Patient Selection

Proper patient selection is vital to the success of an ALIF procedure and is the best predictor of a good surgical outcome. Operative indications for ALIF include degenerative disc disease with intractable back pain. Degenerative spondylolisthesis with back pain often accompanies degenerative disc disease. Other indications for ALIF may include a failure of prior dorsal surgery (pseudarthrosis) or postoperative spondylodiscitis. Most important, the symptoms should have persisted for a prolonged period. In general, symptoms are present for 6 months or longer before a patient undergoes an ALIF operation for back pain. Likewise, a prolonged trial of conservative therapy should have been attempted and failed before planning an ALIF procedure. ALIF is also commonly used in the surgical management of sagittal plane deformities (i.e., flat back syndrome).

Medical Considerations

Contraindications to ALIF are absolute or relative. The only true absolute contraindication is advanced osteoporosis that prevents the vertebral bodies from supporting an interbody graft without subsidence. Relative contraindications occur in varying degrees. Neural compression as a result of either disc disease or stenosis is a relative contraindication. If neural compression is present, it is most safely and thoroughly approached from a dorsal approach. Other relative contraindications to ALIF include prior retroperitoneal surgery, severe peripheral vascular disease, active disc space infection, neoplasm, infrarenal aortic aneurysm, anomalous genitourinary system with only a single ureter, and systemic medical illness that precludes the surgery. Also, ALIF may be relatively contraindicated in men who wish to reproduce because the risk of retrograde ejaculation has been described to occur in varying degrees. As a general rule, an ALIF procedure is associated with less blood loss and is better tolerated than the majority of dorsal lumbar surgical procedures.

A trial of nonoperative therapy is mandatory before undergoing ALIF surgery. Mainstays of nonoperative treatment include optimizing body weight, which usually consists of weight loss in most patients. Cessation of smoking is also important because smoking is known to decrease fusion rates. In addition, a physical therapy program, which typically includes core-strengthening to stretch and strengthen the back and abdominal muscles, should be instituted. Consultations with a pain management specialist, as well as a psychological evaluation, may be useful adjuncts in determining the optimal candidates for this procedure. Chronic pain may cause varying degrees of depression, anxiety, and aggression, and these behavioral changes can be reversed if the cause of pain is discovered and corrected. The efficacy of other conservative therapies such as epidural steroid injections, acupuncture, chiropractic manipulations, and biofeedback are more debatable. A preoperative trial of bracing may be recommended; however, there is not general agreement on the utility of this practice. Advocates of preoperative bracing believe that surgical results are superior if there is a good response to preoperative use of an external orthosis.

It is important to assess the patient’s level of function before surgery. It should be determined whether the patient is able to maintain employment and enjoy recreational activities. In addition, simple parameters such as walking distances in a pain-free state, as well as total walking distance, should be assessed. If the preoperative level of function is equivalent to the level that a good surgical result would yield, then ALIF should not be performed. Back pain of a magnitude sufficient to consider a fusion procedure should substantially limit a patient’s level of function.

Since the early 2000s, the importance of functional outcomes has become increasingly recognized. Merely being “neurologically intact,” evidenced by no objective motor or sensory deficits, is not adequate. The ability to function, which is often impacted by the degree of pain experienced, is important and can be assessed using validated patient controlled outcomes measures. Accordingly, good outcomes in 2015 require success in both the neurologic and the functional realms.

It is essential for both the surgeon and the patient to have realistic expectations of the surgical outcome, and this is one of the reasons for a prolonged, conservative trial. Surgical success should be evaluated by a return to normal activities of daily living rather than by the hope for a return to high-level athletic activities. Surgical success cannot be fully evaluated until a minimum of 6 to 12 months after the operation. In general, it is not until the fusion process has been completed that surgical success can be fully evaluated.

Well-known negative prognostic indicators include involvement in workers’ compensation or litigation cases, because these patients are known to have higher incidences of suboptimal long-term clinical results. This is particularly worrisome because the operative indications for ALIF are often subjective (low back pain) rather than objective (neurologic deficit).

Imaging

Before considering an ALIF procedure, a detailed radiographic imaging workup is required. The initial workup includes a plain-film radiographic series with flexion and extension lateral views. Flexion and extension radiographs may not reveal segmental instability because many painful motion segments are caused by rotational destabilization. These radiographs are used to evaluate spinal stability as well as to confirm the number of true lumbar vertebrae. In preoperative planning, these images are correlated with advanced radiographic imaging studies, such as magnetic resonance imaging (MRI), computed tomography (CT) scans, and myelography with postmyelography CT scanning. It is important to evaluate the lumbar interspaces beyond the zone of the previous spine surgery to identify any additional potential sources of pain.

In cases of an L4-5 fusion, it is important to review the anteroposterior (AP) radiographs to note the position of the L4-5 interspace relative to the pelvic brim. Some surgeons have advocated that patients with a low-seated L4-5 interspace in relation to the pelvic brim have a higher success rate in fusion and clinical outcome than patients with a higher-seated L4-5 interspace. The higher-seated L4-5 interspace may also be complicated by a more difficult exposure.

A myelogram, with a postmyelography CT scan, is considered by some to be the gold standard to rule out neural compression. A myelogram and CT scan are particularly useful in cases of lateral recess stenosis. In addition, CT scanning is an excellent tool for evaluating bony pathology and recording measurements of the cross-sectional diameters of the vertebral bodies on axial views. Lastly, if prior instrumented spinal fusion has been performed, CT-myelograms are less impacted by artifact from the instrumentation than MR images would be.

MRI is extremely useful in defining soft tissue pathology. In particular, T2-weighted sagittal images are valuable in determining disc dehydration as evidenced by decreased T2 signal intensity. In addition, MRI can evaluate motion segments adjacent to a previously operated level, neural compression secondary to a herniated lumbar disc, and central stenosis. Furthermore, axial measurements to be used in preoperative planning can be accurately obtained from MRIs. MRI will also reveal the presence of facet disease or other dorsal disease, which may influence the decision to back up the ALIF with a dorsal fusion. An ideal candidate for ALIF would have an isolated, painful degenerative disc at the L5-S1 level with at least a 50% loss of disc height.

