Fracture fixation | Spine biomechanics & fixation

Spine Biomechanics

Overview

This document covers various aspects of spine biomechanics, including anatomical structures, biomechanical testing methods, and fracture management in the context of mechanical engineering as presented in the course MECHENG 4101.

Learning Objectives

  • Understand spinal anatomy.

  • Comprehend mechanical functions of spinal components.

  • Familiarize with biomechanical testing methods: pure moment, unidirectional and combined/complex testing, and failure tests.

Anatomy of the Spine

Spinal anatomy comprises 29 vertebrae, intervertebral discs, ligaments, and the associated muscles. The anatomical shapes and curves of the spine contribute to its mechanical functions:

  • Sagittal 'S-shape': Essential for maintaining equilibrium in standing posture.

  • Cervical Region: Facilitates head mobility.

  • Thoracic Region: Supports the rib cage.

  • Lumbar (+Sacral) Region: Enables trunk mobility, highlighting variations in anatomy that provide the required flexibility and stability in each region, such as:

    • Lordosis: An inward curve of the lumbar and cervical regions.

    • Kyphosis: An outward curve typically found in the thoracic region.

Spinal Regions and Associated Structures

  • Vertebrae: Classified into cervical, thoracic, and lumbar sections, with sizes increasing from cervical to lumbar to accommodate greater loads and changes in facet orientation.

  • Ligaments: Include two longitudinal ligaments connecting vertebral bodies, with a high collagen content to limit sagittal bending. Elastic ligaments control intervertebral flexion further from the center of rotation and include capsular ligaments.

  • Muscles: Superficial muscles assist in moving the head and shoulders, while deep spinal muscles influence vertebral joint movement, aiding in stability and proprioception.

Functional Spinal Unit (FSU)

The FSU is described as the smallest mechanical unit of the spine characterized by two adjacent vertebrae and their joints, often regarded as a motion or spinal segment.

Intervertebral Spinal Motions

Spinal motions are categorized as follows:

  • Axial Rotation

  • Compression (negative load)

  • Tension (positive load)

  • Extension (negative load)

  • Flexion (positive load)

  • Lateral Shear

  • Shear Forces: Both anterior and posterior shear forces, with posterior shear as negative and anterior shear as positive.

  • Lateral Bending

Spinal Loading

Spinal motions result from loads applied through muscular forces or external sources, influenced by spinal anatomy and joint quality. Key factors affecting kinematics include:

  1. Degeneration

  2. Injury

  3. Surgical intervention
    It is crucial to understand the underlying mechanisms affecting spinal kinematics due to these factors.

Biomechanical Tests

Biomechanical tests characterize the normal and pathological properties of spinal tissues and segments in ex vivo conditions. The reasons for these tests include:

  • Improving understanding of spinal trauma mechanisms.

  • Evaluating surgical efficacy and instrumentation.

  • Informing spinal implants design and Finite Element Analysis (FEA) models.

  • Assisting injury prevention device design based on improved injury criteria.

Testing Types

  • Normal and Pathological Mechanics: Tests can be conducted on intact, injured, or post-surgery spinal segments, either as a single FSU or multi-segment approach.

  • Loading Types: Includes pure moment, unidirectional or combined loading, and can be either non-destructive or pushed to failure.

Mechanical Testing Machines

Testing equipment like the INSTRON series is commonly used for measuring forces during experimentation, including:

  • LVDT (Linear Variable Differential Transformer) for accurate displacement measurements.

  • Actuation devices for applying loads to spinal specimens.

Flexibility Protocol

The flexibility protocol involves applying pure, unconstrained, non-destructive moments to obtain outcome parameters such as:

  • Range of Motion (ROM; degrees)

  • Neutral Zone (NZ; degrees)

  • Flexibility (degrees/Nm) or Stiffness (Nm/degree)

  • Hysteresis Energy (Joules)
    Tests also compare conditions such as pre- and post-injury.

Unidirectional Loading Considerations

Pure moments do not occur in vivo; thus, unidirectional loading represents resultant vectors while using fixed center of rotation boundaries introduces non-physiological conditions.

Failure Testing

Failure tests measure the relationship between applied load and displacement, showing:

  • The yield point (end of the linear region)

  • The failure point (where the structure begins to yield)

  • The peak load point (the maximum load before failure occurs)
    The physiological and subfailure realms are captured within the linear region of load vs. displacement curves, with the first distinct drop indicating the initial failure point.

