Background
This doctoral thesis highlights the occurrence, morphological variations, and clinical aspects of permanent mandibular molars (PMMs) with a supernumerary root. First PMMs have two roots, one mesial and one distal, which are often fused in second and third PMMs, forming a pyramidal shaped root. Occasionally, a supernumerary root can develop on the lingual side (radix entomolaris, RE) or buccal side (radix paramolaris, RP) of the main roots. Supernumerary roots pose a diagnostic and therapeutic challenge in clinical dentistry and their implications extend to related biomedical sciences, including forensic dentistry, oral embryology, and paleodontology. This thesis aims to assess the frequency of both supernumerary roots in all three PMM classes, identify side- and sex-related differences in supernumerary root occurrence and analyse the external and internal root morphology of the affected PMMs in samples from Slovenia.
Methods
The frequency of PMMs with supernumerary roots was assessed retrospectively using intraoral radiographs (periapical and bite-wing) and medium field-of view cone-beam computed tomography (CBCT) scans of 1317 and 1315 individuals, respectively. The sex-, side-, and tooth class-related frequency differences were statistically evaluated. The external root morphology was analysed using the above noted CBCT scans and 17 extracted three-rooted first PMMs from the dental collection. The affected PMMs were categorised according to Carlsen and Alexandersen’s classifications, which consider the mesio-distal position of RE and RP in relation to both main roots. The extracted first PMMs with RE were further classified according to the relative length and curvature of RE. The internal root morphology was analysed in a comparative study of 17 three-rooted first PMMs and 70 age- and sex-matched two-rooted counterparts depicted on retrospectively collected CBCT scans and in a separate CBCT study of extracted teeth.
Results
CBCT study provided the following tooth-level RE frequency estimates for a population of Slovenia: 1.2% (0.7%–2.0%, 95% CI) for the first PMM, 0.9% (0.6%–1.5%, 95% CI) for the second PMM, and 4.9% (3.8%–6.2%, 95% CI) for the third PMM. RE frequency was significantly higher in third PMMs than in second or first PMMs. The respective tooth-level RP frequency estimates were 0.0%, 0.8% (0.5%–1.4%, 95% CI), 2.1% (1.4%–3.0%, 95% CI). RP frequency was significantly higher in third than in second PMMs. The frequency estimates of three-rooted first PMMs were comparable regardless of the radiographic method used. In contrast, the CBCT-based frequency estimates of second and third PMMs exhibiting supernumerary roots were considerably higher than those based on intraoral radiography (tooth-level frequencies of 1.6% vs. 0.3% for second PMMs and 6.5% vs. 1.8% for third PMMs). Results showed no significant male-female and side-related differences in the occurrence of supernumerary roots. In a subgroup of 206 individuals with a full complement of PMMs (bilateral presence of first, second, and third PMMs), at least one affected PMM was detected in 17% of the individuals. A high bilateral concurrency of RE was observed in first PMMs (62.5%). Among 34 first PMMs with RE (17 extracted and 17 depicted on CBCT scans), type A morphology was observed in 33 cases and type AC morphology in one case. In second and third PMMs with RE, type B and AC morphologies were the most prevalent. Considering RP, type A was more frequent in second PMMs, whereas type B predominated in third PMMs. We identified four third PMMs with two RE roots, a variant which we termed ‘double RE’, and three second and five third PMMs with coexistence of RE and RP. The RE unexceptionally contained a single root canal. Based on the comparative study, three-rooted first PMMs exhibit a tendency toward root canal system simplification in both regular roots, which includes a smaller number of identified canal system types, a significantly greater frequency of single-canalled distal roots and a significantly smaller frequency of three- and four-canalled mesial roots. In all extracted three-rooted first PMMs, the RE canal was severely curved bucco-lingually (mean curvature 48.48°) and the average divergence between RE and distal root canals was 53.14°. The average inter-orifice distance from the RE canal orifice to the distal root canal orifice was 3.59 mm (range 2.39–4.50 mm). The positional variability of the RE canal orifice was of the same magnitude in mesio-distal and bucco-lingual directions (approximately 2 mm). In the first PMM with type AC RE morphology, the RE canal orifice was located most mesially.
Conclusions
Overall, the results of four studies composing this doctoral dissertation contribute to better understanding of the occurrence and morphological variation of PMMs with supernumerary roots in a Slovene population. Their transfer into clinical practice will enable more reliable detection and more predictable management of the affected PMMs. The results indicate that supernumerary roots do not represent rare traits in the PMM district. Moreover, when treating both first PMMs in the same patient, a high bilateral concurrency of RE must be considered. Type AC RE represents a rare anatomical variant in this tooth, as confirmed by the results of our literature review. From the endodontic point of view, a special modification of the classical access opening design is required in this variant to gain an unobstructed passage to RE canal for endodontic instruments. In three-rooted first PMMs, a sever bucco-lingual curvature and lingual inclination of RE root canal increase the level of endodontic treatment difficulty. In contrast, the tendency toward root canal simplification in both regular roots facilitates the endodontic procedures. Finally, more complex external morphology of the furcal area challenges periodontal and oral surgery procedures.
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