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The Dahl effect or Dahl concept is used in dentistry where where  a Dahl appliance or anterior bite plane is used to increase the available interocclusal space available for restorations

History
Bjørn L. Dahl from the Dental Faculty of University of Oslo is frequently credited as being the first to fully describe the Dahl concept. Dahl, along with Olaf Krogstad and Kjell Karlsen, first described this phenomenon in 1975 in the paper "An alternative treatment in cases with advanced localized attrition", where a Dahl appliance or anterior bite plane is used to increase the available interocclusal space available for restorations.

Clinical Application
The Dahl concept is commonly used when an increase in the interocclusal space is required together with an increase in occlusal vertical dimension ; for example when restoring a case of severe anterior tooth surface loss.

Typically, restoring the worn anterior teeth with dental composite to the original proportions will result in an increase in OVD, with the posterior dentition held apart out of the occlusion.

Adaptation occurs over a period of some months: compensatory eruption of the posterior teeth will occur, together with some intrusion of the anterior teeth and potential growth of the alveolar bone. This will allow the posterior occlusion to reestablish at the new increased OVD, stabilizing the increased interocclusal space.

Dentoalveolar tissues tend to compensate by remodelling when incisal/occlusal tooth surface loss has occurred to allow the teeth to regain a functional occlusion. A subsequent increase in the OVD with the Dahl approach could exceed the patient's tolerance and adaptive capacity. If the patient is unhappy with the height of their teeth and a Dahl approach cannot be tolerated then crown lengthening may be appropriate.

Dahl appliance
The original material used to construct Dahl's appliance was cobalt chromium. Now, many materials can be used to construct Dahl's appliance as long as the principles of technique are adhered to. A Dahl appliance should fulfil the following aims:
 * A thickness of material should be placed on the incisal/occlusal aspect of those teeth where the creation of interocclusal space is necessary. No mucosal-bone component should be involved
 * Thickness of the material placed should directly correspond to the required amount of inter-occlusal space, which will determine the increased in OVD as measured at specific site in the mouth.
 * An occlusal bite platform should be constructed ideally to ensure the occlusal forces are directed along the long axis of teeth.
 * Stable inter-occlusal contacts should be achieved.
 * Movement of the discluded teeth should not be impeded by the appliance.

Advantages
The advantages of this approach are:
 * minimal removal of tooth substance is required to create the interocclusal space
 * lost OVD can be restored
 * minimisation of facial aging by restoring facial height
 * safety
 * relative simplicity
 * relatively reversible
 * relatively cost effective approach

Success of Dahl concept
Planned occlusal changes can be tested using a removable appliance prior to permanent treatment. Dental composite based approaches to tooth surface loss allow for easy adjustment or removal if required. One study published in the British Dental Journal, 2011 found that patient satisfaction was high when composite restorations were used in the Dahl approach and that the median survival time was between 4.75 and 5.8 years.