MISS-INC (L): bringing precision to the term minimally invasive spine surgery

Throughout our careers, we have been passionate about developing and advancing less disruptive approaches to spinal disorders. At the same time, we have become increasingly aware that the spine community lacks a common language for discussing surgical invasiveness. Different surgeons may perform similar procedures in very different ways. Two operations may both be labeled minimally invasive while creating very different physiological burdens on the patient.

This challenge prompted our international team within AO Spine Knowledge Forum Degenerative to develop the Minimally Invasive Spine Surgery Invasiveness Classification for lumbar procedures, or MISS-INC (L). Rather than focusing on a specific technology, incision length, or surgical philosophy, the goal was to create a practical framework that quantifies the surgical burden experienced by the patient: a scale that named devices could not serve as a neutral comparator between them, which is exactly what the field needs.

We believe this framework offers immediate value for surgeons in daily practice.

Härtl R, Bratescu R, Willett N, et al. A Modified Delphi Consensus Study to Quantify Surgical Burden of Degenerative Lumbar Spinal Procedures: The Minimally Invasive Spine Surgery Invasiveness Classification “MISS-INC (L)”. Global Spine Journal. 2026;0(0).

Moving beyond incision size

One misconception that persists in spine surgery is that invasiveness can be determined primarily by skin incision length. While incision size remains visible and intuitive, it tells only a small part of the story.

A small incision does not necessarily mean minimal physiological stress. Conversely, a somewhat larger exposure may still result in excellent tissue preservation and favorable patient recovery.

When discussing invasiveness, we encourage surgeons to look beyond technical marketing terms and focus on the overall biological impact of the operation. Factors such as muscle disruption, blood loss, operative duration, fusion strategy, anesthesia requirements, and the number of treated levels all contribute to the patient's surgical burden.

This perspective changes the conversation. Instead of asking whether a procedure is minimally invasive, we should ask how invasive it is relative to other reasonable treatment options.

That distinction may seem subtle, but it can substantially improve clinical decision-making.

A frequent question is why the classification does not account for every available minimally invasive technology. This is deliberate. A classification earns its value through reproducibility, not completeness. The Glasgow Coma Scale reduced something as complex as consciousness to three components and fifteen points, was criticized as crude from the day it was published, and became the common language of traumatic brain injury precisely because any two observers could apply it and arrive at the same number. 

The same logic applies here: a scale that named individual devices could never serve as a neutral way to compare them, and would require revision every few years as technology turns over, breaking the very comparability it was built to provide. New technology is not excluded from MISS-INC (L); it is scored by its footprint.

Nine factors every spine surgeon should consider

The MISS-INC (L) framework evaluates nine factors that collectively characterize procedural invasiveness: operative time, number of approaches, estimated blood loss, instrumentation, muscle dissection, anesthesia, surgical levels, fusion technique, and revision.

Operative time and blood loss

Longer surgeries often correlate with increased physiological stress and greater resource utilization. Likewise, blood loss remains an important indicator of surgical burden, even when contemporary techniques have reduced transfusion requirements.

When planning a procedure, surgeons should actively consider whether modifications in approach can reduce operative duration or minimize blood loss without compromising treatment goals.

Surgical access and muscle dissection

The pathway to the spine also matters. Muscle preservation has become one of the defining advances in modern spine surgery.

In my own practice, careful attention to soft tissue handling frequently influences postoperative recovery as much as the actual decompression or instrumentation. The amount of muscular disruption remains a critical element when comparing surgical techniques.

Instrumentation and fusion strategy

Instrumentation provides stability and often improves outcomes when appropriately indicated. However, implants also increase procedural complexity and surgical burden.

Similarly, fusion procedures generally create greater biological impact than decompression-only operations. The decision to add fixation or fusion should therefore be guided by pathology rather than routine preference.

Number of approaches and treated levels

A single-level procedure performed through one corridor differs substantially from a multilevel operation utilizing multiple approaches.

