Anatomy and Approach

ALIF is performed by gaining access to the retroperitoneum. Many techniques have been described to gain access to the anterior lumbar spine (transperitoneal, laparoscopic, open and mini-open retroperitoneal). The mini-open retroperitoneal procedure has become the “workhorse” approach, with mini-open transperitoneal and open procedures saved for revision or salvage procedures. Retroperitoneal and transperitoneal corridors allow complete exposure of the ventral surface of the spine. Centering on the disc space with lateral extension, as needed, allows for a complete discectomy, release of ligamentous structures as indicated for balancing, and placement of a single large interbody implant that matches the vertebral endplate. Avoidance of the posterior musculature reduces postoperative pain and disability associated with extensive muscle stripping, denervation, and devascularization, often noted as a cause of failed back syndrome.

Surgical exposure to the anterior lumbar spine has been described elsewhere. Mini-open access, through either a transverse or a vertical incision, allows visualization from L2 to S1. The rectus can be mobilized medially in its entirety or split in the midline raphe and retracted laterally. Blunt hand dissection allows one to sweep the intraperitoneal contents superiorly, inferiorly, and medially. The ureter is brought medial, after identification, and deep self-retaining retractors can be placed. Vascular dissection is often performed solely with a sponge stick but may require sharp dissection for adhesions. Vascular structures (i.e., segmental vessels across the bodies) are cauterized or ligated and clipped as needed for exposure. Table-mounted or handheld retractors are employed during the remainder of the interspace preparation and interbody implantation. On completion of the procedure, the wound is inspected for any bleeding, ureter damage, or peritoneal tears. The rectus sheath is approximated with absorbable suture, and the skin is closed with a running subcuticular stitch for cosmesis.

Developed in the 1990s, laparoscopic ALIF achieved early success with its pioneers.¹² Laparoscopic ALIF was reported to be less invasive with less blood loss and faster recovery; however, later reports contradicted these findings, noting no identifiable advantages, added technical challenge, and increased specific complications such as retrograde ejaculation.^13,14

Indications for Interbody Fusions

Choosing to proceed with operative intervention on the spine should be a joint decision reached by the patient and the physician. Prospective patients should have tried and failed nonoperative management, and the clinical and radiographic picture should correlate to maximize results and patient satisfaction. ALIF can be chosen to aid patients with (1) DDD and the spectrum of clinical and radiographic changes noted under this umbrella term; (2) pain after prior posterior surgery, such as laminectomy in which significant epidural scar may complicate a revision surgery; and (3) significant low back pain with two or more recurrent herniated discs.

Axial back pain caused by the degenerative process of the disc and spine as a whole is currently the main indication for ALIF. By correctly identifying discogenic pain via radiographs, magnetic resonance imaging (MRI), and provocative discography, one can remove the pathologic disc and stabilize the motion segment with an interbody graft. Using an ALIF procedure, patients with associated radicular leg pain secondary to foraminal narrowing can be indirectly decompressed by restoring the foraminal height, elongating redundant posterior and lateral anulus (if not removed), and realigning overlapping incongruent facet joints and more directly decompressed by removing compressive nuclear material.^15,16 Patients with a herniated nucleus pulposus can undergo direct decompression and discectomy via an anterior exposure. Interbody implants placed through an anterior approach can be used alone or in conjunction with posterior fusion techniques for cases requiring a more robust construct, as discussed later in this chapter.

Relative contraindications to anterior interbody approaches include advanced atherosclerosis of the major vessels and obesity. Some consideration should be given to patients who have previously undergone prior abdominal surgery or have inflammatory diseases because these conditions create significant scarring that may increase the risk or preclude one from successful surgery. A transperitoneal approach may be performed as an alternative in these patients.

Interbody Implants and Graft Material

The race between temporary mechanical support and biology has been run since the origins of orthopaedic care. In the spine, the goal of any interbody device is to provide anterior column mechanical support while a bony fusion develops. A single question remains to be answered: How rigid does a construct need to be to provide early stability without negatively affecting the spine when fusion is present? Creating an unnecessarily stiff construct may lead to stress shielding,¹⁷ additional surgery, and implantation of costly implants; however, too little stiffness leads to biomechanical failure or pseudarthrosis. Pilliar and colleagues¹⁸ showed that small micromotion of 28 µm does not affect bone ingrowth into porous-surfaced implants and large micromotion greater than 150 µm can produce a fibrous interface. Nevertheless, the current consensus seems to be that adequate stabilization must greatly increase the stiffness above the native segment. In vitro studies do not fully recreate in vivo experiences, and individual patient factors, such as bone quality, size, and load demand, are variable and dynamic, complicating the goal.

