occasionally the lowest lumbar vertebra is fused on also - "the socialisation of L5".
The weight of the trunk passes from the vertebral column to the
ilium at the sacro-iliac joint:
below this, the sacrum tapers away and the final part of the vertebral column
- the coccyx - is very small indeed.
The sacrum is potentially unstable in that the weight of the vertebral column above may:
- cause its downward displacement
- cause it to tilt so that its upper end moves forwards and down and its lower end backwards and up.
These movements are prevented by strong ligaments such as the interosseous sacro-iliac, sacrotuberosus and sacrospinous ligaments.
