Local Stresses: Segmental Mechanism of Low Back Pain and Degeneration, and Stresses According to Spinal Orientation—Contact Forces Theory
The human spine assumes the body verticality compared with quadrupeds. This global vertical orientation of the spine and pelvis induces a permanent stronger action of the gravity on the spinal structures. The normal spine physiology can be compared to the function of a crane that is governed by a tripod of mechanical forces: the anterior downward force of the gravity (weight of the patient), the posterior forces of the muscles to erect the spine that allow walking, and the effect of the belly shape in this system, but only two main forces act in pathology: the weight gravity and posterior muscle force (MF). The total contact force on the spinal unit is the sum of these two acting forces. On the other hand, the distribution of the same vertical force is different depending on the sagittal orientation of the vertebrae. This induces different contact forces orientation is the various spinal shapes. Thus, the identification of various spinopelvic morphotypes allows understanding the physical distribution of forces induced by body weight and muscle counterbalance, regarding spinal unit orientation and range of motion. An abnormal overstress may explain the mechanical origin of pain and spine degeneration.
The spinal column forms the axial skeleton and is a complex multiarticular system that supports the head and the trunk with economical transfer of the loads to the lower limbs. This system is under the control of the central nervous system and the local muscles and allows smooth force transfer without any increase of degenerative changes. While assuring this biomechanical role, the spinal column protects the neural elements (spinal cord, cauda equina, and nerve roots) and vascular elements (vertebral artery). While all vertebrates have a common spinal structure by vertebral unit alignment, the human spine must assume the body verticality compared with quadrupeds in which the spine behaves like a bridge between anterior and posterior legs. In quadrupeds, the thoracic and lumbar spine has almost nothing to support regarding the gravity because of its horizontal position. Their intervertebral disks aligned with their horizontal spine do not suffer direct vertical compression forces from gravity (Fig. 9.1). The only pressure is produced by horizontal forces during acceleration by the posterior legs. This propulsion force from the posterior legs is transmitted by a very anteverted pelvis through a vertical sacral plateau positioned forward the femoral heads. In opposition to other bipeds like dinosaurs or birds, human bipedalism is characterized by the verticality. The global vertical orientation of the spine and pelvis induces a permanent stronger action of the gravity on the spinal structures. It is of paramount importance to maintain the standing position in the most physically economical and stable system.
Spinal stability was defined by the American Academy of Orthopedic Surgeons as “The capacity of the vertebrae to remain cohesive and to preserve the normal displacements in all physiological body movements.”1 The spinal stability implies the transfer of power forces between the upper and lower limbs with active generation of forces in the trunk. This allows prevention of early biomechanical deterioration of spine components by reduction of the energy expenditure during muscle action.2 Another factor of stability is provided by the limited segmental mobility, but a global harmonious spinal flexibility linked to the association of the whole segmental unit. The segmental mobility has been extensively studied for each level of the spine3 , 4 and is characterized by a small range of motion rarely exceeding 5° to 6°.
This chapter reviews the mechanisms of low back pain generation focusing on contact forces and forces transmission while applying novel findings of spinal alignment on the generation of these forces. Mechanisms of segmental motion overpassing are also analyzed with its possible painful action.
Even though anatomy of the spine has been quite well described by Hippocrates and then Galen, and very precisely by Vesalius during the 16th Century, it was Giovanni Alfonso Borelli who approached its mechanical function in the book De Motu Animalium. He described the muscles–joint interactions and the forces acting on the spine in various situations (see Chapter 1). Later, Jean Cruveilhier used Euler’s law to demonstrate how successive spinal curvatures favor the spinal resistance. During the 19th century, numerous authors, mainly in Germany, tried to determine the position of the center of gravity and more precisely to define the position of the center of mass at each level of the human body. In 1987, Duval-Beaupère published a technique using a gamma-ray scanner to identify the mass of successive body scans and the position of their respective body mass centers. Based on the notion of spinal unit, many authors (Panjabi, Dimnet) described the intervertebral motion. Recently, using the crane concept, Dimnet defined the combination of different gravity and muscles forces that induce a global contact force on each level.
