Advantages and disadvantages of anterior lumbar spinal surgery are reviewed, including approaches to the anterior lumbar spine, fusion techniques, as well as biologic and biomechanical considerations. The issue of adjacent segment degeneration (ASD) is examined. More recent developments, such as motion-sparing technology and total disc arthroplasty, are also discussed.
Indications for anterior lumbar spinal surgery include degenerative disc disease.
Contraindications for anterior lumbar spinal surgery include prior anterior surgery (relative).
Posterior instrumentation is needed for stability in extension.
Anterior lumbar fusion surgery does not directly address central and lateral stenosis.
Anterior lumbar fusion is a better biologic procedure with high fusion rates.
Anterior lumbar spinal surgery allows restoration of lordosis and indirect decompression of neural foramen with restoration of disc height.
The paramedian retroperitoneal approach is the most common approach.
Vascular surgeon may be needed for access to spine.
Excellent fluoroscopic images are necessary for correct level verification and proper implant placement.
More complete discectomy and end plate preparation is possible anteriorly.
Larger, lordotic interbody implants can be placed.
Motion-sparing techniques may reduce risk for ASD.
Interbody fusion devices have been utilized to prevent collapse and pseudarthrosis well known to bone-only anterior lumbar interbody fusion (ALIF) and posterior lumbar interbody fusion (PLIF) procedures. A variety of these devices are available for use in the lumbar spine but little long-term data exist. U.S. Food and Drug Administration (FDA) trials with interbody devices have shown 95% fusion rates with excellent clinical outcomes. Unfortunately, these results have not been predictably duplicated in subsequent clinical studies. Our fusion habits in the lumbar spine have changed rapidly based on reports of high short-term complication rates with these devices. We are now looking at disc replacement technology to circumvent some of the inherent problems of spinal fusion (nonunion and adjacent segment disease). Are we too hastily embracing a new technology?
We are now aware of the multiple factors that lead to biomechanical alterations of the adjacent motion segment in a lumbar fusion. However, the actual incidence of adjacent segment disease is unknown. All of our current data are class III (retrospective). Recently published and presented studies have shown that the utilization of interbody cages in spinal fusion for single-level degenerative disc disease (DDD) has acceptable fusion rates and outcomes that lend disc replacement less advantageous in the treatment of single-level DDD.
This discussion summarizes the data on interbody fusion cage devices. Fusion rates, outcomes, biomechanics, and complications are outlined. Interbody fusion cages still have a place in the treatment of DDD in the lumbar spine.
INDICATIONS AND CONTRAINDICATIONS
Surgical indications for the anterior approach to lumbar spine include DDD, tumor, infection, deformity, and trauma. Contraindications include active infection, morbid obesity hindering access to the lumbar spine, severe atherosclerosis or calcification of the great vessels hindering mobilization, and significant retroperitoneal scarring caused by prior pelvic, abdominal, or anterior spine surgery.
An anterior approach can be used for a variety of lumbar spine problems, including traumatic, infectious, pathologic, and degenerative conditions involving the lumbar spine. General benefits of the anterior surgical approach to the lumbar spine over the posterior approach include avoiding the paraspinal muscle damage during dissection, less likelihood of dural and nerve root injury, and less or no epidural scarring. However, some potential risks are unique to the anterior approach involving the vascular and the sympathetic system. These risks are discussed in detail in this chapter. A detailed description of the each surgical approach is beyond the scope of this chapter, but the key concepts of each approach are discussed. One may consider placing a bump under the lumbar spine to aid in access to the disc space by extending the spine. A careful consideration should be given to obese patients who require higher lumbar level access, which may not be feasible.
Paramedian (Retroperitoneal Approach)
The left-sided paramedian approach is the most commonly used for the anterior access to the lumbar spine. The incision can be longitudinal, oblique, or transverse depending on the levels needed for surgery. Generally, the anterior rectus sheath is divided longitudinally and the rectus muscles are retracted laterally to gain access to the retroperitoneal space. A blunt dissection is performed to retract the peritoneum and its contents medially. Care should be taken to protect the ureter, retracted medially with peritoneum, and the genitofemoral nerve, lying over the psoas muscle. The iliolumbar vein, the middle sacral vessels, and the segmental vessels should be identified, ligated, and divided as needed to mobilize the great vessels. Care should be taken to avoid damage to the sympathetic chains, located along the lateral portion of vertebral bodies, and the superior hypogastric plexus, located in the retroperitoneal space overlying the lumbosacral junction.
