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Symposium: Minimally Invasive Approaches to the Lumbar Interbody Space

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Patient selection

David F. Fardon, MD: Each year, thousands of lumbar spinal fusions are performed to treat common spinal conditions such as degenerative disc disease, spondylolisthesis or misaligned vertebrae. In the past, spinal surgery involved lengthy operating time and hospital stay, and extensive postoperative therapy. This discussion of minimally invasive spinal technologies will offer alternatives that may result in successful spinal fusion surgery with reduced operating time, shorter hospital stay, less postoperative morbidity and faster recovery. These technologies have been applied to anterior, posterior and transverse interbody fusions. When is spinal fusion considered for patients with back pain?

John Peloza, MD: First, a patient must have a pathological condition that is amenable to spinal fusion. Second, a patient must be significantly disabled by pain and have difficulty with activities of daily living in order to be considered for lumbar spinal fusion. Generally, patients must have completed aggressive conservative care and still have disabling pain and functional deficits. In my practice, patients have completed trunk stabilization and manual therapy, sometimes injection therapy or pain management, but are not making progress. The duration of their symptoms exceeds the natural history of low back pain, and their response to conservative care suggests that their condition will not change without intervention.

Fardon: What are the exclusion criteria for fusion surgery?

James D. Schwender, MD: I cannot stress patient selection enough. I would not perform fusion procedures on patients who do not warrant a surgical stabilization procedure. There are also other conditions of the lumbar spine that do not require fusions such as stenosis and disc herniations. Patients who have radiculopathies do not necessarily need fusions. They need decompressions of their neural elements. Finally, there are patients whose medical comorbidities prevent surgeons from performing potentially advanced surgical procedures.

Fardon: What are some of the biological and mechanical factors that determine successful or unsuccessful fusion?

Martin D. Herman, MD, PhD: Patients with poor bone quality or osteoporosis are candidates for fusion failure. Also, patients who smoke have higher fusion failure rates than patients who do not. There are also technical factors to consider. If a fusion is performed well, then patients are more likely to fuse.

Fardon: What tests are needed to determine whether there is a successful fusion?

Herman: Most surgeons use computed tomography (CT) scans with coronal and sagittal reconstructions, because plain radiographs and flexion-extension radiographs underestimate actual fusion rates. Results of a study by Santos et al¹ showed that roughly 86% of patients appeared to be fused based on plain radiographs and flexion-extension radiographs, whereas only 65% of those patients demonstrated good fusions on CT scans. This means that the CT scan is picking up more than double the fusion failures.

Fardon: What are some of the goals of minimally invasive spinal fusion surgery?

Todd H. Lanman, MD: The goals are to provide a patient with lower morbidity while undergoing a fusion operation. By using minimally invasive techniques in posterior or anterior approaches, the amount of soft tissue and muscle dissection can be reduced. The paravertebral, multifida and paravertebral profunda muscles are spared a traumatic stripping dissection. The vascular supply and innervation to these muscles can be preserved, resulting in a faster recovery.

Fardon: How do some of the minimally invasive procedures differ from the traditional open procedures to achieve spinal fusion?

Kevin T. Foley, MD: Minimally invasive spinal surgery has the same goals and objectives as traditional open spinal surgery. The approach is just different. Most spine surgery patients require decompression and/or reduction of deformity and/or stabilization. Minimally invasive decompression, deformity reduction and stabilization are typically performed following the same principles used with open surgery, but through small incisions with minimal or no muscle stripping and with minimal soft tissue retraction. An analogy can be made to other surgical disciplines where minimally invasive approaches have been proven successful. For example, laparoscopic cholecystectomy or arthroscopic approaches to the knee do not change the surgery at the target structure but, by reducing the surgical trauma related to the approach, result in better patient outcomes.

Herman: Patient expectations are an issue with minimally invasive surgery as it becomes more popular and prevalent. As some patients become more sophisticated, and especially patients who choose to educate themselves with use of the Internet, they may expect a minimally invasive operation when it is not indicated. Certain patients require instruction as to when it is appropriate to do these types of procedures.

Costs associated with minimally invasive surgery

Fardon: How do the costs of minimally invasive procedures compare to traditional approaches?

Thomas J. Kleeman, MD: When performing a cost-analysis, it is important to look at all of the pertinent costs associated with patient care. Many new techniques incorporate expensive technology and, when looking at the cost of the operative equipment and the procedure alone, there may be a misleading impression that the costs exceed the benefit compared to more traditional surgery.

On the other hand, when looking at the combined costs of the hospitalization, recovery and time out of work, the benefits of a shorter hospital stay, faster recovery and earlier return to work drive the analysis in favor of the minimally invasive procedures. It has been estimated that 4% of patients who have significant long-term back pain represent 75% of the cost of caring for back pain. There are a significant number of failed surgeries in that 4%. The trend of improving outcomes through minimally invasive surgery ultimately helps lower the cost.

Fardon: Dr. Peloza, what are the benefits of minimally invasive spine surgery?

