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Review
, 20 Suppl 5 (Suppl 5), 609-18

Biomechanical Analysis of the Spino-Pelvic Organization and Adaptation in Pathology

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Review

Biomechanical Analysis of the Spino-Pelvic Organization and Adaptation in Pathology

Pierre Roussouly et al. Eur Spine J.

Abstract

Introduction: 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.

Conclusion: 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.

Figures

Fig. 1
Fig. 1
a Model and lateral X-ray of a low-grade pelvic incidence pelvis (vertical pelvis). Note the high position of L5 S1 disc with respect to the top of the iliac crest. b High-grade pelvic incidence pelvis (horizontal pelvis). L5 S1 disc and L5 are totally below the top of the iliac crest
Fig. 2
Fig. 2
a For the same pelvis with the same PI, when PT increases (retroverted pelvis), SS increases. A high-grade PI allows a bigger angle of PT, but PT may be limited by hip extension. b A smaller PI permits less retroversion, but PT may reach a greater value for SS = 0 (PT = PI) because of less hip extension
Fig. 3
Fig. 3
Mechanism of compensation of a progressive kyphosis. a Normal situation with a slight pelvis retroversion and C7 PL over the sacral endplate. b With a progressive loss of lordosis, pelvis retroversion permits maintaining C7 PL behind the femoral heads. c In case of severe kyphosis, hip extension (HE) limits the pelvis retroversion. It is compensated by flexion of the knees. C7 PL passes forward to the femoral heads. Due to the tilt of the femoral shaft, PT = PFA + HE
Fig. 4
Fig. 4
The shape of lumbar lordosis depends on SS orientation. Type 1 and 2 have SS < 35°; Type 3 has 35° < SS < 45°; Type 4 has SS > 45°. Generally Type 1 and 2 have a low PI and Type 3 and 4 have a high PI
Fig. 5
Fig. 5
Positioning angles of C7; in case of severe kyphosis (right), SSA decreases strongly
Fig. 6
Fig. 6
Contact force (CF) on the distal lumbar spine is the sum of the gravity force (G) and the force M acting on the posterior muscles to maintain an erect position. The more the system is unbalanced, the more G increases, and the more M has to compensate for increasing CF
Fig. 7
Fig. 7
Distribution of CF with respect to the local intervertebral tilt. When the tilt increases, the sliding resultant force (S) increases; in case of a horizontal plate orientation, the pressure resultant force (P) increases
Fig. 8
Fig. 8
CF positioning with respect to the local spine extension. When the spine is in hyperextension (left distal hyperlordosis of a Type 1), CF acts on the posterior arch. If the local lordosis is poor, CF moves forward on the discs and bodies
Fig. 9
Fig. 9
Compensation of a local iatrogenic hypolordosis (fusion of L4 L5) by hyperextension of L3 L4 on a flexible spine. Treatment by extension of the fusion to L3 maintaining the segmental hyperextension of L3 L4

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