Lumbar fusion has become a widely accepted method for the management of a variety of disorders that require spinal stabilization, such as traumatic, degenerative, infectious, and neoplastic conditions. However, one of the drawbacks of conventional lumbar fusion is the extensive soft tissue dissection that is necessary in order to expose the anatomic landmarks for screw insertion, achieve a proper lateral-to-medial screw trajectory, and develop an acceptable fusion bed. The tissue injury that occurs during the surgical approach can result in increased postoperative pain, lengthened recovery time, and impaired spinal function. Therefore, the development of procedures that minimize tissue trauma without compromising effectiveness represents an important advancement in the field of spine surgery.
This article focuses on the application of minimally invasive techniques to commonly performed lumbar fusion procedures, emphasizing both the apparent benefits and limitations of this emerging technology. Minimally invasive approaches have been applied to a wide range of procedures to include anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), posterolateral onlay (intertransverse) fusion, and pedicle screw/rod placement.
Despite the early encouraging clinical results, it must be remembered that minimally invasive lumbar fusion techniques are in their infancy and the results are preliminary at best. Prospectively conducted outcome studies with long-term follow-up will be the ultimate determinants of the safety and effectiveness of minimally invasive lumbar fusion.
Minimally Invasive Spine Surgery
Minimally invasive techniques have become the gold standard for the management of pathologic conditions in various surgical disciplines. A classic example is in general surgery, where laparoscopic cholecystectomy has supplanted traditional open cholecystectomy as the primary operative treatment for symptomatic gallbladder disease. The laparoscopic approach has been associated with less surgical-related morbidity, better long-term postoperative outcomes, and decreased costs, largely due to shorter postoperative hospital stays. 1,2
Open instrumented lumbar fusion procedures are associated with lengthy hospital stays and significant costs. 3 Additionally, the morbidity associated with these procedures has become an increasing concern for many surgeons. In part, this morbidity is related to the significant iatrogenic muscle and soft tissue injury that occurs during routine lumbar fusion exposures. Multiple authors have documented the harmful effects of the extensive muscle dissection and retraction that normally occur during lumbar procedures. 4–10 Kawaguchi et al 5,6 analyzed the effects of retractor blade pressure on the paraspinous muscles during lumbar surgery. They determined that elevated serum levels of creatine phosphokinase MM isoenzyme, an indicator of muscle injury, is directly related to the retraction pressure and duration. These findings support the work by Gejo et al, 4 who examined postoperative MRIs and trunk muscle strength in 80 patients who previously had lumbar surgery. They concluded that the damage to the lumbar musculature was directly related to the time of retraction during surgery. Furthermore, the incidence of low back pain was significantly increased in patients who had long muscle retraction times. Styf and Willen 10 determined that retractor blades may actually increase intramuscular pressure to levels of ischemia. Mayer et al 7 evaluated trunk muscle strength in patients who had previous lumbar surgery and found that patients who had undergone fusion procedures were significantly weaker than those who had undergone discectomy. Rantanen et al 8 concluded that patients with poor outcomes after lumbar surgery were more likely to have persistent pathologic changes in their paraspinal muscles.
In addition to minimizing the long-term effects of exposure-related muscle injury, minimally invasive lumbar fusion procedures hold the promise of immediate and short-term advantages. Blood loss during open lumbar fusion surgery can be quite significant; not infrequently, patients are asked to donate autologous blood before surgery, or a cell saver is used during the procedure. In comparison, minimally invasive procedures are associated with significantly lower blood loss. 11 Also, the less traumatic surgical approach associated with minimally invasive spinal techniques results in less postoperative pain than do conventional open procedures. This diminished postoperative pain can potentially yield other benefits, such as decreased postoperative narcotic use, earlier mobilization, shorter hospital stay, and a faster return to work.
