Minimally Invasive Midline Lumbar Fusion (MIDLIF)

Introduction

Each year in the United States more than 250,000 individuals undergo spinal fusions for degenerative lumbar spine pathology. Minimally invasive techniques for posterior lumbar interbody fusion offer the benefits of a smaller incision, minimization of injury to muscles and tendons, and shorter hospital stays over traditional open techniques. With regard to patient outcomes, a meta-analysis of 770 patients reported minimally invasive techniques for lumbar fusion also demonstrate significantly lower rate of adjacent level disease. The evolution of minimally invasive approaches is in parallel with technologies allowing safe and adequate surgical access to accomplishing the surgical goals of replacing the disk space with a fusion nidus and placing instrumentation to ensure stability during this process. The MAST MIDLIF procedure, which was developed and introduced in 2011, uses a proprietary retractor and cortical bone screw fixation along with the interbody fusion technique. This technique is unique in offering a midline bilateral minimally invasive alternative without the need for a tubular retractor. This allows for recognition of familiar posterior spinal landmarks and direct access to posterior element pathology including stenosis (canal, lateral recess, foramen) and synovial cysts. For fixation, the more medial bone entry point along with a caudocephalad and mediolateral screw trajectory of cortical screws allows for a more paramedial position of the segmental fixation, obviating the need for a wide lateral exposure ( Fig. 10.1 ).

The biomechanical evaluation of cortical screws validates this novel fixation trajectory. Cortical screws are smaller than traditional pedicle screws; however, the trajectory allows the majority of the screw to pass through dense cortical bone compared with 20% cortical bone purchase of a traditional pedicle screw ( Fig. 10.2 ). The purchase of additional cortical bone fixation despite an overall smaller screw size demonstrates equivalent pullout strength and more dense trajectory bone quality in human cadaveric lumbar spine. Modifying the screw trajectory for the posterior segmental fixation from that of a traditional pedicle screw to a cortical screw trajectory establishes a durable construct using consistent anatomic landmarks even in degenerative spine pathology.

Surgical Indications

Optimal indications are one- or two-level spinal instability or deformity, including spondylolisthesis, lumbar stenosis with instability or stenosis requiring a decompression that may result in postoperative progressive deformity/iatrogenic instability, recurrent disk herniation, adjacent level degeneration to an existing fusion, and pseudoarthrosis. In general, the surgical indications for a minimally invasive posterior lumbar interbody fusion are similar to those for an open posterior lumbar interbody fusion.

Limitations

The originally described technique uses posterior fixation with a cortical bone screw trajectory. Limitations or contraindications would include cases with no competent pedicles (e.g., fracture, neoplasm, infection) and lack of a definitive entry point at the pars and transverse process junction from a prior decompression. Biomechanical studies also identified spondylotic vertebrae as a potentially concerning pathology for placement of cortical trajectory screw fixation.

Surgical Technique

The procedure steps outlined below are one example of many for how to accomplish this surgery. Each surgeon will have preferences to incorporate into the procedure including operative bed, retractor preferences, visualization aids including the microscope and/or endoscope, intraoperative imaging systems, and screw placement technique.

Minimally invasive spine surgery was born from the advancement of retractors focusing on muscle-splitting and visualization systems. Specifically, in our practice we use the MAST MIDLF retractor system; however, there are multiple other types of retractors, including tubular and expandable varieties. Often, we incorporate the use of an operative microscope into the minimally invasive lumbar fusions in place of a fiber optic lighting system. Others routinely use the endoscope.

For intraoperative imaging we use fluoroscopy; however, increasingly computed tomography (CT) navigation systems are utilized. If used, appropriate modifications to the surgical technique, including a registration spin along with compatible hardware must be planned. The advantages to CT intraoperative guidance are surgeon comfort, real-time planning of entry point and screw trajectory, increased screw placement accuracy, and potential confirmation of screw placement. Disadvantages are increased operative time and increased patient radiation exposure.

We present the cortical bone screw trajectory that uses a medial entry point compared with a traditional pedicle screw as this allows adequate visualization of the entry point without the need for further lateral muscle dissection. Percutaneous screw fixation, described in 2001, is also another option to complement a minimally invasive approach to a posterior interbody fusion procedure. Screw fixation can also be accomplished with robotic guidance systems for enhanced trajectory planning and final screw placement accuracy.

Provided with this chapter is a detailed video demonstrating a case of a MIDLIF ( ) that follows the steps outlined below.

Step 1: Positioning. After induction of general endotracheal anesthesia and administration of preoperative antibiotics, we position the patient prone on two chest rolls on a regular table. The correct operative level is identified using fluoroscopy, and a midline incision is marked before preparing and draping in a sterile fashion.

Video 10.1 MIDLIF operative technique. Presented is a case of a 60-year-old male patient with progressive, disabling neurogenic claudication and back pain in the erect position that has not been corrected after nonoperative interventions. His films demonstrate degenerative spondylolisthesis and stenosis at L3-4 with progressive disk height loss and translation when upright. The individual steps for the minimally invasive midline lumbar fusion operative technique are presented as they occur during a live surgery. (Courtesy of Charles L. Branch, Jr., MD.)

Step 2: Incision and superficial dissection. For single level operations the incision is roughly 30 to 40 mm in length and carried down through the fascia in the midline. A speculum is then inserted on the lateral border of the spinous process to develop the subperiosteal plane between muscle and bone ( Fig. 10.3A ). Muscle from the spinous process and lamina of the operative levels is bluntly dissected with the speculum retractor to expose the facet ( Fig. 10.3B ). This process is repeated on the other side of the spinous process to accomplish a bilateral exposure. The speculum is then docked on the lamina, and a ruler on the lateral surface of the speculum blade measures out the appropriate MAST retractor blade length ( Fig.10.3C ). The speculum retractor is rotated 90 degrees so the blades of the speculum open parallel to the spinous process and the handle of the speculum is perpendicular to the spine ( Fig. 10.4A ). The blades are then opened to allow insertion of the MAST retractor blade between the blades of the speculum ( Fig. 10.4B ). Once the retractor blade is seated it is held in place while the speculum is withdrawn from this side of the exposure. It is then inserted on the contralateral side and the retractor blade placement is repeated here ( Fig. 10.4C ). The final position of the MAST retractor blade should be centered over the operative disk space, which can be confirmed with fluoroscopy. The MAST blades are then attached to the retractor device, which is used to angle the blades outward laterally, maximizing the operative field visible through the incision. The retractor blades are then expanded laterally, exposing the operative corridor. The surgeon may either use the operative microscope or attach the light source to the retractor and continue the operation using loupes ( Fig. 10.5 ).