[Therapeutic evaluation of the correction of the severe bi-maxillary protrusion cases by Tweed-Merrifield technique].

OBJECTIVE: To evaluate the influence of Tweed-Merrifield technique in correction of severe bimaxillary protrusion adult patients on the measurement of the dental and skeletal changes after orthodontic treatment by Johnston analysis and the regular cephalomatric analysis.

METHODS: Twelve adolescent patients with severe bimaxillary protrusion were included in this self-control retrospective study. Lateral cephalometric radiographs were taken before and after treatments. All the radiographs were traced and analyzed by the method of Johnston analysis. Other measurements were evaluated using a series of 13 linear and angular measurements including SNA, SNB, ANB, U1-SN, U1-NA, U1/NA, L1-NB, U1/NB, L1/MP, U1-L1, (U1+L1)/2-AB, MP/SN and MP/FH from regular cephalomatric analysis. These measurements were also applied to compare the differences between pre- and post-treatments, which clarify the dental and skeletal changes by Johnston analysis. The effect of orthodontic correction was determined using the non-parameters test.

RESULTS: The maxillary moved backforward by 1.3 mm according to the stable skull base, while the mandible moved forward by 2.12 mm. The relative position between the maxillary and mandible (ABCH) changed 3.42 mm. The upper and lower incisors retracted significantly. The upper and lower molars moved slightly forward and the relative positions of upper and lower molars and anterior teeth after treatment were 3.44 mm and 4.23 mm respectively. After treatment, the parameters of ANB, U1-NA, U1/NA, U1-SN, L1-NB, L1/NB and L1-M were reduced by -(1.98±1.55)°(P=0.012), - (5.08±4.6) mm (P=0.002), -(11.79±1.21)°(P=0.004), -(13.55±6.32)°(P=0.047), -(3.17±3.07) mm (P=0.010), -(6.84±2.55)°(P=0.038) and -(4.13±2.24)°(P=0.048) on average, whose changes had the statistically significant effects.

CONCLUSION: Tweed-Merrifield technique (directional force technique) can stabilize anchorage molar, retract anterior teeth and significantly improve the hard and soft tissue profile for patients with bimaxillary protrusion, and make a good vertical control which means this technique is applicable to the patients who need strong anchorage. Even for the severe bimaxillary protrusion adult patients, the Tweed-Merrifield technique can control the anchoragewell and make the profiles improved greatly.