The aim of this study was to investigate the 3-year morbidity of coronectomy of the lower third molar and to monitor the behaviour and migration pattern of the retained roots postoperatively. A total of 92 patients (111 teeth) who had undergone a coronectomy between October 2005 and July 2009 were investigated. Patients were followed up at 3 months and 1, 2, and 3 years for clinical evaluation and dental computed tomography imaging of the coronectomy sites. In total, 10 cases (9%) required tooth root extraction within the 3 years after coronectomy. In seven of them, the distal pocket of the lower second molars remained connected to the roots within the first year. Of the cases in whom a pocket did not remain at an early stage, none showed peri-apical lesions on transmission images of the retained roots in the apical area, which usually result from necrosis of the pulp. Root migration increased in the first 2 years after coronectomy but stabilized between the second and third years. In addition, a significant difference was noted in root migration between patients of different ages and sex. Retained roots after coronectomy in the lower third molars led to no complications in terms of infection or the development of pathologies within the first 3 years postoperatively.
Extraction of the mandibular third molar, or wisdom tooth, is a relatively minor surgery performed daily in our oral and maxillofacial surgery department. The incidence of postoperative complications such as abnormal sensation and numbness is 1–5%. Such symptoms can reduce the quality of life of patients and sometimes result in a malpractice lawsuit. When performing a mandibular third molar extraction, accurate assessment of the positional relationship between the wisdom tooth and the mandibular canal is necessary to prevent the development of paraesthesia due to injury to the inferior alveolar nerve (IAN). In recent years, dental computed tomography (CT) has been used to obtain detailed information on the positional relationship between the wisdom tooth and the mandibular canal, making it possible to predict the risk of paraesthesia. However, the actual safety measure taken at many dental clinics to prevent paraesthesia is to perform the wisdom tooth extraction carefully. At the same time, coronectomy has received considerable attention in recent years.
When mandibular third molar extraction is likely to damage the IAN, a coronectomy is performed to remove only the crown of the tooth, leaving the root in place. Compared with the conventional extraction method, coronectomy has been found to effectively prevent nerve damage in several randomized clinical trials. Long et al. recently performed a systematic review and meta-analysis of coronectomy and the conventional extraction method and reported the utility of coronectomy as a surgical procedure for preventing nerve damage during wisdom tooth extraction. Our department also performs coronectomy in patients showing contact between the mandibular canal and the third molar on dental CT, and the authors have previously reported the efficacy of coronectomy for preventing nerve damage.
In this study, the authors assessed the retained roots and surrounding tissue on dental CT to investigate the long-term outcome (up to 3 years) of coronectomy in patients who underwent this procedure between 2005 and 2009.
Materials and methods
Coronectomy was performed on 111 teeth in 92 patients (33 teeth in 29 men and 78 teeth in 63 women; mean age 33.8 years), between October 2005 and July 2009. This study included patients who had complete annual follow-ups for 3 years and excluded those who did not return for follow-up observation.
Indications for coronectomy and surgical method
Dental CT is performed in our department when a close positional relationship between the mandibular third molar and the mandibular canal is suspected on panoramic radiographs. Coronectomy is indicated in the presence of at least one of the following seven findings: deviation of the canal; narrowing of the canal; peri-apical radiolucent area; narrowing of the root apex; darkening of the root apex; curving of the root apex; and loss of lamina dura of the canal.
Dental CT images were obtained for 301 patients (365 teeth). The authors excluded patients for whom there was distance between the root and the IAN (146 teeth in 125 patients). When contact between the mandibular canal and the wisdom tooth was verified on dental CT, patients chose traditional extraction (46 teeth in 34 patients) or coronectomy (173 teeth in 142 patients).
Coronectomy was started using the same ordinary incision and tissue separation techniques used to extract an impacted tooth and form a mucoperiosteal flap. The crown of the tooth was then removed and the resection surface was trimmed to 3–4 mm below the edge of the bone to ensure no remaining enamel. Primary closure of the extraction wound was performed by periosteal release to ensure it was tension-free. The authors did not treat the pulp of the retained root. All patients were operated on by the same specialist in oral and maxillofacial surgery certified in Japan. A 3-day course of antibiotics was prescribed after the coronectomy for the prevention of postoperative infection (cefcapene pivoxil hydrochloride hydrate).
