A morphometric analysis of the craniofacial configuration in achondroplasia

Human achondroplasia can be viewed as an experimental model for studying the effects of abnormal endochondral bone formation on the development of the skull as a whole. In this study, lateral cephalograms of 25 adult males and 26 adult females with achondroplasia were converted to a two-dimensional coordinate model of craniofacial morphology and analyzed using 66 linear, angular, and area variables. Lateral cephalograms of 951 normal adults were used for comparison. Significant findings in achondroplasia included enlarged calvaria, frontal bossing, large frontal sinuses, occipital prominence, normal anterior cranial base length, strikingly shortened posterior cranial base length, an acute cranial base angle, a short nasal bone that was deformed and depressed, short upper facial height, recessed maxilla, posterior tilt of the nasal floor, and a prognathic mandible that was anteriorly displaced but of normal size with a normal gonial angle and a high coronoid process. The finding of normal anterior cranial base length in achondroplastic subjects was surprising since the cranial base is preformed in cartilage and hypoplasia and shortening would be expected. Since the brain is enlarged in achondroplasia, the expanding frontal lobes may possibly influence the growth of the anterior cranial base, since it is known to follow a neural pattern of growth. Strikingly short posterior cranial base length was interpreted as resulting from hypoplasia of bone that is preformed in cartilage with possible early closure of the spheno-occipital synchondrosis. The exaggerated closure of the cranial base angle in achondroplasia may be related to an increased brain size and possibly earlier than normal closure of the intersphenoidal synchondrosis. The acute cranial base angle strongly suggests that the natural balance of the achondroplastic head on the spinal column tilts the face downward. The biological interpretation of the differences between achondroplastic and normal subjects was consistent with the results of a stepwise discriminant function analysis. For example, the most discriminating variable with respect to achondroplastic males was the measurement from basion to the anterior nasal spine. In achondroplastic subjects, this dimension crosses a shortened maxilla, a hypoplastic cranial base, and an acutely flexed cranial base angle, all of which are strikingly abnormal and involved by the pathologic process.