The functional shift of the mandible in unilateral posterior crossbite and the adaptation of the temporomandibular joints: a pilot study.

Changes in the functional shift of the mandibular midline and the condyles were studied during treatment of unilateral posterior crossbite in six children, aged 7-11 years. An expansion plate with covered occlusal surfaces was used as a reflex-releasing stabilizing splint during an initial diagnostic phase (I) in order to determine the structural (i.e. non-guided) position of the mandible. The same plate was used for expansion and retention (phase II), followed by a post-retention phase (III) without the appliance. Before and after each phase, the functional shift was determined kinesiographically and on transcranial radiographs by concurrent recordings with and without the splint. Transverse mandibular position was also recorded on cephalometric radiographs. Prior to phase I, the mandibular midline deviated more than 2 mm and, in occlusion (ICP), the condyles showed normally centred positions in the sagittal plane. With the splint, the condyle on the crossbite side was displaced 2.4 mm (P < 0.05) forwards compared with the ICP, while the position of the condyle on the non-crossbite side was unaltered. After phase III, the deviation of the midline had been eliminated. Sagittal condylar positions in the ICP still did not deviate from the normal, and the splint position was now obtained by symmetrical forward movement of both condyles (1.3 and 1.4 mm). These findings suggest that the TMJs adapted to displacements of the mandible by condylar growth or surface modelling of the fossa. The rest position remained directly caudal to the ICP during treatment. Thus, the splint position, rather than the rest position should be used to determine the therapeutic position of the mandible.