DEFINITION

Lumbar disc degeneration is an age-related process heralded by a loss of disc height and gradual changes to the biochemical structure and biomechanical behavior of the intervertebral disc.

Disc degeneration is not painful in most individuals, but in some patients, the degenerative changes do become painful and lead to the clinical entity known as degenerative disc disease (DDD). It is unclear why disc degeneration is painful in some but not in most.

The etiology of DDD is multifactorial, including genetic and environmental determinants.

Discogenic pain is the term used to describe pain due to a degenerative disc.

 

 

ANATOMY

 

The intervertebral disc is composed of the outer annulus fibrosus and the inner nucleus pulposus (FIG 1A).

 

The vertebral endplate is composed of cancellous bone in the center and strong, dense, cortical bone along the periphery.

 

Magnetic resonance imaging (MRI) provides information about the extent of hydration within the disc nucleus. The degenerated disc nucleus will have low signal characteristics (appear dark) on T2-weighted MRI images (FIG 1B).

 

 

Dark discs on MRI do not necessarily correlate with symptomatic low back pain.2

 

 

 

FIG 1 • A. The intervertebral disc is composed of the outer annulus fibrosus (radial orientation of collagen fibers) and the inner nucleus pulposus (relatively higher water content and proteoglycans). The cancellous

center of the lumbar vertebral body is surrounded by a peripheral rim of relatively strong cortical bone. B. T2-weighted sagittal MRI showing DDD at the L4-L5 disc space. The nucleus pulposus is low signal intensity (dark) compared to the adjacent discs, which are high signal intensity (bright) due to relatively higher water concentration. The vertebral body endplates are irregular, with anterior vertebral osteophytes.

 

 

Lumbar vertebral bodies are ovoid shaped with a rounded circumference on axial cross-section, except for L5 which is more triangular with a slightly wider base posteriorly bordering the central canal and flattened sides leading to an anterior apex. Care should be taken with transbody screw placement at this level.

 

PATHOGENESIS

 

Various mechanisms have been proposed to explain disc degeneration with age:

 

 

 

Reduced nutrition and waste transport Decreased concentration of viable cells

 

 

Loss of matrix proteins, proteoglycans, and water Degradative enzyme activity

 

 

Fatigue failure of the matrix Herniated nucleus pulposus

 

Subclinical, indolent disc space infection

 

 

61

 

Alterations to the vertebral endplate microenvironment such as venous pooling and reduced oxygen tension are additional factors.

 

 

Nicotine has known detrimental effects on the intervertebral disc, perhaps via these mechanisms.

 

Several factors have been implicated in the generation of discogenic pain: altered disc structure and function, release of inflammatory cytokines, and nerve ingrowth into degenerated discs, which under normal conditions are only minimally innervated in the outermost portion of the annulus.

 

NATURAL HISTORY

 

Radiographic findings of disc degeneration typically appear around age 30 years.

 

Posttraumatic disc herniations, vertebral endplate injuries, and genetic factors may predispose patients to earlier presentation.

 

As structural changes occur within the intervertebral disc, associated changes in the vertebral body endplate become apparent:

 

 

Anterior, lateral, or posterior osteophyte formation

 

 

Schmorl nodes and cystic cavities along the endplate can be visualized. Endplate sclerosis

 

The degenerative changes at the level of the disc, bony endplate, and ultimately the posterior facet joint complex restrict motion at the affected level or levels. At this stage, patients will typically complain more of back stiffness and soreness rather than pain. Neurogenic claudication due to narrowing of the spinal canal and spinal stenosis typically becomes more limiting than complaints of back pain.

 

The final stage in the natural history of disc degeneration is autofusion.

 

Patients should be counseled that disc degeneration itself is an inevitable process of aging and that any back pain experienced could, but may not necessarily, be associated with the disc degeneration.

 

The overwhelming majority of patients have only occasional episodes of low back pain. Long-term disability resulting from DDD is rare.

 

PATIENT HISTORY AND PHYSICAL FINDINGS

 

No pathognomonic history or physical examination findings exist for the diagnosis of lumbar DDD.

 

Discogenic back pain is typically worst in situations in which an axial load is applied to the lumbar spine, as in prolonged sitting or standing with a forward-bent posture (ie, washing dishes, vacuuming, shaving, or brushing teeth).

 

Conversely, positions such as side-lying (ie, the fetal position) or floating erect in water place the least amount of strain across the intervertebral disc and should therefore provide some pain relief.

 

Leg pain (in the absence of neural compression), if present, is nonradicular and “referred” in that it does not follow lumbar dermatomes into the lower leg and is not typically associated with loss of motor power, reflex changes, numbness, or tingling.

 

Patients will occasionally describe a discrete traumatic disc injury in which they first experienced back pain. Imaging studies that depict an old endplate fracture above or below a degenerative disc help corroborate this history.

