“Minimally invasive” has become one of spine surgery’s most powerful promises. It is also remarkably difficult to define.
An operation can be performed through an endoscope, a tubular retractor or a series of small incisions. A surgeon can use conventional instruments but disturb less muscle and tissue. Two procedures carrying the same “minimally invasive” label can look very different in the operating room.
After decades of technical advances, the field still lacks a standardized way to answer a basic question: How invasive was the operation?
Roger Härtl, MD, thinks it is time to find out.
Dr. Härtl, Hansen-MacDonald Professor of Neurological Surgery and director of neurosurgery spine at Weill Cornell Medicine in New York City, has spent much of his career advancing minimally invasive spine surgery. But his view of the concept begins somewhere more fundamental than incision size or technology.
“One of my mentors said the least invasive operation is the one that you don’t do because the patient didn’t really need it,” Dr. Härtl told Becker’s.
For patients who do need surgery, defining invasiveness becomes more complicated.
“It’s not defined by a specific technology,” he said. “It’s really defined by philosophy to a certain extent.”
Now Dr. Härtl and an international group of spine surgeons are attempting to turn that philosophy into something measurable.
Their proposed Minimally Invasive Spine Surgery Invasiveness Classification, or MISS-INC (L), scores the invasiveness of degenerative lumbar procedures across nine factors, including operative time, blood loss, muscle dissection, instrumentation, anesthesia and number of surgical levels. The system emerged from a modified Delphi process involving 45 spine surgeons and was designed to give researchers a common language for comparing procedures that previously shared a label but not necessarily the same surgical footprint.
Putting a number on invasiveness, however, creates a more consequential question: What does that number mean for the patient?
The assumption hidden inside ‘minimally invasive’
Minimally invasive surgery carries an intuitive appeal: Less disruption should mean less pain, fewer complications and faster recovery.
Dr. Härtl is careful not to make that leap.
“Invasiveness doesn’t mean that it’s going to be better,” he said. “It doesn’t mean that it’s going to be safer, and it doesn’t mean that it’s going to be a quicker recovery necessarily.”
That distinction is central to the project. MISS-INC (L) measures the operation, not whether the operation was successful. The researchers placed procedures along a proposed spectrum. Scores below 5 were classified as minimally invasive, 5 to 10 as less invasive and above 10 as more invasive. An endoscopic single-level discectomy sat at one end; thoracic-to-pelvis fixation sat at the other.
What the score cannot yet say is whether the patient with the lower number did better. The classification has not been validated against clinical outcomes, complications, costs or revision rates. But separating invasiveness from outcome gives researchers a way to begin testing a question the field has struggled to answer: When does a less invasive operation actually lead to better care?
What is a point of invasiveness worth?
A lower invasiveness score may appear better. The calculation changes once cost, risk and outcomes enter the equation.
“If you can bring down the invasiveness score from one operation from five to three, for example, that’s great,” Dr. Härtl said. “But if it’s associated with a huge amount of additional costs, then it may not be worth it.”
The trade-offs are not only financial. An approach from the front of the spine, for example, may reduce certain forms of surgical disruption while introducing other risks, including injury to major blood vessels. The goal, then, cannot simply be to make every operation as minimally invasive as technically possible. The more consequential question is what a reduction in invasiveness actually buys: fewer complications, a shorter hospital stay, faster return to work, lower costs or a better recovery.
Dr. Härtl has begun exploring that relationship in his own practice. In a preliminary review of his most recent 100 patients, he said each one-point increase in invasiveness was associated with a 20% increase in length of stay. The finding does not establish causation, but it illustrates the kinds of relationships that become possible to study once invasiveness can be measured rather than merely described.
“I would love to look at that also in terms of costs, risks of complications, return to work,” Dr. Härtl said.
When less invasive isn’t better
For a surgeon who has spent much of his career advancing minimally invasive spine surgery, Dr. Härtl is equally clear about its limits.
In some complex deformity cases, he said, a traditional open operation may still be the better approach. The field has learned that lesson in part by testing how far minimally invasive techniques can be pushed.
“There were surgeons five, 10 years ago who tried to treat the craziest deformities with a million incisions and approaches from the front and the back,” Dr. Härtl said. “It was a beautifully done minimally invasive operation, but it took forever, and they had a lot of complications.”
An operation can minimize tissue disruption and still take longer, introduce other risks or prove less appropriate for the pathology being treated.
For certain complex deformities, Dr. Härtl said, an experienced surgeon using a more traditional approach may ultimately achieve a better result than one who prioritizes minimizing invasiveness.
The point is not that open surgery is better than minimally invasive surgery. It is that invasiveness is only one measure of an operation, and reducing it should not become an end in itself.
A surgeon’s invasiveness profile
The same measurement could eventually give surgeons a new way to examine their own practice.
Dr. Härtl has begun looking at what he calls his “invasiveness profile,” how his cases score across the classification and how that profile may have changed over the course of his career.
“I’m sure I was probably more invasive 10 years ago than I am now,” he said.
Tracked over time, those scores could show how a surgeon’s practice evolves with experience, new techniques and technology. Paired with outcomes and cost data, they could eventually allow researchers to compare how surgeons approach similar patients with similar pathology, not to assume that the lowest score is best, but to understand what differences in invasiveness actually mean.
A surgeon operating less invasively may have higher costs. Another may perform more invasive operations but achieve fewer revisions or better outcomes. Any meaningful comparison would also have to account for the pathology being treated.
For Dr. Härtl, the opportunity is not to create another surgeon scorecard. It is to give spine surgery another objective variable to study.
He points to cardiac surgery, where mortality provides a clear, measurable outcome that surgeons and hospitals can track. Spine surgery has no comparable single metric. Success is often reflected in pain, function and quality of life, outcomes that matter enormously to patients but are more difficult to quantify.
“There’s not much that we can quantify in the spine world,” Dr. Härtl said. “Because we’re dealing with pain, and we’re dealing with patients who are suffering, and that’s very, very difficult to quantify.”
Invasiveness, he believes, is different. “This is something that we can quantify,” he said.
MISS-INC (L) remains preliminary. Researchers still need to establish how reliably it performs across institutions and whether its scores predict outcomes, complications, costs or revision rates.
But Dr. Härtl sees the ability to measure invasiveness as the beginning of a different kind of conversation, one that moves beyond whether an operation qualifies as “minimally invasive” and asks what being less invasive actually changes.
“I think it really opens up a totally different field of research,” Dr. Härtl said. “It opens up the ability to compare procedures.”
The comparisons, however, will have to be made among similar cases.
“You have to be comparing apples to apples,” Dr. Härtl said. “But right now, we haven’t been able to do that.”
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