Biomechanics of the mandible is a complex topic. Forces applied to the mandible cause varying zones of tension and compression, depending on where the bite force is located. The superior portion of the mandible is designated as the tension zone and the inferior portion is designated as the compression zone.
The mandible is a hoop of bone that deforms with movement based on the origin and insertion of the muscles of mastication.
Tension and compression zones
The superior border of the mandible is the tension zone and the inferior border is the compression zone.
Hunting bow concept
The mandible is similar to a hunting bow in shape, strongest in the midline (symphysis) and weakest at both ends (condyles). The most common area of fracture in the mandible is therefore the condylar region.
A blow to the anterior mandibular body is the most common reason for condylar fracture. The force is transmitted from the body of the mandible to the condyle. The condyle is trapped in the glenoid fossa. Commonly, a blow to the ipsilateral mandible causes a contralateral fracture in the condylar region. If the impact is in the midline of the mandible, fractures of the bilateral condylar region are very common.
Direct trauma to the TMJ area is unusual but may be associated with fractures of the zygomatic complex.
With a condylar fracture, there is very often shortening of the ramus on the affected side. This will result in an ipsilateral premature contact of the teeth. In case of bilateral fractures, the patient may present an anterior open bite. The condylar fragment may be displaced (most often laterally) based on the angulation of the fracture and predominant muscle pull.