Although there are many potential advantages to minimally invasive lumbar procedures, the techniques do have limitations and drawbacks. As with any novel surgical technique, a learning curve is associated with the development of proficient technical skills. The first requirement is a thorough knowledge of the underlying three-dimensional spinal anatomy. In contrast to open procedures, where the surrounding anatomy is directly visualized, minimally invasive exposures are generally limited to the area of surgical interest and certain key anatomic landmarks within this limited field of view. Familiarity with the anatomy allows the surgeon to safely perform the procedure without exposing structures that are not being surgically treated. Minimally invasive spinal techniques are also likely to be more technically demanding than are the corresponding open procedures. Surgeons must become facile working through a significantly smaller exposure using instruments that are longer and are frequently bayoneted. Although these procedures can be performed using loupe magnification, most surgeons presently use either the endoscope or an operating microscope because of the enhanced illumination. The two-dimensional visualization offered by an endoscope and the ergonomics of using a microscope can provide challenges to surgeons attempting to learn minimally invasive lumbar fusion techniques. Consequently, early in the surgeon’s learning curve, the complication rates and duration of these procedures may be increased compared with those of conventional open procedures.
Minimally invasive procedures commonly require the use of intraoperative imaging or image guidance. Not infrequently, the surgeon will rely on live or virtual fluoroscopic images displayed on a monitor or three-dimensional images from an image guidance system for anatomic orientation. This can be challenging for surgeons who have not had significant experience using two-dimensional images to determine their three-dimensional surgical position. There are also financial issues involved, as minimally invasive lumbar fusion requires the use of customized instruments and equipment that can be expensive. Finally, although their preliminary results are promising and look quite good, the long-term efficacy of minimally invasive fusion techniques has not been proven. Many of these techniques are still evolving and need to be validated by longer-term studies.
Evolution of Minimally Invasive Lumbar Fusion.
One of the principal reasons for the development of minimally invasive lumbar fusion has been to minimize the paraspinous muscle injury that occurs with traditional open procedures. For similar reasons, strategies for minimally invasive lumbar fixation have evolved. Magerl 12 first reported the use of percutaneous pedicle screws in conjunction with an external fixator in 1982. This technique was intended mainly for trauma applications, and its most salient disadvantage was the risk of infection. As well, it required the use of a cumbersome external appliance. In 1995, Mathews and Long 13 described the use of percutaneous pedicle screws with longitudinal connectors that were placed under direct vision in the superficial subcutaneous space. As the longitudinal connector (a plate) was internalized, the risk of infection was small. Unfortunately, this instrumentation was associated with a significant nonunion rate, likely secondary to the long lever arms of the hardware. In addition, its superficial location was uncomfortable for patients. More recently, the Sextant system was introduced by Foley and colleagues, 14,15 enabling the minimally invasive placement of percutaneous pedicle screws and rods in a subfascial anatomic position similar to that of traditional open techniques (see Percutaneous Spinal Fixation).
Another means of minimally invasive lumbar fixation is translaminar facet screw insertion. First described for use via an open approach by Magerl, 16 this form of lumbar fixation has been applied in a less invasive fashion by others. 17
In the 1990s, increasing experience with endoscopic techniques for general and urological surgery enabled the development of minimally invasive anterior and retroperitoneal approaches to lumbar fusion. Mathews et al 18 and Zucherman et al 19 reported on laparoscopic approaches for anterior lumbar interbody fusion in 1995. Shortly thereafter, McAfee et al 20 described their experience with a minimally invasive lateral retroperitoneal approach for the placement of lumbar interbody fusion cages. The microscope can also be used for a “mini-open” ALIF, first described by Mayer in 1997. 21 Variations on these techniques continue to be developed, including combinations of open and endoscopic approaches (endoscopic-assisted).
A novel means of performing a minimally invasive fusion and fixation at L5-S1 was described by MacMillan et al in 1996. 22 This procedure required a transiliac, trans-sacral approach using fluoroscopic guidance. Although the technique was successful in the authors’ hands, its use has remained limited.