Peloza: The benefits of minimally invasive spine surgery are predictably successful outcomes. Our data support the Minimal Access Spinal Technologies (MAST) (Medtronic Sofamor Danek, Memphis, Tenn) concept for decompressions and especially fusions. In my experience, it is now unusual to have a poor outcome. That was not the case a few years ago. In fact, lumbar spine surgery was very unpredictable. There were clinical failures even with a reliable diagnosis and technically successful surgery. That was the experience with stand-alone threaded lumbar cages and all other open techniques.

In the beginning of my MAST experience, it was immediately obvious that outcomes were far better in regard to pain relief, function and rapid return to premorbid activity. I have been collecting data prospectively for almost two years and the outcomes have continued to be impressive. I can tell patients with confidence that they should return to their previous level of function after a MAST decompression or fusion.

The benefit of predictably successful outcomes is reflected in medical costs. Some of the direct costs may be higher initially because of the implants and recombinant human bone morphogenetic protein-2 (rh-BMP2). However, overall costs continue to fall secondary to fewer hospital days, less physical therapy, pain medications and/or injections. Most importantly, the incidence of failed back surgery is markedly reduced. This reduces the costs of multiple diagnostic work-ups, revision surgery and long-term disability payments, which are exponentially greater than direct medical costs.

The learning curve

Fardon: Dr. Schwender, what are some risks that you are willing to take for the benefit of your patient?

Schwender: Currently, the majority of orthopedic surgeons and neurosurgeons have not been trained to perform minimally invasive approaches during the formal training period, so they have to learn the procedures on their own. There is a fairly steep and prolonged learning curve for some of these newer technologies. More recently, trained fellows and residents, in some cases, have been trained on these techniques and their learning curves tend not to be as prolonged. Regardless, during the learning period, the procedures do in fact take longer and there may be complications associated with length of surgery.

It is critically important to understand the three-dimensional anatomy, particularly the neural anatomy, to avoid injury to these structures. If, in minimally invasive techniques, a complication occurs intraoperatively, the surrounding anatomy cannot be assessed and repaired as easily in comparison to a more traditional open approach. For instance, dural tears tend to be somewhat more difficult to repair within the confines of minimally invasive approaches compared to traditional open techniques. However, as the surgeon becomes familiar working with the principles of less invasive surgery, benefits such as less tissue trauma, less use of perioperative narcotics, shorter hospital stays and improved outcomes become apparent.

Fardon: Dr. Schwender, can you follow-up on the possible difficulty of dealing with intraoperative complications? Is the recognition of complications any more difficult with minimally invasive procedures?

Schwender: No. For example, in both minimally invasive and open procedures, if a dural tear is created, the cerebrospinal fluid will leak. As the surgeon becomes more comfortable working through a tube or a blade retractor system, complications will be easier to manage.

Foley: Minimally invasive approaches to the spine depend significantly on one’s knowledge of surgical anatomy, as a smaller portion of that anatomy is directly visible compared to a typical open approach. In order to maintain anatomic orientation during minimally invasive spinal surgery, the surgeon must be able to mentally “recreate” this unseen anatomy. This skill has some intuitive aspects to it, but in my experience in teaching surgeons how to do these cases, it typically has to be learned.

Fardon: If an experienced spinal surgeon has not performed surgery using minimally invasive techniques, what is the minimum investment in education?

Foley: If a surgeon already has experience with minimally invasive approaches for decompression, which includes standard microdiscectomy, that surgeon can learn minimally invasive techniques more quickly than a surgeon who has performed only standard open procedures. I do not believe that anyone can master these techniques in a single-day lab or even during a several-day lab. We ask surgeons to start at a basic surgical level and progress as their skill set progresses. Specifically, surgeons should perform minimally invasive decompressions first before moving on to more complex minimally invasive procedures.

Technique preference

Fardon: What is your preferred approach: anterior, posterior or transverse, and why?

Peloza: First, a surgeon must master all approaches. The choice of approach depends upon the pathology. In general, I prefer the anterior lumbar interbody fusion (ALIF) with an LT-cage Lumbar Tapered Fusion Device (Medtronic Sofamor Danek) and rh-BMP2.

I supplement the anterior instrumentation with Sextant Percutaneous Rod Insertion pedicle screws (Medtronic Sofamor Danek). After adding the percutaneous screws, my outcome data improved dramatically. Therefore, I supplement all interbody devices with percutaneous pedicle screws. An anterior approach allows for a mechanically superior implant with a more optimal position on the stronger parts of the vertebral end plates, leading to superior implant fixation and stability. In addition, distraction of the disc space is easier and more predictable, leading to consistent restoration of normal lumbar alignment and lordosis. In cases of a large disc height at the fusion site, the anterior approach enables the placement of a suitably large implant without risk to the neurologic structures.

