TMJ

The glenoid fossa is limited posteriorly by the squamotympanic and petrotympanic fissures◦ limited medially by the spine of the sphenoid◦ laterally by the root of the zygomatic process of the

temporal bone◦ Anteriorly bounded by a ridge of bone described as the

articular eminence, which also is involved in the articulation

The middle part is a thin plate of bone, the upper surface of which forms the middle cranial fossa

(housing the temporal lobe of the brain).

Bones of TMJ

The articular surface of the condyle is strongly convex in the anteroposterior direction and slightly convex mediolaterally

The medial and lateral ends are termed poles.

The articulating surfaces

Histology of articulating surfaces

Unlike most synovial joints, the articular surface of which are covered with hyaline cartilage, the TM articulation is covered by a layer of fibrous tissue.

Glenoid fossa is always covered by a thin fibrous layer. This layer becomes thicker where it covers the slope of the articular eminence.

Earlier accounts of TMJ histology indicated that the surfacecoverings of the joint consist of fibrocartilage rather thanfibrous tissue

with age the fibrous covering layer might contain some cartilage cells, no evidence indicates that this is normal firm evidence indicates that fibrocartilage is associated with the

articulation deep to the fibrous layer, in the condyle and on the articular eminence

Histology of articulating surfaces of the TMJ

1- The most superficial layer forms the articular surface and is composed of fibrous tissue. Although most of the fibers are collagenous, some elastin fibers are also present. 2- Beneath the articular surface layer is a more cellular zone in which proliferation occurs. Providing a source of cells to replenish adjacent layer s.in child (cell from this layer divide to give rise to fibroblast like cell which differntiate in to chonrocytes which form the secodary condylar cartilag )

3- Beneath the cell-rich zone is another fibrous layer the can have a variable appearance. A number of the cells are rounded, and have an appearance reminiscent of cartilage-like cells, the layer has been described as a fibro-cartilaginous layer.

4- Immediately covering the bone is a thin zone of calcified cartilage. This calcified cartilage is a remnant of the secondary condylar cartilage.

A transient growth cartilage also has been found in association with development of the articular eminence.

No eminence exists at birth; its development (involving the same layers as already described for the condyle) situated along the slope of the eminence.

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Articular eminence

In summary, although fibrocartilage is associated with the temporomandibular articulation, it does not form part of the articulation and has no formal functional role to play in the everyday movements occurring between the two bones of the joint.

Recognizing the disk as an extension of the capsule, the capsule of the TMJ can be described as a fibrous, nonelastic (collagenous only except its upper part) membrane surrounding the joint, which is attached :

superiorly tothe squamotympanic fissure posteriorly and laterally the margins of the glenoid fossa anteriorly the articular eminence. Inferiorly, the capsule is attached to the neck of the condyle.

Above the disk the capsule is fairly lax, whereas below, it is attached tightly to the condyle. The lateral aspect of the capsule is thickened to form a fan-shaped ligament known as the temporomandibular ligament, which runs obliquely backward and downward from the lateral aspect of the articular eminence to the posterior aspect of the condylar neck .

capsule

Remember most of our body joints are supported mainly by muscles but in TMJ it is supported mainly by ligaments

TMJ ligament

Outer oblique

(1)Outer obliqueportion arising from the outer surface of the articular eminenceand extending backward and downward to insert intothe outer surface of the condylar neck

(2 )an inner horizontalportion with the same origin but inserting into thelateral pole of the condyle.

Inner horizontal

Two other ligaments are included in conventional descriptions of the joint, although neither has a functional role.

The first is the sphenomandibular ligament, running from the lingula to the spine of the sphenoid. This ligament represents the residual perichondrium of Meckel’s cartilage.

The second is the stylomandibular ligament, running from the styloid process to the angle of the mandible. This ligament represents the free border of

the deep cervical fascia.

Other TMJ ligaments

The articular disk attaches to the inner waIl of the capsule anteriorly and posteriorly, but not medially and laterally, which is where it attaches to the head of the condyle. This structural design requires the disk to be immobile when the head of the condyle moves.

The disc is attached all around the joint capsule except for the strong straps “collagen only” that fix the disc directly to the medial and lateral condylar poles, which ensure that the disc and condyle move together in protraction and retraction.

Disk ( Mesniscus)

The articular disc is a roughly oval, firm, fibrous plate (fibrocartilagenous biconcave).

A-P disk has :1. anterior band “pars anterior” = 2 mm in

thickness, 2. posterior band = 3 mm thick, 3. thin in the centre intermediate band of 1

mm thickness More posteriorly there is a bilaminar or

retrodiscal region.