 

Loss of truncal musculature fitness from abdominal wall hernias, obesity, and prior abdominal wall surgery (ie, rectus muscle transfer procedures) may worsen discogenic back pain.

 

Other causes of back pain should be sought in the history, physical examination, and imaging studies, including muscular strain, spondylolysis or spondylolisthesis, herniated nucleus pulposus, compression fracture, pseudarthrosis, tumor, and discitis.

 

Patients with isolated DDD by definition should have a normal neurologic examination.

 

IMAGING AND OTHER DIAGNOSTIC STUDIES

 

Standing plain radiographs

 

 

Lateral radiographs allow for measurement of the intervertebral disc height and allow comparison to other lumbar intervertebral discs (FIG 2A).

 

Anteroposterior (AP) radiographs allow for determination of asymmetric, coronal plain disc degeneration, which may be a precursor to lumbar degenerative scoliosis.

 

Flexion-extension radiographs may be helpful in diagnosing an occult spondylolisthesis or spondylolysis.

 

MRI provides excellent visualization of the discs, the degree to which they have degenerated, and the relationship of the discs to the adjacent endplate and surrounding neurologic structures (FIG 2B).

 

 

“Modic changes” characterize the endplate on MRI:

 

 

Type 0: no changes

 

Type 1: dark on T1 and bright on T2 (represent marrow edema and inflammation)

 

Type 2: bright on T1 and isointense/bright on T2 (represent conversion from normal red into yellow marrow).

 

Type 3: dark on T1 and T2 (subchondral sclerosis)

 

Modic changes do not always follow DDD but are rare in healthy individuals. They may represent a shifting biomechanical stress distribution across the endplate.13

 

Provocative discography attempts to reproduce the patient's typical back pain by pressurizing the disc with normal saline. The patient needs to be awake to provide subjective feedback as to the quality and intensity of the pain. Architectural changes to the disc are inferred by contrast administered with the saline.

 

 

Studies have shown that provocative discography leads to accelerated disc degeneration.5

 

Computed tomography (CT) discography provides more detailed information about the disc morphology after contrast administration (FIG 2C).

 

Normal laboratory tests, including complete blood count, erythrocyte sedimentation rate, and C-reactive protein, can help rule out a disc space infection; severe disc degeneration can sometimes mimic infection radiologically.

 

DIFFERENTIAL DIAGNOSIS

DDD

Discitis

Pyogenic vertebral osteomyelitis

62

 

FIG 2 • A. Lateral radiograph showing DDD at the L2-L3 level. B. Sagittal T2-weighted MRI of the same patient with low signal intensity in the nucleus of the L2-L3 disc. Anterior and posterior disc bulges are present. C. Sagittal CT discogram of the same patient showing dramatic loss of integrity of the L2-L3 nucleus and annulus with leakage of contrast anteriorly. The patient's pain was concordant at the L2-L3 disc level. The L1-L2 and L3-L4 discs served as negative controls with regard to both disc architecture and pain.

 

 

 

 

NONOPERATIVE MANAGEMENT

 

DDD is analogous to hip and knee osteoarthritis in that the intervening cartilage (in the case of the disc:

collagen, water, and proteoglycans) fails under compressive loads.

 

Weight reduction and activity modification (avoidance of exacerbating activities) may be effective first-line treatments.

 

 

Nonsteroidal anti-inflammatory medications Acupuncture or massage therapy

 

 

Physical therapy with aquatic or dry land exercises Gentle pelvic traction

 

 

Methylprednisolone (Solu Medrol) taper Epidural injections

 

Narcotic medications for severe episodes of pain

 

SURGICAL MANAGEMENT

 

Indications

 

 

Discogenic back pain refractory to nonoperative management

 

 

Discitis with pyogenic vertebral osteomyelitis refractory to nonoperative management Spinal deformity requiring radical discectomy

 

Revision cases for pseudarthrosis

 

A thorough and complete discectomy improves the effectiveness of anterior interbody fusion by creating a wide surface area of exposed bone.

 

Interbody reconstruction and fusion can be accomplished by a variety of methods, including structural autogenous bone graft (iliac crest or fibula), structural allograft (ie, femoral or humeral ring, femoral head, machined bone dowel), or synthetic device (titanium, polyetheretherketone [PEEK], carbon fiber, composite) packed with cancellous bone or collagen sponges impregnated with bone morphogenic protein 2 (BMP-2).

 

Lumbar total disc replacement (LTDR) was initially adopted in Europe for preservation of segmental motion and avoidance of adjacent-level disc disease; however, prospective randomized investigational device exemption (IDE) trials in the United States failed to reveal superiority of LTDR when compared to anterior fusion. This information plus the rare but catastrophic complications associated with LTDR migrations have precluded widespread use in the United States.

