CHAPTER 58 Minimally Invasive Posterior Lumbar Fusion Techniques
Lumbar fusion is a reliable treatment option for a wide variety of spinal pathologies resulting in spinal column instability and/or spinal-related pain. Various spinal fusion techniques are the subject of ongoing clinical investigations with goals to improve surgical technique, graft biomaterials, and implant designs in order to achieve a stable, symptom-free spinal column with the least chance of patient morbidity.
Despite these advances, the morbidity of spinal fusion surgery remains significant. The standard posterior midline exposure is notorious for paraspinal muscle stripping and denervation leading to significant postoperative scar formation. The limitations of this approach for spinal fusion have been well documented, especially regarding a prolonged recovery period and muscle damage that may affect a patient following surgery.1–5
In recent years, less invasive surgical approaches have been developed to minimize damage to the paraspinal soft tissues during surgical exposure. These “minimally invasive” surgical approaches are becoming more popular because they offer the surgeon a method to achieve the goals of spinal surgery while minimizing some of the perioperative morbidity inherent to the classic posterior approach.6,7
Minimally invasive spinal surgery (MISS) is a rapidly evolving field that is supported by a number of technologic innovations. These include the operative microscope, C-arm fluoroscopy, tubular retractor systems, cannulated pedicle screws, and for some, image guidance systems. The basic hand instruments used during a minimally invasive spinal procedure are similar to those used during a traditional spinal case but are often longer and bayoneted to improve visualization through a tubular retraction system. A high-speed burr or drill with a long and thin shaft is useful in decorticating or thinning the bony elements of the spine. To be successful with MISS, a surgeon must be familiar with the microscopic anatomy of the spine. He or she must gain the skills necessary to work safely and efficiently despite a limited field of view and become facile with the use of MISS equipment. This set of skills can only be gained with experience, and thus a significant but definable learning curve should be anticipated by surgeons interested in becoming proficient in MISS techniques. The length and slope of the learning curve will also depend on the individual surgeon’s skill set and prior spinal experience. The learning process is best accomplished in a slow, step-wise fashion, mastering basic skills with simple cases before attempting to approach the more challenging spinal pathologies in a minimally invasive surgical fashion. This chapter provides an overview to the field of MISS as it applies to lumbar fusion techniques for common conditions of the lumbar spine.
Principles of Minimally Invasive Spinal Surgery
All minimally invasive spinal procedures, despite the type and the location, have the common goal of correcting the underlying spinal pathology while avoiding excessive damage to the paraspinal soft tissue envelope. As with other spine procedures, an MISS procedure begins with the careful analysis of the preoperative imaging studies to precisely localize the spinal pathology. Before making a surgical incision, preoperative fluoroscopy is used to localize the involved spinal segments and plan the skin incision. During the surgical approach, the paraspinal muscles are split rather than cut or resected using serial tubular dilators to create a surgical corridor between the skin incision and the spine (Fig. 58–1). Only necessary portions of the vertebral columns are exposed, and excessive use of electrocautery or vigorous retractor pressures should be avoided.
FIGURE 58–1 Serial tubular dilators (A, individual; B, assembled) are used to gain access to the spine through muscle-splitting rather than cutting through the paraspinal muscles.
The surgeon must understand the muscular anatomy of the paraspinal region to design the optimal approach for an MISS procedure. There are two distinct muscular compartments: the multifidus compartment, which overlies the midline spinal structures, and the lateral compartment, which overlies the transverse processes (Fig. 58–2). The multifidus muscle surrounds the spinous processes, lamina, and facet joints. The multifidus muscles receive its nerve and blood supply from the medial branches of the dorsal rami and segmental vessels, respectively, which course from the intervertebral foramen along the base of the transverse process and enter the lateral margin of the muscle in the region of the pars intra-articularis. Care should be taken to avoid rupturing the lateral attachments of the multifidus muscle, which would disrupt the nerve and blood supply to the multifidus muscle, leading to atrophy and scar formation in the substance of the muscle. The lateral muscle compartment contains the longitudinally oriented muscles of the erector spinae group. The lateral compartment overlies the transverse processes and includes the entry site for pedicle screw insertion at the base of the transverse process. The lateral compartment is traversed whenever a posterolateral onlay fusion is performed.
FIGURE 58–2 Deep paraspinal muscle compartment: The multifidus muscle (C) overlies the midline, and the lateral compartment (A, B) contains the longissimus and iliocostalis muscles, overlying the transverse processes.
