There are four factors that may contribute to joint hypermobility.
- the structure of your collagen (a type of protein found in some types of tissue)
- the shape of the ends of your bones
- your muscle tone
- your sense of your joint movements (proprioception)
These are explained in more detail below.
Collagen is a type of protein found throughout your body – for example, in skin and ligaments. Ligaments are tough bands of connective tissue (fibres that support other tissues and organs in your body) that link two bones together at a joint. They strengthen the joint and limit its movement in certain directions.
If the structure of your collagen is altered, it may not be as strong, and the tissues that contain collagen will be fragile. This can lead to weakened or easily stretched ligaments.
Changes in the structure of your collagen are likely to be caused by changes to your genes. Genes are units of genetic material that you inherit from your parents. They contain instructions that tell your body how to work. If the instructions within the genes change, it can alter the structure of your collagen.
Shape of the bone ends
A joint is the junction between two bones. The shape of the bones determines how far you can move your limbs.
For example, your limbs will be more flexible if the socket that the bone moves around in, such as the shoulder or hip socket, is shallow.
Children with joint hypermobility may have a degree of hypotonia (low muscle tone), which makes the muscles "floppy" and could mean the child is able to bend their joints more than usual.
Sense of joint movement
You should be able to sense the position and movement of your joints. For example, even with your eyes shut, you should know whether your arm is bent or straight. The medical term for this sense is "proprioception".
However, some people with joint hypermobility have an abnormal sense of joint movement and are able to sense when a joint is overstretched, giving them a wider range of movement.
In a small number of cases, joint hypermobility is associated with a more serious underlying condition. These are often inherited conditions, which parents pass to their children.
Some conditions that can cause joint hypermobility are described below.
Ehlers-Danlos syndrome (EDS) is the name for a group of uncommon conditions that affect connective tissues. There are four main types of EDS, most of which can affect the joints in some way.
Hypermobile EDS, previously known as EDS type III, is a form of the condition many experts now consider to be the same thing as joint hypermobility syndrome (JHS). This is the most common type of EDS and is estimated to affect around one in every 100 to 200 people.
Like EDS, Marfan syndrome affects the body's connective tissues.
The condition can cause hypermobile joints in addition to a number of typical characteristics, such as being tall and having abnormally long and slender limbs, fingers and toes. It can also cause potentially serious problems affecting the heart and eyes.
Osteogenesis imperfecta is a rare condition sometimes known as "brittle bone disease" because it causes fragile bones. Some forms of the condition can also cause joint hypermobility, along with a range of other problems.