FUNCTIONAL ANATOMY OF THE LUMBAR SPINE

PATRICK HART





FUNCTIONAL ANATOMY OF THE LUMBAR SPINE













INTRODUCTION:



The axial skeleton in human beings has a line of gravity that passes longitudinally through it.  This is a key structural difference between us and other species that allows for our bipedal posture.  The human body is an amazing vehicle that we animate in order to interact with the world around us and the events that lead to its present day structure are debated across the globe. What can be observed in a single person’s lifetime are structural changes brought about by the particular functionality of an individual.



The human body is a marvellous vehicle that adapts and changes according to the stressors that it is subjected to, whether they be seemingly inherent in nature – such as gravity – or personally chosen by an individual according to the way the said individual moves and holds themselves.



Structural differences in regional vertebrae give rise to the differing functionality of each region of the vertebral column.  Without congenital defects, trauma or medical illness, a person’s degree of efficiency of movement largely determines the morphology of the vertebral shape and the health of the intervertebral discs. Via the link system these adaptations then affect the functionality of the appendicular skeleton.





FUNCTIONAL ANATOMY OF THE LUMBAR SPINE:



The five vertebra of the lumbar spine are located above the sacrum and below the cervical and thoracic regions of the axial skeleton, and thus are larger than those above so as to accommodate stronger weight bearing forces.  The laminae, the spinous process, the transverse processes, the superior articulating processes and the inferior articulating processes make up the posterior elements of a vertebrae.  They are attached to the vertebral bodies via the pedicles.



The neural arch is formed by the vertebral body anteriorly, the pedicles laterally and the laminae and spinous processes posteriorly.  The vertebral bodies are specifically designed for dynamic loading and are internally propped with vertical trabeculae that are horizontally braced to prevent bowing.  They are kidney shaped and are separated from each other by intervertebral discs.  Two layers of cartilage called vertebral endplates separate the disc from the vertebral bodies.  Ligaments such as the posterior and anterior longitudinal and the ligamentum flavum, provide stability to the spinal units.  Other ligaments that are particular to the lumbar region are discussed further on.



The zygapophysial joints (z joints) lie behind the transverse processes.  On lumbar vertabrae the superior facets are directed posteromedially and the inferior facets directed anterolaterally, though facet angulations may vary due to tropism.  The above description sounds simple enough but there are important variations within the articulating facet curvatures and orientations.



L1 typically has plane facets while the following vertebrae usually have a j or c curve to them.  There are also variations in the obliqueness of the z joint angles from the sagittal plane.  The shape and orientation of the z joints help to prevent forward displacement and rotary dislocation of the intervertebral joint. 



The other posterior elements are muscle attachment sites.  All muscles attached to the lumbar spine exert pressure that transmits forces through the pedicles to the vertebral bodies.  The pedicles are designed to flex under pressure and act as levers on the vertebral bodies. 



The lumbar spine has good range of motion through extension and forward flexion.  A force coupling action is used during these movements.  The superior vertebrae of a lumbar functional spinal unit (FSU) rotates through the sagittal plane in the direction of movement and simultaneously slides along the inferior vertebra in the same direction.



The forward sagittal plane rotation is restricted by the ligaments of the intervertebral joint and capsular tension of the z joints.  In this regard the capsule acts as a ligament.  Flexion translation is resisted by direct impact of the inferior articulating processes of the superior vertebra impacting the superior articulating processes of the inferior vertebra. 



Axial rotation is limited by z joint facet impact.  The IAF of the superior vertebra is apposed by the SAF of the inferior vertebra.  Axial rotation is also coupled with a small amount of lateral flexion.  Lateral flexion however is coupled with a significant amount of ipsilateral rotation.  Lateral flexion appears to be a complex movement that hasn’t been studied in depth.  We have observed in class the restriction of lateral bending caused by lack of rotation of any of the lumbar vertebrae. 



There is a marked degree of variation in lumbar lateral flexion between adults and adolescents.  This may well be due to an increase in the restrictive effect of the iliolumbar ligament due to either its degeneration over time (becoming more rigid) or a change in its length and use due to altered positioning of the pelvis that an individual may adopt. Structures adapt according to the stressors placed upon them or removed from them, and it has been noted that the iliolumbar ligament changes from a muscle in childhood to a ligament later in life.  Lack of efficient use and biomechanical changes affect the operation of the muscles used in lateral flexion.  The positioning of the pelvis and lumbar spine in relation to each other can alter the effectiveness of the psoas and quadratus lumborum muscles in the performance of lateral flexion. 



