OPTIMAL SPECIFICATIONS FOR MEASURING BONE-TO-IMPLANT CONTACT OF MINI-SCREWS USING MICRO-CT

Abstract

The use of mini-screws (MSs) as temporary anchorage devices (TADs) is becoming more common in orthodontic treatment. With the increased use of TADs and the numerous manufactures producing them, research is needed to aid orthodontists in their selection of MSs. One of the major advantages of using mini-screw implants is that they can be loaded immediately and do not require osseoitegration.1 For this to be successful, the mini-screw needs to have adequate primary stability to retain itself in the bone. The mini-screw bone system relies on the stability of the mini-screw, the stability of the bone, and the stability of their interface.19,20 In the literature, this stability if usually quantified by measuring the pullout force, insertion torque and bone-to-implant contact.25,26 To date, much of the research has demonstrated that the geometric configuration of the mini-screw plays a significant role in its primary stability. Features such as pitch (length between threads), thread body design, screw length and diameter factor into primary stability.5,21,23 In order to observe the bone-to-implant contact traditionally, researchers had to perform histologic section. This method is destructive to the sample and does not allow it to be used for other analyses such as pullout force. With micro-CT technology, it is now possible to study the bone-to-implant relationship without destroying the sample and with great accuracy.6 Currently, it is unclear as to the optimal scanning specification to choose when using a micro-CT to measure bone-to-implant contact. The optimal scan is one, which provides the most accurate measurement within the least amount of time. The highest quality scans increase both time and costs of acquisition, while lower quality scans have the potential of introducing inaccuracies. This study aims to determine the optimal specifications needed to scan a mini-screw in bone using a SkyScan 1172 micro-CT, to measure the bone-to-implant contact. A total of three orthodontic mini-screws from Aarhus (American Orthodontics, Sheboygan, Wisc), 1.4 mm in diameter and 8 mm long were inserted into an adult pig mandible. All three mini-screws were inserted into the lingual area in the molar region. Each mini-screw was inserted until all the threads were buried into the bone. After placement, the blocks of bone containing the individual mini-screw were cut out and shaped to facilitate scanning by micro-CT. Each sample was be positioned and scanned individually using micro-CT (SkyScan 1172; SkyScan, Aartselaar, Belgium) under 5 different specifications, see table 1. SkyScan software was used to process the scans and calculate qualitative and quantitative data. For each sample bone-to-implant contact was measured. The software measured the TS, which is the area of the mini-screws surface and the IS, which is where the bone interfaces with the mini-screws. The IS/TSx100 was determined to be the percentage of the mini-screws surface that is in contact with bone or the bone-to-implant contact. Qualitative and quantitative analyses were performed to determine statically significant differences between the various bone-to-implant contact measurements of the samples. BIC varied greatly between the scanning specifications and samples, from 0% to 70.35%. Quantitative and qualitative analysis was performed to compare the differences in BIC(%) values between the 5 specifications. Two of the three samples displayed an accuracy of greater than 95% for specification 3, thus providing latitude in adjustment/reduction of scanning times with minimal variance in data accuracy for BIC(%). The results show for measuring BIC(%), scanning specifications can be modified/optimized to reduce scanning time while maintaining acceptable accuracy when scanning of large sample sizes are needed. It is recommended that for studies where absolute BIC(%) is needed, specification 5 is recommended since it will provide the most accurate measurement. For studies that are comparing changes in BIC(%), specification 3 is recommended as it will provide an acceptable level of accuracy in a reasonable amount time. However, due to limited sample size, more data is needed.