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Minimally Invasive Spine Surgery
By Joseph K. Weistroffer, M.D., Division of Orthopaedic Spine Surgery
Minimally Invasive Spine Surgery. What is in a name? Evidently plenty if you ever search the internet for “back pain,” or look at the advertisements in the back of an airline magazine. “Minimally Invasive Spine Surgery” has become the catch phrase for slick marketing campaigns when it comes to the treatment of back pain, the most prevalent reason to visit the local emergency room, and a condition that will affect most Americans at some point in their lives. An important thing to remember is that minimally invasive surgery encompasses just some of many surgical techniques utilized to effectively address conditions that afflict the spine. It is also worth noting that minimally invasive techniqueswhich aim for smaller incisions, less disruption of tissue, faster recovery times and improved outcomeshave been applied across the broad spectrum of surgeries for a variety of clinical conditions, such as mitral valve regurgitation, esophageal cancer, and hip arthritis.
Evolution of Minimally Invasive Techniques for Spine Surgery
Spine surgery has been evolving like every other specialty in medicine. As technologies advanced, so has the delivery of healthcare. Improved metallurgy and the development of radiolucent carbon fiber materials have meant better surgical instruments and retractors with improved visualization. Fiber optics have led to better lighting in the surgical theater and have improved the ability to see in smaller, tighter areas. Fluoroscopic advances now let us see in areas that don’t require direct visualization during the procedure. These advances are the hallmark of minimally invasive surgery and have led to many new and innovative procedures about the axial skeleton, but also have improved some of the older procedures.
Many of the established “open” spinal surgical procedures took advantage of specific fascial planes and anatomical features that allowed the surgeon to gain access to the area that required surgical intervention. Often, this approach required large enough wounds and removal of tissue or bone to gain direct visualization, and enough lighting to safely correct the pathology. In more recent years, the advent of more compact retractors and improved lighting combined with fluoroscopy mean that incisions can often be located immediately over the area of pathology. This results in smaller incisions, less soft tissue dissection, and decreased pressure on the surrounding muscles because they don’t have to be pulled apart so far to gain visualization.
An example of the natural progression of spine surgery is the treatment of disc herniation since its first description by Mixter and Barr in 19341. The early surgeries were through large incisions, even though just one vertebral disc was being addressed. The first big advancement in disc herniation excision wasthe use of magnification, such as with loupes and operating microscopes, the latter of which also incorporates high-intensity lighting (Figure 1). This allowed for much smaller incisions and in turn, less tissue dissection, shorter recovery times, and is termed microdiscectomy. The introduction of tubular retractors and the “minimally invasive” surgical approach in the 1980s has meant a smaller incision and the theoretical advantage of not detaching the spinal muscles from the spinous processthe blood supply for the erector spinae. When compared to a traditional open microdiscectomy, minimally invasive microdiscectomy can be performed with similar surgical times, blood loss, complications, and outcomes2.
Application of Minimally Invasive Techniques in Spine Surgery
An example of a well-accepted technique in spinal surgery is a minimally invasive Transforaminal Lumbar Interbody Fusion (TLIF). An interbody fusion is one where the lumbar disc is removed from between two adjacent vertebral bodies and a spacer is placed to preserve or restore the normal height between the two bones. Bone autograft, or allograft, is introduced to help fuse the two vertebral bodies together as one. Instrumentation is added to the construct posteriorly through the pedicle and into the vertebral body in order to add stability and help promote fusion, much like an internal cast. Any tissues pressing on the nerves at this level, such as bone or ligament can be removed, decompressing the stenosis.
This TLIF procedure can be performed through a standard open longitudinal midline incision. The back muscles are stripped off the bone and reflected laterally to gain the exposure to the posterior bony elements. The wound has to be large enough to stretch the tissue laterally to gain access to the facet and bone overlying the pedicle structure of the vertebral body. The minimally invasive technique is an adaptation of an old approach first described in 1968 by Wiltse that takes advantage of a fascial plane between the erector spinae muscles. A 2-cm incision is made laterally from the midline directly over the lumbar facet. A small guide pin is placed through the incision and docked on the bone. Serial dilators are introduced until a tubular retractor is slid down exposing the area directly over the facet to be removed and the pedicles that will carry the pedicle screw. The tissue is under significantly less pressure and the back muscles stay attached to the spinous process, maintaining their main blood supply.
Another example of minimally invasive spine surgery is the technique of anterior lumbar interbody fusion. Standard incisions for approaching the lumbar intervertebral disc started as large abdominal incisions that have become progressively smaller as techniques were improved. Most anterior fusions now are done through a “mini-open” incision horizontally above the pubis symphysis or longitudinally between the area of the navel and the pubis symphysis, gaining access to the spine in a retroperitoneal fashion.
The alternative minimally invasive technique for this type of fusion is called a Direct Lateral Interbody Fusion (DLIF) and is sometimes also referred to as an eXtreme Lateral Interbody Fusion (XLIF), but many physicians refrain from using this term due to a medical device manufacturer claiming this term as their proprietary label. The DLIF approach uses tubular retractors, advanced lighting, and neuromonitoring to serially dilate the muscles of the iliopsoas muscle and avoid the nerves that traverse the midsubstance of this muscle (Figure 2 ). Though some hip flexor weakness can be expected for the short term in most patients, the morbidity of this approach is minimized with less invasive techniques3. The true power of this approach is in its treatment of stenosis from degenerative scoliosis in older patients where the decreased blood loss and less extensive tissue dissection allow for early patient mobilization.
Another innovative minimally invasive procedure for stenosis takes advantage of foraminal volume increase with flexion of the intervertebral body disc space. A device placed between the spinous processes may alleviate some of the compression on stenotic segments and significantly reduce the symptoms of neurogenic claudication (Figure 3). This procedure may be ideal for those patients with isolated back, buttock, and leg pain that is relieved with sitting and only one or two levels of neural compression.
The misnomer about minimally invasive surgery for the spine is that in reality any surgery is maximally invasive from the patient’s perspective. For this reason, every attempt should be made to control back pain or spinal pathology first with nonoperative measures, which include activity modification, medications, bracing, physical therapy, and injections. Other modalities such as chiropractic care, acupuncture, stress management, nutrition, yoga, and pilates also have their role in helping people with back pain.
The last thing to remember about minimally invasive spine surgery is that this is a label for surgical procedures accomplished through smaller incisions. To date there are studies that have suggested these kinds of procedures are less destructive to the soft tissues about the spine, and may reduce operative blood loss and slightly reduce post-operative hospital stays, but there is yet to be any study that has shown them to have better outcomes than today’s standard open procedures. The decision as to which surgical approach is best for a particular patient for a particular condition is best left to the patient and his or her surgeon.
1. Mixtar, W.J., and Barr, J. “Rupture of the intervertebral disc with involvement of the spinal canal.” N Engl J Med 1934;211:210-215.
2. Harrington, J.F., and French, P. “Open versus minimally invasive lumbar micro-discectomy: comparison of operative times, length of hospital stay, narcotic use and complications.” Minim Invasive Neurosurg 2008; 51(1):30-35.
3. Isaacs, R.E., Hyde, J, Goodrich, A., et al. “A prospective, nonrandomized, multicenter evaluation of extreme lateral interbody fusion for the treatment of adult degenerative scoliosis.” J Spinal Disorder Tech 2009; 22(1):34-37.