A.Shamaa • Types of joints • Definition • Anatomy of T.M.J • Development of the T.MJ. • Histology of the T.MJ. • I-Bones of the joint. a) The condyle b)The articular eminence and articular fossa of the temporal bone • II-Articular disc. • III- Capsular ligament. • IV- The joint cavity (synovial membrane). • Innervation and blood supply of the T.MJ. A.Shamaa Types of joints Fibrous joins: Sutures Gomophosis, P.L Syndosmosis tibia, Fibia Cartillagenous joints: primary c.j secondary c.j Synovial joint: TMJ
Definition : it is bilateral diartheroidal joint of modified hinge type between condylar head and the glenoid fossa and articular eminence of temporal bone. It is not weight bearing joint it has a fibrous layer cover not present in other joint it is of the synovial type • Anatomy of T.M.J: Articulation of the condylar head of the mandible with the articular fossa and articular eminence of the temporal bone. The interposed articular disc divides the articular space or the joint cavity into an upper compartment for the gliding movement and a lower compartment for the hinge movement. The temporo mandibular joint (TMJ) consist of two bones each with an articular surface covered by a hyaline fibro-cartilage united and surrounded by the capsular ligament forming the joint cavity this cavity is filled with synovial fluid formed by a synovial membrane that lines its non articular surfaces. • a) The condyle: 15-20 mm long, 8-12 mm thick • Convex ant posterior slightly convex mediolateraly, its medial and lateral end called pole its axis parallel to the articular eminence and at the anterior border of foramen magnum • The fibrous covering of the condyle is of a fairly even thickness. Its superficial layer consists of network of strong collagenous fibers with very occasional elastic fibers, fibroblast are situated between the fiber bundles. Cartilage cells (chondrocytes) may be present, and they may Increase in number with age. The deepest layer of the fibrocartilage is rich in undifferentiated cells as long as the hyaline cartilage is present. • By electron microscope another layer 1—2µ thick, can be identified separating the collagen from the joint cavity. This layer is known as the lamina splendens, it has a smooth articular surface and an irregular under surface following the contours created by the collagen bundles. • The condyle composed of cancellous (spongy) bone covered by a thin layer of compact bone. The trabeculae are grouped in such a way that they radiate from the neck of the condyle and reach the cortex at right angles thus giving maximal strength to the condyle. The large marrow space decrease with progressing age by a marked thickening of the trabeculae. The red marrow in the condyle is of the myeloid or cellular type, in older individual it is sometimes replaced by fatty marrow. • During the period of growth a layer of hyaline cartilage lies underneath the fibrous covering of the condyle. The cartilagenous plate grows by apposition from the deepest layers of the covering connective tissue. At its deepest surface is replaced by bone. Remnants of this cartilage may persist through old age. The hyaline cartilage in the condyle serves as an active growth center till the age of 20 years. • b) The articular eminence and articular fossa of the temporal bone • The roof of the glenoid fossa consist of a thin layer of compact bone. The articular eminence is composed of spongy bone covered with a thin layer of compact hone. The connective tissue covering the articular eminence is calcified immediately next to the bone. • The fibers in this area are irregularly arranged. In the outer layer the fibers are densely arranged and lies parallel to its surface, where as the fibroblasts are flattened and give the appearance of an endothelial cells. It contains Chondrocytes which are either isolated or gathered in small groups and thus form a type of fibrocartilage.this fibrocartilage become thinner towards the glenoid fossa till it disappears completely at the tip of the fossa where we find only a layer of fibrous tissue. • The articular eminince not present at birth and atrophied in old age
