Biomechanical analysis of the spino-pelvic organization and adaptation in pathology
Standing in an erect position is a human property. The pelvis anatomy and position, defined by the pelvis incidence, interact with the spinal organization in shape and position to regulate the sagittal balance between both the spine and pelvis. Sagittal balance of the human body may be defined by a setting of different parameters such as (a) pelvic parameters: pelvic incidence (PI), pelvic tilt (PT) and sacral slope (SS); (b) C7 positioning: spino-pelvic angle (SSA) and C7 plumb line; (c) shape of the spine: lumbar lordosis.
Biomechanical adaptation of the spine in pathology
In case of pathological kyphosis, different mechanical compensations may be activated. When the spine remains flexible, the hyperextension of the spine below or above compensates the kyphosis. When the spine is rigid, the only way is rotating backward the pelvis (retroversion). This mechanism is limited by the value of PI. Hip extension is a limitation factor of big retroversion when PI is high. Flexion of the knees may occur when hip extension is overpassed. The quantity of global kyphosis may be calculated by the SSA. The more SSA decreases, the more the severity of kyphosis increases. We used Roussouly’s classification of lumbar lordosis into four types to define the shape of the spine. The forces acting on a spinal unit are combined in a contact force (CF). CF is the addition of gravity and muscle forces. In case of unbalance, CF is tremendously increased. Distribution of CF depends on the vertebral plate orientation. In an average tilt (45°), the two resultants, parallel to the plate (sliding force) or perpendicular (pressure), are equivalent. If the tilt increases, the sliding force is predominant. On the contrary, with a horizontal plate, the pressure increases. Importance of curvature is another factor of CF distribution. In a flat or kyphosis spine, CF acts more on the vertebral bodies and disc. In the case of important extension curvature, it is on the posterior elements that CF acts more. According to the shape of the spine, we may expect different degenerative evolution: (a) Type 1 is a long thoraco-lumbar kyphosis and a short hyperlordosis: discopathies in the TL area and arthritis of the posterior facets in the distal lumbar spine. In younger patients, L4 S1 hyperextension may induce a nutcracker L5 spondylolysis. (b) Type 2 is a flat lordosis: Stress is at its maximum on the discs with a high risk of early disc herniation than later with multilevel discopathies. (c) Type 3 has an average shape without characteristics for a specific degeneration of the spine. (d) Type 4 is a long and curved lumbar spine: this is the spine for L5 isthmic lysis by shear forces. When the patient keeps the lordosis curvature, a posterior arthritis may occur and later a degenerative L4 L5 spondylolisthesis. Older patients may lose the lordosis curvature, SSA decreases and pelvis tilt increases. A widely retroverted pelvis with a high pelvic incidence is certainly a previous Type 4 and a restoration of a big lordosis is needed in case of arthrodesis.
The genuine shape of the spine is probably one of the main mechanical factors of degenerative evolution. This shape is oriented by a shape pelvis parameter, the pelvis incidence. In case of pathology, this constant parameter is the only signature to determine the original spine shape we have to restore the balance of the patient.
KeywordsLumbar lordosis Spino-sacral angle Thoracic kyphosis Pelvic incidence Pelvic tilt Sacral slope Sagittal balance Pathological balance
The acquisition of a vertical posture, i.e., the ability Man acquired to extend simultaneously the trunk, hips, thigh and legs—and the resulting bipedal locomotion represented definitively the main transformation in the history of the Hominidae, the one that induced slowly the others, the evolution of the hand and the brain, which consequently gave rise to the tools, conscience, culture and the society .
Human beings are the only vertebrates to maintain an upright, totally vertical, bipedal position. In addition to diverse other morphological transformations, as the relations between the foramen magnum orientation and the cervical spine, the spino-pelvic complex played a relevant role in the acquisition of bipedalism. Unlike other vertebrates, the human spine comprises successive, opposing curves that allow the trunk to assume an erect position. The lumbar lordosis is found in no other species. The great apes can achieve an upright position, but only with a semi-erect trunk. Their whole spine looks like a big “C”, a long kyphosis that is incompatible with a constant stable erect posture and walking. Bipedal locomotion in non-human primates is awfully limited. They often use their long forelimbs in pronation for balance when trying to perform a bipedal displacement. Studies have shown that the orangutans are frequently biped, with knees in extension (a characteristic usually associated with human bipedalism . The kangaroo, as well as the extinct Tyrannosaurus rex, walks (or walked) with two legs; however, they have (or had) a stabilizing tail . The biomechanics of birds’ bipedalism also has nothing to do with ours .
