Minimally invasive approach to posterior interbody fusion is predicated on performing the same goals as traditional midline surgery with the emphasis on preserving the lumbar multifidus muscle. The surgical technique can be technically challenging to adapt and requires great care in patient selection. Clinical outcomes are encouraging, but more extensive study is required.
It has similar surgical indications as traditional midline surgery.
Surgeon experience is critical in choosing proper patients for minimal incision surgery.
Coronal/rotational deformities are technically more demanding when performed with minimal incision.
Sound fluoroscopic technique is essential.
Check patient positioning on radiolucent operating room table before prep and draping.
Blunt dissection before placement of the retractor will aid in visualization.
Place pedicle markers before performing facetectomy or hemilaminotomy.
Understand and utilize the “safe zone” in approaching the disc space.
Understand the challenge of operating on obese patients and patients with deformity.
Avoid lateral placement of the retractor by detaching the multifidus insertion.
Facetectomy performed via a minimal incision is technically challenging particularly early in the learning process.
Increase training through use of cadaver labs and training courses.
Minimally invasive surgery (MIS) is muscle-preserving surgery. The goals of traditional lumbar spine surgery are stabilization via fusion and decompression of nervous structures via laminectomy or hemilaminotomy. The goals of muscle-preserving lumbar surgery are to perform these same procedures but perform them with minimal injury to the posterior musculature.
Posterior interbody fusion of the lumbar spine was first reported in 1953 by Dr. Cloward, with the goal of increasing fusion rates and improving sagittal alignment. The technique was further refined by Dr. Brantigan with the use of the fiber carbon implants. Dr. Harms further augmented the procedure to access the disc space via a unilateral transfacet approach known as transforaminal lumbar interbody fusion (TLIF). These procedures are all designed to enable the surgeon to perform the following tasks: (1) access the disc space, remove nuclear material, remove cartilage from the end plates, and place bone grafting material and/or interbody cages; and (2) allow fixation via pedicle screws or facet screws.
Traditionally, this is performed through a posterior midline incision. The main disadvantage of these posterior procedures is the injury to the stabilizing posterior spinal muscles and their nerve supply, which may be a source for continued loss of function and pain, not only from the dissection but also the pressure from the retractors. Because of this muscle injury, Zdeblick coined the term fusion disease for patients with radiographic fusion but poor clinical outcome. These patients often experience early fatigue and inability to perform heavy work after posterior fusion procedures.
A more muscle-preserving approach to the posterior spine is one that leaves the muscle fibers close to its natural preoperative state, whereas still meeting the surgical goals. This approach was originally described in 1968 by Dr. Wiltse, and utilizes an avascular intermuscular plane between the multifidus and longisimus. In this approach, the multifidus is essentially left in situ, whereas the essential portions of the lumbar spine can be accessed to perform surgery.
Furthermore, approaching the posterior lumbar spine via a paramedian approach is a more logical approach. The lumbar pedicles, facet joints, and posterolateral gutter where bone graft is placed for posterolateral fusion all have a paramedian location. Similarly, the approach to the disc space for a TLIF utilizes a paramedian approach. Little logic exists in utilizing a midline incision in performing these surgeries ( Fig. 45–1 ).
This chapter reviews the indications, contraindications, and surgical technique for performing an MIS lumbar interbody fusion. In addition, it covers the postoperative rehabilitation protocol, and reviews the literature on the results and outcomes of MIS interbody fusion.
INDICATIONS AND CONTRAINDICATIONS
Posterior-based interbody fusion is useful in patients where the addition of interbody support can correct a sagittal imbalance; indirectly decompress foraminal, lateral recess, or central stenosis; increase fusion rate; or remove the pain generator. Because the goal of surgery for muscle-sparing posterior interbody fusion is the same as the traditional midline approach, the indications and contraindications are essentially the same. One important caveat exists. The most important factor in determining whether a specific procedure can be performed via a muscle-sparing MIS approach is the experience of the surgeon. It is vital that the surgeon matches the difficulty in treating the pathology of the patient with his or her experience level in MIS surgery. Therefore, contraindications become relative.
The most apparent relative contraindication is obesity. Because the paramedian approach relies heavily on the use of intraoperative fluoroscopy to delineate the bony landmarks for the incision and aid in placement of instrumentation, it can be challenging in the morbidly obese patient and may carry a greater risk for malpositioned pedicle screw placement. Second, many of the surgical instruments and retractor systems commercially available today reach a depth of only 100 mm. Therefore, a morbidly obese patient may have a surgical wound that is too deep for the standard MIS instrument sets.
