Joints

Just a quickie (okay, a quickie following my definition of "quickie") on joints!

There are three main types of joints that can be classified according to range of movement (functional classification) or their structure (structural classification):

• Fixed or fibrous joints: No movement occurs between the bones because they're held in place by fibrous connective tissue. Very difficult to damage this type of joint. Can be found between bones of the skull.
• Slightly movable or cartilaginous joints: Bones held together by cartilage. Slight movement can occur, but not a lot. Examples: between the two pelvic bones (symphysis pubis), between vertebrae, between ribs and sternum
• Freely movable or synovial joints: Most of the joints can move in many directions. Even though their movement is somewhat restricted according to the shape of the joint, they're still classified as freely movable joints.

As you can probably guess, we're going to be focusing on the 3rd type (probably the most interesting type, since there's movement involved!).

There are many types of freely movable joints:

• Ball-and-socket joints: One bone has a spherical head, while the other has a kind of cup-like cavity for the spherical head to fit into. Allows movement in all directions. Examples: shoulder joint, hip joint
• Hinge joints: One bone has a convex surface which slots into the concave surface of the other. It might sound a bit like a ball-and-socket joint, but the surfaces are less rounded (I think) which results in the joint only allowing movement in one direction but not in others. Examples include the elbow and the knee.
• Pivot joints: One bone has a circular, pointed or conical end, which rotates upon the axis of another bone. A prime example of this is the joint between the first vertebra (on which the head is balanced) and the second vertebra.
• Gliding joints: Movement can occur in any direction, hindered only surrounding ligaments or bones. Examples: between carpal bones, between tarsal bones, between the sternum and clavicle, between the scapula and clavicle
• Saddle joints: The two bones of the joint are saddle-shaped (i.e. concave in one direction, convex in the other). This allows both side-to-side and back-and-forth movements. Pretty much only seen in the joint between the thumb and the palm of the hand.

Now for a bit of technical stuff on the structure of synovial (freely movable) joints!

• The whole joint is surrounded by a capsule, which consists of two layers. The outer layer is called the fibrous capsule and consists of dense, fibrous connective tissue, which is attached to the periosteum of the bones (periosteum = the hard white bit. See my post on the skeleton for more info). It's strong yet flexible, allowing movement but resisting dislocation.
• The inner layer of the capsule is called the synovial membrane (hence the name "synovial joint"). It consists of loose connective tissue with blood capillaries. The synovial membrane lines the entire joint cavity aside from the articular cartilages (cartilage that "caps" the ends of the bones) and the articular discs (I'll get to them later).
• The synovial membrane secretes synovial fluid, which lubricates the joint, provides nourishment for the cells of the articular cartilage, and carries phagocytic cells that remove microorganisms and debris resulting from wear-and-tear. Only a small amount of fluid is usually present, but if the joint is injured or inflamed, more fluid may be produced, resulting in swelling and discomfort.
• Articular cartilage is located on the ends of the bones, providing a smooth surface for movement.
• Articular discs occur in some joints and divide the synovial cavity into two, allowing synovial fluid to be directed to areas of greatest friction. In the knee, the articular discs are called menisci (singular meniscus), and consist of fibrocartilage which extends inwards from the articular capsule.
• Bursae are little sacs of synovial fluid and, like articular discs, they are only present in some joints. Bursae are positioned in such a way to prevent friction in certain parts of the joint.
• Accessory ligaments hold the bones together in many joints.

Several forces keep the articulating bones in contact with each other: the shape of the bones, the strength of the joint ligaments and the tension provided by the muscles around the joint.

Now for some more technical terms on the types of movement that occurs at joints:

• Flexion or bending: The angle between bones is decreased (e.g. bending the knee).
• Extension or straightening: Opposite of flexion. Angle between bones is increased (e.g. straightening out the leg after flexion.)
• Abduction: (no, I do NOT mean "kidnapping.") Movement away from the midline of the body (e.g. moving your arm up)
• Adduction: Opposite of abduction. Movement towards the midline of the body.
• Rotation: Bone rotates around its long axis. The humerus, for example, can rotate around quite a bit.

Now for the obligatory "what can go wrong" part...

• Arthritis: Includes many types of inflammation of the joints.
  Rheumatoid arthritis is a severe form involving inflammation of the joint, swelling, pain and loss of function. Firstly, the synovial membrane is inflamed, and then abnormal tissue known as pannus is produced, which grows over the surface of the articular cartilage. It can destroy the cartilage, or even erode the bone. Eventually it becomes invaded by fibrous tissue. In severe cases this tissue ossifies (changes into bone), making the joint entirely immovable.
  Osteoarthritis is much more common, but much less damaging. It involves deterioration of the articular cartilage, causing bony spurs to develop from the ends of the bone forming the joint. Due to these spurs, the space in the joint is decreased, which in turn restricts the movement of the joint.
• Dislocation: A bone is displaced, and ligaments, tendons or capsules are torn. Symptoms include temporary paralysis of the joint, pain, swelling and occasionally shock.
• Sprains: A ligament is torn from the bones. Blood vessels, muscles, tendons, ligaments and nerves may also be damaged. Symptoms include swelling, pain and discolouration due to ruptured blood vessels.
• Slipped disk: Part of the invertebral disc (fibrocartilage between vertebrae) is squeezed to one side, displacing the disc. Depending on how the disc has "slipped," it can put pressure on spinal nerves of the spinal cord, causing severe pain and numbness. This can then result in nerve damage, which then causes weakness and possible degeneration to the tissue of the muscles supplied by the damaged nerves.
• Tendinitis: Inflammation of the tendon sheaths surrounding certain joints. Symptoms include swelling and pain upon movement of the affected joint. RSI (repetitive strain injury) is a form of tendinitis that might affect the wrists of computer keyboard operators or clarinet players.
• Whiplash: The cervical vertebrae allow quite a lot of flexibility which is great most of the time, but in a sharp impact this flexibility can cause the head to fling back and forth. This can then cause ligaments to tear and internal bleeding to occur. Nerves may also be injured. There are many symptoms of whiplash, including headaches, dizziness, nausea, pain and weakness. If the injury is such that the axis is driven into the brain stem, death may occur.

Maybe I should leave it there so that the post can end on an ominous note... nah. I'm too kind for that. Good night and sweet dreams! (Unless you're reading this during class, in which case... GET BACK TO YOUR WORK!)