Discography is a more controversial preoperative radiographic imaging technique. Some advocates of discography promote the use of a provocative discogram to evaluate patients who are considering lumbar interbody fusion. With a provocative discogram, the patient’s typical pain pattern must be reproduced at the involved level in an awake, nonsedated state. This is referred to as a concordant pain response. In addition, injection at other levels of the lumbar spine must not reproduce the patient’s typical low back pain pattern. Discography should be used to confirm MRI findings. It can be most effective in determining the number of levels to be fused in cases of multilevel disease. In patients who show discrepancies between the results of discography and MRI, consideration should be given to delaying surgical therapy and pursuing other conservative treatments. In other words, diagnostic imaging techniques should be used to decrease, not increase , the percentage of surgical candidates. It should also be noted that by penetrating the disc for this diagnostic test, it is possible to accelerate or cause disc degeneration.

The number of levels to be fused is an important yet controversial topic. Many studies have clearly demonstrated that pseudarthrosis rates are increased with each additional level of fusion. Thus, clinical success rates are lessened with each additional fused level. Furthermore, there is an increased chance of adjacent motion segment degeneration with longer fusions, which is particularly problematic at the level immediately rostral to a lumbar interbody fusion. The number of levels fused is most frequently determined by a review of the radiographic imaging studies. Typically, plain flexion and extension radiographs that demonstrate pathologic motion at the proposed fusion site are helpful in demonstrating segmental instability. In addition, MRI scans, especially T2-weighted sagittal images, which demonstrate loss of signal consistent with disc dehydration, may help to define involved segments. Finally, it is the opinion of some authors that a provocative discogram confirming the results of the MRI or the flexion and extension radiographs may be useful in determining the number of levels to be fused.

Before an ALIF surgery takes place, a detailed general medical evaluation should be completed. Smoking is known to decrease fusion rates in spine surgical procedures, and as a general guideline, patients are requested to quit smoking a minimum of 8 weeks before elective surgery and to continue to not smoke until radiographic fusion has been confirmed. Additional factors known to detrimentally affect both clinical and radiographic fusion rates include diabetes mellitus, an immunocompromised state (especially hepatitis C infection), and chronic corticosteroid use.

ALIF is performed for chronic low back pain. As such, patients who have experienced prolonged courses of narcotic analgesic use have a poorer clinical outcome with respect to overall pain control than patients who have not used narcotics preoperatively. It is unclear whether the worsened clinical outcome in patients requiring narcotic analgesics before surgery is due to more severe disease, changes in the pain receptor mechanisms, or psychological factors.

The use of oral contraceptives or any drugs containing estrogens is known to increase the incidence of deep venous thromboses during the postoperative period. As a result, if possible, these drugs should be discontinued for a minimum of 3 months before surgery. If there is any chance that an ongoing spinal infection or neoplastic process is present, it should be thoroughly evaluated before considering ALIF surgery. The presence of peripheral vascular disease should also be investigated preoperatively with a detailed evaluation by physical examination of the aorta and lower extremities. Likewise, if there are genitourinary abnormalities, imaging studies should be performed to confirm the presence of two functioning kidneys and two functioning ureters. Abnormalities of the reproductive system, particularly evidence for ejaculatory dysfunction in a male patient, should be evaluated before surgery. It is essential to document preoperative sexual dysfunction to differentiate this from a problem that may result due to the surgical procedure. The presence of osteoporosis should be evaluated by bone mineral density testing before any consideration of elective surgery.

The preoperative regimen for ALIF should include a bowel preparation the night before surgery. A bowel preparation makes retraction of the intraperitoneal structures easier and increases safety by decreasing the possibility of an iatrogenic bowel injury. In addition, routine preoperative antibiotic coverage directed at Staphylococcus and Streptococcus species should be administered. This is usually accomplished with a first-generation cephalosporin antibiotic.

Positioning

Operative positioning depends on the level to be operated on. Most ALIFs are performed at the two most caudal motion segments of the spine, L4-5 and L5-S1.

General principles of spine surgery should be followed for operative positioning. As a rule, all bony prominences must be padded, with particular attention paid to protection of the ulnar nerve at the elbow and the peroneal nerve at the fibular head. In addition, antiembolic stockings and sequential compression devices are useful intraoperative adjuncts. Because no neural decompression is performed during an ALIF procedure, there is no need for intraoperative evoked potential monitoring or the use of electromyography. The operating table must allow for both AP and lateral fluoroscopy because this feature is essential for accurate confirmation of placement of a bone graft or intervertebral cage.

The best operative positioning is obtained by placing the lumbar spine in a position of lordosis. This position is achieved by reversing the flex on the table and placing a roll or bump under the low back region. These maneuvers help open the interspace ventrally, improve the sagittal plane alignment, and facilitate the discectomy.

When possible, it is best to keep the patient in a true supine position. This is ordinarily achievable without difficulty at the L4-5 and L5-S1 levels. At progressively rostral levels, it may be necessary to position the patient slightly in a right lateral decubitus position. Lateral positioning may be necessary to aid in retraction of the intraperitoneal contents during retroperitoneal exposure. Any lateral positioning leads to increased difficulty in maintaining orientation to the true bony midline, making the placement of the interbody graft more difficult. As a general rule, thin patients can be maintained in a true neutral supine position, whereas more obese patients may require some lateral rotation on the operating table to allow gravity to assist with retraction of the intraperitoneal contents.

Incision

In any surgical procedure, a properly placed skin incision is essential for achieving optimal exposure. The usual approach for an ALIF procedure is by retroperitoneal exposure from the patient’s left side. Most approach surgeons prefer the left side because it allows the great vessels to be approached from the side of the aorta. It is easier to retract the infrarenal aorta from left to right than to retract the inferior vena cava from the patient’s right side. A left-sided retroperitoneal approach minimizes the need for any retraction of the inferior vena cava and thus decreases the possibility of injuring or thrombosing this structure.

The nerves, muscles, and ureters that surround the lumbar spine are ordinarily symmetrical. Only vessel asymmetry determines the need for a left-sided incision. Occasionally, prior abdominal or retroperitoneal surgery may determine the need to perform ALIF from the patient’s right side.