Meniscus

It is shaped like a peaked cap that devides the joint into larger upper compartment and smaller lower compartment .

Sup-inf disk has : Sup : concavo-convix Inferiorly : concave Not innervated and no blood supplay to it

Meniscus

The anterior and posterior bands have predominantly transversal running collagen fibers(which form the discal ligament), while the thin intermediate zone has anteroposteriorly oriented fibers.

Posteriorly, the bilaminar region consists of two layers of fibers separated by loose connective tissue.

The upper layer or temporal lamina is composed of elastin majorly with some collagen and is attached to the postglenoid process, medially extended ridge, which is the true posterior boundary of the joint. It prevents slipping of the disc while yawning.

• The inferior layer of the fibers or inferior lamina curve down behind the condyle to fuse with the capsule and back of the condylar neck at the lowest limit of the joint space. It prevents excessive rotation of the disc over the condyle.

In between the two layers, an expansile, soft pad of blood vessels and nerves are sandwiched and wrapped in elastic fibers that aid in contracting vessels and retracting disc in recoil of closing movements. •The volume of retrodiscal tissue must

increase instantaneously when the condyle translates anteriorly.

80% composed of water .

Like any connective tissue composed of :

Cells : show an outline varying between flattend (fibroblast-like) and rounded ( chondrocyte-like) . More numerous at birth ,contain antiapoptic protein (eg Bcl2)

ECM :

Fibers

collagen type I ( 80% of dry weight),III,VI,XII, II?

Elastic fibers

Meniscus “Histology”

Ground substance :

Constitute about 5% of dry weight .

The major GAGs are chondroitin sulfate ,dermatan sulfate , traces of hyaluronan and heparin sulfate . Also we have fibronectin ,tenascin .

GAGs are negatively charged so they absorb water .

Chondroitin sulfate + protein = LMW proteoglycan resembling aggrecan of the cartilage . These LMW proteoglycans are particularly localized at the periphery cartilage like cells .

Meniscus “Histology”

The capsule is lined on its inner surface by a synovial membrane . Generally, the synovial membrane is considered to line the entire capsule, with folds or villi of the membrane protruding into the joint cavity. These folds increase in number with age and are also more prominent in joints affected by a pathologic process. The synovial membrane does not cover the articular surfaces of the joint or the disk, except for its bilaminar posterior region.

Essentially, any synovial membrane consists of two layers: a cellular intima resting on a vascular subintima and the fibrous tissue of the capsule into which the subintima blends.

The subintima is a loose connective tissue containing vascular elements together with scattered fibroblasts, macrophages, mast cells, fat cells, and some elastic fibers, which prevent folding of the membrane. The intima varies in structure, having one to four layers of synovial cells embedded in an amorphous, fiber-free intercellular matrix. Often cellular deficiencies exist so that the subintimal connective tissue directly borders the joint cavity. These cells are not connected by junctional complexes and do not rest on a basal lamina. The joint cavity therefore is NOT lined by epithelium. The cells forming this discontinuous layer are of two types, a predominant type A (macrophage-like) cell and

a type B (fibroblast-like) cell.

Synovial membrane

Synovial membrane Type A ( Exhibit significant phagocytotic

properties

Type B (fibroblast like ) cells, by contrast, contain many profiles of rough endoplasmic reticulum synthesize the hyaluronate found in synovial fluid.

The synovial membrane secrete the synovial fluid that occupies the joint cavities . Inferior joint cavity hold about 1ml of synovial fluid , while the slightly larger superior joint cavity contain alittle more.

At rest, the hydrostatic pressure of the synovial fluid has been reported as being subatomspheric, but is greatly elevated during mastication === raised fluid pressure may also be clinically relevant in patients who clench their teetn (bruxism)

The chemical composition of synovial fluid indicates that it is a dialysate(ultrafiltrate) of plasma ( contaning cytokines) supplemented with proteins and proteoglycans (hyaluronan). The synovial membrane is responsible for controlling the passage of plasma components and producing the additional components.

Alkaline phosphatase, which may also be present in synovial fluid, is thought to be produced by chondrocytes.

Hyaluronan + fibronectin =non adhernt surfaces

Synovial fluid also may contain a small population of varying cell types such as monocytes, lymphocytes, free synovial cells, and occasionally polymorphonuclear leukocytes. Synovial fluid is characterized by well-defined physical properties of viscosity,elasticity, and plasticity.

Synovial fluid

The function of this fluid is to provide :

(1) a liquid environment for the joint surfaces (2) lubrication to increase efficiency and

reduce erosion.

“ Synovial fluid also is believed to act as a nutrient fluid for the avascular tissues covering the articular surfaces

Synovial fluid

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