Improved clinical use of Twin-block and Herbst as a result of radiating viscoelastic tissue forces on the condyle and fossa in treatment and long-term retention: growth relativity.

Understanding mechanisms of action for orthopedic appliances is critical for orthodontists who hope to treat and retain the achieved corrections in patients with initial Class II mandibular retrognathism. That knowledge can help orthodontists produce clinically significant bone formation and avoid compression at the condyle-glenoid fossa region. It also assists us to understand the differences between short-term and long-term treatment results. It was previously thought that increased activity in the postural masticatory muscles was the key to promoting condyle-glenoid fossa growth. By analyzing results from several studies, we postulate that growth modification is associated with decreased activity, which leads to our nonmuscular hypothesis. This premise has its foundation on 3 key specific findings: significant glenoid fossa bone formation occurs during treatment that includes mandibular displacement; glenoid fossa modification is a result of the stretch forces of the retrodiskal tissues, capsule, and altered flow of viscous synovium; observations that glenoid fossa bone formation takes place a distance from the soft tissue attachment. The latter observation is explained by transduction or referral of forces. Evidence is presented, therefore, that the 3 trigger switches for glenoid fossa growth can similarly initiate short-term condylar growth modifications because the 2 structures are contiguous. These are displacement, several direct viscoelastic connections, and transduction of forces. Histologic evidence further shows that stretched retrodiskal tissues also insert directly into the condylar head's fibrocartilaginous layer. The impact of the viscoelastic tissues may be highly significant and should be considered along with the standard skeletal, dental, neuromuscular, and age factors that influence condyle-glenoid fossa growth with orthopedic advancement. These biodynamic factors are also capable of reversing effects of treatment on mandibular growth direction, size, and morphology. Relapse occurs as a result of release of the condyle and ensuing compression against the newly proliferated retrodiskal tissues together with the reactivation of muscle activity. To describe condyle-glenoid fossa growth modification, an analogy is made to a light bulb on a dimmer switch. The condyle illuminates in treatment, dims down in the retention period, to near base levels over the long-term.

Interactive edgewise mechanisms: form and function comparison with conventional edgewise brackets.

As the frequency of use of the interactive twin (I-twin) edgewise mechanisms and techniques become increasingly prevalent, it is important to consider how they compare with conventional twin (c-twin) edgewise bracket systems. Optimum intrabracket and interbracket forces in I-twins balance with capillary blood pressures. An unbiased, experimental in vitro, scanning electromicroscopy and in vivo clinical investigation of 83 patients was conducted to compare the frictional resistance of three different I-twin bracket systems, type A (Sigma, American Orthodontics), type B (Interactwin, Ormco Corp.), and type C (Damon, A-company) with three c-twins counterparts respectively types D, E, and F. The three interactive twin brackets were each self-seating by an integrated arm component and made significant incremental improvements to the conventional twins in three different ways. First, the I-twin types reduced frictional resistance by using arm engagement with a lower coefficient of friction and a reduced seating force against the arch wire. The reduced seating force friction produced initially small interbracket arch wire deflections for biocompatible tooth movement and, combined with a wide twin bracket, produced accurate rotation corrections. Reduced friction within the I-twin bracket consequently permitted the effective use of light forces for flowing biomechanics that maximized anchorage. In evaluating friction, two distinctly different interaction forces acting to seat the arch wire were also identified. Type A demonstrated active interaction with round arch wires that resulted in a low functional seating force responsible for early and complete tooth control in comparison to the high seating force of c-twins. Types B and C both showed passive interaction with seating force friction approximately equal to zero that required large rectangular dimension arch wires for full bracket expression. Second, a significant reduction in the time taken to change arch wires was found, improving clinical time management. Third, I-twins assisted bracket placement with both center-slot identification markers and coordinated bracket and bonding pad reference planes in three dimensions. In addition, they improved bracket hygiene compared with c-twins by minimizing the use of plaque-retentive conventional ligatures. Despite the reduced elastomer use, experimental and clinical results showed I-twins effectively conserved the traditional four tie-wing design for ease of colored elastomer placement where an increase in friction for anchorage was selectively required and to enhance patient acceptance and motivation. Finally, interactive twins could be used by the practitioner as conventional twins without the interactive arms making them fail-safe.