The Experimental Study Of Biodegradable Mini-implant Anchorage

BackgroundAnchorage control is one of the crucial factors in achieving successful orthodontic treatment. Anchorage, by definition, is the device or unit to resist undesirable tooth movement. Traditionally, anchorage was reinforced by increasing the number of teeth bilaterally or using the musculature, extra oral devices, and the alveolar processes. However, in some cases those strategies are limited. Since the appearance of mini-implant, which is with minimal invasive, easily handling, significant efficacy, flexible implantation site choosing, immediate loading as well as little uncomfortable experience to patient, has been broadly used in orthodontic treatment. Nowadays, all the mini-implants used in orthodontic clinics are metal-made devices. Nevertheless, some properties of metal, including ion releasing, local sensitive, secondary operation to remove, keep some patients from using it. Thus the research on the development of a biodegradable mini-implant would provide a promising solution.MethodSix healthy male beagle, aged between 12 to 15 months and weighted9.5 to 12 kg, were acquired and kept. Three groups of different biodegradable implants with the total of six were placed in the intraradicular spaces of the second premolar, the third premolar, thefourth premolar, the first molar and the second molar in the mandible.The left hemi-arch was the experimental group. The paired biodegradable implants were perpendicularly activated against each other with chromium-nickel traction springs and constant orthodontic load of 250 to300 g was applied. No loading was applied to the control groups in the right hemi-arch. The observation time was three months and six months post treatment. At each time point of three weeks, six week and nine weeks before the animals were killed by overdosed anesthesia,subcutaneous injection of calcine(25 mg/kg), alizarin red(30 mg/kg),tetracycline(30 mg/kg) were administered, followed by retrieving the mandible and being kept in 10% neutral formaldehyde solution for a week. Subsequently, the mandible was trimmed to individual block with the implant in the center and containing at least 2mm of surrounding tissues. After micro-CT analysis, the mechanical properties were evaluated by universal-mechanic test machine. All the implants were also retrieved for SEM observation analysis. Specimen decalcification was conducted by immersing the bone blocks fully in 12% EDTANa2 for six months. Specimens were then mounted in paraffin wax, and 4μm thick sections along the implant’s long axis in the middle of the plane.Hematoxylin and eosin were utilized to stain specimens for descriptive histological analysis. Bone blocks not decalcified were dehydrated and embedded in methyl-methacrylate, sectioned and polished for florescencemicroscopy. Similarly, hematoxylin and eosin staining were also performed for optical histological analysis.ResultAll the animals were well recovered after the surgery. Severe infection was found in the experimental group during clinical observation,demonstrating the red and swelling tissue around biodegradable implant in molar region. In premolar region the inflammation was minor as well as in control group. The overall success rates of biodegradable implant in the experimental group is 52.78% and 72.22% in the control group.The bone integration rate(BV/TV) of the biodegradable implant dropped in the six-month group compared to the three-month group. No significant difference was found between experimental groups and control groups. The mechanical test shows the bending strength of the implant decreased significantly as the implant retained in vivo. The SEM observation shows the degraded particles assembled on the surface of biodegradable implant in the three-month group, while many holes in the six-month group. The histological observation shows no obvious inflammation invasion in the interface between bone and implant. The biodegradable implant was in close contact with bone in the three-month group. In the six-month group some gaps could be seen in the interface.Bone remodeling was active in the interface while enhanced florescence band were shown as well.ConclusionBiodegradable implant has good biocompatibility and can guarantee enough orthodontic strength. Thus, this novel biodegradable implant might provide orthodontist and patients an alternative mini implant in treatment.