Effects of recycling process on the biomechanical characteristics of orthodontic mini-screws

Abstract

The aim of this study was to compare surface properties, mechanical characteristics, and biological responses between recycled and unused orthodontic mini-screws to determine the feasibility of reuse from the biomechanical aspect. We also included both screws with machined surface (MS) and those with etched surface (ES) to further assess the effects of recycling with regard to surface treatments. To simulate initial use, MS and ES were placed in the maxilla of four beagle dogs and maintained for 4 weeks. Retrieved MS and ES were further divided into three subgroups (A,B,C) according to the assigned recycling procedure: group A, air–water spray only; group B, mechanical cleaning only; and group C, mechanical and chemical cleaning. Unused screws (group D) were used as control. Scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS) were assessed for all group and insertion time and maximum insertion torque measurements in artificial bone, and biological responses in the form of Periotest values (PTV), bone–implant contact ratio (BIC), and bone volume ratio (BV) were assessed for group C and D. The results of this study were as follows. 1. Morphological changes after recycling mainly occurred at the screw tip 2. Retrieved MS could produce similar surface composition with unused screws only by mechanical cleaning, while ES need chemical cleaning as well after mechanical cleaning to produce similar surface composition with unused screws. 3. The success rate of recycled screws to penetrate cortical bone was lower than that of unused screws. However, there was no significant difference between recycled screws that successfully penetrated to the cortical bone and unused screws in IT and MIT (P >0.05). 4. PTV, BIC, and BV according to healing time showed remarkable changes. Especially, the increase of PTV and decrease of BIC and BV after 8 weeks of healing time was statistically significant (P < 0.01). 5. There was no statistically significant difference in PTV and BIC between recycled and unused screws (P >0.05). However, BV of the recycled screw was lower than that of the unused screw and this difference was statistically significant (P < 0.05).

In conclusion, the results of our study suggest that used orthodontic mini-screws can achieve a surface composition similar to that of unused screws when subjected to appropriate recycling processes, with mechanical and chemical cleaning showing the best effects together. However, screw tip deformation remains a concern, necessitating pre-drilling or an increase in the vertical force for cortical bone penetration. Furthermore, biological responses may differ between recycled and unused screws. Taken together, reuse of recycled orthodontic mini-screws may not be feasible from the biomechanical aspect.

본 연구에서는 recycled screw와 unused screw의 표면특성, 기계적 특성, 생물학적 반응을 비교하여 교정용 미니스크류의 재사용이 생역학적으로 합당한지를 알아보았다. 또한 표면처리 유무에 따른 recycling의 효과를 추가로 알아보기 위해 machined surface screw (MS)와 etched surface screw (ES)를 비교하여 관찰하였다. 처음 사용하는 것을 재현하기 위해 MS와 ES를 beagle dog의 상악에 식립 후 4주간 유지하였다. 사용 후 제거한 스크류들은 recycling 방법에 따라 3군으로 분류하였다 (A, B, C). A군은 air-water spray만 시행하였고, B군은 기계적 세척까지만 시행하였으며, C군은 기계적 세척 후 화학적 세척까지 시행하였다. Unused screw들은 (D군) 대조군으로 이용하였다. Recycling에 따른 표면 특성의 변화를 관찰하기 위해 모든 군을 대상으로 field emission scanning electron microscopy (SEM) 관찰과 energy-dispersive X-ray spectroscopy (EDS) 분석을 시행하였다. C군과 D군을 대상으로 기계적 특성 차이를 비교하기 위해 insertion time 및 maximum insertion torque를 측정하였으며, 생물학적 반응의 차이를 비교하기 위해, beagle dog 4마리의 하악에 각각 4개의 recycled screw (MS 두 개, ES 두 개)와 4개의 unused screw (MS 두 개, ES 두 개)를 식립하였다. 이 중 두 마리는 식립 4주 후 희생하였으며, 나머지 두 마리는 8주 후 희생하였다. 각 스크류의 안정성 평가를 위해 식립 당일과 희생 일에 periotest (PTV; periotest value)를 시행하였다. 또한 생물학적 반응의 차이를 관찰하기 위해 bone-implant contact ratio (%BIC)와 bone volume ratio (%BV)를 측정하였다. 그 결과 다음과 같은 결론을 보였다. 1. Recycling에 따른 교정용 미니스크류의 외형 변화는 주로 tip 부위에서 발생하였다. 2. Retrieved MS는 기계적 세척만으로도 unused screw와 유사한 표면 조성을 얻을 수 있는 반면, retrieved E...