Articulations

1. Explain how the structure of a synovial joint is adapted to the function it performs 2. Identify the joints and the movement they allow 3. Examine the structure of the knee joint and describe its range of motion
 * __Joints/Articulations__**

1. __**Articular cartilage**__. Glassy-smooth articular cartilage covers the opposing bone surfaces. These thin but spongy cushions absorb compression placed on the joint and thereby keep the bone from being crushed 2. **__Joint cavity.__** A feature unique to synovial joints, the joint cavity is really just a potential space that contains a small amount of synovial fluid 3. __**Articular capsule**__. The joint cavity is enclosed by a two-layered articular (joint) capsule. The external layer is a tough fibrous capsule, composed of dense irregular connective tissue, that is continuous with the periostea of the articulating bones. It strengthens the joint so that the bones are not pulled apart. The inner layer of the joint capsule is a synovial membrane composed of loose connective tissue. Besides lining the fibrous capsule internally, it covers all internal joint surfaces that are not hyaline cartilage. 4. __**Synovial fluid**__. A small amount of slippery synovial fluid occupies all free spaces within the joint capsule. This fluid is derived largely by filtration from blood flowing through the capillaries in the synovial membrane. Synovial fluid has a viscous, egg-white consistency due to its content of hyaluronic acid secreted by cells in the synovial membrane, but it thins, becoming less viscous as it warms during joint activity. Synovial fluid, which is found within the articular cartilages, provides a slippery weight-bearing film that reduces friction between the cartilages. The synovial fluid is forced from the cartilages when a joint is compressed; then as pressure on the joint is relieved, synovial fluid seeps back into the articular cartilages like water into a sponge, ready to be squeezed out again the next time the joint is loaded. This mechanism, called weeping lubrication, lubricates the free surfaces of the cartilages and nourishes their cells. Synovial fluid also contains phagocytic cells that rid the joint cavity of microbes and cellular debris. 5. __**Reinforcing ligaments**__. Synovial joints are reinforced and strengthened by a number of bandlike ligaments. Most often, these are capsular, or intrinsic ligaments; that is, they are thickened parts of the fibrous capsule. In other cases, they remain distinct and are found outside the capsule (extracapsular ligaments) or deep to it (intracapsular ligaments). The articular capsule and ligaments are richly supplied with sensory nerve ending that monitor joint position and help to maintain muscle tone.
 * __1. Explain how the structure of a synovial joint is adapted to the function it performs__** Synovial joints have five distinguishing features:

Besides the basic components described above, certain synovial joints have other structural features. Some, such as the hip and knee joints have cushioning fatty pads between the fibrous capsule and the synovial membrane or bone. These fluid filled sacs located in the areas where muscles and tendons glide over bones are called BURSA sacs. Others have discs or wedges of fibrocartilage separating the articular surfaces. Where present, these so-called articular discs or menisci extend inward from the articular capsule and partially or completely divide the synovial cavity in two. Articular discs improve the fit between articulating bone ends, making the joint more stable and minimizing wear and tear on the joint surfaces.

= = == =**__2. Identify the joints and the movement they allow__**= [|Animation of various joint movement] Click on last animation - A Functional Classification of Synovial Joints

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 * Joints ****- the site where two or more bones meet. Our joints have two fundamental functions: giving our skeleton mobility and structure. Joints are the weakest parts of our skeleton. Nonetheless, their structure resists various forces, such as crushing or tearing, that threaten to force them out of alignment. There are many different kinds of joints.**======


 * Fibrous Joints**- the bones are joined by fibrous tissue; no joint cavity is present. The amount of movement allowed depends on the length of the connective tissue fibers uniting the bones. Most fibrous joints are immovable. Examples of fibrous joints are: the sutures of the cranium and the distal end of the tibia and fibula.
 * Cartilaginous Joints**- the articulating bones are united by cartilage; like fibrous joints they lack a joint cavity. Examples of cartilaginous joints are: the pubic symphysis of the pelvis and the first ribs and the sternum.

[[image:http://www.shelfieldpeonline.co.uk/assets/images/joints.gif width="279" height="379" align="right"]]
Types of synovial joints include: //Plane Joints//- the articular surfaces are essentially flat, and they allow only short gliding or translational movements. Examples are the gliding joints of the intercarpal and intertarsal joints and the joints between vertebral articular processes. It does not involve rotation around any axis. //Hinge Joints//- a cycllindrical projection of one bone fits into a tough-shaped surface on another. Motion is along a singe plane and resembles that of a mechanical hinge. The uniaxial hinge joints permit flexion and extension only by bending and straightening the limb.
 * Synovial Joints**- those in which the articulating bones are separated by a fluid-containing joint cavity. This arrangement permits substantial freedom of movement and all synovial joints are freely movable. Range of motion allowed by synovial joints vary from nonaxial movement to uniaxial movement to biaxial movement to multiaxial movement. There are three general types of movements: gliding, angular, and rotation. Range of motion can also be determined by the overall condition of the body. A small body with weak muscles is less likely to maximize the range of motion as a larger body with strong muscles. Obesity can interfere with the range of motion. Although "double jointed limbs" makes one think of the joints, it actually refers to the ligaments' ability to be flexible and safely stretch beyond ordinary perceived human limitations.