The human pelvis has equally a tremendous importance in the development of verticality. Its intrinsic anatomical relationship with the spine created mechanisms to modulate posture. The pelvis attempts to couple lumbar lordosis with hip extension in the erect position with minimal expense of energy. However, some pelvis can accomplish this task better than others. Progressive knowledge has demonstrated that the shape of the pelvis and its relation to the sacral slope (SS) influence dramatically the type of lumbar lordosis in a single individual .
Thanks to the pelvic parameters described by Duval-Beaupère et al. [5, 6, 7], it became possible to define the pelvic geometry and the relation of this geometry with the position of the pelvis. In other words, for the first time, it was possible to relate the form and function of the pelvis. Recently, some authors have highlighted the correlation between the spino-pelvic organization and an eventual specific degenerative evolution derived from the original organization . The key for it is the pelvic incidence (PI) angle. It is now crystal clear that the PI and the SS are important in determining the type of lumbar lordosis in a specific individual. According to the SS value, Roussouly et al.  described four types of lumbar lordosis. Each type represents a distinct spino-pelvic morphological complex. Each type of spino-pelvic geometry, with respective functional spinal units, will endure, obviously, mechanical stresses, according to the geometry of the lordosis and to the global spinal balance. A better understanding of the local stress on a vertebral unit allows understanding the relations between the geometry of discs and vertebrae, the combination of gravity and muscles forces and the resulting pathological consequences.
Any pattern of degenerative spine is not a static entity. It resulted from an evolution, a transformation. This evolution had certainly, among its causes, mechanical stresses. Both physicians and patients are not satisfied with terms such as non-specific lumbar degenerative disc disease or non-specific low back pain. It is imperative to understand the process to better target treatments. The opening of new horizons of knowledge will help understand the complex challenge that is degenerative disc disease. The comprehension of spinal sagittal balance and the identification of different morphotypes, which may undergo straight degenerative conditions, may imply earlier treatment possibilities, perhaps easier therapies or even prevention measures for specific situations according to each morphotype.
A thorough understanding of how different types of spino-pelvic complexes can lead to different patterns of degenerative spine disease may help the surgeon to tailor more appropriately the treatment strategy for his/her patient.
Several authors [5, 9] have employed pelvic parameters to characterize morphologically and functionally the pelvis. Duval-Beaupère et al. defined the PI as the angle created by the intersection of the line drawn from the center of the femoral heads to the middle of the sacral plate and the line running perpendicular to the middle of the sacral plate. The PI is the angle that describes the relation between the sacral plate and the femoral heads. It is a constant morphological parameter for one person.
The result of PI on pelvis morphology
The pelvic incidence angle provides substantial information regarding the possibilities of pelvic compensation such as ability of pelvic retroversion. The PI determines the relative position of the sacral plate in relation to the femoral heads. In a recent study, we demonstrated that, in an asymptomatic population, the lower value is approximately 35° and the higher around 85°, the average being 51.9° . IP values inferior to 35° are seen in Scheuermann disease and IP higher than 85° are seen in patients with isthmic spondylolisthesis as reported by Labelle et al. . We assume that values outside these bounders are representative of pathological conditions.
The influence of PI on positioning the pelvis
The pelvis can rotate around the femoral heads, following the bicoxo-femoral axis. The pelvic tilt (PT) is the angle between the vertical line and the line drawn from the center of the femoral heads to the center of the upper sacral endplate. When the pelvis rotates backward (retroversion), PT increases; when the pelvis rotates forward (anteversion), PT decreases. PT is a positional parameter, as well as the SS. The SS refers to the orientation of the sacral plate in relation to a horizontal line.
For a person, the ability of retroversion is limited by the value of its own PI. Patients with a small PI have a small capacity to compensate for their sagittal imbalance through pelvis retroversion. The limited posterior offset of the sacrum regarding the femoral heads can be observed in lateral radiographs. The same poor ability of compensation seems theoretically to occur in different pathologies when there is a low PI. This is the case of severe kyphosis and of ankylosing spondylitis with low-grade PI. Debarge et al.  showed that, in ankylosing spondylitis, when there is low-grade PI, the extension of the hips does not limit the pelvis retroversion and the 0° of SS may be easily reached.
