Posterior Lumbar Interbody Fusion

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CHAPTER 49 Posterior Lumbar Interbody Fusion

The technique of posterior lumbar interbody fusion (PLIF) has become an important part of the modern spine surgeon’s armamentarium. This was not always so; for nearly half a century after the introduction of this technique, it was performed routinely by only a handful of surgeons. Most surgeons condemned the technique as unnecessary, technically difficult, or even dangerous.^1,² Controversy persists regarding the safety of PLIF compared with other approaches to the intervertebral space and regarding the benefit that PLIF adds to the outcome of surgical treatment for various spinal pathologies.

PLIF is an operation in which the disc space is exposed from a posterior approach, similar to that used in a discectomy, and a fusion is performed by directly grafting the intervertebral space. Classically, PLIF is performed via bilateral exposure of the disc space, with some retraction of the dura to expose the disc. When PLIF was first described, before the era of spinal instrumentation, advocates of the operation argued that it prevented collapse of the disc space after discectomy and maintained the height of the neural foramen, keeping the root free of bony compression. These advocates also believed that placing bone graft directly in the disc space would more likely result in healing of the fusion because the bone would be subject to compressive forces. Critics argued that the operation required excessive epidural dissection and nerve root retraction with a high risk of dural tear, epidural fibrosis, nerve root injury, and chronic arachnoiditis and that graft displacement could lead to late neurologic compromise.

Historical Perspective

Attempts to identify the first surgeon to perform an interbody fusion from a posterior approach are muddled by several reports appearing in the published literature beginning in 1944,³^–⁵ but no one disputes that credit for initially developing the techniques and key principles of the operation as it is practiced today belongs to Cloward,⁶ who emphasized the importance of a wide exposure of the spinal canal to minimize nerve root injuries, the use of structural graft to prevent intervertebral collapse, and the complete removal of nuclear material from the disc space and replacement with bone to promote fusion. Cloward was widely criticized for his operation, probably as much because he advocated that it be included as part of the routine surgical treatment for all lumbar disc herniations as for the frequency of poor outcomes and complications when it was attempted by other surgeons. A few surgeons embraced Cloward’s operation, and several large surgical series were published in the literature reporting good outcomes and high fusion rates.⁷^–¹⁰ Widespread interest in and acceptance of PLIF as a valuable technique did not occur, however, until the introduction of pedicle screw instrumentation.

With the early generation of pedicle instrumentation, screw fracture was a common occurrence. Fracture especially occurred if a dramatic change in spinal alignment was achieved, such as in the reduction of a high-grade spondylolisthesis. Even without large changes in alignment, screw fracture occurred commonly in overweight patients or when the disc space was distracted.¹¹ In those cases, the mechanical loads that the instrumentation was subjected to exceeded the fatigue properties of the screws. Surgeons, in particular Steffee and Sitkowski,¹² recognized that a structural graft placed in the disc space would divert some of the load, decreasing the forces on the posterior instrumentation, and reduce the frequency of instrumentation failure. Steffee and Sitkowski¹² reported their experiences with reduction of high-grade spondylolisthesis. In each of the first three patients treated with pedicle screws, the instrumentation failed, and alignment was lost. In the next 11 patients, an interbody graft was placed. All of these patients developed solid fusions with no loss of alignment. With PLIF, the load-sharing anterior column support could be added to protect the pedicle screws without the need for a separate anterior incision (Fig. 49–1).

FIGURE 49–1 Screw fracture and loss of alignment is a common complication after reduction of spondylolisthesis if anterior column support is not added.

Simultaneous with the benefit of PLIF on reducing failures of the new instrumentation, pedicle instrumentation had a beneficial effect on the outcome of PLIF surgery. For the first time, rigid fixation was available that could effectively stabilize the lumbar spine after laminectomy. This fixation virtually eliminated the complication of graft displacement that had previously been associated with PLIF in some surgeons’ hands. The addition of instrumentation also improved the fusion rate of PLIF. Many of the previously held objections to PLIF surgery were no longer applicable when PLIF was combined with pedicle instrumentation.

Another development that favored the adoption of PLIF was the invention of the interbody fusion cage by Brantigan and the titanium mesh cage by Harms. Their designs eliminated the need to harvest a structural graft from the iliac crest, which was a major source of morbidity previously associated with the PLIF procedure. The cage provided the structure,¹³ and the osteogenic potential for fusion came from cancellous bone packed into the cage, which could be obtained from the iliac crest more easily than a structural graft, with less injury to the patient. Before the invention of the cages, it was possible to avoid the morbidity associated with graft harvest by using allograft bone, as advocated by Cloward.^14,¹⁵ Concerns about availability, disease transmission, healing potential, delayed healing, and wide variation in the structural integrity of available grafts all contributed to the limited enthusiasm for the use of allograft bone for interbody fusion.^16,¹⁷ The availability of cages overcame another common objection to the PLIF operation.

