22.2 Cephalometric Landmarks, Analysis & Biology of Orthodontic Tooth Movement

Orthodontic diagnosis and biomechanical treatment planning require an objective quantification of dentofacial relationships through lateral cephalometry, coupled with a rigorous biological understanding of the tissue response to mechanical forces within the periodontal ligament (PDL).

---

Lateral Cephalometric Landmarks and Reference Planes

Standardized lateral cephalograms are obtained with the patient's head stabilized in a cephalostat (anode-to-midsagittal plane distance of 5 feet [152.4 cm]; subject-to-receptor distance of 15 cm) in centric occlusion with lips in repose.

                    S (Sella)
                     •
                    / \
                   /   \
                  /     \  Po (Porion)
                 /       \    •
                /         \    \
   N (Nasion)  •           \    \       Frankfort Horizontal (Po - Or)
               |\           \    \-----------------------------------• Or (Orbitale)
               | \           \    \
  ANS •--------+--\-----------• PNS\
      |            • A              \
      |            (Subspinale)      \
      |                               • Go (Gonion)
      |                                \
      |            • B                  \
      |            (Supramentale)        \
      |                                   \
      +------------• Pog (Pogonion)--------• Me (Menton)
                   • Gn (Gnathion)

Hard Tissue Anatomical Landmarks

1. Cranial Base Landmarks:
   • S (Sella): The geometric center of the pituitary fossa (sella turcica) in the sphenoid bone. A constructed midpoint.
   • N (Nasion): The most anterior point of the frontonasal suture in the midsagittal plane.
2. Maxillary Landmarks:
   • ANS (Anterior Nasal Spine): The tip of the sharp anterior bony process of the maxilla at the lower margin of the anterior nasal aperture.
   • PNS (Posterior Nasal Spine): The posterior spinous process of the palatine bone forming the posterior limit of the bony hard palate.
   • A (Subspinale / Point A): The deepest midline concavity on the anterior contour of the maxillary alveolar process between ANS and the maxillary alveolar crest (prosthion).
3. Mandibular Landmarks:
   • B (Supramentale / Point B): The deepest midline concavity on the anterior contour of the mandibular alveolar process between the mandibular alveolar crest (infradentale) and pogonion.
   • Pog (Pogonion): The most anterior point on the bony mandibular chin in the midsagittal plane.
   • Gn (Gnathion): The point located on the curvature of the mandibular symphysis midway between Pogonion and Menton.
   • Me (Menton): The most inferior point on the bony mandibular symphysis.
   • Go (Gonion): The constructed point at the intersection of the lines tangent to the posterior border of the mandibular ramus and the inferior border of the mandibular body.
4. Facial & Cranial Profile Landmarks:
   • Po (Porion): The superior-most point of the external auditory meatus (anatomic Porion) or the top of the ear rod of the cephalostat (machine Porion).
   • Or (Orbitale): The lowest point on the infraorbital margin of the bony orbit.

Primary Cephalometric Reference Planes

• S-N Plane (Sella-Nasion): Represents the anterior cranial base. Because the spheno-ethmoidal synchondrosis ossifies early (age 7–8), this plane ceases anteroposterior growth early, serving as the standard reference plane for Steiner analysis.
• Frankfort Horizontal (FH) Plane: Line passing through Porion (Po) and Orbitale (Or). Best approximates the natural physiological horizontal head position.
• Mandibular Plane (MP): Represented as Go-Gn in the Steiner analysis (SN-GoGn) or as a tangent to the lower border through Menton (Downs/Tweed). The angle formed between the mandibular plane and the S-N plane (SN-MP, normal $32^\circ \pm 3^\circ$) defines the vertical facial pattern:
  • Hyperdivergent (High-Angle) Pattern ($>35^\circ$): Steep mandibular plane, backward/downward mandibular growth vector, anterior open bite tendency, long lower facial height.
  • Hypodivergent (Low-Angle) Pattern ($<28^\circ$): Flat mandibular plane, forward/upward mandibular growth vector, deep bite tendency, square jaw, strong masseteric musculature.
• Functional Occlusal Plane (FOP): A line drawn through the maximum intercuspation of the permanent molars and premolars (or primary molars).