When reviewing treatment plans with trainees and colleagues, we often encourage a simple question: Is every planned step necessary to achieve the patient's goals? That discussion frequently helps refine surgical strategy.

Anesthesia and revision status

Revision surgery presents unique challenges. Scar tissue, altered anatomy, and increased technical complexity often elevate procedural invasiveness even when the objective remains the same.
Likewise, anesthesia requirements contribute meaningfully to overall surgical burden.

Recognizing these factors helps create a more complete picture of invasiveness beyond what can be appreciated from procedure names alone.

Using invasiveness scores in clinic tomorrow morning

The practical value of quantifying invasiveness becomes apparent during patient consultations. Patients increasingly arrive with information gathered from internet searches, social media, or marketing materials. Many request minimally invasive surgery without necessarily understanding what that term represents.

Rather than relying on subjective descriptions, surgeons can frame discussions around measurable procedural characteristics.

For example, a patient with lumbar stenosis may be evaluating a decompression-only procedure versus an instrumented fusion. The conversation can move beyond labels and focus on the expected surgical burden associated with each option.

This approach also helps establish realistic expectations regarding recovery, postoperative discomfort, rehabilitation needs, and potential complications.

Importantly, a lower invasiveness score should not automatically be interpreted as a better procedure. The least invasive operation is not always the most appropriate operation. The correct procedure remains the one that best addresses the patient's pathology and clinical needs.

Improving research and communication

As already highlighted, the term minimally invasive has been used inconsistently in spine literature. Researchers often group procedures together under a common label despite significant differences in technical execution and patient impact. This makes interpretation of outcomes challenging. A standardized classification creates an opportunity to improve scientific rigor.

When investigators report outcomes, invasiveness scores can provide additional context that helps readers more accurately compare studies. Procedures with similar diagnoses and indications may demonstrate different results simply because they impose different levels of surgical burden.

The same principle applies to multidisciplinary communication. Surgeons, anesthesiologists, rehabilitation specialists, hospital administrators, and patients all benefit when procedural complexity is described using a common framework.

One caveat is essential for anyone applying the classification in research. Invasiveness scores should be reported and compared within a defined pathology group—lumbar disc herniation, lumbar spinal stenosis, spondylolisthesis with stenosis, degenerative scoliosis. A high score in degenerative scoliosis may represent entirely appropriate surgery, while the same score for a single-level disc herniation would not. 

When pathologies are pooled, a difference in mean invasiveness tells you which patients were operated on rather than how they were operated on. In that sense a MISS-INC (L) score behaves like a TNM stage: the number carries meaning only alongside the diagnosis it was assigned in.

A tool, not a judgment

Invasiveness is only one dimension of surgical care and MISS-INC (L) is not intended to rank surgeons or determine whether one procedure is superior to another. An operation with a higher invasiveness score may be the optimal treatment for a particular patient. Similarly, a procedure classified as minimally invasive may not necessarily deliver better long-term outcomes.

The classification simply provides a way to quantify and communicate procedural burden. This distinction allows surgeons to have more informed discussions while preserving clinical judgment and individualized treatment planning.

The next step for minimally invasive spine surgery

The creation of MISS-INC (L) represents an important milestone and a starting point for future work. We will examine how invasiveness scores correlate with clinical outcomes, complications, patient satisfaction, recovery trajectories, healthcare utilization, and value-based care metrics.
These relationships will help determine whether quantifying invasiveness can improve patient selection and treatment optimization. For now, we believe the most immediate contribution is conceptual clarity.

For many years, the term minimally invasive has meant different things to different surgeons. We now have the opportunity to describe surgical burden using a common language grounded in expert consensus and practical clinical factors.

As spine surgeons, our objective has never been simply to perform smaller operations. Our objective is to perform the right operation with the least necessary disruption while achieving durable patient outcomes. A standardized framework helps us pursue that goal more effectively.