The extraction of mandibular impacted third molars, a routine outpatient surgery performed by oral and maxillofacial surgeons, is associated with common complications, such as trismus, pain and infection (1). The most severe cases of trismus usually occur at 2 days post-operatively. The primary causes include elevation of flap beyond the external oblique ridge, low-grade infection following local anesthesia and repeated stimulation of the medial pterygoid muscle (inferior alveolar nerve block), as well as other causes (2). The symptom of trismus is alleviated by the post-operative local injection of dexamethasone (3,4). Generally, trismus is gradually alleviated or disappears within approximately 1 to 2 weeks post-operatively; however, in very rare cases, trismus persists for >1 month. The present study reports the case of a patient who exhibited trismus for 45 days following mandibular third molar extraction. The patient received local and systemic anti-inflammatory treatment, as well as incision and drainage therapy under local anesthesia. Furthermore, the factors associated with the occurrence and development of trismus were also analyzed, and appropriate management strategies are discussed in order to provide an effective treatment method for affected patients, as well as to prevent the occurrence of trismus in the future.

The extraction of third molars can easily lead to tissue injury, inflammation and other post-operative complications, among which trismus is one of the most common (1). It is closely related to post-operative edema, and is typically substantially alleviated or disappeared within 1 to 2 weeks post-operatively. A number of factors may contribute to the onset of trismus. For example, during the extraction of mandibular third molar from the buccal side, the muscle tendon may be severed; this painful stimulus causes muscle protection, which results in trismus (2). Moreover, during inferior alveolar nerve block anesthesia, masticatory muscle spasms may be caused by incomplete disinfection and infection. Brooke (6) reported that multiple needle injections during anesthesia may cause inflammation in that area, while the stretching of intraoral muscles and muscle spasms may be elicited by the use of narcotic drugs. However, the symptoms of progressive trismus in the patient presented herein were not greatly affected by the above factors.

Early during the course of the disease, the patient of the present study exhibited mild trismus. However, this did not receive extensive monitoring as it was presumed to be related to postoperative edema and anxiety. However, when more severe trismus was observed in combination with marginal osteomyelitis, the attending physician intervened with a treatment plan for management of marginal osteomyelitis. Due to the delayed diagnosis by the attending physician, the disease treatment was delayed, which affected the patient's post-operative recovery and daily activities. Based on the intraoperative findings, the absence of the lingual bone plate in the apical region was presumed to be the main cause of marginal osteomyelitis. It was hypothesized that the following was the possible cause of trismus: An infection after tooth extraction spreads along the lingual space of the mandible. Inflammatory mediators can cause irritation associated with muscle tendon attachment to the mandibular ramus and the anterior section of the medial pterygoid muscle, thereby resulting in masticatory muscle spasms that lead to trismus.

For the patient in the present study, at 33 days post-operatively, the surgeon performed incision and drainage in the area of tooth 38 to eliminate a large amount of bloody exudate. The progressive trismus was presumed to result from long-term contact between low-toxicity inflammatory exudate and the muscle tendon. Tight suturing had been performed at the end of surgery. Notably, the inflammatory exudate could not drain with sufficient speed; thus, it spread along the lingual space of the mandible and interacted with the medial pterygoid muscle, which caused fascial space infection associated with marginal mandibular osteomyelitis. Treatment of chronic osteomyelitis has been a major focus in the field of maxillofacial surgery; the main sources of chronic osteomyelitis include odontogenic infection and tooth extraction-related infection (7). Bamberger encouraged the use of systemic antibiotic treatment for 4 weeks post-operatively to manage chronic osteomyelitis; using this approach, surgical treatment was suggested to be unnecessary (8). By contrast, Baur et al (7) reported that surgical treatment was generally more effective than single drug treatment; moreover, conservative drug treatment often leads to multiple recurrences of chronic osteomyelitis. For the patient presented herein, the incision and drainage in the area of tooth 38 achieved ideal results by combination with drug treatment; this was potentially due to the fact that the patient was a young adult with robust overall health and the drainage of inflammatory exudate was performed relatively rapidly. Importantly, long-term follow-up was necessary for our patient.

Tolstunov et al (9) found that the thickness of the lingual bone plate of mesial or horizontal impacted mandibular third molars was thinner than that of vertical impacted mandibular third molars; thus, they stated that the lingual bone plate should be protected during tooth extraction procedures. Lang et al (10) reported that the post-operative use of antibiotics could effectively reduce the incidence of infection. If tooth extraction causes extensive trauma and a large volume of blood is lost, a drainage strip should be placed in the wound for 1 day to prevent infection. This approach also may aid reducing postoperative edema of the patient's cheek. However, some studies have demonstrated that when the mandibular third molar is tilted to the lingual side, less post-operative inflammation will occur when the lingual approach is used to remove the lingual bone plate, compared with the buccal approach; thus, edema and pain will be significantly reduced (11,12). Accordingly, during the extraction of impacted third mandibular molars, protection of the soft and hard tissues around the buccal and lingual sides should be prioritized and trauma should be minimized to avoid unnecessary injury. When fracture or loss of a lingual bone plate occurs, close monitoring should be performed during follow-up to ensure timely management of symptoms.

In conclusion, the extraction of an impacted mandibular third molar is a very common oral surgery procedure. During the surgery, the surgical area should be protected, and careful assessment is necessary regarding fracture or loss of the lingual bone plate. Effective drainage of inflammatory exudates should be performed to prevent muscle spasms and trismus caused by chronic contact with low-toxicity inflammatory exudates.