---

Steiner Analysis & The Wits Appraisal

Steiner analysis provides a standardized, angular evaluation of sagittal jaw and dental relationships:

              S (Sella)
                 •
                / \
               /   \  SNA = 82° ± 2°  (Maxillary Sagittal Position)
              /     \ SNB = 80° ± 2°  (Mandibular Sagittal Position)
             /       \ANB =  2° ± 2°  (Intermaxillary Relationship)
 N (Nasion) •─────────•
             \       /
              \     /
               \   • Point A
                \ /
                 • Point B

Steiner Angular Measurements

1. SNA Angle ($82^\circ \pm 2^\circ$):
   • Evaluates the anteroposterior position of the maxilla relative to the anterior cranial base.
   • $SNA > 84^\circ$: Maxillary skeletal prognathism (protrusion).
   • $SNA < 80^\circ$: Maxillary skeletal retrognathism (retrusion / midface hypoplasia).
2. SNB Angle ($80^\circ \pm 2^\circ$):
   • Evaluates the anteroposterior position of the mandible relative to the anterior cranial base.
   • $SNB > 82^\circ$: Mandibular skeletal prognathism.
   • $SNB < 78^\circ$: Mandibular skeletal retrognathism (mandibular deficiency).
3. ANB Angle ($2^\circ \pm 2^\circ$):
   • Represents the difference between SNA and SNB, quantifying the sagittal relationship between the maxilla and mandible.
   • Skeletal Class I: $0^\circ \le ANB \le 4^\circ$ (mean $2^\circ$).
   • Skeletal Class II: $ANB > 4^\circ$ (indicates maxillary protrusion, mandibular retrusion, or a combination).
   • Skeletal Class III: $ANB < 0^\circ$ (negative ANB angle; indicates mandibular protrusion, maxillary retrusion, or a combination).

Limitations of the ANB Angle and the Wits Appraisal

The ANB angle is susceptible to geometric distortion caused by anatomical variations outside the dental arches:

• An abnormally long or short anterior cranial base (S-N length).
• An anterior or posterior spatial displacement of Nasion (N).
• Clockwise or counter-clockwise rotation of the S-N plane or the jaw bases (e.g., a high mandibular plane angle rotates Point B downward and backward, artificially inflating the ANB angle into a false Class II reading).

To overcome these geometric distortions, Alex Jacobson (1975) introduced the Wits Appraisal (named after the University of the Witwatersrand):

                        Point A
                           •
                           │
                           │  Perpendicular to FOP
                           ▼
        ===================AO====BO=================== Functional Occlusal Plane (FOP)
                                 ▲
                                 │  Perpendicular to FOP
                                 │
                                 •
                              Point B

        Normal Wits: Females = 0 mm (AO coincides with BO)
                     Males   = -1 mm (BO is 1 mm anterior to AO)
        Class II:    BO is posterior to AO (Positive Wits: +4 to +8 mm)
        Class III:   BO is anterior to AO  (Negative Wits: -4 to -10 mm)

• Methodology: Perpendicular lines are dropped from Point A and Point B directly onto the Functional Occlusal Plane (FOP), establishing contact points AO and BO.
• Normative Values:
  • In females: Points AO and BO coincide (Wits = 0 mm).
  • In males: Point BO is located approximately 1 mm anterior to Point AO (Wits = -1 mm).
• Diagnostic Interpretation:
  • Skeletal Class II: Point BO is positioned significantly behind Point AO (positive value, e.g., $+4\text{ to }+8\text{ mm}$).
  • Skeletal Class III: Point BO is positioned significantly in front of Point AO (negative value, e.g., $-4\text{ to }-10\text{ mm}$).

---

Biology of Orthodontic Tooth Movement

Orthodontic tooth movement represents a sterile, bio-mechanical inflammatory reaction within the periodontal ligament (PDL, average width $\approx 0.25\text{ mm}$) and the adjacent alveolar bone.

