Defining Minimally Invasive Spine Surgery

McAfee and colleagues provided a concise, in-depth definition of MIS: “An MIS [minimally invasive spine] procedure is one that by virtue of the extent and means of surgical technique results in less collateral tissue damage, resulting in measurable decrease in morbidity and more rapid functional recovery than traditional exposures, without differentiation in the intended surgical goal.” To justify its use, an MIS must demonstrate decreased iatrogenic tissue damage, quantifiable perioperative benefits such as lower operative morbidity, sustained clinical efficacy, and economic cost that is comparable to familiar open surgeries. MIS represents an evolution in spine surgery and must match or improve upon the validated successes of traditional, open surgeries.

Any development in technology or technique that has led to an attainment of the goals listed in the prior paragraph may be construed as MIS. The introduction of the operative microscope for lumbar discectomies, by Yasargil in 1977, permitted better visualization and less iatrogenic tissue disruption. Galibert and Deramond’s development of the vertebroplasty, injecting polymethylmethacrylate into a vertebral body hemangioma, provided immediate improvement in pain via a percutaneous, outpatient procedure. Since its description, the use of vertebroplasty and its more recent iteration, kyphoplasty, has been a commonplace minimally invasive therapeutic option for osteoporotic and pathologic compression fractures. A comprehensive evaluation of the evolution of spine surgery is beyond the scope of this chapter. Instead, we will focus on more recent adaptations.

Recent Developments in Minimally Invasive Spine Surgery

Developments in MIS spine surgery have largely focused primarily on modifications of “access.” Foley and Smith introduced the microendoscopic lumbar discectomy (MED) via a tubular retraction system in 1997. This muscle-splitting technique with sequential tubular dilatation allowed for smaller incisions, preservation of the tendinous attachments of key paraspinal muscles, mitigation of crush injury induced by self-retaining retractors, and the use of anatomic tissue planes while providing visualization comparable to open exposures. A tubular retractor permits access to perform unilateral and bilateral laminotomies, foraminotomies, and discectomies, as well as interbody fusions such as minimally invasive posterior lumbar interbody fusions (PLIF) and transforaminal lumbar interbody fusions (TLIF). Laminotomies through a tubular retractor can be performed in the cervical, thoracic, and lumbosacral segments. Percutaneous pedicle screw constructs are utilized in conjunction with minimally invasive decompressions and interbody fusions for stabilization when indicated.

MIS innovations are not limited to posterior approaches. Anterior and lateral minimally invasive access has been described throughout the spinal axis. Mini-open anterior lumbar interbody fusions are performed through narrow retroperitoneal corridors. A laparoscopic, direct lateral, transpsoas approach was described by Pimenta in 2001. Utilizing technologic advancements in neuromonitoring and a novel tubular retractor, this approach was further developed, and the extreme lateral interbody fusion (XLIF) was introduced. The XLIF has been modified for minimally invasive lateral access to the thoracic spine. These approaches are commonly used to treat thoracolumbar degenerative disc disease, trauma, tumor, infection, and for deformity correction.

New technologies, the desire for improved patient care, and marketing have provided the impetus for a multitude of MIS innovations. Many of these technologic advancements have specific indications in niche markets with limited clinical efficacy data available for review. Furthermore, it would be difficult to compare many of these procedures to a corresponding traditional open procedure. The authors will focus on comparisons between posterior MIS procedures and open surgical techniques.

Minimally Invasive Spine Surgery Versus Open Surgery: a Comparison

High-quality studies designed to provide class I evidence comparing surgical techniques are difficult to produce due to the time, expense, and issues related to having clinical equipoise among the treated patients. These problems substantiate the relative paucity of such data in the spine surgery literature. Convincing patients to enroll in randomization for surgical therapy, the infeasibility of blinding for both subjects and providers, and the need for lengthy longitudinal follow-up are some of the obstacles encountered when comparing MIS to open spine surgeries. Additionally, this lack of quality, long-term efficacy data is a frequently cited weakness in the debate regarding the value of MIS techniques.

The theoretic advantages of MIS can be placed into one of three categories: (1) decrease in perioperative morbidity; (2) reduction of immediate postoperative pain, which hastens recovery; and (3) maintaining long-term stability through limited iatrogenic collateral tissue injury. Potential advantages favoring open procedures include shorter operative times, higher fusion rates due to more fusion surface exposure, and less radiation exposure. Perhaps the most significant advantage for open techniques lies in the history of spine surgery. The clinical efficacy of many traditional spine techniques is supported by decades of scientific investigation and clinical experience. Other factors such as cost effectiveness comparisons and a purported learning curve associated with minimally invasive spine procedures will be addressed.

The early literature assessing the efficacy of minimally invasive discectomies and comparing it with that of open procedures was overwhelmingly in favor of MIS. Minimally invasive discectomies were associated with lower operative times, reduced hospital stays, and shorter return to work intervals with low complication and reherniation rates. The early data for MIS for lumbar stenosis reported similar findings including decreased postoperative narcotic usage. None of the reports represented class I evidence, and these studies often suffered from small cohort sizes, retrospective analyses, and lack of controls. More recent studies have been less favorable toward MIS. In 2009, Arts and coworkers presented their randomized controlled trial comparing 1-year outcomes after minimally invasive discectomies to traditional open discectomies. There was a significant advantage in leg and back pain on the visual analog scale (VAS) as well as the mean Roland-Morris Disability Questionnaire for the 159 patients who underwent conventional surgery versus the 166 patients in the MIS cohort.