Incisions vary based on the level of surgery. Single-level ALIF at either the L4-5 or L5-S1 interspace may be performed using a right-angled incision. The standard retroperitoneal incision includes a horizontal limb extending from the midline to the linea semilunaris (lateral border of the rectus abdominis muscle). This horizontal limb is located at a point between the umbilicus and the pubic tubercle. The vertical limb of the incision extends up the linea semilunaris for a variable length, depending on the number of levels to be fused and the physical size of the patient. For a single-level ALIF, the vertical limb may be approximately the same length as the horizontal limb ( Fig. 76-1 ). In some thinner patients, the vertical limb may be considerably less or even nonexistent. When multiple levels are to be fused, the vertical limb extends a longer distance than the horizontal limb. At progressively higher levels, it is necessary to orient the incision in an oblique fashion, extending toward the tip of the left 12th rib. This oblique incision may be necessary to gain access to the retroperitoneal space of the L1-2 and the L2-3 interspaces. At more rostral levels, care must be taken to avoid retraction on the spleen, which is susceptible to minor blunt trauma from the retractors.

Incision for an L5-S1 or L4-5 anterior lumbar interbody fusion.

(Copyright University of New Mexico, Division of Neurosurgery, Albuquerque, with permission.)

Exposure

The most commonly used technique is a left-sided retroperitoneal dissection. Other methods include a transperitoneal open procedure as well as a laparoscopic approach. The difficulties with transperitoneal surgery include prolonged postoperative ileus, as well as problems with fluid management and “third spacing” secondary to fluid shifts from an edematous bowel. As such, the open transperitoneal approach is less frequently used in favor of retroperitoneal exposure. Laparoscopic techniques are discussed elsewhere; however, they do not have long-term follow-up data that support their use because they typically have a steep learning curve and prolonged operative time. Although laparoscopic approaches were popularized in the 1990s, they have fallen out of favor and become less often utilized due to prolonged operative times and increased incidence of complications.

It is mandatory that a surgeon who is comfortable with the surgical approach provide the exposure. The spine surgeon may elect to perform his or her own exposure. Alternatively, a general surgeon or a vascular surgeon familiar with the retroperitoneal region as well as the management of the potential complications that may occur during the operation can perform this portion of the procedure. Proper techniques of vessel ligation are vital to successful exposure of the disc spaces. With a left-sided retroperitoneal approach, the intraperitoneal contents are dissected away from the retroperitoneal space from the left side toward the midline. The ureter must be identified and protected. During the exposure, it is best to avoid cutting longitudinally oriented structures. The dissection must be performed ventral to the psoas muscle. In thin patients, it is possible to dissect into the retropsoas space, which can result in excessive postoperative fluid collections, blood loss, and the possibility of injury to the genitofemoral and ilioinguinal nerves.

In patients who have not undergone prior retroperitoneal surgery, gentle blunt dissection is the safest, quickest, and easiest technique. If the peritoneum is violated during the retroperitoneal approach, it should be immediately closed to prevent development of a postoperative hernia. Closure is accomplished by isolating the edges of the peritoneum and using an absorbable suture with a continuous stitch. An unrecognized peritoneal violation can result in a postoperative hernia and subsequent bowel obstruction or incarceration. If the bowel proper is violated during retroperitoneal exposure, it should be treated with immediate irrigation, followed by layered closure, with the mucosal and serosal layers repaired separately, and termination of the ALIF procedure. In addition, broad-spectrum antibiotics, with both aerobic and anaerobic coverage, should be instituted immediately. In spite of the excellent bony and vascular anatomic landmarks present in the ventral lumbar spine, radiographic confirmation of the correct level is mandatory before performing an ALIF. Intraoperative radiographs should be compared with preoperative plain radiographs to confirm the appropriate level. Surgery at the incorrect level has been previously reported.

Each level of the lumbar spine has distinct anatomic structures that must be addressed during a retroperitoneal approach. In general, lumbar segmental arteries should be preserved unless specific arterial damage has occurred. There is no need routinely to ligate these vessels, which are involved in supplying blood to the vertebral body proper.

At the L5-S1 interspace, there is typically no need to mobilize the iliac vessels. This interspace is routinely located between the right and left common iliac vessels and is easily visualized without vessel dissection. Important structures at the L5-S1 level include the autonomic nerves that traverse the prevertebral space at this level. These nerves may be damaged by use of monopolar electrocautery, and this can lead to autonomic dysfunction. In men, this is manifested as retrograde ejaculation. To avoid this complication, a monopolar electrocautery should not be used in the prevertebral space. In addition, a vertical opening of the midline prevertebral space should be used with gentle blunt dissection laterally to free all overlying soft tissues and, in the process, to gently dissect the hypogastric plexus away from the ventral disc space at the L5-S1 level ( Fig. 76-2 ). Rates of postoperative retrograde ejaculation have been reported as ranging from 0.42% to 22.5%, with the transperitoneal approach carrying an approximately 10-fold greater risk.

After retraction of the retroperitoneal and abdominal structures, a vertical incision is made in the prevertebral space. A, Gentle retraction laterally gains access to the L5-S1 and, occasionally, the L4-5 disc interspace. B, Vessels are ligated and clipped or tied.

(Copyright University of New Mexico, Division of Neurosurgery, Albuquerque, with permission.)

At the L4-5 interspace, the left common iliac vessels routinely traverse the prevertebral space and thus prevent direct access to the space. These vessels must therefore be mobilized from left to right to expose the bony midline. Before mobilizing the iliac vessels, however, vessel loops should be passed proximally and distally to obtain vascular control of both the left common iliac artery and the left common iliac vein. In addition, the left iliolumbar vein routinely enters the left common iliac vein laterally at a variable distance caudal to the inferior vena cava. It is necessary to locate and ligate the left iliolumbar vein before medially mobilizing the left common iliac vein. This ligation will allow for better exposure of the L4-5 interspace because the vessel typically courses over the L5 vertebral body and tethers the iliac vein. If the left common iliac vein is mobilized without ligation of the iliolumbar vein, extensive hemorrhaging can occur secondary to avulsion of this vessel ( Fig. 76-3A ). If an iliac vein is damaged, direct pressure should be applied and proximal and distal vascular control obtained. Fine, nonabsorbable suture is used to close the venous defect primarily with a continuous suturing technique. Common iliac vein injuries have been described in 2% to 4.5% of cases. Although they have been reported, injuries of the common iliac artery and inferior vena cava are rare.