Effect of PI on the shape of lumbar lordosis
The relation between lumbar lordosis and sacral slope was originally demonstrated by Stagnara et al. . The more SS was tilted, the more the lumbar curvature was high (“dynamic back”); contrarily, when SS was rather horizontal, lumbar curvature was flat (“static back”). Several studies presented this strong correlation between SS and the global angle of lordosis [15, 16, 17].
Another important observation is that lordosis can be reconstructed into two tangent arcs of circle, with a superior (proximal) one and an inferior (distal) one . The purpose of this division is to better understand the geometry of lordosis. Lumbar lordosis must not be studied as a single curve.
Low sacral slope (<35°)
Type 1 the distal arch is almost absent, the apex is very low, close to L5 and LL is quite constructed by only the proximal arch. LL is short and, therefore, the kyphosis is long, with an extension on the thoraco-lumbar area. In summary, it is a non-harmonious back with thoraco-lumbar kyphosis and short hyperlordosis.
Type 2 the distal arch is low; it is longer but flat, close to a straight line. It is a harmonious flat back.
Type 3 LL is well balanced between its two arches. It is a harmonious regular back.
Type 4 the distal arch is highly increased in angle and number of vertebrae; length and curvature of LL increase. It is a harmonious hypercurved back.
Due to the correlation between SS and PI (when PI values are higher, SS also is higher), Type 1 and 2 have generally a low-grade PI; Type 3 and 4 have generally a higher grade of PI.
Assessment of the global spinal balance: C7 positioning
The C7 PL is the reflection of the global balance of the spine. Its position in the sagittal plane is analyzed in relation to two anatomical landmarks: the femoral head axis (HA) and the posterior edge of the sacral plate. In a population with a balanced spine, the C7 PL is generally located behind or at the level of the posterior edge of the sacral endplate . If C7 PL locates between HA and the posterior edge of the sacral plate, the system may be considered as slightly unbalanced. When it is forward HA, the system is probably severely unbalanced. Later in this article, it will be presented that the positioning of C7 must be correlated to PT, which is another indicator of balance failure.
The second method of employing C7 vertebra as an indicator of sagittal balance is through SSA, an angle that quantifies the global kyphosis of the whole spine. In a well-balanced spine, SSA remains proportional to SS. In case of kyphosis, or loss of lumbar lordosis, SSA decreases (Fig. 5). In severe kyphosis, SSA decreases strongly. These relations may provide a guide to evaluate the need of correction for the kyphosis.
Orientation of the spine and local stress: the contact force
When acting on a functional spinal unit, there is a distribution of CF between the intervertebral disc in front and the facet joints behind. The more the lumbar spine is curved, the more is the impact of CF acting on the posterior elements. On the contrary, in case of low curvature or flat back, CF acts mainly on the discs.
Biomechanical adaptation of the balance in pathological kyphosis
When the kyphosis occurs acutely, promoting “flexible” deformation on the spine, such as traumatic kyphosis, the compensation is obtained by the “flexible” non-fractured spine below the kyphosis. Generally, there is no compensation at the pelvis level. When the kyphosis is highly located, the lumbar lordosis is able to compensate the balance. When the kyphosis extends into the thoraco-lumbar area, the length of lordosis could be too short to compensate. Compensation is relative to the length of lordosis and the extensibility of the spine.
When the kyphosis occurs on a rigid spine, such as degenerative spine or ankylosing spondylitis, or in iatrogenic situation such as arthrodesis, the only and very first way of compensation is pelvic tilting, aiming at retroversion. This mechanism increases the horizontal length between the femoral heads and the sacral plate and decreases the SS. Due to the geometrical relation between the PI and the PT, a vertical pelvis with a low PI is less able to compensate for this mechanism than a horizontal pelvis with a high PI.
In case of very severe kyphosis, the flexion of the knees increases the effect of posterior PT and improves the positioning of the head and the gaze direction.
Spino-pelvic assessment in pathology
Which degenerative evolution with which type of spine?
- Type 1 is a combination of kyphosis and hyperextension area:
In the thoraco-lumbar kyphosis area, there is an increased risk of disc degeneration.
