CEPHALOMATRIC ANALYSIS

The introduction of radiographic cephalomatrics in 1934 by Hofrath in Germany and Broadbent in
the United States provided both a research and clinical tool for the study of malocclusion and the underlying skeletal disproportions.
Cephalomatric films could be used to evaluate dentofacial proportion and clarify the anatomic basis for a malocclusion. Any malocclusion is the result of an interaction between jaw position and dental compensation or adaptation. It is possible to have a normal occlusion in spite of an underlying jaw discrepancy through the mechanism of dental compensation. or to have a significant dental malocclusion within a normal skeletal pattern. Another possibility is additive rather than compensatory dental and skeletal deviations, so that a combination of moderate jaw discrepancy and moderate dental displacement adds up to a severe malocclusion. Two apparently similar malocclusion as evaluated from the dental casts can turn out to be quite different when evaluated more completely, using cephalomatric analysis to reveal difference in dentofacial proportions.



TECHNIQUE OF TRACING CEPHALOMATRIC RADIOGRAPHS.

It has been found that direct measurements on the headplates are often difficult. For this reason, a 'TRACING' is made. The tracing is a pencilled outline of the major and pertinent structures, made on a semi-transparent sheet of acetate, usually .003" thick, and superimposed on the headplate itself.


MOUNTING TRACING PAPER:

Place the cephalomatric radiograph on the tracing table, with the profile facing to the left. Place a piece of acetate paper on top of the reproduction of the radiograph so that the two edges on the top are even. The dull side of the acetate should be facing you. Now take two pieces of draftman's tape about one to one and one-half inches long and place one piece on the upper left of the acetate sheet and the other one on the upper right side. Leave about half the length extending beyond the upper edge. Bend the free ends of the tape down and seal them to the back of the radiograph. The acetate sheet and the radiograph are now sealed to each other at the top, and open at the bottom. This permits folding the tracing paper up frequently to check directly on structual details while tracing.

TRACING:

1. Trace the soft tissue outline with a sharp pencil. Draw accurately, without erasure if possible.

2. Draw the bony skeleton of the head.

3. Draw the Sella Turcia. This is the pituitary fossa of the sphenoid bone.

4. Draw the mandible and the condylar head.

5. Trace the molars and the central incisors.

6. Tace the porion.

7. Trace the palate and the pterygo-maxillary fissure.

8. Trace the orbit.


.LANDMARKS:

· SELLA TRUCICA (S).
The centre of the Pituitary Fossa of the Sphenoid Bone.

· NASION (N)
The intersection of the Internasal Suture with the Nasofrontal Suture in the midsagittal plane

· ORBITALE (OR)
The most inferior point on the External Border of the Orbital Cavity.

· PORION (PR)
The most surerior point of the External Auditory Meatus.

· IR POINT (IR)
The most convex point along the Inferior Border of the Mandible.

· MENTON (M)
The most Inferior Point of the Symphysis.

· POGONION (PO)
The most Anterior Point of the Mandibular Symphysis.

· GNATHION (GN)
The Point of intersection between the plane connectimg N and Po, and the plane connecting Me and Ir points.

· MOLAR POINT (MP)
The bisection of the over bite of the Upper and Lower Molars as seen on the Lateral Cephalogram.

· A POINT (A)
The deepest point of the curve of the Anterior Maxillary Border between Anterior Spine and the Alveolus. .

· B POINT (B)
The most Posterior Point in the Concavity along the Anterior Borber of the Symphysis as seen in the Lateral Cephalogram .

· A 1 INCISOR (A 1)
The Incisal tip of the Upper Central Incisor.

· AR POINT (AR)
The Root Apex of the Upper Central Incisor.

· B1 INCISOR (B1)
The Incisal tip of the Lower Central Incisor.

· BR POINT (BR)
The Root Apex of the Lower Central Incisor.

· UPPER LIP (UL)
The most Anterior Point on the Curve of the Upper lip.

