Even those suffering from temporomandibular disorders (TMD) may not understand the anatomy of the temporomandibular joint (TMJ) itself. Understanding the anatomy will help you better understand the disorder, and, more importantly, how to protect yourself from painful flare-ups of the jaw. In this post, we’ll provide an introductory lesson on the anatomy of the TMJ and associated structures.
The Temporomandibular Joint (TMJ)
The TMJ is the most complex joint in the body. Each person has two of them, one on the right side and one on the left, working in tandem to provide movement of the mandible (lower jaw). The TMJ is the only joint in the body built to be both stable and unstable. Why? Because it is the only joint designed to regularly come out of its socket during function. It rotates within the socket at the beginning of opening, but will slide out of the socket with sideways movement or wide opening.
The TMJ capsule is the outer covering of the joint and seals it from the surrounding environment. The inner layer has a membrane providing fluid for lubrication, facilitating joint function. The capsule is fibrous with a thick side that forms a ligament called the temporomandibular ligament.
Ligaments of the TMJ
There are generally three ligaments named as being associated with the TMJ. These include the Sphenomandibular ligament, the Stylomandibular ligament, and the Temporomandibular ligament. These ligaments work together to guide and limit extreme movements of the lower jaw. These may become stretched or damaged with high impact injuries to the face.
Condyle of the Mandible
The ball in the socket forming the TMJ is called the condyle. It is the top part of the mandible, on either side, forming a rounded part for jaw movement. High impact injury, autoimmune diseases, or prolonged pressure and inflammation may sometimes affect the integrity of this bone. Arthritic changes to this bone will actually cause it to alter its shape, further affecting jaw movement and the bite.
This is the socket of the TMJ. It is a bony cave under the temporal bone of the skull. The condyle rotates within and slides forward of this socket. Injury and arthritic changes can also affect the mandibular fossa, leading to alterations in shape, jaw movement, and the bite.
The articular disk is a cartilage-like cushion between the ball and the socket. Its shape fits the ball and the socket, allowing them to turn and slide against one another. The disk divides the TMJ into two compartments: an upper and lower joint cavity. The disk attaches to the condyle on either side, to muscles at its front, and to the back of the socket by retrodiscal tissue. The disc acts as a shock absorber and helps keep movement smooth. The disc can become damaged in many ways. It may get displaced, compressed, torn, or thinned. Damage to the disk can lead to TMJ problems and symptoms.
Upper and Lower Joint Cavities
These joint cavities are divided by the disk, one above and one below. They are filled with joint fluid called synovial fluid, a very special liquid with properties to minimize friction and allow smooth movement of the TMJ parts. Synovial fluid is believed to provide nourishment to the working surfaces of the joint. The volume of fluid may change in relation to barometric pressure and inflammation. These can lead to jaw pain or changes in the bite.
Retrodiscal tissues are the tissues behind each TMJ disk. These tissues attach to the socket by an upper band, and attach to the ball’s neck by the lower band. The upper band is elastic, so it helps hold the disc in its proper position. The lower band does not have elastic fibers, which is why sometimes extreme stretching of this lower band leads to disk injury or displacement. The tissues between the bands hold most of the blood and nerve endings of the TMJ. These tissues can get squished in an unstable bite or when clenching and grinding our teeth. This leads to decreased blood flow. Inflammation of the tissue is painful and may affect the bite.
Lateral Pterygoid Muscle
The lateral pterygoid muscle is one of the main positioning muscles of the TMJ. It is split into an upper and lower head. The upper muscle has fibers attaching to the ball and disk, helping stabilize their position relative to each other. The lower head is larger and pulls on the ball to allow opening, thrusting, and side movements of the jaw. These muscles may be asked to work overtime in an unstable joint system. Muscle cramping, soreness, and fatigue may result.
We hope this introduction to the anatomy of the TMJ has improved your understanding of the complexity of this joint. The temporomandibular joint is an orthopedic system, requiring stability and mobility. When stability is lost, mobility will also frequently decline. Our goal is to conservatively restore stability to your jaw, and allow more freedom of movement, less jaw pain, and better quality of life. If you believe your jaw stability has been compromised, we invite you to call or visit us online to schedule an appointment. At MedCenter TMJ, we do everything we can to improve the health of your jaws. It’s all we do!
– Nathan J. Pettit DMD, MSD