Techniques that facilitate posterior and posterolateral approaches to minimally invasive lumbar fusion have been described only recently. Leu and Hauser 23 developed a biportal percutaneous endoscopic approach to interbody fusion in 1986 and subsequently reported their work. The fusion was carried out with cancellous autograft placed through a 7.5-mm diameter cannula that was inserted into the disc space via a posterolateral, percutaneous approach. Unfortunately, the autograft resorbed and nonunion resulted. Leu then combined his original technique with an external spinal fixator. 12 Although this approach proved successful, it required three surgeries on separate days (placement of the external fixator, percutaneous fusion, removal of the fixator) and was associated with a 16% nonunion rate and an infection rate of 8%. 23
Using cadavers and experimental animals, Boden et al 24 developed a minimally invasive technique for posterolateral fusion in 1996. Instead of autograft, the group used bone morphogenetic protein (rhBMP-2) to successfully induce fusion in the animals.
A tubular retractor system was first developed for microdiscectomy in 1994 by Foley and Smith, 25 and its basic concept is the foundation on which several contemporary approaches to minimally invasive posterior lumbar fusion are based. The system consists of a series of concentric dilators and thin-walled tubular retractors of variable length. The spine is accessed via serial dilation of the natural cleavage plane between muscle fascicles, instead of a more traumatic muscle-stripping approach. The use of a tubular retractor, rather than blades, allows the retractor itself to be thin-walled (0.9 mm), even when the wound is quite deep. Also, unlike blades, the tube circumferentially defines a surgical corridor through the paraspinous tissues. This helps prevent muscle from intruding into the exposure. All of the midline supporting musculoligamentous structures are left intact with this technique. An appropriately sized working channel is created that permits spinal decompression and fusion. Surgery can be performed using the operating microscope, loupes, an endoscope, or a combination of techniques, depending on the preference of the surgeon. The tubular retractor approach can be utilized for minimally invasive lumbar fusion via posterolateral onlay, PLIF, or TLIF. Extreme lateral approaches to the lumbar spine are also possible (ELIF). Detailed descriptions of the various techniques for minimally invasive lumbar fusion follow.
Minimally Invasive Anterior Lumbar Interbody Fusion.
Carpenter 26 first described ALIF for the treatment of spondylolisthesis in 1932. Since that time the procedure has undergone a number of modifications and has become a widely accepted method for lumbar arthrodesis. The potential advantages of the procedure include avoidance of epidural scarring, preservation of posterior spinal elements, and diminished risk of neural injury. A wide assortment of implants have been used for ALIF, including autologous iliac crest, cylindrical bone dowels, femoral ring allografts, carbon fiber cages, 27 and cylindrical 28 and tapered metallic cages.
ALIF can be performed as a stand-alone procedure, but concerns regarding low rates of fusion have prompted many surgeons to place supplemental posterior fixation. This is particularly useful in cases of spondylolisthesis, spinal instability, or in patients with relatively preserved disc height. The development of percutaneous pedicle screw and rod systems has made the placement of a supplemental posterior tension band simpler and more attractive. Percutaneous translaminar facet screws can be used for a similar purpose. ALIF was originally performed through an open retroperitoneal exposure with a large incision; more recently, mini-open retroperitoneal, 21,29 laparoscopic transperitoneal, 18,19,30 and endoscopic retroperitoneal 20,31 approaches have been introduced.
Laparoscopic Transperitoneal ALIF.
Laparoscopic transperitoneal ALIF is performed through three or four 1- to 2-cm incisions, depending on the size of the patient and amount of bowel retraction required. There are usually one to two retraction ports, one camera port, and one working port. Laparoscopic approaches to the L5-S1 level are most commonly performed, as the bifurcation of the great vessels is located rostral to this interspace. The posterior peritoneal wall is opened over the interspace, and the overlying retroperitoneal fat is bluntly dissected in order to avoid injury to the parasympathetic plexus. Once the disc space is well exposed, the fusion can be performed through the working port. Exposing the L4–L5 level can be quite difficult, as this is the typical site for the bifurcation of the great vessels. Depending on the individual patient anatomy, the disc may be accessed between the aorta and vena cava, or the great vessels may be retracted together to either the left or the right. Another factor that makes the laparoscopic approach to L4–L5 significantly more difficult is the descending iliolumbar vein that must be ligated and divided.
Mini-open Retroperitoneal ALIF.