At the L4-5 interspace, lateral retraction initially exposes the left common iliac vein beneath the left common iliac artery ( A ). The iliolumbar vein is then clipped and ligated or tied. After this, further retraction gains access to the ventral L4-5 and L3-4 disc interspaces ( B ).

(Copyright University of New Mexico, Division of Neurosurgery, Albuquerque, with permission.)

At the L3-4 disc space, the iliac vessels must be mobilized medially. In addition, a minimal amount of mobilization of the distal aorta from left to right is necessary ( Fig. 76-3B ). There is no need to mobilize the inferior vena cava during a left-sided approach to the lumbar spine at any level. If surgery is necessary at the L1-2 levels, the aorta will need to be mobilized from left to right to gain exposure to the true bony midline. In addition, care must be exerted at the L1-2 level to avoid iatrogenic injury to the left renal artery if overly vigorous medial displacement of the aorta is performed.

Retraction

After exposure of the retroperitoneal space, a table-mounted, self-retaining retraction system is useful. It should have malleable, broad-bladed retractors to disperse pressure evenly. Laparotomy pads are used to cover the retractor blades to protect the sensitive intraperitoneal structures. After the self-retaining retractor system is positioned and the correct level identified, a lateral fluoroscopic image can confirm the appropriate level. An AP fluoroscopic image is necessary to mark the true bony midline with an indelible marking pen or methylene blue dye. Maintaining orientation to the bony anatomic midline is critical to the optimal performance of an ALIF procedure.

Excessive retraction can lead to injuries to the peritoneum, the intraperitoneal structures, or the major blood vessels. The intraperitoneal contents are best retracted using the self-retaining retractor system; however, the iliac vessels and the aorta should not be retracted by this method. The use of self-retaining retractors on these vessels can be associated with an increased risk of vascular injury and an increased risk of thrombosis of either the artery or the vein. The major vessels are best retracted by handheld instruments manipulated by a surgical assistant under direct vision. Thus, pressure on the vessels can be frequently released, which helps to prevent development of an intramural thrombus. If one of the iliac vessels or the aorta is injured, the umbilical tapes, which provide distal and proximal vascular control, are used to halt the bleeding. This is followed by primary vessel repair using nonabsorbable sutures.

Iliac artery thrombosis can be detected by carefully monitoring peripheral pulses and skin color in the lower extremities. If a thrombus is detected or strongly suspected, an intraoperative angiogram should be obtained immediately. Thrombectomy is the treatment for a positive angiogram. In addition, if the ureter is injured during the retraction or exposure, placement of a ureteral stent is ordinarily performed by a general or urologic surgeon.

Discectomy

After the correct level has been confirmed with lateral fluoroscopy and an AP fluoroscopic image has confirmed the midline, the midline is marked on the vertebral bodies above and below the disc space to help maintain orientation. At this stage in the operation, long-handled instruments are useful. Commercially available curettes and rongeurs 13 to 15 inches long facilitate performance of the discectomy. To determine the depth of the discectomy, confirmation should be made based on the preoperative MRI study or CT scan. Particularly at the L5-S1 interspace, it is important to confirm that all tissue overlying the disc space has been gently and bluntly swept laterally before incising the ventral anulus fibrosus. Once again, avoidance of monopolar electrocautery diminishes the incidence of postoperative autonomic dysfunction.

A symmetrical window is cut into the ventral anulus fibrosus, centered off of the anatomic midline, which takes care to preserve the lateral aspects of the disc ( Fig. 76-4 ). It is essential to maintain orientation to the midline and stay within the anulus fibrosus, with care being taken not to violate the anulus laterally or dorsally. The nucleus pulposus is removed, up to the dorsal anulus fibrosus. This can be confirmed with lateral fluoroscopy. Calibrated tips on the instruments are useful at this stage. With use of some of the newer cage techniques, a predetermined cylinder of disc material is removed. In a standard lumbar interbody fusion, all articular cartilage is removed from the bony end plates above and below the disc space, with care taken to preserve the subchondral bone of the end plates. As a general rule, disc material is removed 25 to 30 mm in depth and 30 to 35 mm in width, centered at the bony midline. These measurements are consistent for the lower lumbar motion segments. Variations exist in the AP dimensions of the vertebral bodies. The oval shape of the lumbar vertebrae accounts for a larger depth in the midline than in the lateral portions of the vertebrae. It is important to avoid the use of bone wax for bleeding cancellous bone because foreign bodies may decrease the postoperative fusion rates.

An annulotomy is performed as depicted, taking great care not to proceed too far laterally.

(Copyright University of New Mexico, Division of Neurosurgery, Albuquerque, with permission.)

Excessive depth of the discectomy can be catastrophic. The dorsal anulus fibrosus is not well visualized from an ALIF approach, and if it is violated, epidural bleeding or a dural injury can occur. These are usually not well controlled by a limited exposure and are best avoided. If the dorsal anulus fibrosus is violated, the disc space must be observed for bleeding or a cerebrospinal fluid (CSF) leak. Bleeding is best controlled with copious amounts of irrigation, as well as with the use of thrombin-soaked Gelfoam. Patience is the key because access in this area is extremely poor. It is unrealistic to attempt bipolar cautery on a bleeding epidural vessel unless it is directly visualized. Monopolar electrocautery must be avoided at all times in the epidural space. If a CSF leak occurs, free muscle and fascia grafts, with or without fibrin glue, are used to attempt to control the egress of CSF. Consideration should be given to turning the patient into the prone position at the completion of the ALIF surgery and placing a lumbar subarachnoid drain to divert CSF. Safe and reliable decompression of the thecal sac or nerve roots is not realistic from an ALIF approach, and if neural decompression is necessary, it should be performed using a dorsal procedure or as part of a combined procedure.

Distraction

Several commercially available systems have been developed for ALIF. These systems have disc space distractors with calibrated tips to prevent excessively deep disc space distraction ( Fig. 76-5 ). Improved distraction can also be obtained by proper patient positioning. This helps to maintain lumbar lordosis and in the process opens up the ventral aspect of the interspace. Particularly in larger patients, the operating surgeon may choose to wear a headlamp to improve visualization.

An interbody strut, several types of which are available, can be used to increase disc interspace height.

(Copyright University of New Mexico, Division of Neurosurgery, Albuquerque, with permission.)