 

Regardless of the method used, prerequisites are that the interbody spacer be strong enough to resist intervertebral compressive loads and provide an appropriate biologic environment for healing.

 

Preoperative Planning

 

Plain radiographs, MRI, or CT scans should be carefully evaluated for undiagnosed spondylolysis or spondylolisthesis, which may alter the surgical plan.

 

Templates can be used with plain radiographs or MRI scans to gauge the size of the final implant to be used.

 

 

Oversized implants can lead to undesired stretch on neurologic structures and reduced motion of lumbar disc replacements.

 

Screw trajectories in stand-alone anterior lumbar interbody fusion (ALIF) devices must be templated to avoid violation of the central canal or neuroforamina.

 

The level of the confluence of the common iliac veins into the inferior vena cava and the bifurcation of the aorta can be located on the axial MRI scans.

 

At L5-S1, the pubic symphysis occasionally precludes appropriate visualization and instrumentation of the disc space in patients with a deep-seated L5-S1 relative to the pelvis. Evaluation of the lateral radiograph with the pubis on the film is critical to visualize the trajectory into the disc space and avoid this miscalculation.

 

Positioning

 

See Chapter SP-35.

 

The patient is placed over an inflatable pillow over a 1-inch thick foam pad, which is placed on the mattress of the operating table. The pillow allows for modulation of lordosis throughout the procedure and the foam pad props the patient up, allowing the arms to be tucked posteriorly out of the plane of the spine during imaging.

 

 

63

 

Positioning over the break in the table allows for increased lordosis if needed.

 

The use of fluoroscopic C-arm imaging is crucial for appropriate patient and implant positioning. It is helpful to verify that adequate fluoroscopic imaging of operative landmarks can be achieved after the patient is positioned but before the incision is made.

 

Approach

 

See Chapter SP-35 on Anterior Lumbar Approach.

 

 

Anterior retroperitoneal approaches will typically allow access to the lumbar discs from L2 to L3 to the sacrum. The renal vessels limit more proximal extension of the exposure.

 

 

Lateral exposures to the lumbar spine are required for access to the L2 vertebra and above.

 

TECHNIQUES

• Anterior Lumbar Radical Discectomy

Exposure

Identify the intervertebral disc and mark the midline with a spinal needle or screw placed into the vertebral body (we prefer not to place a needle into the disc space because this may create unwanted disc injury) (TECH FIG 1A).

Use AP and lateral fluoroscopic imaging to check the midline. The midline marker also serves to verify the spinal level.

At L5-S1, retract the left common iliac artery and vein to the patient's left and the right common iliac artery and vein to the right. At levels above L5-S1, the aorta and inferior vena cava must be mobilized to the patient's right.

The great vessels can be held in their retracted position using handheld Hohmann retractors, custom-designed pins, or Kwires, all of which can be advanced directly into the vertebral bodies (virtually eliminating the risk of vessel migration into the field of interest) (TECH FIG 1B).

Alternatively, stainless steel vein retractors or radiolucent retractors can be fixed to the arms of an abdominal retractor system (Omni) or floating, Endo ring-type retractor system. These blade retractors have the disadvantage of allowing vessel migration into the field by sliding under the retractor blades as motion occurs during the procedure. The advantage of the radiolucent retractors is that better visualization of the operative field is possible with fluoroscopy. In addition, blade-type retractors can be

easily manipulated during the procedure without having to reinsert into the vertebral body.

 

 

 

TECH FIG 1 • A. Lateral radiograph showing the spinal needle inserted into the L4 vertebral body above the L4-L5 disc to be removed. B. Lateral radiograph showing sharp Hohmann retractors placed into the L4 vertebral body above and L5 vertebral body below. Blade-type retractors can be left in place lateral to the Hohmann retractors for additional visibility, as shown.

 

 

Attempt to retract the vessels as far lateral as you can to allow for the widest possible view of the intervertebral disc. Poor visualization at this stage will compromise the quality of the discectomy and any ensuing interbody device placement.

Removing the Disc

 

Using a no. 10 blade on a long handle, incise the intervertebral disc starting laterally along the superior endplate and move toward midline. Always move away from the vessels to avoid an accidental lateral plunge into the great vessels. The blade should be inserted between the cartilage endplate and bone if possible, and we use both hands on the knife shank for optimal control and coordination (TECH FIG 2A,B).

 

A large, sharp Cobb elevator is then used to release as much of the cartilaginous endplate as possible from the superior and inferior endplates. By angling the Cobb blade toward the bone and pronating and supinating the hand, almost the entire disc (annulus and nucleus) can be removed, as if peeling an orange in one large piece (TECH FIG 2C).

