Biomechanics – the back
Biomechanics is an important part of understanding the neurological symptoms of the spine. It is also important to understand how trauma or various illnesses can lead to problems such as instability and its consequences. Above all, it is important with biomechanical knowledge to determine if the patient is best suited for surgical or conservative treatment.
Interaction between anatomy, force and movement
When you talk about biomechanics you usually describe it as “The interaction between anatomy, force and movement”. One thus applies a force and observes the movement in the structure itself. When you want to investigate biomechanics in the back, you usually want to see how the movement between different vertebrae and the disks occurs. By observing this, one can see how large the range of motion is normal in each movement (latflex, flexion, extension, rotation, etc.) and when you know what is perceived as normal, you can also account for instability or mobility. What can have been seen is that the movements differ between individuals and different regions of the spine.
The invertebral disk consists of a capsule (anulus fibrosus), a nucleus (nucleus pulposus) and cartilaginous endplates that limit the vertebrae upward and downward.
Forms the outer boundary of the disk and is made up of collagen fibers. The outer layers of the anulus fibrosus are
Shaped like bands, with each band’s fibers going in the same direction
Adjacent bands have an opposite direction with a 30 degree slope to the end plate
This structural structure means that when the straps are stretched on one side (flexion, extension), the straps on the opposite side are slackened. In a rotation, then half will also tighten up and slow down the movement. Thus, a combined rotation with forward bending will have a reduced number of fibers to withstand force.
Intervertebral Disc – Studies
In biomechanical studies, the disk has been loaded in different directions and then seen that compression is more likely to cause a fracture of the end plate than damage to the disk itself. It has also been seen that a degenerate fish was stronger than a healthy fish. However, if a compression was performed in combination with rotation, it increased the risk of disk damage. Thus, it is believed that a disk hernia is often due to a composition of force against the disk:
Studies have shown that the most provocative position of the spine is:
Standing with 10 degree flexion
In combination with 20 degree rotation
Ligaments are structures that are similar to rubber bands. When stretching and stretching the ligaments they hold and in relaxed shortened position it folds. The function of the ligaments is:
Allow physiological range of motion with minimum movement energy
Limit range of motion between the vertebrae to reduce the risk of spinal cord and nerve roots being damaged by physiological stress.
Protect structures from sudden trauma from external forces
Stability between the vertebrae
Anterior and posterior longitudinal ligament
The anterior and posterior longitudinal ligaments consist of the anterior and posterior longitudinal ligaments (ALL and PLL). They run along the front and rear part of the pusher and attach to both the pusher and the counter.
The posterior ligament (PLL) is considered to be of crucial importance in assessing trauma stability. If all the structures behind and in front of the ligament PLL have ruptured, the stability is determined by whether the posterior ligament is intact or not. Assessment is done by MRI examination.
Interspinal and supraspinal ligament
The interspinal and supraspinal ligaments run between the spinal projections and tighten at full flexion of the spine.
Ligamentum Flavum (the yellow ligament) is a ligament that attaches to the underside of the vertebrae and connects them. The ligament is yellowish due to its elastic fibers and it is the most elastic ligament we have in the entire body. Liq. Flavum also has a certain amount of rest, which, however, decreases when the back is extended. With flexion, the tension in the body increases. Flavum which increases the body’s “recapture” of a more neutral position of the spine.
Liq. With increasing age, Flavum has an increased risk of ingrowth of connective tissue in the ligament, leading to hypertrophy (becoming thickened / enlarged). A hypertrophied Flavum can lead to spinal stenosis.
The vertebra has the ability to absorb compression forces. It’s built for that. It has been seen that the different types of vertebral bodies have different limit values before fracturing.
Cervical vertebrae 1500-2000N before fracture
Lumbar vertebrae 7000N before fracture
One can see that a vertebra’s limit value before fracture is greatly reduced if you have developed osteoporosis. A reduction in bone mass of 25% resulted in a lowered limit value of 50% in one study.
Spinal cord and dura
The spinal cord is protected in the spinal canal formed by the vertebrae of the vertebral column. The spinal cord is surrounded by three membranes and the spinal fluid (liquids). Next is the pia mater, which is surrounded by the spinal fluid, then the arachnoidean and the outermost layer consists of dura mater.
The length of the spinal canal varies depending on whether we bend the upper body forward, backward or sideways.
The operating segment
The smallest segment of motion is in the vertebral column between the two adjacent vertebrae with the disc, ligaments and joint capsules connecting the vertebra.