The Pressure-Tension Theory

When a sustained orthodontic force is applied to a tooth, the root displaces within the PDL space, creating two distinct mechanical environments:

                  ORTHODONTIC FORCE VECTOR (--->)

          [ TENSION SIDE ]                 [ PRESSURE SIDE ]
    PDL fibers stretched             PDL space compressed
    Capillaries patent / dilated     Capillaries constricted / occluded
    Osteoblast recruitment           Osteoclast recruitment
    Alkaline phosphatase, OPG       PGE2, IL-1beta, TNF-alpha, RANKL
    ---------------------------      --------------------------------
    BONE APPOSITION                  BONE RESORPTION
    (Osteoid formation &             (Direct frontal or indirect
     mineralization)                  undermining resorption)

1. Tension Side:
   • PDL fibers (Sharpey's fibers) are placed under mechanical tension.
   • Blood flow is maintained or elevated.
   • Mechanical stretch upregulates osteoprotegerin (OPG), alkaline phosphatase, and bone morphogenetic proteins (BMPs).
   • Fibroblasts align, osteoblasts are recruited, and unmineralized osteoid is laid down along the alveolar lamina dura, which subsequently calcifies, resulting in bone apposition.
2. Pressure Side:
   • PDL fibers are compressed; fluid is extruded from the PDL space.
   • Vascular and cellular responses are dictated entirely by the magnitude of the applied force relative to the capillary blood pressure.

Light Continuous Force vs. Heavy Continuous Force

The physiological hydrostatic blood pressure within human PDL capillaries averages $20\text{ to }26\text{ mmHg}$ ($2.5\text{ to }3.5\text{ kPa}$).

LIGHT CONTINUOUS FORCE (< Capillary Pressure: 20-50 g)   HEAVY CONTINUOUS FORCE (> Capillary Pressure: >100 g)
------------------------------------------------------   ----------------------------------------------------
• Capillaries remain patent; partial compression only.   • Capillaries completely collapsed; thrombosis.
• Chemical messengers released (PGE2, RANKL).            • Total ischemia -> Sterile coagulative necrosis.
• Osteoclasts recruited from local patent PDL vessels.   • Formation of cell-free, glass-like HYALINIZED ZONE.
• DIRECT (FRONTAL) RESORPTION of lamina dura.            • Local osteoclasts cannot be recruited.
• Smooth, steady tooth movement without lag phase.       • Recruitment from distant marrow spaces behind bone.
• Negligible root resorption risk.                       • UNDERMINING (INDIRECT) RESORPTION.
                                                         • Clinical LAG PHASE (7-14 days) with zero movement.
                                                         • Severe risk of EXTERNAL APICAL ROOT RESORPTION.

1. Light Continuous Force ($0.2\text{–}0.5\text{ N} / 20\text{–}50\text{ g}$)

• The applied mechanical force does not exceed capillary blood pressure.
• Blood flow is slowed but never occluded, maintaining cell viability.
• Prostaglandins ($PGE_2$), interleukins ($IL-1\beta$), and receptor activator of nuclear factor $\kappa B$ ligand ($RANKL$) are synthesized locally.
• Within 48 to 72 hours, multinucleated osteoclasts differentiate on the internal surface of the alveolar bone facing the PDL.
• Frontal (Direct) Resorption: Osteoclasts continuously dissolve the lamina dura from the PDL side. Tooth movement begins almost immediately and proceeds smoothly and continuously.

2. Heavy Continuous Force ($>1.0\text{ N} / >100\text{ g}$)

• The applied force greatly exceeds capillary blood pressure.
• PDL microvasculature is instantly compressed and thrombosed, leading to absolute ischemia.
• Cells within the compressed PDL undergo sterile necrosis. The tissue loses cellular architecture and nuclear staining, appearing optically translucent and glass-like under light microscopy—a histological process designated as hyalinization.
• Because the hyalinized zone is avascular, osteoclasts cannot be recruited through local blood vessels. Instead, osteoclasts must be recruited from adjacent viable endosteal and bone marrow spaces behind the lamina dura.
• Undermining (Indirect) Resorption: Osteoclasts excavate the alveolar bone from the inside out. Clinically, this manifests as a lag phase of 7 to 14 days during which the tooth remains entirely motionless while marrow osteoclasts undercut the thick bone plate.
• Once the necrotic lamina dura collapses, the tooth jumps forward abruptly. Heavy force concentration strips cementum protection, causing permanent external apical root resorption (EARR) and severe patient discomfort.