The spinal biomechanical unit is the functional spinal unit (FSU). An FSU is formed by two adjacent vertebrae with one intervertebral disk and corresponding ligaments (Fig. 9.2).5 The FSU’s main function is to provide spinal movement while protecting the neural elements. Another function of the FSU is to transmit the weight of the body to the lower limbs via the pelvis and the femoral heads.5 The FSU function relies greatly on the anatomy of its components as well as on the interactions between the bony structures, the surrounding muscles and ligaments, and the control of the central nervous system.2
In everyday activity, the vertical loads on the spine are thought to be around 500–1000 N, nearly twice the body weight. These compressive forces increase to nearly 5000 N with lifting, achieving nearly half the failure load.6 The transmission of these loads is assured by the anatomy of the vertebral bodies, the intervertebral disk, and the facets and the sagittal curvatures of the spine.2
The vertebral body anatomy and architecture plays an important role in the load transmission. The load-bearing ability increases from 200 N in cervical vertebra to around 8000 N in the lower lumbar vertebrae. This is caused by the increasing size of the vertebral body going from the cervical spine to the lumbar spine (C1 has no vertebral body and S1 has the largest vertebral body). This increase of size of the vertebral bodies abides by Wolff’s law: “bones adapt their mass and architecture in response to the magnitude and direction of forces applied to them.” Bony microarchitecture also plays an important role in the load transmission. In normal vertebrae, the trabecular bone is arranged with vertical and horizontal trabeculae. While the vertical trabecular system transmits the compressive forces, the horizontal system decreases the bending forces on the vertical system by transmitting the bending forces to the outer cortical shell.7 With osteoporosis, there is a rarefication of the horizontal system with elongation of the vertical system. This phenomenon is more marked in the anterior body, thus the prevalence of anterior wedge osteoporotic fractures.8
The intervertebral disk is another actor in the load transmission. It has the proprieties of both a ligament (tension resisting) and a synovial joint (compression resisting). The orientation of the fibers of the annulus fibrosus limits the movements of the FSU in axial (limits rotation), sagittal (limiting flexion, extension), and coronal planes (limiting side bending). On the other hand, the nucleus pulposus acts as a spacer between the two endplates and as a shock absorber and load transmitter. It is interesting to note that the same unit has very different functions in quadrupeds compared to humans. In quadrupeds, intervertebral disks are used for motion stabilization and for propulsion forces transmission but no gravity support, whereas in humans, gravity loads are the main constraints the FSU has to sustain. This may explain the apparent disk fragility in humans who develop more diskopathies than quadrupeds.
The final load-transmitting components are the articular facets. The articular facets contribute to the posterior column (in the Denis model) and acts primarily counteracting shear forces. Similar to vertebral bodies, the shape, size, and orientation of the facets change from C1 to S1 to accommodate the increasing load transmission requirements. Classically, it has been stated that for a normal spine, the intervertebral disk receives more than 90% of the loads. There is concern on the definition of a normal spine. Contemporary studies have shown that there are various spine shapes from more straight (or less curved) to more curved and that the stress/load bearing is divided between disk forward and facets backward, depending on the sagittal orientation of the FSU. With increasing disk degeneration, causing loss of disk height, the loads are transmitted via the posterior facets that receive more than half of the compressive loads. This induces facet joints hypertrophy and osteoarthritis.
As the disk acts as a synovial articulation, it could be compared to another osteoarticular articulation as, for example, the knee or the elbow. Every joint has a normal range of motion that, when beyond it, movement is painful (hyperextension and hyperflexion). This principle is applied to an FSU albeit with a lesser range of motion (5°) compared to the elbow. With the degenerative cascade, the range of motion is ultimately decreased by degenerative changes. Disk degeneration limits flexion and brings the hyperflexion threshold closer to the average position, whereas facet joints arthritis limits hyperextension and brings the hyperextension threshold close to the average position (Fig. 9.3). When the FSU is in a hyper lordosis state, the remaining possibility of increasing extension is limited. In this kind of structure (i.e., type 1 lordosis), an increasing thoracolumbar kyphosis induces a painful caudal compensation by increasing the distal lordosis in an already much-extended area. As the painful caudal position in extension is very close to the neutral posture, an excessive postural lordosis such as supine with knees extended may be quickly intolerable. If hyperextension is needed (like in osteoporotic fracture in the thoracolumbar area in type 1 lordosis), the hyperextension is painful and generates low back pain and there is increased risk of facet hypertrophy (Fig. 9.4). Therefore, the back pain of these patients is relieved by lying on the side and rounding the back. On the other hand, an FSU in hypolordosis has the average positioning closer to a hyperflexion threshold. This phenomenon is seen is type 2 spines where the patient may adopt a lumbar kyphosis alignment as a result of multilevel diskopathies. Back pain is generated by the inability to escape the hyperflexion zone and the patients are relieved by lying in a hyperextended position (Fig. 9.5). In addition, different degenerative mechanisms would also modify this range of motion of the FSU (Fig. 9.6).
