29.3 Skeletal Pattern and Cephalometric Analysis

3. Skeletal Pattern Assessment & Cephalometric Analysis

Skeletal Relationship Evaluation Modalities
  │
  ├── Clinical Bedside Assessment
  │     ├── Two-Finger Technique (Palpation of soft tissue A-point and B-point)
  │     └── Zero-Degree Profile Line (Perpendicular from Nasion to soft tissue Pog)
  │
  └── Radiographic Lateral Cephalometry
        ├── Sagittal Skeletal Relationships (SNA, SNB, ANB Difference, Wits)
        ├── Vertical Skeletal Relationships (MMPA, Jarabak ratio, LAFH %)
        └── Dentoalveolar Angulations (UI-MxP, IMPA, Interincisal Angle)

Clinical Assessment of Skeletal Pattern

• Two-Finger Palpation: With the patient seated upright looking straight ahead, the clinician places the index finger at soft tissue Point A (subnasale / base of upper lip) and the middle finger at soft tissue Point B (mentolabial sulcus above the chin):
  • Class I: Hand is approximately vertical (index finger rests $2-3\text{ mm}$ anterior to middle finger).
  • Class II: Index finger is markedly anterior to middle finger (hand tilts backwards).
  • Class III: Middle finger is anterior to index finger (hand tilts forwards).

Standard Cephalometric Tracing Landmarks

Cephalometric analysis utilizes standard anatomical and constructed hard tissue landmarks on a standardized lateral skull radiograph:

Standard Lateral Cephalometric Landmarks

               S (Sella)
                 *                     N (Nasion)
                                         *

                                            * Point A (Subspinale)

                                            * Point B (Supramentale)
         Go (Gonion)                        * Pog (Pogonion)
             *                              * Me (Menton)

• S (Sella): Geometric center of the pituitary fossa (sella turcica) in the sphenoid bone.
• N (Nasion): Most anterior point on the frontonasal suture in the midline.
• Point A (Subspinale): Deepest midline concavity on the anterior contour of the maxillary alveolar process between anterior nasal spine and prosthion.
• Point B (Supramentale): Deepest midline concavity on the anterior contour of the mandibular symphysis between infradentale and pogonion.
• Pog (Pogonion): Most anterior point on the bony mandibular symphysis.
• Me (Menton): Most inferior point on the bony mandibular symphysis.
• Go (Gonion): Most postero-inferior point at the angle of the mandible (constructed by bisecting the tangents to the posterior ramus and inferior mandibular border).
• ANS (Anterior Nasal Spine) & PNS (Posterior Nasal Spine): Define the maxillary plane.

Cephalometric Angular Norms and Interpretations

UK orthodontic cephalometric interpretation relies primarily on the Eastman Normal Values (British Caucasian standards):

[!NOTE] The Geometric ANB Correction Rule: The ANB angle is valid only when the anterior cranial base angle (SN to maxillary plane) and SNA angle are within normal limits. If the cranial base is abnormally steep or SNA is abnormal (e.g. $87^\circ$), the apparent ANB difference will be artificially distorted. Clinicians must apply geometric adjustments or utilize the Wits appraisal (linear projection in millimeters of points A and B onto the functional occlusal plane; normal is $0\text{ mm}$ in females, $-1\text{ mm}$ in males) to verify true skeletal discrepancy.

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Assessing the Patient Before Reaching for a Radiograph

Clinical assessment comes first, and a cephalometric radiograph is justified only when it will change the treatment plan — a requirement of IR(ME)R, not merely good practice. The clinical assessment is made with the patient seated upright, unsupported, with the Frankfort plane horizontal and the teeth in centric occlusion. In the anteroposterior dimension, the relationship of the maxillary and mandibular skeletal bases is judged by palpation of points A and B or by visual assessment in profile, giving Class I, Class II or Class III. In the vertical dimension, the Frankfort–mandibular planes angle is estimated by extending imaginary lines, and the lower anterior face height is compared with the middle third; equal thirds with a planes angle meeting at the occiput indicates average proportions. In the transverse dimension, asymmetry is assessed from in front and from above.

Interpreting the Numbers Sensibly

The standard Caucasian norms — SNA about 81°, SNB about 78°, ANB about 3°, maxillary incisor to maxillary plane about 109°, mandibular incisor to mandibular plane about 93°, Frankfort–mandibular planes angle about 27°, and lower face height about 55 per cent — are means with standard deviations of two to four degrees, and they are population-specific. Applying Caucasian norms uncritically to patients of other ethnic backgrounds produces systematic misdiagnosis, most commonly labelling a normal bimaxillary proclination as pathological. Examiners expect candidates to know that cephalometric values support clinical judgement rather than replace it.

A second common error is misreading ANB in isolation. ANB is affected by the position of nasion and by the rotation of the jaws, so a large ANB does not always mean a large skeletal discrepancy; Eastman correction and alternative measures such as the Wits appraisal exist for that reason. Finally, incisor inclination must be interpreted relative to the skeletal pattern: a lower incisor at 93° to the mandibular plane is normal only when the mandibular plane angle is itself average, which is why the value is often quoted as 120° minus the Frankfort–mandibular planes angle.

Finally, candidates should know that a cephalometric radiograph carries a measurable radiation dose and must satisfy the same justification requirement as any other exposure. It is taken when the skeletal pattern cannot be adequately assessed clinically, when a functional appliance or orthognathic treatment is planned, or when incisor inclination must be quantified before extractions are decided. Taking one routinely for every orthodontic assessment is not justifiable, and a candidate who selects that option in an SBA has missed the regulatory point as well as the clinical one.