For the exposure of L5-S1 level, the area below the bifurcation of the great vessels is approached. Care should be taken to mobilize the soft tissue anterior to the lumbosacral junction and the superior hypogastric plexus from left to right. Injury to the superior hypogastric plexus can cause retrograde ejaculation in men. For other higher lumbar levels, the great vessels are mobilized to the right after the segmental lumbar arteries and veins are ligated and divided as needed.
Given the difficulties encountered during revision approach to the anterior lumbar spine, a right paramedian approach to a single-level L5-S1 can be considered. The left-side paramedian approach can be saved for a possible revision or approaches to the other lumbar level in the future.
Midline (Transperitoneal Approach)
The superficial dissection is similar to the paramedian approach. However, a peritoneal incision is made after the superficial dissection to enter the abdominal cavity. The small bowel is gently retracted to reveal the lumbosacral junction and the great vessels. The posterior peritoneum is divided to gain access to the vertebral bodies and the disc space. An important aspect of this approach is to remember that the superior hypogastric plexus is anterior to the sacral promontory and often courses along the left iliac vessels preferentially. One way to minimize the injury to the superior hypogastric plexus is to gently elevate the soft tissue as a unit from the right common iliac vessels toward the left.
In general, a revision approach to the anterior lumbar spine is difficult because of intense inflammatory reactions and adhesions. This is particularly true if the revision approach is not done acutely (>7 days). For approaches during the early postoperative period (<7 days), the original surgical approaches can be used. However, for delayed approaches to the lumbar spine, alternative approaches must be considered. Even for an experienced access surgeon, the revision approach can be quite challenging. The use of a cell saver, use of pulse oximeter on the left great toe as a monitor for the left iliac artery perfusion, preoperative placement of ureteral stents, and preoperative vascular imaging studies are all useful aids in revision approach.
A right-sided approach to L5-S1 is an option in a revision setting, especially in patients with prior left-sided approach. The right-sided approach can be retroperitoneal similar to the left-sided retroperitoneal approach. If the patient had a prior left-sided approach at higher levels, a lower left-sided approach to L5-S1 may be possible by taking advantage of the virgin territory in the lower retroperitoneal planes.
In revision approach to the L4-L5 level or higher lumbar levels, a left retroperitoneal approach is ideal if the prior surgery to L5-S1 was done from the right side. Even if the prior surgery was performed from the left side, a left-sided revision approach is still possible given that the prior approach was done through the lower retroperitoneal planes with limited dissections. Another option is the anterolateral approach or the flank approach to the retroperitoneal space. In this approach, the psoas muscle is identified first and the retroperitoneal plane is developed toward the spine.
Advantages of Anterior Approaches for Fusion
The anterior approach to the lumbar spine for discectomy and fusion has several advantages to posterior techniques for the treatment of lumbar pathology. Without the muscle stripping and violation of posterior stabilizing structures inherent in posterior lumbar surgery, the anterior approach allows for more rapid healing and functional recovery. Persistent decreased abdominal and back muscle strength has been shown at 1 year after surgery after posterior approach discectomy. So-called fusion disease is a recognized complication of chronic pain after posterior lumbar surgery. This constellation of symptoms is caused by muscle stripping and prolonged retraction during the posterior approach. The anterior approach also avoids direct entrance into the canal and the subsequent risk for dural injury, and postoperative scarring and fibrosis.
The anterior lumbar approach allows a wide exposure to the disc space, which enables the surgeon to perform a more complete discectomy and end plate preparation. Direct compression of the neural elements by herniated nucleus pulposus and osteophytes can be addressed anteriorly by thorough discectomy and partial or complete corpectomy if required.