This procedure begins with a 3- to 4-cm transverse incision centered slightly left of the midline and allows a proper trajectory to the interspace of interest as confirmed by fluoroscopy. The dissection is carried down through the anterior rectus sheath and lateral to the rectus abdominus until the peritoneum is encountered. The peritoneum is bluntly separated from the lateral abdominal wall, and the retroperitoneum is entered. A self-retaining retractor system is then used to retract the peritoneal contents, and hand-held retractors are used for the great vessels. The appropriate level is exposed, followed by discectomy and placement of the spinal implant and autologous bone graft or bone morphogenetic protein.
Endoscopic Retroperitoneal Lumbar Fusion.
The endoscopic retroperitoneal approach was first developed for urological surgery 32 and later adapted by McAfee et al 20 for lumbar spine fusion. The procedure can be performed using CO2 insufflation, balloon insufflation (gasless), or a combination of both techniques. The patient is usually placed in the lateral decubitus position, but the supine position has also been described by LeHuec. 31 A 2- to 3-cm transverse skin incision is created at the appropriate level that is centered on a line between the eleventh rib and anterior superior iliac spine. Blunt dissection is carried down through the muscle layers using an endoscopic trocar until the fatty retroperitoneal space is reached. Once the space has been manually confirmed, the dissection balloon is placed and then slowly inflated until a retroperitoneal cavity has been created. At this point the balloon is removed and either a self-retaining retractor system or CO2 insufflation is used to maintain the retroperitoneal cavity. A minimum of three ports are placed for retraction, endoscope, and working instruments. Once the appropriate level has been confirmed fluoroscopically, the psoas muscle is elevated from the spine. The discectomy and fusion are then performed using standard techniques.
Results of Minimally Invasive ALIF Techniques.
Laparoscopic transperitoneal ALIF was initially greeted with enthusiasm by spine surgeons. However, experience with this procedure has shown that it has some significant drawbacks compared with mini-open retroperitoneal ALIF. One of the major disadvantages of laparoscopic transperitoneal ALIF is the risk of retrograde ejaculation in men, presumably caused by injury to the superior hypogastric plexus during the approach. Zdeblick 30 reported a 6% rate of retrograde ejaculation in a series of 68 patients who underwent laparoscopic ALIF. In comparison, Flynn and Price 33 noted an incidence of 0.42% in a series of 4500 open ALIF procedures. Additionally, the bifurcation of the great vessels can significantly increase the difficulty of a laparoscopic L4–L5 ALIF. In a series of 50 patients who underwent L4–L5 ALIF (25 mini-open and 25 laparoscopic), Zdeblick and David 34 reported a significantly higher complication rate (20%vs. 4%) in the laparoscopic group. They found that 16% of the laparoscopic cases were associated with inadequate exposure, and only one cage could be placed in these patients; two cages were placed in all of the mini-open cases. Another potential concern with laparoscopic transperitoneal ALIF is the rate of other complications (including conversion to an open procedure due to inadequate exposure and vascular/visceral injury), which ranges from 10% to 20%. 29,35
In contrast, mini-open ALIF offers many of the same benefits as the laparoscopic approach, such as decreased blood loss, reduced postoperative pain, and shorter hospital stay, while minimizing some of the aforementioned disadvantages. 35 Mini-open ALIF also has a much simpler learning curve for both spine and access surgeons. Despite its limitations, laparoscopic transperitoneal ALIF is a procedure that can be performed effectively and safely. It is unlikely, though, to supplant its open counterpart as has been seen with laparoscopic cholecystectomy. The situation is analogous to that of laparoscopic appendectomy, a procedure preferred by some general surgeons and not others, 36 which has not been shown to offer short- or long-term results significantly different from those of open appendectomy. 37,38
Minimally Invasive Posterior Lumbar Interbody Fusion.
The PLIF procedure was first performed by Cloward in 1943 39 as a method to achieve simultaneous nerve root decompression and interbody fusion in a large series of patients with herniated lumbar intervertebral discs. Iliac crest bone graft was packed into the interbody space following discectomy. Cloward reported good clinical outcomes and fusion rates; however, many other surgeons did not have the same success with PLIF. 40 Their complication rates were relatively high and fusion rates low, and interest in the procedure waned.