 

Long-handled no. 2 and no. 3 Cobb curettes are used to remove the remaining disc, taking the dissection all the way to the posterior longitudinal ligament (TECH FIG 2D). Systematic removal of endplate cartilage enhances thorough removal. Thus, start anteriorly on the superior endplate and move posteriorly. Then start anteriorly on the inferior endplate and move posteriorly.

 

The curette will function much more effectively if it is used as a cutting instrument rather than a scraper. For this reason, we prefer that curettes be sharp, nonangled, and used with a pronating-supinating motion with the edge of the curette between the cartilage endplate and the endplate bone.

 

64

 

 

 

TECH FIG 2 • A,B. Direction of movement of the surgical blade. At L5-S1, the surgical field is within the bifurcation of the great vessels, so the surgical knife should always be directed toward the midline and inferiorly—away from vascular structures. At L4-L5 and above, the vascular structures are retracted to the patient's right, and therefore movements with the knife blade are directed to the patient's left and inferiorly. C. A large Cobb is used between the disc cartilage and the vertebral body to remove as much as possible in one large piece. D. Lateral radiograph showing a no. 2 Cobb curette used to remove the cartilaginous disc endplate. E. Lateral radiograph demonstrating a lamina spreader creating distraction within the disc space. The distractor enhances visualization of the posterior portion of the disc space.

Care should be taken to make sure that the distractor is seated anteriorly and laterally on strong endplate bone to avoid damage to the central cancellous region. F. Lateral radiograph showing the use of a 4-mm long Kerrison rongeur to decompress the neural foramen.

 

 

65

 

The posterior longitudinal ligament is not routinely removed, but the posterolateral corners of the disc space must be thoroughly débrided of disc material for several reasons:

 

Periphery of the endplate is the strongest bone and therefore provides the most stable support of an interbody device.

 

Disc material that is left over can be pushed posteriorly into the epidural space, causing an iatrogenic disc herniation during implant insertion.

 

If anterior decompression of the neural foramen is one of the goals of surgery, visualization and removal of a herniated disc or disc-osteophyte complex will not be possible without proper visualization in this region.

 

The lateral extent of the discectomy is determined by the width of the device to be inserted, but care must be taken to maintain the width of the discectomy posteriorly as the natural tendency is to remove less disc laterally in the posterior portion of the disc space.

 

A lamina spreader can be gently distracted in the anterolateral interbody region to gain enhanced visibility of the posterior disc space (TECH FIG 2E).

 

Removal of a posterior or foraminal disc herniation can be accomplished by passing an angled Kerrison rongeur posteriorly and into the neural foramen. Identification of the ventral aspect of the dura enhances the safety of this maneuver (TECH FIG 2F).

 

Epidural bleeding can be brisk during posterior disc removal, but thrombin-soaked Gelfoam gauze and removal of intervertebral distraction can be used to control it.

Anterior Lumbar Interbody Fusion

Threaded Devices

 

Once the discectomy has been completed, disc space distractors are inserted to gauge the size of the final implant (TECH FIG 3A). Appropriate distractor size can be gauged by comparing the operative level with a normal disc above or below. In addition, the interface between the distractor and the bony endplate should be less than 1 mm. This ensures good interference fit of the final device.

 

For threaded devices such as the lumbar tapered (LT) cage, a cannulated guide channel is inserted over the disc distractors. This working channel serves to prevent inadvertent migration of the great vessels into the disc space.

 

Endplate reamers are then inserted to appropriate depth as determined by lateral fluoroscopic imaging (TECH FIG 3B). Care should be taken to aim the reamer for the midportion of the disc space posteriorly on lateral fluoroscopy rather than through one endplate or the other.

 

Asymmetric reaming will result in excessive removal of one endplate compared to another and the final implant will be more likely to fail in subsidence. Because the reamer tends to follow the path of least resistance, an exceptionally sclerotic endplate will predispose one to asymmetric reaming by this mechanism.

 

Alternatively, an “exact-fit” approach may be selected in which the endplate is left intact. However, proper sizing of the implant may be difficult, and the intact endplate does not provide an optimal vascular environment for fusion.

 

 

 

TECH FIG 3 • A. Lateral radiograph showing a radiopaque disc distractor within the intervertebral disc. The distractor approximates the height of the disc space above (L3-L4) and there is at most 1 mm of space between the intervertebral endplate and the distractor. B. Lateral radiograph showing reaming of the intervertebral channel for the anterior interbody device. Because the vertebral body is shallower in the AP plane away from the midline, reaming should stop shy of the posterior vertebral body line, as shown. C. Lateral radiograph showing threaded cage entry into the disc space. The cage is directed parallel to the vertebral endplates. (continued)

 

 

Final threaded implants are then screwed into the appropriate depth and orientation (TECH FIG 3C,D). The first cage (in a dualcage system) is inserted in the same trajectory as the reamers, and lateral fluoroscopic imaging during cage placement ensures that the cage is not placed too anteriorly or posteriorly. The cage should not be inserted beyond the depth of the reamer or else

 

66

the threads will strip and the cage will lose a large percentage of its fixation strength.