The first lumbar interbody fusion was reported by Burns¹⁹ in 1933, using a tibial peg to treat spondylolisthesis. Interbody fusion for DDD was described by Harmon²⁰ in 1963 and then by Crock² and Stauffer and Coventry.⁸ Early days of interbody grafting incorporated the use of bicortical or tricortical spacers harvested from iliac crest. When used alone, this graft is associated with significant rates of mechanical failure, loss of correction, and pseudarthrosis.^21,22 Sterile allograft (i.e., bone dowels or tricortical grafts) have often been used because they are stronger than equivalent fresh, autologous bone and eliminate the need for autologous harvesting.²³

Femoral ring allograft (FRA), used frequently, obviates the need for cortical autograft and provides a strut with significant compressive strength,^22,23 incorporates with host bone, and provides a medium easier than metal to evaluate graft incorporation (Fig. 48–3). Previously, allograft rings were fashioned by surgeons on back operating room tables. Now precision machined grafts provide surgeons the option to trial size and implant the most appropriately sized graft to fit the patient’s anatomic needs and obtain the most stable construct with the allograft reaching the dense peripheral ring of subchondral bone. To augment fusion, allograft or autograft cancellous bone can be placed in the center of the cortical ring. FRA as a stand-alone intervertebral spacer has been shown to have a high rate of pseudarthrosis and subsidence.^24,25 Anterior or posterior augmentation has been recommended.

FIGURE 48–3 Single-level 360-degree fusion with FRA and bilateral pedicle screws and rod fixation.

Holte and colleagues²⁶ reported in 1994 on the use of FRA with and without supplemental posterior fixation. These investigators achieved fusion rates of 96% in some cases, depending on the number of levels fused. Sarwat and colleagues²⁷ reviewed 43 patients undergoing combined anterior-posterior fusions using FRA packed with cancellous allograft with supplemental posterior fixation; 100% of the one-level fusions and 93% of the two-level fusions were radiographically deemed solid fusions.

The addition of posterior fusion, instrumentation, and cortically lined grafts added to the mechanical stability of these constructs, but surgical time and complications led surgeons to seek other options. Transforaminal lumbar interbody fusion and posterior lumbar interbody fusion allowed surgeons to place interbody grafts from posterior approaches, providing anterior column support and negating anterior exposures. Posterior based surgery has been shown, however, to have increased operative time, patient morbidity, and complications compared with anterior based surgery.^28,29 Also, patient anatomy and the degree of pathology can make achieving an adequate discectomy and ideal graft size and position and placement a challenge. These factors have contributed to less restoration of disc height, less than complete fusion bed preparation, and less mechanical stability at the time of implantation than with ALIF.

Horizontal threaded cylinder cages, typically made from titanium, were brought on the market with huge fanfare but have fallen out of favor. These were originally developed to treat wobbler syndrome, which is a chronic cervical instability causing myelopathy in thoroughbred horses.³⁰ The U.S. Food and Drug Administration (FDA) investigational device exemption (IDE) studies for the BAK (Zimmer, Warsaw, IN) and Ray TFC (Stryker, Kalamazoo, MI) indicated that threaded cages used as stand-alone devices without supplemental posterior fixation performed well (Fig. 48–4).^31,32 Ray³² reported his results during the FDA IDE clinical trial using the Ray titanium fusion cages in 236 cases showing 91% fusion rate and 80% average clinical improvement.

FIGURE 48–4 Anteroposterior radiograph of threaded fusion cages at 1 year, with bone growth across cages.