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//Pivot Joints-// the rounded end of one bone protrudes into a "sleeve" or ring composed of bone of another. The only movement allowed is uniaxial rotation of one bone around its own axis. An example of the joint allows you to move your head from sided to side to indicate "no".======

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//Condyloid Joints//- the oval articular surface of one bone fits into a complementary depression in another. The important characteristic is that both articulating surfaces are oval. The radiocarpal (wrist) joints and the metacarpophalangeal (knuckle) joints are typical condyloid joints.======

//Ball-and-Socket Joints// - are the spherical or hemispherical head of one bone articulates with the cuplike socket of another. These joints are multiaxial and the most freely moving synovial joints. Universal movement is allowed in all axes and planes including rotation. The shoulder and hip joints are examples. Injuries such as a torn rotator cuff or dislocated/separated shoulder are common in the arm regarding the shoulder. When a lot of force is placed on the shoulder it can have serious damage to the arm and can become separated or dislocated. A torn rotator cuff is a common injuries among baseball pitchers and separated and dislocated shoulders are very common in a big contact sport like football. Examples of ball- and- socket joints: the femur and pelvis and the humerus and scapula.

[|Description of Joints Video]

 =__**3. Examine the structure of the knee joint and describe its range of motion**__= The knee joint is one of the most complex joints and is most prone to injury. Bones in the knee provide the "rigid structure" of the joint. The four bones that make up the knee are the femur, fibula, tibia, and patella. The patella slides on the knee joint while the knee bends. The knee is able to flex and extend (bend and straighten). The ligaments in the knee joint hold it all together, and help stabilize the knee. They are strong elastic bands of tissue that connect one bone to another. The four main stabilizing ligaments of the knee are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). The ACL and PCL are the major stabilizing ligaments of the knee and help control how far the tibia can move in relation to the femur. The knee joint also contains the meniscus (a.k.a meniscal cartilage) which provides some strength. The bursa is what helps the muscles and tendons slide freely. A bursa is a small fluid sac around the knee joint. When alot of pressure is put on the knee sometimes these bursa sacs can pop. These will need time to heal themselves and can cause pain to the knee. There are two basic groups of muscles at the knee. In the front of the knee are the quadriceps muscles that attach to the front of the tibia and originate at the top of the femur, also helping to straighten the leg out, as seen in the photo above, to the left. Your quads are also the strongest and leanest muscles in the human body which can be divided into four separate portions. In the back of the knee are the hamstring muscles which are the tendons that make up the borders of the space behind the knee, and help to flex the knee, seen in the photo above, to the right. Injuries to the MCL and ACL are very common in sports and can take a long time to rehab. These are caused by sharp cuts and blows directly to these ligaments and since they are ligaments they take longer to heal. The way to heal these injuries is to use arthroscopic knee surgery. Arthroscopic knee surgery refers to a procedure in which a joint is viewed using a small camera. It allows an orthopaedic surgeon to diagnose and treat knee disorders by providing a clear view of the inside of the knee. [|Link to video of knee ligament anatomy]

Link to knee articulation information- info starts at about 3:30 into the video. []

[|Video Reconstructing ACL]
 * Knee joint[[image:http://www.arthroscopy.com/knee3.jpg align="right"]]**



=Anatomy of the Shoulder= The shoulder joint is composed of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). (See diagram.) Two joints facilitate shoulder movement. The acromioclavicular (ah-KRO-me-o-klah-VIK-u-lahr; AC) joint is located between the acromion (ah-KRO-me-on; part of the scapula that forms the highest point of the shoulder) and the clavicle. The glenohumeral joint, commonly called the shoulder joint, is a ball-and-socket-type joint that helps move the shoulder forward and backward and allows the arm to rotate in a circular fashion or hinge out and up away from the body. (The “ball,” or humerus, is the top, rounded portion of the upper arm bone; the “socket,” or glenoid, is a dish-shaped part of the outer edge of the scapula into which the ball fits.) The capsule is a soft tissue envelope that encircles the glenohumeral joint. It is lined by a thin, smooth synovial membrane. Shoulder The two main bones of the shoulder are the **humerus** and the **scapula**(shoulder blade). The joint cavity is cushioned by **articular cartilage** covering the head of the humerus and face of the glenoid. The scapula extends up and around the shoulder joint at the rear to form a roof called the **acromion**, and around the shoulder joint at the front to form the **coracoid process**. The joint is stabilized by a ring of fibrous cartilage surrounding the glenoid called the **labrum**. || || All of these components of your shoulder, along with the muscles of your upper body, work together to manage the stress your shoulder receives as you extend, flex, lift and throw. || ||
 * The end of the scapula, called the **glenoid**, meets the head of the humerus to form a glenohumeral cavity that acts as a flexible ball-and-socket joint.
 * [[image:http://www.scoi.com/images/scoi-shoulder-ligs.jpg width="200" height="180"]] || Ligaments connect the bones of the shoulder, and tendons join the bones to surrounding muscles. The **biceps tendon** attaches the biceps muscle to the shoulder and helps to stabilize the joint. ||
 * muscles originate on the scapula and pass around the shoulder where their tendons fuse together to form the **rotator cuff**. (Get the FAQs on [|rotator cuff tears]).