Figure 2. Mechanical apparatus. A, a: Instron machine; b: The testing bracket bonded on the upper side of the universal bevel protractor with the mesial-distal direction of the slot parallel to the plump line direction; c: The lateral view of the universal bevel protractor. The scale was used to regulate the angulation; d: The horizontal scale was used to measure the length that could counterbalance the deviation. B, The trigonometric function used to avoid the deviation by bonding height. A point was the midpoint of the bracket. When the rotational angle was set, the bracket not only rotated but also moved downward and forward. AB was the horizontal displacement and perpendicular to BC, while the vertical displacement could be neglected as it did not influence the findings. AB = sin∠ACB × AC. The brackets were moved backward previously to compensate for the horizontal error.