The availability of pedicle screw instrumentation and intervertebral cages contributed to progressively greater acceptance of the operation first pioneered by Cloward. Better alignment and higher fusion rates are routinely achieved using these devices. The training of surgeons and marketing of these devices by their manufacturers have also contributed to wider adoption of PLIF in surgical practice. Not all of these devices have proven to be successful, however. Cylindric threaded fusion cages enjoyed brief popularity as PLIF devices that did not require supplementary fixation, but mediocre results and high complication rates associated with their use resulted in their virtual disappearance as a posterior spinal implant.¹⁸ The clinical experience with threaded implants and the study of the relevant biomechanics have helped surgeons understand the PLIF operation better, however, and develop the operation that is currently performed. Although more recent advances, such as the development of the transforaminal and direct lateral approaches to the disc space, have decreased the frequency with which PLIF is performed, PLIF remains an important and powerful tool for spinal surgeons.

Indications for Interbody Fusion

Some surgeons believe that an interbody fusion is indicated whenever a lumbar fusion is done. They argue that the intervertebral space is biologically and mechanically superior for fusion compared with the intertransverse plane because of the larger surface area of the highly vascular bony endplate and because the interbody bone graft is subject to compressive forces. In addition, they criticize the large amount of muscle damage that occurs from exposure of the spine for posterolateral fusion. Despite these theoretical advantages, it is difficult to show clinical superiority of interbody fusions over posterolateral fusions for most lumbar degenerative conditions, and most published studies comparing the techniques reveal similar outcomes regardless of what fusion technique is used.¹⁹^–²¹ There are, however, certain circumstances when interbody fusion offers definite advantages. Adding an interbody fusion to a posterolateral fusion increases the rate of achieving successful arthrodesis. This is especially true when a long fusion extends to the sacrum, which is historically associated with a high risk of lumbosacral pseudarthrosis.²²

Some authors have argued that interbody fusion should be combined with posterolateral fusion in other patients at high risk for failed fusion, such as smokers.²³ In patients with pseudarthrosis after failed posterolateral fusion, an interbody fusion is a good salvage operation, allowing the fusion to occur in a well-vascularized bony bed rather than attempting fusion again in the scarred devascularized posterolateral space.²⁴ Interbody fusion offers a particular advantage in the treatment of scoliosis and kyphosis or flatback deformities. The destabilizing effect of disc space preparation facilitates rotational correction, and distraction of the intervertebral space allows correction of segmental kyphosis and asymmetrical tilt of the vertebrae. In patients with isthmic spondylolisthesis who undergo deformity reduction, it has been well shown that an interbody graft offers a biomechanical advantage, which protects instrumentation by load sharing, helping to maintain the alignment achieved at surgery. Also, the L5 transverse process is often hypoplastic in these patients, so the increased bone surface area of the vertebral endplate offers a vastly larger area to which bone can fuse.

Most series evaluating the role of interbody fusion in isthmic spondylolisthesis show substantial clinical and radiographic benefit.^25,²⁶ Finally, the most common indication for interbody fusion, but probably the most controversial, is ablation of the disc space in patients with discogenic pain.^27,²⁸ Many advocates for surgical treatment of this entity argue that only interbody fusion can completely remove motion of the painful disc and that complete removal of nuclear material is necessary to eliminate the anatomic source of pain.

Regardless of the indication, interbody fusion can be performed from anterior and posterior approaches. The relative advantages of these approaches are influenced by many factors, and it is essential that the spinal surgeon be well trained in various approaches so that the most appropriate one can be chosen for each unique clinical situation. Specific advantages of the posterior approach include the ability simultaneously to decompress the spinal canal directly and stabilize the spine more effectively with instrumentation. Although safe techniques of anterior fixation have become available more recently, if multilevel instrumentation is needed, posterior approaches are preferred. In patients with spinal deformity, bilateral facet resection allows more effective destabilization of the motion segment, allowing greater corrections than can be obtained by anterior disc removal alone, again favoring a posterior approach to the intervertebral disc. Posterior approaches avoid manipulation of the great vessels, of special concern in patients with atherosclerosis or a history of venous thrombosis. Also, posterior approaches avoid injury to the hypogastric plexus, which can result in retrograde ejaculation and sterility, so they are generally preferred in young men.

When there has been extensive posterior surgery and scarring, anterior approaches to the disc space avoid the increased risks of dural tear and nerve root injury associated with reoperation on the spinal canal. If a conjoined root is identified on preoperative imaging, the risk of root injury is also higher, and an anterior approach is preferred. A wider exposure of the disc space can also be achieved anteriorly, theoretically allowing better preparation of the endplates and more complete packing of the interspace with bone graft. Some surgeons prefer the anterior approach in patients at high risk for pseudarthrosis. An anterior approach may also be preferred when trying to increase lordosis. Dividing the anterior longitudinal ligament may allow for greater anterior distraction and more lordosis. Despite this theoretical advantage, in the author’s experience, anterior approaches are more effective than posterior approaches in achieving lordosis only when the facet joints are highly mobile or have first been resected through a posterior approach.

