Joints And Joint Tissue

JOINTS AND JOINT TISSUE JOINTS

Fibrous Joints They do not have synovial cavity. Two bones remain held together by thin layer of fibrous tissue or dense fibrous tissue or cement or sutures. (i) Sutures Synostoses – (Suture during infancy but fusion afterwards as in Frontal bone). Bones are thin and plate like, held together by inter digitations. e.g., skull bones. (ii) Gomphosis One bone remains embedded in the socket of other attached through fibres or cement layer. e.g., Thecodont teeth of humans. (iii) Shindylases One bone fits into slit of other. e.g., ethmoid bone into vomer. (iv) Syndesmosis Two bones are united by dense fibrous tissue. e.g., joint between skull bones and bones of upper jaw, distal ends of tibia and fibula. Cartilaginous Joints No synovial cavity, articulating bones are united by cartilage. (i) Synchondrosis Connecting material is hyaline cartilage. e.g., temporary joint between diaphysis and epiphysis of a long bone and permanent joint between true ribs and sternum. (ii) Symphysis Connecting material is broad flat disc of fibrocartilage. e.g., Intervertebral disc and symphysis pubis. Synovial Joints Are the Most perfect, freely movable and most common type of joints. They allow free movement in one or more directions. There is no direct contact between two bones, end is covered with hyaline cartilage cap (articular cartilage) and the whole structure is covered with ligament. The synovial cavity between two bones is lined with synovial membrane and is filled with synovial fluid secreted by this membrane. Synovial fluid acts as lubricant and shock absorber and also provides nourishment to articular cartilage. In old age stiffness of joints occur due to decrease in synovial fluid and erosion of cartilaginious part. Synovial membrane is composed of loose connective tissue with elastic fibres and a variable amount of adipose tissue. Synovial fluid also contains phagocytic cells and removes microbes and debris resulting from wear and tear of joints. It also contains Hyaluronic acid and interstitial fluid formed from blood plasma and is similar in appearance, consistency to uncooked egg white. When there is no movement, the fluid is quite viscous, but as movement increases, fluid becomes less viscous. Amount of synovial fluid varies in different joints of body, ranging from a thin viscous layer to about 3.5 ml of free fluid in large joint such as knee. It also removes metabolic wastes from the joint. Arthroscopy is examination of interior of a joint, usually knee by an arthroscope.

One interesting feature of some synovial joints is their ability to produce a cracking sound when pulled apart. (i) Articular discs (menisci) These are pads of fibrocartilage that lie between articular surfaces of some bones. These allow 2 bones of different shapes to fit tightly, these modify the shape of joint surfaces of the articulating bones. Articular discs help to maintain the stability of joints and direct the flow of synovial fluid to areas of greatest friction. (ii) Bursae Sac like structures situated between tendons and bones, muscles and bones, ligaments and bones, skin and bones. Their wall has connective tissue lined by synovial membrane, they are also filled with a fluid similar to synovial fluid. Inflammation of Bursa is called Bursitis.

Types of Synovial Joints Ball & Socket joint Articulate end of one bone is like a ball whereas other bone end is like a cup shaped socket. It permits triaxial movements, i.e., movement in three planes. e.g., Acetabulum of pelvic girdles and head of femur, glenoid cavity of pectoral girdles and head of humerus. Angular/Condyloic/Ellipsoid joint Half ball and socket type joint with one biaxial movement in two planes, back and forth and side to side. Oval condyle of one bone fits into elliptical cavity of other bone like joins of phalanges with metacarpals, radius and carpals. (radiocarpal joints) Gliding/Sliding joint or Arthrodia Articulating surfaces of bones are usually flat. Only side to side and back and forth movements are permitted. Twisting and rotation are inhibited because ligaments or adjacent bones restrict the range of movement. Since gliding joints do not move around an axis, they are referred to as Non-axial. e.g., joints between carpal bones, tarsal bones, sternum and clavicle, scapula and clavicle, pre and post zygapophysis of vertebrae. Hinge joint or Ginglymus Convex surface of one bone fits into concave surface of another bone, movement is primarily in a single plane and the joint is known as monoaxial/uniaxial. Motion is similar to that of a hinged door. e.g., elbow, ankle, interphalangeal joint.

Saddle/Sellaris joint Articular surface of one bone is saddle shaped and that of other bone is shaped like a rider sitting in a saddle. It is a modified ellipsoidal joint in which movement is somewhat freer (biaxial). e.g., joint between trapezium of carpus and metacarpals of thumb (carpometacarpal joint).

Pivolt joint/Rotatorial/Trochoid A rounded, pointed or conical surface of one bone articulates within a ring formed partly by another bone and partly by ligament. Primary movement permitted is rotation and the joint is Monoaxial. e.g., joint between atlas, and axis, and between proximal ends of radius and ulna (Atlantoaxial and Radioulnar joint).

Different types of joints

Levers For performing movements bones and muscles form levers at the joints. The force generated by muscle contraction pulls or lifts bone through tendons. Movement occurs along the joint, hence the bones actually act as lever having 3 parts : (i) fulcrum (F), the fixed point, (ii) point of effort (E), and (iii) the point of Resistance (R) or load.

(i) In the body joint acts as F, point of attachment of muscle to bone is E. (ii) There are 3 classes of lever. 1st class lever : F comes between E and R, very rare in body, one such example is head resting on the vertebral column. 2nd class lever : R comes between F and E, only few in the body e.g., raising body on the toes. 3rd class lever : E remain in between the F and R, the most common type lever in the body e.g., flexing fore arm at elbow joint. 1.

Fist Order Levers. They are levers in which fulcrum lies in the middle, weight (resistance) at one end and force (or force at the other end. Such a joint occurs between occipital bone and atlas vertebra of neck. The joint functions as fulcrum. Head is the weight or resistance. Muscles of the back of neck provide force or energy.

2.

Second Order Levers. They are levers in which fulcrum is at one end, weight or resistance in the middle and energy or force at the other end. Human body resting on toes form the second lever where toes at one end function as fulcrum, resistance or weight is over the front part of heel while force or energy is provided by calf muscles. Third Order Levers. They are those levers in which force or energy is in the middle with weight or resistance at one end and fulcrum over the other end. Lifting of a book or any other load involving flexing movement over the elbow joint represents the third order lever. Here end of arm with a load functions as weight or resistance, elbow as fulcrum and biceps muscles attached to radius functions as force or energy.

3.