Radiographic Cephalometry: From Basics to 3-D Imaging, Second Edition by Alexander Jacobson Richard L. Jacobson

Author:Alexander Jacobson, Richard L. Jacobson
Language: eng
Format: epub
Tags: ebook, book
Publisher: International Quintessence Publishing Group
Published: 2013-04-18T04:00:00+00:00

Fig 15-1 The effect of variation in the position of sella (S) relative to nasion (N) resulting in an upward (cephalad) or downward (caudad) inclination of the SN reference line and consequently in an overestimate and underestimate of prognathism as measured with the facial angle. (Pog) Pogonion; (MD) mandible.

The inclination of the SN line also affects the values of sella-nasion-subspinale (SNA) and sella-nasion-supramentale (SNB) and the interpretation of sagittal jaw relationship. Consider a patient with the following angular measurements: point A–nasion–point B (ANB) = 10 degrees (norm: 2 to 3 degrees), SNA = 82 degrees (norm: 82 degrees), and SNB = 72 degrees (norm: 79 to 80 degrees). One would assume that the sagittal discrepancy is caused by a retrognathic mandible. If, however, the cant of SN is at 15 degrees to the horizontal recorded from the natural head position, instead of a norm of 8 degrees (7 to 9 degrees), the difference of 7 degrees (15 – 8 degrees), when added to the values of SNA and SNB to correct for the inclination of SN, would yield the following angular measurements: SNA = 82 + 7 = 89 degrees; SNB = 72 + 7 = 79 degrees. Consequently, the correct diagnosis is maxillary prognathism and “normal” mandibular position.

These findings may be detrimental when relied on in facial surgery, where drastic changes are possible by advancing the maxilla, retruding the mandible, or both. Clinical judgment may be a better guide than reliance on cephalometric analyses, which do not include compensation for the variation in the inclination of intracranial reference lines. This confusion can be avoided when taking lateral and frontal head radiographs and photographs by adhering to the natural head position concept, defined in 1861 by Broca12 as follows: “When a man is standing and when his visual axis is horizontal, his head is in the natural position” (see chapter 14).

The natural head posture can be readily registered in the cephalostat by using common-sense judgment to prevent an occasional strained position when the patient turns his or her head upward or downward. Although it is standardized, natural head position lacks mathematic precision; nevertheless, the resultant small variations in its reproducibility are smaller than the often marked differences encountered in the inclination of the FH or the anterior skull base (SN).

Mesh coordinate system study

In a landmark study, Moorrees et al used standardized lateral head radiographs of 46 men and 47 women from the greater Boston area, representing a broad range of normal occlusal variation, to construct male and female norms for the mesh diagram.4 The radiographs were obtained as part of the Longitudinal Studies of Child Health and Development by the Department of Maternal and Child Health, School of Public Health, Harvard University.

In addition, a subsample from the longitudinal study of twins conducted at the Forsyth Institute (formerly the Forsyth Dental Center) was used in which 414 twin pairs participated during various age ranges, generally entering the study between 4 and 8 years of age with an average follow-up between 9 and 10 years.

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