Lumbar spine biomechanics
Now that you have an understanding of some basic spinal anatomy, as well as neck anatomy. This post will focus on the biomechanics of the low back region, also known as the lumbar spine. Remember, it is important to note that this discussion leaves out some of the finer details and anatomical description of the lumbar region as a whole, and is meant to facilitate a general understanding of the importance of biomechanics on low back function. If you have any questions regarding the material, or would like our clinic to assess your function please leave a question or call the clinic.
Low back pain is the most common musculoskeletal problem encountered by healthcare providers.
The lumbar spine Camp4 Human Performance, is the next to lowest portion of the axial skeleton, immediately above the sacrum and coccyx. The lumbar spine consists of 5 vertebral bodies and their intervening discs. The curvature of the lumbar spine is similar to that of the cervical spine (neck) and is termed lordosis, which is concave to the posterior (backside). The primary function of this curvature is one of shock absorption.
The lumbar spine Camp4 Human Performance, must be able to sustain enormous loads while still providing enough mobility to perform daily tasks. In the lumbar spine, there are two ‘junctional zones’ where large forces are concentrated. These junctional zones, also termed transitional segments, in the lumbar spine are at the thoraco-lumbar region as well as the lumbo-sacral region. The lumbar spine also must protect and provide a pathway for the neural structures for the low back, pelvis, and lower extremities.
The lumbar spine vertebral bodies are composed primarily of vascularized (having a bood supply) cancellous (less dense) bone and in the upright posture assume 80-90% of the compressive load. This bone, along with the intervening blood and its cells provide reinforcement for axial loads.
The lumbar vertebrae have many structural features similar to other vertebrae of the spine. Including a posterior arch, connected by thick cortical structures called pedicles, laminae that form off the pedicles, and a spinous process which together creates the space for the spinal cord, and after the second lumbar level the caude equina. The caude equina is the peripheral nerves to the lower extremity that exit off the cord, which ends at the L1/L2 level as the conus medullaris. They also have transverse, accessory, and mamillary processes, together termed the posterior elements, which provide areas for muscle attachment.
An important thing to note is that the forces acting on a vertebrae are first delivered to the posterior elements, and the forces that act on a spinous and articular process are transmitted first to the laminae.
The pars interarticularis is subjected to considerable bending forces as the forces transmitted from the lamina undergo a change of direction into the pedicle. This is the reason why the cortical bone (the outer portions) in the pars is thicker than any other part of the lamina. As mentioned already, the pedicles are the only connection between the posterior elements and the vertebral bodies and transmit both tension and bending forces. The pedicles are basically a hollow cylinder designed to resist bending forces.
One important thing to note about the articular processes of the lumbar spine Camp4 Human Performance, or the facet/zygapophyseal joints, is that they provide a locking mechanism that resists forward sliding and twisting of the vertebral bodies. This is important in preventing dislocation at end ranges of motion. Even though the primary function of these joints is to guide segmental motion, they also are structures that bear loads. These will be discussed further in detail shortly.
18-20% of the compressive loads acting on the lumbar spine in normal standing pass through these joints.
The lumbar spinal nerves exit below the vertebral bodies of the same name. For example, the first lumbar spinal nerve, or L1 nerve exits below the L1 pedicle and is between L1 and L2.
Discs of the lumbar spine
Now lets make a quick discussion on the intervertebral discs of the low back given their important role in spinal mechanics. Again, primary functions are to bear and distribute load, as well as allow motion between two adjacent vertebrae. Similar to all regions of the spine these discs are composed of a nucleus pulposis (in the middle), annulus fibrosis (collagen outer fibers), and the vertebral endplate (top and bottom end portions).
The nucleus pulposis is a semifluid mass of mucoid material with the consistency of toothpaste. An important property of the nucleus is its large quantity of water, which is approximately 70-90%. This allows the nucleus to deform under pressure however, because it is mostly water and acts like a fluid because its volume cannot be compressed. As you continue to read, you will realize that the nucleus is enclosed in all directions by a concave disc. The concavity of the discs shape actually provides more strength due to an increase in collagen per unit area.
The annulus fibrosis is primarily collagen fibers arranged in a highly ordered pattern. There are multiple lamellae (thin plates or flakes), roughly between 10 and 20 layers thick, oriented at approximately 65-70 degrees from the vertical, thicker towards the center of the disc, and thinner in the back where they are more tightly packed. Adjacent lamellae are oriented in opposite directions from each other creating a criss-cross pattern, that promotes tensional support in both directions when we rotate the lumbar spine. Even though the annulus is also 60-70% water, the collagen lamellae give the annulus bulk, which allows load to be distributed in a passive way. In order to transmit load in this way, the fibers cannot buckle because the nucleus adds additional bracing. When putting compression on the lumbar intervertebral discs from above, the nucleus expands to each side and puts tension on the fibers. An equilibrium is reached when radial pressure exerted by the nucleus is balanced by tension developed in the annulus. If the annulus is healthy, it will resist any tendency to bulge laterally, upward or downward.