Biomechanics of Interbody Fusion

An appreciation of the mechanical properties of the spine after interbody fusion is essential to understanding the benefits and pitfalls of this operation. It seems intuitively obvious that placing a solid block in the disc space would prevent flexion of the vertebral motion segment. This is what occurs—whether the graft is placed from an anterior or a posterior approach. This fact has important implications for protection of posterior instrumentation. Interbody grafts significantly decrease the strain in posterior spinal implants when they are subjected to compression or flexion loads, which are the common modes of failure of these constructs in clinical practice. Various authors have shown that placing intervertebral cages decreases forces and strain in posterior implants by 56% to 80%,^29,³⁰ and the clinical benefit of establishing anterior load sharing to prevent implant and construct failure is well established (Fig. 49–2).³¹

FIGURE 49–2 These photographs dramatically show reduction in strain on posterior implants when a cage is placed in intervertebral disc space. This model of a spinal motion segment with pedicle screw fixation has no anterior column support. A, Alignment with no load applied. B, Loss of alignment with only 700 N of load, the physiologic equivalent of standing in place. C, Protective effect of an implant placed in disc space.

(Courtesy Hassan Serhan, PhD.)

Increasing stability when the spine is subject to flexion loading does not mean that the spine is more stable when subject to forces in every direction. After interbody graft placement, the motion segment is more unstable in certain directions. The pattern of instability is determined to a great extent by the direction from which the graft is placed and represents the destabilizing effect of the surgical approach ³² and the preparation of the disc space. Grafts placed from an anterior approach result in increased motion when force is applied in extension; this likely represents the effect of dividing the anterior longitudinal ligament. Grafts placed from a posterior approach result in increased axial rotation of the motion segment owing to the resection of the posterior facet complexes necessary to avoid excessive dural retraction.³³^–³⁷ The design of the cage—vertical, box, or threaded and whether or not the cage engages the endplates—does not seem to matter with regard to the patterns of instability created by intervertebral placement. Only the structures sacrificed during the approach to the disc are important in determining instability of the final construct (Fig. 49–3).^38,³⁹

FIGURE 49–3 These data represent a summation of five biomechanical studies of stability after anterior cage placement in human cadaveric models and three studies of posterior cage placement. Data for each direction of testing are presented as ratio of range of motion after interbody cage placement compared with motion of unoperated segment. Ratio of 1.0 implied same motion as intact specimen. Placement from anterior approach stiffens spine in flexion, axial rotation, and lateral bending, whereas placement from posterior approach stiffens the spine only in flexion. When testing axial rotation, placing a cage from posterior approach creates more motion than unoperated spine.

(Redrawn from Oxland TR, Lund T: Biomechanics of stand-alone cages and cages in combination with posterior fixation: A literature review. Eur Spine J. 9:S95-S101, 2000.)

Most surgeons make the assumption that a PLIF procedure is a stabilizing operation. The biomechanical data show that this is not entirely true. Understanding that placing an interbody graft or cage can lead to destabilization of the motion segment in certain directions is important for understanding why interbody fusion done without supplementary fixation has a high failure rate, especially when done from a posterior approach. Proponents of threaded fusion cages, which were designed to be used without supplementary fixation, argued that distraction of the disc space would stabilize the motion segment through ligamentotaxis. This argument is true. Increasing the size of the cage used to distract the disc space results in greater stabilization of the motion segment on mechanical testing.⁴⁰ After cyclic loading, some subsidence is inevitable, however, and biomechanical tests done after cyclic loading of threaded cage constructs reveal decreased stability in all directions.⁴¹

The destabilizing effects of interbody fusion are overcome by adding pedicle screw instrumentation; this is true whether the fusion is done from an anterior or a posterior approach. These biomechanical data correlate well with the substantially greater clinical reliability of achieving an arthrodesis when interbody fusion is done with supplementary pedicle fixation compared with the variable results reported with stand-alone interbody fusion procedures.

Although cage design characteristics do not seem to have a significant effect on segmental motion, these characteristics do influence the risk of subsidence, the healing of bone within the cage, and the alignment achieved. In addition, endplate preparation and vertebral bone density have important effects on the extent of subsidence.

