Treatment timing for Twin-block therapy.

This cephalometric study evaluated skeletal and dentoalveolar changes induced by the Twin-block appliance in 2 groups of subjects with Class II malocclusion treated 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, and at 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 12 years 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. A modification 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 to the 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 produces more 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 of skeletal maturity in individual patients with Class II disharmony to be treated with functional appliances is emphasized.

Treatment effects produced by the twin-block appliance and the FR-2 appliance of Fränkel compared with an untreated Class II sample.

This retrospective cephalometric study compares the treatment effects produced in 40 patients treated with the Twin-block appliance to those seen in a matched sample of 40 children treated with the FR-2 appliance of Fränkel and to changes undergone in 40 untreated Class II controls from The University of Michigan Elementary and Secondary School Growth Study. The average starting ages for the Twin-block, Fränkel, and control groups were 10 years 5 months, 10 years 2 months, and 9 years 11 months, respectively. The T(2) to T(1) observation period was adjusted to an average of 16 months for all groups. Significant decreases in overbite and overjet were observed at the end of treatment in the Twin-block and Fränkel groups. Compared with the untreated subjects, statistically significant increases in mandibular length were observed in both treated groups. The Twin-block patients achieved an additional 3.0 mm of mandibular length, whereas the Fränkel group increased 1.9 mm more than did the controls. No significant restriction of midfacial growth was observed in either functional appliance group relative to controls. A significant increase in lower anterior facial height was evident in both treatment groups. Vertical increase in the Twin-block patients was significantly greater than in the FR-2 group. In general, more extensive dentoalveolar adaptation was observed with the tooth-borne Twin-block appliance than with the more tissue-borne FR-2 of Fränkel. The Twin-block and FR-2 samples both showed significant retroclination and extrusion (eruption) of the maxillary incisors. The Twin-block patients also exhibited distal movement of the upper molars; however, there was no extrusion. Slight lower incisor proclination was noted in both treatment groups, and lower molar extrusion was found to be significantly greater in the Twin-block group compared with the other 2 samples. No horizontal differences were detected in the lower molars among groups. The present study suggests, therefore, that Class II correction with the Twin-block appliance is achieved through normal growth in addition to mandibular skeletal and dentoalveolar changes. Class II correction with the FR-2 is more skeletal in nature, with less dentoalveolar changes noted.

Two distinct apolipoprotein B alleles in mice generated by a single 'in-out' targeting.

'In-out' gene targeting using a hypoxanthine phosphoribosyltransferase (HPRT) minigene was applied to generate two new alleles in the gene (Apob) coding for apolipoprotein B (apo B) in murine embryonic stem (ES) cells. Homologous integration of the targeting vector during the 'in step' disrupted the Apob gene leading to an allele encoding apo B81, having a 19% carboxyl-terminal truncation. All six targeted cells obtained had more than one insert at the locus, and the chromosomal target sequence in four of them was changed during the recombination. These results suggest that concatenation of the targeting vector prior to insertion was needed to generate sufficient gene product to yield the HPRT+ phenotype, and that recombination between the concatenated DNA and endogenous DNA was a gene replacement more frequently than a simple insertion. The 'out step' recombination event which occurs between sequences duplicated in the 'in step', was planned to replace the sequences encoding the putative LDL receptor-binding domains of apo B100 with sequences encoding human beta-globin peptides (designated apo B100-beta). 6-Thioguanine (6-TG) resistant colonies were obtained from all the 'in-step' cell lines tested at frequencies of 10(-5) to 10(-4), but the frequency of physical loss of the HPRT sequences accompanied by retention of the modified Apob sequence was variable, indicating that mechanisms other than a simple excision are responsible for the generation of 6-TG resistance. Mice from the 'in-step' produce apo B81 and display characteristics of familial hypobetalipoproteinemia; some homozygotes develop hydrocephaly or exencephaly. Mice from the 'out-step' produce apo B100-beta and secrete lipoprotein particles containing the modified protein; their phenotypic changes are subtle, suggesting the lack of the putative LDL receptor-binding domains is not sufficient to increase the steady-state level of apo B100-beta particles above that of apo B100 particles in control mice.