The anterior approach also lends itself to rapid multi-level MAST surgery now that multi-level Sextant screw fixation has been developed. If decompression is required, it can be performed after the anterior surgery just prior to the placement of percutaneous pedicle screws through a METRx cannula (Medtronic Sofamor Danek).

I consider posterior approaches if I need to decompress the spinal canal or if there is a relative or absolute contraindication to anterior surgery (e.g., vascular disease, previous anterior surgery at the surgical site).

I prefer the transforaminal lumbar interbody fusion (TLIF) approach because it is quicker and unilateral with less distraction on the neural structures. In addition, bilateral and multi-level decompressions can be performed through a unilateral portal technique by angling the METRx retractor. Recently, the TLIF interbody implants have been improved and combined with rh-BMP2. I am more comfortable with the fixation. I consider the posterior lumbar interbody fusion (PLIF) technique if I need to do a bilateral decompression with fusion and I cannot adequately decompress the contralateral side from the ipsilateral TLIF approach. As you can see, the options and versatility of the surgical approaches change as a surgeon’s skill improves.

Foley: I perform all of these procedures in my practice. There are no scientific data to tell us there is a significant difference in outcome among TLIF, PLIF or ALIF.

In the Swedish Lumbar Spine Study Group’s series² of fusion for low back pain, there was no difference between on-lay fusion, on-lay fusion with metal fixation and 360º fusion. In my experience, I find an anterior approach offers a better ability to restore lordosis or open a collapsed disc space, and I prefer it for patients with grade 2 spondylolisthesis.

On the other hand, a posterior approach is mandatory for direct decompression of the spinal canal. A patient with a large disc fragment or significant stenosis that requires a decompression should be approached posteriorly. If a fusion is necessary, whether to perform a TLIF or PLIF procedure is a surgeon’s choice. I prefer a PLIF when a bilateral direct decompression is required. Most of the time, when a posterior approach for decompression in conjunction with a fusion is necessary, a TLIF suffices. When approached unilaterally, TLIF is a faster procedure than the PLIF and decreases neural morbidity.

Schwender: Most minimally invasive technology, particularly fusion technology, is appropriate for one- and two-level procedures. We have not mentioned multi-level procedures, and minimally invasive technology has not advanced at this time to do more than two-level procedures with reproducibility. I prefer the minimally invasive TLIF procedure for its versatility. Restoration of height is best achieved anteriorly but can be done posteriorly. Restoration of lordosis can be done posteriorly as well. Currently, a posterior-only approach unilateral TLIF can be done in a two- to three-hour time frame. There is no risk of retrograde ejaculation, and there is a minimal risk of vascular injury.

Lanman: Each approach has a role. The anterior approach is suited particularly well for the patient with a grade 3 or 4 spondylolisthesis. However, most patients with degenerative one- and two-level disease, which is the majority of patients, do not have pure biomechanical pain, but significant radiculopathy is associated with it as well. As a result, decompressing the neural foramen directly through a TLIF approach is my preferred method, as this relieves root compression and stabilizes the segment.

Height restoration is less of an issue in the TLIF approach to the degree that it might be when reducing a significant spondylolisthesis. Through the TLIF approach, a surgeon removes the entire foramen, so the root is decompressed without having to restore height to decompress the foramen via distraction.

Herman: For the majority of my patients who have spondylolisthesis or degenerative disc disease, I perform ALIFs. TLIFs are my second most common procedure, which I perform on patients with unilateral radiculopathy due to recurrent disc herniation, synovial cyst or facet hypertrophy with spondylolisthesis. For bilateral radiculopathy, a PLIF can be performed, but as with a unilateral radiculopathy, visualizing and decompressing the nerve is necessary. For high-grade spondylolisthesis, the anterior approach is preferred because of the potential for improved correction of sagittal balance. In patients where fusion is a question due to poor bone quality, steroid use or smoking, I prefer an ALIF because of the ability for excellent end plate preparation.

Kleeman: My preference is laparoscopic ALIF. The challenges of the laparoscopic approach are largely associated with the difficulty in accessing the L4-L5 disc space. It is frustrating that while 90% of back pathology is equally distributed between L4-L5 and L5-S1, L5-S1 is relatively easy to approach laparoscopically whereas L4-L5 is quite difficult. By focusing on the laparoscopic technique, studying the anatomy and researching alternative laparoscopic approaches, I have made the L4-L5 level less intimidating and much safer. The unique challenges of the laparoscopic approach require a significant learning curve but the outcomes can be outstanding.

Posterior lumbar interbody fusion

Fardon: What are some of the indications for a PLIF? Describe the procedure and potential advantages and disadvantages.

Maurice Smith, MD: After spinal instability has been diagnosed at a certain level and neurologic decompression is necessary, a PLIF provides the best way to stabilize the motion segment without performing additional surgery.

