Kyphosis may be caused by poor posture during childhood or be the result of abnormally shaped vertebrae or developmental problems with the spine.

Your spine extends from your skull to your pelvis. It's made up of 24 individual rectangular-shaped bones called vertebrae, which are stacked on top of one another.

The vertebrae are separated by soft pads, or discs, which act as shock absorbers. The vertebrae are held together by tough bands of tissue called ligaments. Together with the spinal muscles, the ligaments give the back its strength.

In cases of kyphosis, the middle section of vertebrae, known as the thoracic vertebrae, are curved out of position.

Everyone has some amount of curvature in their spine to allow space inside the chest for organs such as the heart and lungs. There is a range of curvature which is considered normal. However, excessive curvature can lead to symptoms.

There are several reasons why the vertebrae can be affected in this way.

Poor posture in childhood, such as slouching, leaning back in chairs and carrying heavy schoolbags, can cause the ligaments and muscles that support the vertebrae to stretch. This can pull the thoracic vertebrae out of their normal position, resulting in kyphosis. 

Kyphosis caused by poor posture is known as postural kyphosis.

Kyphosis can also be caused when the vertebrae don't develop correctly. They can take on a wedged, triangular shape, rather than the normal rectangular, box-like shape. This leads to the vertebrae being out of position and is known as Scheuermann's kyphosis.

In people with Scheuermann's kyphosis, the ligaments surrounding the vertebrae can also be thicker than normal, which may contribute further to the condition.

It's not known what disrupts the normal formation of the spine. One idea is that the blood supply to the vertebrae becomes disrupted, affecting the growth of the vertebrae. There also appears to be a genetic link, as the condition occasionally runs in families.

Congenital kyphosis is caused when something disrupts the normal development of the spine before birth. In many cases, two or more of the vertebrae fuse together.

It's often unclear why certain children are affected in this way. However, some cases of congenital kyphosis run in families, so it seems genetics also play a role in this type of kyphosis.

Conditions that can cause kyphosis include:

• osteoporosis – where the bones become weak and fragile and more likely to break (fracture)
• cervical spondylosis – a term that describes the general "wear and tear" that occurs in the bones, discs and ligaments of the spine as a person gets older
• spina bifida – a condition where the spine hasn't formed properly
• Paget's disease of bone – a condition in which the development of new bone cells is disrupted, resulting in the bones becoming weak
• neurofibromatosis type 1 – a genetic disorder that affects the nervous system
• muscular dystrophy – a genetic condition that causes progressive weakening of the muscles
• tuberculosis (TB) – a bacterial infection that mainly affects the lungs
• cancer that develops inside the spine or spreads to the spine from another part of the body

Kyphosis can also sometimes develop as a result of an injury to the spine.