Associations between incisor and mandibular condylar movements during maximum mouth opening in humans

This study evaluated the common clinical assumption that condylar translati on and mouth opening at the incisor are closely related. The sample studied comprised 27 adult females (23-35 years), selected for normal temporomandi bular function, occlusion, and skeletal patterns. Condylar and mandibular c entral incisor movements (straight-line distances and curvilinear pathways) were recorded in three dimensions (3D) for 20 s with an optoelectric (Opto trak(R)) jaw-tracking system while each participant performed multiple maxi mum opening cycles. Masticatory analysis and multilevel statistical program s computed the 3D movements of the incisors and condylar hinge axis during opening and closing. The incisor point moved an average straight-line dista nce of 46.6 mm during opening and 45.9 mm during closing, the lengths of th e opening and closing curvilinear pathways were 48.6 and 47.7 mm, respectiv ely. The condyles moved average straight-line distances of 11.9 and 12.2 mm during opening and closing, respectively. The condyles' curvilinear pathwa ys during opening and closing were 14 and 14.6 mm, respectively. Ranges of condylar movement varied widely between individuals. The straight-line dist ances and curvilinear pathways were highly correlated for the incisors (R = 0.98) and the condyles (R = 0.98). Neither the straight-line distances nor curvilinear pathways of the incisors were correlated with those of the con dyles. Incisor straight-line distances and curvilinear pathways were modera tely correlated with mandibular rotation (R-between (subjects) = 0.82 and R -between (repeats) = 0.88) During repeated openings, both condylar and inci sor excursions increased, but during repeated closings only incisor excursi ons increased. It is concluded that (1) maximum incisor opening does not pr ovide reliable information about condylar translation and its use as a diag nostic indicator of condylar movement should be limited, (2) healthy indivi duals may perform normal opening with highly variable amounts of condylar t ranslation, (3) the straight-line distances of the incisor and condyles pro vide adequate information about the length of the curvilinear pathway, and (4) variation in maximum incisor opening is largely explained by variation in the amount of mandibular rotation. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.