Clinical Case - Infratemporal Fossa & Oral Cavity

An 18-year-old woman went to see her dentist for a regular checkup. During the visit, the doctor discovered that her lower third molars ("wisdom teeth"), which had appeared to be erupting fine on her last visit, had begun to cause crowding in the patient's mouth. He was afraid that they might eventually cause her discomfort, and since her teeth were currently straight, he recommended that she have the third molars extracted. She agreed, and returned the following week for the procedure. Before the dentist began to work, the patient told him that she had a low pain tolerance and asked that he make sure that her mouth was fully anesthetized. The dentist promised that he would take care of it , and then proceeded to inject the anesthetic in the mucous membrane on both sides of the patient's mouth. The teeth were removed without incident; however, when the dentist finished, the patient found that she could not close her mouth. The dentist apologized, saying that he must have dislocated her jaw, but that he could fix it easily and she would suffer no lasting effects. He then reduced the dislocated jaw by pressing downward on the remaining molars until the mandible slipped back into place. He then warned her not to attempt to chew any food until the anesthetic had worn off and she was able to feel her tongue and lower lip again, lest she damage them by chewing on them.

Questions to consider:

1. What nerve(s) would need to be anesthetized in order to prevent pain during a lower third molar extraction?

   The inferior alveolar nerve, a branch of the mandibular nerve (V3), provides sensory innervation to all of the teeth of the lower jaw.

2. Why were the patient's lower lip and tongue numb?

   The lower lip was numb because the chin and lower lip receive sensory innervation from the branch of the inferior alveolar nerve called the mental nerve. When the inferior alveolar nerve is anesthetized near the mandibular foramen, as in most dental procedures, all sensory stimuli from branches distal to that point will be blocked. The tongue (as well as the mandibular gingiva) is deadened at the same time that the inferior alveolar nerve is anesthetized because the lingual nerve passes beneath the mucosa in close proximity to the inferior alveolar nerve medial to the ramus of the mandible.

3. What joint was dislocated during the tooth extraction and how did this occur?

   The temporomandibular joint (TMJ) was dislocated during the tooth extraction, probably due to excessive downward stress during the procedure that caused the head of the mandible to pass anterior to the articular tubercle to a position beneath the zygomatic arch. Downward and backward pressure on the patient's lower molars generally reduces this dislocation fairly easily.

4. Some patients occasionally experience temporary paralysis of the muscles of facial expression or of mastication following a dental procedure. Why do you think this might happen?

   If a sufficiently large amount of anesthetic is injected (or if the parotid fascia is pierced by the needle), it may spread through the soft tissue to the parotid gland, where the facial nerve will be anesthetized, leading to paralysis of the muscles of facial expression. An analogous event would be true for paralysis of the muscles of mastication.

5. When dental work is performed on the lower teeth, anesthetic injected at a single point is often adequate to produce complete local anesthetization of all the teeth on a given side. The same is not true for the upper teeth. Why not? How do they have to be anesthetized?

   The teeth of the upper jaw are innervated by a series of branches from the posterior, middle, and anterior superior alveolar nerves, which are derived from the maxillary nerve (V2). There is no convenient way to anesthetize the entire maxillary nerve and thus provide nerve block to all of the maxillary teeth, as can be done with the mandibular teeth. Instead, the nerves to each tooth must be anesthetized by injecting the anesthetic into the gingiva around each tooth.

6. What are the anatomical structures that prevent joint dislocation?

   The fibrous capsule of the TMJ is thickened laterally to form the lateral ligament, which reinforces the lateral part of the capsule. Other ligaments (the stylomandibular and sphenomandibular) have a minor role in stabilizing the joint.

You were asked to assess the case of 20-year-old woman whose impacted right lower wisdom tooth was surgically removed. The operation lasted about an hour, and the dental surgeon suspected that some nerves might have been bruised during the operation. The patient presented with loss of sensation in the gums of her lower jaw, and her mouth was slightly dry. You examined taste sensation in the tongue and found that it was diminished in the anterior two-thirds but it was normal in the posterior portion.

Questions:

1. Which nerve is most likely to have been bruised in this patient?
   

   The lingual nerve, which is a branch of the posterior division of the mandibular nerve (CN V₃).
   

2. How would you explain the patient's complaints?
   

   The lingual nerve supplies general sensory fibers to the anterior two-thirds of the tongue, the floor of the mouth and the gums of the mandibular teeth. It carries the chorda tympani nerve, which carries taste fibers from the anterior two-thirds of the tongue and parasympathetic innervation to the submandibular and sublingual salivary glands.
   

3. Which nerve is responsible for sensation in the posterior third of the tongue?
   

   The glossopharyngeal (CN IX) supplies taste and general sensation to the posterior third of the tongue.