 

Saving the C-arm image of the final reamer depth allows the surgeon to reference this image when inserting the cage.

 

 

 

TECH FIG 3 • (continued) D. Final cage placement should not extend beyond the depth of the reamer. E. Lateral radiograph showing final cage placement. The overlapping pedicles confirm true lateral positioning. F. AP radiograph showing parallel positioning of paired cages.

 

 

The second cage is inserted using the first cage as a reference for trajectory and depth. Final images should be true AP and lateral projections showing the cage devices to be in good position. Overlapping pedicles on the lateral image will appear sharp, confirming true lateral positioning (TECH FIG 3E,F).

 

Cages should be positioned within the footprint of the endplate as near to the periphery as possible to dissipate compressive force over a wider area. Cages placed anteriorly may have a biomechanical

advantage through increased compressive loading.6

Nonthreaded, Stand-Alone Anterior Lumbar Interbody Fusion Devices

 

A subtotal discectomy, whereby the lateral most aspects of the annulus are preserved, is advised to provide increased stability via the remaining annulus. A trial implant can serve as a guide for optimal disc resection.

 

The interspace can be distracted, and then the entire cartilage endplate region covered by the trial implant is removed on both vertebral bodies.

 

 

 

TECH FIG 4 • AP (A) and lateral (B) fluoroscopic images showing midline positioning and screw trajectories of stand-alone ALIF device.

 

 

Typically, a single interbody cage that spans the disc space is selected, with graft material packed on both sides within the implant. This is positioned in the center of the interspace using product-specific instruments (TECH FIG 4A).

 

Locking screws are directed cephalad and caudad either through the cage device or through a metal faceplate that attaches to the cage (TECH FIG 4B). Knowledge of productspecific screw trajectories and starting points is paramount as they may be either symmetric about the midline or translated left or right to allow safe passage of the drill between the iliac vessels.

Adjunct Treatments

 

Autograft material or a BMP-2-impregnated collagen sponge may be used to fill the graft. BMP-2 has been approved by the U.S. Food and Drug Administration (FDA) for single-level ALIF in the titanium LT

interbody devices (Medtronic Sofamar Danek, Memphis, TN).4

 

Concomitant posterior fusion increases construct stiffness and is indicated in the setting of instability. Transpedicular fixation theoretically reduces the force transmitted through the ALIF cage and provides immediate stability. Facet screw fixation provides less stability but reduces stiffness to permit more force

 

anteriorly. Scant evidence exists to differentiate these options based on outcomes.8

 

• Lumbar Total Disc Replacement

  67

   

  Under fluoroscopic guidance, determine the midline on a true AP image of the vertebral body above or

  below the disc (TECH FIG 5A). A bone screw may be inserted as a reference.

   

  Sizing guides are trialed to fill the entire footprint of the endplate (TECH FIG 5B). Height and lordosis are then set using trial wedges (TECH FIG 5C-E) and should match the resected gap and preoperative templating.

   

   

   

  TECH FIG 5 • A. True AP fluoroscopic image. The distance between the midpoint of the vertebra and the pedicles should be the same. The cortical margins of the pedicles themselves should be the same size (ensuring the spine is not rotated). Finally, the spinous processes should bisect the vertebra. The spinous processes are the least reliable landmark as they can be malformed, especially at L5 and S1. B. A sizing guide, or “lollipop,” demonstrates how well the endplate will be covered by the final implant. The largest size that allows good peripheral endplate coverage in both the sagittal and coronal planes is desired. C-E. Using radiolucent trial wedges of varying height and lordosis allows the final device to be individualized to the patient's anatomy. F. Introduction of the channel cutter into the disc space. G. Lateral fluoroscopic image showing implant insertion. The insertion instruments are still connected, which allows for fine adjustment to the final positioning. H,I. Lateral and AP fluoroscopic images of the final TDR placement with all of the instruments removed. The final implant should be in the center of the vertebral

  body on the AP image and in the center (sagittal midline) or just posterior to the center of the vertebral body on the lateral image. (B-E: Courtesy of DePuy Spine, Raynham, MA.)

   

   

  An implant-specific chisel is then directed straight posteriorly through the bodies to cut a groove for the keel or teeth that align the implant and prevent rotation (TECH FIG 5F) and the final implant inserted (TECH FIG 5G-I).

