Normal Joints

=Objectives=

(No Objectives were supplied for this lecture, this are entirely student-created)

Define

 * Synarthrosis: a joint that is fused together and has no movement, aka "fibrous synarthroses". Ex: skull sutures, roots of teeth and jaw.
 * Amphiarthrosis: a joint that is a bit more flexible and usually has a fibrocartilage disk involved in the joint, aka "cartilaginous synarthroses" or "synchondroses" Ex: symphysis pubis, intervertebral disk.
 * Diarthrosis: highly mobile joint with hyaline cartilage and a synovium ("synovial joint"). Ex hip joint.  Types of these joints are: ball & socket (flex, extend, abduct, adduct, rotate), hinge (flexion), saddle (flex, extend, limited adduct/abduct) and plane (translate).
 * Articular cartilage: also known as hyaline cartilage. It has a matrix full of things mentioned below and nests of chondrocytes that make it all.
 * Collagen: alpha chain, smallest unit of fibril, has lots of glycine, proline and hydroxyproline. 50-60% of dry weight of cartilage, which mostly has Type II collagen.
 * Proteoglycan: Big bottle brush looking molecules that are composed of hyaluronic acid and Chondroitin sulfate with a bit of keratin sulfate. It functions to hold a ton of water and make the joint more flexible and squishy.
 * Hyaluronic acid: a glucosamine that is the core protein in Proteoglycans. Major component of synovial fluid
 * Chondrocyte: a differentiated cell that makes collagen, proteoglycans, and synthesizes enzymes, growth factors, and cytokines also respond to these things. Articular cartilage chondrocytes are the link between synthetic and degradative processes.  They alter synthetic function in responses to changes in matrix degradation.  The frequency and intensity of joint loading influences chondrocyte metabolism.  Immobilization of a joint leads to increased proteoglycan degradation that exceeds synthesis.  Joint health and function is linked to regular use.
 * Synovium: surrounds the joint but is NOT A MEMBRANE!!! Has fenestrated capillaries and a matrix of collagen and proteoglycans.
 * Hydrodynamic lubrication: Pressure is put on the opposing joint the bone squeezes water out of the fluid rich matrix into the joint space, when the bone is not pressing on the opposing surface the matrix of the cartilage sucks water back up in to it so it can do it again.
 * Cartilage Extracellular Matrix: This is the fluid and framework. The collagen framework is filled with water and proteoglycans.  The interaction of the fluid with the framework gives mechanical properties.  The tissue fluid is 80% water and 20% gases, small proteins and metabolites.  Electrostatic interactions link water and proteoglycans inside the collagen matrix producing the biomechanical properties.
 * Articular cartilage zones: The superficial zone contains superficial cells and the protein Lubricin, decorin and biglycan. The Middle Zone is the pericellular region with Decorin and Type IV collage.  The Deep Zone has the territorial region where there is more intact aggrecan, and the interterritorial region where there are degraded aggrecans.  The Calcified Zone is the deepest layer and it has a tidemark, type X collagen, hypertrophic chondrocytes, subchondral bone and subchondral bone marrow.
 * Synovial Fluid: It is a protein-rich ultrafiltrate of plasma and hyaluronan synthesized by synoviocytes. There is a high hyaluronic acid concentration and rapid turnover with a T1/2 of 13 hours.  Concentration of proteins in synovial fluid is inversely proportional to molecular size.  The fluid is cleared by lymphatics in the synovium, assisted by pressure generated during joint movement.

Give examples of each of the 3 major types of joints.
Synarthrosis -> skull, amphiarthrosis -> pubic symphysis, diarthrosis -> knee

Describe the major components in and immediately around a synovial joint.
In the joint we have the capsule, the synovium, the fibrocartilage pad, the hyaline articular cartilage, and the bones. Outside the joint we may or may not have: a bursa or two, tendons, muscles, ligaments, enthesis (where the tendon attaches to the bone) fat and subcutaneous tissue. You know the ligaments may be considered in the joint if you want to be fancy.

Metabolic Supply of Cartilage
Cartilage derives nutritional support from synovial fluid. There is a highly specialized nutrient system. Molecules pass through double diffuse barrier that limits the size, charge, and unique molecular configurations of what reaches the chondrocytes. The first barrier is the synovial tissue and synovial fluid. The second barrier is the cartilage matrix. Due to the low synovial fluid O2 concentration, chondrocytes primarily use anaerobic metabolism.

Three factors which confer stability on joints
The joint shape, the size and number of ligaments and muscles involved at the joint, the synovial fluid that makes an adhesive like seal, and the negative intra-articular pressure. He said the hip was more stable than the knee.
 * Muscle conditioning is an important part of joint health; muscles and tendons absorb more force than the cartilage or bone.

Draw the synovium and label all the nifty parts to it



 * The synovium is made of: a matrix of collagen and proteoglycans, a rich bed of fenestrated microvessels, and synoviocytes.
 * It has no basement membrane.
 * Type A synoviocytes (~90%) are macrophage-like. They have intracellular inclusions, lysosomes and stellate processes.  They are phagocytic and clear particulate matter from the joint.
 * Type B synoviocytes (~10%) are fibroblast-like. The have ER and protein synthetic organelles.  They make collagen and proteoglycan as well as Lubricin.
 * Synovial blood flow is autocrine, neural and hormonally regulated.

Describe 2 roles the synovial fluid plays in normal joint physiology
Its main role is the passive transport of fluid, metabolites, and other macromolecules to/away from the cartilage surface to keep it well nourished and free of metabolic byproducts. It also plays a minor role in lubrication and stability of the joint.

Joint Function

 * There is an extremely low coefficient of friction due to the boundary layer which contains lubricin and lipids, the synovial fluid (hyaluronate) and the hydrodynamic forces (squeeze film, or boundary lubrication as defined above). It functions as stress protection and distribution.  Joints can repair themselves.

Outline the 4 major classes of musculoskeletal pain, and in turn the two major classes of arthritis.
The 4 classes are Arthritis, periarticular which is from things around the joint like tendons, ligaments, bursa etc, Non-articular which is due to bone or muscle pains, and finally Referred pain which is often neurologic in origin. Arthritis can further be broken down into Inflammatory and non-inflammatory Arthritis.