Peloza: First, the surgeon should consider whether a patient needs a decompression (central, lateral recess or foraminal) and if the decompression needs to be done unilaterally or bilaterally. If the pathology is bilateral and mostly in the ventral part of the spinal canal, then a PLIF is a better option than a TLIF. A previous posterior decompression can make a PLIF more difficult. In this case, a TLIF might be easier and safer. Second, a large disc height at the operative level is more difficult to do safely because there are limits to the implant size without risk to the neural structures. Third, spinal alignment is important. One must restore lumbar lordosis with an appropriate size wedge-type implant to restore and maintain lordosis. Fourth, contraindications to anterior surgery (e.g., previous anterior surgery, vascular disease) or surgery at L4-L5 are indications for a PLIF.

Fardon: A battered root is a potential complication of the PLIF procedure. In a patient who already has a battered root, is it a contraindication to perform a PLIF procedure?

Peloza: A battered root is a relative contraindication because there would be more potential injury to the root. This might be a situation for a unilateral procedure, a TLIF-type procedure on the opposite side or an anterior approach.

Fardon: When performing a minimally invasive PLIF, what instruments do you use?

Peloza: First, I use fluoroscopy. The surgical site is identified under fluoroscopy and the skin is marked. Then a small incision is made and a wire is advanced to the lamina. Sequential METRx dilators are then used to spread the paraspinal muscles to the desired size and a METRx retractor is placed. The dilators are removed and a microscope is brought into the field. I do a partial laminectomy above and below the disc space, a partial facetectomy, expose the disc, then do a discectomy. The same steps are repeated on the contralateral side. From this point, the technique is the same as the Tangent PLIF technique (Medtronic Sofamor Danek) except I am doing it through two METRx retractors simultaneously. Once the interbody implants are in place, I remove the METRx retractors, undermine the subcutaneous tissue just dorsal to the paraspinal fascia and retract the skin lateral (same incision) so that I can make a more lateral approach to the pedicle entry sites. Next, surgical technique is the same as the Sextant pedicle screw system. Once the screws and rods are placed, compression is applied to the screw/rod construct. The Sextant system is removed and the small incisions (25 mm to 30 mm) are closed.

Smith: Instrumentation has improved as a result of surgeons, performing more interbody work posteriorly. Improved instrumentation allows for safe access between the exiting and traversing nerve roots. The amount of room between these two nerve roots defines the operative corridor. Making a large open incision and retracting significant amounts of muscle, in many cases do not make sense anymore. Exposure beyond the exiting and traversing nerve roots do not make the operative corridor any safer or easier to work through.

The key to minimal access technology is to use the appropriate size tubular retractor to gain adequate exposure to the operative corridor. In terms of PLIF and TLIF, a 22-mm to 26-mm tube allows good access to the entire facet joint, disc space, and traversing and exiting nerve roots. Through a tubular retractor, discs can be removed, end plate prepared and instrumentation delivered in a safe and effective manner.

Fardon: Is there a clear distinction between a PLIF and a TLIF?

Smith: The PLIF procedure I perform is more like a bilateral TLIF than what would be considered a classic PLIF. These are semantics, and what one surgeon calls a PLIF or TLIF may be the same operation in terms of access to the disc space. The PLIF that I perform requires a complete facetectomy, which allows exposure between the exiting and traversing nerve roots on both sides. With this method, there is exposure to the volar epidural space and disc space with minimal retraction to the exiting and traversing nerve roots.

Foley: PLIF antedates TLIF by decades, as it was first performed by Cloward³ in the 1940s. Although Cloward reported positive results with the technique, other surgeons did not have a similar experience. PLIF was initially a stand-alone technique, done with allograft and/or autograft. Fusion rates, pain control and neural morbidity were issues to consider. When I was training in the late 1970s and early 1980s, PLIF was not a popular procedure. The advent of instrumentation and the combination of pedicle fixation with an interbody fusion technique improved the surgery significantly. Minimally invasive PLIF is a variation on the theme of open, instrumented PLIF and involves working through muscle-splitting approaches with tubular retractors, but follows the same basic principles as the open procedure. TLIF, which was popularized by Harms and Jeszenszky,⁴ is a unilateral interbody approach from a slightly more oblique angle and involves more resection of a single facet joint. Due to the facet resection, there is less need for neural retraction. That is a potential advantage of TLIF over PLIF. Also, because the approach to the interbody space is unilateral, TLIF is a timesaver.

Fardon: What results, such as fusion rates and other long-term data, are available to date?

Foley: In terms of results, it is important to keep in mind that the first successful minimally invasive PLIF with Sextant fixation was performed in Memphis in April 2001. Thus, these procedures have a relatively short history, and long-term data are being collected. That being said, we have seen fusion rates with minimally invasive PLIF and TLIF that are akin to what we have seen with open PLIF and TLIF. The clinical differences we have seen with the less invasive surgeries are shorter hospital stays and quicker returns to activity. A PLIF or TLIF performed in a minimally invasive fashion achieves the same goals and objectives as the open surgery with less approach-related morbidity.

Anterior lumbar interbody fusion

Fardon: Describe the indications for an ALIF. Which patient factors do you consider?

