach include familiarity, ease of decompression of multiple levels, and the ability to extend the fusion and/or fixation rostrally to the occiput and/or caudally to the thoracic spine. The major disadvantage of laminectomy and laminoplasty is the obligatory disruption of the dorsal tension-band, resulting in a high rate of postlaminectomy kyphosis.^5,7 This complication has led some authors to caution against use of these procedures in patients with preexisting cervical kyphosis or even in the relative kyphosis of the “straightened” cervical spine.^2,7,9,19 Moreover, ventral compression cannot be addressed from the dorsal approach. In some cases, the use of dorsal cervical fixation, in combination with dorsal decompression, may be used to correct a mild degenerative kyphosis.²⁰ However, this technique requires the presence of a flexible deformity to facilitate correction of the kyphosis, without the aid of a ventral release.⁶ Needless to say, this situation is a rare occurrence in the degenerative spine. Furthermore, the degree of deformity correction achieved via an isolated dorsal strategy may be limited.⁶

A number of authors have advocated the use of a ventral approach for dealing with cervical myelopathy associated with kyphosis.^12–1416 This approach affords the spine surgeon the ability to both address ventral compression, as well as perform a ventral release via corpectomy or multiple discectomies, prior to the actual correction of the cervical deformity. Moreover, because the majority of cervical kyphotic deformities are idiopathic and the result of prior dorsal procedures, use of a ventral strategy has the added benefit of avoiding much of the morbidity associated with revision surgery.^3–5 Although, this strategy has been shown to be highly effective for patients with short-segment stenosis and kyphosis, historically high rates of pseudarthrosis, bone graft subsidence, and graft dislocation have been seen with long-segment or multisegment constructs.^2,16 These complications are largely a manifestation of the often suboptimal bony fixation sites, the poor mechanical advantage, and the reliance on screw fixation as the only available method of bony fixation afforded by the ventral approach.⁷ Some of these factors may be mitigated by the use of intermediate points of fixation and by the introduction of dynamic implants, although their true effectiveness in this regard has yet to be proven.⁷ A purely ventral approach, much like the purely dorsal approach, neither permits access for dorsal decompression of the spinal cord nor affords the surgeon the ability to recreate the dorsal tension-band in patients who exhibit incompetent dorsal elements.

The use of a combined dorsal and ventral strategy affords all of the aforementioned advantages of each and limits the disadvantages of both approaches used alone. The surgeon is able to perform a 360-degree decompression and thereby address both dorsal and ventral neural compression. Optimal correction of sagittal plane cervical deformities can be achieved by making use of the mechanical advantage of dorsal constructs and osteotomies, in conjunction with ventral releases and reconstruction of the ventral load bearing column via interbody fusion techniques.^7,17,21 This affords the spine surgeon the ability to both lengthen the ventral column and shorten the dorsal column to achieve an optimal correction. Moreover, in the ankylosed spine with a fixed deformity, it facilitates the releases of both dorsal and ventral elements necessary for deformity correction.^17,21 Furthermore, the addition of dorsal fixation to a ventral construct may help further minimize the risk of pseudarthrosis by loading the construct via a compressive moment and provide further translational and torsional resistance.^7,22,23 This may optimize fusion rates via Wolff’s law and may be especially useful in patients with poor bone quality, comorbidities, or prior failed surgeries. Lastly, a combined ventral and dorsal fusion strategy may obviate the need for external orthoses in many cases.^17,22 The chief disadvantage of the combined approach has been the relative morbidity when compared with either purely dorsal or ventral strategies. Overall complication rates as high as 33% have been reported in the literature with circumfrential strategies for correction of cervical kyphosis.^22,23 However, both Schultz et al.²¹ and McAfee et al.¹⁷ have reported acceptably low rates of complications and long-term morbidity (5% and 11%, respectively).

Summary

Kyphotic deformities of the cervical spine may be approached dorsally, ventrally, or via a combined strategy. Although both the dorsal and ventral approaches have many advantages and many patients can be successfully treated by these strategies, a combined approach may be appropriate for the optimal management of only a subset of patients. This subset may include patients with severe multilevel degenerative disease and a fixed deformity, those with both dorsal and ventral neural compression, those with poor bone quality, those with comorbidities that may inhibit bony fusion and wound healing, and those with failed prior attempts at deformity correction.