In the junctional area, between thoraco-lumbar kyphosis and lumbar lordosis, the discs are very tilted with a risk of retrolisthesis.
In the hyperlordotic area, we see the “kissing” of the spinous processes; there is a risk of facet hyperpressure and L5 spondylolysis by “crack nuts” effect. The discs L4 L5 and L5 S1 are generally protected.
In case of increasing thoraco-lumbar kyphosis (e.g.,: fracture) the only way to compensate is by increasing the short distal lordosis. Due to a low PI, the range of posterior pelvis tilt is limited. It is mandatory to reduce the thoraco-lumbar kyphosis, if possible, to decrease the hyperlordosis.
Type 2 is the flat back. The orientation of the discs is horizontal; therefore, the disc pressure is maximum. There is an important risk of early disc degeneration with central disc herniation. Barrey showed that the more the patients with disc herniation were young, the more was PI low . The risk of early distal discopathy increases in patients with low PI and flat back. Mechanically, this back is not the one adapted for weight bearing, or sports or activities with high pressure on the back.
A kyphotic evolution at the lumbar level may result in a severe unbalance because of the limitation of pelvis retroversion, but complete retroversion bringing SS to 0° is possible.
Types 3 and 4 have a bigger lumbar lordosis, mainly Type 4. They are generally associated with a horizontal pelvis with a high-grade PI. The more the increases in PI, the more was SS high, with a risk of spondylolisthesis through L5 isthmic lysis by a “sliding” mechanism.
Due to the big amount of curvature of lumbar lordosis, the stress is mainly posterior on the facet joints. The risk of posterior facet degeneration is significant. A proof of limits of extension is the kissing of the spinous processes. When spinous processes are in contact, the local extension is at its maximum and may be an important cause of pain. Posterior facet arthritis may have different pathological presentations, such as subluxation through the sliding mechanism, osteophytes, synovitis and cyst formation. When the contact between two facets is no more efficient, there is an anterior instability with sliding producing degenerative spondylolisthesis, mainly between L4 and L5 [14, 15].
In aging people, to prevent pain in extension, there is a tendency to decrease lordosis. Progressive degenerative discopathies enhance the loss of lordosis, compensated by an increasing posterior tilt of the pelvis. We may differentiate a true Type 2 with discopathies from a “false” Type 2, resulting in degenerative evolution of a Type 4. The first has a low-grade PI and a small PT, while the second has a high-grade PI and a large PT. Thus, it is important to understand the sagittal spino-pelvic parameters as their knowledge will allow the surgeon to establish the correct surgical strategy regarding the sagittal balance of the spine.
Diagnostic and strategic applications
The only way to assess the global spinal sagittal balance and its eventual consequences is the long-standing lateral X-ray. It is necessary to visualize the whole spine and pelvis, from the skull to the proximal femurs.
Pelvic angles: PI, PT and SS
Type of lumbar lordosis
Positioning of C7 PL
Whatever the technique of treatment, the restoration of the sagittal balance is the best way to obtain a good result. In a pathological situation, it is always difficult to recognize how much the present position of the spino-pelvic complex represents its original anatomy and how much it represents its pathological evolution or its functional adaptation (antalgic position, mainly). The only signature of the initial situation is the PI, which remains approximately stable in adult life. When treating a patient with a low-grade PI, it is necessary to restore a Type 1 or 2, without increasing the lordosis. In case of higher PI, the more the PI increases, the more the lumbar lordosis has to be augmented in angle and length, to reduce the posterior tilt of the pelvis.
In case of large PT, hips are forced in extension with an increasing anterior unbalance when walking. This situation may impair a hip arthritis or a total hip prosthesis function due to the more vertical orientation of the acetabulum.
In severe hypolordosis with high-grade PI, a posterior subtraction osteotomy may be used.
Sagittal balance of the spine is not only a matter of positioning the spine over the pelvis. There is a strong correlation in shape and positioning, and form and function, between the pelvis and the spine. The morphology of the pelvis is identified by PI, with consequences on the morphology of the spine. With time, depending of the individual morphology of a person, specific degenerative evolutions may occur. Sagittal parameters may be considered as predictive regarding the respective shape of the spine and pelvis. A better understanding of this relation may lead to improved diagnosis of degenerative spine diseases and a better strategy of treatment.
Conflict of interest
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