Is the chemical and mechanical ageing of clear aligners during use the reason why predicted results don't always align with final clinical outcomes?

Data sources The online databases of Scopus, Web of Science, ScienceDirect, PubMed, ProQuest and the World Health Organisation (WHO) International Clinical Trials Registry Platform (ICTRP), from inception to September 2021, were searched with no restrictions to identify eligible studies measuring the changes in surface morphology, chemical and mechanical properties of clear aligners before and after intraoral usage.Study selection Only studies investigating changes in Smart Track (LD30) material met the inclusion criteria. In vitro studies, studies with a sample size of less than ten, studies not assessing the properties of aligners after usage and those assessing the old material of Invisalign were excluded.Data extraction and synthesis Two reviewers independently assessed the studies, and disagreements were resolved through consultation with a third reviewer. Data was pooled using Review Manager (RevMan) version 5.3. Inconsistency test (I2) was used to examine heterogeneity between studies. Meta-analysis was performed based on a random-effects model. Qualitative synthesis was used to assess outcomes from a single study or where different outcome measures were used between studies.Results Two randomised non-control studies and two cohort studies met the inclusion criteria and were included in this systematic review. All included studies were judged to have a moderate risk of bias when assessed using Cochrane collaboration tools (RoB 2 and ROBINS-I). Qualitative assessment of all studies showed LD30 exhibited changes in surface morphology after clinical usage. Mechanical properties were analysed using the pooled estimate of data from 70 aligners. Significant decreases in both Martens hardness (SMD = -3.37; 95% CI [-6.24, -0.52]; P = 0.02) and indentation modulus (SMD = -2.41; 95% CI [-3.39, -1.43]; P = 0.02) indicate changes in mechanical properties before and after usage.Conclusions Within the limitations of the included studies, the authors concluded that LD30 exhibited changes in surface morphology, chemical and mechanical properties during usage, which may have an impact on the differences between predicted and final clinical results. There is currently insufficient evidence to precisely determine the best material that could serve treatment needs.