DualPlane Cephalometric Analysis

The Cannon DualPlane Cephalometric Analysis uses two planes: 1. Occlusal plane (as defined by Jacobson), draw a line that joins the midpoint of the overlap of the mesio-buccal cusp of the first molar and the buccal cusp of the first premolar). 2. A line joining A point to Pogonion . The occlusal plane is used in establishing the linear, apical base, relationship between the maxillary and mandibular arches, and the facial lingual inclination of the maxillary and mandibular incisors. The A point - pogonion plane is use to establish the anterior posterior position of the maxillary and mandibular incisors, the relationship of the soft tissue to the dental arches, and the relationship of the middle face to the lower face.  It must be remembered that the average given for all cephalometric measurements are just averages, not standards.  It is always better, when evaluating cephalometric measurements, to compare the variances from their average rather than just  comparing all the actual measurements.  In narrowly defined populations all like angles, and like linear measurements are not always the same.  Any properly selected sample of individuals will be distributed along the Bell curve.  When the average is established is is possible that no single individuals measurement of any angle or any linear distance will be the same as the average of all the individuals in the selected population. 

The diagnostic measurements of the Cannon DualPlane Cephalometric Analysis are divided into three categories; Skeletal, Dental and Soft Tissue.

Illustration1: Tracing of a cephalometric xray utilizing the Cannon DualPlane Analysis

Skeletal

1. Wits

The Wits Analysis is used to establish the apical base relationship between the maxillary and the mandibular arches as measured along the Jacobson occlusal plane. The Wits measurement is established by drawing a line from A point and B point, that proceeds perpendicular to the Jacobson occlusal plane.  If the B point line intersects the occlusal plane posterior to the A point line the Wits measurement is postive.  If the B point line intersects the occlusal plane anterior to the A point line the Wits measurement is negative. The millimeter distance between the lines is the Wits measurement.  Zero to plus one millimeter Wits is considered ideal.  A Wits of minus 2 to plus 2 millimeters is considered as apical base class I. Over plus two, Wits is considered as apical base class II. Less than minus two, Wits is considered as apical base class III. Since the vertical overbite is not arbitrarily bisected as with the Downs occlusal plane, a more accurate evaluation as to the cause of the abnormal overbite can be established. ( Note: If the first bicuspid is not present, use the first primary molar). The Wits lets you know if the case can be finished utilizing the predetermined angles of the pre-adjusted appliance or if it is necessary to modify these angles through the utilization of the vertical slot or if the discrepancy is so great that surgery is necessary. If the SN - Mp angle is low to average and the Wits is as much as a negative 4 millimeters, surgery might be indicated. However, if the SN - Mp angle is high and the Wits measurement is as much as a negative four millimeters, surgery might be contraindicated since more than likely the patient will already have a straight profile. 

3. A point - pogonion to Frankfort Horizontal

This measurement is the superior posterior angle formed between the intersection of the Frankfort Horizontal and the A point - pogonion plane.  This measurement gives the relationship between the middle and lower face.  If the Frankfort plane is considered the true horizontal plane of the face, then the ideal measurement would be 90 degrees.  As this angle increases above 90 degrees the convexity of the face increases.  As the angle decreases below 90 degrees the face becomes more concave. This measurement should react with the Wits measurement. As Wits increases this measurement should increase and vice versa.  The exception to this general rule is in high SN to Mp angle cases where the profile is straight.  The Wits can be considerably above the average and have an average A point - pogonion to Frankfort Horizontal angle.  In these cases the linear measurement of Porion to B point will be unusually high.


4. Sella turcica - Nasion 
5. Porion - Nasion
6. Porion - A point

These three linear measurements establish the relationship of the maxilla to cranial base.  In apical base discrepancy cases, these measurements help to establish whether or not the maxilla is contributing to the discrepancy by being improperly developed and/or positioned. If Frankfurt horizontal is accepted as the true horizontal plane of the face, and if Pr - Na and Pr - point A are of equal length, then Na and point A must occupy the same anterior/posterior vertical plane. If Pr - point A is shorter than Pr - Na with Na positioned properly, then the maxilla is positioned posterior to Na. If Wits is a minus three millimeters or less, this would strongly suggest a deficient maxilla. If Wits is plus one millimeter or greater, perhaps Na is simply positioned anteriorly and the maxilla is therefore not deficient.

