Stage IVb is the locking stage of patients who have medial pole discal displacement. In Stage IVb, there has been significant deformation of the disc at the medial pole. These patients are not able to reduce their mandibular condyle beneath the disc during any phase of mandibular movement. Hence, they continuously function on their retrodiscal attachment. They may or may not have limited movement. Occasionally, the limited movement can occur because of a true lock or blockage of forward translation. Alternatively, they may have a reduced range of motion simply because of muscle hyperactivity and splinting. Some patients in Stage IVb remain in this stage forever. A smaller number of these patients may be able to be reverted back to a Stage III patient. Occasionally, some of these patients develop perforation of their retrodiscal attachment and develop Stage V.