Correct orientation of the patient is of utmost importance before exposing him/her to X-ray radiation. The cephalostat head holder is rotated 90 degrees so that the patient faces the X-ray cassette/sensor. The central X-ray beam passes through the skull in a PA direction bisecting the trans meatal axis perpendicularly.

The patient’s head is held in a fixed position by the ear rods of the head holder. The standard distance from the X-ray source to the ear rods is 5 ft. Reproduction of the head position is crucial because if the head is tilted, the vertical-dimension measurements will not be accurate.

The correct head orientation is the basis of accurate measurements in PA cephalometry. The head orientation can be achieved using the Frankfurt horizontal plane (FHP) as a reference or held in a natural head position (NHP).

The methods of correct head orientation are as follows:

• 1.

  Conventionally, the head can be positioned with the tip of the nose and forehead in light contact with the film cassette holder sensor. This position is good for the evaluation of craniofacial anomalies that require special attention to the upper face.

• 2.

  The standard method is by keeping the FHP parallel to the floor while the patient is facing the X-ray film cassette as close as permissible within the limits of nose prominence.

• 3.

  To ensure correct orientation of the head in the FHP, patient positioning should be guided by scribing a line on the ear rod assembly at a point 15 mm above the ear rod. The height of the orbit is about 3 cm, and the lateral canthus is essentially at the centre of the orbit or 15 mm. The patient should be oriented such that his ear canals tuck snugly against the top of the ear rods with the head positioned so that the lateral canthus of the eye is located at the same level as that line.

• 4.

  Cephalograms should be taken with the mouth of the patient slightly open in cases of significant mandibular displacement.

PA cephalogram can also be obtained by orienting the head in the NHP.

Signs of a good head position on PA cephalogram X-ray film include:

• 1.

  In a properly oriented PA cephalogram, the top of the petrous portion of the temporal bone lies near the centre of the orbit.

• 2.

  The head position and inter-maxillary occlusal relationship that appear on an X-ray should first be checked using an intraoral photograph of the patient, study casts or by clinical evaluation of the occlusion.

Tracing/digitisation of a PA cephalogram should be carried out by orienting it in front of the examiner as he/she is looking at the patient (i.e. the patient’s right should be on the examiner’s left). Tracing should begin with the midline structures. The bilateral points marked on a PA cephalogram are conveniently abbreviated with the addition of R and L for the right and left side. Important landmarks used in a PA cephalogram are given in Table 28.1 and Figs 28.1 and 28.2 .

A PA X-ray is taken with the face towards the sensor or film; however, it is analysed as shown here.

Note L for the left side. A PA cephalogram is developed and oriented for the purpose of tracing in such a way as to give the anteroposterior view (i.e. the film is placed in such a way that the orthodontist is actually facing the patient). This orientation greatly helps the orthodontist simultaneously compare facial photos and dental casts when evaluating frontal dysplasia.

Commonly used landmarks for PA cephalogram analysis.

Tracing a PA cephalogram requires considerable experience. It is a much more cumbersome process compared to that happens with tracing a lateral cephalogram. An orthodontist must be fully conversant with the detailed anatomy of the skull and its structures. The landmarks on a PA cephalogram are as follows. A6L , The most prominent contour of the upper left first molar; A6R , the most prominent contour of the upper right first molar; AGL , antegonial left; AGR , antegonial right; ANS , anterior nasal spine; B3L , tip of left lower canine; B3R , tip of right lower canine; B6L , most prominent contour of lower left first molar; B6R , most prominent contour of lower right first molar; Cg , crista gall; C’ , conca left; C , conca right; JL , jugal process left; JR , jugal process right; Me , menton; ZAL , zygomatic arch left; ZAR , zygomatic arch right; ZL , zygomatic suture left; ZR , zygomatic suture right.

Various horizontal and vertical planes are drawn on a PA cephalogram so that different analyses can determine asymmetry, linear dimensions and angles.

The MSR plane has been selected as a key reference line because it closely follows the visual plane formed by the sub-nasale and midpoints between the eyes and eyebrows. It generally runs in a vertical direction from the Cg through the anterior nasal spine (ANS) to the chin area and is typically nearly perpendicular to the Z plane. The Z plane is defined as a line joining the zygomaticofrontal suture of one side to the other.

If the location of the Cg is in question, an alternative method of drawing the MSR plane is to draw a line from the midpoint of the Z plane through the ANS. The position of the ANS will be altered in facial asymmetry involving the maxilla.

If there is upper facial asymmetry, the MSR plane can be drawn as a line from the midpoint of the Z plane through the midpoint of the Fr-Fr line (foramen rotundum of one side to the other). To avoid any such bias, a best-fit vertical line is drawn in the centre connecting the midpoints of lines joining the zygomaticofrontal sutures (Z-Z), centres of the zygomatic arches (ZA), medial aspects of the jugal processes (J) and the antegonial notch (AG-GA) of both sides.

The best-fit line and all lines constructed as perpendiculars through the midpoints between pairs of orbital landmarks have shown excellent validity.

Besides vertical reference lines, horizontal best-fit lines have to be constructed to ascertain asymmetry in the vertical plane. All horizontal lines connecting bilateral cranial landmarks can adequately serve as reference lines to analyse vertical asymmetry from PA cephalograms as long as landmark identification error is acceptable ( Fig. 28.3 ).

Horizontal planes used in PA cephalometric analysis.