   

  68

• Anterior Lumbar Corpectomy

Vertebra Removal

 

The indications for anterior corpectomy in the lumbar spine are lumbar burst fracture, catastrophic failure of lumbar disc replacement or interbody device (ie, vertebral fracture), lumbar vertebral osteomyelitis, correction of kyphosis, and vertebral body malignancy.

 

In cases of corpectomy for vascular tumors, preoperative embolization should be performed (TECH FIG 6A).

 

In cases of corpectomy, radical discectomies are performed above and below the vertebral body to be removed (see discectomy technique discussed earlier).

 

This enables the surgeon to become oriented to the midline and also to judge the depth and width of the corpectomy to be performed.

 

The discectomy space also allows the surgeon to use a large rongeur efficiently to remove the vertebral body (TECH FIG 6B).

 

Retractors should be placed above and below the entire vertebra to be removed so there is an unobstructed view for the surgeon and the assistants. The vertebral body bleeds more rapidly than the endplates, so the assistants need to be able to visualize the operative field to suction effectively.

 

A Leksell rongeur can be used to remove all of the vertebral body back to the level of the posterior cortex. If this needs to be removed, angled curettes are used to develop the plane behind the vertebra, starting at the disc space. Kerrison punches or angled curettes are then used to lift the posterior cortex off the ventral dura.

 

Healthy vertebral body bone should be saved for interbody fusion.

 

 

 

TECH FIG 6 • A. Preembolization angiogram depicting the aortic bifurcation in a 65-year-old patient with metastatic renal cell carcinoma to the L4 vertebra. Note the degree of vascularity of the L4 vertebral body.

B. Postembolization angiogram depicting a striking reduction in contrast entering the L4 vertebral body. Small embolization coils are seen in the vascular network surrounding the vertebral body. C. Anterior discectomy enables the surgeon to use a large rongeur to gain access to the edge of the vertebra and thereby remove the vertebral body bone.

Filling the Interbody Space

 

Once the corpectomy is completed, bone graft or an interbody device is contoured to fit into the defect. The wooden end of a cotton-tipped applicator can be cut to the length of the defect and can then be used as a size gauge for the final interbody device. This is particularly useful when cutting and contouring a bone graft because calipers and rulers do not always fit easily into the central portion of the corpectomy defect to give an accurate height measurement.

 

Check the height of the corpectomy defect with the wooden applicator throughout its entire depth from anterior to posterior. Keep in mind that the shape of the corpectomy site may be lordotic, and thus the bone graft or implant needs to be fashioned appropriately.

 

Autogenous tricortical iliac crest and autogenous fibula have the greatest healing potential but are also associated with significant harvest site morbidity.

 

Metal cages generally are the easiest to fashion to fit the corpectomy space and can be packed with morselized corpectomy bone (TECH FIG 7A). The disadvantages are their expense and relatively reduced surface area at the endplate for fusion compared to bone.

 

69

 

 

 

TECH FIG 7 • A,B. AP and lateral postoperative radiographs of a patient in whom posterior element resection followed by fusion and instrumentation with pedicle screws was performed as a first stage followed by complete anterior corpectomy and reconstruction with a cylindrical titanium mesh cage packed with autogenous bone graft. An anterior side plate was applied as the lateral vertebral body wall was completely removed. C. The corpectomy strut device should fit snugly against the cut edge of the vertebral body to promote side-to-side fusion from host bone to strut graft. D. Intraoperative image of anterior allograft reconstruction after corpectomy, irrigation, and débridement of the L3 vertebra in a 62-year-old man with L3 vertebral body destruction from pyogenic vertebral osteomyelitis. 4.5-mm cortical screws with washers are used to prevent allograft kickout. E,F. PEEK polymer and titanium expandable cages should fill the interspace snugly and are useful for precise reconstruction of the defect space. (Courtesy of Globus Medical [Fortify], Globus Medical Inc., Audubon, PA.)

 

 

70

 

The width of the corpectomy should be kept as narrow as possible without compromising decompression or removal of pathologic bone (TECH FIG 7B).

 

Allows bone ingrowth from the corpectomized vertebral body into the interbody bone graft Enhances the stability of the interbody strut

A bone screw with a washer can be used above and below large defects as an “anti-kickout” buttress for allografts (TECH FIG 7C).

Allograft strut grafts such as femoral head, humerus, or fibular shafts can be cut using an oscillating saw to fit snugly into the interbody space (TECH FIG 7D). The advantages of allograft are it can be packed with morselized autogenous bone, it has a similar modulus of elasticity to host vertebral bone, and it will become osseointegrated over time.

Expandable corpectomy devices have been developed to facilitate anterior column reconstruction following single- or multilevel corpectomy. These devices are inserted and expanded until there is good interference fit at the endplates (TECH FIG 7E).

Their use in single-level cases also permits more accurate height restoration and simplifies the carpentry because the height of the device can be expanded to fill virtually any defect.