The second relative contraindication is in treating patients with sagittal, coronal, or rotational deformity. Mild sagittal deformity such as grade I spondylolisthesis can be treated by paramedian approach with no increased risk for complication compared with the traditional midline approach. However, grade II or higher increases the difficulty of the procedure, and care should be paid toward the decision to perform this surgery through an MIS approach. With the decrease in the exposure of the posterior bony elements with an MIS approach, small changes in the anatomic relations in the coronal and rotational plane can result in errant placement of interbody implants and pedicle screw instrumentation with catastrophic results ( Fig. 45–2 ).
The last relative contraindication is patients with previous surgery or anatomic anomalies. For the same reasons as stated earlier, even small variations in the anatomic relation can increase the risk for complications when performing surgery through a paramedian approach.
Comparing traditional midline TLIF with the paramedian TLIF reveals great differences between the two procedures. First, performing surgery through smaller incisions requires a better understanding of the anatomy than the open approach. Second, utilizing a paramedian approach requires the recognition of anatomic structures from a different perspective. The traditional TLIF approach begins with a midline incision with identification of the spinous process, a superficial landmark, to guide the exposure deeper and laterally to the lamina, pars intra-articularis, facet, and transverse process. This medial-to-lateral approach is familiar to most spine surgeons. Conversely, the MIS TLIF utilizes an intermuscular plane between the multifidus medially and the longissimus laterally. The first bony landmark encountered is the facet joint and transverse processes often 6 to 8 cm deep to the skin. From this starting point, the surgeon must identify the pars intra-articularis and lamina in a lateral-to-medial fashion. Therefore, the surgeon must be able to not only recognize the anatomy through a smaller incision, but be familiar with the anatomic relationships from a different starting point. Third, with smaller incisions and decreased direct visualization of anatomic structures, an increase in the use of fluoroscopic imaging occurs. Therefore, it is vital the surgeon is well trained in the use of fluoroscopy.
The first step in performing successful minimal incision surgery is proper patient positioning. It is important that the patient is squared on the operating room table with no rotation. Before prepping, an AP fluoroscopic image should be obtained to ensure the spinous processes are centered between the pedicles. To minimize the dissection required for the approach, it is important to properly place the incisions.
The C-arm needs to be angled in the correct trajectory to be on plane with the operative disc space (Ferguson view). This angle is roughly 15 to 30 degrees at L5-S1, 5 to 15 degrees at L4-5, and vertical at L3-4. These angles are important because they also represent the angle of the approach to the operative level. Bony anatomic landmarks can now be marked on the skin using fluoroscopic guidance ( Fig. 45–3 ). Important landmarks are the pedicles to be instrumented and the midline. For the average-sized patient undergoing surgery in the lower lumbar spine, the incision is approximately 4.5 cm off midline centered between the operative pedicles. This distance is decreased in thin patients and when operating on more cephalad levels.
Next, adequate visualization is accomplished through the use of loupe magnification, intraoperative microscope, or an endoscope. Loupe magnification allows for three-dimensional (3D) visualization but does not provide maximum magnification and requires an additional light source. A surgical microscope provides maximum magnification and 3D visualization but can be cumbersome when working down a small portal. It also can be difficult to maneuver if a C-arm needs to be brought into the surgical field. The endoscope can provide visualization into spaces and around corners where the loupe magnification and microscope cannot, but provides only 2D visualization and also requires an additional light source.
Once the patient is prepped and draped, the skin incision is made. Most often, the size of the incision is determined by the type of retractor system used. The dissection is carried deeper to the lumbosacral fascia. Once the lumbosacral fascia is identified, the interval between the multifidus and longissimus can be palpated. The fascia is incised in line with the incision within this interval. This is a natural intermuscular and usually avascular plane that will guide the surgeon just lateral of the facets to the transverse process. This surgical plane is slightly lateral to the proper starting point to successfully perform a TLIF. Therefore, a small portion of the multifidus muscle, which inserts onto the lateral aspect of the superior articular process of the facet, must be detached to allow adequate visualization of the facet. Only three bony landmarks need to be exposed to perform a TLIF: the facet, the pars intra-articularis, and the transverse processes of the operative level. The surgeon should be able to palpate all these structures before placement of the retractor. If these structures can be palpated, they will be visualized when the retractor is placed.
Once correct operative level has been confirmed with fluoroscopy, your retractor of choice can be placed usually over a series of dilators. Once the retractor is in proper position ( Fig. 45–4 ), if the dissection before placement of the retractor was adequate, only a small amount of tissue will need to be removed to directly visualize the facet, pars intra-articularis, and transverse processes. Minimizing the amount of muscle removed is the cornerstone to successful MIS surgery; therefore, care should be taken not to be overexuberant in tissue dissection. Remove only the tissue required to perform the goals of the procedure.