Kuslich and colleagues³¹ reported on 247 ALIF patients with 24-month follow-up; they observed a fusion rate of 98% among single-level procedures and 80% among two-level procedures. They reported clinical pain improvement from a mean of 5 to 2.9 at 2 years, on a scale of 0 to 6. The functional scores improved from 20.9 preoperative mean to 15.2 at 2-year follow-up, on a 7 to 32 point scale evaluating sitting, walking, and other activities of daily living and recreational activities. Tran and colleagues³³ reported that only 5.2% of their group underwent additional posterior surgery at the same level for unresolved or new-onset pain, supporting the use of stand-alone cages. Preparation of the disc space for threaded cages violates the peripheral subchondral ring of bone, theoretically increasing the risk of subsidence. Although approved for stand-alone interbody fusion, threaded cages were later questioned for their amount of resistance to motion in the unstable spine; some authors adopted a 360-degree approach.^34,35

The advent of more stable anterior column support allowed surgeons to use less invasive posterior surgical options with stronger biomechanical constructs than with an associated posterolateral fusion.⁶ Translaminar screws, facet screws, and percutaneous pedicle screw constructs (termed 270-degree fusions) provided surgeons the additive effect of immediate mechanical strength and long-term fusion stability without necessitating significant posterior fusion bed creation, associated soft tissue devitalization, and patient morbidity (Fig. 48–5).^36–38 Ferrara and colleagues³⁹ compared pedicle and translaminar facet screws and found both to be reliable constructs with similar properties after 180,000 cycles and recommended that surgeon preference and patient-specific needs could drive the choice. In 2001, Schofferman and colleagues⁴⁰ prospectively compared the addition of posterior spinal fusion in patients who had ALIF with pedicle screw instrumentation (360 degrees vs. 270 degrees). These investigators found that posterolateral fusion was associated with greater blood loss, operating room time, and cost, with no significant improvement in the arthrodesis rate; in addition, 68% of posterolateral fusions failed to show radiographic fusion.

FIGURE 48–5 Two-level 270-degree fusion with translaminar facet screws.

Previously, stand-alone anterior interbody grafts were fraught with reported complications, such as migration, subsidence, and pseudarthrosis. As interbody graft technology has progressed, there has been renewed interest, however, in stand-alone anterior interbody fusion, single-approach procedures. Biomechanical studies have reported that stand-alone anterior interbody grafts without fixation are weakest in sheer strength, rotation, and extension.⁴¹ The addition of a blocking screw or plate for FRA and synthetic spacers such as polyetheretherketone (PEEK) has minimized the occurrence of anterior migration. Currently available in PEEK, metal carbon fiber, and machined allograft and other various materials and combinations, interbody synthetic grafts have been optimized compared with their predecessors. Cage design from cylindric to a box shape has led to better matching of the endplate geometry and has been shown to decrease motion at the intervertebral segment.⁴² Grant and colleagues,⁴³ performing a human cadaveric biomechanical investigation, showed that lumbar endplate density and thickness increased toward the periphery, with the strongest being posterolateral just in front of the pedicles.

With the addition of anterior intersegmental instrumentation, many authors are performing stand-alone anterior interbody fusions. In 1958, Humphries and Hawk⁴⁴ first reported the use of an anterior plate to stabilize the motion segment after ALIF. Potential danger to the great vessels and limited use of the ALIF technique contributed to the early demise of this technique, however. Tzermiadianos and colleagues⁴⁵ showed marked in vitro reduction in range of motion using interbody graft with the ATB (Synthes Spine, West Chester, PA); they posited that the ATB may be a biomechanically sound construct, especially compared with additional posterior surgery and its morbidity. Gerber and colleagues⁴⁶ noted slightly better motion segment immobilization using pedicle screws versus an anterior plate in cadaveric spines with double-threaded metallic cages.

Larger, more robust, locking and nonlocking plates have been used to augment a stand-alone interbody graft; although effective, these plates require increased exposure and have a large anterior profile. To negate the exposure necessary for plating and minimize soft tissue and vessel irritation, anterior interbody synthetic products with fixation through the graft into the vertebral bodies have been developed. By increasing their stand-alone biomechanical strength to minimize extension and sheer forces, one hopes these products may negate the need for a robust anterior plating and posterior surgery.

Many studies have shown the stability-enhancing effect of integrated anterior instrumentation and a biomechanical argument for stand-alone ALIF (Fig. 48–6). Kuzhupilly and colleagues³⁷ looked at stand-alone FRA versus industrial FRA with integrated crossed cancellous screws into the adjacent vertebral bodies. They found significant improvement in extension stability only. Le Huec and colleagues⁴⁷ tested an anterolateral threaded cage connected to an anterolateral cage and found significant increase in stiffness in all loading directions. No statistical significance was noted whether or not the plate was attached to the cage.

