Anatomy & Biomechanics

Overview

The human spine is a complex, segmented column providing both mobility and stability for the body. It consists of 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal segments. These vertebrae are interconnected through discs, ligaments, and muscles, forming a biomechanically dynamic structure that supports axial load, enables movement, and protects the spinal cord.

Each vertebra comprises a vertebral body and a posterior arch. The body, primarily cancellous bone, functions as the main weight-bearing element. The posterior arch, composed mainly of cortical bone, includes pedicles, laminae, and spinous and transverse processes, which provide attachment points for ligaments and muscles. Between adjacent vertebral bodies lie the intervertebral discs, acting as flexible cushions that absorb compressive forces while allowing controlled motion.

Cervical Spine

The cervical spine includes seven vertebrae (C1–C7), forming a lordotic curve in the sagittal plane.

C1 (Atlas): ring-shaped, lacks a body, and transmits cranial load through its lateral masses. The atlanto-occipital joint allows flexion–extension (“nodding”) but limited rotation.
C2 (Axis): characterized by the odontoid process (dens), which forms a pivot for C1 rotation, allowing approximately 50% of total cervical rotation.
C7: has a prominent, non-bifid spinous process; vertebral arteries typically do not pass through its transverse foramen.

Cervical motion includes flexion, extension, lateral bending, and rotation, made possible by the facet orientation and ligamentous configuration.

Major cervical ligaments include:

Anterior longitudinal ligament (ALL): runs along anterior vertebral bodies, limits hyperextension.
Posterior longitudinal ligament (PLL): along posterior vertebral bodies, limits hyperflexion.
Ligamentum flavum (LF): connects adjacent laminae; elastic fibers assist in returning to extension.
Interspinous and supraspinous ligaments: resist flexion.
Nuchal ligament: a continuation of the supraspinous ligament from C1–C7, providing attachment for deep cervical muscles.
Transverse ligament of the atlas: stabilizes the dens, maintaining atlantoaxial integrity.

Thoracic Spine

Composed of 12 vertebrae (T1–T12), the thoracic spine is kyphotic and articulates with the ribs via costovertebral and costotransverse joints.

Typical vertebrae (T2–T8) are heart-shaped and smaller than lumbar bodies.
Facet joints are oriented in the coronal plane, permitting limited rotation but restricting flexion–extension.
The rib cage enhances structural rigidity, reducing mobility but protecting thoracic organs.
Normal thoracic kyphosis ranges between 20° and 50° (average ~35°).

Lumbar Spine

The five lumbar vertebrae (L1–L5) form a lordotic curve, designed to bear increasing axial loads toward the sacrum.

Vertebral bodies enlarge caudally to support greater weight.
Facet joints are oriented in the sagittal plane, allowing flexion–extension while limiting rotation.
The vertebral foramen is triangular, smaller than cervical but larger than thoracic.

Intervertebral discs constitute about 20% of total spinal length, consisting of the annulus fibrosus, nucleus pulposus, and cartilaginous endplates.
Discs are largely avascular in adults, adapted for compression but less resistant to torsion.

Under normal conditions:

The vertebral body carries ~80% of axial load.
Facet joints bear the remaining ~20%.
In degenerative disc disease, disc height loss shifts up to 70% of load to the facets, increasing posterior stress.

Flexion contribution:

75% at L5–S1,
15–20% at L4–L5,
5–10% at L1–L4.

Ligaments of the Lumbar Spine

Lumbar stability depends on both static (ligamentous) and dynamic (muscular) structures.

Anterior longitudinal ligament (ALL): limits hyperextension; major stabilizer of the anterior column.
Posterior longitudinal ligament (PLL): resists hyperflexion and confines disc herniation, especially posterolaterally.
Ligamentum flavum (LF): elastic, connects laminae; thickens with age, contributing to spinal stenosis.
Interspinous ligament (ISL): between spinous processes; tenses during flexion.
Supraspinous ligament (SSL): connects the apices of spinous processes from C7 to sacrum; resists excessive flexion.

Muscles of the Lumbar Spine

Lumbar musculature plays a vital role in postural control, motion, and load transfer.

Erector spinae group (spinalis, longissimus, iliocostalis): composed mainly of slow-twitch type I fibers; maintain posture and control trunk extension.
Multifidus muscle: short, segmental fibers with a large physiological cross-sectional area, providing segmental stability.
Rich connective tissue increases endurance and contributes to passive stiffness.
Dysfunction or atrophy is strongly linked to chronic low back pain.

Sacrum and Coccyx

The sacrum consists of five fused vertebrae forming a wedge-shaped bone that articulates laterally with the iliac bones via the sacroiliac joints.

The sacral canal continues from the lumbar spinal canal, terminating at the sacral hiatus, where the coccyx begins.
There are four pairs of anterior and posterior foramina, through which the sacral spinal nerves exit.
The coccyx, formed by fusion of four small vertebrae, represents the vestigial tailbone and provides attachment for pelvic floor muscles.

The sacrum serves as the keystone of the pelvis, transferring spinal load to the lower extremities and contributing to the stability of the lumbopelvic complex.

References:

Gray H, Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier, 2020.
White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Lippincott Williams & Wilkins, 1990.
Bogduk N. Clinical Anatomy of the Lumbar Spine and Sacrum. 5th ed. Elsevier, 2012.
Adams MA, Dolan P. Spine biomechanics. J Biomech. 2005;38(10):1972–1983.
Pal GP, Routal RV. Anatomy and biomechanics of the human vertebral column. Clin Anat. 1999;12(5):324–339.