Bushang

ORIGINAL ARTICLE Treatment timing for Twin-block therapy
Tiziano Baccetti, DDS, PhD,a Lorenzo Franchi, DDS, PhD,a Linda Ratner Toth, DDS, MS,b and James A. McNamara, Jr, DDS, PhDc Florence, Italy, Ann Arbor, Mich, and Los Angeles, Calif This cephalometric study evaluated skeletal and dentoalveolar changes induced by the Twin-block appliance in 2 groups of subjects with Class II malocclusiontreated at different skeletal maturation stages in order to define the optimal timing for this type of therapy. Skeletal maturity in individual patients was assessed on the basis of the stages of cervical vertebrae maturation. The early-treated group was composed of 21 subjects (11 females and 10 males). Mean age of these subjects at time 1 (immediately before treatment) was 9 years ± 11 months, andat time 2 (immediately after discontinuation of the Twin-block appliance) was 10 years 2 months ± 11 months. According to the cervical vertebrae maturation staging at times 1 and 2, the peak in growth velocity was not included in the treatment period for any of the subjects in the early group. The late-treated group consisted of 15 subjects (6 females and 9 males). Mean age of this group was 12years 11 months ± 1 year 2 months at time 1 and 14 years 4 months ± 1 year 3 months at time 2. In the late group, treatment was performed during or slightly after the onset of the pubertal growth spurt. Both treated samples were compared with control samples consisting of subjects with untreated Class II malocclusions also selected on the basis of the stage in cervical vertebrae maturation. Amodification of Pancherz’s cephalometric analysis was applied to the lateral cephalograms of all examined groups at both time periods. Linear and angular measurements for mandibular dimensions, cranial base angulation, and vertical relationships were added to the original analysis. Annualized differences for all the variables from time 1 to time 2 were calculated for both treated groups and contrasted tothe annualized differences in the corresponding untreated groups by means of nonparametric statistics. The findings of this short-term cephalometric study indicate that optimal timing for Twin-block therapy of Class II disharmony is during or slightly after the onset of the pubertal peak in growth velocity. When compared with treatment performed before the peak, late Twin-block treatment producesmore favorable effects that include: (1) greater skeletal contribution to molar correction, (2) larger increments in total mandibular length and in ramus height, and (3) more posterior direction of condylar growth, leading to enhanced mandibular lengthening and to reduced forward displacement of the condyle in favor of effective skeletal changes. The importance of the biological evaluation ofskeletal maturity in individual patients with Class II disharmony to be treated with functional appliances is emphasized. (Am J Orthod Dentofacial Orthop 2000;118:159-70)

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wide range of functional/orthopedic appliances is available for the correction of Class II skeletal and occlusal disharmonies (eg, Bionator,1-3 FR-2 of Fränkel,4-7 fixed and removable Herbst appliances8-13). Among these, theTwin-block originally developed by William J. Clark of Fife, Scotland, has gained increasing popularity during the last decade. The appliance consists of maxillary and mandibular acrylic plates with bite blocks that posture the mandible forward on closure, and it is indicated for the correction of Class II

Department of Orthodontics, University of Florence. Orthodontic Program, The Universityof Michigan, Ann Arbor, Mich., and in private practice of orthodontics, Los Angeles, Calif. cThomas M. and Doris Graber Endowed Professor of Dentistry, Department of Orthodontics and Pediatric Dentistry, School of Dentistry; Professor of Anatomy and Cell Biology, School of Medicine; and Research Scientist, Center for Human Growth and Development, The University of Michigan; and in private...