Cage Characteristics

The optimal cage material is unknown. Cloward ^41a advocated the use of ethylene oxide sterilized allograft bone. Brantigan and colleagues¹⁶ tested 18 tricortical iliac specimens obtained from bone banks and determined that 3 of them were not strong enough to sustain anticipated loads without collapse. Current allograft products available for intervertebral use are made primarily of cortical bone and have adequate compressive strength. In designing his cage, Brantigan chose carbon fiber–reinforced polymer because of its strength and because its modulus of elasticity is similar to that of cortical bone.¹³ In the United States, other materials commonly used in commercially available cages are polyetheretherketone (PEEK), a nonreinforced polymer, and titanium, which has a modulus of elasticity significantly greater than that of cortical bone. The stiffness of the cage is determined not only by the mechanical properties of the material but also by the shape and design characteristics of the cage. The ideal stiffness and its role in limiting subsidence or enhancing fusion are unknown. Finite element data have suggested that stiffer cages are more likely to subside,⁴² but animal data have not supported this hypothesis,⁴³ and clinical reports have not convincingly shown that metal cages are more likely to subside than cages made of polymer (Fig. 49–4).

FIGURE 49–4 A-F, Anteroposterior and lateral radiographs of successful interbody fusions with carbon fiber–reinforced polymer cages (A and B), titanium mesh cages (C and D), and allograft wedge spacers (E and F). Incorporation of bone graft and replacement by new bone is most easily visualized with polymer cages because they are radiolucent. Outline of polymer can be seen on anteroposterior view as four radiolucent squares at periphery of implant. With titanium mesh cages, healed bone graft is visualized in front and behind implants, but assessment of graft within cage is difficult with standard x-rays. When nonunion occurs with allograft implants, radiolucency is usually seen adjacent to allograft.

(A and B, Courtesy John Brantigan, MD.)

Kanayama and colleagues ⁴⁴ studied the effect of cage design on stress shielding of the graft inside the cage. They determined that it was the pore size, not the stiffness of the material, that determined the load experienced by the graft. The total pore size was not important; rather, it was the size of the largest contiguous opening that determined how much force was transferred to the graft. It was implied that minimizing stress shielding is important to promote fusion, but no data were presented to support that presumption. One clinical study consistent with this hypothesis compared narrow and standard carbon fiber–reinforced polymer cages. The pore in the narrow cage was 36% smaller than the pore in the standard cage. There was a small but statistically significant reduction in fusion rate, 91.1% compared with 98.9%, but the conclusions have limitations because of methodologic flaws.⁴⁵ A larger pore size seems desirable as long as the design of the cage provides adequate structural support for the endplate.

Cage design also influences the alignment achieved. An increasingly large body of data show that maintaining or restoring lumbar lordosis is an important surgical goal and that lordosis correlates with the outcome of surgery.⁴⁶ Early cage designs were rectangular in shape and tended to force the vertebral endplates into parallel alignment. It is possible to achieve lordosis with rectangular cages, either by resecting bone posteriorly or by compressing the posterior disc space and causing the posterior portion of the cage to subside.⁴⁷ A disadvantage of this approach is that it results in loss of posterior disc space height, which can lead to foraminal narrowing. A better solution is to use a cage that is tapered to achieve lordosis.⁴⁸^–⁵⁰ It is commonly said that anterior distraction is necessary to achieve lordosis, but distraction of the disc space with an implant that is not tapered flattens the spine. Placing large cages to achieve lordosis is a mistake. They require more root retraction to place, and this is more likely to result in nerve injury. A better solution is to use a tapered device and not to strive for maximal anterior height restoration (Fig. 49–5).

FIGURE 49–5 Using a tapered cage or spacer is the best way to achieve lordosis. Facet resection is sometimes needed to allow back of disc space to narrow adequately. An advantage of allograft bone is the ease with which it can be sculpted intraoperatively to achieve the desired shape. A, In this case, more lordosis was achieved than would be typically possible using commercially prepared devices by using a bur to increase taper of graft. B and C, Using larger grafts and distracting the disc space more does not increase lordosis; it increases amount of dural retraction needed and increases difficulty of surgery.

(Courtesy Depuy Spine.)

Subsidence

In addition to achieving fusion, avoiding subsidence to maintain alignment is an important goal of interbody fusion surgery. In mechanical testing, endplate failure has a linear correlation with decreased bone density,⁵¹ and severe osteoporosis is considered to be a relative contraindication to interbody fusion because of the risk of endplate collapse. The strength of the endplate is not uniform, and the central portion, where most surgeons place their grafts, is the weakest. Mapping studies have shown that the posterolateral portion of the endplate is most resistant to compression.^52,⁵³ Placing the cage or graft in the area where the bone is strongest makes biomechanical sense⁵⁴ but may not be consistent with the goals of an individual surgical case. If it is necessary to increase lordosis, better results are achieved by anterior placement. Putting the graft in the posterolateral position might be stronger but would block compression of the posterior disc space and limit lordosis. This is especially true with rectangular cage designs or vertical titanium mesh cages.

