Malformations of the sacrum and base of the spine are very common, with variations too numerous to cite. Being the seat of spinal movement, these structural anomalies subtly influence the free-flowing function of the spine. These two congenital disorders go by the name of lumbarisation and sacralisation.
In the ‘modern’ human skeleton the sacrum is a solid bony mass of 5 fused vertebrae at the back of the pelvis on which the upright spine sits. However, in earlier evolutionary forms the segments of the sacrum were not fused. They were free to move – like a tail – and participated as an extension of the spine in normal activity.
Lumbarisation is where the uppermost segment of the sacrum is not fused. Rather it is free to move and participates, along with the neighbouring lumbar vertebrae in spinal activity. The first sacral segment is said to be lumbarised.
With lumbarisation, anatomists and clinicians have taken to referring to this additional mobile lumbar segment as an ‘extra’ vertebra, which has led to some confusion in the minds of the patients. There is no extra vertebra jammed into the length of the spine. Simply an extra mobile vertebra and one less fixed one.