Evaluation and analysis
The coronectomy was assessed on the basis of clinical evaluation, panoramic radiography, and dental CT performed at 3 months and 1, 2, and 3 years after the procedure. Evaluation items are listed below.
Analysis of cases with root extraction
Histopathological testing was performed to investigate the conditions of cells and tissues in the root in cases where the extraction of the retained root was necessary during the postoperative observation period and where the root was extracted en bloc.
Condition of the retained roots and surrounding tissue at the 3-year follow-up
Gross clinical examination for infection was performed at 3 months and 1, 2, and 3 years after coronectomy. In addition, dental probes were used to directly palpate the surrounding tissue and periodontal probes were used to examine the distal pocket (≥4 mm) of the mandibular second molar. Diagnostic imaging was performed to evaluate coronal bone formation over the retained root, the condition of the root apex, and contact with the mandibular canal.
Analysis of root migration during the 3-year follow-up
Dental CT images were used to set the baseline for calculating the distance of root migration. Three lines were identified on follow-up CT images: line 1 was tangent to the distal part of the mandibular second molar; line 2 connected the root apex and the centre of the crown in the mandibular third molar; and line 3 was perpendicular to line 1 and passed through the root apex of the mandibular third molar. If the root of the mandibular third molar was bifurcated, lines 2 and 3 were generated from the midpoint of the bifurcation. After placing these lines on the CT images, a specific formula was used to calculate the migration distance ( Fig. 1 ).
The statistical analysis was performed using IBM SPSS Statistics for Windows, version 19.0 (IBM Corp., Armonk, NY, USA). Migration distances were compared using Friedman’s repeated measures two-way analysis of variance (ANOVA). To reveal the factors influencing root migration, the Mann–Whitney U -test was used to compare results by sex as an independent variable, and Kruskal–Wallis one-way ANOVA was used to perform multiple comparisons of age, axial angle, root morphology, and depth of the impacted root. Significance was set at P < 0.05.
Of the 173 teeth in 142 patients who had undergone coronectomy between October 2005 and July 2009, the authors followed 111 teeth (64.2%) in 92 patients. Despite our requests, 50 patients (62 teeth) did not return for follow-up observation for various reasons and were excluded from the study. None of the excluded patients contacted us even though all coronectomy patients had been requested to inform the department in the event of abnormal postoperative complications. The state of the impacted wisdom tooth and its positional relationship to the mandibular canal were analyzed using patient information and preoperative panoramic radiographs ( Table 1 ). The mean age of the patients was 33.8 years.
|Patients ( n = 92)
Teeth ( n = 111)
|Depth of impaction (Winters class)|
|Position A||47.7 (53/111)|
|Position B||37.8 (42/111)|
|Position C||14.4 (16/111)|
|Radiographic signs of an increased risk of inferior alveolar nerve injury|
|Deviation of the canal||19.8 (22/111)|
|Narrowing of the canal||30.6 (34/111)|
|Peri-apical radiolucent area||0.9 (1/111)|
|Narrowing of the root apex||1.8 (2/111)|
|Darkening of the root apex||25.2 (28/111)|
|Curving of the root apex||15.3 (17/111)|
|Loss of lamina dura of the canal||38.7 (43/111)|
Development of postoperative paraesthesia
One subject (1.0%), a 28-year-old woman, developed hypoesthesia of the left lower lip on postoperative day 2, but the hypoesthesia improved after 2 months.
