Anthropometric analysis of mandibular asymmetry in infants with deformational posterior plagiocephaly

The incidence of deformational posterior plagiocephaly has increased dramatically since 1992. We tested the hypothesis that mandibular asymmetry, associated with this condition, is secondary to anterior displacement of the ipsilateral temporomandibular joint. The response to molding helmet therapy was also evaluated.

A caliper was used to measure mandibular dimensions in 27 infants (16 boys and 11 girls) with deformational posterior plagiocephaly; the mean age was 6.2 months (range, 3 to 12 months). Anthropometric measures included ramal height (condylion-gonion), body length (gonion-gnathion), and condylion-gnathion. Gonial angle was calculated from the law of cosines: C(2) = A(2) + B(2) - 2AB cos c. The position of the temporomandibular joint was accepted as corresponding to auricular position and measured from tragion to subnasal. Cranial asymmetry was measured, in the horizontal plane, from orbitale superius to the contralateral parieto-occipital point at the level of inion. Ten of 27 patients were remeasured 5 months after beginning helmet therapy to evaluate change in mandibular dimensions.

Two thirds of infants (67%) had right-sided and one third (33%) had left-sided deformational posterior plagiocephaly. The mean auricular anterior displacement was 79.7 mm on the affected side and 83.4 mm on the unaffected side. The mean difference of 3.8 mm between the sides was statistically significant (P <.001). Transverse cranial dimension averaged 136.0 mm on the affected side and 146.8 mm on the unaffected side; this was also significant (P <.001). There was a significant positive correlation between auricular displacement and cranial asymmetry [R(23) =.59, P <.01). Auricular (temporomandibular joint) displacement also resulted in an apparent mandibular asymmetry with rotation of the jaw to the affected side. Mean mandibular measurements on the affected and unaffected sides were ramus height of 35.2 and 36.4 mm, body length of 59.0 and 60.3 mm, and gonial angle of 127.1 degrees and 126.8 degrees, respectively. Comparison of the affected with the unaffected sides, using a paired-samples t test, was not statistically significant. Improvement in cranial asymmetry occurred with helmet therapy, but there was no correction of auricular and temporomandibular joint position.

This study supports the clinical observation that the mandibular asymmetry in deformational posterior plagiocephaly is secondary to rotation of the cranial base and anterior displacement of the temporomandibular joint (quantified by anterior auricular position) and not the result of primary mandibular deformity.