The vertebral endplates are layers of cartilage about .6-1 mm thick and covers the area on the vertebral body called the ring apophysis. The annulus around, and the enplates on the top and bottom, encloses the nucleus in a concave sphere of collagen fibers. Also during compressive loading the endplates resists upward and downward pressure displacement of the nucleus under loaded conditions. This actually lessens the load on the annular fibers by making it brace and prevents them from buckling.
Important take home to remember is that disc health is all about water!! The water content of the nucleus makes the disc an engorged body that resists compression. Normal mechanics of the disc depend on water content and thus proteoglycan content. Any change in either of those changes the biomechanical properties of the disc.
The nucleus transmits 1.5x the applied compressive load per unit area and the annulus bears 4-5x the applied compressive load per unit area as tensile stress.
As the nucleus expands radially (around) it stretches the collagen fibers of the annulus like springs which stores energy. Once the load is removed, the stored energy is exerted back on the nucleus and restores any deformation that was undergone. If the load is very rapid, the force will be diverted to stretching the annulus. This weakens the speed at which force is transmitted from one vertebrae to the next. Full force will eventually be transmitted but the slowing of forces has a protective effect. This is why spinal rehabilitative therapy has everything to do with movement quality and joint centration.
Movements at the intervertebral discs
Compression (rocking forwards and backwards) lowering one end of the vertebrae while raising the other end. Outer annulus will compress and buckle while the nucleus will be compressed on one side but able to deform to accommodate on the other.
Distraction (two vertebral bodies moved away from the disc), all points on one vertebral body move an equal distance perpendicularly from the surface of the other vertebral body. Every fiber is strained to resist distraction. The annulus in particular strongly resists this motion.
- When bending compression deforms the nucleus. Add to this a load at the same time and nuclear pressures rise dramatically and exert significant pressure on the back of the annular fibers.
- Normal annular fibers will resist this combination of tension and pressure however, an already compromised posterior annulus is at an increased risk. Previous injury, disc disease (erosion) that may weaken lamellae may lead to insufficient resistance to tension and disc pressures leading to annular rupture and nuclear extrusion.
Translation (sliding of a vertebrae on its disc) all points of the top vertebrae move equal that are parallel to the vertebrae below. Only half the fibers have their points separated by movement and only half the fibers resist forward sliding. Remember the 65-75 degree orientation from the horizontal plane.
Rotation (twisting from right to left) all points on one vertebrae will move in relation to the adjacent one circumferentially in the direction of the twist. Annulus resists twisting movements with only half the fibers. This is the most likely mechanism to cause annular injury.
Compression (bending) and rotation (twisting) are the most common movements responsible for advanced disc injury.
Any of these movements, at the disc level, change the instantaneous axis of rotation of each motion segment. The locations of these differ slightly throughout each motion and these changes in axis are known as the centrode of motion. In normal discs, the centrode is tightly packed around the center of the vertebral endplate of the vertebrae below. In injured discs, the centrode is abnormal in location, size, and shape.
Discs as pain generating structures
There are three potential mechanisms for discs to be a source of pain.
- Injury to pain sensitive outer annular fibers (outer 1/3 are innervated)
- Nuclear herniation through the annular fibers that causes mechanical pressure and/or chemical irritation of pain sensitive structures.
- Degenerative loss of disc height, which causes the vertebral bodies to approximate (move closer) each other leading to ligament laxity and instability of motion segments.
Not all herniated discs cause symptoms, and most degenerative discs do not cause symptoms.
This makes both evaluating and treating patients with disc pathology who also have low back pain very challenging.
Ligaments of the lumbar spine are essentially the same as other parts of the spinal column and will not be renamed here given their presence in other blog posts. I will note however that the ligamentum flavum, which connects adjacent lamina stacked on top of each other together, blends with the anterior (front) portion of the joint capsule. This ligament has a high proportion of elastic fibers and can be elongated to 40% of its resting length without creating failure. This allow elongation with motion and then a re-shortening without buckling when it is unloaded. This means that forward flexion (bending) creates tightening of the ligament and a return to neutral posture releases that tension, so long as it is not done repetitively all day.
Another important ligament is the iliolumbar ligament. Before we discuss this ligament remember the importance noted of the transitional segments of your spine. When you move from the lumbar to the sacral portion of the spine, the curvature changes and the iliolumbar ligaments prevent anterior (forward) translation, twisting, flexion (bending forward), extension (bending backward), and lateral flexion of the L5/S1 (fifth lumbar and first sacral joint) joint. This ligament is quite broad and strong, connecting the L4/5, L5/S1 vertebral bodies to each ilium (hip bone).
Traditionally lumbar ligaments have been classified as the primary stabilizers of the lumbar spine, which we now realize is not the case, muscle contractions are responsible for stabilizing motion segments”
Recent work has shown that the lumbar spine will buckle under only 2kg of loading without muscular support.
Then what is the role of the lumbar ligaments?
Lumbar ligaments are interconnected with many other structures such as fascia, muscles, and tendons.
