Injection Needle Injury of Oral Sensory Nerves
The ADA estimates conservatively that there are over 300 million local anesthetic injections given each year by dentists in the U.S. Of the 300 million-plus injections, an estimated 50% were likely directed to the inferior alveolar nerve supplying the lower teeth, floor of the mouth, buccal mucosa, tongue, lip, and skin of the chin (see Figure 1). Since many pain conditions involving the trigeminal nerve also exhibit referred pain, it is important to the physician and dentist to discriminate from pain at the temporomandibular joint, ear, or lower lip and teeth. Some medical schools such as Quillen College of Medicine in Johnson City, Tennessee also teach dental injections to increase the family physicians’ role with regard to minor oral trauma, as well as temporary pain relief in rural settings. A physician friend who graduated from Quillen commented to me that it was frustrating to attempt to give the inferior alveolar nerve (IAN) block since the region seemed vague and very difficult to relate to the anatomical landmarks given. He and other medical students found the block to be very difficult to implement. Even for an experienced dentist, it can still be a difficult block to give since the anatomical landmarks are not static, and reference points for injection can vary from patient to patient. Therefore the block is essentially being given “blind” since there is no other way to administer the injection. Approximately one patient in 26,000 experience a needle stick, or puncture of the nerve.2 It is speculated that in some instances, the doctor may accidentally inject the epimesium of the nerve with local anesthetic with resulting toxic injury as well. The current literature suggests that the injury to the IAN or the lingual nerve is either a result of needle puncture, needle nick, partial transection, bent needle tip being withdrawn through the nerve after contacting bone, or a hypersensitivity reaction to the local anesthetic or to the preservative often found in the local anesthetic. The aforementioned preservative is included in the anesthetic medium to stabilize the epinephrine when added as a vasoconstrictor and so extend the anesthetic duration and intensity. Without a preservative, the epinephrine would only be good for 48 hours or so. Dental anesthetics now contain sodium bisulfite as a preservative for the epinephrine whereas in multiple dose bottles the preservative is still methylparaben. Methylparaben was at one time the preservative found in dental anesthetic carpules as well.
The more serious the actual injury to the nerve, the less likely it will heal and be resolved over time. Currently, most experts invoke the 6-week time-line as the point of no return for healing of the nerve with a corresponding diminution of symptoms. If the nerve has not healed itself in six weeks, then an effort to help stimulate the nerve potential to heal is made with several medications and protocols. Much research is being done in an effort to understand nerve injury and repair. Currently, researchers are studying a neurotransmitter protein found throughout the human body called neuronal nitric oxide synthase (nNOS). This chemical compound is ubiquitous and has a number of functions. There is some evidence that persisting nerve pain suffered by a patient is a result of migration of nNOS from the cell body to the axonal site of nerve injury. It is believed that the nNOS accumulates at the site of injury and, with enough concentration, causes ectopic activity or hypersensitization of the nerve membrane.3
There is also research that shows sprouting of large diameter afferent pain fibers in the medullary dorsal horn from laminae III-IV to lamina II in the spinal cord at seven days after nerve injury. This may be related to the condition following peripheral nerve injury whereby allodynia occurs—a phenomenon where non-noxious touch stimuli may induce a pain sensation.4
When there is IAN injury, the patient may experience numbness throughout the distribution of the IAN or the lingual nerve, or the patient may have a sharp pain or even a burning sensation. Interestingly, many patients who exhibit injury of the IAN or lingual nerve find that the act of chewing gum or food seems to temporarily abate the pain. The likely reason for this is that different mechanoreceptors have different threshholds for activation. Neurophysiologists explain that pressure, temperature, and touch receptors have a competitive dominance over pain receptors. Even though chewing often turns off the pain receptor, it is only a temporary benefit.
Half of all patients who suffer IAN or lingual nerve pain do not remember an electric shock feeling while the needle was inserted into the area of the nerve. Approximately 70% of injection injuries affect the lingual nerve and 30% affect the IAN. The increased lingual nerve involvement is likely due to the anatomy of the lingula of the mandible where the course of the lingual nerve is not as protected, and the monofascicular nature of the lingual nerve at the site of injection.2
A 60-year-old female patient presented with a long history of multiple general surgery procedures. To the best of her knowledge, she has had right TMJ surgery twice—one in 1994 and the second in 1997. Her persisting pain was located at her right TMJ and the right IAN.
Examination revealed a strain or sprain of the right temporal tendon at the mandibular coronoid process. The pain was localized at the TMJ and zygomatic arch and was remitted by local anesthetic infiltration of the lateral tendon of the temporalis muscle. It is a common experience for “TMJ” pain to be a result of strain or injury at the insertion of the lateral tendon of the temporalis muscle.
Once the temporalis pain was remitted, the IAN pain was addressed. A cardinal diagnostic key for IAN injury is relief of pain while chewing as was the case for this patient. The patient was informed of the nature of her problem and she agreed to have an IAN nerve infiltration with 3% nonvasconstrictor local anesthetic. The infiltration blocked the painful symptoms. The patient was advised of the potential anti-inflammatory benefit of Sarapin injection at the IAN site and the injection was given. Dr. Wesley Shankland of Columbus Ohio also prescribes Medrol dose pak and has had great success with the regimen of local anesthetic mixed with Sarapin and oral Medrol. In this patient’s case, it was difficult to determine when the IAN pain started since she had an overlying condition involving the right lateral temporalis tendon.
This patient was injected three consecutive days and was then dismissed to her home area. Follow-up was managed by phone and 60 days post-injection the patient was pain free. The patient had two conditions, right lateral Temporalis tendon pain, and right IAN pain. Both conditions resolved with injectable medications and did not require more advanced treatment.