4.1. Summary of Main Findings
This systematic review included one randomized and three case-control studies, with a total of 214 patients. The overall results demonstrate that Invisalign seems to be significantly associated with less discomfort rather than fixed appliances. Although at 1 and 3 days the results are not significant, at 7 days this difference is conclusive.
Notwithstanding, by accounting the type of material, SmartTrack appear to provide better comfort for orthodontic patients than the previous standard material. Notably, this differences becomes significant at 3 days after appliance insertion and turns more pronounced at 7 days. In fact, these results comply with a previous research where patients were transitioned to the aligners made with the SmartTrack material and experienced less pain intensity, pain duration, and less pressure upon insertion 7. In addition, Invisalign patients reported less analgesics consumption compared to patients with fixed appliances, and present significant differences.
4.2. Quality of the Evidence, Limitations and Potential Biases in the Review Process
The strengths of this systematic review include the extensive unrestrictive literature search, with a rigorous and predetermined protocol implemented in each phase. However, there are limitations worth to mention among the included studies.
The included investigations were of small samples, and two of them lack sample size calculation 25,26. As well, there are a diversity of buccal brackets description since one did not refer the type of fixed appliances 25, two used different self-ligated wire appliances 26 27 and another used a traditional buccal fixed appliances 6. Though passive self-ligating systems result in minor periodontal ligament ischemia and therefore less discomfort 28, literature evidences that pain experience in the beginning of treatment is independent of bracket type 29–32. The type and size of archwires was only specified in two studies 6,26, which is also a limitation. However, previous studies have found no significant differences in the pain perception using different archwires types 29,33,34.
Moreover, only one study 6 performed allocation concealment, while the remaining three 25–27 did not randomly assigned the treatment modalities due to their cohort design nature. In fact, well-conducted and adequately informed “gold standard” RCTs are the epitome in clinical research 35,36. The main advantage lies in the random allocation of patients minimizing patients selection bias 37. However, randomizing adult patients is not simple, since some of whom are unwilling or unable to comply with the random assignment due to esthetic reasons 26. This difficulty limits the ability to completely randomize the study. The fact that the patient has a choice demonstrates personality traits, which can impair the perception of pain.
Additionally, pain experience is a notoriously subjective response and there is a non-linear relationship upon multiple factors such as age, gender, individual pain threshold, the magnitude of the force applied, present emotional state and stress, cultural differences, and previous pain experiences 12,15,38–42. A hypothetical limitation would be the fact that there is an unbalanced gender ratio. However, gender has no significant effect on orthodontic pain perception 12,15,41,43, except in adolescents, where females have less pain tolerance than males 44,45.
Finally, the placement of SmartForceⓇ attachments since the beginning of the treatment with Invisalign is relevant in pain perception, because they make more pressure during the insertion of the aligner. They were introduced by Align in September of 2009. In the included studies, Miller et al 25 in 2007 did not place attachments since it was not protocol, Shalish et al 26 do not describe the placement of attachments in the first set of aligners, White et al 6 placed attachments already in the first aligners, and Almasoud 27 delayed the attachments placement until the third set of aligners. The high heterogeneity among the studies prevent a definitive conclusion and, therefore, the influence of the attachments in pain perception and comfort should be considered in the future.