What is apparent about all of the above movements is the large role played by the zygapophysial joints.  Considerable amounts of forces move through them.  The parsinterarticularis, the bone between the SAP and the IAP on a vertebra, are also subjected to large movement stressors.  The human body is a dynamic engineering marvel that loves to move as can be seen by the fact that joints deteriorate and seize if not exercised.  Small bones such as the vertebrae and their parts are easily able to handle so much of what the body is put through.  Of course, like any piece of engineering, inefficient use can lead to degeneration as can lack of rest to allow regeneration to take place; both of which are significant precursors to intervertebral disc prolapses.



The intervertebral discs, which are situated between the vertebrae, contain a fluid like centre called the nucleus pulposus.    It is a malleable soft tissue composed of water, collagen and proteoglycans.  Concentric rings of a more fibrous collagenous material surround the nucleus.  These rings are collectively known as the anulus fibrosis.  The discs purpose is to allow a rocking motion between vertebrae during movement of an FSU.  While facilitating movement of vertebrae, the discs also distribute weight across the top surface of a vertebra and shape the spine longitudinally, giving the lumbar region its lordotic curve.  This curve is initiated inferiorly by the angle of the superior articulating surface of the sacrum.



Discs are avascular and rely on the diffusion of nutrients; oxygen, glucose and substrates; to enable fibroblasts and chondrocytes to continue synthesis of the tissue.  The low oxygen concentration, due to reliance on anaerobic metabolism, means that the discs operate in a slightly acidic environment with the optimum PH being between 6.8 and 7.1.  There is a steep decline in performance when the PH drops below 6.8 (becoming more acidic).  This is an important factor to take into account when considering the health of a disc.  As an example, smoking raises the acidity of blood and smoking has been noted to be a risk factor in disc prolapses. 



It is difficult to define where the inner edge of the anulus meets the outer edge of the nucleus but the centre of the nucleus is very distinct from the anulus.  When degeneration of the nucleus occurs it becomes more fibrous and loses its fluid ball like properties.  Degeneration of the anulus fibrosis may lead to tears and fissures appearing in it.  In most cases it is a relatively healthy nucleus; one that is still able to push outwards when compressed; combined with a degenerated anulus fibrosis that leads to a disc prolapse.  The pressure of the nucleus pushes on the weakened tissue causing the anulus to bulge outwards (called a protrusion) or rupture all together.  With a disc rupture, part of the nucleus either leaks out while still remaining attached to the rest of the nucleus (known as extrusion) or, in a complete prolapse (sequestration) it is expelled from the disc and no longer attached to it.



The majority of disc prolapses occur posteriorly.  That is, the disc bulges towards the rear.  During forward flexion, the anterior portion of the disc is compressed while the rear portion of the anulus fibrosis is stretched and thinned.    The layers of fibres of the anulus fibrosis are arranged in concentric rings around the nucleus.  The fibres of each layer run obliquely to the transverse plane with every second layer directed in the opposite direction.  Thus, during axial rotation only half of the fibres are able to resist the pulling forces.  Therefore, during forward flexion with rotation, the disc is at its most vulnerable; only half of its structure is able to resist the stressor and done so while stretched.  It is not surprising then that the most common action that produces a prolapse is forward flexion combined with axial rotation.



Most prolapses occur between the ages of 30 and 50.  Generally after the age of 50, even though the anulus fibrosis may have degenerated in most of the populations studied, the nucleus has also deteriorated to such an extent as to have lost its fluid like nature and thus be unable to exert lateral pressure on the surrounding tissue.  In fact, the more a nucleus becomes fibrous the more it resembles the anulus.  While less of a risk factor for a prolapse it does mean reduced mobility of the interbody joint which can lead to further structural failures throughout the link system. 



Many prolapses are asymptomatic (24% - Adams et all p53).  When symptoms are evident it can be for a number of reasons. The bulging or herniated portion of the disc pushing on a dorsal root ganglion or inflammatory chemicals affecting the nerve root, either of which produce radicular pain.  Back pain can be present without either of the above scenarios.  Nociceptor nerves in the dura matter may be stimulated by the prolapsed disc material irritating the dura of the nerve root sleeve; the prolapsed material may stretch the anulus fibrosis or posterior longitudinal ligament; or back pain may even be associated with the internal disc disruption (IDD) that preceded the prolapse.  Surgery that cuts out the prolapsed disc material has been effective in relieving leg pain but not necessarily easing the back pain (Adams et all p54), hence showing a correlation between back pain and IDD.