The interspace can be distracted with either a system developed specifically for ALIF or with a laminar spreader, a straight osteotome, or a Cobb periosteal elevator. The danger of excessively deep placement of a distractor is violation of the dorsal anulus fibrosus, with subsequent entrance into the epidural space. This can be associated with epidural bleeding, which may be difficult to control, or neurologic injury or durotomy, which cannot be adequately visualized and repaired. In most cases, no distraction is necessary after an aggressive discectomy is completed in a well-positioned patient.

Bone Graft Harvest

Excellent fusion rates are obtained when autograft bone is used as a fusion substrate. This bone can be used alone, in combination with allograft bone, or with a cage. If the iliac crest is within 5 cm of the retroperitoneal skin incision, dissection through the same skin incision allows access to the iliac crest for harvesting autologous bone graft. If more than 6 cm separates the skin incision from the iliac crest, a separate skin incision will be necessary to harvest the bone graft.

When harvesting iliac crest autograft, the most medial aspect of the skin incision should be at least 2 cm lateral to the anterior superior iliac spine (ASIS). The lateral femoral cutaneous nerve is subcutaneous in this location and typically runs within 2 cm of the ASIS. If this nerve is transected, permanent anesthesia of the ventrolateral skin of the thigh can result. If it is compressed, a temporary sensory deficit, caused by neurapraxic injury, results. When ALIF is performed using structural iliac crest autograft as the sole graft source, the medial and lateral aspects of the crest need to be exposed and an oscillating saw used to harvest tricortical strut grafts. Osteotomes can also be used; however, they have been shown to be associated with an increased incidence of microfractures of the tricortical bone graft. If the iliac crest autograft is being used to augment allograft bone or a cage, then a small cortical window can be opened on either the medial or lateral surface of the iliac crest, and curettes and gouges can be used to remove cancellous bone from the iliac crest. In extremely thin patients, the medial crest is favored for its cosmetic advantages; in heavier patients, the lateral crest is more easily accessible. Ordinarily, harvesting iliac crest autograft does not result in cosmetic deformity.

Excessive exposure of the medial iliac crest can lead to a hernia through the transversalis fascia. If the transversalis fascia is violated, it should be repaired immediately to prevent development of a postoperative hernia. After removal of the iliac autograft, the wound should be copiously irrigated and packed with thrombin-soaked gauze. Bone wax should be avoided because it is associated with an increased incidence of wound infections. After removal of all packing and copious irrigation, hemostasis is obtained. A drain can be considered, followed by a routine layered closure.

Complications associated with iliac crest autograft harvesting include lateral femoral cutaneous nerve palsies that result from medial placement of the skin incision. A postoperative hematoma may develop in the event of inadequate hemostasis. This complication can be avoided by paying attention to hemostasis, and perhaps with the use of a drain. Postoperative wound infections and subsequent pain may also occur and are avoided by use of generous quantities of irrigation, drainage of the wound, use of prophylactic antibiotics, and consideration of pulse lavage after graft harvest.

Bone Morphogenetic Protein

In 2002, the U.S. Food and Drug Administration (FDA) approved the use of recombinant human bone morphogenetic protein-2 (rhBMP-2) for use in the Medtronic (Minneapolis, MN) LT Cage at the L4-5 and L5-S1 interspaces. The study compared the use of rhBMP-2 with iliac crest autograft, citing clinical success with fusion rates of 96.9% versus 92.6% at 12 months and 94.5% versus 88.7% at 24 months. The rhBMP-2 group also did not complain of donor site pain or cosmetic appearance, as opposed to the small percentage (2% to 10%) of the iliac crest group who did experience these complaints.

To date, rhBMP-2 is the only osteoinductive growth factor that is FDA approved for use in ALIF. Its presumed mechanism is the stimulation of pluripotent stem cells from bleeding cancellous bone to form bone. Other, subsequent studies have used rhBMP-2 in femoral ring allograft, threaded cortical bone dowels, and threaded cylindrical titanium cages, and they found similar clinical and radiographic success rates and comparably low complication rates. In the initial FDA study group’s 6-year follow-up, the clinical and radiographic results remained durable. In a systematic review of the safety of rh-BMP-2 in spine surgery published by Carragee and colleagues, the authors concluded that the risk of adverse events in the use of rhBMP-2 is 10 to 50 times higher than estimates earlier estimated. This, however, is controversial and not uniformly agreed upon.

Fusion

Anterior lumbar interbody fusion is performed for low back pain, and long-term bony fusion is necessary for the best postoperative results. Fusion can be judged both clinically, as evidenced by the relief of back pain, and radiographically, as evidenced by radiographic incorporation of the graft. The meticulous performance of the ALIF surgical procedure is critical.

After the discectomy has been completed, a spacer must be placed in the involved interspace. A variety of materials have been used to function as spacers. Iliac crest autograft can be used as a spacer, in which case two or more tricortical grafts are typically placed side by side. Allograft bone can also be used. The different forms of allografts used in ALIF procedures include femoral ring, tibia, humerus, and iliac crest. Of these, femoral ring is the best choice because it can be sculpted to anatomically fit the defect and then be packed with cancellous autograft bone. The use of iliac crest allograft is a poor choice because this bone does not have the same structural strength as comparable autografts. Threaded dowels of bone derived from femoral allograft have also been used. They are placed in predetermined channels, a method similar to the Cloward technique used for the cervical spine. The use of cages as spacers in ALIF procedures has gained popularity. These cages received FDA approval in 1996 and are available in many forms; threaded titanium cages, polyetheretherketone (PEEK) cages, and carbon fiber–reinforced PEEK (CFRP) cages can be obtained. These cages are routinely packed with iliac crest cancellous autograft bone or rhBMP-2 soaked sponges. Of note, when rhBMP-2 is used in these surgeries, it is considered an “off-label” usage, as FDA approval has not yet been obtained for use in this fashion despite the fact that it is widely used.

Titanium cages can be threaded cages that lie flat within the interspace, side by side, and are filled with autograft iliac crest cancellous bone. In addition, a titanium cage is available that stands upright as a single trapezoidal or oblong cage within the interspace. Typically, carbon fiber–reinforced PEEK (CFRP) cages are placed as two cages flat within the interspace, side by side. Also, PEEK cages have been used to allow a more customizable footprint. These PEEK cages have been used in stand-alone fashion and with a ventral plate, an interference plate, or an integrated plate.

