- Measuring the overjet at retruded position.
- Measuring the overjet at maximal protrusive position.
- The difference is the protrusive capacity of the patient, and the sagittal activation of the Twin Block should not be more than 70% of that capacity
In this way the patient can tolerate the activation and use the appliance full time, even when eating.
W.Clark suggests of 10mm as a border for activation but again this is patient depended and some patients can easily tolerate 12-13mm of activation at once, even with much more robust and non removable appliances such as the Herbst.
W.Clark however suggests that especially in vertical growing patients the tolerance is usually lower and the approach should be of small and gradual activations. He refers to a paper from Charmichael et al, 1999 BrJO where a modified Twin Block is suggested with horizontal screws that can be activated sagittally at the chair. A picture of that modification is presented:
Clinical Management and Guided Eruption:
W.Clark suggests the cementing of the appliance for the first 2 weeks. With that small trick he advocates of enhanced compliance for the crucial first weeks which will affect the overall compliance.
The vertical dimension is as important as the sagittal. This can be facilitated with guided eruption during the Twin Block therapy. Guided eruption dates back to Woodside and the ergenzinger treatment approach. The principle is exactly the same with the Twin Block.
At increased overbite patients the appliance should not have clusps at the lower molars to allow for eruption. Every appointment 1-2mm is grinded on the upper bite blocks at the region of 1st and 2nd molar. Once good interdigitation is achieved, the upper plate could be transformed to a simple retention plate with an inclined plane at the front to retain the sagittal.
A full fixed appliance finishing phase should follow when and if needed.
1-2mm trimmed upper bite blocks at the region of lower 6 and 7s. No more than 2mm per appointment otherwise there is risk of the tongue inhibiting the eruption. In addition separation elastics could be placed to facilitate faster eruption during appointments
Same procedure followed...
... until proper interdigitation is achieved between upper and lower molars. At this phase the lower plate could be discarded and the upper used as retention plate, while transitioning to the full fixed phase.
Summing up, W.Clark suggests if the appliance is constructed correctly as advised there is no need for labial bow as the upper front will be retroclined (Class II Div.1 cases) by the lip seal forces. Also he suggests that the main mistake that he sees in Twin Block fabrications is the Adam clusps at the lower first molars and the three ball clusps at the lower front.
11 year old female with Class II Div.1 malocclusion, 9mm of overjet and 6mm of overbite with palatal impingement.
Appliance in the mouth, note wire distal of the lower right canine is ending to a lip bumper shield that was added to the lower block of the Twin Block in this case.
Immediate profile change, first picture is without the appliance and second one is with the appliance. Note again the lip bumper creating the mentalis strain visible at the second profile picture
Lateral open bite created by the Twin Block therapy, occlusion is unstable as the guided eruption mentioned above was not completely achieved before moving to full fixed appliances.
Before and after result from an intra-oral frontal aspect. First picture is at treatment start and second at treatment finish.
First picture is a profile blow up at the start of treatment.
The other two pictures show the treatment outcome at the dental and soft tissue level