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A Modification to Enable Controlled Progressive Advancement of the Twin Block Appliance

The Twin block appliance was introduced by Clark (1982). Since then, the original design has evolved with reduced emphasis on the need for headgear and intra-oral elastic wear (Trenouth, 1989; Clark, 1995). This system is currently one of the most popular functional appliances used in the United Kingdom, and recent evidence would suggest that it is probably the most successful in the treatment of Class II division 1 malocclusions (Chadwick et al., 1997). One of the drawbacks of the original design was the inconvenience of reactivating the appliance to achieve an edge-to-edge protrusive position in some patients to enable full overjet reduction, or to allow a degree of overcorrection. In the treatment of patients with large initial overjets, the appliances usually require block augmentation or even appliance remakes, since many patients are unable to tolerate mandibular protrusion greater than 7 mm. Patients with dolichofacial patterns tend to have weak craniomandibular musculature and are less able to tolerate large initial protrusions, sometimes showing a tendency to bite the occlusal blocks one upon the other, instead of in the correct protrusive position. In such cases, Clark (1995) recommends gradual bite advancement. Woodside (1977) recommended bite registration for the Andresen activator in a position where the mandible is advanced approximately 3 mm distal to the most protruded position the patient can achieve, while vertically the bite is registered with the limits of the freeway space. In Class III treatments using the Twin block, difficulty may be experienced with reactivating the appliance. In these cases, a gradual reactivation of the bite would facilitate better patient compliance and a more favourable treatment progression. There has been more recent evidence that gradual incremental advancement of the working bite during treatment of Class II divisions 1 malocclusions may give a more favourable response to the growth modification process with a reduced effect on incisor tilting (Petrovic et al., 1981; Falck and Frankel, 1989; De Vincenzo and Winn, 1989). Compliance may be greater with small increments of mandibular advancement by reduction of tension in the craniomandibular musculature. It has been suggested that this improves patient comfort and speech (Bass, 1996) with greater likelihood of maintaining the correct appliance position during sleep. Currently, reactivation of the Twin block appliance requires the addition of cold curing acrylic at the chairside or, alternatively, time-consuming laboratory modification. The former has the following disadvantages: