Minimally Invasive Transforaminal Lumbar Interbody Fusion (MITLIF)

Introduction

Minimally invasive transforaminal lumbar interbody fusion (MITLIF) allows a surgeon to restore intervertebral disk height, lumbar lordosis, and achieve an indirect decompression of the spinal canal and neural foramen while preserving vital posterior soft tissues. Adequate exposure of anatomic landmarks in open transforaminal lumbar interbody fusion (OTLIF) mandates a longer muscle-splitting incision and use of large self-retaining retractors at high tension that can generate pressures in the erector spinae musculature from 61 to 158 mm Hg. The resultant prolonged retraction induces ischemic changes resulting in reduced muscle fiber diameter, fibrosis, and fatty infiltration. These degenerative changes, in combination with disruption of stabilizing ligamentous structures, have been implicated as a major source of chronic back pain, failed back syndrome, and reductions in patient-reported functional outcome scores following open lumbar spine surgery.

Foley et al. introduced MITLIF in 2003 as a means of achieving the same objectives as OTLIF while minimizing iatrogenic soft tissue injury. Use of a tubular retractor placed via sequential dilation maintains continuity of muscle fibers and allows equal distribution of pressure around the wound edges. Critics of the MITLIF will cite technical limitations including loss of surface area for fusion, as the posterolateral recess is typically not exposed, and a heavy reliance on fluoroscopic imaging that increases radiation exposure to the surgeon, patient, and operating room staff. Lack of exposure of anatomic landmarks and limited working space also contribute to a significant learning curve for a surgeon looking to adopt MITLIF. To limit exposing the patient and surgical staff to risk, it is imperative that the surgeon follows a reproducible process of patient evaluation and perioperative care. The goal of this chapter is to describe in detail the MITLIF focusing on preoperative evaluation, intraoperative technical pearls, patient outcomes, and complication avoidance.

Surgical Indication

The MITLIF carries the same indications as OTLIF; both allow restoration of disk height with a structural interbody graft resting in the anterior column, which is responsible for bearing 80% of the load transmitted through the spine. This recreates the normal sagittal alignment between the two vertebrae and opens the facet joints to their native apposition, thereby achieving an indirect decompression of the spinal canal and contralateral nerve root. An MITLIF is an effective treatment for symptomatic spondylolisthesis, lumbar stenosis with instability, recurrent disk herniations, as well as instability secondary to trauma, pseudarthrosis, or iatrogenic sources.

Limitations

Contraindications and limitations with respect to MITLIF can be divided into those that are absolute and those that are relative. Absolute contraindications to both OTLIF and MITLIF include conjoined nerve roots, acute trauma, or active infection. Aberrant location and connections seen with conjoined roots make safe performance of a TLIF nearly impossible, as limited mobility of the nerve root restricts access to the intervertebral disk ( Fig. 9.1 ). With acute trauma to the vertebral endplate there is no stable foundation to distract upon or to support the interbody cage, greatly increasing the risk of nonunion or cage migration. Active systemic infection is a contraindication to TLIF as well as other elective orthopedic surgeries where metal hardware is implanted into the body. However, it should be noted that implantation of titanium interbody cages with posterior spinal fixation has been shown to be safe and effective in the treatment of discitis or vertebral osteomyelitis as it provides the necessary stability for healing to occur. Treatment includes focused antibiotic therapy, typically for 12 weeks, and radical debridement of infected tissue mandating an open approach. Relative contraindications would include severe epidural scarring, severe osteoporosis, or grade III and IV spondylolisthesis. MITLIF may be preferable to OTLIF in the setting of morbid obesity or soft tissue compromise owing to burns, trauma, or cutaneous lesions.

Perceived complexity of the MITLIF increases when there is severe collapse of the disk space or significant osteophyte formation, especially over the posterior edge of the disk space ( Fig. 9.2 ). When encountering these cases, the preoperative images should be scrutinized for the presence of a mobile spondylolisthesis or vacuum phenomenon within the disk. Both of these signs indicate laxity of the soft tissues around the disk space, which should allow for distraction and restoration of height. When osteophyte overgrowth covers the disk, the experienced surgeon can use an osteotome or bur to debride the lip and gain entry to the disk space.

In general, no specific situations exist in which OTLIF is overtly better suited than MITLIF; however, this depends on the surgeon’s experience with each technique. Notably, the progressive narrowing of the interpedicular distance at each cranial level in the lumbar spine makes performing an MITLIF much more difficult above the L3-4 disk. Performing a facetectomy in this region reveals the dural sac and a small Kambin’s triangle, which increases the risk of dural tear or postoperative radiculopathy from retraction of neural structures.

Surgical Technique