One potential disadvantage of the expandable corpectomy devices is the reduction in bone graft volume and bone graft apposition to the vertebral endplate. Concomitant posterior fusion is advocated given the large amount of resection and potential instability.

 

PEARLS AND PITFALLS

Use of a pulse oximeter on the left great toe

provides real-time feedback to the surgeon about perfusion to the distal extremity during great vessel retraction.

• There should be a low threshold for prophylactic

inferior vena cava filter placement in patients with venous injuries requiring repair, as pulmonary embolism, although rare, carries potentially catastrophic consequences.

Perforation of the cancellous vertebral body

endplates with Cobb curettes or the lamina spreader increases the likelihood of implant subsidence.

• Early (<2 weeks) implant malpositions or

migrations can be easily revised as the anterior tissue planes are still preserved.

Epidural bleeding can be effectively

controlled quickly with thrombin-soaked Gelfoam gauze and release of any disc space distractors.

• Overdistraction of the disc space with a lumbar

disc replacement implant will result in compromised motion and may be associated with new postoperative leg pain related to stretch injury to the lumbosacral nerve trunks.

Marking the location of the dorsalis pedis

and posterior tibial pulses with a marking pen facilitates reassessment of pulses in the postoperative setting when lower extremity swelling is more prevalent.

 

POSTOPERATIVE CARE

 

As soon as the patient emerges from anesthesia, a complete neurologic examination and brief history should be performed. Specifically, patients should be asked if they have any new leg pain. If present, CT myelography or plain CT scans should be obtained to ensure that no bone, disc material, or portion of an implanted device is impinging on the lumbar nerve roots.

 

Nasogastric tubes for the first 12 to 24 hours help to minimize abdominal wall distention and postoperative ileus.

 

Patients are encouraged to walk on postoperative day 1.

 

Lumbar corsets or abdominal binders are prescribed at the discretion of the surgeon and may reduce the tension on the abdominal incision in the early postoperative period.

 

Return to heavy manual labor is restricted in patients undergoing anterior interbody fusion until the fusion is solid. Fine-cut CT scans are useful in documenting solid fusion if there is doubt on AP, lateral, or flexion-extension radiographs (FIG 3). Manual labor should be restricted in patients undergoing disc replacement until the bone-prosthesis interface is judged to be stable.

 

 

 

 

FIG 3 • Sagittal fine-cut CT image depicting trabecular bone bridging across the disc space 3 months after

anterior interbody fusion with a threaded titanium cage packed with collagen sponges impregnated with BMP-2.

 

 

71

 

 

OUTCOMES

Level IV evidence reported by Tropiano et al17 showed significant improvements in back pain, radiculopathy, and disability at mean of 8.7 years after insertion of the ProDisc lumbar disc replacement.

However, a Cochrane review of seven randomized controlled trials of LTDR versus fusion failed to demonstrate a clinically significant difference in outcomes and called into question selection and

reporting bias in studies supporting LTDR.9

ALIF with titanium cages and iliac crest bone graft has been shown to yield significantly greater fusion rates (97%) versus allograft dowels packed with iliac crest bone graft (48%).14

BMP-2 is FDA approved for use in LT interbody devices for single-level ALIF. Improved fusion rates and clinical outcomes were reported in patients who had ALIF cages packed with BMP-2-impregnated

collagen sponges compared to patients in whom the cages were packed with iliac crest bone graft.4 Subsequent investigation has called into question the safety and efficacy of BMP-2. A longitudinal study6 of 472 patients and an independent review of industry data15 both demonstrated an increased

rate of retrograde ejaculation (7% vs. 1%) with nearly equivalent functional outcomes in groups treated

with ALIF and BMP-2 as opposed to autograft. They also noted a small, but clinically insignificant, increased cancer risk associated with the use of BMP-2.

A review of a cohort of 146,278 Medicare patients undergoing lumbar fusion surgery revealed no difference in new cancer diagnosis rates between those who received BMP-2 and those who had not

(15% vs. 17% of patients with a new cancer diagnosis, respectively).7 The relationship between BMP-2 and carcinogenesis is uncertain at this point.

Clinical outcomes and flexion-extension range of motion correlate with surgical technical accuracy of lumbar disc replacement.10

The addition of posterolateral fusion with or without instrumentation (360-degree fusion) may be efficacious in carefully selected individuals; however, identification of the most appropriate patients and

the outcomes of this approach have not been clearly established in the literature.12

 

COMPLICATIONS

Most complications associated with anterior lumbar discectomy, interbody fusion, disc replacement, and corpectomy are approach-related1,3,11,17 (see Chap. SP-35).

The most common complications of ALIF are pseudarthrosis and device failures such as migration or breakage.