Herman: Patients eligible for the ALIF procedure should have discitis, pseudarthrosis, spondylolisthesis and/or degenerative disc disease.

The ALIF is the best direct approach to the disc space for debridement of infected disc or failed interbody fusion material. The ALIF offers a great mechanical advantage for spondylolisthesis correction and restoration of disc height. In addition, there is the biological advantage offered by excellent exposure to the entire interbody space, which allows for meticulous end plate preparation and engagement of the graft with a large end plate surface area. This is particularly important in biologically compromised patients who smoke, chronically use steroids, have osteoporosis or have had a previous fusion failure (pseudarthrosis).

Patients who are not good candidates for the ALIF approach include patients with radicular symptoms due to nerve compression from current disc herniations, stenosis or synovial cysts. Also, patients who have had previous anterior spine infections or inflammatory bowel problems with multiple abdominal procedures may have vessels adherent to the anterior spine, which could make exposure difficult. Obese patients may be better candidates for the laparoscopic than the mini-open ALIF.

Fardon: Describe the ALIF procedure and the advantages and disadvantages of the open and laparoscopic procedures.

Kleeman: Open and laparoscopic procedures were both designed to successfully approach the anterior interbody space. The anterior approach allows a greater mechanical advantage for restoring disc height and sagittal alignment. Most surgeons who perform the open procedure use a retroperitoneal approach, which results in less incidence of certain complications, particularly retrograde ejaculation.

With the laparoscopic approach, the surgeon uses a transperitoneal approach, providing excellent access to the midline as long as the vascular tissues can be safely retracted.

Laparoscopic procedures have the advantage of insufflation, which provides 15 mm of pressure within the surgical area reducing bleeding. One of the difficulties with the laparoscopic approach, especially at L4-L5, is the challenge of working around the vascular anatomy. With the laparoscopic approach, a patient has to be placed in a Trendelenburg^5,6 position to allow the intestines to fall out of the way. The actual spinal procedure is highly dependent on fluoroscopy to visualize the spine. The scopes and instruments are challenging to use because they have to be placed through portals and manipulated from a distance of 20 cm to 30 cm.

The open approach uses a larger incision, which allows better direct visualization but at the expense of more postoperative pain. Even a minimally open approach requires a 3-cm to 4-cm incision, whereas with the laparoscopic approach, the largest incision is 18 mm. The incision size has an effect on the patient’s postoperative recovery. Intestinal motility in particular recovers faster with the laparoscopic than the open technique.

Herman: It might be valuable to highlight some statistical differences between the mini-open and laparoscopic approaches, because there are several published papers⁷ that describe differences in the incidence of retrograde ejaculation with the two procedures. In fact, surgeons in Dr. Schwender’s group have stopped performing laparoscopic procedures because the laparoscopic incidence of retrograde ejaculation is approximately 10 times higher than with the open procedure.⁸

Kleeman: The superior hypogastric plexus is a variable meshwork of nerves. It appears to extend along the left anterior side of the aorta, then sweeps across the L5-S1 disc space in a variable manner. In two-level fusions, the chance of retrograde ejaculation is significantly higher than in one-level fusions. By avoiding the left side of the aorta and staying to the right side at L5-S1 and using minimal dissection during surgery, the incidence of retrograde ejaculation in the laparoscopic population can be brought close, if not equal to, the open procedure.

Fardon: Dr. Herman, what instrumentation and technique do you use with an open procedure?

Herman: There are several important factors, which include the size of the exposure window (between the vessels), the height of the disc and the type of the anatomical correction which is necessary (spondylolisthesis vs. degenerative disc disease).

I have used a femur cadaver graft with either iliac crest bone graft or BMP inside the lumen of the cage, which is an off-label use of BMP. I have also used a type of titanium mesh cage for the implant, bone dowels, titanium screw-in cages, LT ramped cages, carbon fiber and polyaryletheretherketone (PEEK) cages. Using a single implant takes up less room than two cages, which can be helpful if a surgeon finds himself in a situation with narrow exposure. For example, if a surgeon is coming anteriorly at L5-S1 and finds it difficult to spread the vessels apart, placing a single cadaver femur graft is easier than putting in two threaded cages. Likewise, when coming obliquely at L4-L5, it is easier to put a cadaver graft or one transverse bone dowel through the inner space. Also, a taller cadaver femur graft can be placed through the same exposure window for correction of kyphosis in contrast to cylindrical cages, which require a wider exposure the larger the cage. On the other hand, a threaded or titanium mesh cage may provide better bone engagement, which might be helpful for correction of spondylolisthesis.

Fardon: What implant do you use in a laparoscopic procedure?

Kleeman: Cage implants are optimal for laparoscopic use because they are easily placed through a round portal. We are in an era now where achieving a solid fusion is really not the issue it was five years ago. With BMP available, the fusion is assured. Thus, the design of the material used for implantation becomes less important, allowing the implant to be tailored to the specific approach without jeopardizing the fusion.

