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Biomechanical Modeling of the Spine
Summary of Key Points
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Computational spine models are valuable options for conducting comprehensive biomechanical analyses, especially when clinical and experimental approaches are difficult or impossible.
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Multibody models and finite element models are complementary techniques used to analyze the biomechanics of the spine and the treatment of pathologic conditions.
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Multibody models are adapted to assess loads and displacements along the spine and with surgical instrumentation constructs and to evaluate surgical techniques for determining optimal outcomes.
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Finite element models allow analysis of stress and strain in anatomical structures and instrumentation constructs with complex geometry.
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Verification, validation, and quantification of uncertainty are essential steps to ensure that the models correctly represent reality, to verify their applicability in the context of use, and to properly establish their credibility and predictive capability.
Acknowledgment
The work presented in the chapter was financially supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grant program and Industrial Research Chair program with Medtronic of Canada), the Canada Research Chair Program, and the Scoliosis Research Society.
Three-Dimensional Morphofunctional Anatomy and Biomechanics of the Asymptomatic and Scoliotic Spine
The human spine, rib cage, and pelvis are complex biomechanical structures. The bones are constrained by joints, ligaments, and muscles to provide the right body shape, protect vital organs, bear internal and external loads, and allow posture control and functional movement while maintaining the balance and stability of the whole body. Each pair of adjacent vertebrae are connected by an intervertebral disc, facet joints, and ligaments, forming a functional spinal unit (FSU). The three-dimensional (3D) spine biomechanics can be analyzed in each of the three anatomical planes. In the coronal plane, the profile of a normal spine is straight. In the sagittal plane, the accepted normal values are between 20 degrees and 50 degrees for thoracic kyphosis (TK), and between 30 degrees and 80 degrees for lumbar lordosis. A normal spine is offset by less than 4 cm from the sagittal vertical axis (horizontal distance between a plumb line drawn from the middle of the C7 vertebral body and the posterosuperior corner of the S1 vertebral body). For the pelvis, the normal values are between 34 degrees and 84 degrees for pelvic incidence, from 20 degrees to 65 degrees for sacral slope, and from 5 degrees to 30 degrees for pelvic tilt. The biomechanical stability of the spine ensures the ability to support physiological loads, constrain motion patterns so as not to damage or affect the spinal cord and nerves, and prevent structural changes. Knowledge of the 3D biomechanics of asymptomatic spine is essential for the understanding, evaluation, and treatment of various spinal disorders.
Scoliosis is one of the most common spinal disorders, and is classically defined as a lateral deviation of the spine. The deformity of the scoliotic spine is not limited to the coronal plane and can be accompanied by an anomaly in the sagittal plane, uneven shoulders, pelvic abnormalities, and transverse plane rotation of the vertebrae and rib cage, as well as intrinsic deformities of the bones and modifications of the mechanical properties. Additionally, the regional geometry of the spine can be characterized by a curved spline defining planes of maximum curvature. In a healthy spine, which is straight in the coronal plane, the planes of maximum curvature are in the sagittal plane, but in a scoliotic spine the planes of maximum deformity are rotated because of the lateral deformity. It was found that the lateral curvature seen in patients with adolescent idiopathic scoliosis (AIS) altered the mechanical stress distribution across the spine, which is generally recognized as a self-sustaining biomechanical process resulting in asymmetrical growth of the vertebrae and further regional deformation of the spine.
Biomechanics of Deformity Correction
The main treatment option for severe scoliosis remains surgical instrumentation and fusion to correct the deformity, stabilize the spine, and achieve and maintain a well-balanced trunk. Implants used in this technique have two complementary mechanical roles. The first is to apply corrective forces to intraoperatively reduce the spinal deformity and to maintain the achieved correction subsequently. The second is to immobilize the spine until bony fusion has occurred. In patients for whom several years of growth are still expected, fusionless growth modulation surgical techniques are available. One of these techniques uses a compressive approach. Based on the Hueter–Volkmann principle, compression forces are applied to the convexity of the curve through a flexible tether to slow down the growth of the vertebral part on the convex side of the curve and favor the growth of the vertebral part on the concave side. The modulated residual growth of the spine allows correction of the spinal deformity over time.
Outcomes of spinal instrumentation depend on many factors; some are inherent to the particular spinal deformity of each individual patient, such as the presenting Cobb angles, sagittal alignment, spine stiffness, and spinopelvic parameters, whereas others pertain to implant design, construct configuration, and surgical techniques. Great variation persists between surgeons in preoperative planning regarding implant types, construct configuration, and surgical techniques for similar cases; consensus on the most appropriate surgical strategies for optimal results has yet to be reached. The biomechanics of surgical instrumentation for the treatment of spinal deformities is not yet fully understood and mastered. In the following sections, a few examples of 3D spine models, their applications to investigating the biomechanics of the spine, and spine surgery instrumentation are presented.
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