The complications of lumbar disc replacement depend on the exact type of device being inserted but generally can be categorized as follows16,18:

Device failures: metal endplate breakage, core dislodgement or fracture, polyethylene degradation Bone implant failures: subsidence, vertebral body fracture, implant migration or dislocation

 

Iatrogenic deformity: kyphosis, scoliosis

Host response: osteolysis, heterotopic ossification Infection

Revision approaches to the anterior lumbar spine carry six times the risk of major bleeding or

thromboembolic complications.11 Preoperative intravenous filter insertion, ureteral stenting, and percutaneous venous access wires are critical to reduce these risks.

 

 

REFERENCES

1. Bertagnolli R, Zigler J, Karg A, et al. Complications and strategies for revision surgery in total disc replacement. Orthop Clin North Am 2005;36:389-395.

    

    

2. Boden SD, McCowin PR, Dina TS, et al. Abnormal magnetic-resonance scans of the lumbar spine in asymptomatic subjects: a prospective investigation. J Bone Joint Surg Am 1990;72(3):403-408.

    

    

3. Brau SA, Delamarter RB, Schiffman ML, et al. Vascular injury during anterior lumbar surgery. Spine J 2004;4:409-412.

    

    

4. Burkus JK, Heim SE, Gornet MF, et al. Is INFUSE bone graft superior to autograft bone? An integrated analysis of clinical trials using the LT-CAGE lumbar tapered fusion device. J Spinal Disord Tech 2003;16:113-122.

    

    

5. Carragee EJ, Don AS, Hurwitz EL, et al. 2009 ISSLS Prize Winner: does discography cause accelerated progression of degeneration changes in the lumbar disc: a ten-year matched cohort study. Spine 2009;34(21):2338-2345.

    

    

6. Comer GC, Smith MW, Hurwitz EL, et al. Retrograde ejaculation after anterior lumbar interbody fusion with and without bone morphogenetic protein-2 augmentation: a 10-year cohort controlled study. Spine J 2012;12(10):881-890.

    

    

7. Cooper GS, Kou TD. Risk of cancer after lumbar fusion surgery with recombinant human bone morphogenic protein-2 (rh-BMP-2). Spine 2013;38(21):1862-1868.

    

    

8. Hueng DY, Chung TT, Chuang WH, et al. Biomechanical effects of cage positions and facet fixation on initial stability of the anterior lumbar interbody fusion motion segment. Spine 2014;39(13):E770-E776.

    

    

9. Jacobs W, Van der Gaag NA, Tuschel A, et al. Total disc replacement for chronic back pain in the presence of disc degeneration. Cochrane Database Syst Rev 2012;9:CD008326.

    

    

10. McAfee PC, Cunningham BW, Holtsapple G, et al. A prospective, randomized, multi-center FDA IDE study of lumbar total disc replacement with the CHARITETM Artificial Disc vs. lumbar fusion: part II: evaluation of radiographic outcomes and correlation of surgical technique accuracy with clinical outcomes. Spine

    2005;30: 1576-1583.

     

     

11. McAfee PC, Geisler FH, Saiedy SS, et al. Revisability of the CHARITE Artificial Disc Replacement: analysis of 688 patients enrolled in the U.S. IDE study of the CHARITE Artificial Disc. Spine 2006;31: 1217-1226.

     

     

12. Mummaneni PV, Dhall SS, Eck JC, et al. Guideline update for the performance of fusion procedures for degenerative disease of the lumbar spine. Part 11: interbody techniques for lumbar fusion. J Neurosurg Spine 2014;21(1):67-74.

     

     

13. Rahme R, Moussa R. The modic vertebral endplate and marrow changes: pathologic significance and relation to low back pain and segmental instability of the lumbar spine. AJNR Am J Neuroradiol 2008; 29(5):838-842.

     

     

14. Sasso RC, Kitchel SH, Dawson EG. A prospective, randomized controlled clinical trial of anterior lumbar interbody fusion using a titanium cylindrical threaded fusion device. Spine 2004;29(2):113-122.

     

     

15. Simmonds MC, Brown JV, Heirs MK, et al. Safety and effectiveness of recombinant human bone morphogenetic protein-2 for spinal fusion: a meta-analysis of individual-participant data. Ann Intern Med 2013;158(12):877-889.

     

     

16. Tortolani PJ, McAfee PC, Saiedy S. Failures of lumbar disc replacement. Semin Spine Surg 2006;18:78-86.

     

     

17. Tropiano P, Huang RC, Girardi FP, et al. Lumbar total disc replacement: seven to eleven year follow-up. J Bone Joint Surg Am 2005;87(3):490-496.

     

     

18. van Ooij A, Oner FC, Verbout AJ. Complications of artificial disc replacement: a report of 27 patients with the SB Charité disc. J Spinal Disord Tech 2003;16(4):369-383.