Enthusiasm for the procedure was rekindled decades later after several modifications to Cloward’s original PLIF technique were made. These refinements addressed concerns about the success rate of this technically demanding procedure and were stimulated, in part, by advances in spinal instrumentation. Steffee and Sitkowski 41 reported the use of pedicle screw and plate fixation to supplement the interbody fusion. This permitted surgeons to perform a more generous bony decompression without risk of instability, thereby improving visualization and lessening the risk of neurologic injury. 42,43 The development of titanium cages and precision-machined allografts improved structural support, decreased subsidence, and promoted bony fusion. 44,45
The latest refinement of this procedure has been the application of minimally invasive techniques using tubular retractors. 11 The minimally invasive PLIF procedure begins by creating 25-mm skin incisions centered on the disc space, located 25 mm lateral to the midline bilaterally. The paraspinous muscles are bluntly separated by the METRx dilators under fluoroscopic guidance, and the appropriate length tubular retractor is positioned at the lamina-facet junction overlying the disc space. The tubes are 0.9 mm in thickness and range from 3 to 9 cm in length. We typically use 22-mm diameter tubes for lumbar fusion procedures, although 26-mm diameter tubes may also be used (Figure 1). A hemilaminotomy with medial facetectomy is performed, and the autologous bone is saved for the fusion. The ligamentum flavum is then resected, and the nerve root is gently retracted medially. A complete discectomy is followed by disc space distraction and endplate preparation using a customized set of instruments. The disc space is then packed with the autologous bone and machined cortical allografts (cages can also be used). Percutaneous placement of Sextant pedicle screws is performed through the same incisions once the tubular retractors have been removed.
To date, 15 patients (7 male, 8 female; mean age, 61 years) have undergone this procedure who have had at least 1-year of follow-up (range, 12 to 20 months). Ten patients had L4–L5 involvement, and 5 had L5-S1 involvement. Six patients had degenerative disc disease and/or disc herniation, 8 patients had Grade I spondylolisthesis, and 1 had Grade II spondylolisthesis. One patient had neurogenic claudication only, 1 had mechanical back pain only, 4 had mechanical back pain and unilateral radiculopathy, and 1 had mechanical back pain and bilateral radiculopathy.
Minimally invasive PLIF was performed successfully in all 15 patients, with no patient requiring conversion to open surgery. Bilateral pedicle screws were placed in 14 patients, and unilateral pedicle screws were placed in 1 patient. Figure 2 illustrates the incisions resulting from a minimally invasive PLIF with percutaneous pedicle screw and rod insertion. Figure 3 shows a lateral lumbar spine plain film following a minimally invasive PLIF and percutaneous instrumentation. There were no complications. Average operative time was 290 minutes. Average blood loss was 190 mL. Average length of hospitalization was 2.4 days (range, 2–4 days). All patients showed improvement in clinical symptoms and developed solid fusions by radiographic criteria (bony bridging, no motion on flexion-extension, intact instrumentation).
Minimally Invasive Transforaminal Lumbar Interbody Fusion
Transforaminal lumbar interbody fusion (TLIF), a unilateral posterior approach for achieving an interbody arthrodesis, has gained recent popularity. 46–48 The interspace is accessed by performing a unilateral facetectomy, which exposes the posterolateral disc space. If necessary, the exiting nerve root, traversing nerve root, and adjacent dural sac can be identified and decompressed. To facilitate discectomy, distraction is performed through the posterior elements. Following discectomy, structural support (allograft bone or various cage designs) and autograft bone are inserted into the interspace. Supplemental pedicle fixation is added. Depending on surgeon preference, a facet and/or intertransverse fusion may also be performed.
The unilateral TLIF approach for interbody fusion offers several advantages over the PLIF technique. Nerve root and dural retraction are minimized because of the lateral entry point, thereby reducing the risk of neural injury. This lateral entry point to the disc space also makes revision surgeries less difficult, as there is less need to mobilize nerve roots that may be surrounded by epidural scar tissue. Although both procedures require supplemental posterior internal fixation, the TLIF technique preserves the majority of the supporting midline ligamentous and bony structures, whereas these are frequently removed with the PLIF procedure. Lastly, TLIF is associated with a shorter operating room time because of the unilateral approach. The most salient disadvantage of unilateral TLIF is that direct nerve root decompression can only be performed unilaterally, and patients with bilateral radicular symptoms may not respond to the indirect contralateral neural decompression. In this instance, a contralateral direct decompression may need to be performed.
