Wisdom teeth are classified using several overlapping systems that describe their angle, depth, available space, and relationship to surrounding bone and nerves. Dentists and oral surgeons combine these classifications to assess whether a wisdom tooth is likely to cause problems and how difficult it would be to remove. The two most widely used systems are Winter’s classification (which describes angulation) and the Pell and Gregory classification (which describes depth and space).
Winter’s Classification: Angle of the Tooth
Winter’s classification sorts wisdom teeth by the angle they sit at relative to the neighboring second molar. The angle is measured by drawing a line through the long axis of each tooth and seeing where they intersect. There are four main types:
- Vertical impaction (10° to -10°): The wisdom tooth is essentially upright, parallel to the second molar. This is generally the most straightforward position.
- Mesioangular impaction (11° to 79°): The tooth is tilted forward, toward the second molar. This is the most common type of impaction in lower wisdom teeth.
- Horizontal impaction (80° to 100°): The tooth is lying on its side, nearly perpendicular to the second molar.
- Distoangular impaction (-11° to -79°): The tooth is angled backward, away from the second molar. This orientation can make extraction more complex because the path of removal works against the natural curve of the jawbone.
Angles that fall outside all four ranges (101° to -80°) are grouped into an “other” category, which includes rare inverted or transverse positions.
Pell and Gregory: Depth and Space
The Pell and Gregory system measures two things independently: how deep the wisdom tooth sits and how much room it has between the second molar and the back edge of the jaw (the ramus of the mandible). This system applies to both upper and lower wisdom teeth, though the ramus measurement is most relevant for lower ones.
Depth Relative to the Second Molar
Depth is described using three levels, based on where the highest point of the wisdom tooth sits compared to the tooth next to it:
- Level A: The top of the wisdom tooth is at or above the chewing surface of the second molar. The tooth is relatively shallow.
- Level B: The top of the wisdom tooth sits between the chewing surface and the neck (cervical line) of the second molar. This is the most common depth in upper wisdom teeth, seen in about 39% of impacted maxillary third molars in one large study.
- Level C: The wisdom tooth is entirely below the neck of the second molar, buried deep in the jaw. This typically means a more involved extraction.
Space Available in the Jaw
The second part of Pell and Gregory looks at how much of the wisdom tooth is blocked by the ramus, the vertical portion of the lower jawbone that rises behind the last molar:
- Class I: There is enough space between the second molar and the ramus to accommodate the full width of the wisdom tooth.
- Class II: The space is smaller than the width of the wisdom tooth, so the ramus partially covers it.
- Class III: There is no space at all. The wisdom tooth is entirely embedded within the ramus.
As the class number increases, the extraction generally becomes harder because there is less room to work with and more bone may need to be removed.
Classification by Tissue Coverage
A simpler classification, widely used by dental insurance companies and in treatment planning, sorts wisdom teeth by what’s covering them:
- Soft tissue impaction: The tooth has emerged through the bone but remains covered by gum tissue.
- Partial bony impaction: Part of the tooth is still encased in jawbone, with some portion visible or covered only by gum.
- Full bony impaction: The tooth is completely surrounded by bone and has not broken through at all.
This classification matters practically because it determines the surgical approach. A soft tissue impaction usually requires only a gum incision, while a full bony impaction means the surgeon needs to remove surrounding bone to access the tooth. Insurance coding and fee structures often follow these categories directly.
The Pedersen Difficulty Index
To predict how challenging a lower wisdom tooth extraction will be, many oral surgeons use the Pedersen difficulty index. It combines all three measurements from Winter and Pell and Gregory into a single score. Each category gets a point value:
For angulation, mesioangular scores 1 (easiest), horizontal scores 2, vertical scores 3, and distoangular scores 4 (hardest). For depth, Level A scores 1, Level B scores 2, and Level C scores 3. For ramus relationship, Class I scores 1, Class II scores 2, and Class III scores 3.
The three scores are added together. A total of 3 to 4 indicates a minimally difficult extraction. A score of 5 to 7 means moderately difficult. Anything from 7 to 10 suggests a very difficult case. A mesioangular tooth at Level A with Class I spacing (score of 3) is about as straightforward as an impacted wisdom tooth gets. A distoangular tooth at Level C with Class III spacing (score of 10) represents the most challenging scenario.
Proximity to the Nerve Canal
For lower wisdom teeth specifically, the relationship between the tooth roots and the inferior alveolar nerve canal running through the lower jaw is an important part of classification. This nerve supplies feeling to the lower lip and chin, and damage during extraction can cause temporary or permanent numbness. The risk of temporary numbness is estimated at 1% to 5%, with permanent changes occurring in under 1% of cases.
On 3D imaging, the nerve canal’s position relative to the roots is classified into four types: sitting directly below the roots (apical), running along the cheek side (buccal), running along the tongue side (lingual), or passing between the roots (interradicular). The contact between the tooth and canal is further graded by whether there is a visible white line of bone separating them, whether that line is incomplete, or whether the roots actually penetrate into the canal. When standard panoramic X-rays show warning signs of close proximity, such as darkening of the root, narrowing of the canal, or root curving around the canal, a 3D cone-beam CT scan is often ordered to get precise information before surgery.
Root Shape and Complexity
Root morphology adds another layer to classification. Wisdom teeth can have fused roots (a single conical root), two or three separate roots, or roots with unusual curvature called dilaceration. Roots that are widely splayed, sharply curved, or hooked around the nerve canal increase surgical difficulty because they resist a clean path of removal. In complex cases, the surgeon may need to section the roots individually, separating them with a drill and removing each one on its own. Fused, straight roots are the simplest to extract, while dilacerated or divergent roots are among the factors that can make a case a contraindication for straightforward removal.
Upper vs. Lower Wisdom Teeth
Most classification systems were developed primarily for lower wisdom teeth, where the rigid jawbone and nearby nerve make extraction more complex. Upper wisdom teeth use the same Winter’s angulation categories and Pell and Gregory depth levels, but the ramus space measurement (Class I, II, III) is less relevant because the upper jaw doesn’t have the same bony wall blocking access from behind.
Upper wisdom teeth have their own anatomical concern: proximity to the maxillary sinus, the air-filled space above the upper teeth. Standard panoramic X-rays don’t reliably show the relationship between upper wisdom tooth roots and the sinus floor. When there’s concern about roots projecting into the sinus or complex root anatomy, a cone-beam CT scan provides the three-dimensional detail that a flat X-ray cannot.