This retrospective study included adult patients, who visited for the mandibular third molar extraction on the Department of Oral and Maxillofacial Surgery of Armed Forces Capital Hospital from May 2018 to February 2019. The inclusion criteria of the patients were as follows: (1) healthy status without any underlying disease, (2) impacted mandibular third molar which is horizontal (80 to 100°) with less than half of the third molar crown above the CEJ of the adjacent second molar and contacted with the inferior mandibular canal on computed tomography (Fig. 1) [8, 9], and (3) facial MRI within 48 h after surgical extraction of their mandibular third molar. The exclusion criteria were as follows: (1) simultaneous extraction of impacted maxillary third molar with incision, ostectomy, or odontomy, (2) poor oral hygiene control, and (3) uncontrolled systemic diseases.
Surgical extraction of the mandibular third molar was performed with operculectomy. After operculectomy, the extraction was performed through odontomy on the exposed third molar with or without additional mesial incision [8]. Patients were instructed to take oral antibiotics (625 mg, amoxicillin, Ilsung Pharmaceutical, Korea) and NSAID (500 mg, dexibuprofen, Samil Pharmaceutical, Korea) thrice daily for 5 days and daily mouth rinse with a chlorhexidine solution.
In this study, the facial MRI (Discovery™ MR750, GE Healthcare, USA) was taken within 48 h after the extraction. The MRI protocol included T2-weighted nonfat-saturated fast spin-echo sequence (TR, 2800 ms; TE, 90 ms; matrix, 320 × 224; slice thickness, 5 mm; gap, 1 mm; FOV, 34 × 34 mm2) and T1-weighted-echo sequence with fat suppression (TR, 3.5 ms; TE, 1.6 ms; matrix, 288 × 160; FOV, 40 × 28 mm2; section thickness, 5 mm). The postoperative edema space was determined by the high signal in the T2-weighted MRI image and the low signal in the T1-weighted MRI and was evaluated independently by one expert oral and maxillofacial surgeon (Y.K.J) and radiologist (B.S.H).
Ethical approval was approved by the Institutional Review Board at Armed Forces Capital Hospital (No. AFCH-19-IRB-008) and followed the STROBE Guidelines with the Helsinki Declaration.
In MRI, the spaces around the mandibular third molar were divided into the buccinator muscle, supra-periosteum space, buccal space, parapharyngeal space, and lingual space (sublingual and submandibular space) [10, 11]. Each space was defined as follows:
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1.
Buccinator muscle (Fig. 2a). An anatomical component organized from the origin (from the alveolar processes of the maxilla and mandible, buccinators crest, and temporomandibular joint) to insertion (in the fibers of the orbicularis oris muscle).
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2.
Supra-periosteum space (Fig. 2b). When enhancement is observed along the outer border of the mandible on MRI, it corresponds anatomically to the periosteum, but it is defined as the supra-periosteum space because the periosteum cannot be stretched enough to allow fluid collection and is attached firmly to the underlying bone.
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3.
Buccal space (Fig. 2c). The fascial space with a buccal fat pad consists of anterior (angle of the mouth), posterior (masseter muscle), superior (zygomatic process of the maxilla and zygomaticus muscles), inferior (depressor anguli oris muscle and attachment of the deep fascia to the mandible), medial (buccinator muscle), and lateral (platysma muscle, subcutaneous tissue, and skin) borders.
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4.
Parapharyngeal space (Fig. 2d). The fascial space consists of anterior (pterygomandibular raphe), posterior (deep lobe of the parotid gland), superior (lateral pterygoid muscle), inferior (attachment of medial pterygoid of the mandible), medial (medial pterygoid muscle), and lateral (medial surface of ramus of the mandible) borders.
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5.
Lingual space (sublingual and submandibular space, Fig. 2e). The fascial space on the lingual side of mandible consists of anterio-lateral (medial surface of the mandible), superior (mucosa of the floor of mouth and the tongue), posterior (hyoid bone), and inferio-lateral (platysma muscle and superficial layer of the deep cervical fascia) borders.
Statistical analysis
The inter-examiner reliability was calculated using Kappa statistics. Kappa values were rated as follows: < 0.200 was considered poor, 0.201–0.400 fair, 0.401–0.600 moderate, 0.601–0.800 good, and > 0.800 excellent [12]. Except for the inconsistent postoperative images of edema between the two examiners, the correlation of the postoperative incidence of edema among each space was analyzed using a Pearson Chi-square test. In addition, the differences between the patients who underwent extraction only mandibular third molar or with maxillary third molar, and under local or general anesthesia were analyzed using a Fisher exact test and Chi-square test. Two-sided P values of < 0.05 were considered significant. The analysis was performed using SPSS 25.0 for Windows (SPSS Inc., Chicago, IL, USA). The continuous variables were expressed as mean ± standard deviation (SD) and qualitative variables as absolute and percent frequencies.