Ventral Surgery

The ALL and the PLL, as well as the anulus fibrosus, contribute significantly to the stability of the spine.^3–5 The PLL is weaker than the ALL and is often intentionally destroyed during dorsal, ventral, or lateral spine surgery. However, the ALL is often not totally disrupted, even with a wide ventral exposure. A strong and wide ligament, the ALL provides a significant proportion of spinal stability in extension. This function may be considered as a tension band that limits extension. As a result, ventral decompressive spine surgery (e.g., corpectomy), which adequately decompresses the dural sac, generally causes a disruption of the PLL, with preservation of at least a portion of the ALL. The width of the PLL significantly narrows in the middle portions of the vertebral body, thus making it susceptible to surgical disruption. In conjunction with existing bony disruption, surgical decompression usually causes significant instability of the spine. The extent of this destabilization can be assessed via intraoperative manipulation, such as vertebral body distraction. If significant instability is iatrogenically created, an interbody strut graft is necessary, with or without supplementation by instrumentation. The PLL limits flexion and distraction.

Dorsal Surgery

Resection of the interspinous ligaments may lead to instability. Although the interspinous ligaments are relatively weak, their long moment arm (i.e., distance from the instantaneous axis of rotation to the ligament attachment site) provides a mechanical advantage with regard to their function as a tension band.¹ The capsular ligaments are strong. Although they function through a short moment arm, their relative strength allows them to provide a significant stabilizing effect, if they are intact.

Ventral Surgery

Bone destruction and additional surgical bone removal have a significant impact on spinal stability. Both the amount of vertebral body destruction and its location play an important role in the surgical destabilization process (Fig. 153-1). The first issue is the extent of ventral bony destruction. A complete vertebrectomy causes an obvious instability (see Table 153-1). The extent of instability is closely related to the amount of bone removed.

FIGURE 153-1 Resection portions of the ventral vertebral body in most frequently performed ventral spine surgeries. A, Cervical corpectomy—resection of the middle horizontal section of the body. B, Oblique cervical corpectomy, with resection of the lateral and dorsal sections of the vertebral body. C, Ventral thoracic or lumbar surgery. D, Lateral extracavitary surgery. In the latter two approaches, resection of the dorsolateral portion of the body is more extensive in ventral surgery.

(Copyright Division of Neurosurgery, University of New Mexico.)

White and Panjabi⁵ used a three-column model to explain the effects of element disruption on spinal column stability. To determine the effect of a partial vertebral body resection on spinal stability, Benzel¹ used a hypothetical design that divides the vertebral body into 27 equal, small cubes (Fig. 153-2). In this regard, resection of the ventral portion of the vertebral body affects spinal stability more than a corresponding resection of the middle or dorsal portion of the vertebral body (Fig. 153-3), because the largest force to which the spine is subjected is that of flexion. The more ventral portion of the vertebral body is farther from the instantaneous axis of rotation, and it therefore exerts its resistance through a longer moment arm in resisting flexion. Also, resection of the middle horizontal section of the vertebral body affects stability more than does resection in the middle vertical sections (Fig. 153-4).

FIGURE 153-2 A vertebral body seen, for theoretical purposes, as a cube composed of 27 equal, small cubes (3 × 3 × 3). A, Oblique view; B, lateral view.

(From Benzel EC: Biomechanics of spine stabilization: principles and clinical practice, New York, 1995, McGraw-Hill.)

FIGURE 153-3 Resection of the components of the cubic vertebral body. Partial vertebrectomy involving removal of the ventral component in the coronal plane of the vertebral body (A) affects the stability more than resection of the middle (B) or dorsal portion (C) in the coronal plane. Resection of both the middle and the dorsal third of the vertebral body (in the presence of an intact posterior column and an intact ventral third of the vertebral body) may not significantly disrupt spinal integrity (D).

(From Benzel EC: Biomechanics of spine stabilization: principles and clinical practice, New York, 1995, McGraw-Hill.)

FIGURE 153-4 Resections of portions of the cubic vertebral body. A, Resection (or disruption) of the middle axial (horizontal) third of the vertebral body in its sagittal dimension, as may occur after trauma. B, Resection of the middle (vertical) third of the vertebral body.

(From Benzel EC: Biomechanics of spine stabilization: principles and clinical practice, New York, 1995, McGraw-Hill.)

Minimizing bone removal helps decrease postoperative instability. To attain this goal, vertebral body resection in cervical corpectomy should be carefully determined. In this regard, oblique corpectomy is an approach that does not significantly interfere with the stability of the spine.^6,7 This approach protects the ventral portion of the vertebral body but sacrifices the dorsal and lateral aspects (see Fig. 153-1B).