The nerves that exit this region of the spine effect the large intestine, appendix, sex organs, uterus, bladder and the prostrate gland.  They innervate the muscles of the abdomen, upper and lower legs and knees and the feet and all of the associated joints.  The lumbar plexus stems from L1 to L4 and travels behind the psoas major muscle and anterior to the quadratus lumborum muscle.  The femoral nerve is the largest nerve that stems from this plexus. 



Trauma or degenerative changes that effect these nerves are implicated in conditions such as; chronic constipation, colitis, cramps, difficulty breathing, incontinence, impotency, knee problems, backache, poor circulation in the legs and a myriad of other problems in the regions innervated by the lumbar plexus. 



Degenerative changes in any part of the human body are often attributed to ageing but this begs the question – what is the aetiology of ageing?  Ageing appears to be used as a label to excuse degeneration.  While nutrition, and environment may play some part, we have seen in class that simply altering the way that a person moves can enhance their functionality and also relieve symptoms of pain, if not the cause of the dysfunction and pain all together. The most prolific substance in the makeup of the intervertebral discs is water, the content decreasing with age, from 80 -90% in the young to less than 70% in the elderly.  Disc hydration has been shown to be influenced by external pressures exerted on the discs by body weight, spatial positioning of the spine and muscle action on the spine.  These factors are to be considered when discussing the concept of ageing as biomechanical functioning tends to become less efficient with age.  But which factor is more influential on the other; ageing or biomechanical functionality?



Structurally we are dynamic beings in control of a fabulous system of links.  By exerting or relieving forces in one area of the skeletal structure, corresponding outcomes can be observed further along the chain.  For instance, it has been shown that dysfunctional curvatures in the thoracic spine often stem from the lumbar region.  Due to the limited ability of the lumbar region to axially rotate, forces that are resisted (due to dysfunction in the lumbar spine) through the transverse plane transfer up to the more mobile area of the thoracic spine and present themselves in a pronounced manner.



The Lumbosacral Joint:

The lumbosacral joint is where the mobile axial skeleton meets a vastly less mobile structure.  Therefore the ability to absorb or dissipate forces through the sacrum is reduced.  It is the last joint located at the bottom of the vertebral column and thus, like a building’s foundations, has to bear more weight than the upper structures – any defects above it also transferring even more stress to it.  The angulations of the LSJ articulating processes help to concentrate a lot of force in a small area.



The LSJ is structured in such a way that L5 wants to slide forward and downwards but is prevented by its inferior articulating processes being pushed onto the sacrum’s superior articulating processes.  Due to this, the parsinterarticularis of L5 is placed under enormous stress.  Excessive and frequent loading of the joint through activities that it can be argued we weren’t designed for, such as football ‘scrumming’, stress the LSJ and parsinterarticularis.



Poor biomechanics may also lead to larger forces, than are necessary to complete an action, being placed on the joint.  Poor function, combined with lack of recovery to allow for tissues to resume their optimal starting position after creep, may likely lead to degeneration of the structures involved.  According to Palastanga et all (p410), in our early years (2 – 13) 75% of sagittal plane flexion and extension occurs at the LSJ yet is much reduced by the relatively young age of 35.  Lateral flexion declines from 7 degrees at this joint in children to 0 degrees in the elderly.  This loss of functionality is going to jar and stress the region and considering this I wouldn’t be surprised if L5 spondylosis is common amongst the population and often undiagnosed.  Any amount of degeneration is going to cause the surrounding structures, such as the lumbosacral disc and the paravertebral muscles, to degenerate and function in a less than optimal capacity.  In relationship to disc prolapse, it is wise to consider this when 95% of herniations appear at L4-L5 and L5-S1 (Adams et all, p244).



Movement at an interbody joint is accompanied by movement at the z joints.  If synovial joints are not regularly taken through their ranges of motion they begin to seize as the synovial fluid becomes viscous and soft tissues adhere to each other.  So it is apparent that vertebral joints should be exercised to help maintain their integrity.  It is observable in mechanical systems that inefficiencies in operation lead to loss of energy and greater strain on their moving parts.  Disc prolapses often resolve themselves spontaneously (Liebenson, p76, quoting Bush et all) and it has been observed that a strategy of management with an over emphasis on bed rest “…has been negative…We have actually prescribed low back disability!” (Liebenson, p.77, quoting Allan & Waddell).