When approaching the spinal canal, laminae, or facet joint, a trans-multifidus compartment approach is required. When placing percutaneous pedicle screws or performing a posterolateral onlay fusion, a translateral compartment approach is necessary. To operate within a particular compartment, the fascia over that compartment should be opened and then the muscular tissues dilated or split to reach the spinal structures of interest. When moving from compartment to compartment, the surgeon should never transgress the fascial barrier between the compartments because this would disrupt the nerve and blood supply to the multifidus muscle. Instead, the retractor should be withdrawn and the fascia should be opened over the other compartment followed by a muscle-splitting approach to the spinal contents of the compartment. The same skin incision can generally be used for two separate fascial incisions used to access the compartments.
When discussing a minimally invasive surgical approach with a patient, the surgeon should ensure that the patient understands the available surgical options (both MISS and open surgery) for treating the spinal condition. In addition, the surgeon should include in the surgical consent process the possibility that the less invasive procedure may need to be converted to a larger, open approach to achieve the ultimate goals of surgery. Both the surgeon and patient should remember that correction of the spinal pathology is the most important issue, whereas the approach and incisional size are lesser considerations.
Surgical Setup for a Posterior Fusion Procedure
Setup and Imaging
Following placement of surgical monitoring equipment and the induction of general anesthesia, the patient should be positioned prone on a radiolucent spinal frame (Fig. 58–3). The abdomen should be free of compression, and free access to the lumbar region for fluoroscopy should be confirmed. The preoperative imaging studies should be available in the room with the operative plan clearly marked. The surgeon should ensure the availability of the proper implants and instruments prior to commencing with the operative procedure.
After a sterile skin preparation and draping, the C-arm mobile fluoroscopy unit is used to demarcate the location of bony landmarks, which are drawn on the skin. A critical step is to ensure that the skin incisions are localized in an optimal position to allow access to the underlying spinal pathology. In some cases, it may be useful to introduce a spinal needle along the proposed trajectory of the surgical incision and check the position of the needle on both anteroposterior (AP) and lateral fluoroscopic views to ensure an optimal path to the pathology is achieved.
Surgical Incisions and Approach
The number and length of skin incisions should correspond to the surgical plan, which must be more “thought out” compared with a traditional open surgery. A single skin incision may be used during different phases of the surgical procedure to reach different areas of the spine. For instance, one incision may initially be used to decompress the neural elements (multifidus compartment). Subsequently, the same skin incision may be used to perform a posterolateral fusion and place pedicle screw instrumentation (lateral compartment). Although a single skin incision is used, separate fascial incisions should be used to reach each individual compartment.
When working through perimedian incisions, two distinct fascial layers are encountered. The superficial layer corresponds to the thoracodorsal fascia, while the deeper layer is a thin fascia that overlies the muscle of the compartment. Both fascial incisions should be a little longer than the corresponding skin incision because this will allow the subsequently placed tubular retractors to be maneuvered and angulated freely as needed to reach the various areas of the spine necessary to perform the operation. The muscles of the compartment can be split with the surgeon’s digit or with an instrument such as a Cobb elevator. It is often helpful to palpate bony landmarks such as the facet joint or transverse processes to assist with placement of the initial instruments through the skin and muscle portal to the vertebral column.
When operating through a tubular retractor, the smallest dilator is then docked at the appropriate bony site and serial dilation is used to expand the operative corridor. Care should be taken into bringing each subsequent dilator in contact with the bony elements. The correct length of the tubular retractor can then be selected, inserted, and secured using an operating table–mounted retractor holder. Once the tubular retractor is in place, the position of the retractor should be verified using fluoroscopy (Fig. 58–4).
The topic of spinal decompression is substantial and exceeds the goals of this chapter; however, a few points deserve mention. The authors prefer to perform the decompression first when performing both a decompression and fusion of the lumbar spine. This allows the surgeon to obtain local autogenous bone from the decompression site that may be used for the spine fusion and also exposes important bony landmarks such as the pedicle, which is useful in subsequent stages of the procedure. The decompression is done by entering and traversing the multifidus compartment. Thus it is useful to perform a facet fusion during this phase of the procedure, before exiting the multifidus compartment.
Posterior Interbody and Transforaminal Interbody Fusion
When performing a posterior or transforaminal lumbar interbody fusion (PLIF and TLIF) via a minimally invasive approach, it is important to align the tubular retractor collinear with the disc space on the lateral view (Fig. 58–5). When performing a TLIF procedure, the tubular retractor must be aligned with enough lateral to medial angulation to allow the surgeon to reach the contralateral side of the disc space for preparation of an adequate fusion bed (Fig. 58–6). During the exposure, adequate facet joint must be removed to minimize retraction of the neural elements and provide working access to the disc space.8
FIGURE 58–5 Lateral fluoroscopic view shows the tubular retractor positioned in a proper alignment (i.e., collinear with the disc space).