As an aside, the uncovertebral joints add stability during extension, lateral bending, and torsion.⁸ In general, if the (1) ALL, (2) ventral section of the vertebral body, (3) dorsal column integrity, and (4) dorsal column ligaments remain intact, a significant instability does not develop.

Dorsal Surgery

A laminectomy can cause instability because of destabilization of the spine. The frequency of iatrogenic instability is proportional to the width of the laminectomy.^9,10 Often, the extent of the injury is not readily apparent shortly after surgery. The prediction of its subsequent occurrence is even less obvious. If a ventral (vertebral body) lesion already exists, the incidence of postlaminectomy kyphosis is even higher.

Laminectomy often creates distortion of the dura mater and spinal cord, with flexion and distraction over the ventral fulcrum (Fig. 153-5). Even in the absence of the ventral pathology, the disruption of the laminae, facet joints, and dorsal ligamentous complex may result in progressive deformity, the so-called postlaminectomy kyphosis (Fig. 153-6). Postlaminectomy kyphosis occurs more commonly in the more mobile portion of the spine—the cervical spine. Laminoplasty may preserve a portion of the dorsal tension band and thereby diminish the instability observed after laminectomy.¹¹ Another alternative that minimizes the destabilizing effect of laminectomy is the addition of a stabilization strategy such as dorsal fusion or external orthosis.

FIGURE 153-5 Development of postlaminectomy kyphosis. In the case of preexisting ventral pathology, laminectomy (A) can cause severe kyphotic deformity (B). In the cervical spine, even in the absence of ventral pathology, progressive kyphosis may develop (C and D).

FIGURE 153-6 The “clamping down” phenomenon in the cervical spine. If the posterior longitudinal ligament and the anterior longitudinal ligament are intact, the graft firmly fits into the mortises of both end plates (A). If the ligaments are destroyed, their contribution to resisting distraction is lost, and the graft does not fit into the mortises firmly (B).

In summary, there are three important issues that should be addressed during decompressive laminectomy: (1) the presence, or absence, of ventral spinal instability, (2) the extent of resected laminae and facet joints as well as the extent of ligamentous disruption, and (3) the location of the laminectomy (i.e., cervical, thoracic, or lumbar spine). The cervical spine is more prone to instability after laminectomy.

The contribution of the facet joints to dorsal column stability is very important. With axial loading, the anterior and middle columns transmit only 36% of the applied load, whereas each pillar (facet) transmits 32% of the total applied load.¹² Therefore, regardless of the region of the spine involved, excessive facet joint resection can result in instability. In the cervical spine, the tolerable limit of resection is one third to one half of the facet joint.¹⁰ In the lumbar spine, facet resection may often result in glacial instability. However, the value of fusion and instrumentation after partial facetectomies for spinal stenosis is controversial.¹³

The shape and angulation of the facet joints are also important. A ventral translational deformity is more likely to result if vertically oriented joints and a hyperlordotic posture are present. The L4 and L5 facet joints are sagittally oriented, whereas L5-S1 joints are coronally oriented (Fig. 153-7). Therefore, L5-S1 joints resist translational deformity, whereas L4-5 joints can easily glide on a sagittal plane. Degenerative listhesis frequently involves this level.

FIGURE 153-7 Facet joint angles at the L4-5 and L5-S1 levels. A, The L4-5 facet joints are sagittally oriented and do not resist translation. B, The L5-S1 facet joints are coronally oriented and resist translation well.

Trauma Surgery

Because trauma itself causes instability, additional iatrogenic instability caused by a decompression operation may be catastrophic. Therefore, most operations for spine trauma include a stabilization component.

The site of decompression (ventral vs. dorsal) is usually dictated by the type and location of the pathology. Spine stabilization can also be achieved from the same orientation.

Tumor Surgery

An important issue that the spine surgeon cannot avoid is the iatrogenic instability created by radical tumor surgery. Until recently, the surgical treatment of spinal tumors was accomplished predominantly via laminectomy, despite the fact that the neural compression was often ventral to the spinal cord. However, laminectomy was often ineffective. Ventral surgery is often the procedure of choice for treating most bone tumors of the spine.^14–16 Because the pathology in most patients with these bone tumors lies ventral to the spinal canal, attempts at tumor resection can cause loss of ventral spinal integrity.

Tumors involving both ventral and dorsal elements cause even greater instability. For oncologic tumor surgery, extramarginal resection of bone tumors of the spine is desirable. Although the spinal cord and nerve roots do not allow such a resection in many instances, there is an increasing trend toward accomplishing total spondylectomies by only a dorsal approach,^14,15,17,18 or by circumferential surgery.^19,20 Because the iatrogenic destabilization of the spine is so significant in these cases, radical measures to reconstruct and stabilize the spine are mandatory.

