Purpose Improved superelastic nickel-titanium (Ni-Ti) alloy wires (ISWs) can be used not only for aligning but also for closing the extraction space. The objective of this study was to measure and compare the force and moment generated during mandibular molar protraction using an ISW combined with either short or long hooks under simulated crowded dentition conditions. Materials and methods Assuming crowded dentition following mandibular first premolar extraction, we designed a three-tooth model simulating the canine, the second premolar, and the first molar. First, 0.018 × 0.025-inch slot self-ligating brackets were bonded to the simulated canine and first premolar, and a tube was bonded to the first molar. Next, a 0.016 × 0.022-inch ISW was ligatured into the brackets. Either a long hook or a short hook was attached next to the first premolar bracket, and the hook and molar tube were pulled with a 150-gf Ni-Ti closed coil spring. We investigated the force and moments exerted on the teeth using an orthodontic simulator attached to a high-precision six-axis sensor. Results and discussion Using a short hook for molar protraction caused the simulated teeth to collapse into the extraction space, generating an insufficient counter moment (average moment: 2.0 ± 0.4 N·mm). In contrast, combining ISW with a long hook generated significantly (p<0.01) higher counter moments on the first molar (average: 6.5 ± 0.6 N·mm), resulting in controlled bodily movement without the need for rigid wires. While Ni-Ti wires are generally limited to leveling, our system utilizes a single archwire throughout all treatment stages. This approach demonstrated that early stage space closure is achievable by applying appropriate counter moments through molar protraction using the long hook.
Watanabe et al. (Tue,) studied this question.