FIGURE 58–6 Angulation of the tubular retractor system allows access to the contralateral side of the disc space and therefore preparation of the fusion bed.
The detrimental effects of over-retraction of the neural elements with the PLIF procedure have been well documented in the literature.9 Facet removals for a PLIF or TLIF can be achieved with either osteotomes or a high-speed burr. It is helpful to skeletonize the upper and medial portions of the caudal pedicle (e.g., L5 pedicle for a L4-5 TLIF) to gain adequate access to the disc space and allow safe retraction/protection of the dural/neural elements.
Once the disc space has been adequately exposed, the posterolateral annulus is incised with a scalpel and the posterior margin of the disc is removed. The posterior “lip” of the vertebral body should be resected so that the opening is flush with the most concaved portions of the disc space. Disc material and the cartilaginous endplates are thoroughly débrided from the interbody space using curettes, shavers, and/or pituitary rongeurs until the interspace is clean, leaving only intact bony endplates to support the interbody cage. If the disc space is collapsed, the endplates should be dilated to restore the foraminal height and improve the sagittal contour of the spine.
After disc space preparation, the interspace should be packed with autogenous bone graft or an adequate fusion substrate. An interbody fusion cage, of appropriate size, is selected and packed with the graft material, before impacting the cage into the disc space. The optimal position of the cage is toward the anterior portion of the disc space.10,11 This produces better reconstruction of the sagittal contour of the spine and allows ample bone graft material to be packed around and behind the cage.
Instrumentation, most commonly with pedicle screws, is a standard component of both the modern PLIF and TLIF procedures. Following the insertion of pedicle screws and rods, compression of the interbody construct is performed to restore the lumbar lordosis and ensure compressive loading of the interbody grafts.
Posterolateral Fusion (Intertransverse Onlay Fusion)
From the traditional midline approach, access to the intertransverse region for onlay fusion requires complete stripping of the paraspinal muscles to the tips of the transverse processes, an act that causes destruction, or at least disruption, of the multifidus muscle and significant postoperative scarring.5 Using the paraspinal muscle-splitting approach (Wiltse approach), exposure of the intertransverse region is simple to achieve without major muscle stripping. This provides direct access to the intertransverse region for fusion.
The skin incision for a paraspinal approach for intertransverse fusion is made at least 3.5 to 4 cm lateral to the midline. The fascia is divided in line with the skin incision, and the paraspinal muscles are split in line with their fibers to expose the transverse processes. For fusion purposes, the entire transverse process at both levels should be exposed. Either a tubular retractor (preferably an expandable tubular retractor) or side-to-side (e.g., McCullough retractor) retractor can be used to visualize the intertransverse interval. The authors prefer to use an expandable tubular retractor, which allows both transverse processes to be simultaneously exposed (Fig. 58–7).
FIGURE 58–7 The use of an expandable tubular retractor allows simultaneous exposure of both transverse processes.
Once the intertransverse region has been exposed, the soft tissues are meticulously cleaned away from the transverse processes and intertransverse membrane. The transverse processes are decorticated using a high-speed burr and then the interval is packed with autogenous bone graft or a suitable graft material. When withdrawing the retractors, care should be taken to not displace the graft materials from the fusion bed.
Fusion of the facet joints is useful as an adjunct to interbody or intertransverse fusion but has not been well accepted as a stand-alone fusion due to the relatively small surface area of the facet joints. However, the facet joint offers a number of theoretical advantages as a fusion site including the ease of access to the joint, the small gap across which the fusion must heal, and the compression of the fusion site that is achieved during normal upright posture of the patient. In addition, a facet fusion is a quick, simple, and low-morbidity procedure.
To perform a facet fusion, the retractor should be docked on the facet, which resides in the lateral portion of the multifidus compartment (see Fig. 58–2). If decompression of the spinal canal is required, facet fusion can easily be performed during the exposure through the multifidus compartment. Once the facet is exposed, the capsule is removed with electrocautery and the articular surfaces of the inferior and superior articular processes are identified. A high-speed burr is used to decorticate the facet joint along its entire length, and the joint space is packed with fragments of autogenous bone or a suitable bone substitute.
In some cases osteophytic bone material may overlie the true facet joint, and this should be removed to expose the native joint surfaces. The surgeon should be cognizant of the normal anatomy of the facet joint with the superior articular process lying lateral and deep to the inferior articular process. The specific topography of the facet joint can also be defined preoperatively by analyzing imaging studies (magnetic resonance imaging [MRI] or computed tomography [CT]).