Spinal Biomechanics

Spinal biomechanics centers on the functional spinal unit—two adjacent vertebrae plus intervening disc, facet joints, and ligaments—representing the smallest functional motion unit with 6 degrees of freedom. The Denis three-column theory defines spinal stability: Anterior column (ALL + anterior 50% of vertebral body), Middle column (PLL + posterior 50% of body), Posterior column (neural arch structures)—disruption of 2 or more columns indicates instability. The anterior column bears approximately 70% of axial load, with the nucleus pulposus behaving as an incompressible fluid distributing forces, while facet joints bear 10-30% depending on posture and guide motion patterns (sagittal in lumbar, coronal in thoracic). The instantaneous axis of rotation (IAR) normally lies within the disc space; abnormal IAR migration indicates degeneration or instability. White and Panjabi criteria quantify clinical instability: greater than 3.5mm translation or greater than 11° angulation in the cervical spine, greater than 4.5mm or greater than 20° in the thoracolumbar spine.

What is Spinal Biomechanics?

Spinal biomechanics is the study of mechanical principles governing spinal motion, load distribution, and stability. Understanding these principles is essential for interpreting spinal pathology, assessing instability, and planning surgical interventions. The spine functions as a flexible column that must balance competing demands: mobility for daily activities and stability to protect neural elements.

The Functional Spinal Unit

The functional spinal unit (FSU), also called a motion segment, is the smallest physiological unit of the spine capable of exhibiting biomechanical characteristics similar to the entire spine. It consists of two adjacent vertebrae, the intervening intervertebral disc, all adjoining ligaments, and the paired facet joints.

Degrees of Freedom

Each spinal motion segment has six degrees of freedom: three translations (anterior-posterior, lateral, vertical) and three rotations (flexion-extension, lateral bending, axial rotation). The range of motion varies by spinal region.

Understanding degrees of freedom is critical for analyzing instability.

Load Transmission Through the Spine

The spine transmits load through two parallel systems: the anterior column (vertebral bodies and discs) and the posterior column (facet joints and neural arch). In neutral standing, approximately 70% of axial load passes through the anterior column, while the posterior elements bear 10-30%. This distribution changes with posture, increasing posterior load in extension and anterior load in flexion.

Three-Column Stability Model

Denis proposed a conceptual framework dividing the spine into three structural columns to assess stability. This model recognizes that the middle column is the critical determinant of spinal stability.

Motion Control and the Instantaneous Axis of Rotation

During spinal motion, each vertebra rotates about a point called the instantaneous axis of rotation (IAR). In a healthy spine, the IAR is located within the disc space or adjacent vertebral body. Abnormal IAR location outside these boundaries indicates instability or degeneration. Understanding IAR is crucial for evaluating pathological motion patterns.

Denis proposed a three-column model in 1983 to classify thoracolumbar fractures and predict stability. Disruption of two or more columns indicates spinal instability.

Axial Load Sharing

The anterior column (vertebral body and disc) bears approximately 70% of axial load in neutral standing. The posterior elements (facet joints) bear 10-30%, increasing with extension.

Intradiscal Pressure

Nachemson demonstrated that intradiscal pressure is highest in sitting flexion (275% of standing), intermediate in standing (100%), and lowest in supine (25%). This has implications for patient positioning and rehabilitation.

The instantaneous axis of rotation (IAR) is the point about which a vertebra rotates during motion. In a healthy spine, the IAR is located within the disc space or adjacent vertebral body. Abnormal IAR location indicates instability or degeneration.

Clinical Significance

• Normal IAR: Within disc space, consistent motion pattern
• Abnormal IAR: Outside disc space, indicative of instability
• Degenerative changes: IAR shifts posteriorly with disc height loss
• Fusion effect: Eliminates motion at that segment, IAR moves to adjacent levels

Application to Spinal Trauma

Understanding spinal biomechanics is essential for interpreting fracture patterns and assessing stability. The Denis three-column theory guides surgical decision-making: isolated anterior column fractures (simple compression) are often stable, while burst fractures involving anterior and middle columns require careful evaluation. Two-column disruption is the threshold for surgical stabilization in most cases.

Application to Degenerative Disease

Biomechanical principles explain the natural history of spinal degeneration. Disc degeneration leads to loss of disc height, which shifts the IAR posteriorly and increases facet loading. This creates a degenerative cascade: increased facet stress accelerates facet arthropathy, which further alters load distribution. Understanding this cascade informs treatment decisions, including motion preservation versus fusion.

Surgical Decision-Making

Biomechanical understanding informs surgical approach selection. For example, posterior fixation addresses the tension band but may not adequately restore anterior column height in burst fractures. Combined anterior-posterior approaches restore both columns. In cervical spine, understanding that C1-C2 provides 50% of rotation helps explain why C1-C2 fusion significantly limits neck rotation.

Patient Education and Rehabilitation

Nachemson's intradiscal pressure measurements provide evidence-based guidance for activity modification. Patients with disc pathology benefit from understanding that sitting flexion creates the highest disc pressure (275% of standing), while supine positioning is lowest (25%). This informs posture recommendations and lifting technique education.

Australian Epidemiology and Practice

Spinal Biomechanics in Australian Practice:

• Spinal biomechanics forms a core component of FRACS Basic Science examination
• Understanding Denis three-column theory, White-Panjabi criteria, and IAR concepts is essential for Australian orthopaedic training
• Major spinal centres (Austin Hospital, Royal North Shore, Princess Alexandra Hospital) manage complex spinal trauma and degenerative conditions

RACS Orthopaedic Training Relevance:

• Spinal biomechanics is heavily tested in both written and viva examinations
• Candidates must understand load distribution, instability criteria, and regional biomechanical differences
• Denis three-column theory and TLICS scoring are commonly examined topics
• Viva scenarios frequently test application of biomechanical principles to clinical decision-making

Australian Spinal Trauma Management:

• Major trauma centres follow standardised spinal clearance protocols incorporating biomechanical principles
• Victorian State Trauma System and NSW Trauma Networks coordinate complex spinal injury management
• Pre-hospital spinal immobilisation protocols based on mechanism and biomechanical injury patterns
• Aeromedical retrieval services (CareFlight, RACQ LifeFlight) follow evidence-based spinal handling protocols

PBS (Pharmaceutical Benefits Scheme) Considerations:

• Bone graft substitutes for spinal fusion have variable PBS coverage
• rhBMP-2 available through Special Access Scheme for specific spinal fusion indications
• Analgesic medications for spinal conditions follow PBS prescribing guidelines

eTG (Therapeutic Guidelines) Recommendations:

• Pain management guidelines for acute and chronic spinal conditions
• Antibiotic prophylaxis recommendations for spinal surgery
• VTE prophylaxis protocols for spinal surgery patients

Australian Research Contributions:

• Australian spinal surgeons contribute to international biomechanics research
• Collaboration with AO Spine for education and research initiatives
• Cadaveric and biomechanical testing facilities at major Australian universities