Two neighbouring vertebrae, together with the disc between them, the small facet joints and the ligaments, form a motion segment. Unstable means that the segment moves more under load than it should. The most common form is spondylolisthesis, in which one vertebra slips forwards in relation to the one below.
In wear-related spondylolisthesis, the disc and facet joints weaken, usually between the fourth and fifth lumbar vertebrae. Another form often begins in adolescence with a gap in the vertebral arch. Less commonly, instability develops after fractures, inflammation, tumours or previous operations.
Typical symptoms are deep low back pain that increases when standing, walking and lifting and eases at rest. If the displaced vertebra presses on nerves, pain, numbness or weakness in the legs can follow, sometimes also a shorter walking distance. However, how far a vertebra has slipped says little about how severe the symptoms are.
Most symptoms can be treated without surgery. Fusion – specialists speak of spondylodesis – becomes an option if thorough conservative therapy does not allow a satisfactory quality of life or if nerves are under lasting pressure. Because it changes the spine permanently, psychological strain and your personal circumstances are also part of the decision.