Minimally invasive techniques have been adopted more slowly in spine surgery than in other surgical disciplines, primarily due to the difficulty of accessing and visualizing critical structures through small, closed working channels. But beginning in the 1990s, better understanding of spinal biomechanics, more-sophisticated instrumentation and refined techniques led to greater implementation of microsurgical procedures, including lumbar decompression and fusion. Though now widely used, the indications and limitations of novel techniques in spine surgery are not always well-understood, and until recently, there has been a lack of scientific evidence to support their safety and effectiveness.
The goal of minimally invasive spine surgery (MISS) is to achieve outcomes equivalent to those of open surgery while minimizing muscle dissection, disruption of ligament attachment sites and collateral damage to soft tissues.
In conventional diskectomy, for example, the paraspinal muscles are dissected from the posterior aspect of the lumbar spine and portions of the lamina are removed to gain access to the spinal canal. This allows the removal of disk herniation and relieves pressure on spinal nerves, but the dissection of spinal muscles and supporting tissues can lead to pain and possible instability.
Minimally invasive approaches can spare these tissues and reduce collateral damage. A tubular retractor system is used that dilates rather than dissects muscle. By utilizing sequentially larger tubes, the working channel is expanded without cutting muscle fibers. The reduction in trauma has been shown to reduce immediate negative effects, such as pain and disability, but not long-term outcomes, says Brett A. Freedman, M.D., an orthopedic surgeon specializing in spine surgery at Mayo Clinic's campus in Rochester, Minnesota.
"When minimally invasive lumbar decompression is performed well with the right patient, there are advantages in the early phase, but the final outcome should be the same as with open procedures. There are no long-term outcomes reported in the literature where minimally invasive techniques led to a better end result than traditional approaches," he says.
Norwegian researchers confirmed the equivalence of the clinical effectiveness of the two procedures in a multicenter observational study published in The BMJ in 2015. Using prospective data from a large national spine surgery registry, they compared outcomes for more than 800 patients who had undergone open laminectomy or microdecompression for stenosis of the lumbar spine.
Favorable outcomes, as measured by change in the Oswestry Disability Index, were equivalent at one year. Complication rates and length of surgery were also similar after propensity matching, but patients who underwent microdecompression had consistently shorter hospital stays.
At Mayo Clinic, minimally invasive approaches are used when indicated. But Dr. Freedman cautions that it is far more important to perform all the functions necessary to fully and safely decompress the nerves than to worry about the method used to expose them.
"It is essential to do the same tasks in minimally invasive surgery that are done in open procedures," he says. "You need to select patients with focal disease that can respond best to small windows of exposure, and you cannot compromise on the aspects of surgery that have been proved to provide full decompression just because you are using minimally invasive techniques. You need to be able to see what you need to see. Otherwise, MISS becomes a mistake."
Minimally invasive fusion surgery
Spinal fusion has been used to manage a variety of disorders of the lumbar spine, including tumors, spinal instability, deformity and stenosis. But traditional open anterior or posterior surgery requires extensive soft tissue dissection to expose the anatomic landmarks for screw insertion, achieve a proper screw trajectory and develop an acceptable fusion bed.
The tissue injury that occurs is not only associated with increased postoperative pain and a lengthy recovery time but also with significant complications. Anterior surgery requires a relatively morbid incision and may cause vascular complications, postoperative colonic obstruction or injury to the sympathetic chain. Posterior surgery, including posterolateral fusions, posterior lumbar interbody fusions and transforaminal lumbar interbody fusions, can lead to dural tears and neural complications such as radiculitis.
One alternative to traditional anterior and posterior approaches is lateral interbody fusion, which is performed using a lateral trajectory that can avoid abdominal and vascular structures as well as the spinal canal and nerves. This approach enables placement of an interbody graft into the disk space while minimizing the risks associated with anterior and posterior exposures. The procedure often requires supplemental fixation, most often in the form of pedicle screws, which can also be placed in a percutaneous minimally invasive fashion in the same setting or in a second-stage operation.
"Lateral interbody fusion allows access to the front of the spinal canal in a trajectory that has the least amount of tissue disruption," Dr. Freedman says. "It is a new and powerful technique that is gaining more favor."
Clearing up a misconception regarding MISS, Dr. Freedman says it's not uncommon to need multiple small incisions to complete a minimally invasive fusion, whether approaching the spine from the anterior, lateral or posterior direction.
"The total extent of the skin incision is probably as long as or longer than a standard midline incision. It's not the length of the skin incision that defines minimally invasive techniques but rather the minimization of collateral tissue damage incurred while trying to reach the spine. The surgery needs to accomplish certain goals in order to correct the pathology. We have to continue to achieve what we have been achieving surgically for decades, and if we can do that with less collateral damage, then that would be ideal," he explains.
Minimally invasive lumbar fusion is associated with reduced intraoperative blood loss and postoperative pain as well as greater and earlier restoration of function. Although these benefits are significant, especially for patients, Dr. Freedman says that in the long term, it can be difficult, if not impossible, to detect a benefit to minimally invasive spine procedures compared to open ones.
He explains: "The increased chance of complications, especially during the steep learning curve, must be balanced against the uncertain long-term benefit of MISS approaches. That said, our primary promise to the patient is to 'Do no harm.' If we can limit the collateral damage and still perform all of the key elements of the surgery to an equivalent or superior degree of completion, then MISS approaches are most appropriate.
"There is a constant desire to do things in a less invasive manner, and this will be increasingly possible as our experience grows and our implants and instrumentation get better. For now, MISS has a limited but growing role in spine surgery. The most important aspect of spine surgery is what the surgeon does to the spine. How he or she gets there is of less consequence. That said, less is more when it comes to collateral damage, so if you can achieve the goals of surgery through less invasive methods, then you have made the best case for use of MISS techniques."
For more information
Nerland US, et al. Minimally invasive decompression versus open laminectomy for central stenosis of the lumbar spine: Pragmatic comparative effectiveness study. The BMJ. 2015;350:h1603.