FIGURE 48–6 A and B, Anteroposterior (A) and lateral (B) intraoperative fluoroscopic images with L5-S1 single-level stand-alone interbody graft with incorporated hardware.

Schleicher and colleagues⁴⁸ compared the SynFix-LR (Synthes GmbH, Solothurn, Switzerland) and the STALIF (Centinel Spine, Minneapolis, MN), two stand-alone ALIF cages, and found statistically significant increase in stiffness over the native in vitro segment, with the SynFix-LR, a locking four-screw implant, showing a higher stabilizing effect in lateral bending than the STALIF. Flexion-extension finite element analysis revealed that the cage bears most of the force in flexion, and the screws and the screw-plate interface take most of the stress in extension. Cain and colleagues⁴⁹ showed the SynFix-LR had equal stiffness versus 270-degree and 360-degree constructs. The test device showed a higher ability to withstand axial torque compared with standard pedicle screw instrumentation. At the present time, a few products have FDA approval for stand-alone interbody use, with many more devices seeking approval.

Most current implants allow the incorporation of graft material to augment the fusion process. Historically, the “gold standard” has been autogenous morcellized bone graft; however, to minimize patient morbidity, allograft and biologic materials have been developed. Multiple studies have evaluated allograft efficacy in fusion patients as extenders and stand-alone with good results.⁵⁰ The addition of the recombinant human bone morphogenetic protein (rhBMP-2) InFUSE (Medtronic, Memphis, TN) has been shown to promote osteoinduction and to stimulate early incorporation of grafts (Fig. 48–7).^51,52 Burkus and colleagues⁵² reported 100% fusion rate with stand-alone interbody fusion using threaded cylinder allograft dowels with rhBMP compared with autograft. Pradhan and colleagues⁵³ showed, however, that the use of rhBMP-2 with stand-alone FRA can lead to an aggressive early osteoclastic response causing graft and endplate osteolysis with potential subsidence risk. These authors believed augmenting with intersegmental instrumentation can support the FRA with bone morphogenetic protein during this mechanically vulnerable time.

FIGURE 48–7 Six-month follow-up lateral radiograph of graft using femoral ring allograft and bone morphogenetic protein, with posterior bilateral pedicle screws showing bone bridging and solid fusion.

Circumferential Fusion

Circumferential fusion is often indicated in patients with significant instability (i.e., trauma, postlaminectomy), significant bone loss (i.e., trauma, osteomyelitis), high risk for nonunion (i.e., multilevel, tobacco users), or osteoporosis.^54,55 To increase fusion rates, decrease subsidence risk, and maximize construct stiffness, many surgeons adopted posterior instrumentation, including pedicle screw–based systems, translaminar screws, or facet fixation. There is no question that the addition of posterior instrumentation provides greater construct rigidity; however, the ideal amount of micromotion to stimulate fusion while structurally stabilizing the motion segment until fusion occurs has yet to be determined. Pedicle screw–based instrumentation has been shown to decrease the subsidence rate associated with interbody allografts and cylindric cages.⁵⁶ The disadvantages of posterior supplementation include increased operative time, increased blood loss, increased cost, and potentially a more difficult recovery for patients. Factors for and against posterior instrumentation are present in all cases and must be weighed carefully to optimize patient outcome and minimize morbidity.

Postoperative Management

Patient-controlled analgesia is instituted for pain control and weaned as bowel function and oral tolerance allow. The patient is mobilized the day of surgery, starting with chair transfers and assisted ambulation. A liquid diet is started after surgery and progressed as tolerated. For stand-alone one-level anterior fusions, hospital stay is typically 1 to 3 days, based on patient type and surgery performed. Bracing is variable, dependent on stability of fixation and surgeon specific, ranging from an abdominal binder for comfort and incision healing to a rigid thoracolumbosacral orthosis until clear evidence of fusion has occurred. The addition of instrumentation, bone morphogenetic proteins, and similar substances may obviate the need for rigid bracing. Activity should include limited bending and twisting motions, and lifting initially should be limited to 10 lb or less. Return to activities of daily living is progressively allowed and encouraged. Return to work and sport depends on the patient and surgeon, based on the patient’s requirements, status of fusion, and fixation method.

