Biomechanical rationale for surgically facilitated expansion of the maxilla in the cleft palate patient.

AIM: To evaluate in vitro the biomechanical effects of surgically assisted rapid palatal expansion in a photoelastic unilateral cleft palate analog by observing stresses produced during appliance activation.
METHODS: A photoelastic analog of an adult skull with unilateral cleft palate was fabricated using birefringent materials to simulate bone and teeth. A customized hyrax appliance was applied to the anchor teeth and incrementally activated. Resulting stress patterns on both the cleft side and the intact side were observed and recorded photographically in the field of a circular polariscope. Subsequently, the pterygomaxillary junctions were sequentially cut, the appliance activations repeated, and the resulting stresses recorded.
RESULTS: With intact pterygomaxillary junctions, the highest stresses were localized at the zygomaticomaxillary and zygomaticofrontal sutures, with higher intensity on the intact side. Stresses concentrated in the pterygoid plates evidenced resistance to the expansion forces of the hyrax appliance. On separating the defect pterygomaxillary junction, increased stresses were located from the zygomaticomaxillary suture to the zygomaticofrontal suture and at the zygomatic arches on both sides, as well as the pterygomaxillary junction of the non-defect side. After cutting both the pterygomaxillary junctions, decreased stress was located from the zygomaticomaxillary suture to the zygomaticofrontal suture, the zygomatic arch, and the frontonasal sutures on both sides.
CONCLUSIONS: The pterygomaxillary junctions acted as the main resistive elements to expansion forces generated by a hyrax appliance. Separating the pterygomaxillary junctions assisted the bodily displacement of the lateral maxillary segment, which would facilitate correction of maxillary arch constrictions in the adult unilateral cleft lip patient.