---

Biomechanics of Orthodontic Tooth Movement

Predictable tooth movement requires precise control of the force system applied at the bracket:

1. Center of Resistance ($C_{res}$)

• Definition: The point within a restrained body through which a single line of action of force must pass to produce pure translation (linear movement) without any rotation.
• Anatomical Location: For a single-rooted tooth with healthy, normal alveolar bone height, the Center of Resistance lies approximately 33% to 40% (one-third to one-half) of the root length apical to the alveolar crest.
• Alveolar Bone Loss: As periodontal disease reduces crestal bone height, the Center of Resistance moves progressively more apically, increasing the tipping moment generated by coronal bracket forces.

2. Center of Rotation ($C_{rot}$)

• Definition: The single point about which a tooth rotates during movement. The location of $C_{rot}$ is entirely variable and is governed mathematically by the Moment-to-Force ($M/F$) ratio.

3. Moment of a Force ($M_F$) and Moment of a Couple ($M_C$)

• Moment of a Force ($M_F$): When an orthodontic force $F$ is applied at the bracket (which is separated from $C_{res}$ by a perpendicular distance $d$), it creates a rotational moment: $M_F = F \times d$. This tends to tip the crown in the direction of the force and the root in the opposite direction.
• Moment of a Couple ($M_C$): A couple consists of two parallel forces of equal magnitude directed in opposite directions along different lines of action. An archwire engaged in a rectangular bracket slot applies a couple that creates a rotational moment ($M_C$) to counteract $M_F$.

  TYPE OF MOVEMENT             M/F RATIO     CENTER OF ROTATION (Crot)          STRESS PATTERN
  ----------------             ---------     -------------------------          --------------
  Uncontrolled Tipping           0:1         Slightly apical to Cres (mid-root) Extreme crest/apex
  Controlled Tipping            ≈ 7:1        At the root apex                   Crestal compression
  Bodily Movement (Translation) ≈ 10:1       At infinity (no rotation)          Uniform throughout
  Root Torquing                 > 12:1       At the incisal edge / bracket      Extreme apical
  Intrusion                      0:1         N/A (Pure vertical)                Apex only (Lightest!)
  Extrusion                      0:1         N/A (Pure vertical)                Uniform tension

4. Specific Tooth Movements and Force Requirements

1. Uncontrolled Tipping ($M/F = 0$):
   • A simple horizontal force applied at the bracket with no counteracting couple.
   • The crown tilts in the direction of the force; the root apex kicks in the opposite direction.
   • $C_{rot}$ is slightly apical to $C_{res}$ (near mid-root).
   • Optimum force: 35 to 60 g (Proffit).
2. Controlled Tipping ($M/F \approx 7:1$):
   • A moderate counteracting couple is generated by the wire to restrain the root apex.
   • The crown tips in the force direction while the root apex remains stationary.
   • $C_{rot}$ is located at the root apex.
   • Optimum force: 35 to 60 g (tipping range).
3. Bodily Movement / Translation ($M/F \approx 10:1$):
   • The counteracting couple exactly balances the force moment ($M_C = M_F$).
   • Crown and root move equally in the same direction with zero rotation.
   • $C_{rot}$ is at infinity; stress is uniformly distributed along the entire root surface.
   • Optimum force: 70 to 120 g.
4. Root Torquing ($M/F > 12:1$):
   • The couple exceeds the force moment ($M_C > M_F$).
   • The root apex moves while the incisal crown edge remains stationary.
   • $C_{rot}$ is at the incisal edge or bracket slot.
   • Optimum force: 50 to 100 g (root uprighting/torque).
5. Intrusion:
   • Pure axial force directed toward the tooth apex.
   • Because force is concentrated onto the tiny surface area of the conical root apex, hydrostatic stress is exceptionally high.
   • Requires the lightest force in orthodontics: 10 to 20 g per tooth.
   • Exceeding 20 g rapidly causes apical ischemia, pulp necrosis, and severe apical root blunting.
6. Extrusion:
   • Pure axial force directed coronally.
   • Generates tension across the entire PDL, stimulating bone apposition along the crest without hyalinization.
   • Optimum force: 35 to 60 g.

---

Clinical Comparison Table: Orthodontic Tooth Movements