Stability Principles

Spinal Stability Principles

Definition:

 Stability maintenance of vertebral alignment under physiological loads, ensuring load transmission, motion, and neural protection without neurological injury, deformity, or pain.

Spine Core Functions:

• Transmits and distributes spinal loads

• Permits multidirectional motion

• Protects the spinal cord and neural structures

Important components of stability

1- Spinal Stability Subsystems

· Defined by Panjabi

· In a healthy spine, stability is maintained through the integrated function of three subsystems: passive, active, and neural control.

• Passive subsystem: Includes the intervertebral discs,        ligaments, facet joints, vertebrae, and passive muscle support.

• Active subsystem: Comprises spinal muscles, tendons, and        thoracolumbar fascia.

• Neural control subsystem: The nervous system regulates muscle        activation based on proprioceptive feedback.

2- Functional Spinal Unit (FSU)

· Smallest motion segment; 70% load via vertebral body/discs, 30% via facets;

ensures motion and neural protection (Figure 1).

3- Three-column Model

· Spinal trauma and stability have been classified using the “Three-column model”  proposed by Denis in 1983 (Figure 2).

· This system divides the spine into three anatomical columns, with the anterior column comprising the anterior longitudinal ligament and the anterior two-thirds of the vertebral body.

· The middle column comprises of the posterior one-third of the vertebral body and posterior longitudinal ligament.

· The posterior column consists of the vertebral facets and posterior ligaments.

4- Posterior ligamentous complex (PLC)

· Includes facet capsules, interspinous ligaments, ligamentum flavum, and supraspinous ligaments (Figure 2).

· Major stabilizer (supraspinous, interspinous, flavum, facet capsule);

· Interspinous ligament; A Thin membrane between spinous processes; resists hyperflexion; easily torn in trauma.

· Supraspinous ligament; Continuation of ligamentum nuchae below C7, runs along the spinous processes, first to fail in flexion

· Ligamentum flavum: Connects adjacent laminae; posterior to thecal sac;

hypertrophy/infolding → canal stenosis.

· Facet joint capsules are major passive stabilizers; their failure increases motion and decreases stiffness

· Disruption = mechanical instability.

· Central in Denis’ 3-column model (posterior column) and TLICS PLC integrity decisive for surgery vs. conservative.

5- Spinal ligaments;

· C2–sacrum, Predominantly collagenous; ligamentum flavum is elastin-rich, supporting spinal stability (Figure 3)

· ALL: Extends occiput–sacrum, broader over bodies, narrower at discs; deep fibers span 1 level, superficial 3–5; resists hyperextension.

· PLL: Runs posterior to vertebral bodies; widest at discs, narrow over bodies; thins laterally → cervical disc herniation risk

· Intertransverse ligaments: Span between transverse processes; usually preserved in posterolateral fusion.

· Ligamentum nuchae: Continuation of supraspinous (C7–occiput);

Dorsal raphe (muscle attachment), ventral septum (C2–

C6 to interspinous + AA/AO), some fibers to dura →

tension in flexion (Figure 4).

(AA: Atlanto Axial, AO: Atlanto Oksipital)

· Atlantoaxial (AA) Joint Ligament stability: Transverse ligament = principal stabilizer; alar ligaments + tectorial membrane = secondary, but become primary if transverse ruptures (Figure 5).

· Regional variation: Lumbar ligaments are strongest → stability; upper cervical is weakest → flexibility (Figure 6).

6- Spinal motion unit:

· Each vertebral junction has 3 joints — intervertebral disc–endplate complex + bilateral facet (zygapophyseal) joints.

· Facet joint orientation varies regionally, directing motion and limiting instability.

· Cervical (45° transverse–frontal): Allows flexion–extension, lateral bending, rotation (high mobility, less stability)

· Thoracic (60° transverse, 20° frontal): Allows lateral flexion + rotation; limits flexion–extension

· Lumbar (sagittal): Promotes flexion–extension; resists rotation + shear

Clinical instability

· Dysfunction in one of the three subsystems (passive, active, neural) may be compensated by the others; however, when compensatory capacity is exceeded, mechanical instability ensues, leading to clinical symptoms.

Clinical manifestations of spinal instability

• Pain: Predominantly mechanical back or neck pain,      exacerbated by motion and alleviated by rest, often localized to the      unstable segment.

• Mechanical symptoms: Sensations of “catching,”      “locking,” or “giving way” during movement, reflecting abnormal      intersegmental motion.

• Neurological signs: Radiculopathy or, in advanced      cases, myelopathy due to dynamic neural compression.

• Postural abnormalities: Progressive deformity,      altered sagittal or coronal alignment, and impaired global spinal balance.

• Muscle-related findings: Paraspinal      overactivation → fatigue, spasms, reflexive guarding as compensatory      stabilization.

• Functional limitations: Reduced endurance for      standing, sitting, or walking, with activity-related symptom aggravation.

· Abnormal motion signs: Instability catch (sudden block/release), painful arc (transient pain range), crepitus (click/grating), shake phenomenon (uncontrolled trembling in flex–ext).

Surgical Importance of Stability

• Preservation or restoration of spinal stability is a primary      surgical goal.

• Adequate stabilization is critical to:

o Protect neural elements (prevent compression or further injury)

o Prevent deformity progression (kyphosis, listhesis, collapse)

o Maintain functional mobility and quality of life

• Failure to achieve stability may lead to neurological deficits,      pain, and structural collapse.

Surgical Options for Stabilization

• Instrumentation and Fusion

  • Pedicle screw–rod constructs (open or percutaneous)

  • Anterior or posterior interbody fusion (ALIF, PLIF, TLIF,       LLIF)

  • Posterolateral fusion techniques

• Cement Augmentation

  • Vertebroplasty, kyphoplasty for osteoporotic or tumoral       instability

• Decompression with Stabilization

  • Laminectomy or corpectomy combined with fixation to avoid       iatrogenic instability

• Corrective Procedures

  • Osteotomies (Smith-Petersen, pedicle subtraction, vertebral       column resection) for deformity correction while restoring alignment and       stability

References

1- Ramachandran M, editor. Basic orthopaedic sciences. CRC Press; 2018 Sep 3. (Figure 1)

2-https://www.anatomystandard.com/  (Figure 4,5)

3- https://www.coloradospineinstitute.com/ (Figure 3)

4- White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia: JB Lippincott; 1990(Figure 6)

5-Benzel EC. Spine Surgery: Techniques, Complication Avoidance, and Management. 5th ed. Elsevier; 2021.

6-Rothman RH, Simeone FA. The Spine. 7th ed. Philadelphia: Elsevier; 2018.