7. Pr - B point

This measurement establishes the length of the mandible without consideration as to the direction of growth or the anterior/posterior position of pogonion.  A straight profile does not necessarily indicate that the mandible and maxilla are balanced. Mandibular length and the shape of the profile are not necessarily related. Pr - B point can be used in conjunction with Wits, the SN - Mp angle to evaluate convex profiles. As previously stated, convex profiles as a result of a high SN - Mp angle usually have a Wits measurement of such a dimension that it would indicate an apical base class I between the arches. In these cases, the Pr - B point measurement should have the same variance from its average as does the Pr - A point measurement. Convex profiles, as a result of a deficient mandible, should have a Pr - B point measurement with a variance from its average greater than the variance of the Pr - A point measurement from its average. Variance being defined and expressed as the plus or minus difference between any linear measurements of a given patient, when compared to its appropriate average as established from a like population
Since people of the same age and sex are not necessarily all equal in size, cephalometric comparisons of linear measurements to averages for a given population are limited in value. If, however, we compare the variances to each other, a more accurate comparison of the linear measurements of a single individual can be accomplished. Therefore, when evaluation S - Na, Pr - Na, Pr - point A, and Pr - point B, do not compare their actual lengths, but rather, compare their variances.

Note: Dual Plane cephalometric measurements, seven, eight, nine and ten above, were derived in an unpublished study, by calculating the average of these measurements taken from lateral head plates of 250 patients. These patients were categorized by age and sex. All the patients were considered to have a good anterior/posterior balance between nasion, A point and pogonion. The anterior/posterior relationship between nasion A point and pogonion was established as follows: A vertical line perpendicular to Frankfurt horizontal that passes through A point was drawn. If both nasion and pogonion were within plus or minus two millimeters to this line, the patient was considered to have a good anterior/posterior balance between nasion, point A and Pogonion. It appears that a straight profile is considered more aesthetically pleasing than a convex profile; however, this does not mean that all convex profiles are abnormal. Successful orthodontic treatment many times shall and will result in a convex profile.

  Dental 

135 degrees is considered ideal.

2. UI - OP 

60 degrees is considered ideal.

3. LI - OP

75 degrees is considered ideal.

Note: These are the pre-determined "torque" angles that are established when finishing cases with full size rectangular archwires while utilizing the so called Roth Torque. These angles vary according to which "Straight Wire" prescription is utilized.  It must be understood that these predetermined angles are best suited for apical base class I cases. In apical base class II and III, UI - OP and LI - OP must be customized. In cases that finish with an apical base class II relationship, the lower incisor to OP angle will be less than 75 degrees with the upper incisor to OP increasing above 60 degrees. The reverse of this would be true in cases that finish with an apical base class III relationship. The upper incisor to the lower incisor should remain approximately 135 degrees.

Plus two to three millimeters are considered ideal. Apical base class I cases, treated with lower incisor placement within two to plus three millimeters of the plane demonstrate maximum stability. This measurement is used to determine the degree of retroversion or protrusion of the lower incisors. In apical base class II and class III cases, this measurement can be misleading. This is due to the change in the anterior/posterior relationship between pogonion and A point. In apical base class II ands III cases, the angle formed between the long axis of the LI and OP might be a better indicator. Variances in the size of the chin button can also affect this measurement.

6. Incisor vertical overbite

When utilized in conjunction with Jacobson’s occlusal plane, it establishes which incisor teeth need to be intruded in those case where bite opening should be accomplished by intrusion of the anterior teeth.

7. Incisor horizontal overbite

This measurement establishes the horizontal overbite between the maxillary and mandibular incisors, and should be related to the difference in the maxillary and mandibular incisor protrusion measurement.

 Soft Tissue

1. Upper lip to A point - pogonion

2. Lower lip to A point - pogonion

3. Soft tissue pogonion to A point - pogonion

4. Superior labial sulcus to A point - pogonion

These four measurements establish the relation of the soft tissue to the A point - pogonion plane. Again, the best procedure is to establish the the variance of each measurement from its average and compare the variances.  These measurements are not affected by convex or concave profiles nor large or small chin buttons or noses.

Hopefully, the Cannon DualPlane Cephalometric Analysis will help to point out that some assumptions that are routinely made in orthodontics are false assumptions. For example: (1) straight profiles always result in linear balanced arches; (2) all cases with an ANB of plus six degrees must have deficient mandibles; and (3) all convex profiles are abnormal.