Without the exposure limitations of the nerve root and thecal sac encountered with posterior approaches, a larger implant can be placed anteriorly. This allows greater disc height restoration, and thereby indirect neural decompression by increasing the cross-sectional area of the neural foramina by distraction. The sagittal balance of the lumbar spine can be addressed and lordosis recreated with use of lordotic cages. The larger cages placed from the anterior approach also reduce the risk for subsidence because of their larger surface area and the fact that they are loaded peripherally on the stronger cortical vertebral bone. Threaded cages placed centrally showed a tendency to subside because they cut into the bony end plate and were loaded centrally on cancellous bone ( Fig. 20-1 ).
Anterior lumbar fusion surgery is a better biologic procedure and, therefore, carries a low nonunion rate. Wolff’s law tells us that the potential for bony fusion is increased when grafts are compressed. The vertebral bodies carry approximately 80% of the load of the body and constitute approximately 90% of the surface area of the functional spinal unit. The vertebral bodies also have rich vascularity. The anterior lumbar interbody implant is therefore in a position of compression with high surface area and good blood flow, all of which are important factors promoting bony fusion. The FDA-approved use of recombinant human bone morphogenetic protein-2 (rhBMP-2) in ALIF has been shown to yield excellent fusion rates and spare the patient the need for iliac crest bone graft (ICBG) harvest. Burkus et al. performed a multicenter, prospective, randomized, nonblinded, 2-year study of ICBG versus rhBMP-2 in ALIF. They showed a fusion rate of 94.5% in the rhBMP-2 group at 2 years versus 88.7% in the ICBG group. Surgical time and blood loss were less in the rhBMP-2 group. Both groups had similar outcomes in mean postoperative Oswestry, back and leg pain scores, and neurologic improvement. Of those patients who had ICBG harvest, 32% reported graft site discomfort at 2 years.
Disadvantages of Anterior Approaches for Fusion
One of the disadvantages of anterior approach is the potential need for an access surgeon. In most situations, a vascular surgeon can provide a safe and adequate access to the anterior lumbar spine. The availability of a good access surgeon is can be a limiting factor in this approach. However, having a vascular surgeon involved in the case can be beneficial in dealing with potential vascular injuries during the surgery.
In a literature review of reported complications from anterior surgical approach from 1963 to 1990, Regan et al. finds the most common complications to be urinary retention (9.9%), warm leg (8.8%), and thrombophlebitis (8.7%). Sasso et al. report on complication rates in ALIF procedures and found a statistically significant difference between L4-5 (6.1%) and L5-S1 levels (1.2%).
One of the most dreaded complications is the retrograde ejaculation in men. This can occur with the injury to the superior hypogastric plexus of the sympathetic system, which leads to an incompetent closure of the bladder neck during ejaculation. The incidence rate is somewhat variable but generally low with modest resolution of symptoms in some cases. Flynn and Price report an incidence rate of 0.42% in 4500 cases with a 25% resolution rate. A literature review by Regan et al. from 1963 to 1990 reports an incidence rate of 1.7% (16/942). A review of 148 cases from 1992 to 1995 reported a 4.7% incidence rate (7/148) with 6 of 7 cases having resolution of symptoms in 6 to 9 months. Rajaraman et al. report a 3.3% incidence rate (2/60) with resolution of symptoms in 1 patient by 15 months. Brau reports an incidence rate of 0.3% (1/345); this patient did not recover after 1 year.
Sasso et al. reports on the risks associated with the transperitoneal versus the retroperitoneal approaches. He found a 13.3% incidence rate (4/30) in the transperitoneal group versus the 1.7% incidence rate (2/116) in the retroperitoneal group. After 2 years, one patient from each group had resolution of symptoms, thereby reducing the incidence rate to 0.86% in retroperitoneal group and to 10% in the transperitoneal group. Increased incidence of retrograde ejaculation also has been reported with the use of laparoscopic approaches. Escobar et al. report 25% versus 2% incidence rates of retrograde ejaculation in transperitoneal video-assisted approach versus open approach, respectively. Kaiser et al. also report an incidence rate of 45% versus 6% in comparing laparoscopic versus open approach to the anterior lumbar spine.