Cage designs that optimize contact with the strongest portions of the endplate are desirable and include cloverleaf designs and large round cages, both of which have peripheral endplate contact.⁵⁵ Practical use of large round cages is limited to anterior surgery because of the excessive dural retraction that would be necessary to place such an implant. Lordotic cages placed laterally also achieve near-optimal endplate contact. The surface area of the endplate that needs to be in contact with the graft is unknown but is commonly stated to be 30%. This percentage is based on a study of thoracic vertebrae subjected to a physiologic load of 600 N. Failure of the endplate occurred in 80% of specimens subjected to that load when the grafts were 25% or less of the endplate surface area. When the grafts exceeded 30% of the endplate surface, 88% of the specimens remained intact (Fig. 49–6).⁵⁶

FIGURE 49–6 In this case, subsidence of cage into inferior endplate of L5 is obvious. Technical factors that may have predisposed to subsidence include use of single small cage placed centrally. Endcaps were not used, which would have increased surface area of metal supporting endplate. Inadequate grafting of interspace around cage may also have been a factor, suggested by absence of bone in disc space. A solid posterolateral fusion was achieved.

Preservation of the endplate is important to maintain the structural integrity of the vertebra. Oxland and colleagues ⁵⁷ removed slightly less than 1 mm of cortical endplate with a power bur to expose trabecular bone; this resulted in a 33% reduction in compressive resistance of the vertebra. If the endplate is decorticated where it is not in contact with the cage, the strength of the remaining endplate is preserved.⁵⁸ Consequently, it is recommended that the endplates be carefully preserved during preparation of the disc space wherever they are in contact with the implant, but that areas of the endplate that are not load bearing can be decorticated to increase contact of graft placed outside the cage with trabecular bone.

Author’s Preferred Technique

The author’s experience with PLIF dates back to 1994, and the method of interbody fusion described here has been used without modification since 1999. Early results were reported in 2000 with an interbody fusion rate of 92%.⁵⁹ It is the author’s practice always to perform a posterolateral fusion simultaneously with the PLIF to ensure the highest chance of achieving an arthrodesis. The technique for PLIF evolved from that described by Steffee and Biscup⁶⁰ for insertion of trapezoidal “ramps” made of carbon fiber–reinforced polymer. Their method was to insert the ramps sideways and rotate them into their final position, minimizing the amount of root retraction needed and simplifying placement of a lordotic graft.

The author prefers to use a commercially prepared trapezoidal allograft wedge for two reasons. The first is that its size and shape can be easily customized intraoperatively with a power bur to match the individual patient’s anatomy. The second is that, in contrast to titanium and polymer cages, this structural device has the potential to incorporate in the fusion. Although the graft is placed primarily as a spacer and it is the autologous bone placed around the graft that is relied on to achieve arthrodesis, incorporation of the allograft generally occurs, resulting in a more robust fusion. There is no potential for polymer or metal to incorporate and contribute to the fusion, so a smaller area of the endplate is available to participate in healing when using devices made of those materials. Ceramic blocks have been used in Asia because of the potential for bone ingrowth, but there is no experience with that material in the United States, and it does not share the ability to be easily sculpted intraoperatively. Currently, several manufacturers produce allograft implants of this design.

The technique described here for preparation of the intervertebral space is applicable to any intervertebral cage or graft, and the principles outlined help prevent neural injury and other complications that have been associated with this operation. The procedure begins with proper positioning, which is essential to achieve a good outcome. The patient is positioned in hyperextension to help create lumbar lordosis. The abdomen should hang free for unimpeded venous return; this decompresses the epidural venous plexus and helps reduce bleeding. A radiolucent positioning frame is useful so that intraoperative fluoroscopic images can be obtained to confirm correct placement of screws and grafts (Fig. 49–7). Pedicle screws should be placed before beginning the interbody fusion. PLIF destabilizes the motion segment and should not be done unless adequate pedicle fixation can be achieved.

FIGURE 49–7 The patient is positioned in hyperextension, with pillows under thighs and head of bed elevated to help achieve maximal lordosis. Abdomen hangs free to allow unimpeded venous return. Electromyographic monitoring is used to help prevent nerve injury resulting from excessive traction.

A wide laminectomy and resection of the medial portion of the facet joints is essential to minimize retraction of the dura and nerve roots. The wide exposure helps avoid traction injury to the lumbar roots, which can cause neuropathic pain and weakness. Generally, the exposure of the disc space should extend lateral to the medial border of the pedicle below. Adequate exposure may require sacrifice of the facet joints, especially in the upper lumbar spine. This sacrifice is of little consequence because stability is restored by pedicle screw fixation. The facets should always be excised when correcting deformity because this helps loosen the spine and facilitates correction. The superior edge of the upper lamina should be preserved to maintain the posterior ligamentous attachments to the vertebra above the fusion.

An abundant epidural plexus is usually encountered at the lateral border of the spinal canal. Bipolar electrocautery and packing with cottonoid patties can prevent the excessive bleeding that sometimes accompanies interbody fusion procedures. A paste made from absorbable gelatin foam powder and saline is very helpful in controlling bleeding. The venous plexus should be sharply divided to facilitate mobilization of the nerve roots and dura. A nerve root retractor is used to protect the dura, and a rectangular opening is cut in the anulus with a scalpel. When extending the annular opening laterally, care is taken to protect the exiting nerve root. This is easily done by sweeping any soft tissue toward the root with a No. 1 Penfield elevator and leaving the elevator in place to protect the root while working laterally.

