Normally, the spine is shaped in an even, slight S-shape with all of the elements fitting together. When spondylolisthesis occurs, one or more of the vertebrae slip out of their normal position.
This instability can have a number of different causes. Some congenital deformities of the spine can cause vertebrae to slide out of their normal position. Equally, gradual changes such as wear and tear, bone loss or previous fractures can also cause spondylolisthesis. Well-trained ligaments, muscles and tendons slow this displacement, since they provide additional grip.
Essentially, there are four degrees of severity (categorised according to Meyerding):
- Grade I: less than 25 % displaced
- Grade II: 25 to 50 %
- Grade III: 51 to 75 %
- Grade IV: more than 75% displaced