It is apparent that the body likes to be moved; it is not a static system.  Even during sleep, the body performs a myriad of functions; from pumping blood, to breathing and maintaining the core body temperature at an optimum level.  We digest and assimilate nutrients, excrete wastes, and hundreds of thousands of chemical reactions take place every few seconds.  And all this occurs without conscience thought.



In the past surgical treatment has often been prescribed for symptomatic disc prolapses.  But research does suggest that non-surgical treatment can resolve up to 90% of cases that haven’t already resolved spontaneously (Baldwin, 2002).  When considering massage therapy as treatment, in the non-acute phase, symptomatic relief is obtainable via reducing muscle hypertonicity and confusing sensory nerve pathways.  (During the acute phase care needs to be taken as muscle spasms act as a protective mechanism.)  Combined with postural correction and education of the client as to efficient biomechanical function, a therapist can be a powerful aide during rehabilitation.



CONCLUSION:



The human body is a dynamic structure and nothing works in isolation.  The lumbar spine and its associated soft tissues work in a coordinated manner.  They influence the development and functionality of each other.  For the link system as a whole to operate at its most optimum, efficiency of movement must be developed and maintained throughout. The facilitation of efficient biomechanical function, including sufficient periods of recovery, is indicated to prevent disc prolapse.  Further to this, a holistic approach would consider nutrition and the thoughts that people entertain; these being precursors to biochemical stressors.



With an aim to increase efficient functionality of the lumbar region, a combination of massage therapy and corrective postural alignment techniques and at home exercises is suggested; with an emphasis on finding the cause of the dis-ease.  While the human body may be a complex structure consisting of many interrelated systems, the maintenance of it may actually be simple.  Rather than chase symptoms from one part of the body to another, finding and treating the most likely cause or causes should relieve all symptoms.



When reviewing the literature, it should be noted that research is often performed on cadavers and animals.  Further, many people present in a similar manner that suggests a pattern that is then considered to be normal.  Yet a ‘normal’ shape seen across the population may not reflect an optimally healthy structure nor one shaped for peak performance. 





























BIBLIOGRAPHY.



Adams M, et all, 2006, The Biomechanics of Back Pain 2nd ed, Elsevier.



Baldwin, N G., Lumbar Disc Disease: The Natural History, Neurosurgical Focus

Published: 11/04/2002, http://www.medscape.com/viewarticle/442526, Accessed 30/08/09.



Bogduk N, 2005, Clinical Anatomy of the Lumbar Spine and Sacrum 4th ed, Elsevier.



Geddes, K, Class Lectures and Notes.



Liebenson C, Rehabilitation of the Spine: A Practitioner's Manual,  http://books.google.com/books?id=mLMLFOlua3YC&pg=PA76&lpg=PA76&dq=disc+herniatio... , Accessed 30/08/09



Marieb E, 2001, Human Anatomy and Physiology 5th ed, Benjamin Cummings, San Francisco.



Oliver J, Middleditch A, 1991, Functional Anatomy of the Spine, Butterworth Heinemann, Oxford.



Palastanga N, et all, 1994, Anatomy and Human Movement Structure and Function 2nd ed, Butterworth Heinemann, Oxford.





Postacchini, F, Lumbar Disc Herniation,  http://books.google.com/books?id=i-An_XbHpJcC&pg=PA420&lpg=PA420&dq=disc+herniat... Accessed 31/08/09.



R. Prasad, M. Hoda, M. Dhakal, K. Singh, A. Srivastava & V. Sharma : Epidemiological Characteristics Of Lumbar Disc Prolapse In A Tertiary Care Hospital.  The Internet Journal of Neurosurgery. 2006 Volume 3 Number 1 http://www.ispub.com/journal/the_internet_journal_of_neurosurgery/volume_5_numbe...



Further considerations:



Sacrotuberous ligament

Aorta

Hip joint mvt & lumbar spine mvt – non exclusive.

SIJ

Internal hip rotation anomaly across pop. – r = restricted

Seated torso rotation “  “        “          “  - l = restricted

Disc research anomaly – Adams et all claim don’t thin.