VALIDITY OF CEPHALOMETRIC LANDMARKS - AN EXPERIMENTAL-STUDY ON HUMAN SKULLS

Cephalometric landmark validity (the difference between the estimated landmark and the true landmark) has surprisingly not previously been c omprehensively evaluated, and no previous study has examined the valid ity of cephalometric angles and distances. The aim of this study was t o investigate the validity of 15 commonly used skeletal and dental cep halometric landmarks, and the subsequent effects on 17 angles and dist ances. Small steel balls were glued on to 30 Chinese dry skulls to rep resent the true anatomical landmarks. The skulls were mounted in a pur pose-designed skull holder and two cephalograms recorded of each skull , one with and one without the steel balls on the landmarks. Validity was expressed as the difference in the measurements between the assess ments made with and without the steel ball markers. Measurements were made relative to X and Y co-ordinates which were constructed from refe rence points (steel balls) glued intracranially to the skulls. Seven o ut of the 10 skeletal landmarks and all five dental landmarks, were fo und to be non-valid along the X or the Y axes (P<0.05). The standard d eviations of the validity errors were large, being 1.0-2.5 mm, along a t least one axis, for eight of the skeletal landmarks and three of the dental landmarks. Four of the cephalometric angles (SNA, SN/MnP, MxP/ MnP, and LI/MnP) and three of the distances (N-Me, MxP-Me, and lower i ncisor edge to APg) were also found to be invalid (P<0.05). The validi ty errors were greater for angles involving dental landmarks and for a ngles dependent on four landmarks compared to those dependent on three . The standard deviations of the validity errors for the skeletal angl es ranged from 0.9 to 1.8 degrees, except for ANB (0.4 degrees), and f or the dental angles from 3.2 to 5.8 degrees.