Extraction of retained roots
Of the 111 coronectomy cases, 10 roots (9.0%) in 10 patients (two men and eight women; mean age 37.8 years) were extracted for incomplete wound closure due to a remaining pocket connected to the root ( Table 2 ). The roots in seven patients were palpable. The surgical wound in two patients was closed at one point, but the root was exposed and became palpable due to migration during the first postoperative year. The last patient developed pulpitis in the retained root due to a dry socket. Extraction of the retained roots was performed within 3 months of surgery in three patients, between 3 months and 1 year after surgery in five patients, and between 1 and 2 years in two patients. The surgical sites stabilized after 2 years, resulting in no need for root extraction. No postoperative sensory deficit developed in any patient. Histopathological examination of the root was possible in seven of the 10 cases, and the pulp and root appeared to have been vital in five cases. The patient with the dry socket showed mild infiltration of inflammatory cells in the pulp, and three patients showed dentine bridge formation over the vital pulp of the resected wisdom tooth.
|Case||Age, years||Gender||Root morphology||Axial angle||Cause||Post-surgery month of extraction||Pulp||Dentine bridge|
|1||38||F||Horizontal||A||Incomplete wound closure||7||Vital||+|
|2||34||F||Angular||B||Incomplete wound closure||12|
|3||39||F||Horizontal||A||Incomplete wound closure||12||Vital||+|
|4||38||F||Horizontal||A||Incomplete wound closure||3||Non-vital||−|
|5||37||M||Horizontal||C||Incomplete wound closure||3.5|
|7||40||F||Angular||A||Incomplete wound closure||10||Vital||−|
|8||33||F||Horizontal||A||Incomplete wound closure||24||Vital||+|
Conditions of the retained roots and surrounding tissue at the 3-year follow-up
Diagnostic imaging findings, such as contact between the mandibular canal and the mandibular third molar apex, coronal bone formation of the retained root, and abnormal transmission images of nearby bones, were evaluated in 101 cases for up to 3 years after surgery ( Fig. 2 ). Retained roots were no longer in contact with the mandibular canal in 56 cases (55.4%) at the 1-year follow-up, in 64 cases (63.4%) at the 2-year follow-up, and in 69 cases (68.3%) at the 3-year follow-up. Coronal bone formation was visible in 86 cases (85.1%) at the 1-year follow-up, 99 cases (98.0%) at the 2-year follow-up, and 99 cases (98.0%) at the 3-year follow-up. In the two cases with no bone formation at the 3-year follow-up, part of the crown could not be removed because it was attached to the IAN. However, as there was no root exposure into the oral cavity and no findings indicative of infection, these patients were placed under observation. In addition, no abnormal radiolucent images were observed in the bone surrounding the retained root in all cases. Three years postoperatively, the pocket of the mandibular second molar in eight of the 101 cases (7.9%) was ≥4 mm; five of these eight cases had a prosthetic crown. Advanced periodontal disease was thought to be the cause of the pocket in the remaining three patients, who were placed on follow-up observation because no pus or inflammation of the surrounding gum was observed.
Root migration during the 3 years after surgery
In the 101 cases, mean root migration was 1.84 mm at the 3-month follow-up, 2.88 mm at the 1-year follow-up, 3.41 mm at the 2-year follow-up, and 3.51 mm at the 3-year follow-up, with a significant difference in the values between postoperative month 3 and year 1 ( P < 0.001) and between years 1 and 2 ( P < 0.001) ( Fig. 3 ). However, no significant difference was observed between years 2 and 3.
No root migration was observed in 25.7% of cases from the 1-year to the 2-year follow-up and in 82.2% of cases from the 2-year to the 3-year follow-up ( Fig. 4 ). These findings suggest active root migration and bone formation over the resected surface within the first year after surgery and that subsequent root migration decreases and eventually stabilizes.
At the 3-year follow-up, factors significantly associated with root migration were sex and age ( Table 3 ). The migration distance was 3.71 mm in women and 3.08 mm in men, with a significant difference between the sexes ( P = 0.030). Migration by age group was 4.28 mm, 3.21 mm, and 2.83 mm for subjects aged ≤29 years, 30–39 years, and ≥40 years, respectively. A significant difference was observed between subjects aged ≤29 years and those aged 30–39 years ( P = 0.001) and between subjects aged ≤29 years and those aged ≥40 years ( P < 0.001). No significant differences in root morphology, axial angle, or depth of impacted root were observed at 3 years after surgery.