Characterization of the Temporo-masseteric Nerve Block (TMNB) in Healthy Subjects

1.1 Purpose/Specific Aims The Temporo-masseteric Nerve Block (TMNB) is a local anesthetic injection targeting the deep temporal and masseteric branches of the mandibular division of the Trigeminal Nerve, that supply the temporalis and masseter muscles. Emerging evidence supports the TMNB's therapeutic potential in alleviating both acute and chronic myogenous pain of temporalis or masseteric origin. The TMNB injection is safe and well tolerated by patients.

The nerves targeted by the TMNB are mixed nerves and supply both sensory and motor innervation to the muscles. In addition, pain relief from the TMNB injection is sustained beyond the duration of the local anesthetic's duration of action. It is important to characterize the TMNB block, particularly its effects on the target muscles to understand the mechanism for its sustained pain relief.

The temporalis and the masseter muscles mediate jaw closure and clenching actions. The proposed pilot study is aimed at preliminary characterization of the TMNB injection in 20 healthy volunteers. Assessment of bite-force distribution and surface electromyographic activity of the temporalis and the masseter muscles, at rest and clench, before and after unilateral TMNB injection, will be the primary outcome measures. Post-injection assessments will be performed at 30 minutes, 2 hours, and 2 weeks after the injection. Participants will be surveyed regarding their experience receiving the TMNB injection (secondary outcomes).

1. Objectives The proposed pilot study is aimed at preliminary characterization of the TMNB injection in 20 healthy volunteers. Assessment of bite-force distribution and surface electromyographic activity of the temporalis and the masseter muscles, at rest and clench, before and after unilateral TMNB injection, will be the primary outcome measures. Post-injection assessments will be performed at 30 minutes, 2 hours, and 2 weeks after the injection. Participants will be surveyed regarding their experience receiving the TMNB injection (secondary outcomes).
2. Hypotheses / Research Question(s) (Alternate hypotheses)
3. The TMNB injection alters bite force distribution
4. The TMNB injection alters the surface electromyographic activity of the temporalis and masseter muscles on the side of the injection

1.2 Research Significance The TMNB injection is a local anesthetic injection technique that targets the anterior deep temporal and masseteric branches of the mandibular division of the Trigeminal Nerve. It has been in clinical use for over seven years. It has value as a diagnostic tool in helping differentiate between a toothache and jaw muscle pain when it is difficult for the patient and the clinician to otherwise differentiate between the two. In addition, it also has the ability to ease chronic jaw muscle pain that impacts the masseter and temporalis muscles. However, the mechanism of its sustained pain relief is not known. Given that the local anesthetic action usually lasts for over an hour, it is important to decipher how the TMNB brings about sustained pain relief for longer periods of time. In fact, the pain relief achieved by the TMNB injection in chronic myogenous Temporomandibular disorder involving the masseter and temporalis muscles is comparable to trigger point injections, a standard management modality that requires specialty training and hence is not readily accessible to all patients. The investigators speculate that the nerves to the masseter and temporalis muscles being mixed nerves (i.e., carrying both sensory and motor nerve supply), administration of the TMNB injection could ease muscle tone and activity, relieving chronic pain. Interruption of the pain-contraction-pain cycle may help relieve chronic pain. In order to verify this speculation, the investigators propose the use of TMNB in healthy subjects followed by assessment of jaw muscle function, as measured by bite force distribution and surface electromyographic assessment of the masseter and temporalis muscles. Elucidating the mechanism of TMNB's action has important implications on understanding its therapeutic potential in mTMD as well as elucidating the mechanisms of chronic myogenous TMD itself.

1.3 Research Design and Methods This study is a non-randomized prospective pilot study with twenty study subjects. Assessment of bite-force distribution and surface electromyographic activity of the temporalis and the masseter muscles, at rest and clench, before and after unilateral TMNB injection, will be the primary outcome measures. Post-injection assessments will be performed at 30 minutes, 2 hours, and 2 weeks after the injection, to capture the impact of onset, known duration of action of the local anesthetic used for the study (standard dental local anesthetic, 2% lidocaine with 1:100,000 epinephrine) after a period of 2 weeks.5 Participants will be surveyed regarding their experience receiving the TMNB injection (secondary outcomes).

1. Research Procedures The study will be conducted at the Clinical Research Center at Rutgers School of Dental Medicine. Healthy subjects with no history of TMDs will be recruited (see Eligibility criteria, Figure 1a and b).6 10 male and 10 female subjects will be recruited as per criteria outlined elsewhere.

Subject eligibility and initial evaluation will be conducted by PI SA, a Board-certified Oro-facial Pain Specialist. Informed consent will be obtained from all study participants and all participants can withdraw study consent at any time during the study. The TMNB injection will be delivered using the published technique. The needle position will be verified using a Nerve conduction study (a standard technique for verification of motor nerve blocks). The subsequent assessments including occlusal analysis (OA) and surface electromyography (sEMG) will be administered by either PI GS, PI SA or Co-I SYPQ, using T-scan/BioEMG (Tekscan, Inc., Boston, MA and BioRESEARCH, Inc., Brown Deer, WI). The side of TMNB administration will randomly assigned as left or right side.

Participants will be surveyed regarding their experience receiving the TMNB injection (secondary outcomes). The questionnaire will be offered to the subjects on the day of their injection and again at the two-week post-TMNB injection.