Initial reports of minimally invasive fusions mirrored those of discectomies. Minimally invasive fusions were associated with similar or improved clinical outcomes along with lower blood loss, shorter hospital stays, and less postoperative pain. However, the conclusions of several literature reviews range from improved clinical outcomes with lower complication rates for MIS fusion to therapeutic equivalency. The short-term results of MIS procedures have invariably succeeded due to the lesser amounts of tissue disruption; however, long-term outcomes that typically require a successful fusion have been more difficult to demonstrate with the MIS approaches. Opponents of minimally invasive TLIFs and PLIFs often note potentially higher pseudarthrosis rates for MIS fusion procedures, inevitably due to the sole reliance on the disc interspace to fuse. In 2010, Wu and colleagues published a meta-analysis on fusion rates for minimal access versus open TLIFs. Fusion rates were over 90% and largely equivocal for both groups. However, the use of bone morphogenic proteins, a powerful bone induction product that is utilized in an “off-label” manner for MIS procedures, was 50% in the MIS patients and only 12% in the open cohort. The purported higher incidence of increased cerebrospinal fluid (CSF) leaks encountered in MIS decompressions as well as fusions has also been studied with wide-ranging conclusions.

Radiation exposure for spine surgery is often appreciable for both the operating room staff and the patient, and it can vary considerably depending on the operation and the surgeon’s technique. Aside from localization, some surgeons may rely solely on visible landmarks, whereas others utilize digital fluoroscopy. For most minimally invasive procedures, real-time dynamic imaging is essential because the spine is mostly unexposed. An open microdiscectomy typically requires two intraoperative x-rays: one to approximate the incision and one to confirm the correct level once exposed.

Tubular discectomies require multiple fluoroscopic images to dilate and dock the sequential dilators and retractors. A prospective study compared the average radiation exposure to the primary surgeon during 10 minimally invasive lumbar discectomies with 10 traditional open discectomies. A greater than 10-fold, and statistically significant, increase in radiation exposure to the thyroid/eye, chest, and hand occurred during minimally invasive discectomies. Interestingly, the authors noted that a surgeon would exceed the exposure limit for whole-body radiation after 1623 MIS discectomies. Innovations in imaging and computer-assisted navigation have led to increased efficiency, accuracy of instrumentation, and significantly decreased radiation exposure. At present, however, MIS usually results in increased radiation exposure when compared to open surgery.

With the development of any new skill set, a surgeon will likely undergo a learning curve made up of the first set of cases where operative times and complication rates may be higher than accepted standards. The number of cases required to attain proficiency at any given task will vary depending on the complexity of the task and the characteristics of the individual. This initial learning phase has been described for MIS. Aside from longer operative times, surgeons can learn both minimally invasive decompression and fusion techniques while avoiding significant complications during the learning curve period. It is unclear if the unique challenges posed by performing minimal access surgery leads to a longer or more complication-prone learning phase compared to open spine procedures.

The cost effectiveness of spine operations has come under scrutiny. MIS often necessitates higher upfront costs for operative equipment such as digital fluoroscopy, three-dimensional navigation equipment, specialized surgical instruments, and implants. The use of expensive fusion promoting substances such as recombinant bone morphogenic proteins in MIS fusion cases, to offset the lack of a posterolateral fusion, may further sway cost effectiveness in favor of open spine surgery. In contrast, the potential perioperative and immediate postoperative benefits associated with MIS (i.e., fewer operative complications, less postoperative pain, and a shorter length of stay) may balance the higher upfront expense.

A 2010 literature review assessing the cost and clinical efficacy of open versus minimally invasive spine surgeries suggested that MIS may be more cost effective but noted the dearth of available data. Parker and associates reported a 2-year cost utility analysis comparing multilevel hemilaminectomies performed with tubular retractors versus open midline retractors and found that they were cost equivalent. Differences in cost effectiveness may become more apparent when comparing MIS fusions to their open analogs. Wang and coworkers found that in a retrospective analysis of a single center cohort as well as a separate multi-institutional database review, MIS fusions had lower overall charges when compared to open fusions. This was in large part driven by shorter lengths of stay and lower rates of discharge to inpatient rehabilitation in the MIS cohort. Furthermore, the difference was more pronounced in two-level versus one-level procedures.

These data simply compare the immediate short-term charges accrued and do not account for many indirect charges that may account for up to 85% of total costs. If the long-term clinical efficacy of MIS proves inferior to open cases, overall cost effectiveness will likewise diminish. Further study is required to assess the long-term clinical efficacy and well as cost efficiency of MIS.

Additional issues that have been raised, but not formally studied, include the volume of bone graft placed in MIS versus open surgeries. In most MIS procedures, a single intervertebral cage, filled with bone or rhBMP-2, is relied upon to achieve a bony arthrodesis across the interspace. Open surgeries more frequently utilize bilateral cage placements and may also be supplemented by lateral fusions with bone placed over the transverse processes. The quantity of bone graft placed is clearly greater in open surgeries; however, whether this translates into improved clinical outcomes has not been formally studied in any good prospective clinical investigations.