As for minimally invasive PLIF, a METRx tube is used for the minimally invasive TLIF procedure, but the 1-inch skin incision is placed 45 to 50 mm lateral to the midline. On the opposite side, a “mirror-image” incision is utilized for Sextant percutaneous pedicle screw insertion. This contralateral incision can also be used for segmental distraction of the posterior elements and for direct decompression of the contralateral nerve root, if necessary (see below). Proper localization is verified fluoroscopically before making the incisions. Sequential dilators are used, and the distal end of a 22- or 26-mm diameter tube of appropriate length is positioned over the facet joint complex. A total facetectomy is performed using a bayoneted osteotome and Kerrison rongeurs or a high-speed drill. This bone is denuded of all articular cartilage and soft tissue and saved for later use as interbody graft material. At this point, the ligamentum flavum may be resected laterally in order to visualize the ipsilateral exiting and traversing nerve roots. This allows for direct neural decompression and, if the surgeon prefers, direct visualization of these structures during the subsequent procedural steps. The anulus is exposed medial and inferior to the exiting nerve root with little or no need for neural retraction. To improve visualization of the anulus, provide better access to the interbody space, and further protect the exiting root, distraction is performed. With the use of the contralateral 1-inch incision mentioned above, a specially designed laminar-type spreader can be placed between the spinous processes for maximal distraction. Alternatively, the contralateral Sextant screws and rod can be inserted, distracted, and provisionally tightened. After a discectomy has been performed, further distraction can also be carried out using interbody distractors inserted into the disc space via the ipsilateral METRx tube (Figure 4). This is followed by provisional tightening of the contralateral Sextant screw-rod construct and removal of the final interbody distractor. These techniques may also be combined.
The lateral to medial trajectory of the tubular retractor allows the surgeon to reach the contralateral side of the interspace to complete the discectomy, using customized instruments. Structural allograft bone or cages (depending on surgeon preference) are placed into the interspace along with autologous bone graft. If necessary, cancellous bone can be harvested from the iliac crest using a trephine technique through a 1- to 2-cm incision. Alternatively, BMP can be utilized. The tubular retractor is removed, and a Sextant pedicle screw-rod construct is inserted using the same incision. Compression is applied to this construct before final tightening, restoring lordosis and providing compression of the bone graft in the middle column. Contralateral Sextant instrumentation is placed through the contralateral incision. Before this, the contralateral nerve root can be directly decompressed, if necessary. A METRx tubular retractor is inserted through the contralateral incision if this is the case. A facet and/or intertransverse fusion can also be performed through this METRx tube.
The preliminary clinical results for the first 12 patients operated on with this technique have been analyzed. The diagnosis was degenerative disc disease in 7 patients and spondylolisthesis in 5. Operative time averaged 240 minutes. There were no intraoperative complications. Electromyographic monitoring was used in all cases. Estimated blood loss averaged 75 mL. Hospital stay averaged 1.7 days; 4 patients were discharged the day after surgery, and the remaining 8 on the second day after surgery. Seven of 8 patients presenting with preoperative radiculopathy had resolution of symptoms immediately after surgery. Narcotic use was generally limited to 2 to 4 weeks after surgery. To date there have been no early failures or complications to report. Outcomes and radiographic data continue to be collected and will be reported at a minimum of 2-year follow-up (Figures 5, 6).
Minimally Invasive Posterolateral Lumbar Fusion
Posterolateral fusion using the transverse processes, pars interarticularis, and facet joint was first described by Watkins in 1953. 49 Since that time it has become the most widely performed method of lumbar fusion and can be used in a variety of clinical conditions. In 1996, Boden et al 24 described a video-assisted minimally invasive approach for intertransverse process fusion without supplemental internal fixation. This technique was developed in the cadaver lab and applied to rabbits (3-cm midline incision) and monkeys (2.5-cm transverse skin incisions). Moskovitz 50 mentioned using the same approach to perform human arthrodesis, but no clinical details were given. Foley et al 15 performed a successful minimally invasive posterolateral onlay fusion with percutaneous pedicle fixation in 2000 and published the results in 2001. The METRx and Sextant systems were used as described below.
