Direct laryngoscopy requires the operator to reduce the primary curve enough to create a line of sight from the eye to the glottis.

Video laryngoscopy changes that relationship by relocating the eye.

The camera travels with the blade, moving around the primary curve and closer to the larynx. It can reveal the glottis without fully aligning the airway or displacing the tongue to the same degree.

That is the defining power of VL.

It is also the source of its most important limitation.

The camera can look around a corner that the tube must still physically negotiate. The camera and tube do not occupy the same position, follow the same path, or approach the glottis from the same angle.

The screen then compresses this three-dimensional airway into a two-dimensional image. It shows the glottis with remarkable clarity while concealing much of the journey required to reach it:

• The curved path behind the camera
• The working space remaining inside the mouth
• The depth and position of the blade
• The approach angle of the tube
• The turn from the glottis into the tracheal axis

The image feels complete because it is bright, enlarged, and centered.

It is not complete.

To use VL effectively, the operator must supply what the screen removes: a three-dimensional mental model of the airway. You must see the primary curve behind the image, the secondary curve beyond it, and the path the tube must follow through both.

The glottis is not the endpoint. It is the point where the journey changes direction. A clear view tells you where the trachea begins. Your mental model tells you how to get there.

The screen shows the destination. You must still understand the route.

The Airway Is More Than a Collection of Landmarks

Traditional airway education teaches us to identify structures: the tongue, epiglottis, and arytenoid cartilage. These landmarks tell us where we are. They do not tell us how to get from one place to another.

Intubation is not merely an identification task. It is a problem of movement through space.

The tongue occupies space. The blade changes that space. The camera observes it. The tube must travel through it. The glottis is not simply an image to acquire; it is an opening that must be approached from the correct direction.

Two Curves. One Airway.

The Two-Curve Theory offers a practical model of the airway as a serpentine path formed by two opposing curves. It is a more modern understanding of the intubation pathway for the age of video laryngoscopy, and it will help you engineer first-pass success with VL by improving your understanding of the spatial relationships between anatomic structures and how VL dynamically changes them.

The primary curve begins at the mouth, follows the surface of the tongue, and continues through the oropharynx toward the laryngeal vestibule. This is the curve the laryngoscope must follow, reduce or look around to expose the glottis.

The secondary curve continues through the laryngeal vestibule and glottic opening before turning downward into the trachea. This is the curve the tube must enter after it reaches the vocal cords.

The two curves meet within the laryngeal vestibule—the space between the epiglottis and the glottis. Here, the path changes direction. What approaches the larynx must now turn and descend into the trachea.

The curves are not simply anatomical features. They describe the work of intubation.

The blade negotiates the first curve. The tube must negotiate the second.

The Airway Was Always Curved. Our Explanation Was Straight.

For much of modern airway history, direct laryngoscopy was explained through the Three Axes Alignment Theory.

The model proposed that positioning the head and neck brought the oral, pharyngeal, and laryngeal axes into alignment, creating a straight line from the operator’s eye to the glottis. It offered a tidy explanation for the sniffing position and became deeply embedded in airway education.

But the airway was never three clean lines.

Later imaging studies questioned whether those axes could actually be aligned as described. Greenland and colleagues revisited the historical foundations of the theory and found that it rested largely on a small collection of descriptive texts, diagrams, and radiographs published between the nineteenth century and 1944—not on modern anatomic validation. Greenland et al., Anaesthesia and Intensive Care

The old model remained useful as a teaching story.

The anatomy simply refused to cooperate with it.

A Different Way to Describe the Airway

In 2008, anesthesiologist Keith Greenland proposed a different model for direct laryngoscopy and tracheal intubation.

Rather than imagining the airway as three straight axes waiting to be aligned, he described the airway passage as a continuous curve—more like a road than a set of lines. For practical use, that pathway could be divided into two opposing curves:

• The primary oropharyngeal curve
• The secondary pharyngo-glotto-tracheal curve

The curves meet at a point of inflection within the laryngeal vestibule: the space between the epiglottis and the glottis. In simplified clinical terms, the first curve carries us from the mouth, around the tongue, toward the larynx. The second changes direction through the glottis and descends into the trachea.

This was more than a change in terminology.

It replaced the idea of alignment with the idea of navigation.

What Greenland Actually Studied

In 2010, Greenland and colleagues tested the model using MRI scans from 42 normal adult volunteers.

The investigators traced the airway passage as a mathematical curve and examined how its configuration changed in four positions:

• Neutral
• Head extension
• Head lift
• Sniffing position

They identified the point where the two curves changed direction and measured the relationship between the airway passage, the laryngeal vestibule, and the proposed line of sight.

The point of inflection remained within the laryngeal vestibule in all four positions. Head lift and the sniffing position rotated the vestibular region toward a more horizontal orientation. Head lift, extension, and sniffing also reduced the area between the airway curve and the line of sight compared with the neutral position.

