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.