Spacers are used to maintain disc height after discectomy and to facilitate bony fusion. Ideally, a spacer is slightly larger than the disc height as measured by preoperative sagittal plane imaging studies. Likewise, an optimal spacer has a slightly trapezoidal configuration, with the larger end placed ventrally to maintain or increase lumbar lordosis. The depth of the spacer is determined by careful examination of the preoperative axial images from CT scans or MR imaging studies. Spacers perform a variety of functions, including restoration of lumbar lordosis and improvement of sagittal plane alignment of the lumbar spine. In addition, a spacer improves disc height and, in so doing, increases the size of the corresponding neural foramina. The PEEK cage implants are available in a variety of lordotic or straight angles and may be customized to improve or preserve a patient’s lordosis and coronal and sagittal balance.

Two principles of bony fusion are applicable to ALIF: (1) the need for compression and (2) the need for immobilization to increase the chances of acquiring a stable bony fusion. Unlike dorsal fusions, with ventral fusions, the graft can be maintained in a position of compression. Compression is best achieved by distracting the interspace and placing a slightly oversized spacer in this disc space. Compressive forces felt by a spacer in the intervertebral space are many times greater in magnitude than any compression that can be achieved by an onlay graft dorsally. Immobilization is obtained by firmly fitting a spacer into the intervertebral disc space. Modular devices that have integrated fixation to the end plates to confer initial stability have also had excellent outcomes with solid anterior interbody fusions being identified in 71 of 73 motion segments (97%). In addition, an external orthosis may be used to diminish motion at the lower lumbar spine. There is less need for internal instrumentation with ALIF. Historically, interference plates were placed to prevent cage backout. Newer devices have been used for implantation with ventral plate/screw fixation or with integrated plate/screw fixation in the cage itself.

In sizing the spacer or choosing the number of spacers to place, recent data recommend 60% vertebral body end-plate coverage. Because approximately 80% of the load on the spine and 90% of the articular surface area is supported ventrally, this larger footprint is biomechanically more advantageous in stability and allows for optimum fusion across the intervertebral space.

The grafting technique itself involves removal of all articular cartilage overlying the surfaces to be fused. The interspace is distracted, and a slightly oversized spacer is selected. A portion of cancellous bone from the vertebral body immediately above and below the interspace is exposed using a drill, curette, or gouge. The exposed cancellous bone of the vertebral body should be aligned with the exposed cancellous portion of the bone graft or the center of the cage, which allows for optimal fusion as well as providing immediate hemostasis. The ventral-most aspect of the spacer should be slightly countersunk beneath the ventral-most surface of the vertebral bodies above and below. Bone wax should be avoided because it may decrease fusion rates; hemostatic agents or grafting agents may be used instead. When placing a spacer, meticulous attention to maintenance of the orientation of the bony midline is important. If two spacers are used, they should be symmetrically placed on each side of the midline, and if a single spacer is used, it should be centered on the anatomic midline. After placement of the spacer, AP and lateral fluoroscopic confirmation should be obtained immediately. If any modification of the positioning of the spacer is necessary, it should be addressed immediately during the index surgical procedure. A particular concern regarding the placement of two spacers side by side is assurance that when the second spacer is positioned, it does not cause loosening of the first spacer. This must be confirmed after placement of the second spacer, and if the initial spacer has been loosened, modification must be performed prior to wound closure. Strict adherence to symmetrical placement of the spacers with respect to the midline helps avoid this problem.

Plating

Biomechanical studies have shown improved stabilization with ventral fixation devices. Stand-alone interbody implants without instrumentation do not confer significant stabilization in extension or rotation compared with flexion and lateral bending. In a study by Liu and colleagues, the authors investigated the biomechanical stability in a cadaveric model of an ALIF followed by either posterior screw fixation (PSF) or anterior lumbar locked screw plate (ALLSP). Results showed that the ALIF+ALLSP group had significantly greater axial stiffness under applied axial compression and significantly less angular displacement under rotational forces than the ALIF+PSF group. The ALIF+ALLSP group showed less angular displacement under flexion than that of the ALIF+PSF, and the angular displacement under lateral bending and extension was greater; however, these differences were not statistically significant. The historic buttress/interference plate has evolved into a ventral plate, similar to a cervical plate, to allow for immediate stabilization. Ventral plates can eliminate the need for a separate dorsal approach for instrumentation. The presumed increase in biomechanical stability would protect against graft migration and allow for better bony healing. Biomechanical studies have shown that ventral plating significantly enhances stability in flexion, extension, and lateral bending. Clinical studies have also supported the effectiveness of ventral plating for ALIF with excellent fusion rates and improvement in visual analog pain scores (VAS) and the Oswestry Disability Index (ODI).

Newer technologies have incorporated the plate and screw into the interbody graft. These devices have also shown improved biomechanical stability over stand-alone ALIFs. The long-term clinical outcomes of these devices are still under investigation. The benefit of an integrated plate with screws is that it allows for less dissection of soft tissues and less mobilization of blood vessels. Some surgeons have reported blood vessel complications such as erosion when the vessels overlie the ventral plate, lending further advantage to cages with integrated plate and screws.

Ventral plating entails the placement of a plate ventrally or ventrolaterally to the interbody implant overlying the superior and inferior vertebral bodies. Screws are then placed in a convergent manner in the superior vertebral body in a rostral trajectory and convergently in the inferior vertebral body in a caudally aimed trajectory. Care should be taken to maintain native blood vessel anatomy with respect to the ventral plate when the retractors are removed.

Dorsal Supplementation

In situations in which circumferential instability is present, anterior/posterior circumferential fusion should be considered. This procedure involves a significantly larger operation, more operative time, and greater blood loss. If neural compression is present, requiring a dorsal operation, and there is a need for anterior column restoration, then both a ventral and dorsal operation may be preferable. In addition, ventrodorsal fusions may be indicated in conditions with known diminished fusion rates, including smokers, patients with diabetes, immunocompromised patients, and patients with chronic corticosteroid use.

