Three-dimensional Assessment of the Dentoskeletal Changes Associated With Herbst Appliance Therapy: A Controlled Clinical Trial

1. BACKGROUND Class II malocclusion is highly prevalent worldwide, and it is a major reason for orthodontic treatment to be initiated. This type of malocclusion can be restricted to the dentoalveolar region, but the majority of Class II patients exhibit a strong skeletal component to the malocclusion, including most frequently mandibular skeletal retrusion and an increased lower anterior facial height. With increased severity of the mandibular deficiency, the orthodontist is challenged to treat the underlying skeletal problem appropriately.

For growing patients, the use of fixed and removable mandibular advancement appliances has been advocated for many decades. A variety of fixed Herbst appliance designs have achieved worldwide acceptance in that they eliminate most major patient compliance factors. Today the Herbst appliance (HA) is by far the most frequently used mandibular jumping device in the United States, with more Herbst appliance being fabricated than all other functions appliances combined.

The Herbst appliance originally was introduced by Emil Herbst in the early years of the twentieth century, but it did not achieved worldwide clinical acceptance until its re-introduction by Hans Pancherz 70 years later. Since then, a significant number of clinical and scientific studies has been conducted about HA, particularly by Pancherz and colleagues.

Two-dimensional cephalometric studies have reported increases in the length of the mandible and in forward displacement of the mandible following HA in comparison to matched untreated controls. On average, 2 mm of mandibular length gain (measured from Gonion to Pogonion) and 1.5 degree of improvement in the SNB angle can be observed following Herbst appliance therapy.

Investigations in experimental animals have provided the histological evidence of the changes in the condyles, rami, and articular fossae when the mandible is advanced in a laboratory setting. Previous reports have showed an increased proliferation of the condylar cartilage in monkeys that had their mandible advanced with the Herbst appliance. These adaptations occurred primarily in the posterior and posterosuperior regions of the condyle. They also reported significant bone deposition along the posterior border of the mandibular ramus during the early part of the experimental period as well as significant deposition of new bone on the anterior surface of the postglenoid spine at the articular fossae. It should be noted, however, that the post-glenoid spine is not well-defined in humans.