Sunday, 19 December 2010

Design and Management of Twin Blocks

William Clark published recently an article at the British Journal of Orthodontics.


Design and Management of Twin Blocks: Reflections After 30 Years of Clinical Use


As the abstract reads:
There is an amount of misconceptions concerning the design of the Twin Block appliance. Those misconceptions might lead to poor compliance and poor treatment results and discourage some clinicians from the use of the appliance.


Small note: This post will not go into the principle and the basics of the Twin Block therapy as it's aim is to discuss the common mistakes when constructing the appliance. As such it is mend for readers with prior clinical experience using the appliance. 




Height of Occlusal Blocks


The height of the occlusal blocks should be enough to overcome the freeway space and not allow the patient to retrude the mandible when at rest. However constructing very high blocks would give other undesired complications.


Patient has to be able to close the lips without major strain, to be able to incise and chew posteriorly without difficulty, not to affect his/her speech, and not compromise the aesthetics when worn. These will ensure compliance and full time wear of the appliance which is the goal for the Twin Block therapy.


What is commonly used among clinicians is the rule of 2mm inter-incisal clearance, that will give a 5-6mm opening at the first premolar region. However this is not the case for all starting malocclusions, and that is one of the most common mistakes.


Summing up (in short):

  • Class II Div.1 deep bite patients, with a 2mm clearance construction bite, will give a 5-6mm opening at the first premolar region which is the favorable construction for those patients.
  • Class II Div.2 deep bite patients however, should have edge to edge and contact anteriorly. This will give the 5-6mm  opening at the first premolar region. In those patients 0mm of interincisal clearance and inscisal contact at edge to edge should be the aim.
  • Lastly at anterior open bite, Class II patients, in order to achieve a 5-6mm opening at the premolar region, the interincisal clearance should be increased. For example in a 2mm anterior open bite patient, a 4mm anterior clearance should be the optimal.
Due to the increased opening required for the anterior open bite patients, the sagittal activation as it will be discussed further should be in steps and more "gentle".

Overall unfavorable construction of the twin block. Notice the height posteriorly which gives an enormous clearance in the front. It is the appliance design fault that this twin block will certainly not be used by the patient, as no lip closure can be achieved without strain and the appliance cannot be worn outdoors for obvious aesthetic reasons.

In that sense, there is no guidline, and fabrication should be specific for each patient. When the patient is a deep bite, with a pronounced curve of Spee, it is understandable that small interincisal clearance would give a large opening at the premolar region. The opposite happens with anterior open bite patients, or retroclined upper front deep bite.

The aim should be 5-6mm of opening at the first premolar region, and to achieve that different construction bites are taken for different starting malocclusions.


Sagittal Activation

This is another factor of major importance for the outcome of the treatment. Again numerical guidelines are not easily given and each case should be addressed individually.

Each patient has a certain ability of protruding the lower jaw:
  • 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.


Case Report:



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

Case treated by postgrad Dimitris Galaktopoulos, DDS under the supervision of clinical instructor of the Aarhus Orthodontics department, Jytte Karnbak Pedersen, DDS

Reference - Literature:


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