The anulus is opened bilaterally, and the disc space is distracted with intervertebral spreaders. These are flat bars of increasing width with rounded edges. They are inserted into the disc space horizontally and rotated 90 degrees, distracting the disc space. If the disc space is initially very narrow, it is sometimes helpful to use a smaller instrument first, such as a No. 4 Penfield elevator, to identify the path. It is important not to use force so as to avoid inadvertent penetration into the vertebral body. Lateral fluoroscopy can be helpful to confirm that the starting tools are correctly placed in the interspace. The disc is gradually distracted, working from side to side with increasingly large spreaders until resistance is met. The interspace should not be overdistracted, or the endplates may collapse. It is not necessary to maximize distraction to achieve lordosis, and decompression of the nerve roots should be accomplished by aggressive foraminotomy, not aggressive distraction. Placing a maximally tapered graft is the best way to increase lordosis. Trying to place a tall graft has many negative effects. It increases the amount of dural retraction necessary, it increases the volume of bone graft needed to fill the interspace, and it increases the distance over which the fusion must occur (Fig. 49–8).

FIGURE 49–8 A-C, Disc is gradually distracted with flat bars of increasing width that are inserted and rotated. D, Intraoperative photograph shows that wide exposure allows intradiscal work to be done with minimal dural retraction. E-H, Collis four-sided curets and standard curets are used to clean disc space fully. I and J, Lordotic allograft spacers are inserted horizontally and rotated into position. Autogenous bone is packed around spacers. K, Axial CT image at level of disc space shows wide decompression, posterolateral fusion, and abundant bone packed around spacers.

(Courtesy Depuy Spine.)

After distracting the space to a comfortable working distance, usually 11 or 12 mm, one of the spreaders is removed, and a four-sided Collis curet 1 or 2 mm smaller than the largest spreader used is inserted horizontally, then rotated clockwise and counterclockwise to separate the cartilage from the bony endplate. This step is repeated at various depths and angles to clean as much of the endplate as possible. These curets can be very aggressive, and it is important to be careful not to cut into the endplate. A Kerrison rongeur is used to resect loosened anulus flush with the vertebral endplates; this allows better visualization of the disc space so that a more complete discectomy can be done.

The discectomy is completed with standard curets and pituitary rongeurs. A reverse curet is very effective in removing any remaining cartilage from the endplates. It is helpful to mark the curets so that they are not inserted past a depth of 30 mm. In most cases, the anterior anulus prevents protrusion anterior to the vertebra, but the anulus is sometimes deficient, especially in patients with spondylolisthesis, and it is important to avoid anterior protrusion of the instruments and visceral injury. After cleaning the interspace, the intervertebral spreader is reinserted, usually one size larger than was possible before removing the disc and cartilaginous endplate. The opposite side of the disc is now prepared in the same fashion.

It now is obvious that the interspace has been grossly destabilized. If a rotational deformity is present, it can be corrected by rotating the bilateral intervertebral spreaders simultaneously and “walking” the displaced vertebra into alignment. If no rotational deformity is present, the spreaders should be turned in opposite directions so that a rotational deformity is not induced. A graft is chosen to fit into the interspace. The graft should not be taller than the widest spreader used. It can be tapered to achieve the desired lordosis. After ensuring that the dura and exiting nerve root are protected, the trapezoidal allograft is inserted horizontally into the disc space and then rotated into its final lordotic position. The intervertebral spreader is removed from the opposite side of the disc space.

Cancellous graft is packed from the opposite side into the middle of the interspace; a syringe with the end cut off is useful for this step. Finally, the second trapezoidal allograft is inserted into position. Additional graft is packed around the allograft wedges to fill the disc space maximally with bone. The foramina and the midline under the dura are inspected to ensure no cancellous bone or disc material was displaced into the spinal canal or neural foramen. Lastly, the instrumentation is tightened in gentle compression to secure the grafts and achieve lordosis (Figs. 49-9 through 49-11).

FIGURE 49–9 Lateral listhesis and rotational deformity can be corrected efficiently with posterior lumbar interbody fusion. Spine is destabilized by bilateral facet excision and complete discectomy. Distraction paddles can be used to translate vertebra laterally, correcting axial rotation, lateral listhesis, and asymmetric disc space collapse.

FIGURE 49–10 Acute rotational deformity causing degenerative scoliosis is corrected by posterior lumbar interbody fusion. Deformity at L45 was flexible and resolved spontaneously with correction of structural deformity at L34.

FIGURE 49–11 In this patient, severe degenerative disease of lumbar spine has resulted in spinal stenosis, degenerative spondylolisthesis, and asymmetrical disc space collapse. Because posterior decompression is needed to treat the patient’s stenosis and pseudoclaudication, posterior lumbar interbody fusion is the most efficient way to correct the patient’s deformity and achieve fusion simultaneously.