· LOWER LIP (LI)
The most Anterior Point on the Curve of the Lower Lip.

· CHIN (DT)
The most Anterior Point on the Curve of the Soft Tissue Chin.



LANDMARKS - PLANES.

1. SELLA NASION: - SN

The plane connecting Sella Turcica and Nasion. Known as the Cranial Base, SN defines the Lower Border of the Cranium.

2. FRANKFORT HORIZONTAL PLANE: - FH.

The plane formed by connecting Porion and Orbitale. This plane is approximately parallel to the ground when the patient is looking straight ahead. This plane is often used as a superimposition line for cephalomatric tracing.

3. OCCLUSAL PLANE: - OP

The bisection of points A1 and B1 is connected to the Molar Point to form this plane. Although the patient's actual Occlusal Plane is rarely completely flat, this constructed plane is a useful approximation

4. MANDIBULAR PLANE:- MP

The plane connecting Menton and the Ir Point. This plane defines the Base of the Mandible.

5. APo PLANE:- APo

This plane defines the Anterior Border of the Facial Skeleton. It is formed by connecting A point with Pogonion.

6. SOFT TISSUE LINE:-

A line connecting Dt (chin) and UL (upper lip). This line defines the Anterior Border of the Soft Tissue in the Lower Face.

LANDMARK - ANGLES.

1 SNA:-
The Angle between the SN Plane and a Plane connecting Nasion and A Point. The SNA angle defines the anteroposterior position of point A relative to the anterior cranial base. Its mean value , 81 degrees, indicates a normal relationship between maxilla and anterior cranial base. If the angle is less than normal, the maxilla lies more posterior in relation to the cranial base , if the angle is too large, the maxilla lies more anterior. The angle therefore defines the degree of prognathism for the maxilla. A large SNA angle (greater than 84 degrees ) makes the anteroposterior position of the maxilla prognathic, a small angle (less than 78 degrees) makes it retrognathic.

Variations due to age and sex are minimal with this angle (80.5 - 82 degrees).


2. SNB :-
The Angle between the SN Plane and a Plane connecting Nasion and B Point (NB). The SNB angle determines the anteroposterior position of the mandible in relation to the anterior cranial base, analogous to the SNA angle for the maxilla. This angle defines prognathism for the mandible, the mean value being 79 degrees. If it is greatewr than 82 degrees, the mandible is prognathic relative to the cranial base, if it is less than 77 degrees, the mandible is retrognathic. The mandible is described as orthognathic if the angle is between 77 and 82 degrees.

The size of this angle increases with age (from 76 degrees at 6 years to 79 degrees at 16 years of age). Retrognathism may thus be compensated in the course of growth, and it is often difficult to distinguish the effects of therapy from those of growth when Class II anomalies are treated.


3. ANB :-
SNA minus SNB. This represents the diffrence between the SNA and SNB angles and defines the mutal relationship, in the sagittal plane, of the maxillary and mandibular bases.The SNA is positive if point A lies anterior to NB. If NA and NB coincide, the angle will be zero. If, however, point A lies posterior to NB, ANB will be negative. Apart from establishing the relationship between the maxillary and mandibular bases, the angle also largely determines the position of the incisors. On average, the angle is 2 degrees. High positive occurs in Class II, negative in skeletal Class III.


Comparison of SNA, SNB and ANB
The three angles refered to above (SNA, SNB and ANB) define the relationship of the maxillary and mandibular bases to the anterior cranial base, and also the mutual relationship of the maxillary and mandibular bases. A number of combination are possible.

Normal SNA and SNB
This indicates a normal position of the maxillary and mandibular bases relative to the cranial base and also to each other.


Normal SNA.
Normal SNA angles indicate normal relations between maxilla and cranial base with:
(a) Small SNB angle = mandible retrognathic.
(b) Large SNB angle = mandible prognathic.


Normal SNB
Normal SNB angle indicates normal relationship between mandible and cranial base, with :

(a) Small SNA angle = maxilla retrognathic.
(b) Large SNA angle = maxillprognathic.