The METRx system can be used to place a 22-mm diameter tubular retractor of appropriate length at the junction of the facet and transverse process. This is done via a 2.5-cm skin incision following sequential soft tissue dilation under fluoroscopic guidance. Residual soft tissue is dissected from the lateral aspect of the facet complex and the dorsal aspects of the intervening transverse processes. The tubular retractor is mobile and can be manipulated to view several adjacent spinal levels through the same skin incision. The bony anatomy is decorticated, and previously harvested cancellous iliac crest graft (trephine technique) is placed on the intertransverse membrane and over the decorticated bony surfaces. Alternatively, BMP with an appropriate carrier can be used. Percutaneous pedicle screws and rods are placed through the same incisions using the Sextant system once the tubular retractor has been removed. Alternatively, an expanding METRx tube (X-tube) that opens to 4 cm at depth can be used to span the intertransverse interval. Pedicle screws and a connecting rod or plate can be inserted directly through the expanding tube, followed by placement of autologous iliac graft.
Endius has recently developed a posterolateral fusion system that uses a similar muscle-splitting approach. Sequential dilation is used to expand the incision, and a specialized retractor is positioned on the lateral facet between the two pedicles. An endoscope is connected to the retractor and provides visualization. The retractor has a skirt that expands from 21 mm superficially to 42 mm at the depths of the incision, allowing the area between the adjacent transverse processes to be visualized simultaneously. Pedicle screws are now placed through the retractor using fluoroscopic guidance. The bony surfaces are decorticated, and autologous bone graft is placed. A fixation plate is placed over the screw heads through the retractor, and nuts are used for final tightening of the construct.
To our knowledge, there are no published series of minimally invasive posterolateral lumbar fusion. We have performed several cases with the use of METRx and Sextant (Figure 7), all of which have resulted in a successful fusion, but we tend to prefer the minimally invasive interbody techniques.
Percutaneous Spinal Fixation
Current options for percutaneous lumbar fixation include facet screws and pedicle screws. Facet screws fix the spine in situ and should be used only when the posterior spinal elements are intact (e.g., following an ALIF). Percutaneous pedicle screws, on the other hand, can be used following a posterior decompression (e.g., after a minimally invasive PLIF with hemilaminotomy and medial facetectomy) or when the posterior elements are deficient (e.g., lytic spondylolisthesis). As well, pedicle screws can be used to apply corrective forces to the spine and to compress interbody grafts. For these reasons, we prefer pedicle screws for minimally invasive lumbar fixation.
Percutaneous pedicle screw placement begins by determining the proper skin entry point using standard fluoroscopy or image guidance. A 22-gauge spinal needle is inserted into the skin approximately 4 to 5 cm lateral to the midline so that its tip appears to bisect the pedicle on a lateral fluoroscopic image. For pedicle fixation following a minimally invasive PLIF, where the skin incision is only 2.5 to 3 cm lateral to the midline, the incision is stretched laterally. An AP projection is then obtained, and the needle’s medial-lateral trajectory is modified until its tip appears to be at the lateral cortical margin of the pedicle. The entry point has now been determined, and a 15-mm skin incision is centered on this point. Alternatively, if the lumbar lordosis allows, a single 1-inch incision centered between two adjacent pedicles can be used to place two screws. Using fluoroscopic guidance, an 11-gauge bone needle is advanced down the pedicle to the posterior third of the vertebral body following a lateral to medial trajectory that is similar to performing a vertebroplasty. A K-wire is passed into the vertebral body through the needle and the needle is removed, leaving the wire in place. Sequential METRx dilators are passed over the K-wire; the final dilator is left in place as a working channel. The pedicle is then tapped over the K-wire, using the final dilator as a protective sheath. Alternatively, a customized pedicle awl and probe may be utilized to create a pilot hole in the pedicle. If the surgeon desires, the pedicle entry site can be directly visualized via a METRx tube.