The normal spine physiology can be compared to the function of a crane that is governed by a tripod of mechanical forces: the anterior downward force of the gravity (weight of the patient), the posterior forces of the muscles to erect the spine that allow walking, and the effect of the belly shape in this system. The belly shape is different between men and women and changes with age. In younger patients, contraction of abdominal muscles increases the rigidity of the abdomen and helps the spine in supporting forward forces. In older patients, abdominal muscles weaken and the belly size increases. Women’s fatty depositions are concentrated around the hips and buttock areas before menopause (gynoid disposition), whereas the fat is concentrated around the belly in men and postmenopausal women (android deposition). In addition, the belly shape is somewhat different in men and postmenopausal women. Men’s bellies are round and puffy (with more musculature) allowing it to function as a shock absorber by maintaining higher intra-abdominal pressure and helps in fighting the anterior angulation of the spine and kyphosis. On the other hand, the postmenopausal woman’s belly shape is fluffy and, in ptosis, acting as an increasing force to the anterior angulation of the spine (Fig. 9.7). In pathology, the belly action is poor and, to simplify the crane system, the belly shape function will be removed from the equation and we will only discuss the two main acting forces: the weight gravity and posterior muscle force (MF).
Weight gravity is not caused by the total weight that applies on the ground but at each spinal level if we consider the vertical force linked to the mass over the level of the spine where this force acts. The more caudal is the level (like in lumbosacral area), the higher the force. This was the center of the barycentrometry theory of Duval-Beaupère. Using a gamma-ray scan, the authors were able to determine the body mass and the corresponding center of mass of each spinal segment above the pelvis. With the Duval-Beaupère technique, each functional segment unit has its own evaluation and the gravity acting on it is calculated by the sum of gravity of the different segments above. When using the force plate technique, the gravity on the whole body is calculated. It is not sure that both gravity on one vertebral unit and global gravity are equivalent. But as they are close, the idea of approximation is acceptable. Nonetheless, this study was experimental and difficult to use in clinical routines. To define more easily the global gravity line, some authors combined a force plate and standing X-rays to position the vertical projection of the center of mass on the ground and report it on the X-rays.
Center of mass falls always down on the foot surface and passes close to the center of femoral heads. Even if it is not totally accurate, we may consider that the centers of mass of each anatomical segment over the pelvis are, by approximation, close to the global gravity line.
Posterior muscles action is a result of paraspinal, posterior abdominal muscles that go against the body mass forces (BMFs). This mechanical system may be assimilated as a crane with the spine corresponding to the pylon. On one side of the pylon, body mass forces are acting forward, and on the other side, posterior muscles are counteracting backward. Contact force acting on the pylon is the sum of BMFs and MFs. To balance the system, if “A” is the distance from BMF to the pylon and “B” from MF to the pylon, the equation would be A × BMF = B × MF. When BMF is closer to the spine, only a small MF is necessary. If BMF displaces forward, MF cannot compensate by a backward displacement and MF must increase, increasing the contact force applied on the spine, because the contact force is an addition of BMF + MF (BMF is constant and MF increases) (Fig. 9.8).
The main objective of this system is to have the gravity force line behind the femoral heads to maintain an economical balance and the ability of the humans to walk in a vertical stance. In fact, as shown in Fig. 9.8, the moment of the force is equal to the force times the level arm of this force. Because the erector muscle level arm is constant, and to maintain a balanced spine, the increase of the moment of the gravity force will induce a higher workload from the posterior erector muscles, generating increased potentials and the fatigability of this muscular group. The result of this system is a contact force on the lower disks that increases with the imbalance of this system (Fig. 9.8).
When the Euler law is applied to spinal curvatures, the resistance for compression is proportional to the squared number of curvatures plus one (R=[(Number of curves)2 + 1] × K) with a minimum of 1 when there is no curve as in a straight spine. In fact, within an FSU, the normally vertical compressive forces will be divided into two forces: one force is parallel to the intervertebral disk and tries to displace the vertebra anteriorly or posteriorly (the shear force), and the other is perpendicular to the disk that tends to stabilize the FSU with the drawback of higher intradiskal pressures (Fig. 9.9).