• Articular disc: (meniscus); • It is avascular biconcave oval plate whose periphery fused with the capsular ligament, and it divides the joint cavity completely into an upper and lower compartment. The upper is the larger compartment, since the upper articular surface is the wider. The upper surface of the disc is concavo convex to fit the articular eminence and fossa of the temporal bone, its lower surface is concave to fit the head of the condyle, while its center is thin and sometimes is perforated. • Histologically the disc consist of dense a vascular fibrous tissue the fibers are straight and tightly packed. Elastic fibers are formed only in small numbers. The fibroblast are elongated and have cytoplasmic processes. Chondrocytes are found with advancing age. The large central area of the disc is devoid of blood vessels and nerves. • Medially and laterally the disc blends with the capsule where both are attached to the medial and lateral poles at the condyles i.e. the head of the condyle and disc move in union. Anteriorly the disc splits or divide into two lamellae, with the upper sheet attached to the anterior edge of the articular eminence and the lower one to the anterior surface of the condylar head, In between these two lamellae, muscles are inserted into the disc and the neck of the condyle. • Posteriorly the disc again divides into inferior and superior lamellae. the inferior lamella is a thin extension of the fibrous disc running over the posterior surface of the condylar head, to fuse with the periostium of the neck of condyle. The superior lamella, the fibrous discs consist of loose connective tissue containing blood vessels, nerve and elastic fibers that blend posteriorly with the capsular ligament. Blood vessels and nerves are only found in the peripheral parts of the disc. The disc fuses with the fibrous capsule on its anterior circumference, at its posterior margin it is connected to the capsule by retrodiscal pad of loose connective tissue which is well vascularized and innervated. • III- The capsular ligament • The capsular ligament is attached superiorly to temporal bone around the margins of the articular fossa and eminence and inferiorly to the neck of the mandibular condyle. Its lateral part is thickened to form the temporomandibular ligament. This is a triangular band attached by its base to the tubercle of the root of the zygoma and by its apex to the lateral side of the neck of the condylar process. Histologically the capsule consists of an outer fibrous layer and an inner synovial layer (synovial membrane) which is a thin layer of connective tissue which has a rich blood supply. The synovial membrane lined the entire capsule, with folds or villi of the membrane protruding into the joint cavity. • IV- The joint cavity: • Consists of the synovial membrane and containing the synovial fluid Histologically the synovial membrane is formed of two layers, cellular intima resting upon a vascular sub intima which blends with the fibrous tissue of the capsule. • The sub intima is a loose connective tissue containing vascular elements together with scattered fibroblasts, macrophages, mast cells and fat cells. The intima varies in structure, having one to four layers of synovial cells embedded in an amorphous fiber-free intercellular matrix. The intimal cells are of three types, fibroblast like or B cell rich in rough endoplasmic reticulum. The second type is called macrophages like or A cell rich in Golgi complex and lysosomes and contains little or no R.E.R. The third type has a cellular morphology between type A & B these cells are not connected by junctional complex and do not rest on a basement membrane. The synovial membrane is responsible for the production of synovial fluid which is viscus fluid contains a varying cell types such as monocytes, lymphocyte free synovial cells and occasionally polymorphnuclear leucocytes and macrophage. • The synovial fluid consists of plasma with some added protein and mucin, which is a product of the intimal cells. Its function is to provide a liquid environment for the joint surfaces and lubrication to reduce erosion. It also provides nutrition to the disc and the articular surfaces, the synovial membrane is also responsible for the removal of foreign material shed into the joint cavity. The cells have marked phagocytic properties. • Innervation and blood supply of the T.MJ. • The T.M.J. is innervated by the trigeminal nerve (mandibular division through its auriculotemporal branch) and masseteric nerve. • The arterial supply to the joint comes from the internal maxillary artery through its deep auricular branch and the superficial temporal arteries. The pterygoid plexus provides the main drainage system. The vascular plexus is present in the wall of the capsule where it is responsible for the production of the synovial fluid in the joint cavity. • Movements: • Protrusion by lat and Med. Ptrygoid muscle • Retrusion: by Lat. Ptrygoid muscle • Elevation: by Masseter, Temporalis and Med. Ptrygoid muscle • Lateral movements: by Lat. And Med. Ptrygoid muscles • Age changes: • Flattened condylar surface • Fibrosis • Osteoporosis • Thin hyaline disc • Fibrosis of synovial folds • Thickened walls of the blood vessels • Decreased no. of nerves. • Decreased fluid and lubrication