The apical maturity of MTM typically occurs between 18 and 19 years of age [12], but may be delayed until 20 to 23 years [9, 13, 14]. Asif et al. [8, 9] used a 3D approach to measure the apical surface area of MTM and reported a correlation with chronological age. The present study applied the same approach in IMTM to examine this correlation through ASAM, and to compare differences between impaction levels. Developing teeth are typically described as ‘unerupted’ but may also be classified as ‘impacted’ if eruption is hindered by local or systemic factors and the average age of alveolar emergence has passed [15]. In this study, all MTM meeting these criteria were categorised as impacted.

The inverse correlation observed between IMTM apical surface area and age is consistent with previous findings in impacted and non-impacted MTM [9], maxillary canines [16], and studies based on 2D imaging [13, 14, 17]. These results confirm the developmental pattern in which apical surface area decreases as root formation progresses. The decrease in surface area between ages 15 and 18 was the most notable, with the steepest decline between 17 and 18 years. These findings support the application of ASAM, regardless of impaction status, as an indicator of dental maturity for age estimation, especially in identifying and distinguishing individuals having attained the age of majority (18 years) in the Malaysian population [4].

Despite similarities in the general pattern, apical narrowing in IMTM was relatively more gradual than in ungrouped MTM [9]. The direct relationship observed between impaction depth and apical foramen measurement corroborates earlier reports of developmental delay in IMTM [5, 18]. The present study further demonstrated that, although the difference was subtle, the delay at level C was particularly significant, as same-age comparisons across levels, together with the effect size, provided credible support for this observation. Friedrich et al. [19] proposed that such delays are more likely attributable to physical factors, such as limited eruption space or mechanical obstruction, rather than pathological causes. This reflects the broader physiology of tooth eruption, in which root development is closely associated with eruption and may be halted or slowed when the path is obstructed. Interestingly, despite this delay, the strength of the correlation between ASAM and age remained consistent across impaction depths. This seemingly paradoxical finding presents a new challenge, particularly in estimating age for level C MTM, as current methods may yield misleading results with wider margins of error.

Previous studies on IMTM have reported population differences in the distribution of impaction levels [20,21,22]. In the present study, which included Malaysian nationals from the west coast of Peninsular Malaysia, level B was the most frequent impaction, which contrasts with findings from a northeastern Malaysian population, where level A was most common, followed by B and C [22]. Another pattern among Asian-Indian groups was reported with a more even distribution across levels B (39.0%), C (34.0%), and A (27.0%) [21]. Such variations may, in part, be explained by age differences between study samples, as younger individuals are less likely to exhibit level A impactions due to incomplete root development, whereas older samples, such as those in the northeastern study, are more likely to do so. Biological variation among populations and potential misclassification due to inconsistent classification criteria may also have contributed. To minimise such inconsistencies and improve repeatability, the modified level B/C category was introduced.

One key implication of comparing disproportionate samples is the potential for bias, as smaller sample sizes may inadequately represent the population [23]. In the present study, this concern was partly mitigated by the comparable and consistent ages across the four groups. While standardising sample sizes across levels could allow for more balanced and comprehensive analyses, non-parametric tests remain appropriate when the assumption of normality is not met. In addition, this limitation was addressed through the use of effect size, which reflects the magnitude and practical significance of the findings, particularly the ASAM difference observed at level C.

Another limitation of this study was the exclusion of other impaction parameters such as angulation [24] and ramus position [10]. Other factors, including sex and root number, were also not considered. While some studies suggest these may influence development, their effects appear minimal when assessed independently [6]. Their combined impact remains unclear; however, future studies using multivariate approaches with these parameters could provide a more robust understanding of their potential interactions and overall implications for dental age estimation.

In conclusion, this study demonstrated a clear inverse correlation between the apical surface area of IMTM and chronological age, with the most marked reduction occurring around 18 years. A significant delay was evident only in level C impactions, and IMTM can therefore serve as reliable indicators of maturity for age estimation, particularly in identifying individuals at or above 18 years of age. However, caution is required when assessing level C impactions due to their delayed development and the potential for increased estimation error.