External and internal morphological analysis in fused-rooted mandibular second molars: a micro-computed tomographic study

Abstract Objectives To investigate the root canal morphology of fused-rooted mandibular second molars (MSMs) based on the pulp chamber floor (PCF) and to analyze the correlation between external morphology of the radicular groove and internal morphology of the PCF and root canal configuration. Materials and Methods Two hundred and ninety-one fused-rooted MSM were selected from a Chinese population, and were scanned and reconstructed using micro-computed tomography. The morphology of the PCF was observed under a dental operating microscope (DOM). The classification of the PCFs and root canals configuration were identified according to modified Min’s and Gao’s classifications, respectively. A new radicular groove classification was proposed. The correlation among these morphological characteristics were investigated using the Chi-square test and Fisher’s exact test (α = 0.05). Results A total of 216 (74.2%) teeth had C-shaped PCFs; 61 (21.0%) had NC-shaped PCFs; and 14 (4.8%) had no PCF. The root canals were categorized as follows: 109 (37.5%) teeth were identified as merging type, 119 (40.9%) as symmetrical type; 43 (14.8%) as asymmetrical type, 6 (2.0%) exhibited taurodontism, and 14 (4.8%) had a single root canal. Statistical analysis showed a correlation between the type of PCFs and the root canals (P = 0.000). For teeth with C-shaped PCFs, the majority of the root canal was classified under merging type and symmetrical type, and asymmetrical type was not identified in NC-shaped PCFs. Besides, the results revealed a morphological association between the root canals configuration and radicular grooves types (P = 0.000). Conclusions The internal morphology of fused-rooted MSM is very complex. To a certain extent, it is feasible to predict the morphology of root canal by using PCF observed under a DOM. Clinical Relevance An understanding of the morphological aspects of the root canal based on the PCF will assist the clinician in the prediction and identification of canals below the visible orifice and further achieve more predictable clinical outcomes.