4. Angle of Convexity.
The angle defined by the intersection of the line connecting N and A , and the APo plane.


5. SN - MP (Mandibular Plane).
The Angle formed by the intersection of SN and the Mandibular Plane. This angle gives the inclination of the mandible to the anterior cranial base.


6. FMA (Frankfort - Mandibular Angle).
The angle formed by the intersection of the Frankfort Horizontal and the Mandibular Plane.


7. OM (Occulasal - Mandibular Angle).
The angle formed by the intersection of the Occlusal Plane and the Mandibular Plane.


8. Y - Axis Angle.
The Angle between SN and a line connecting Sella Turcica with Gnathion. This angle determines the position of the mandible relative to thr cranial base, as an additionall check. It has a mean value of 66 degrees. if it is greater than that, the mandible is in a posterior position, with growth predominantly vertical. If the angle is less than 66degrees, the mandible is in an anterior position relative to the cranial base, and growth is predominantly anterior.


9. Upper 1 to SN.
The Angle between SN and a line connecting A1 (upper incisor tip) to AR (upper incisor root apex). The posterior angle is measured. It has a mean of 102 degrees +- 2 degrees. Up to the 7th year, it is only 94-100 degrees, with 1o2 angulation achieved only 1 or 2 years after eruption. Large angles usually indicate maxillary incisor protrusion, smaller angles very upright incisors.


10. IMPA.
The Angle between the Mandibular Plane and the line connecting B 1 and BR. It has a mean value of 90 +- 3 degrees. From the 6th to the 12th year the angle increases from 88 to 94 degrees. A wide angle denotes protrusion of the mandibular incisors, a smaller than normal angle, very upright incisors. Treatment planning, even for simple forms of treatment, always calls for diagnostic analysis of lower incisor angulation.


11. Interincisal Angle.
The intersection of the two lines A 1 - AR and B1 - BR. It has a mean value of 135 degrees. A good incisal angle on conclusion of treatment is a major factor in denture stability and prevention of relapse.

LANDMARKS - LINEAR MEASUREMENTS.

1. Lower Incisor to APo.
The linear distance from Point B1 to the APo Plane, measured perpendicular to APo. Apart from determining the angles, linear measurements are also used to assess incisor position. The distance of the lower incisal edges from the APo plane is determined. The average distance being i mm. This figure is of considerable importance in treatment planning. The aim of treatment, at least with the permanent dentition is to achieve those normal relations to the APo plane This particular measurement therefore, is frequently the key factor in deciding:

(a) whether extraction is indicated.
(b) whether the lower incisors can be moved forward.
(c) whether anchorage is critical.

Until the 9th year, the metric relations in the mandible are not sufficent to stabilised to serve as the basis for major diagnostic decisions, In the mixed dentition period, interpretation must consider the phases of active growth still to come.


2. Holdaway Ratio.
The Ratio of the following two distances.
1). The perpendicular distance from Po to the line connecting Points N and B (NB).
2). The perpendicular distance from B1 to the line connecting N and B.


3. Lower Lip to Harmony Line.
The Hormony Line connects DT (Soft Tissue Chin) with UL ( Upper Lip).
The distance from the most anterior point on the lower lip (LT) to the Harmony Line
is measured parallel to FH


4. Wit's appraisal.
The 'Wit's (Univ. of Witwatersrand) appraisal of jaw disharmony, which is a measure of the extent to which jaws are related to each other anteroposteriorly. Perpendiculars lines are drawn on the lateral cephalometric head flim tracing from point A and B on the maxilla and mandible respectively, onto the occlusal plane which is drawn through the region of maximum cuspal interdigitation. The points of contact on the occlusal plane from A and B are labelled AO and BO respectively. It was found that with normal occlusion, point BO was approximately 1mm anterior to point AO. In skeletal Class II jaw dysplasias, point BO would be located well behind point AO, whereas in skeletal Class III jaw disharmonies, point BO will be forward of point AO.