American Board of Orthodontics (ABO) Practice Exam 2026 - Free Orthodontics Practice Questions and Study Guide

The upper compartment of the temporomandibular joint (TMJ) is responsible for translation during jaw movement. This compartment includes the glenoid fossa of the temporal bone and the articular disc. When the jaw opens, the condyle of the mandible moves forward and downwards within this upper compartment. This movement, known as translation, involves anterior and inferior movements of the condyle, which allows for greater mouth opening and movements like protrusion and retraction.

In contrast, the lower compartment is involved primarily in hinge-like rotation of the mandible. This compartment allows for movements such as the initial opening and closing of the jaw, where the condyle rotates in the concavity of the articular disc. The articular disc itself acts as a cushion and facilitates smooth movement, but it does not account for translation as it is not a compartment but rather a structure within the TMJ. The coronoid process is an anatomical feature of the mandible that serves as a site for muscle attachment, but it does not play a direct role in the TMJ movements. Therefore, understanding the function of each component helps clarify the primary role that the upper compartment plays in translation during jaw movement.