Evaluation of fusion progression can be difficult in the presence of an interbody graft. Evaluation of fusions that is 100% reliable is available only with histologic examination, which is rarely possible or done. Radiolucent and nonferromagnetic implants such as carbon fiber and polymer synthetics have been developed to allow better visualization of surrounding bone and soft tissues on plain radiographs, MRI, and computed tomography (CT). Cizek and Boyd⁵⁷ evaluated cadaveric implant models radiographically and found neither CT nor radiographic interpretation reliable. Evidence of a fusion can be evaluated by the following radiographic features: (1) no motion on flexion-extension films (<5 degrees has been accepted as fused in the literature and by the FDA, which the authors believe is too lenient); (2) anterior or posterior bridging bone across the disc space, termed sentinel sign⁵⁸; (3) bridging trabecular bone across the intervertebral space and endplates; and (4) the absence of “a windshield wiping halo,” or lucency around instrumentation indicating implant motion.

Complications

The incidence of complications in ALIF is often difficult to assess because of the different types of procedures, surgeons performing them, implants used, and experience of the operative team. Poor outcome is the number one surgical complication, and poor patient selection is often to blame; one must be vigilant in diagnostic workup and adhere to surgeries that address the patient’s pain generators.

Vascular injury to the common iliac vessels with L5-S1 exposure is most commonly seen. Clear identification of the disc space before discectomy must be obtained and maintained during the operation. In particular, the common iliac vein, being compressible and dorsal to the artery, can be mistaken for soft tissue during the approach. The iliolumbar vein, also termed the ascending lumbar vein, is at risk during approaches to the L4-5 interspace and should be controlled as dictated by the amount of exposure necessary. Some surgeons believe ligation of this vessel should be obtained in 100% of exposures to minimize the risk of tearing during retraction. Arterial thrombosis from aggressive retraction or injury or both has also been reported.^59,60

Retrograde ejaculation as result of hypogastric plexus injury has been reported to range from 8%⁶¹ to 0.4%.⁶² In one of the largest single ALIF trials to date, Kuslich and colleagues³¹ reported a 4% rate in 591 patients. Loguidice and colleagues¹¹ in 58 patients and Brau⁶³ in 686 exposures each noted only 1 case of retrograde ejaculation. The preaortic (prevertebral) sympathetic plexus runs along the anterolateral edge of the vertebral bodies, adjacent to the psoas, then traverses over the aortic bifurcation and common iliac vessels forming the hypogastric plexus. Blunt dissection to mobilize the more cephalad prevertebral plexus before the hypogastric plexus can aid the exposure. Aggressive electrocautery should be minimized during the approach in this area and during the disc space preparation. Male patients should be counseled on this potential adverse event and advised that there also is a chance of spontaneous recovery. If the patient is concerned, he can predonate and store sperm.

Injury to the alimentary tract can be minimized by packing the peritoneum behind self-retaining retractors. Postoperative ileus occurs and can be effectively managed with restricted oral intake, proper fluid hydration, and gastric suction as indicated. Damage to the ureter, rare in primary cases, can be minimized in revision cases with proper identification and the placement of preoperative ureteral stents.

Implant-related complications, such as graft subsidence, malposition, extrusion, and pseudarthrosis, as alluded to previously in this chapter, can be minimized with careful patient selection, careful implant selection, and meticulous discectomy to optimize the fusion bed without disrupting the structural integrity of the subchondral bony endplates.

Revisions

Indications for returning to the anterior spine include adjacent level disease, implant failure or migration causing neurovascular compromise, and pseudarthrosis. A meticulous approach minimizing dissection during the primary surgery can reduce the risks associated with dissection and grafting of reoperations for adjacent level disease. In patients undergoing an L4-5 procedure after prior L5-S1 exposure, the risk of vascular, ureteral, and neurogenic injury can be minimized by avoiding the prior L5-S1 exposure between the iliac vessels and concentrating on the cephalad tissue dissection. Products to decrease postoperative scar and to allow easier revision dissection are being developed and tested. A product creating a safe plane for dissection and mobilization of the major vessels would be useful. One must also be cautious when approaching inflammatory patients with disease or postinfections because these patients often form abundant fibrous material and adhesions anterior to the vertebral motion segments and can be as difficult to treat as patients undergoing revision surgery.