Fardon: Please discuss whether you prefer an open technique or a laparoscopic technique.

Peloza: I use the mini-laparotomy retroperitoneal approach as described by Blumenthal et al.⁹ The technique uses fascial incisions without cutting, denervating or devascularizing the muscle. The retroperitoneal exposure is done bluntly with finger and Kitner dissection. Electrocautery is not used and the vessels are retracted with handheld vascular retractors. The technique is fast (average exposure time is 10 minutes and average fusion time is 30 to 40 minutes for a single-level and 50 to 60 minutes for a two-level), reproducible and inexpensive using standard laparotomy sets without special equipment. The procedure is also safe with negligible incidence of vascular injury, thrombosis or retrograde ejaculation.

Foley: My colleagues and I have experience with the laparoscopic technique, but I have stopped using the laparoscopic approach for ALIF procedures. Published data demonstrate higher rates of retrograde ejaculation in the laparoscopic patient population than in the retroperitoneal patient population. Higher rates of retrograde ejaculation are seen not just laparoscopically, but also with open transperitoneal approaches.

Thus, I prefer a mini-open retroperitoneal approach to ALIF. I also prefer a mini-open approach to the anterior lumbar spine because the instruments available for restoring disc space height and lumbar lordosis via this approach are superior to the laparoscopic instruments.

Fardon: With the improved fusion rates that will probably follow the use of BMP, do you perform stand-alone ALIFs or combine ALIF with a posterior approach?

Peloza: In the past, I performed many successful stand-alone ALIFs with and without BMP. The Food and Drug Administration (FDA) trial of LT-cage/rh-BMP2¹⁰ showed impressive clinical results. However, there was a significant percentage of patients who did poorly in spite of solid diagnostic criteria and technically successful surgery. The cause of the poor outcome can be difficult to determine as diagnostic imaging and pain provocative testing are unreliable with titanium cages. With the development of percutaneous pedicle screws, I have supplemented the anterior cages with screws. I have followed patients for almost two years with validated outcome instruments and the results are far better than stand-alone cages with BMP in one-level and especially two-level cases.

Kleeman: At about three months following implantation of an anterior interbody device, the implants are slightly loose due to bone remodeling. While BMP produces the formation of bone, more importantly, it drives the complete cycle of bone remodeling, which includes bone resorption. Thus, at three months following implantation, patients who are otherwise doing well will often experience discomfort when they roll over in bed. In the recumbent position, the stabilizing effect of gravity is lost, allowing for micro motion. By adding the percutaneous posterior instrumentation, patients experience less discomfort during the period of bone remodeling where the implants are experiencing micro motion. At the same time, the additional fixation provides better axial support and less subsidence of the implants.

Foley: Many surgeons are seeing better outcomes with supplemental percutaneous posterior fixation of patients in whom they would previously have performed a stand-alone ALIF. Investigational Device Exemption data with BMP and lordotic cages showed high fusion rates. Oswestry scores improved, but moderately so. There seems to be substantial improvement in the patients in whom posterior percutaneous fixation is added to stand-alone ALIF. Perhaps, by biomechanically stabilizing patients at an earlier stage of recovery, long-term outcomes are improved.

Transforaminal lumbar interbody fusion

Fardon: Dr. Schwender, would you choose a TLIF over a PLIF or an ALIF?

Schwender: I utilize the unilateral TLIF technique at this time for all posterior interbody applications. If a patient has unilateral symptoms, this is the side that I perform the facetectomy and decompression followed by interbody work. If symptoms are bilateral, then bilateral decompressions are performed; however, only unilateral interbody exposure is required.

In addition, as part of the fusion technique, the unilateral facetectomy allows for the contralateral facet to be utilized for fusion purposes. Even if decompression is required, a trumpet-type laminectomy and foraminotomy will spare the majority of the facet, which again will be utilized for fusion purposes. This contralateral foramen can be directly decompressed and will be indirectly decompressed with restoration of height from the interbody procedure.

Finally, posterior compression of pedicle screw instrumentation helps restore lordosis. Once this is performed I again utilize a foraminal probe to ensure the foramen is adequately decompressed and all neural elements pass without impingement.

Fardon: In a patient with central or unilateral symptoms, should the contralateral side be exposed to supplement the fusion?

Schwender: My current technique with the minimally invasive approach is to place the METRx cannula contralaterally in a 22-mm or 26-mm portal. This allows the surgeon to perform decompression if necessary. I place the cannula in the contralateral side to identify the facet joint, fuse the facet joint with a posterior but not a posterolateral fusion, then directly visualize the landmarks for pedicle screw insertion. Visualizing obviates three-dimensional imaging other than fluoroscopy.

Fardon: Dr. Lanman, with the TLIF technique, you mentioned taking the inferior facet off with a burr. What do you do with the superior facet? Do you use the burr close to the nerve root?