Preoperatively, the extent of tumor spread is the main determinant of stability. Instability is often not the sole reason for operation. The extent of neurologic deficit, the biology of the tumor, its radiosensitivity, and its sensitivity to chemotherapy are also important reasons. In selected cases, radiotherapy of a radiosensitive tumor and external bracing may be suitable.

Degenerative Spine Surgery

Cervical spondylotic myelopathy is often treated via a decompressive operation. Ventral and dorsal operations both may cause significant iatrogenic spine destabilization. This is similarly true for the thoracic and lumbar spine.

Ventral decompressive surgery of the cervical spine causes bony destruction and also some form of ligamentous disruption. The PLL is intentionally removed during ventral osteophyte excision for cervical spondylotic myelopathy. In this case, interbody strut graft stability, or “clamp down,” a phenomenon that would have been provided by an intact ligament, is not realized.

Ventral cervical plates can help resist the distractive forces caused by the disruption of ligamentous resistance.

The anulus fibrosus contributes to the spinal stability in a similar manner on the PLL and ALL. Its disruption in degenerative conditions may cause transitional instability.

Adjacent-Level Spondylosis

It is common to observe degenerative changes above or below the level of a multilevel fusion. This type of instability is obviously iatrogenic. To avoid this, the use of more flexible (dynamic) fixation devices has been proposed.¹ Short-segment fixation and fusion may minimize the incidence of this complication. Another technique for dealing with this problem is to create a “transitional” level by using instrumentation that is less rigid at the first segment adjacent to the fused segments. For example, an L3 burst fracture might be treated by the use of rigid screw instrumentation, along with bony fusion from L2 to L4. The “transitional” segment might then be created by the use of laminar hooks (hooks are less rigid than screws) at L1.

Segmental fixation with the use of pedicle fixation systems can be very rigid. Although increasing rigidity may improve fusion rates, it also increases the rate of degeneration of adjacent segments.^21,22 Rahm and Hall²² have reported an incidence of adjacent-segment degeneration of 35% in cases with lumbar fusion and internal fixation. They also noted that the degeneration was associated with increasing patient age, use of interbody fusion, and worsening of clinical results with time.

Lumbar spine fusion remains a pillar of degenerative lumbar spine surgical procedures. However, lumbar spine fusion surgery may increase the loads on the functional lumbar segments and cause advanced disc degeneration, facet joint arthritis, and an increase in intradiscal pressures in adjacent segments.

“Adjacent-level degeneration” and “adjacent-level disease” (both adjacent-segment degeneration [ASD]) are different entities. In a study by Sugawara et al.,²³ asymptomatic adjacent-disc degeneration was detected in 50% of the patients by their measurement methods. However, symptomatic adjacent-disc degeneration occurred in 5% of the patients, and only 2% required additional surgery. ASD may also cause a worsening of the clinical outcomes,²⁴ especially with the ASD after multiple-segment fusion. The most sensitive technique to evaluate ASD is MRI.²⁵

Postlaminectomy Instability

Although a dorsal approach is convenient and appropriate for most spine lesions, it causes a significant defect in the structural integrity of the posterior column as well as a loss of the dorsal tension band.

After cervical laminectomy in children, the incidence of kyphosis is very high.^41,42 The important factors that affect cervical instability after laminectomy are (1) patient’s age, (2) number of laminae excised, (3) curvature of the cervical spine, and (4) degree of facet joint violation.⁴¹ Although the relative incidence of instability after cervical laminectomy is controversial, there is a tendency to not perform multilevel laminectomies in pediatric patients and to provide additional stabilization measures in patients with cervical spondylotic myelopathy undergoing laminectomy.^6,41 In patients with cervical spondylotic myelopathy, these measures may either be a laminoplasty or fixation (e.g., lateral mass plating) and fusion after laminectomy.

Extensive lumbar laminectomy is often necessary for patients with spinal stenosis. Because of the nature of the compression, this procedure often includes a partial facetectomy. Although the exact incidence of instability after extensive lumbar laminectomy for lumbar stenosis is not well known, some surgeons have suggested the use of bilateral hemilaminectomies and partial facetectomies, without destruction of interspinous ligaments, while preserving the majority of the laminal and spinous process. Performing a fusion, with or without instrumentation, is controversial, however.^1,43,44

Bone Graft Harvesting

Instability related to bone graft harvesting is a rare but important problem. It was first reported in 1962 by Lichtblau,⁴⁵ who observed that dorsal iliac crest graft harvesting can cause dislocation of the sacroiliac joint.

Cervical Spine

Iatrogenic destabilization is most often caused by multilevel decompressions for spondylosis. This may be via a multilevel corpectomy or discectomy, or via laminectomy. Multilevel corpectomy or discectomy is usually accompanied by fusion. Therefore, stability is addressed by the fusion. A multilevel central corpectomy injures or destroys the ALL and the PLL, and it also affects bone integrity. In this circumstance, the destabilizing effect results from destruction of ligaments and bone.

