Natural reference structures in the human mandible: a systematic search in children with tantalum implants

The purpose of this study was to locate stable natural reference structures (NRS) in the mandible as seen on lateral cephalometric radiographs. The material consisted of pairs of serial radiographs of 34 children (15 males and 19 females aged 8.7-14.3 years at the initial radiograph, with a mean interval of 4.1 years between radiographs). Each child had three Björk type tantalum implant markers in the left side of the mandible. The pairs of radiographs were converted to digital image format, superimposed, and registered using stable implants. Potential NRS were located by visual examination of the superimposed images. The morphological and spatial stability of these potential NRS were tested using automated computer-based analysis employing cross-correlation. NRS were located in each subject (range 4-15 NRS). The number of NRS was statistically significantly correlated with age at the initial radiograph (r(rank) = 0.39, P = 0.02) but not with the interval between the radiographs (r(rank) = 0.21, P = 0.23). Two groups of NRS were found: a small group of amorphous bony condensations and prominent trabeculae mainly in the body and ascending ramus, and a larger group of nine anatomical structures consisting of neurovascular canals and bony striae close to the endocortical surface of the inferior cortex. Two bony structures previously reported as stable (the inner inferior contour of the symphysis and prominent trabeculae within the symphysis) were generally found to drift, presumably because of gradual differential remodelling, making them unreliable as NRS. Based on these findings, two alternative methods are proposed for accurate mandibular superimposition.