A segment moves three-dimensionally and what limits the movement are:
The joint capsule
Lead surface (and how it is oriented)
Kinematics studies the movement of bodies in a three-dimensional. For a therapist, it is therefore important to be able to interpret e.g. X-ray images.
When it comes to your back you want to observe the movement itself and what range of motion the movement has (ROM, Range of motion). You can measure the range of motion in degrees or millimeters depending on the movement that occurs:
Clinical instability is defined as the loss of the ability of the spine during physiological loads to maintain relationships between the vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots and in addition, there is no development of incapacitating deformity or pain due to structural changes
White & Panjabi 1990
Clinical instability means that the spine loses the ability to maintain a normal balance between the vertebrae so that no injury or irritation occurs on the spinal cord or nerve roots. Deformity or pain may occur as a result of structural changes.
It can also occur because of:
Combination of these
One sign of clinical instability is often that one has a movement that is larger than what is considered normal.
Upper cervical spine – occipitoatlantoaxial complex (Occiput-C1-C2)
One can regard the upper neckline as a distinct anatomical and functional unit. Occiput and Axis are like two rotating orbs with the atlas arch as a layer between them.
Occiput Atlas (C0-C1)
The paths between the occiput and the C1 are dome shaped in a horizontal plane. This design allows flexion and extension up to 15-20 degrees as well as lateral flexion which gives less than 15 degrees of movement. There is basically no rotation or translation of this segment. There are no coupled movements across this segment due to the tight ligaments surrounding it.
An instability or luxation between occiput and atlas is uncommon and is usually seen in occasional high-speed trauma. It is usually such a high-energy trauma that the patient does not survive.
Atlas and axis guide surfaces are easily convex and oriented horizontally. Because of this, the joint mainly allows rotation up to 50 degrees, which incidentally accounts for about 50% of the entire rotation that can occur in the cervical spine.
The rotational motion is limited by the alar ligaments, which originate from the pinnacle and attach into the foramen magnum. Upon rotation of the head, it is first initiated in the atlantoaxial joint and then transferred to the lower cervical segments.
In addition to rotation, flexion extension also takes place at about 10 degrees. Due to the design of the joints, no lateral flexion occurs. Unlike C0-C1, translation between atlas and axis occurs both anteriorly and laterally. This translation of C1-C2 is in adults 3 mm and in children about 4 mm.
Between C1-C2 one can also have coupled movements. At a maximum rotation with a simultaneous lateral translation, due to the convexity of the guide surfaces, C1-C2 will approach each other, while in neutral position are separated.
The atlantoaxial complex is more often involved in various types of pathology resulting in instability. This by Lig. The transverse runs dorsally about its from one side of the atlas to the other. It stabilizes the peak at the axis toward the atlas. For example, a Rheumatoid Arthritis, this ligament can degenerate and rupture leading to instability. Its can also compress the spinal cord and cause a myelopathy. Under these conditions, you may need to fix atlas and axis surgically. If you need to be surgically fixed, the patient will also have a reduced rotational capacity in the spine.
Lower Neck – Subaxial Complex (C2-C7)
In the lower neck, the movements are more homogeneous than in the upper part of the neck. In the subaxial complex, flexion, extension, rotation and lateral flexion with approximately the same degree number can also be performed. The largest range of operations is considered to exist between the C5-C6 segment. You can also perform a light translation (approx. 3mm) in the lower neckline segments. You can therefore suspect that if there is a translation larger than 3mm then you should see it as an unstable neck which may need further investigation.
Coupled movements are especially characteristic of the lower neckline segments. This is especially noticeable at a lateral flexion in the cervical spine as the ridge projections on the cervical vertebrae move toward the convexity at a lateral bend in the cervical spine. This coupled movement explains why one can get a unilateral facet joint flexion in a traumatic side bend, e.g. traffic accident.
The range of motion in the chest is more limited than in the other, since each vertebra also leads to two ribs. Flexion and extension occur with about 4-6 degrees in the upper and middle parts and with 12 degrees in the lower part. Conversely, the rotation is slightly larger in the upper chest (8-9 degrees) than the lower (about 2 degrees).
The range of motion in the lumbar spine increases in caudal direction regarding flexion and extension. Rotation and lateral flexion are relatively limited in all segments of the lumbar spine. This fact, combined with the fact that these segments are the ones that are most stressed under normal physiological conditions, have been considered as a reason why most disc hernias in the lumbar spine occur between L4-L5 and L5-S1.