Most biomechanical studies have shown that pedicle fixation offers the strongest construct to resist forces in flexion, extension, axial rotation, and lateral bending. Novel methods in minimally invasive percutaneous pedicle fixation may be used in patients with facet disease, instability, or dorsal pathology. These patients may have dorsal fixation for enhanced biomechanical stabilization with minimal blood loss and tissue disruption. In experienced hands, surgical times are often shorter for these percutaneous procedures.

Postoperative Complications

Postoperative complications related to ALIF can be divided into four categories: (1) neurologic complications, (2) complications related to the operative approach, (3) complications related to the bone graft, and (4) complications related to the fusion. Neurologic complications are unusual in properly performed ALIF procedures. They tend to appear in the first 2 days after the operation and require prompt diagnosis and intervention. Complications related to the operative approach and those related to the iliac crest donor site both occur between 3 and 14 days after surgery. Complications related to the fusion proper tend to consist of the development of chronic pain syndromes as a result of a delayed nonunion or pseudarthrosis. Fusion problems usually cannot be definitively detected until 6 to 12 months after surgery.

Operative Approach Complications

After an ALIF, it is possible to develop a postoperative hernia, which can lead to bowel obstruction and possible bowel infarction. Violation of the peritoneum during ALIF with the retroperitoneal approach, or violation of the transversalis fascia during iliac bone graft harvest, can lead to the development of both internal hernias and subsequent bowel obstruction. When the peritoneum or the transversalis fascia is violated, postoperative hernias are best prevented by immediate operative repair of the defects during the index surgical procedure. If these structures are violated and are inadequately repaired or not appreciated, a hernia may result.

An injury to the bowel may occur during retroperitoneal exposure, and this extremely rare complication requires immediate detection and treatment. The treatment is aggressive irrigation followed by direct repair of the bowel, with separate, layered closures of the mucosal and serosal layers. Prophylactic antibiotics are administered and the ALIF procedure is aborted at this point. If not already present during the surgery, a general surgeon should be summoned to perform or assist in the bowel repair. Bowel injuries are best avoided by using a preoperative bowel preparation, including an enema to help decompress the loops of bowel, as well as placement of a nasogastric tube before the retroperitoneal procedure is performed.

Major blood vessel injuries are also rare during operative exposures. Injuries to the iliac arteries or veins, the inferior vena cava, and the aorta occur in 2% to 4% of ALIF operations. To avoid vessel injuries, a self-retaining retractor should not be used on the vessels during the exposure portion of the operation. In addition, before mobilizing the iliac vessels or the aorta, proximal and distal control should be obtained with vessel loops, which are used when a major vessel is injured. Should this type of injury occur, a vascular surgeon should perform or assist in vessel repair. Major vessel injuries are repaired primarily using nonabsorbable sutures. Sometimes smaller vessels are injured. The left iliolumbar vein can be avulsed if it is not ligated before the left common iliac vein is mobilized. This vessel is safely ligated at any time and should be ligated before common iliac vein mobilization. Likewise, lumbar segmental arteries can be ligated safely at any level; however, these vessels ordinarily can be spared during the surgical procedure.

Retrograde ejaculation is a serious complication that may occur in male patients after ALIF. It occurs when the autonomic nerves are injured, usually at the L5-S1 level. The hypogastric plexus is a continuation of the preaortic sympathetic chain that extends down from the thoracic region ventral to the aorta and lumbar vertebrae in the retroperitoneal space. As stated, the use of monopolar electrocautery over the L5-S1 interspace has been associated with an increased incidence of retrograde ejaculation and should be avoided. The rates of retrograde ejaculation have been reported as ranging from 0.42% to 22.5%, with a transperitoneal approach carrying a 10-fold greater risk. The mechanism of retrograde ejaculation involves the presence of a dry ejaculate secondary to relaxation of the internal bladder sphincter, with retrograde flow of ejaculate into the bladder. There is no surgical management for this complication, and it becomes prograde in 25% to 33% of patients by the end of the second year.

Thrombosis of either venous or arterial structures may occur after ALIF. Venous thrombosis has been reported in 1% to 11% of all ALIF procedures, but arterial thrombosis is extremely rare and reported to be around 0.45%. To avoid thromboses, retraction should not be prolonged, and self-retaining retractors should not be used on major vessels. If thrombosis is suspected, an immediate angiogram or venogram should be obtained. The treatment is open surgical thrombectomy. Likewise, an embolus in the arterial vascular tree in the lower extremities can occur rarely and is particularly likely if the patient is elderly. To avoid arterial embolization, retraction should not be prolonged and care should be taken not to use self-retaining retractors on blood vessels. An arterial embolus is detected in the perioperative period by loss of distal pulses and a cool extremity and is best treated by immediate vascular surgical intervention.

General systemic medical problems may arise during the postoperative period. Urinary retention occurs in 5% to 27% of cases and is usually temporary. It may be related to the use of perioperative narcotic analgesics. If urinary retention occurs, it is important to rule out an injury to the ureter or the development of a cauda equina syndrome. If urinary tract dysfunction occurs in the late postoperative period, obstruction of the distal ureter from retroperitoneal scarring must be ruled out. Ileus is also common during the postoperative period and it typically resolves less than 1 week after the operation. Prolonged postoperative ileus has been reported in 1% to 8% of all ALIF procedures and also may be related to the use of perioperative narcotic analgesics. An internal hernia that causes bowel obstruction secondary to violation of the peritoneum or transversalis fascia must be ruled out in cases of prolonged postoperative ileus. Serious general medical problems that have been reported include both fatal and nonfatal cardiac arrest, fatal and nonfatal pulmonary embolus, and aortic aneurysm rupture.

Neurologic Complications

Neurologic complications are rare after ALIF because the epidural space ordinarily is not entered and no attempt is usually made to decompress the neural elements during this procedure. Most neurologic complications are related to injuries to the nervous structures during the operative procedure. If a neurological deterioration occurs during the postoperative period, immediate advanced neuroimaging studies with either an MRI or a CT myelogram are performed. If an epidural hematoma is detected, the patient should be returned to the operating room immediately. Epidural hematomas are best treated using a dorsal approach, which provides the safest and easiest access to the thecal sac and nerve roots to decompress the hematoma. An incomplete neurologic deficit requires that imaging studies be promptly performed. The surgeon’s judgment determines whether repeat surgery or expectant observation is the solution. Cauda equina syndrome must be ruled out and, if detected, treated with a laminectomy. The decision to observe or explore a patient for postoperative bowel and bladder dysfunction is based on postoperative neuroradiographic imaging studies and the degree of clinical concern.

