Pedicle screws remain an important adjunct for spine surgeons. Their use in complex reconstructions provides for immediate stability until fusion occurs. Placing pedicle screws as a minimally invasive technique is another important innovation, one that potentially spares the patient from sequelae that result from conventional open surgery.

Conventional open spinal approaches require extensive dissections to allow for the identification of anatomic landmarks for instrumentation and graft placement. Multiple studies have demonstrated that these approaches lead to paraspinal muscle atrophy and scarring with impaired spinal function.

Open surgery requires prolonged tissue retraction that is associated with increased contact pressures, tissue ischemia, decreased muscle density, and paraspinal electromyographic abnormalities. Gejo et al. studied 80 patients who underwent open lumbar surgery. They note that prolonged retraction time resulted in a decrease in trunk muscle strength. The study also correlates prolonged retraction times with an increased incidence of low back pain. In addition, Datta et al. demonstrate that muscle retraction time was proportional to pain and disability scores.

Minimally invasive surgery (MIS) techniques have the potential to decrease the amount of muscle injury by using tubular retractors and tissue dilators that produce less tissue ischemia. In addition, these techniques eliminate muscle stripping that may denervate the paraspinal musculature. Several postoperative comparisons between open and MIS approaches have supported this claim. Kim et al. have compared patients with open and MIS posterior fusion. The study demonstrates that there is a significant decrease in the multifidus muscle cross-sectional area (CSA) and a clinically significant decrease in trunk extension strength seen only in the patients with open posterior fusion. In a similar study, Stevens et al. compares the magnetic resonance images taken from patients who underwent posterior lumbar fusion by either a standard open or a minimally invasive approach. They report “striking visual differences in muscle edema” in the open posterolateral group of patients compared with the minimally invasive patient group. Hyun et al. retrospectively assessed a group of patients who underwent unilateral transforaminal lumbar interbody fusion (TLIF) with ipsilateral instrumented posterior spinal fusion via a standard open midline approach. An additional contralateral instrumented posterior spinal fusion was performed at the same level, using a paramedian intermuscular approach. After surgery, a significant decrease in the CSA of the multifidus on the side of the midline approach occurred. On the contralateral side where the paramedian approach was used, no reduction in the multifidus CSA was measured.

The aforementioned muscle and soft-tissue injury that results from open surgery have led to an increased focus to perform less tissue-disruptive spinal procedures. MIS procedures strive to follow several key concepts. First, the exposure should be limited to only the target surgical site, limiting the unnecessary resection of bone, tendons, and muscle. Through preoperative planning and use of intraoperative imaging, the most direct surgical approach to the pathology is identified. When possible, the surgeon should utilize intermuscular, internervous, and/or intervascular planes to develop the surgical corridor. Second, increased contact pressures that result from retraction must be avoided. Finally, surgeons should exploit the interbody space to achieve fusions. The interbody compartment not only provides for the best fusion environment because of natural axial compressive forces, but also constitutes an isolated tissue compartment. These strategies entail methods to preserve tissue vital to the patient long after a successful decompression or arthrodesis. The ultimate goal of any MIS spine surgery is to limit surgical morbidity while achieving outcome rates comparable with conventional open surgery.