Joints

Joints hold bones together but permit movement.  There are three different types of contact between tissues.  Bone to bone, cartilage to bone and teeth to bone.  Arthrology is the study of joints and kinesiology is the study of motion.  Joints are classified by both structure and function.  Structural classifications are based on the presence or absence of a synovial cavity and type of connecting tissue.  There are three kinds: 1. fibrous (bones are held together by dense collagen fibers), 2. cartiliginous (joints are held together by cartilage) and 3. synovial (held together by ligaments).  Functional classification is based upon how much it can move and there are also three kinds.  1. immovable or synarthrotic, 2. slightly movable or amphiarthrotic and 3. freely moveable or diarthrotic.

Fibrous joints do not have a synovial cavity.  The articulating bones are held closely together by dense irregular connective tissue.  They permit little or no movement.  They are also sometimes called sutures (ragged edges of adjacent bones re held together by fibers).

Cartilaginous joints lack a synovial cavity as well.  There are two types.  1. synchondroses which are bones held together by hyaline cartilage that are synarthrotic and 2. symphyses that are bones held together by fibrocartilage that are amphiarthrotic.

Synovial joints have a synovial cavity that allow them to be diarthrotic.  The articular cartilage covers ends of bones to reduce friction and absorb shock.  The articular capsule has two layers that surround the joint.  Fibrous capsule is the ligaments that attach to periosteum around the bone.  The synovial membrane is the inner lining of the capsule and is loose connective tissue.  The synovial membrane secretes synovial fluid.  The synovial fluid brings nutrients to articular cartilage because cartilage is avascular.  There are two accessory ligaments: the extracapsular ligaments and the intracapsular ligaments (acl).  The menisci stabilize the joint by providing a better fit of articulating bones.  There are medial and lateral menisci of the knee: c-shaped like 2 half stadiums facing each other.  Bursae are the saclike structures of connective tissue that are lined with synovial membrane and filled with synovial fluid  They function in cushioning and reducing friction.

The types of movement at synovial (diarthrotic) joints include:
1. gliding movement: the simple movement of back and forth and side to side.
these are mostly found in intercarpal joints.  They permit mainly sliding, back and forth and twisting movements.
2. angular movement: increases or decreases the angle between articulating joints.
a. flexion: decreasing the angle between bones. ex: bending the trunk forward
b. extension: increasing the angle between bones.  ex: in anatomical position, the body is in extension.
c. hyperextension: continuation of extension beyond anatomical position.
d. abduction: movement away from the midline
e. adduction: movement toward the midline
f. circumduction: making a cone with the distal end of the bone.
3. rotation: a bone revolves around it's longitudinal axis ex: turning your head side to side to say no.
4. special movements.
a. supination: turning palms up
b. pronation: turning palms down
c. plantar flexion: standing on tippy toes
d. dorsiflexion: press soles of feet together and lift arch medially
e. eversion: press soles away from each other and press arch medially
f. depression: lovering a body region
g. elevation: raise a body region
h. protraction: move body part anteriorly
i. retraction: move protracted body part back to normal.

Types of synovial joints:
Planar joints primarily permit back and forth or side to side movement.  These include intercarpal and intertarsal joints.
Hinge joints have a spool that fits into a concave area.  They permit flexion-extension movements.  The knee and elbow are good examples.
A pivot joint is when the rounded surface of bone articulates with a ring formed by second bone and ligament.  Monoaxial since it allows only rotation around the longitudinal axis.  Examples include the proximal radioulnar joint of the hand.
Condyloid or Ellipsoidal joints are oval shaped projections that fit into an oval depression.  They are biaxial and can flex/extend or abduct/adduct.  The best example is your wrist.
A saddle joint looks like a person riding on a saddle.  Examples include the thumb joint.
The ball and socket joint has a ball that fits into a cuplike depression.  Movement is capable in all planes and rotations.  The only examples include the hip and shoulder.
A sprain is a stretch or tear of a ligament.  More serious sprains involve complete tears of one or more ligaments.  A common knee sprain involves the anterior cruciate ligament.
Strains are defined as a partially or completely torn muscle or tendon.  Patellar tendinitis or jumper's knee is a common strain that usually results from overuse.  Tendonitis is an inflammtion of the tendons and synovial membranes surrounding certain joints.