Thoracic and Lumbar Spine

Because of the widespread use of ventral surgery for spinal tumors, more reconstruction materials and stabilization problems are discussed. In general, if the lesion is strictly ventral, and the operation has destroyed only the anterior column and part of the middle column, ventral reconstruction and stabilization would be adequate. For single- or two-level lesions in the upper and middle thoracic spine, only a ventral reconstruction can be used. However, for lesions below level T10, ventral instrumentation, in addition to reconstruction, may be necessary. If, however, two or three columns are involved in lesions below level T10, supplemental dorsal instrumentation may be used after ventral reconstruction and ventral stabilization.⁴⁶

Some surgeons perform en bloc resections of the dorsal and ventral elements of the spine from a dorsal route.⁴⁴ Tomita et al.¹⁷ have described this approach as “total en bloc spondylectomy.” From an oncologic point of view, this method is superior to traditional spinal tumor surgery, curettage, and intralesional removal. The investigators have used a reconstruction material and additional dorsal fixation with instrumentation. It can be used in one stage and one position only. However, its disadvantage is the risk of spinal cord ischemia, resulting from bilateral occlusion of the segmental arteries.

Limiting the extent of dorsal element removal or disruption during laminectomy can minimize spine destabilization by limiting the disruption of the dorsal tension band (e.g., by performing multilevel laminotomies instead of laminectomy). Laminotomy, theoretically, restricts the amount of important bone and soft tissue removed. However, laminectomy provides a trajectory advantage that most likely outweighs the laminotomy advantage (Fig. 153-8).

FIGURE 153-8 A, Trajectory to the lateral recess achieved via a laminotomy. Note that the Kerrison punch cannot adequately decompress the lateral recess without significant bone removal. B, Trajectory to the lateral recess achieved via a laminectomy. Note that a more optimal trajectory is achieved. An undermining of the lateral recess and foramina may thus be achieved without significant facet disruption.

(Copyright Division of Neurosurgery, University of New Mexico.)

Lumbosacral Spine

The sacrum is a transitory structure between the vertebral column and pelvis. The sacroiliac joint is the immobile joint that functions as a shock absorber for the spine. The pelvis itself transfers loads from the sacroiliac joints to the hip joints, with the help of strong ligamentous structures. White and Panjabi examined the weight-bearing capacity of the sacroiliac joint. They found that, despite the lower half of the S1 vertebral body and half of the sacroiliac joint being resected, only 50% of weight-bearing capacity was lost. This, in fact, is adequate for ambulation, and there is no need for stabilization in most cases (Table 153-2).⁵ If the sacral lesion is a primary tumor, and complete tumor removal with extra marginal resection is desired, combined ventral and dorsal approaches, with a significant amount of bone and ligamentous tissue destruction, is necessary. In this case, an adequate reconstruction is required.^18,47–49

TABLE 153-2 Resection Levels and the Need for Reconstruction in Sacral Tumor Surgery

The key point for reconstruction is whether the sacroiliac joints are involved.^50,51 If the tumor is not large and the sacroiliac joints are preserved, stability is not lost. However, involvement of one or both joints dictates the need for internal fixation and fusion. Kostuik and Esses¹⁵ suggest surgical reconstruction if 50% or more of the sacroiliac joint is removed.

If the sacrum above the S2 segment is resected with some of the ilium, the L5 vertebral body drops into the pelvis.^18,52 In this case, an implant connecting the lumbar spine to both sides of the ilium is necessary. Three alternatives for the implant procedure are described in Box 153-1. Because the defect is so massive, the patient’s own bones do not allow an autograft fusion, and allograft bone is usually used. The forces from the lumbar spine are distributed to both lower extremities via the sacrum and pelvis. This complicates the lumbopelvic instrumentation decision-making process.^53,54

Because the sacrum is a transitory structure between the vertebral column and the pelvis and lower extremities, adding the sacrum into the long segment fixations for deformity correction is also an important point.^9,55 Achieving strong lumbosacral fixation and fusion poses unique problems because of the shape of the sacrum and the presence of a large amount of cancellous bone. For this reason, different adaptations, such as the use of larger-diameter screws, application of multiple screws, and insertion of the rod into the iliac cristae after significant bending (Galveston technique), have been recommended (Table 153-3).^3,9,55,56

TABLE 153-3 Methods of Sacropelvic Fixation and Reconstruction

The modified Galveston L-rod technique is a reasonable reconstructive method after total sacrectomy.^53,57 It prevents caudal migration and axial rotation of the spinal column. Adding a threaded transiliac rod can prevent the open-book phenomenon and provide stability around the horizontal axis of the spinal column; at the same time, it also prevents rotation around this axis.