Practical Cephalometry

Aims
Background
Tracing
Common Cephalometric points
Common Lines/Planes
Analysis/Interpretation
Self assessment

Aims

a)To enable the identification of cephalometric points and planes used in orthodontic diagnosis and treatment planning.
b)To trace a lateral skull radiograph.

Background

Cephalometrics is the interpretation of lateral skull radiographs taken under standardized conditions.
The patient is placed in a Cephalostat. This positions the patient with their head oriented at 90o to the X-Ray beam at a distance of 5ft from the tube. The film is placed 15 inches from the head. This is a standard under which all cephalometric radiographs are taken worldwide. It ensures that radiographs taken at different centers are directly comparable.


The film is then traced and various standard landmarks, lines and angles are measured and recorded. This allows comparison with normal values for a population and assessment of growth and/or effects of treatment.
For the purposes of this practical we will use a set of analysis widely used in orthodontics. It is known as 'Eastman Analysis'

Below is cephalometric radiograph. You will be able to see the soft tissue out line of the face as well as the tongue and pharynx. All the hard tissue references points should be clearly visible. In order to trace the image you need a light box and a dark room. This means all lights turned off and all areas around the radiograph blanked off so your eyes can adapt to the dark. Only in these conditions will you be able to make out all the landmarks clearly

 

Note the following:

The patient is in the 'Natural Head Position', this is the patient holding their head as if they looking off into the distance.

There is a scale to allow calculation of the radiographs magnification

Collimation has been used so the soft tissues are clearly seen

 

The diagram on the left shows the relationship of a number of key cephalometric points in relation to the skull

Tracing technique

This should be undertaken in a darkened room; block out all light from the light box other than that seen through the radiograph.

Use high quality tracing paper securely taped to radiograph along the top edge of the tracing paper, directly to the radiograph - like a hinge. This allows the tracing paper to be lifted to examine the radiograph directly, yet replace it in the same place for tracing


Use a sharp (HB) pencil to outline the following :

1. Soft tissue profile of face (forehead to chin)
2. Sella turcica
3. Frontal bone and nasal bone
4. Orbital floor
5. External auditory meatus
6. Maxilla, upper lst molar and upper central incisor
7. Mandible, mandibular symphysis, lower lst molar and lower central incisor

NB: Tracing the incisors is difficult - make sure you draw the incisor the correct shape and size. The lower incisor apex is difficult to locate as the canine apex is very close. Remember your tooth anatomy!

Your tracing should now look like this:
 

 

Cephalometric points:

Next Identify and mark the following landmarks

S Sella: Mid point of sella turcica
N Nasion: Most anterior point on fronto-nasal suture
Or Orbitale: Most inferior anterior point on margin of orbit
Po Porion: Upper most point on bony external auditory meatus
ANS Anterior Nasal Spine
PNS Posterior Nasal Spine
Go Gonion: Most posterior inferior point on angle of mandible
Me Menton: Lower most point on the mandibular symphysis
A point: Position of deepest concavity on anterior profile of maxilla
B point: Position of deepest concavity on anterior profile of mandibular symphysis

NB: Gonion is difficult. You can construct Gonion by drawing a line along the lower border and down the posterior of the ramus, then bisecting the angle, the bisected angle is then projected to the mandible - that point is Gonion.
If there are 2 lower borders of the mandible (due to poor patinet positioning) draw both borders then draw the average and construct Gonion on the average lower border

Your tracing should now look like this:

Lines/planes

Then draw in the following lines/planes

Frankfort Plane Po - Or Equivalent to the true horizontal when patient is standing upright.

Maxillary Plane PNS - ANS Gives inclination of maxilla relative to other lines/planes.
Mandibular Plan Go - Me Gives inclination of mandible relative to other lines/planes.

The angle MMPA - Maxilla to Mandibular Planes Angle (Maxillary plane to Mandibular plane) Gives an inclination of the maxilla relative to the mandible, this in turn indicates the relative proportions of face height and acts as an indicator for future growth direction.

S - N Line: Indicates orientation of anterior cranial base.
N - A indicates relative position of maxilla the cranial base
N - B indicates relative position of maxilla the cranial base

The angles SNA; SNB; ANB indicates relative position of maxilla/mandible to each other and to the cranial base

Long axis of upper central incisor/lower central incisor (root apex to incisal edge) - allows measurement of the angulation of incisors to maxilla/mandibular planes.

Your tracing should now look like this:

Finally measure the angles (to 1/2 degree accuracy)

Analysis/Interpretation of tracing

By comparison of angular measurements with normal values you can interpret the results of your analysis to give a diagnosis of the patient's presenting skeletal pattern. Comparison of the findings from the original and final cephalometric radiographs will allow you to assess the outcome of treatment.
The UK (white Caucasian) standard values are: (standard deviation in brackets):

SNA = 81o (3)
SNB = 79o (3)
ANB = 3o(2)

Interpretation of SNA/SNB/ANB angles:

If SNA or SNB greater or less than the normal - this indicates that the mandible or maxilla is either positioned anterior or posterior. This may be due to a difference in jaw growth and size.

ANB indicates the relative position of maxilla to mandible, and allows the measurement of the extent of the jaw size/position discrepancy.

ANB 2-4o= Class I skeletal pattern
ANB > 4o= Class II skeletal pattern
ANB < 2o= Class Ill skeletal pattern

Interpretation of MMPA :

MMPA (max/mand planes angle) 27o(4)

Gives an inclination of the maxilla relative to the mandible, this in turn indicates the relative proportions of face height and acts as an indicator for future growth direction (Forward or Backward rotation).

Interpretation of Incisor to maxilla/mandible angles :

Gives a measurement of the extent of the proclination or retroclination of the incisors

1 - Mx - Upper incisor to Maxilla angle - 109o (6)
1 - Mn - Lower incisor to Mandible angle - 93o (6)
OR
if MMPA is not normal (ie greater or less than 27o) the 'normal' lower incisor - Mn angle is calculated as 120o minus MMPA

Self Assessment

Having undertaken a cephalometric tracing discuss the tracing and measuremenst with your teacher(s)

What was the patient's original skeletal pattern?
Were the incisors originally proclined or retroclined?
What changes occurred during orthodontic treatment?
What was the patient's final skeletal pattern?

The results of your tracing in the practical must be discussed with your teachers as a number of cephalometric radiographs are used.

Under Construction A set of on-line exercises for practicing identification of cephalometric points and the interpretation of the analysis

Watch this space.....

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