Lumbosacral Biomechanics

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Original Editors Bert Lasat

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Search Strategy

First I searched for books about lumbosacral biomechanics in the library of the University of Brussels. After I read the literature, I searched on multiple scientifically sites like PubMed, Web of Knowledge,… for information about the lumbosacral biomechanics.
Then I had enough information and I started to write this publication.

Definition/Description

Mechanics, the study of forces and their effects, when applied to humans, is called biomechanics. Logically means ‘lumbosacral biomechanics’ the study of forces and their effects at the level of the lumbosacral region.

Clinically Relevant Anatomy

The lumbar spine has normally 5 vertebrae (normal range 4-6) with a discus intervertebralis between 2 vertebral bodies. There is a cartilaginous endplate between the discus and the vertebral bodies. The discus itself has three components: the nucleus pulposus, annulus fibrosus and the endplates. The lumbar discs are larger than the cervical discs but the components are the same. The nucleus (in the middle) of the disc and has a larger water component but does not have much collagen fibers. The endplate is a thin layer of cartilage tissue between the vertebral body and the disc. The annulus fibrosus consists of concentric rings of collagen fiber layers that surround the nucleus. The sacrum is a triangular bone with a concave and convex surface, the facies pelvina and the facies dorsalis, and an apex. It consists of a series of 3, 4, or 5 fused sacral vertebrae. On the anterior surface of the sacrum the superior and inferior edges of the vertebral bodies correspond as transverse ridges. Between these ridges lays the lumbosacral disc that connects the lumbar spine with the sacrum.

Biomechanics of the lumbar spine and sacrum (L4-L5 L5-S1)

The three main movements in the spine and sacrum are flexion, extension and lateral bending. There is a larger movement possible in the thoracic region than in the lumbar region. Then we have the different forces acting on the lumbar spine and sacrum: compressive force, tensile force, shear force, bending moment and torsional moment. Not only forces act on the spine and sacrum, also the mass, body weight, stress and displacement have their influences. The vertebral bodies and their discs form a column that permits movements, they resist compression and resist the body weight and forces of the thorax and upper limbs. Forward bending is possible because each intervertebral disc compresses, anteriorly, and is resisted by tension developed in the posterior part of the annulus fibrosus. Extension and lateral bending are possible because of the actions of the contralateral elements in the opposite direction. In the normal position the sacrum is bending forward so that the upper surface is below horizontal at an angle of 50° . The L5-S1 intervertebral disc is wedge-shaped, by about 16° (between sacrum and bottom of L5). The inferior processi articulares of L5 conect with the sacrum so the sacrum can not slide forward. The mobility in the sacrum contains 2°. The differences in ROM between L4-L5 and L5-S1 in the three movements are :

- More flexion in L4-L5
- More Extension in L5-S1
- More lateral bending in L4-L5
- More rotation in L4-L5

The differences in stiffness are :
- More compression in L5-S1
- More shear forces lat and ant/post in L4-L5
- More bending in L5-S1
- More rotation in L4-L5

Mechanism of Injury / Pathological Process

In degenerative disc disease the L4-L5 disc will be first to cause an injury because there is an increased compressive force on the spine in upright position that reduces the ability to distribute stress evenly, so the posterior annulus gets a high concentration of compressive stress in upright position and the anterior annulus in flexed position. That’s because the L5-S1 disc is 16° below horizontal angle and so the posterior part of the annulus gets less compression than the L4-L5 disc.
Shear force is a force which acts parallel to the intervertebral disc. Shear force is more commonly from gravity acting on the body, and so there’s a greater force on the lumbar spine where the disc’s lay in a steep angle to the horizontal (90°) and in bending postures. So the L5-S1 disc will probably have earlier damage because of the shear force.
Because there is a 50° angle from the L5-S1 disc to the horizontal (L4-L5) there can be a nucleus extrusion or annulus protrusion sooner than in the L4-L5 disc due to repetitive bending moments and compression forces. This can result in a disc herniation and instability .

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Recent Related Research (from Pubmed)

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References

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