Extraction of the mandibular third molar is one of the most common dental surgeries. Postoperative swelling is a common complication after surgery that affects the physical, psychological, and aesthetic quality of life for a few days. The adverse effects on quality of life were reported to be three-fold higher with postoperative complications¹. Even if facial swelling is not prominent after surgery, postoperative edema may occur from exudates or transudation resulting from surgery. Because postoperative edema and pain are significantly less severe with smaller incisions², postoperative edema related to surgical incisions is an area of interest. Some studies reported more severe postoperative swelling with a vertical incision^3-5. However, others showed that surgical flaps do not appear to have a significant role in edema^6-8.

When evaluating postoperative swelling after third molar extraction, several limitations need to be overcome, including the surgical control strategy. Strategic considerations concern the incision, odontomy, ostectomy, suture pattern and technique, and postoperative care along with surgical difficulty and the quantifying method for evaluating swelling^2,7. Several different techniques have been used to measure postoperative swelling including visual measurement^5,7, patient questionnaire⁸, direct measurement by a ruler⁹, and magnetic resonance imaging (MRI)¹⁰. Among these, MRI is the most objective and reproducible assessment tool, but the high cost may limit its use. In 2021, Jeong et al.¹⁰ suggested qualitative analysis by MRI by classifying postoperative edema based on its anatomic division: one anatomic component (buccinator muscle) and four fascial spaces (supra-periosteum space, buccal space, parapharyngeal space, and lingual space). These researchers reported that most postoperative edema spread though the buccinator muscle and supra-periosteum space¹⁰ and suggested that an incision that avoided these anatomic structures could reduce the level of postoperative edema.

A few studies have reported significant adverse effects of third molar surgery in a comparison of incision techniques^2,6. Kleinheinz et al.¹¹ made proposals concerning incision design according to cadaver results based on the vascular area that traverses parallel to the alveolar ridge in the vestibulum. Depending on the degree of inflammation and micro-leakage, vertical incisions may create discontinuity of these vascular structures and affect postoperative edema¹². Buccal envelope flaps without a vertical incision and modified triangular flaps with a vertical incision have been used as traditional approaches for impacted third molars.

Few studies have evaluated postoperative swelling by MRI, the most accurate diagnostic tool. Several issues deserve consideration when comparing surgical approaches, including surgical difficulty. Kim et al.¹³ proposed a more practical classification system that considered the validity of the existing classification. In 2020, Ku et al.¹⁴ validated the modified difficulty index for an impacted third molar based on the molar’s three-dimensional relationships in space and with the ramus, as well as its depth, associated pathologic conditions, and surgical time with a unified extraction method. The present study examined the effects of a vertical incision on comparative split-mouth MRI evaluation of postoperative edema after third molar extraction with homogeneous difficulty based on the three-dimensional modified difficulty index.

The Institutional Review Board of the Armed Forces Capital Hospital approved this study (AFCH-20-IRB-012). This study was conducted according to the principles of the Declaration of Helsinki for research on humans. Written and verbal informed consent was obtained from each patient.

Two patients with homogeneous bilateral impacted mandibular third molars assigned the same grade based on the modified difficulty index by computed tomography¹⁴ were enrolled in this study. These patients had undergone facial MRI within 24 hours after extraction surgery under general anesthesia.

Regarding postoperative swelling, considerable attention has been given to radiography, dental morphology and depth of impaction, patient age, and experience of the surgeon, but few studies have analyzed incision design². In this split-mouth study, the effects of a vertical incision were noted while controlling other variables. Two pairs of homogeneous extractions showed that a vertical incision was qualitatively and quantitatively related to extensive postoperative edema and additional buccal space edema.

The results of some randomized controlled trials (RCTs) conducted on postoperative swelling after a vertical incision (triangular flap) have been controversial. In 2007, Kirt et al.³ reported greater extraoral swelling after a vertical incision. In 2012, Baqain et al.⁴ found significantly greater facial swelling and reduction of mouth opening after a vertical incision. In 2021, Xie et al.⁵ observed milder swelling in the absence of a vertical incision. In accordance with the RCTs for evaluating postoperative swelling, the two patients in this study showed more severe postoperative edema on the vertical incision side. This edema spread to the buccal space, which can cause facial swelling¹⁰.

In contrast to the second patient who complained of facial swelling and pain on the left side with a vertical incision, the first patient did not complain of facial swelling and pain, even though MRI showed more severe postoperative edema on the right, vertical incision side. Thus, clinical postoperative edema, which is expressed as facial swelling or patient discomfort, may not reflect the actual amount of edema around the surgical site. Studies have suggested that there is no relationship between postoperative swelling and flap design, although that could be due to underestimated postoperative edema measured from facial swelling according to method^7-9.

In the present study, a vertical incision caused more severe postoperative edema, supporting an anatomic rationale. The discontinuity of the parallel vascular system could impair circulation and cause leakage of edema toward adjacent fascial spaces, such as buccal, parapharyngeal, and lingual spaces^10-12. Among these spaces, the buccal space might be associated with postoperative facial swelling because of its location on the buccal side of the mandible. Although facial swelling and discomfort vary by patient depending on the extent of postoperative edema and anatomic characteristics, a flap design without a vertical incision is recommended to prevent postoperative spread of edema to the buccal space.

This study qualitatively showed that a vertical incision was associated with edema spreading to the buccal space. In addition to the incision design, the extraction time was longer and the extent of postoperative edema was greater in the second patient. A prolonged operation time can increase postoperative complications¹⁶. In general, a triangular flap with a vertical incision could provide a comprehensive surgical field that could reduce the operation time. Further study is needed to determine if flap design can prevent edema spread to the fascial space even if extensive swelling occurred due to aggressive surgical damage or prolonged operation time.

In conclusion, MRI can qualitatively evaluate postoperative edema by anatomical space. Within the limitations of this study, the flap design without a vertical incision helped prevent the spread of edema toward the buccal space after mandibular third molar extraction.