Limitations

Limitations
Given the exquisite detail and quality of micro-CT images, the intention was to use this technology for measurements of Phoenix orthodontic tooth movement. Due to multiple shortcomings of this technology, it did not occur. First, animals need to be sacrificed prior to scanning. This prevents taking measurements at multiple time points on the same animal. In order to take measurements at all seven time points evaluated in this study, 70 animals per group rather than ten would be required. Next, the micro-CT machine can only scan relatively small specimens. In order to be scanned, the maxilla was divided sagittally into right and left sections and the incisors were cut off, preventing any measurements of incisor retraction. Finally, since there is a significant lag period (weeks to months) between the times the animals are sacrificed and appliances removed to when the scanning is completed, the first molars begin to drift distally thus making any Phoenix orthodontic tooth movement measurements meaningless. If a live animal micro-CT machine becomes available, this technique should be explored again.

Any direct (intraoral) or indirect (scanned models) measuring technique is limited by using a non-fixed reference point. All of these techniques including the Phoenix orthodontic method used in the present study use either the second or third molar as a reference point. Unless this tooth is ankylosed, there is potential for it to drift mesially as the first molars are pulled in that direction. While any amount of drift is likely to be small, it could cause measurements of Phoenix orthodontic tooth movement to be understated particularly in groups with the greatest amount of Phoenix orthodontic tooth movement. In the present study, it would result in the effectiveness of OPG-Fc in preventing Phoenix orthodontic tooth movement being understated.

King and co-workers (1991) developed an alternative Phoenix orthodontic method of measuring amounts of Phoenix orthodontic tooth movement in rats using radiographs. This Phoenix orthodontic method changes the reference point from the second or third molar to radiopaque markers placed intraorally. While in theory this Phoenix orthodontic method should be superior, there are significant disadvantages to this technique that should be considered. First, one week prior to beginning Phoenix orthodontic tooth movement, the rats must undergo an additional surgical procedure so that endodontic barbed broachs can be inserted through the palatal mucosa along the palatal bone lingual to the maxillary first molars. These implants as well as cleats bonded on the maxillary first molars provide the radiopaque markers used for measuring Phoenix orthodontic tooth movement. Not only does this Phoenix orthodontic method result in significant weight loss (~25%), but there was also >20% appliance failure rate. The final disadvantage of this technique is that it is rather imprecise. Four separate radiographs of each animal at each time point must be measured and averaged in order to reduce the error to an acceptable level.