Biomechanics and injury mechanisms
The overwhelming majority of injuries (over 90%) are non-occupational, with domestic trauma leading the way. The nature of bone destruction is closely linked to its anatomy.
Four mechanisms of fracture are distinguished (according to Wassmund):
- Bending. The most frequent variant. It occurs in the thinnest and most curved areas. If the impact is delivered over a narrow area, a direct fracture occurs at the site of impact. With a wide impact area, the bone breaks far from the point of force application — this is an indirect (reflected) fracture.
- Shear. An unsupported bone section is displaced relative to a fixed one. Characteristic of longitudinal fractures of the ramus when struck from below and upwards anterior to the angle.
- Compression. Opposing action of forces. Causes a transverse fracture of the ramus (most commonly in the middle section), as the ramus is wedged into the glenoid fossa.
- Avulsion. Reflex spastic contraction of the powerful temporal muscle (m. temporalis) avulses the thin coronoid process upon a blow to the chin (with tightly clenched jaws).
Muscle anatomy and fragment displacement
The mandible (mandibula) unites two muscle groups into a single system. Upon fracture, this connection breaks, and the fragments begin to displace independently.
Dislocation involves:
- Elevator muscles (posterior group): masseter (m. masseter), temporal (m. temporalis), medial and lateral pterygoid (m. pterygoideus medialis et lateralis). They are physiologically stronger.
- Depressor muscles (anterior group): digastric (m. digastricus), mylohyoid (m. mylohyoideus), geniohyoid (m. geniohyoideus), etc.
The direction of dislocation depends on where the fracture line passes and which muscles are attached to each fragment. For example, in a single fracture of the angle region, the small fragment (ramus) is pulled upward and inward by the elevator muscles. The large fragment (body) is pulled downward and outward by the muscles of the floor of the mouth.
In a symphyseal/parasymphyseal fracture, the picture changes: if there are three fragments, the central small fragment displaces downward and posteriorly, while both lateral fragments displace inward and superiorly.
Clinical presentation and symptoms
Fractures of the mandibular body are always open. This is due to the fact that the mucosa of the alveolar part is tightly fused with the periosteum and tears with the slightest bone displacement. The second reason is the fracture line passing through the periodontal ligament of the tooth.
Key symptoms:
- Malocclusion. Due to asymmetric muscular traction, dental contact is lost. Often, occlusion is preserved only on the molars of the affected side.
- Deviation. When attempting to open the mouth, the chin deviates toward the fracture.
- Specific hematoma. A reliable sign of a mandibular body fracture is a bilateral hematoma (both in the oral vestibule and lingually). In a simple contusion, blood accumulates only vestibularly.
- Neurological deficit. Injury to the inferior alveolar nerve (n. alveolaris inferior) results in numbness of the skin of the lower lip and chin strictly on the side of the fracture.
Diagnostic criteria
In addition to complaints of pain and impaired chewing function, the physician relies on objective tests and imaging diagnostics.
- Load test (referred pain sign). Pressure on a healthy area (e.g., pressure on the chin) causes sharp pain in the hidden fracture zone due to micro-displacement of the fragments.
- Mobility sign (pathognomonic sign). The physician fixes the mandible with both hands on either side of the suspected fracture line and gently attempts to displace the fragments to test for mobility.
- Palpation of the condylar processes. To assess their integrity, the physician inserts index fingers into the patient's external acoustic meatuses and asks them to move the jaw. The absence of synchronized condylar head movement indicates injury.
- Radiography. Mandatory standard. Images are taken in at least two projections (or an OPHT/orthopantomogram is used) to assess the position of tooth roots relative to the fracture line and check for bone fragments.
Briefly: answers to topical questions
What are the absolute clinical signs (symptoms) of a mandibular fracture?
Reliable (absolute) clinical signs of a fracture include pathological mobility and crepitus.
- Pathological mobility — defined as bone mobility outside of a joint.
- Bone crepitus — a sound or specific crunching sensation upon palpation of the fracture area in the early stages. The mechanism of this symptom is the mutual friction of bone fragments against each other.
Which anatomical structures pass through the mandibular canal and can be injured during a fracture?
The neurovascular bundle passes through the mandibular canal (canalis mandibulae), and its elements can be injured during a fracture.
- Inferior alveolar nerve (n. alveolaris inferior / inferior alveolar nerve) — a branch of the mandibular nerve.
- Artery — the blood vessel within the bundle.
- Vein — the venous vessel of the bundle.
Trauma or rupture of the inferior alveolar nerve due to bone fragment displacement causes neurological symptoms. They manifest as a loss of pain sensation (anesthesia or analgesia) of the skin of the lower lip and chin on the affected side.
Which radiographic views and projections are used to diagnose mandibular fractures?
Various radiographic methods and projections are used to diagnose mandibular fractures depending on the location of the injury.
- Anteroposterior projection — used to examine the body and ramus of the mandible.
- Lateral projection — used to assess the body and ramus of the mandible.
- Intraoral projection — indicated for examining the alveolar process.
- Panoramic radiography (orthopantomography) — allows a single image of the entire dentition and jaw system.
Frontonasal, mentonasal, semi-axial (submento-vertical), and axial projections are also used for radiography of the facial bones.
Wassmund's fracture mechanisms can be remembered with the acronym: Bending (most frequent, in weak points), Avulsion (of the coronoid process), Shear (along the ramus), Compression (across the ramus).
Class and exam questions
Why are fractures within the dental arch always open?
The mucosa of the alveolar part is fixed and tears together with the periosteum during bone dislocation. If the line passes through the socket, the wound communicates with the oral cavity through the torn periodontal ligament of the tooth.
How to distinguish a fracture of the mandibular body from a soft tissue contusion?
In a fracture, a hematoma forms on both sides of the alveolar process (vestibular and lingual). In a contusion, the hematoma is always unilateral (vestibular only).
Why does chin numbness occur during trauma?
This is a consequence of stretching, entrapment, or rupture of the neurovascular bundle (primarily the inferior alveolar nerve) passing inside the mandibular canal.
What else to study on this topic
- Management of alveolar process fractures and immobilization rules
- Management tactics for a tooth located within the fracture line
- Features of surgical debridement in complete avulsion of a bone fragment
- Specialized radiographic views for temporomandibular joint visualization
- Biomechanics of displacement in multiple and bilateral fractures
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Other topics in the section "Chapter 11. Non-ballistic and ballistic facial injuries. Thermal and radiation lesions of the maxillofacial region (V.V. Afanasyev)"
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