What happens inside a joint?

What do joints look like inside?

The most obvious joints in the body are synovial joints – the knee, hip, ankle, wrist and shoulder are all examples.  These are quite complex, and consist of the following characteristics:

 

1. Cartilage – The ends of the bones are covered with a smooth, white, shiny substance called cartilage.  This protects the bone tissue and helps to reduce friction between the bones. The tissue making up cartilage contains a high percentage of water which ‘cushions’ the joint and enables it to withstand high tensile forces/loads.

 

2. Articular Capsule – A strong, fibrous tissue that is attached to the bones and surrounds the joint. This adds stability to the joint and prevents unwanted material from entering.

 

It consists of two layers:

 

    • the outer fibrous membrane that may contain ligaments

    • the inner synovial membrane which is a fine membrane that lines the inside of the capsule but does not cover the cartilage.  It secretes synovial fluid. 

The articular capsule is highly innervated, but it lacks blood and lymph vessels.  It receives nutrition from the surrounding blood supply via either diffusion (slow process) or by convection, which is much more efficient and is achieved through exercise.

 

Synovial fluid is a yellowish oily fluid that:

 

    • Lubricates the ends of the bones where they meet.
    • Forms a fluid cushion between the bones.
    • Provides nutrients for the cartilage.

Many but not all synovial joints also contain:

 

  • Accessory Ligaments (extracapsular and intracapsular) -    Strong fibrous bands that attach joining bones.  They assist in the control of movement and stability of the joint.

  • Articular discs (menisci) -   Fibrocartilage pads between opposing joint surfaces. They act as shock absorbers, help with stability and protect the bone surfaces.

  • Articular fat pads – Adipose (fatty) tissue pads that protect the articular cartilage.

  • Bursae – Closed sacks filled with synovial fluid.  They are located where friction may develop, such as between the muscles and the bones.  They help lubricate the joint.

  • Peripheral labrum – a ring of fibrocartilage around the edge of the articular (joint) surface of a bone.

    Interesting Fact – ‘labrum’ was borrowed from the Latin for ‘lip’.