Optimal Graft Material

The “gold standard” for achieving a solid fusion is cancellous autograft bone, regardless of which cage device or intervertebral spacer is chosen. Brantigan ⁶¹ reported a 97.7% fusion rate when using autologous cancellous graft in the cage he designed. Numerous authors have reported similar fusion rates using bone harvested from the lamina, spinous processes, and facet joints, without using iliac graft.⁶² Miura and colleagues⁶³ reported a 100% fusion rate at 12 months in 32 patients treated with carbon fiber–reinforced polymer cages. Kim and colleagues⁶⁴ reported a 95% rate using local bone in titanium vertical mesh cages in a prospective study of 50 patients. Kasis and colleagues⁶⁵ reported improved clinical outcomes in a prospective comparison when using local bone, attributing at least some of the benefit to avoiding the pain and complications of iliac graft harvest. McAfee and colleagues⁶⁶ reported a 98% fusion rate in 120 patients using local bone in carbon fiber–reinforced polymer cages placed via a unilateral transforaminal approach.

The aforementioned reports of fusion rates determined radiographically must be interpreted in light of previous studies that suggested that radiographic methods have limited reliability for the determination of fusion success.^67,⁶⁸ Other authors have cautioned that the use of local bone may be associated with pseudarthrosis, and if local bone is used, careful preparation and cleaning is necessary to avoid incorporation of soft tissue with the graft material.^69,⁶¹ Synthetic ceramic bone graft substitutes combined with iliac crest aspirate have also been reported to be highly effective for achieving interbody fusion,⁷⁰ but experience is limited.

More recently, there has been a surge of enthusiasm for the use of bone morphogenetic protein to achieve spinal fusion.⁷¹ Recombinant human bone morphogenetic protein (rhBMP-2) was shown to be highly effective in achieving spinal fusion when used inside a titanium threaded cage placed anteriorly in the intervertebral space. In a 2-year prospective randomized trial of 279 patients, fusion was achieved in 94.5% of patients compared with 88.7% in the control group in whom iliac autograft was used.⁷² Although not approved by the U.S. Food and Drug Administration (FDA) for posterior use, many surgeons have extrapolated the outcomes documented in the anterior clinical trial to posterior intervertebral applications. Review of the published data on outcomes of posterior interbody use of this material reveals conflicting results. Although some authors report universal fusion with few complications,⁷³ others have reported a worrisome list of unexpected outcomes.

A clinical trial of rhBMP-2 used in posterior threaded fusion cages was stopped by the FDA because of a high rate of new bone formation (28 of 34 patients) in the spinal canal or neuroforamina, although the authors of the study were enthusiastic about the fusion results and believed that there was no clinical significance to the new bone formation.⁷⁴ Radiculopathy and neurologic deficit associated with ectopic bone have been reported by other authors,^75,⁷⁶ and radiculitis has been reported with the use of rhBMP-2 even when no ectopic bone has formed. The use of a barrier between rhBMP-2 and the neural elements has been reported to eliminate this complication.^77,⁷⁸ Osteolysis and endplate resorption resulting in subsidence and cage migration have also been reported as a complication of using rhBMP-2. Reports regarding the clinical significance of these radiographic findings vary significantly, and many surgeons continue to advocate for the use of rhBMP-2 in posterior interbody applications.⁷⁹^–⁸² Optimal dosing and methods of application of this recombinant protein are unclear at this time. Although fusion rates seem to be high with the use of rhBMP-2, questions remain regarding safety, superiority, and cost-effectiveness.

Posterior Lumbar Interbody Fusion Versus Transforaminal Lumbar Interbody Fusion

Although Cloward, Steffee, Branch, and others emphasized the importance of a wide exposure to minimize dural retraction and nerve root injuries during PLIF, other authors described a more medial approach, which was often associated with a high incidence of nerve root injuries. This association was especially true before the advent of posterior instrumentation, when it was often recommended that the facet joints be preserved to avoid destabilizing the motion segment.⁸³ Dural and root injuries were particularly prevalent when threaded fusion cages were used posteriorly because of the emphasis on using large devices to achieve stability through annular distraction.⁸⁴ Because the cylinders were designed to cut into the endplates, the diameter needed was sometimes 50% greater than the height of the interspace achieved.³⁹ Even when using smaller devices, rates of neurologic complications of 17% were reported.⁸⁵

In 1997, Harms and colleagues ⁸⁶ reported a technique for performing a posterior interbody fusion through a more lateral approach than previously described.⁸⁷ These investigators advocated a unilateral approach to the disc space through the neural foramen by removing the facet joint. This approach allowed access to the disc space through the triangular “safe zone” between the exiting nerve root and the lateral dural edge.⁸⁸ In this way, an interbody fusion could be performed with minimal or no retraction of the dura. The technique has been enthusiastically adopted by many surgeons, and at least one group showed a reduction of nerve root injuries when comparing PLIF and transforaminal lumbar interbody fusion in their hands.^89,⁹⁰