Lanman: I use a burr and a Kerrison rongeur. In the beginning, surgeons may want to use a diamond burr. I use a standard Midas Rex AM8 burr (MicroAire Surgical Instruments, Charlottesville, Va) and cut from the inferior edge of the lamina in an arc to the pars interarticularis, because the ligamentum flavum acts as a protective barrier that runs under the lamina and inserts immediately underneath the facet joint and pars interarticularis.

Once the inferior articular facet is resected, I place a small angled curette on the superior medial edge of the superior articular facet and remove the ligamentum from the medial edge of the facet. The exiting root is above and the traversing root is medial to this location at the superior medial edge of the superior articular facet. Then, I run a Kerrison rongeur down the medial edge of the superior articular facet, disarticulating the ligamentum flavum.

Once that is complete and the top superior margin of the inferior pedicle is reached, I cut along the superior edge of that inferior pedicle with a drill and cut laterally and then upward, cutting the rostral edge of the inferior pedicle to provide access to the disc space without encroaching on the exiting root and its dorsal root ganglion.

It is important to shave the superior edge of the inferior pedicle down to provide access for the bone chisel into the disc space. I do not expose the dorsal root ganglion or the exiting root. I take the epidural fat and epidural veins, which I make bipolar and transect, and sweep the fat rostrally over the exiting root to protect the dorsal root ganglion. Then, the discectomy and interbody fusion can be performed with BMP or whatever interbody carrier the surgeon prefers.

Implant preference

Fardon: What is your choice of an implant?

Lanman: I have been using the resorbable copolymer Hydrosorb (Medtronic Sofamor Danek), which is a copolymer of poly-L-lactide and poly DL lactide, a 70%/30% mix. This implant resorbs over about 36 months, and bone is laid down as the implant resorbs completely. I have also used PEEK, which is a nonabsorbable construct used as a carrier for the BMP. These implants have shown faster fusion rates than autologous bone in my experience. There is a slight inflammatory response induced with the resorption process of Hydrosorb, but any inflammatory response in the site of a bony fusion promotes faster bone growth. That is why we do not give patients anti-inflammatory medications after fusions for three months, because they reduce the chance of fusion.

Fardon: What is the shape of the Hydrosorb? Is the morphology of the implant similar to the others?

Lanman: There are many shapes available with the resorbable copolymer. The Hydrosorb spacer or cement restrictor is similar in shape to tangent bone, with different heights. Hydrosorb is approximately 1 cm wide and hollow in the middle. Right now the Hydrosorb is an off-label use. It is currently FDA-approved as a cement restrictor for the long bones, but I ask patients for consent for the use of the Hydrosorb in an FDA off-label manner. The Hydrosorb has a 3° wedge angle and resembles clear plastic. Surgeons should insert the Hydrosorb as they would a tangent bone, using complete discectomy, meticulous end plate preparation and then cortical end plate bone cutting.

Kleeman: Dr. Lanman, could the bone formation with the Hydrosorb be due to less stress shielding as the implant reabsorbs and the resultant compressive forces on the remodeling bone?

Lanman: My colleagues and I considered bone formation vs. an inflammatory response. The Hydrosorb is not supposed to have any loss of its structural integrity at 12 weeks after implantation. Typically, we have seen bone bridging across the disc interspace at 12 weeks. However, when I look at the films of some patients, the Hydrosorb bends in a few patients at the 12-week mark, although bone can be seen through the Hydrosorb. One Hydrosorb graft I explanted at six weeks was structurally intact. We are biomechanically testing this to determine the structural integrity of the implant, but the Hydrosorb resorption with stress shielding may be a factor in the rapid bone formation that is occurring.

Complications

Fardon: Dr. Schwender mentioned worries about elution of BMP and overgrowth of bone with posterolateral and posterior exposures. Has that been a complication or is it a potential complication?

Foley: In my experience, elution of BMP and overgrowth of bone have not been a problem. However, the use of BMP posteriorly and for posterior interbody purposes is off-label. With BMP, I have seen patients fuse in about half the time than in my experience with autograft, and I have seen a correlating clinical improvement with solid fusion. To minimize the elution of BMP into the epidural space and the potential for ectopic calcification, I soak BMP for at least 1.5 hours in its collagen sponge carrier before I implant it. The longer BMP soaks in collagen sponge carrier, the more adsorbed the BMP becomes. There is far less BMP in aqueous solution at 1.5 hours than there is after 15 or 20 minutes of soaking. I also try to minimize epidural bleeding because blood is a BMP carrier. For the same reason, I try not to leave gel foam adjacent to neural structures.

Lanman: In initial FDA studies by Alexander et al¹¹ with BMP placed posteriorly where there was posterior bone formation, the grafts were not countersunk. The titanium cages were flush against the posterior end plate margins. Although the grafts formed some ectopic bone, none had radiculopathy related to that ectopic bone formation. I have had few difficulties with any radiculopathy or ectopic bone formation other than a slight bulge on the initial cases where I did not countersink the grafts enough. Soaking the BMP sponges early is important. With any other excess sponge protruding posterior to the implant I will use a pituitary instrument and trim it off. I seal the posterior hole of the Hydrosorb or polyaryletheretherketone with bone wax and remove the excess BMP on the dura and epidural compartment by irrigation. I then seal the annulus and dura with the Tisseel VH Kit (Baxter Healthcare, Deerfield, Ill).

