What is the function and clinical significance of the posterior belly of the digastric muscle?

Posterior Belly of the Digastric Muscle: Function and Clinical Significance

Anatomical Function

The posterior belly of the digastric muscle (PBDM) works in coordination with the anterior belly to either depress the mandible during mouth opening or elevate the hyoid bone during swallowing, chewing, and speech. 1, 2

• The PBDM originates from the mastoid notch of the temporal bone and connects to the intermediate tendon, which is typically anchored to the hyoid bone 1, 2
• When both digastric muscles contract together, they function as a single unit to facilitate jaw depression or hyoid elevation, depending on which structure is fixed 2
• The muscle plays essential roles in deglutition (swallowing), mastication (chewing), and phonation (speech production) 3

Surgical Landmark Significance

The PBDM serves as a critical anatomical landmark during neck dissection procedures, defining boundaries of the carotid, submandibular, and submental triangles. 3

• Surgeons rely on the PBDM to identify surgical boundaries during neck dissection, particularly in oncologic procedures 3
• The muscle helps define the submandibular triangle laterally and contributes to orientation during complex head and neck surgeries 4
• Recognition of the PBDM's location is essential to avoid inadvertent injury to adjacent structures, including the facial nerve and major vessels 1

Clinical Palpability Controversy

Despite inclusion in many examination protocols, evidence demonstrates that the PBDM is NOT palpable due to anatomical reasons, and postmandibular tenderness should not be attributed to this muscle. 5

• A systematic review of dental and medical literature found only two publications addressing palpability, both concluding the PBDM cannot be accessed through palpation 5
• The postmandibular region is typically tender upon palpation even in healthy subjects, which can lead to false-positive findings and unnecessary diagnostic or therapeutic interventions 5
• Clinicians should avoid attributing postmandibular tenderness to PBDM pathology, as this may result in incorrect clinical judgments 5

Anatomical Variations and Imaging Implications

Anatomical variations of the PBDM occur less frequently than anterior belly variations but can be mistaken for neck masses on imaging, making recognition important to avoid unnecessary invasive procedures. 1, 3

• The prevalence of digastric muscle variations is approximately 31.4% in cadaveric studies, though most involve the anterior belly rather than the posterior belly 3
• Duplicate PBDM has been documented, where a superior duplicate overlies the stylohyoid muscle 3
• With increasing use of CT and MRI for neck imaging, recognizing PBDM variations prevents misdiagnosis as pathologic masses and helps surgeons plan appropriate approaches 1

Intraoperative Considerations

When PBDM variations are identified intraoperatively, surgeons must maintain awareness to ensure thorough dissection of neck structures, particularly when oncologic clearance is paramount. 3

• Anatomical variants typically do not cause clinical symptoms but require surgical awareness to prevent complications 1, 3
• The PBDM's relationship to the stylohyoid muscle and its proximity to neurovascular structures necessitates careful dissection 3
• Variations should not prohibit or limit complete neck dissection when required for cancer treatment 3