The incidence of retrograde ejaculation appears to be heavily dependent on the surgical techniques and the handling of the soft tissues during surgery. The superior hypogastric plexus tends to course over the lumbosacral junction with a bias toward the left iliac vessels. Regardless of the approach, the soft-tissue anterior to the sacral promontory and the vessels should be elevated and mobilized as a unit, and the dissection along the middle of the plexus should be avoided. Limiting the use of monopolar electrocautery is also helpful in minimizing the risk. For the patients who wish to have children, it is advisable to have their sperm preserved at a sperm bank before surgery.
A review of 102 cases by a vascular access surgeon reported a vascular injury incidence of 15.6%. These injuries were injuries to the great vessels requiring suture repairs. Interestingly, injuries occurred most commonly during the exposure, and the common iliac vein was most often involved. Other reported incidence rates range from 0.7% to 6.6%. Another study of 686 cases reported a 1.7% vascular injury incidence rate. Of the injuries, half were arterial thromboses and the other half were venous injuries that required repair. The use of a pulse oximeter on the great toe was discussed as a simple way to intraoperatively monitor adequate perfusion to the lower extremities.
The sympathetic trunk lies on the lateral portion of the upper and midlumbar vertebral bodies. These structures are vulnerable to an injury during the anterior approach and can lead to ipsilateral lower extremity warmth because of loss of sympathetic vasoconstriction of the vessels. Often, the first sign of sympathetic trunk injury is the contralateral lower extremity coolness in comparison with the affected extremity and can be mistaken for thrombosis of the unaffected side. Fortunately, this condition is mostly self-limiting. However, periodic swelling, dysesthesia, and discoloration may persist in some patients. One study reported an incidence rate of 10% (6/60) with complete resolution of symptoms in half of the patients by 4 months. Other potential complications that occur in less frequency include lumbar plexus and its peripheral nerve branch injury, infection, ileus, incisional hernia, and deep vein thrombosis.
As it is true of many procedures, a learning curve is associated with the ALIF. After the initial enthusiasm for ALIF procedures, the failure rates with poor outcomes have been reported to be as great as 17% to 30%. Some important lessons can be learned from the literature.
One of the reasons for the observed poor outcomes can be attributed to poor surgical techniques. Togawa et al. histologically reviewed failed ALIF cages and found poor bone formation within these cages. Based on their findings, they stressed the importance of graft and implant site preparation in minimizing nonunion. Subsidence has been recognized as another problem. We know that the end plate strength is in the periphery at the apophyseal ring. Newer techniques and designs of cages have focused on adequate preparation of end plates, preservation of cortical bone, using cages with larger footprint (>6 cm ), and placement of the cages along the apophyseal ring (see Fig. 20-1 ).
High enthusiasm for minimally invasive surgery has made an impact on the anterior lumbar surgery. In the experienced hands, this approach can theoretically provide short-term benefits such as reduced blood loss, less postoperative pain, and shorter hospital stay. However, many of the current reports are less promising. Zdeblick and David report a significantly greater rate of complication in the laparoscopic group (20%) versus a mini-open group (4%) for L4-5 ALIF, without a significant difference in operating time, blood loss, or length of hospital stay. Another recent study comparing laparoscopic versus mini-open ALIF at L5-S1 level showed no clinical benefit with minimum of 2 years of follow-up. However, they did report a statistically longer operative time in the laparoscopic group. The increased complication rates related to retrograde ejaculation have been discussed earlier. Overall, the laparoscopic approach is technically demanding and has been associated with greater complication rates, longer preparation time, longer operative time, a steep learning curve, and significant rate of conversion to an open procedure ranging from 10% and 11%.
Another valuable lesson learned is the instability of the stand-alone cages in extended lumbar spine. Oxland et al., in a cadaveric study of anterior cages, show that these constructs are unstable in extension. However, with the addition of translaminar screws, the stability of the spine increased significantly even in extension. The importance of supplemental stabilization of the anterior cage construct was also reported by Patwardhan et al. In a cadaveric study, they show an increased stability in extension when an external compressive preload was applied to the cage construct. Several options exist for supplementing an ALIF cage including pedicle screws and rods or transfacet screws. Recently, instruments for percutaneous placements of these devices have been developed and are now available. Additional options are available that may obviate the need for posterior instrumentation. These devices are anterior cages with screw fixation capabilities into the vertebral bodies ( Fig. 20-2 ).