The mechanical environment that drives distraction osteogenesis is believed to be tensile strain, but additional, unintended displacements of the osteotomy site may alter loading. Such displacements could arise from contracting jaw muscles. Objective: To measure osteotomy displacements at a distraction site during natural and stimulated masticatory muscle activity. Methods: Pigs (Sus scrofa), 6-8 weeks old, had mandibular surgery at the right ramal/body junction (n=16). A distractor (Synthes® or KLS-Martin®) was installed. Lateral osteotomy widths were measured with digital microsonography using pairs of piezoelectric crystals placed superior to, on, and inferior to the distractor. Medial widths inferior to the distractor were measured using a differential variable reluctance transducer (DVRT). One pair of crystals was placed on the intact left side. Dynamic width changes at the undistracted osteotomy were measured during natural chewing and during stimulation of the masseter and medial pterygoid. Results: The two distractors gave similar results. During mastication, crystals on the appliance showed 0.1-0.2 mm displacements, and larger excursions were seen superior and inferior to the device. The crystals and DVRT revealed sagittal bending; the inferior part of the osteotomy was compressed during the power stroke (-0.28 mm ± 0.15) while the superior part was generally tensed. Osteotomy-side contractions of masseter and medial pterygoid produced similar sagittal bending but had opposite effects on transverse bending. The masseter enlarged the inferior osteotomy medially (average 0.17 mm) and diminished it laterally (average -0.21 mm), whereas the medial pterygoid shortened the medial aspect even more than the lateral (-0.64 vs –0.27 mm). Conclusions: The distraction osteotomy site is not stable. The ipsilateral masseter and medial pterygoid cause similar sagittal but opposite transverse distortions. Sagittal bending predominates during mastication, in a pattern that suggests that the muscles pull the ramus up while bite force pushes the body down. Supported by PHS award DE14336.

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