Disc displacement refers to an alteration in the normal positioning of the cushioning disc within the temporomandibular joint (TMJ). This condition is a common form of internal joint derangement, describing a mechanical problem affecting the relationship between the jawbone and the skull. Displacement typically involves a forward, or anterior, movement of the disc relative to the head of the mandible.
Anatomy of the Temporomandibular Joint
The temporomandibular joint is the hinge connecting the lower jaw (mandible) to the temporal bone of the skull, located just in front of the ear. Within this articulation sits a specialized, oval-shaped piece of fibrocartilage known as the articular disc. The disc is biconcave, thinner in the center and thicker at its edges, allowing it to fit precisely between the bony surfaces.
The primary function of the articular disc is to act as a smooth, shock-absorbing cushion, preventing the bones from rubbing against each other during movement. It divides the joint space into two separate synovial cavities containing lubricating fluid. The disc’s attachments, particularly the elastic fibers in the posterior retrodiscal tissue, help pull it back into its correct position as the mouth closes. When the jaw opens, the mandibular condyle moves forward and downward, sliding directly underneath the disc.
Classifying Disc Displacement
Disc displacement is categorized based on whether the disc returns to its correct position when the jaw moves. The most common type is anterior disc displacement with reduction (DDR). In this scenario, the disc is positioned too far forward when the mouth is closed, but upon opening the jaw, the condyle “recaptures” the disc, causing it to snap back into place.
This reduction is often accompanied by a distinct, audible clicking or popping sound. A second, reciprocal click may occur upon closing the mouth as the disc slips forward again. While initially painless, the repetitive mechanical snapping can eventually strain surrounding tissues and lead to discomfort.
Alternatively, the condition can progress to anterior disc displacement without reduction (DDNR), which represents a more advanced stage. Here, the disc remains permanently displaced forward, acting as a physical block that prevents the condyle from moving into its full range of motion. Because the disc never returns to its normal position, the characteristic clicking sound ceases.
This non-reducing displacement causes a mechanical obstruction, leading to a restricted ability to open the mouth fully, a condition often described as a “closed lock.” Research indicates that this progression from a reducing to a non-reducing disc is not uncommon, especially if factors like ongoing clenching or joint hypermobility are present.
Etiology and Risk Factors
The reasons for disc displacement are multifactorial, stemming from mechanical, structural, and behavioral influences. Macrotrauma, such as a direct blow to the jaw or a whiplash injury, can physically stretch or tear the surrounding ligaments, allowing the disc to shift out of alignment.
More frequently, microtrauma from chronic habits contributes to the displacement over time. Parafunctional activities like bruxism—the involuntary clenching or grinding of teeth—or excessive gum chewing place undue and repetitive load on the joint structures. This chronic stress can eventually lead to elongation of the discal ligaments and a breakdown of the disc’s shape and integrity.
Underlying systemic conditions also represent significant risk factors for joint instability. Certain connective tissue disorders can affect the elasticity and strength of the ligaments that hold the disc in place. Degenerative joint diseases like osteoarthritis can erode the bony surfaces and alter the joint mechanics, compounding the effects of disc displacement.
Clinical Signs and Patient Experience
Disc displacement is characterized by distinct signs and symptoms. The most recognizable sign is the presence of joint sounds, which directly correspond to the type of displacement. A single, loud click or pop upon opening and sometimes closing the mouth is the hallmark of displacement with reduction.
When displacement advances to the non-reducing stage, the patient reports a sudden onset of limited jaw movement. Maximum mouth opening is significantly reduced, sometimes to 30 millimeters or less, and the jaw may deflect toward the affected side upon opening. Pain is commonly felt in the periauricular area (just in front of the ear) and may radiate into the face, temple, or neck.
The pain does not originate from the fibrocartilaginous disc itself, which lacks nerve endings. Instead, the discomfort arises from the compression and inflammation of the highly innervated retrodiscal tissue, which is forced into the joint space by the displaced disc. In later stages, a grinding or grating sound, known as crepitus, may replace the click, indicating degenerative changes on the bony surfaces of the joint.
Treatment Modalities and Intervention
The initial approach to managing disc displacement prioritizes conservative, non-invasive therapies. Self-management strategies, including a soft diet, moist heat application, and avoiding extreme jaw movements, are recommended first to reduce strain and inflammation. Pharmacological interventions may involve nonsteroidal anti-inflammatory drugs (NSAIDs) for pain and muscle relaxants to address associated muscle spasms.
Physical therapy, incorporating gentle jaw exercises and manual manipulation, can help restore muscle function and improve the range of motion. A common, reversible intervention is the use of an oral appliance, such as a stabilization or repositioning splint, worn primarily during sleep to reduce joint load and prevent clenching. An anterior repositioning splint attempts to temporarily hold the jaw forward, encouraging the disc to remain on the condyle.
For persistent or severe cases, particularly non-reducing displacements that cause significant functional limitation, more invasive procedures may be considered. Minimally invasive options include arthrocentesis, which involves flushing the joint space with fluid to remove inflammatory byproducts and free up the disc. In rare instances where conservative measures have failed, surgical intervention, such as arthroscopy or open-joint surgery for disc repair or repositioning, may be necessary.