There were several unifying themes that emerged from the peer-review discussion of the articles in this Focus issue on minimally invasive spine (MIS) surgery. Although it is somewhat intuitive that the goals of less-invasive surgery include minimization of the surgical footprint and its resultant effect on the patient, an authoritative definition of MIS surgery remains elusive.
We propose a definition based on identifying the common goals and principles of MIS surgery—“An MIS 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.”
The justification of a procedure as “MIS” includes (1) reduced surgically induced tissue damage; (2) measureable clinical benefits such as lower blood loss, reduced surgical morbidity, reduced postoperative analgesic requirements, reduced length of hospitalization, and early resumption of activities; (3) clinical effectiveness; and (4) a favorable socioeconomic effect. A number of the articles in this Focus issue touch on these topics.
It is difficult to identify a consistent comparable technique against which MIS exposures may be measured as “less than.” What is considered a “traditional” exposure? In reality, there is not a single MIS procedure, but rather the category encompasses various gradations of surgical exposures, including what is commonly referred to as “mini-open,” “tubular,” “percutaneous,” or even combined approaches. All of these approaches represent an attempt to minimize surgically induced tissue damage.
Posterior-approach lumbar procedures are a good example of this, where percutaneous—literally “through the skin” or stab incision—techniques are common for pedicle screw placement. Decompressions and interbody fusion procedures can be performed down narrow surgical corridors using fixed tubes or expandable retractors with endoscopic or direct visualization, all defined as minimally invasive because of limiting the extent of exposure and more importantly, preservation of specific structures such as the multifidus muscle insertion on the spinous processes. These issues are highlighted in the articles by Kim, Lauryssen, and Karikari.
Anterior-approach lumbar procedures have a similar continuum, with modifications of anterior lumbar interbody fusion performed through direct anterior open transperitoneal approaches toward less-invasive exposures including laparoscopic approaches, direct anterior retroperitoneal approaches with smaller exposures under direct visualization (“mini-open ALIF”), and lateral retroperitoneal transpsoas approaches under direct visualization using expandable retractors (XLIF).
The nature and complexity of the condition being treated are important in evaluating the potential benefits of MIS procedures compared with traditional surgeries. For example, in the surgical treatment of complex spinal deformities which has traditionally been associated with high complication rates and substantial morbidity, multilevel MIS lateral approach interbody fusion with posterior fixation may be less invasive than traditional multilevel interbody fusion from an open anterior or posterior approach. Mundis, Akbarnia, and Phillips and also Isaacs highlighted this in the articles discussing treatment of adult scoliosis. With respect to pathology, distinction must be made between scoliotic deformity from an adult idiopathic etiology and those primarily due to degenerative conditions. Adult degenerative scoliosis and adult idiopathic scoliosis are distinct entities with different characteristic and natural histories that often require different management. The degenerative scoliosis patient typically presents at a more advanced age, with symptoms of neurogenic claudication and radiculopathy, whereas adult idiopathic scoliosis patients usually present at a younger age with a more advanced degree of deformity and complaints related primarily to their progressive deformity. Surgical goals for degenerative scoliosis may focus on symptoms resulting from spinal stenosis and sagittal imbalance. In adult idiopathic scoliosis, the goals also include correction of deformity as a large lumbar or thoracic deformity can be both a pain generator and a cosmetic problem for the patient. If sagittal imbalance is part of the pathology, care must be taken to realign the spine for optimal outcomes.
It is incumbent on us to seek less-invasive alternatives to the most morbid procedures. Historically, these have been those least likely to be attempted through MIS techniques. MIS treatment of traumatic spinal injuries or tumors, for example, has had varied success. Video-assisted thoracoscopic surgical techniques have limited adoption due to high technical demand and the difficult learning curve. In the Supplement, Smith and Uribe discussed addressing these complex pathologies using less-invasive surgical access with direct visualization of the pathology.
Complexity may stem from the patient demographic in addition to the pathology. It is the frailest of patients who may benefit most from the least invasiveness. MIS may allow for treatment of those who previously have had few options for improved quality of life. The elderly, for example, need not be excluded from surgery on the basis of age alone, if the surgical treatment has an acceptable morbidity and rapid recovery. Rodgers highlights this approach in this Supplement.