One theoretical disadvantage of the transforaminal lumbar interbody fusion technique is that because only a unilateral approach to the disc space is done, it is impossible to remove the disc material completely. Javernick and colleagues ⁹¹ showed that, on average, 31% more disc material could be removed when a bilateral approach was done. This finding leads to concern that the fusion rate might not be satisfactory with a unilateral approach. McAfee and colleagues⁹² showed that for anterior interbody fusions, 100% of patients who underwent complete discectomies achieved a solid arthrodesis, whereas only 86% of patients with incomplete discectomies achieved solid fusion. In an animal model, the inclusion of disc material with autologous bone in cages led to a significant impairment of fusion.⁹³ Despite these concerns, although no prospective randomized comparison has been done, the unilateral approach to the disc space seems to result in adequate fusion rates.

In the author’s experience, the unilateral transforaminal approach is useful when extensive postoperative scar makes mobilization of the dura or exposure of one side of the disc difficult. When trying to correct deformity, the greater laxity of the motion segment achieved by a bilateral approach makes PLIF the preferred technique, however. The technique of PLIF described in this chapter involves extensive or complete facet resection, eliminating any differences in risk of nerve root or dural injury between PLIF and transforaminal lumbar interbody fusion.

Outcomes and Complications of Posterior Lumbar Interbody Fusion

The outcome of PLIF surgery depends on many factors. Probably the most important, as with any operation on the lumbar spine, is proper patient selection. Lumbar fusion done for the wrong reasons or on the wrong patient uniformly yields bad results. Review of the published outcomes of PLIF surgery makes it clear that PLIF is a technically demanding procedure. Variations in technique, care, and skill have significant influence on the rate of fusion and the frequency with which complications occur.

In large series, fusion rates greater than 95% have routinely been reported with PLIF, regardless of whether carbon fiber–reinforced cages, polymer cages, vertical titanium mesh cages, allograft bone spacers, or stand-alone cylindric threaded cages have been used.^19,⁹⁴^–⁹⁶ In contrast, only 77% of the patients reported by Rivet and colleagues⁹⁷ achieved solid arthrodesis, despite the use of cancellous iliac graft and supplementary pedicle fixation. Fuji and colleagues⁹⁸ reported a nonunion rate of 72% in their series of threaded cages placed posteriorly without supplementary fixation. It is unclear whether poor technique or other factors are responsible for the poor fusion results reported in those series.

Probably the most devastating complication that can occur with PLIF procedures is a nerve root injury.⁹⁹ The reported incidence of this complication varies widely, suggesting that surgical technique is an important factor affecting the frequency with which this complication occurs. Hosono and colleagues, in their review of 240 patients operated on by four different surgeons, correlated complication rates with the experience of the surgeon.⁸⁵ In that series, 41 patients had some kind of nerve injury, mostly transient. In Brantigan’s series,⁶¹ despite previous failed discectomy and the associated epidural scar in 83% of his patients, postoperative radiculitis was reported in only 2 of 100 patients, and one patient experienced a footdrop.

In the larger series of carbon fiber–reinforced polymer cages submitted to the FDA in which 221 patients were operated on by 15 surgeons, 3 cases of reflex sympathetic dystrophy and 3 cases of motor deficit, 1 of which was permanent, were reported.⁹⁴ Davne and Myers¹⁰⁰ reported only a 0.4% rate of traction root injury in their series of 384 PLIF procedures. Krishna and colleagues¹⁰¹ reported postoperative neuralgia in 7.1% of their patients, but they were able to reduce the incidence of this complication by half by modifying their technique and removing the superior facet to widen the exposure. Barnes and colleagues⁸⁴ reported a 13.6% incidence of permanent nerve root injury when using threaded fusion cages but no nerve root injury when using much smaller allograft wedges. These results reinforce the principle that wide exposure, careful technique, and avoiding oversized grafts can minimize the risk of neurologic injury in PLIF.

Graft displacement and loosening is another complication that was associated with PLIF when the technique was first described. This is a rare complication, however, with the addition of posterior pedicle screw stabilization. Subsidence of the implants can occur with PLIF. Most authors have not carefully documented this phenomenon. Factors predisposing to subsidence include inadequate graft technique and sizing and endplate injury during preparation. Patient factors such as weight and osteoporosis are likely important.¹⁰² Implant type may be a factor, but convincing evidence of this has not been presented.^103,¹⁰⁴ Pedicle screws alone do not prevent subsidence after PLIF.¹⁰⁵ Other complications of PLIF are similar to the complications encountered in all lumbar spine surgery. Epidural hematoma, wound infections, and other non–implant-related complications seem to occur with similar frequency in PLIF as in other reconstructive operations on the lumbar spine.