Visual Guide for Individualized Inclination of Twin Block Appliance

Abstract

The Twin Block appliance is one of the most preferred functional appliances for the treatment of skeletal Class II malocclusion in growing children. Conventionally, the inclines of the Twin Block appliance are kept at a 70° angle, based on the angle between the hinge axis of opening, which is approximately 70° to the occlusal plane. But this angle is not constant for each individual and can vary based on individual cuspal inclines and condylar morphology. The present article describes a simple visual method for individualized Twin Block angulation based on the individual’s own cuspal inclines.

Introduction

In human dentition, cuspal inclines are essential for proper occlusion and function, and their deviations can contribute to various forms of malocclusion. In Class II malocclusion, these cuspal inclines can contribute to the jaw being locked in a retruded (backward) position. The Twin Block appliance modifies the natural occlusal inclines with specially designed bite blocks, encouraging the mandible to move forward and upward into a corrected position. ¹ Conventionally, the Twin Block appliance is made with a 70° inclination between the blocks. This was based on the angle between the hinge axis of opening and the occlusal plane. ² But this angle is variable (69.1° ± 4.2°) and is based on individuals’ condylar anatomy with respect to the glenoid fossa and cuspal inclines. ³ Hence, making the Twin Block inclination within this range based on individuals’ cuspal inclines would be a more logical solution. This clinical pearl describes a simple visual guide method to achieve this individualized inclination of blocks.

Technique

1. This method uses the mesial slope of the maxillary first premolar as a reference plane. This is because, out of all posterior teeth, from the buccal aspect, the mesial slope of the buccal cusp in the first premolar is longer and straighter. Its distal slope is shorter and more curved. Moreover, from the buccal aspect, the mesiobuccal line angle of the maxillary first premolar meets the occlusal table at a sharp, nearly right angle, and the most prominent mesial point is at the contact point with the canine. ⁴ This peculiar morphology of the maxillary first premolar makes it an ideal site for the formation of a reference line to be used during the construction of the Twin Block appliance.

2. To make a reference line, two points are marked on the mesial slope of the first maxillary premolar. One point is at the cusp tip, and another is just above the contact point with the canine (Figure 1).

3. A line is drawn on the cast joining these two points. In almost all individuals, the angle between this reference line and the occlusal plane is within the range of 60°-70° (Figure 1).

4. Appliance fabrication is done in a conventional way after taking the bite registration.

5. While making the inclines of the blocks, the constructed line on the mesial slope of the maxillary first premolar is taken as a reference. The inclines are made parallel to this line. To make similar incline angles on both sides, the angle between the reference line and the occlusal plane is measured and kept the same by adjusting the reference line, if needed (Figure 2).

Figure 1.

Construction of Reference Plane.

Figure 2.

Intraoral Presentation of Twin Block Inclination Parallel to Reference Line on the First Maxillary Premolar.

Discussion

The present clinical pearl describes the use of the mesial incline of the maxillary first premolar to construct a reference plane, which is used as a guide during the fabrication of the Twin Block inclination. The angle between this reference plane and the occlusal plane is quite similar (60°-70°) to the inclination range suggested for the Twin Block appliance based on the angle between the hinge axis of opening and the occlusal plane. ³ Moreover, since the reference plane is placed just mesial to the intended contact between two blocks, using it makes the fabrication of Twin Block angulation very simple and less time-consuming. It also eliminates the use of any tool or prefabricated blocks, and it is quite effective since the block inclination is individualized according to the patient’s cuspal inclines.