Large densities of free nerve endings and mechanoreceptors (motion detectors) are found in the lumbar ligaments making them sensitive to joint position and pain”
The ligament system may be a large part of the reflex arc of the lumbar spine. Lets give an example of one of these reflex arcs. Remember, there is a ligament which connects each spinous process, called the supraspinous ligament. When this ligaments is loaded, as in flexion of the spine, it causes a reflexive muscle contraction of the multifidus muscle which stiffens the motion segments of the spine at that level. In fact, the magnitude of muscle contraction is dependent on the tensile load applied. If you remember, the multifidus muscle is mentioned in the performance rehabilitation page and the video sections of the website and is a primary target for spinal rehabilitative therapy. A blog post will be created for this muscle given its commonly discussed importance in spinal function. Other reflex arcs may exist, not to date, but this highlights the interconnected relationship between the bones, soft tissues, and neuromuscular components of the lumbar spine.
Another important piece of interconnected anatomy is that of the thoracolumbar fascia. This word sounds complicated but simply means a piece of connective tissue (fascia) that connects the thoracic and lumbar spines. This connective tissue directly connects the spine to the deep abdominal muscles and is an important dynamic stabilizer of the lumbar spine. It also directly connects the proximal humerus (arm bone close to the armpit) with the proximal femur (leg bone close to the hip) by creating an x-shaped connection on the back. This X is created by the lattisimus dorsi muscle on one side and the opposite side gluteus maximus muscle.
Lumbar spine Camp4 Human Performance motion
An accurate assessment of impairment of lumbar motion and its relationship to a persons symptoms and function are very important in their functional evaluation. Abnormal motion can manifest itself as various combinations of reduced, excessive, or poorly timed joint displacements. Unfortunately, measuring ranges of motion in the spine isn’t as easy as was once thought and a generalized analysis of a patients range of motion misses a few important things.
(1) the plane of the facet joints truly dictate possible displacement directions.
(2) the thickness of an individuals discs also help to allow for appropriate displacements at the joint surfaces.
Lumbar spine flexion (when you touch your toes) is the entire lumbar spine leaning forward and taking out the natural lordotic curve. At the end range, the lumbar spine is straight or curved slightly forward. During this motion the facet joints are lifted slightly off each other and moved a little behind their previous position. Also during this motion there is a slight gap created between the facet joints, which is then closed by either gravitational pull or muscular contraction until the articular surfaces contact one another as the body slides forward. There is about 5-7 millimeters of motion and 5 degrees of forward rotation due to capsular tension at each facet joint during a flexion motion.
The bony locking mechanism of the facet joints, as well as the ligamentous structures are the major players in resisting excessive spinal flexion, which ultimately protect excessive load to the intervertebral discs. Lumbar spine extension, as when you lean backwards as far as you can is mostly limited not by ligamentous structures but by bony impaction of the spinous and articular processes.
Lumbar spine rotation is restricted by torsion of the discs and most importantly, impaction of the facet joints. All fibers oriented along the plane of rotation are put under strain while the other half become relaxed. The amount of rotation of an intervertebral disc without injury is roughly 3 degrees. The annulus exhibits roughly a strain of 1% per degree of rotational strain and the collagen fibers become damaged past 4% of their resting length. Between 3 and 12 degrees of rotation the annulus shows microfailure, and beyond 12 degrees macroscopic failure occurs.
The space between facet joints is quite small and the compression of articular cartilage is what allows such motion to occur. When the cartilage is compressed, water is squeezed out of the cartilage and is reabsorbed when compression, so long as it is, is released. There is .5mm of compression that must occur with every 1 degree of rotation. Beyond 3 degrees of rotation the upper vertebral facet joint must pivot on the impacted joint, which causes lateral shear on the underyling disc, in addition to torsion. This causes excessive tension on the facet joint capsule and is a significant cause of capsular dysfunction and pain. If the force of rotation is strong enough the disc becomes strained and torn, the same side facet joint becomes strained in compression and the opposite side facet joint becomes strained in tension and torn.
If the lumbar motion segment does not rotate beyond 1.5 degrees, it can sustain up to 10,000 repetitions without visible damage. Failure occurs with fewer reps when a greater range is used.
As long as the facet joints limit rotation to less than 3 degrees the annulus if protected from injury
Lumbar spine distraction is not as well studied as other motions. The facet joint capsules are strong when subjected to longitudinal tension. In fact a pair of capsules can bar twice the body weight in distraction. Also, the discs are relatively strong in distraction. The application of a 20lb weight to stretch the lumbar spine results in an initial mean lengthening of 7.5 mm. Sustaining that traction over 30 minutes results in an addition 1.5 mm of length. Removal of that load reveals and immediate “set” of 2.5 mm, which reduces to .5mm 30 minutes after removal of the load.
Roughly 40% of the lengthening from traction is due to flattening of the lumbar curvature (lordosis) and 60% due to vertebral body separation. Only about .9mm of separation is gained at each intervertebral joint.
In reality, all these motions are considered “coupled” to other motions. This simply means that none of these motions happen alone literally. For example, with extension of the spine there is also some translation backwards as well as rotation. This happens with all motions and means that it is challenging to note global “abnormal” motions of the lumbar spine, which brings importance to physicians assessing segmental ranges of motion which have more diagnostic utility.