Measureable Clinical Benefit: Minimal Morbidity
The MIS discussion is not simply about the extent of exposure. The minimization of the exposure must result in a measurable clinical benefit. The major clinical benefit of MIS procedures is in the short term; typically reported outcomes include lower intraoperative blood loss, fewer infections, less intensive care utilization, less postoperative analgesia, and shorter hospitalization than traditional exposures or procedures. These outcomes are discussed in the articles of Buvenendadran, Lauryssen, Isaacs, Youssef, Mundis, and Rodgers. By minimizing tissue damage, these short-term outcomes are optimized. MIS approaches typically require the use of intraoperative imaging or advanced real-time imaging techniques, and the associated risks and benefits of these are worthy of further study.
One area of confusion when reporting the short-term results is complications. There is a lack of consistency in the literature-reported definitions and rates of complications, regardless of whether related to traditional or MIS surgery. The interest in complications in MIS procedures is heightened by the novelty of the approach. That is, there is a fairly well-understood complication profile for any spinal surgery, such as neural, vascular, infection, etc., despite inconsistency in reporting. The introduction of new approaches or techniques may introduce additional concerns to this complication profile, whether these are increased risk of injury or the technicality of how to address a problem should it occur: pulmonary complications following deflation of the lung for thoracoscopic procedures, for example, or emergent conversion to open from endoscopic procedures. The lateral transpsoas approach is thought to have a lower vascular complication profile than a direct anterior approach, and a lower neural complication profile than a posterior approach. But a new concern has arisen with the novelty of the lateral approach—postoperative thigh symptoms. Inconsistency in complication reporting confuses the topic, and postoperative thigh pain and/or weakness have been reported as a complication by some, and as an expected side effect by others. In this Issue, Isaacs and colleagues reported all cases of thigh or leg weakness after XLIF, but defined only significant or prolonged motor or sensory loss as a complication, including 2 cases that occurred after posterior stabilization procedures and 2 cases that had contralateral thigh pain after surgery. Traversing the psoas muscle is an inherent part of an MIS lateral lumbar approach procedure. It can therefore be argued that during the acute postoperative period, a patient's complaint of thigh pain is to be expected and is not a complication.
Given that MIS surgeries generally require specialized equipment and are typically dependent on advanced access technologies, it is important to appreciate that the complications with one particular MIS system may not be applicable to other systems. For example, in the lateral transpsoas approach, intraoperative neural monitoring is used to traverse the psoas muscle and identify the nerves of the lumbar plexus. The neural complications with lateral approach surgery using integrated neural monitoring may not be reproduced without the use of such monitoring. The role of neural monitoring in MIS surgery is discussed in detail by Uribe.
MIS procedures need to be as effective as traditional procedures in achieving the intended surgical goals (e.g., neural decompression, alignment correction, stabilization, pain and functional improvement). Long-term clinical and radiographic outcomes from studies on MIS procedures have been limited to date. This Focus issue reports several examples of demonstrated effectiveness.
An often-cited weakness in the argument for MIS surgery is the absence of high quality randomized controlled trials. However, it is clear that randomizing patients between open and MIS procedures is difficult in today's postmarket environment. Not only are patients unwilling to be randomized, but some institutional review boards have already denied prospective randomized trials, arguing that the “obvious” benefits of MIS make it unethical to subject patients to randomization. One requirement of a clinical investigation in prospective randomized studies is equipoise, meaning that each treatment option has to be considered as having an equal likelihood of achieving a successful clinical outcome. This is difficult to reconcile, given that MIS approaches by definition reduce the surgical assault and muscle injury. This is perhaps contrary to the movement of evidence-based medicine. Given the challenges of randomized controlled trial in this area, nonrandomized concurrently controlled trials or prospective registries will provide valuable data.
Presuming clinical effectiveness, one would expect that the measureable benefits shown in the short term support the cost effectiveness of MIS procedures. In today's economic environment, it is incumbent on us to show the value of any surgical treatments, not just MIS. However, there is an assumption of MIS procedures as having higher upfront costs associated with newer technologies. Validation of cost effectiveness will require documentation of the procedural outcome using validated metrics, as well as the cost of the procedure over time. Allen and Garfin discussed this in this issue. Economic analyses published to date comparing open and MIS approaches tend to show favorable economics for MIS surgeries despite higher up-front procedural costs, largely related to the cost savings associated with reduced length of hospital stay and lower reoperation rates for infection. Given the potential increased up-front costs for MIS procedures, only if the durability and value of the intervention is shown will premiums on implant pricing and perhaps surgeon reimbursement be tolerated.
Patients will not and should not accept being subjects in a learning curve as inexperienced surgeons adopt new technology. For this reason, hands-on didactic training courses are necessary to educate medical practitioners in newer specialized MIS techniques. Over time, many of these procedures will become part of the residency and fellowship training programs.