The distribution of the same vertical force is different depending on the sagittal orientation of the vertebrae. In fact, vertebrae that are located in the apex of a curve (apex of the spinal lordosis or the apex of spinal kyphosis) are practically horizontal. The vertical force of weight transmission is divided into a small shear component and a much higher compressive component. On the other hand, vertebrae located at the ends of the curves tend to be more inclined with a higher shear component and a lesser compression component (Fig. 9.9).
This biomechanical concept may explain several findings in normal and pathological conditions. First, it is well documented that the most common location of lumbar diskopathies is at L4-L5.9 This could be explained by the fact that the L4-L5 disk is horizontally oriented in the majority of the cases and is subject to higher compressive loads and, thus, higher intradiskal pressures. Second, the degeneration profile is different depending on the spinal shape as defined by Roussouly et al10 (Chapter 6). As a matter of fact, Roussouly et al stated that type 2 spines have a higher prevalence of multilevel disk degeneration while type 4 spines had higher prevalence of spondylolisthesis.11 The degeneration in type 2 spines is mainly caused by the higher compressive component in a relatively straight spine. Degeneration of the disk is the primum movens of the degenerative cascade in this spinopelvic type. On the other hand, the inclination of the L4 and L5 vertebrae induces high contact forces on the posterior facets, determining facet joints hypertrophy and osteoarthritis as well as high shear forces favoring vertebral slippage and spondylolisthesis.
One spinal type is less explored in the literature: type 1 or thoracolumbar kyphosis. This spinal type is associated with low PI, low SS, lower number of vertebrae comprised in the lordosis, and a lordosis apex located in the L5 vertebra. In this type, there are practically no shear forces on the L5-S1 articulation with higher pressures on the posterior facets and thus higher L5-S1 facet joint degeneration.12 There are low rates of L5-S1 degeneration as most of the compressive forces are focused on the facets. Even more, there are higher rates of L2-L3 retro spondylolisthesis, mainly because of the great inclination of the L2-L3 FSU, rendering it more susceptible to the increased shear forces and then to retrolisthesis.
The combination of FSU orientation and positioning with the global shape of the spine may be used for explaining mechanical spinal disorders. An observation of spinal mechanics shows that an overstress in flexion or in extension may explain local pain by hypermotion and hyperpressure because of a local spinal orientation. For example, in type 4, back where spinal lordosis is overcurved in extension, pressure on the facets is favored by a double effect: localization of the contact forces on the posterior elements and FSU hyperextension. This mechanism may generate low back pain even before evidence of degeneration. As explained in Chapter 10, mechanisms and localization of degeneration are strongly linked with specific mechanical stress according to each spinopelvic morphotype.
It is evident that proximal junction kyphosis (PJK) has an important mechanical component and the theory of stress distribution according to the spinal architecture allows possible new explanations. Based on two different situations, we constructed the local junctional contact force (Fig. 9.10). In case 1, the insufficient lumbar reduction is compensated by pelvic retroversion. The posterior displacement of the spine positions the junctional area far backward from the gravity projection. This fact combined with a vertical straight thoracic spine may explain the junctional overstress and the PJF (proximal junctional failure). In case 2, a too-long spinal lordosis inflicts on the thoracic spine a backward position compared to the head. The head gravity has a very strong force moment that cannot be compensated by posterior muscles inducing a PJF.
Fig. 9.10 (a) Case 1 showing insufficient balance restoration, causing thoracic hyperextension. This increases muscles activity and induces dramatic increase in contact forces. The result is a proximal junctional failure. (b) Case 2 shows a forward displacement of the gravity line with insufficient muscle compensation inducting the proximal junction kyphosis.
Verticality of the human body has imposed specific mechanical stresses on the set spine and pelvis to counteract the gravity. The aim of an ideal balance is to minimize these stresses on the different spinal structures: vertebral bodies, disks, and facet joints. The identification of various spinopelvic morphotypes allows understanding the physical distribution of forces induced by body weight and muscle counterbalance, regarding spinal unit orientation and range of motion. An abnormal overstress may explain the mechanical origin of pain and spine degeneration. After surgical treatment, position induced by an arthrodesis may disturb the mechanical forces distribution and induce unexpected complications such as PJK.
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