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.
L6 in lumbarisation is not 'an extra vertebra'. Rather it is one more mobile lumbar segment and one fewer fused segment of the sacrum
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.
A sacralised lumbar segment (also referred to as 'one less vertebra') can be semi-fused on one side only, or both
The other congenital anomaly is where the bottom lumbar segment (L5) is fused to the sacrum below, or to the ilium at the side (the large ear-shaped bones of the pelvis). Sacralisation with the sacrum can be termed central sacralisation, whereas to the sides it can be either uni- or bi-lateral transverse sacralisation. Being fused or semi-fused the L5 segment has more in common with its sacral neighbours than its lumbar ones, so it is said to be sacralised.
Just to confuse matters, the vertebra in question is often not fused all the way across. It may be fused on one side but not the other. Its participation with spinal movement on one side only makes for very wonky movement and the other working lumbar links above have to cope with that. Where strain sets in there will be pain.
With sacralisation there may be fusion of the transverse process to the pelvic bone (ilium) on one or both sides, or there may be a false joint (pseudarthrosis).
This makes for wonky spinal movement
It has always been assumed that congenital sacralization and lumbarization are clinically unimportant, but that may not be the case. Generally speaking, both anomalies of the sacrum and base of spine cause trouble, usually because of the incomplete nature of each. With lumbarisation, it is rare for this additional joint to be completely free. With sacralisation it's rare for it to be totally fused. The pure extreme of both is rarely found, rather a no man’s land in between.
There is a certain safety to be had in a joint being completely fused, which helps explain the orthopaedic surgeon’s penchant for fusing problem joints. The rationale had always been – drastic as it sounds - if a joint hurt to move then join it up to neighbouring bones to stop it moving.
That surgical fusion inevitably led to other problems later on in nearby healthy joints, which were sometimes as unwelcome as the original complaint, is another matter; the case for fusion can be clearly seen. However, if the fusion was not quite solid, everybody was left in a quandary. Indeed, surgical fusions which do not quite ‘take’ are the bane of the back surgeon’s life.
Sarah Key's iconic book, in print since 1986, discusses this topic in the section 'Do I Have An Extra Vertebra?'
Exactly the converse is true of spines that are sacralised at the first sacral segment. Although the first sacral segment is said to be moble, in truth it's not usually free enough. Again this joint is neither fish nor fowl, and struggles to be part of everyday function of the spine. Invariably, this not-quite joint is not up to the task. Furthermore, its debility becomes more obvious with advancing years.
Being not quite mobile enough a lumbarised first sacral segment is more susceptible to incidental trauma. As we get older and all our joints aren't as loose, an errant everyday movement can tweak it; just as the joint that is meant to be fused also gets hurt.
This phenomenon explains the latter-day diagnosis of a congenital anomaly that has been there all along but only recently started causing trouble.
The solution is to get both types of congenital anomalies moving freely again – the
semi-fused and the semi-free. Attempting to get movement into a
radiologically fixed joint always strikes fellow medical
practitioners as bizarre (if not mad). But treatment is usually
short-term and highly rewarding for the patient. All the usual
appeasing, self-mobilisation and strengthening exercises described in exercises for back pain can be put into action here.
With sacralisation and lumbarisation there is a vast variety in the anomalies of bone making the pseudarthrosis (false joint). With unilateral transverse sacralisation for example, instead of the transverse process being a fine lateral sprig it can be a huge bony mass like an elephant's ear, that articulates with both the ilium and the top of the sacrum.
Conservative treatment of the above type of anomaly involves manual mobilisation of the false joint with the hands to make it move better. Treatment also involves muscle strengthening the segmental union between all the lumbar vertebrae, not unlike the techniques used to shore up an unstable vertebra and developmental instability as a result of surgical fusion.
A typical elephant's ear sacralisation on (your) right
Sacralisation of the fifth lumbar segment also has another far-reaching effect, one not associated with joint mobility. This relates to the altered centre of gravity of the base of the spine. With the L5 fused to the sacrum, the seat of spinal movement is raised. L4, the new base of spinal movement, lacks the secure shoring afforded L5 and this can lead to problems.
Normally, L5 sitting on the sacrum means the spine originates deep within the pelvis where it is firmly anchored by a three-dimensional array of very strong ligaments. The dense strands of the stellate (star-shaped) ligament fan out either side in robust semi-circles from the fifth lumbar vertebra to various parts of the the inner pelvic bones, lashing the spine securely to the sacral table.
The most mobile segment in the spine in terms of forward bending is the second vertebra up from the bottom, L4. Its inherent mobility means the usual problem with a sacralised L5 is that L4 is too flimsy to cope and the L4-5 joint becomes over-used. Usually, by the middle to late decades, it starts to suffer excessive strain and eventually becomes painful.
The extensive ilio-lumbar ligament firmly secures the base of the spine to the sacrum and inner pelvic bowl
For these reasons, treatment for lumbar sacralisation must always involve strengthening exercises of the muscles both front and back of the spine. The spinal intrinsic muscles (multifidus) must be re-educated to bolster the bending strength of L4 as the raised seat of spinal movement and you can see here about spinal strengthening and the Roman Chair. At the same time, stronger abdominal muscles help create a stronger retaining wall at the front. You can read all about the best abs exercises for bad backs here.
You may also like to see Sarah Key on video showing what you can do yourself. Whatever the pictures say, you need to be making your spine like it used to be; more plastic and compliant. See Sarah demonstrate how to decompress the spine, making the not-quite joints in your back work better. Yes, it really is as simple as that! See the Complete Back Pain Video Package