Temporomandibular joint growth adaptation in Herbst treatment: a prospective magnetic resonance imaging and cephalometric roentgenographic study

The aim of this investigation was to analyze three possible adaptive TMJ growth processes contributing to the increase in mandibular prognathism accomplished by Herbst appliance therapy: (1) condylar remodeling; (2) glenoid fossa remodeling; and (3) condyle-fossa relationship changes. The subjects were 15 consecutive Class II malocclusions (11 males and four females, aged 11.5-17.5 years) treated with the Herbst appliance for an average period of 7 months. Condylar remodelling, glenoid fossa remodelling, and condyle-fossa relationship changes were analyzed by means of magnetic resonance imaging (MRI). From each subject, four MR images were evaluated: before treatment, start of treatment (when the Herbst appliance was placed), during treatment (6-12 weeks after appliance placement), and after treatment (when the appliance was removed). 'Effective condylar growth' (= the sum of condylar remodelling, fossa remodelling, and condyle-fossa relationship changes) was analyzed with the aid of pre- and post-treatment lateral cephalometric roentgenograms. In all 15 subjects, Herbst therapy resulted in an increase in mandibular prognathism. After 6-12 weeks of treatment MRI-signs of condylar remodelling were seen at the posterior-superior border in 29 of the 30 condyles. MRI-signs of glenoid fossa remodelling at the anterior surface of the postglenoid spine were noted in 22 of the joints. Condylar remodelling seemed to precede fossa remodelling. The condyle-fossa relationship was, on average unaffected by Herbst therapy. 'Effective condylar growth' during treatment was, on average, approximately five times larger in the Herbst group than in an untreated group with ideal occlusion (Bolton Standards) and the direction of the growth changes was relatively more horizontal in the treated cases. The results indicate that condylar as well as glenoid fossa remodelling seem to contribute significantly to the increase in mandibular prognathism resulting from Herbst treatment, while condyle-fossa relationship changes are of less importance. MRI renders an excellent opportunity to visualize temporomandibular remodelling growth processes.