The findings provided an anatomic and mathematical explanation for why head and neck positioning can make direct laryngoscopy easier: positioning does not perfectly align three straight axes. It changes the shape and orientation of a curved airway, reducing the amount of tissue that must be displaced to expose the glottis. Greenland et al., British Journal of Anaesthesia

What the Evidence Shows—and What It Does Not

The study was elegant.

It was also limited.

It examined MRI images from normal adult volunteers. It did not study difficult airways, anesthetized patients, actual laryngoscopy, tube delivery, or clinical outcomes. No video laryngoscope was used. No intubation was performed. First-pass success was not measured.

The investigators themselves were careful. Their conclusion was that the model helped explain airway configuration and supported the use of the sniffing position for direct laryngoscopy. They specifically stated that its application to other forms of laryngoscopy required further investigation.

That distinction matters.

The original research supports the existence of a curved airway passage whose configuration changes with head and neck position. It does not prove that teaching the Two-Curve Theory improves intubation performance. It does not establish the model as the definitive mechanism of laryngoscopy. And it does not directly validate its application to video laryngoscopy.

The theory also attracted legitimate criticism. Lee argued that laryngoscopy succeeds primarily by displacing the tongue, not by aligning abstract airway curves, and questioned whether the anatomical regions joined by the model behave as a single biomechanical structure. Lee, British Journal of Anaesthesia

Greenland and colleagues responded that their curve represented the central airway passage rather than a single anatomical structure. They emphasized that direct laryngoscopy modifies this passage by lifting the mandible and displacing the contents of the submandibular space. They also identified devices such as flexible endoscopes and hyperangulated video laryngoscopes as tools that can follow the primary curve without flattening it to the same degree. Greenland et al., author reply

The debate remains useful because it exposes the theory for what it is:

A model of airway geometry—not a complete mechanical explanation of intubation.

Why the Model Still Matters

A model need not explain everything to reveal something important.

The Two-Curve Theory describes a clinical experience familiar to anyone who has used a video laryngoscope: seeing the glottis and reaching the trachea are related tasks, but they are not the same task.

Direct laryngoscopy asks the operator to position the patient, displace the tongue, and reduce the primary curve enough to establish a line of sight.

Video laryngoscopy changes the visual problem. The camera moves around the primary curve, bringing the device’s eye closer to the larynx. A hyperangulated blade can reveal the glottis while preserving much of the curvature that direct laryngoscopy attempts to reduce.

The camera can therefore complete its task before the tube completes its own.

The camera only needs to see around the first curve. The tube must travel around that curve, reach the laryngeal vestibule, change direction, and enter the second.

This is where the theory becomes valuable for VL.

It reminds us that the image on the screen represents only a thin slice of a much larger three-dimensional pathway. The primary curve remains behind the camera. The secondary curve continues beyond the glottis. Neither is fully visible on the screen.

The operator must mentally reconstruct both.

Evidence From Real-World Performance

Later clinical research has not directly validated the Two-Curve Theory, but some findings are consistent with the geometric problem it describes.

In a randomized trial of 163 patients undergoing GlideScope intubation, a deliberately restricted glottic view resulted in faster, easier tube delivery than a full view. The restricted-view group had a median intubation time that was 9 seconds shorter, although first-attempt success and complications were not significantly different. Gu et al., Canadian Journal of Anesthesia

That study did not test the Two-Curve Theory.

What it demonstrated was the view–delivery gap: advancing the blade to improve the image can change the geometry in ways that make tube delivery harder. Withdrawing slightly may make the glottis appear smaller while restoring working space and creating a more favorable approach to the trachea.

The image becomes less impressive.

The intubation becomes easier.

This is exactly the kind of clinical contradiction that a geometric mental model helps us understand.

Translating the Theory Into Practice

The Two-Curve Theory does not give us a single correct technique. It gives us better questions.

When the glottis is visible, but the tube will not advance, ask:

• What path did the blade create?
• How much of the primary curve remains?
• From what direction is the tube approaching the glottis?
• Where does the secondary curve turn away from the screen?
• Has advancing the blade improved the image while worsening the route?
• Would pulling back, lifting, tilting, rotating, or changing the tube shape produce better geometry?

These questions turn failure into information.

A collision with the arytenoids suggests one relationship. Anterior tracheal-wall contact suggests another. A tube that never enters the screen suggests that the problem began outside the camera’s field of view.

The screen shows the result.

The curves help explain why it happened.

The Deep-Cut Bottom Line

The Two-Curve Theory is neither a proven law nor an obsolete diagram. It is a disciplined way of thinking about the three-dimensional airway surrounding a two-dimensional image.

Its original evidence comes from MRI studies of airway configuration and positioning—not from clinical trials of video laryngoscopy. Its extension to VL is a reasoned application supported by clinical experience and compatible evidence, but it remains a model rather than a validated performance tool.

That intellectual honesty does not weaken the theory.

It tells us how to use it.

The camera supplies the view. The Two-Curve Theory helps reconstruct the space around it.