Other Testing Considerations

When testing, consider:

  • Viscoelastic properties (loading rate, creep, stress relaxation)

  • Load vs. motion control, complexities in loading magnitudes and directions, cyclic loading versus ramp loading

  • Pre-loads and muscle simulation effects

  • Conditions of specimen hydration, temperature, and storage methods (fresh, frozen-thawed, embalmed)

  • Specimen handling fixtures and sample length/type (FSU vs. segment vs. tissue)

  • Additional instrumentation influencing the testing outcomes.

Other Testing Outcomes

Outcomes from tests may include:

  • IVD Pressure: Measurement of intervertebral disc pressures during movement.

  • Bone Strains and deflection measurements.

  • Analysis of Intervertebral Kinematics: the movement of vertebrae in relation to each other.

Biomechanics of MSK Injury

Learning Objectives

  • Describe long bone fracture types and healing processes.

  • Understand the mechanics in healing.

  • Familiarity with bone fracture repair devices and strategies.

Long Bone Fracture Classification

Fractures are classified as follows:

  • Group A: Simple fractures

  • Group B: Wedge fractures

  • Group C: Complex fractures
    The fracture types are characterized into specific sub-types such as spiral, oblique, transverse, and multifragmentary, with relevant forces including torsional, bending, and axial compressive forces.

Fracture Healing Stages

  1. Inflammation: Formation of hematoma and inflammatory cell migration occurring post fracture.

  2. Soft Callus Formation: Gradual reduction of pain and swelling, where fibrous tissue and cartilage replace granulation tissue, stabilizing the fracture fragments.

  3. Hard Callus Formation: Connection of fracture ends via woven bone, increasing structural stiffness.

  4. Remodeling: Replacement of woven bone with lamellar bone to restore normal morphology and function.

Fracture Healing Mechanics

The rigidity of the fracture site correlates with mechanical properties and callus geometry: (E I), where I is calculated as Ibending = rac{ ext{π} d^4}{64} and Itorsion = rac{ ext{π} d^4}{32}. Increased callus diameter contributes to stabilization and reduced interfragmentary motion over time.

Contact vs. Gap Healing

Healing can occur through direct contact between bone fragments (contact healing) or through a fibrous gap (gap healing). Mechanics and osteoblast/osteoclast activity vary based on fracture stabilization techniques.

Interfragmentary Strain Theory

According to Perren's interfragmentary strain theory, a delicate balance of strain must occur throughout the healing process, with high strains tolerated in early stages. The goal is to foster an environment conducive to bone healing while managing mechanical loading effectively.

Fracture Fixation Goals

When addressing fractures, fixation aims to provide:

  • Initial mechanical stability

  • Resistance to loading stresses while achieving optimal interfragmentary strain

  • Adequate conditions for callus formation without excessive strain or gap widths

Factors Affecting Biomechanics of Healing

Critical factors that influence healing include interfragmentary motion, fracture type, distribution of fragments, and blood supply, all of which can delay union and affect the healing process.

Fracture Fixation Devices

Fracture fixation devices can be categorized into:

  • External Fixators: Provide stabilization from outside the body, often utilized in open wound scenarios.

  • Internal Fixators: Such as nails and plates, support bones internally, restoring alignment and compressing fracture sites.

Spinal Disorders and Surgical Considerations

Several diseases and conditions affect spinal function and biomechanics, including:

  • Osteopenia and Osteoporosis

  • Spondylosis and Disc Degeneration

  • Osteoarthrosis/Osteoarthritis

  • Disc Prolapse/Herniation

  • Scoliosis

  • Spondylolisthesis

  • Spinal fractures and infections

Spine Surgery Goals include:

  • Decompression of the spine and restoration of stable curvature ranges

  • Achieving positive patient outcomes such as reduced pain and improved neurological functions.

Advanced Topics in Spine Surgery

In recent years, innovations in spinal surgery have made strides in instrumentation and techniques:

  • Navigated and Robotic Surgery to enhance precision.

  • Augmented Reality tools and Endoscopic Techniques for minimally invasive approaches to spinal interventions.

This document encompasses detailed notes necessary for understanding spine biomechanics, mechanical testing protocols, and the complex interplay between anatomical factors and surgical interventions, aimed at supporting effective study and application in the field of Biomechanical Engineering.