Specific nerve root syndromes are manifested as radiculopathies and should be worked up immediately with neuroimaging studies. Based on the results of these studies, either expectant observation or dorsal exploration should be undertaken. Injuries to the genitofemoral or ilioinguinal nerves may occur after an ALIF procedure. These injuries are characterized by postoperative numbness in the groin or the medial thigh region. They are most common in patients who undergo ALIF procedures at the upper lumbar levels. Palsies in these nerves frequently resolve spontaneously and are usually treated by observation alone.

Compression of the thecal sac during the postoperative period secondary to hematoma, infection, or retropulsed disc material is best treated with dorsal exploration. A dural tear is ordinarily detectable during surgery. However, it usually cannot be adequately treated through an ALIF approach. Placement of a lumbar subarachnoid drain to divert the CSF during the first week after surgery is the usual treatment for an intraoperative dural tear resulting from an ALIF procedure. Injury to the sympathetic nerves on the side ipsilateral to the retroperitoneal approach may occur. Usually, partial sympathectomy occurs and is manifested by vasodilation of vessels in the ipsilateral foot. The usual patient complaint is a cold sensation in the contralateral foot. Sympathectomy symptoms usually resolve spontaneously in the first 3 to 6 months after surgery. Discitis and osteomyelitis each occur in approximately 1% of all ALIF procedures. Their treatment involves reexploration for a confirmed abscess, using either a ventral or a dorsal approach, and wound culture and drainage of the abscess. If infection is present without an abscess, treatment with prolonged antibiotics and immobilization can be utilized. Surgical exploration may also be necessary in some cases.

Bone Graft Harvest Complications

Postoperative infections of the iliac crest donor site wound occur in 1% to 9% of all ALIF procedures and are usually detected secondary to pain, which is followed by eventual purulent discharge from the wound. Iliac wound infections are best prevented by avoiding the use of foreign material in the wound (e.g., bone wax) and using perioperative prophylactic antibiotics, copious irrigation, and meticulous intraoperative hemostasis. The use of large-bore drains in iliac wounds may prevent the accumulation of a hematoma and decrease wound infection rates; however, they lead to an egress of organisms through and around the drain. When iliac crest wound infection occurs, treatment requires surgical reexploration of the wound, cultures, and drainage.

Postoperative iliac crest wound hematomas usually are manifested as pain in the iliac crest region, which may radiate toward the groin. These hematomas are best avoided by meticulous hemostasis and generous intraoperative wound irrigation. The use of a postoperative large-bore drain may diminish the incidence of postoperative hematomas. Most of these hematomas resolve spontaneously and can be managed by observation. Based on the surgeon’s preference, it may be necessary to drain them if they become large or painful.

Fusion Complications

Problems related to the bony fusion portion of the operation are the most common complications of ALIF. Persistent low back pain is the most frequent complaint and is usually the result of pseudarthrosis, or nonunion, of the fusion. This complication is reported at extremely variable rates, ranging from 3% to 58%. It is not possible to diagnose lumbar pseudarthrosis secondary to an ALIF procedure until 6 to 12 months after surgery. The workup includes plain radiographs to detect evidence of a persistent lucent line between the graft and the vertebral body. Tomograms and CT scans with sagittal reformations also may be useful for detecting this lucent line. Flexion and extension lateral radiographs can detect persistent motion consistent with a pseudarthrosis. MRI is difficult to interpret after ALIF and is not particularly useful in revealing postoperative pseudarthrosis.

The treatment of symptomatic pseudarthrosis secondary to ALIF is prolonged immobilization of the patient, which can be accomplished with a thoracolumbosacral orthosis or with a body cast. Electrical stimulation or a bone growth stimulator may also be helpful. In addition, reoperation using a ventral or dorsal approach may be necessary. Rarely, a ventrodorsal combined procedure is necessary for the treatment of a pseudarthrosis. Several authors have stated that there is no correlation between radiographic pseudarthrosis rates and clinical outcome results. No treatment is required for asymptomatic pseudarthrosis, and the patient should be observed with serial radiographs and clinical evaluations.

Graft collapse occurs in 1% to 2% of all ALIF procedures and ordinarily results from excessive removal of subchondral bone from the vertebral body bony end plate. A kyphotic deformity may develop. The treatment is surgical reexploration through a ventral approach. Graft resorption may occur and is particularly likely in patients who are smokers, have diabetes, or are immunosuppressed. Graft dislodgement occurs in 1% of all ALIF procedures, and the treatment is reoperation using a ventral approach. Graft displacement may be minimized using a ventral buttress/interference plate, ventral/ventrolateral plate fixation, or dorsal pedicle fixation to enhance stability.

Other revision strategies include midline open dorsal fusion and stabilization with pedicle fixation and dorsolateral fusion. Alternatively, minimally invasive dorsal fusion options, from percutaneous pedicle fixation to paramedian tubular retractor–assisted fusion and stabilization, can be performed, although their long-term outcomes are still under evaluation. The extreme lateral transpsoas approach can also be used as a salvage technique at the L4-5 level, if above the pelvic brim, or at the more rostral segments. Using this technique, the disc space can be approached laterally through the psoas muscle to allow removal of the failed implant, placement of a new interbody device, and placement of a lateral fixation plate.

After a successful ALIF procedure, the motion segments immediately adjacent to the fusion are exposed to increased stress. Months to years after a successful procedure, adjacent-level disc herniation or degeneration may occur and is particularly likely to take place at the motion segment rostral to the ALIF. Detection of adjacent-level disc problems is best accomplished with diligent long-term follow-up.

Multilevel Anterior Lumbar Interbody Fusion

Complication rates increase when multiple motion segments are fused. Several investigators have described a direct correlation between the number of levels fused and postoperative pseudarthrosis rates. With each additional level fused, the overall pseudarthrosis rate increases. Likewise, clinical success rates decrease for each additional level fused. In addition, because of the longer lever arms that result from multilevel ALIF, adjacent-level disc problems increase after multilevel procedures. Complications with the retroperitoneal approach also increase because of the larger surgical exposure. Furthermore, systemic medical problems secondary to the longer duration of surgery with increased blood loss are more common in multilevel procedures.