Chapter 8 - Articulations

     Wherever two bones come together there is an articulation, or joint. The function and mobility of each joint depends on its anatomical design. Joints that permit no movement or slight movement are common in the axial skeleton where basic structural support and protection  is expected. Joints that are freely moveable are common in the appendicular skeleton where mobility is required. 
Classification of Joints
     Joints can be classified according to the range of movement permitted. This puts joints into functional categories. Within each functional category joints can be further subclassified according to structural differences. This puts joints into structural categories. 
Functional Categories:
1. Synarthrosis (Immovable Joint)
     The range of movement in this type of joint ranges from none to very slight depending on its design. Structural categories are:
a. Suture
     In these joints the bones interlock and are held together by fibrous connective tissue. Sutures are common between bones of the skull. 
b. Gomphosis
     This joint holds the roots of the teeth firmly in their sockets. The fibrous connection between the root and the socket is the periodontal ligament. 
c. Synchondrosis
     This joint holds bones together with hyaline cartilage. An example is the epiphyseal cartilage that holds together the epiphysis and diaphysis of long bones. 
d. Synostosis
     This joint does not appear to be a joint because it represents the fused boundary between two bones. This fusion happens at some sutural joints and at the epiphyseal plates when they close. 
2. Amphiarthrosis (Slightly Movable Joint)
     The range of movement in these joints is limited. 
a. Syndesmosis
     The articulating bones in this joint are held together by a ligament. The distal articulation between the tibia and fibula is an example. 
b. Symphysis
     The bones in this articulation are held together by a pad of fibrocartilage. Examples include intervertebral discs and pubic symphysis. 
3. Diarthrosis (Freely Movable Joint)
     These joints permit free movement between the apposing articulating surfaces of bones. Movement is limited only by the shape of the articulating bones and the accessory structures that hold the bones together. There is only one type of diarthrosis, the synovial joint.
  Structural Features of Synovial Joints
     The touching surfaces of bones in synovial joints are lined by articular cartilage that reduce friction and act as shock absorbers. Friction is further reduced by synovial fluid. Synovial fluid is a viscous fluid that is produced by the synovial membrane that lines the interior of the joint capsule. The joint capsule surrounds and supports the synovial joint and is composed of dense connective tissue. 
     Synovial fluid functions include:
a. Provides lubrication
     Synovial fluid contains a glycoprotein called lubricin that enhances the lubricating qualities of the fluid. 
b. Nourish chondrocytes
     Articular cartilage is avascular and depends upon synovial fluid for nourishment and waste removal. 
c. Absorb shock
     Synovial fluid helps to cushion and evenly distribute compression shocks. 
  Accessory Structures of Synovial Joints
     Accessory structures strengthen synovial joints and make them work more efficiently.
Cartilage and Fat Pads
      In complex joints such as the knee:
     Menisci (articular discs) are fibrocartilage pads that help to channel the flow of synovial fluid and stabilize the joint as it moves. 
     Fat pads acts as packing material that provides protection for the articular cartilages. 
     Accessory ligaments strengthen and reinforce synovial joints. Intrinsic ligaments are within the joint capsule. Extrinsic ligaments are either localized thickenings of the joint capsule or separate ligaments outside of the capsule. 
     The tendons of muscle that cross the joint provide support for the joint. An example is the tendons that form the "rotator cuff" of the shoulder joint. 
     Bursae are packets of connective tissue filled within synovial fluid and lined by synovial membranes. They reduce friction between bone, tendons, ligaments and soft tissue. Synovial tendon sheaths surround tendons that move across bony surfaces.
Structural Subclassification of Synovial Joints
     Synovial joints can be further subclassified according to the type and range of movement they permit:
Plane (Gliding) Joints
     Plane joints have apposing flat to slightly curved articular surfaces that slide over one another. The movement is slight. Examples of plane joints include intertarsal and intercarpal joints and the joints between the articular facets of vertebrae.
Hinge Joints
     These joints permit angular movement in a single plane. Examples include the elbow and knee joints.
Pivot Joints
     Pivot joints only permit rotation. The joint between the atlas and the axis is an example.
Condylar (Ellipsoidal) Joints
     In condylar joints, an oval convex articular surface moves in an oval depression. This permits angular movement in two planes. Condylar joints exist between the proximal phalangeal bones and the metatarsal and metacarpal bones in the hands and feet.
Saddle Joints
     In saddle joints, two saddle-shaped articular surfaces, orient at right angles to one another, appose one another in an interlocking fashion. This kind of joint permits a wider range of angular movement than the condylar joint. The joint at the base of the thumb is an example.
Ball-and-Socket Joint
     In this joint the hemispherical articular surface of one bone fits into the cup-shaped depression of the other. Angular and rotational movements are permitted. The shoulder and hip joints are examples.
Knee Joint
     The knee joint will serve as an example of a synovial joint. The knee joint is a complex joint in which the wheel-shaped femoral condyles "roll" on the flat tibial articular surfaces. The knee joint can be divided into three separate joints with there own separate joint capsules:
     the femoral medial condyles articulate with the tibial medial condyles;
     the femoral lateral condyles articulate with the tibial lateral condyles; and
     the patella articulates with the patellar surface of the femur.
     The medial and lateral condylar articulations between the femur and tibia have between them pads of fibrocartilage called the medial and lateral menisci. These menisci
1. absorb the compressive forces generate at this weight-bearing joint;
2. more evenly distribute the force transferred from the femoral articulations to the tibial; and
3. provide lateral stability.
  Supporting Ligaments
  Seven major ligaments support the knee joints:
     The patella (knee cap) is present in the tendon of the muscle that extends the knee. The patella ligament extends from the patella to its attachment to the tibia and supports the anterior side of the knee joint.
  The remaining ligaments are classified as either extracapsular or intracapsular depending on their position relative to the fibrous joint capsule. The extracapsular ligaments are:
     The tibial collateral ligament reinforces the medial side of the knee joint and the fibular collateral ligament reinforces the lateral side of the knee joint. Two popliteal ligaments reinforce the posterior side of the knee joint.
  The intracapsular ligaments are:
     The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) limit the anterior and posterior movement of the femur. The names of these ligaments derive from the way they cross one another and their relative attachments on the tibia.
Types of Movement
     All the movements described below assume the anatomical position.
Gliding - opposing flat surfaces slide over one another
Angular Movement
abduction - movement away from longitudinal axis
adduction - movement toward longitudinal axis
flexion - reduces angle of articulating elements in anterior-posterior plane
extension - increases angle of elements in anterior-posterior plane
hyperextension - movement past the anatomical position
circumduction - moving arm, leg or hand in a loop (complex movement)
Rotation (movement around longitudinal axis)
medial rotation - inward movement in reference to anterior
lateral rotation - outward movement in reference to anterior
pronation - movement of hand to palm-facing-backward
supination - movement of hand to palm-facing-forward
Special Movements
eversion - twisting of foot to sole outward
inversion - twisting of foot to sole inward
dorsiflexion - flexion of ankle to elevate sole
plantar flexion - flexion of ankle to elevate heel
lateral flexion - movement of vertebral column to side
protraction - anterior movement in horizontal plane
retraction - reverse of protraction
opposition - grasping movements between thumb and fingers
elevation - movement of structure in superior direction
depression - movement of structure in inferior direction