“flat back syndrome” or “fixed positive sagittal imbalance”
The normal shape of the spine
The normal spine is balanced with regards to the position of the head over the pelvis. This is true for the front view(coronal) plane and side view (sagittal plane). Scoliosis refers to abnormal curve of the spine in the coronal plane and these curves may be balanced or imbalanced where the head sits off to one side or the other compared to the centre of the pelvis. The spine has a normal curved shape in the sagittal plane. In the lumbar and cervical region there is a concavity at the back termed lordosis, whereas in the truncal or thoracic spine there is a convexity (kyphosis) to the spine at the back. Overall the head sits well balanced over the pelvis to neutralize the forces and the amount of work the spine muscles need to do to maintain upright posture.
When it all goes wrong
Sagittal imbalance occurs when the spine cannot be maintained in an upright position without significant compensation from the hips or knees. It is important to understand that as the spine undergoes age related changes it loses much of the normal lordosis and there is a gradual trend toward kyphosis. This occurs because of disc degeneration that leads to loss of disc height more at the front than the back and with osteoporosis the spine can undergo spontaneous wedge fracture that also add to the kyphosis. This is a “normal” aging process and for most people causes no major symptoms apart from the bent posture. If the hips remain mobile then most spines self adjust by allowing extension of the pelvis through the hips.
There are other causes of positive sagittal balance such as post-surgical when fusion is performed without adequate lordosis (hypolordotic), or when the spine falls off the top of a long fusion segment due to altered stiffness and accelerated degeneration. Also some patient’s are born with abnormal shape vertebrae that can lead to kyphosis in one area of the spine.
Why does it hurt?
The spinal extensor muscles (erector spinae) have to work overtime to bring the head and shoulders back over the pelvis in a spine with excessive forward bend (kyphosis). The constant activity in these muscles can lead to increased forces across the facet joints and degenerative arthritis. The erector spinae can also fatigue and cause muscle aching when trying to stand and walk. If the kyphosis is stiff then the pull of the muscles will not be effective and the muscles will fatigue more quickly.
However if the kyphosis becomes too much for the pelvic compensation mechanisms then there will be imbalance and hips and knees must flex to allow an upright position. This leads to inefficient use of the leg muscles and fatigue in these muscles also. Walking and standing can become very difficult and exhausting. This can be a very disabling condition once it has decompensated.
The first step in management is to recognise that there is a mechanical cause for the disabling back pain and document the deformity with long spinal x-rays. The size of the problem can then be appreciated.
Non-operative options include activity modification and the use of walking aids such as a stick or frame. These can help share the loads through the upper limbs and reduce the strain on the spine extensor muscles. Excercises to strengthen the CORE muscles are prescribed and sensible weight loss should be considered where appropriate.
If non-operative options fail and surgery is considered then it must be carefully planned depending on the nature of the deformity and underlying bone quality. Frequently an extension osteotomy of the spine will need to be part of the operative plan in order to increase the extension of the spine and bring the head back over the pelvis.