The Sextant system consists of specially designed, cannulated pedicle screws with polyaxial heads, screw extenders, a rod inserter, and a precontoured rod. 14,15 The screw extenders are connected to the pedicle screws before insertion. Their manipulation aligns the screw heads so that a percutaneously inserted rod can be received. The extenders also contain the lock plugs that secure the rod to the pedicle screw heads. The rod inserter is attached to the screw extenders, creating a geometrically constrained pathway that intersects the screw heads. With the use of the inserter, the rod is percutaneously passed into the screw heads and the lock plugs are tightened. Before final tightening, compressive force (of distractive force, if appropriate) can be applied to the construct via the screw extenders. The rod inserter and screw extenders are removed, leaving a completed construct.
Percutaneous spinal fixation with the Sextant system has been carried out in 63 patients at the University of Tennessee since March 2000. Thirty-nine of these patients have been followed up for at least 12 months (range, 12–37 months; mean, 22 months). Of these 39 patients, 22 were male and 17 were female; their ages ranged from 23 to 80 years (mean, 46 years). Seventeen patients had isthmic spondylolisthesis (11 Grade I, 5 Grade II, and 1 Grade III), 15 had degenerative spondylolisthesis, 6 had degenerative disc disease, and 1 was a trauma patient (distractive flexion injury). Two patients had undergone prior open fusion and had symptomatic nonunion. All patients underwent a concomitant minimally invasive fusion: 25 ALIF, 1 minimally invasive retroperitoneal lumbar fusion, 1 METRx posterolateral onlay fusion, and 12 minimally invasive PLIF or TLIF. Thirty-seven were single-level cases (1, L2–L3; 1, L3–L4; 16, L4–L5; 19, L5-S1) and 2 were two-level (1, L3–L5; 1, L4-S1). Average blood loss for the percutaneous pedicle screw/rod placement was 25 mL. Average length of hospitalization was 2.1 days.
All of the patients improved clinically and developed solid fusions by radiographic criteria. Twenty-six had excellent outcomes and 12 had good ones, as determined by the modified MacNab criteria. 51 One patient required replacement of a loose lock plug 1 month after surgery. The patient did well clinically, and the event was asymptomatic. Reoperation to replace the lock plug was carried out on an outpatient basis. This event occurred early in our clinical experience and led to a redesign of the lock plug. No other device-related problems have been experienced. As the patient required repeat surgery, though, his outcome is classified as poor by the MacNab criteria.
Future Direction of Minimally Invasive Spine Surgery
The future for minimally invasive spine surgery appears promising. New technologies will allow surgeons to effectively perform more complex spinal procedures using techniques that minimize tissue injury. Percutaneous procedures are someday likely to supplant many of the conventional open procedures as the gold standard. Some of the technologies that will enhance the impact of minimally invasive spine surgery include advances in image guidance, robotics, and genetically engineered products such as the bone morphogenetic proteins.
A number of commonly performed lumbar fusion procedures can now be carried out in a minimally invasive fashion. These procedures hold the promise of decreased iatrogenic soft tissue injury and approach-related morbidity, while allowing the surgeon to perform the operation as effectively as the conventional open surgery. Preliminary results suggest that these procedures can be performed safely and effectively. However, the current debate regarding laparoscopic versus open ALIF should be kept in mind, as the issues it raises pertain to minimally invasive spine surgery in general. Although minimally invasive spinal techniques have a logical basis and are appealing to patient and surgeon alike, only prospectively conducted, long-term studies will clearly determine their advantages and disadvantages compared with conventional open surgeries.
- Recent technical advances have allowed minimally invasive lumbar fusion to be performed in a safe and effective manner.
- Minimally invasive techniques exist for posterolateral onlay fusion, anterior and posterior interbody fusion, and lumbar instrumentation.
- Prospectively conducted studies with long-term follow-up will be necessary to clearly define the relative merits of these procedures.
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Keywords:© 2003 Lippincott Williams & Wilkins, Inc.
fusion; lumbar; minimally invasive; percutaneous ] Spine 2003;28:S26–S35