Schwender: I utilize the products of decompression as well as the facetectomy bone to supplement the interbody spacer and BMP. Preparation of the local autologous bone includes removal of all soft tissue including articular cartilage and placing this bone through a bone mill. This makes the bone easy to work with and compactable.

I have been using a structural implant placed anteriorly followed by Infuse Bone Graft (Medtronic Sofamor Danek) placed through the foramen to the contralateral side and finally placing this compactable bone through the annulotomy and recessing this bone with compaction by approximately 5 mm. This local autologous bone acts as a biologic barrier for the BMP. Thus far, with thin-cut CT scans I have not seen any heterotopic formation of bone within the foramen.

Future

Fardon: What does the future hold for minimally invasive techniques? What research may lead to future improvements and changes?

Foley: Minimally invasive technology will be extended to multi-level procedures, and effective ways to address deformity in a minimally invasive fashion will be developed as well. One area that we have not touched upon is image guidance. Image guidance has not been widely adopted in minimally invasive spinal surgery, but it allows surgeons to “see” inside the human body while minimizing radiation exposure and aids in orientation to hidden anatomy. A newer technology is the isocentric fluoroscope with its ability to generate a CT scan in the operating room and provide automated registration of the anatomy. We will see the integration of automated image guidance systems into minimally invasive approaches to spine surgery in the next several years.

Peloza: Eventually there will be ways to supplement a disc replacement through a posterior technique to protect an artificial disc implant anteriorly. Dynamic stabilization through a posterior approach has been done in Europe, but it is being done with open techniques. If the goal is to provide dynamic stabilization to the spine, it seems counterintuitive to interfere with the dynamic stabilizers with a surgical approach. Performing a dynamic stabilization through a minimally invasive technique is definitely obtainable.

The future is very bright for minimally invasive spinal surgery. Surgeons are already performing multi-level and bilateral decompressions through single portals in the cervical and lumbar spine.

As tools improve, we will push the techniques further. I have also performed many multi-level minimally invasive fusions with ALIF, TLIF and PLIF techniques.

Presently, spondylolisthesis is routinely managed with MAST procedures, and I have started treating selective degenerative scoliosis cases with minimally invasive decompressions and fusion. Another concept is dynamic stabilization. Professor Wolfgang Rauschning¹² champions this theory. Basically, the spine can be stabilized by pedicle or nonpedicle fixation devices that still allow motion. The spine can be directly or indirectly decompressed with this technique but will still move. Since the dynamic stabilizers of the spine (muscles) are paramount, MAST procedures make the most sense for putting in these devices.

Motion technology — total or partial disc arthroplasty — is generating much interest. I can see the need to implant these devices with minimally invasive techniques from anterior, posterior, lateral or far lateral approaches. In addition, a posterior MAST implant may be a way to protect a total disc replacement from premature failure. Lastly, I have already salvaged a failed total disc replacement with MAST implants and techniques and see that as a viable alternative to open techniques or redo anterior surgery.

Schwender: At this time, a minority of surgeons are routinely utilizing minimally invasive procedures to treat patients with lumbar pathology. However, the use of minimally invasive technologies will eventually be driven by patients to improve quality-of-life and potentially reduce associated morbidity of traditional techniques. Both PLIF and TLIF techniques have been performed for approximately two years via a minimally invasive technique. Outcomes look promising, which will further drive the popularity of this technique.

Kleeman: With percutaneous minimally invasive procedures, there is the potential for interventional pain managers to use the discogram approach to inject BMP in the disc space hoping to achieve a fusion. Minimally invasive procedures are not just about performing fusions, but about understanding the anatomy, identifying the pathology and finding the generator of the pain. Then, the surgeon must design an appropriate procedure, attempting to restore disc space, perform indirect decompressions and restore the sagittal alignment and lordosis. The surgeon must apply basic surgical principles and perform the procedure consistently well in a minimally invasive environment. As we look to the future, we must not lose sight of the fact that treating back pain is not just about procedures; it is about taking care of patients.

Foley: It is important to keep in mind the traditional principles of spine surgery. As surgeons continue to explore minimally invasive approaches to the spine, it is important to honor these traditional principles. Minimally invasive should not equate with minimally effective. These approaches are about being a spine surgeon and doing what spine surgeons do well, but with less approach-related morbidity.

Fardon: I would like to thank Orthopedics Today for organizing this symposium and Medtronic Sofamor Danek for sponsoring this event. Special thanks also goes to the faculty for their participation. We hope that the information presented in this discussion will enlighten surgeons in the use of minimally invasive spinal procedures. In the near future, we hope to present further data on the studies and research currently being conducted on these procedures.

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