Lumbosacral weight bearing & impacts of lumbar/sacral anomalies

The sacrum weight transmission factors: The direction of the trabecular bone indicates the route of load transmission in the sacrum. From the various parts of the sacrum (body, facets, alae and laminae) distinct sets of trabeculae extend towards the auricular surface.

L5-S1 weight transmission

Weight transmitted from the fifth lumbar vertebrae to the sacrum is distributed in 3 separate components i.e.

(a) Anteriorly- through vertebral bodies
(b) Intermediately- through transverse elements
(c) Posteriorly- through lumbosacral facet joints

The posterior components of the fifth lumbar vertebra share greater proportion of load in comparison with the posterior elements of the upper lumbar vertebral levels.

The forces acting on the body and articular facets, at the upper end of the sacrum, are ultimately transmitted through the two auricular surfaces with an appreciable part of the load passing directly from the transverse process of the fifth lumbar vertebra to the ala of the sacrum through the lumbosacral ligament.

There are a few anatomical alterations pertaining to this area that alters the biomechanics of this area & weight transmission.

Anatomical alteration in sacrum & lumbosacral junctions:

1. Variable positions of scaral auricular surface & it’s varieties

According to a recent study by Mahato NK sacral auricular surface not only defines the magnitude of weight transmission to the hip bones but also the position of the auricular surfaces influence load bearing patterns at the sacrum.

The position of auricular surfaces may cause vertical shifts in weight-bearing patterns between the L-5 and S-1 segments, altering weight distribution at the lumbosacral and sacroiliac regions.
The auricular surfaces may be higher or lower than normal. Hence there are 3 varieties of auricular surfaces. The higher auricular surface occupied a high-up position (from upper S-1 to low S-2 segments) & lower auricular surface occupied a high-up position low-down auricular surface (from the low S-1 to low S-3 sacral segments). A normal position of the auricular surface is from the S-1 to the middle of the S-3 segments.

The position of the auricular surface varies in human sacra. These variations are associated with differential load bearing at the sacral joints. Only the high-up sacra demonstrated the presence of accessory articulating facets between L-5 and S-1. The position of the auricular surface can explain or possibly predict low-back pain situations.

a. Features of low-down sacra: The low-down sacra transmitted load predominantly via lower (S2-3) segments and exhibited stouter, broader, and efficient weight-bearing lower sacral elements, and a prominent gap between the S-1 segment and the rest of the sacrum. The low-down sacra are longer than they were broad, had a substantially broad body span at S-1, possess the smallest interauricular distance, and showed considerable depth of the plane of the facet joints.

b. Features of high-up sacra: They demonstrate unilateral or bilateral accessory articulating facets on the alae that articulates with extended transverse processes of the L-5 vertebrae. The high-up sacra:

1) are shorter and broader in comparison with the normal sacra
2) at times presented accessory articular facets on their alae
3) has a smaller body span and a wider ala
4) are found to have the plane of the facet joints nearer to the posterior aspect of the S-1 body
5) had the smallest of the facet areas.

2. Presence of rudimentary lumbosacral facet(s):

According to Pal et al, observations on the sacra with the anomalous articular processes provided strong evidence for the role of the neural arch elements in the load transmission. In specimens where the articular facet was absent on one or both sides, there was always an accessory facet on the ala of the sacrum so that the load was transmitted to this facet from the transverse process of the fifth lumbar vertebra.

In a recently published study by Mahato et al 16 out of 20 selected cases presented unilateral rudimentary facets, and the remaining four had facets that were bilaterally rudimentary facets. Rudimentary lumbosacral facet articulation(s) is found to affect load sharing at this region. This study also found:

a. Accessory articulating area: Rudimentary facet in sacrum is also show accessory articulating area on the upper surface of the ala on the same side as the rudimentary zygapophyseal facet. Sacrum with bilateral rudimentary facets demonstrates bilateral accessory L5-S1 articulations.

b. Strong ligamentous attachments: Strong ligamentous attachments between the L5 and S1 transverse elements on the sides of the rudimentary facets.

Development of strong L5-S1 lumbosacral ligamentous attachments or accessory articulations at the transverse elements serve a compensatory mechanism for load sharing at lumbosacral junctions bearing a rudimentary facet joint.

3. Sacralized fifth lumbar vertebrae:

Sacralizations represent a transitional state at lumbosacral junctions and are more susceptible to degenerative changes resulting from the altered load-bearing patterns at these regions.

Features of Sacra with fused L5:

a. Sacra with fused L5 possess significantly smaller heights than the normal ones if the fused L5 vertebra was excluded from the measurements.

b. On inclusion of the L5, these sacra presented
i. a grossly reduced distance between the zygapophyseal facets,
ii. a greater distance of the coronal plane of the facet joints from the posterior aspect of the L5,
iii. a narrower interauricular distance,
iv. slightly increased body width at the top of the sacrum (now L5),
v. a comparable auricular surface area (with inclusion of the fusion of L5 transverse process at the lateral mass) to the normal ones, attenuated facet area, and occasionally, small intervertebral space between L5 and S1.

c. The unilaterally fused variety exhibit overall smaller dimensions. None of these sacra show accessory articulations at their ala. The auricular surfaces in these sacra spanned from mid-L5 to mid-S2 segments. The fusion of L5 increased the sacral height, width, and auricular surface. The auricular surfaces appeared to be situated "low-down".

Sacralization of L5 vertebra entails morphological alterations in the sacrum. The remnants of the original sacra in these specimens presented grossly diminished parameters.

Sacralization of L5 possibly represents a structural and biomechanical adjustment to compensate for reduced joint interfaces associated with smaller sacra. It may correspond to one end of the transitional "spectrum," the other end being defined by lumbarization of the S1 sacral segment.

References:

1. Mahato NK; Clin Anat. 2010 Jun 8. (Association of rudimentary sacral zygapophyseal facets and accessory and ligamentous articulations: Implications for load transmission at the L5-S1 junction.)
2. Pal GP; J Anat. 1989 Feb;162:9-17.(Weight transmission through the sacrum in man.)
3. Mahato NK; Neurosurg Focus. 2010 Mar;28(3):E12. (Variable positions of the sacral auricular surface: classification and importance.)
4. Mahato NK; Spine J. 2010 May 21. [Epub ahead of print]. (Complete sacralization of L5 vertebrae: traits, dimensions, and load bearing in the involved sacra.)