The Articular System

I've spoken a fair bit about the specifics of the vertebral column, but it's time to go a bit more general and talk about the joints that are found between bones all over the body. Yay!

Types of Joints

Joints can be classified in different ways. They can be classified in terms of moveability (i.e. moveable vs. immoveable) or via their structure (fibrous, cartilaginous or synovial). These are probably best represented in a table. Good thing I had one prepared earlier (for my end-of-topic revision thing, hehe).

I don't think there's much else that I can add to the above diagram, other than a few more random factoids about each joint type. (I'll only cover the fibrous and cartilaginous joints, as I'm going to ramble on about synovial in a bit.)

• Suture
• Fibrous tissue used: dense regular connective tissue
• Examples: Coronal suture, frontal suture etc. (basically sutures in the skull)
• Tend to fuse with age
• Syndesmosis
• Fibrous tissue used: interosseous ligaments, which are made of dense regular connective tissue
• Examples: radioulnar syndesmosis, tibiofibular syndesmosis
• Synchondrosis
• Examples: epiphyseal growth plates (in children), between the first rib and manubrium. (Synostosis is the fusion of two bones, such as when the epiphyseal plate closes in children.)
• Symphysis
• Movement varies according to thickness
• Cartilage used: articular hyaline cartilage (capping the bones) and fibrocartilage (between bones)
• Examples: pubic symphysis, intervertebral discs, between sacral and coccygeal bones

Synovial Joints

Synovial joints are kind of special as they have both fibrous and cartilaginous bits. Articular hyaline cartilage covers the surfaces of bones involved in synovial joints while the entire thing is enclosed in a fibrous capsule. The inside of the capsule is lined with a synovial membrane that produces synovial fluid, which lubricates and nourishes the joint.

As mentioned in the picture above, cartilaginous bits (i.e. articular cartilage and menisci- more on menisci later) get no nerve supply. The joint capsule, ligaments and tendons do, however. There's also a neat little law called Hilton's Law that states that if a nerve supplies a muscle that crosses a joint, it will also supply sensory fibres to that joint.

The blood supply is a little different. Bones and muscles have their own blood supply. The synovial membrane is supplied by small arteries, and the fluid it produces nourishes the articular cartilage. These arteries also have many anastomoses (i.e. alternative routes for blood) just in case some of the arteries are squashed due to the positioning of the joint. Fibrous and cartilaginous parts do not require a rich blood supply.

Special Features of Synovial Joints

Not all synovial joints are created equally. Some have special features which make them more suitable for various purposes. Let's take a look at some of these special features:

• Labria- these are like "lips" that deepen the socket of ball and socket joints (probably allowing for greater stability?)
• Bursae- these are like synovial joints without the articular cartilage. These normally form between tendons and bones.
• Intra-articular discs- these are fibrocartilaginous discs that divide the joint cavity in half. Usually different movements occur on different sides of the disc. The temporomandibular joint (between the temporal bone and the mandible) is one example of a joint with an intra-articular disc. Above the disc, protraction and retraction occur; below the disc, elevation and depression occur.
• Menisci- these are like partial intra-articular discs. There are different movements above and below these as well.
• Fat pads- these are like "soft menisci." They are more likely to be found in facet joints.

Movement of Synovial Joints

The movement of synovial joints depends a lot on the shape of the joint and the bones that form it.

Uniaxial joints have one degree of freedom- for example, they can usually flex/extend, or rotate, but not both. These tend to be bicondylar in shape (i.e. have two little "bumps" or condyles), or pivot joints.

Biaxial joints have two degrees of freedom, usually flexion/extension and abduction/adduction. These tend to be condylar or saddle joints.

Finally, multiaxial joints can flex/extend, abduct/adduct and rotate. These include ball and socket joints as well as plane/facet joints.