15.4 Back & Spinal Column: Vertebrae, Curvatures, Joints, Back Muscles, Spinal Cord, Meninges & Cord Syndromes

Why this area matters

The TOS lists:

• the vertebral column (structure, function, curvature, joints, ossification, regional characteristics);
• the muscles of the back (superficial and deep back muscles, suboccipital and deep neck muscles);
• the spinal cord and meninges (structure of spinal nerves, spinal meninges and CSF, vasculature of the spinal cord).

Back pain, disc herniation, spinal tuberculosis (Pott disease, still common in the Philippines), trauma and lumbar puncture make this practical anatomy.

The vertebral column

• There are 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral and about 4 coccygeal.
• Typical vertebra: body, vertebral arch (pedicles and laminae), spinous and transverse processes, superior and inferior articular processes.
• Regional features:

• Curvatures:
  • Primary (kyphotic) curves, present at birth: thoracic and sacral.
  • Secondary (lordotic) curves: cervical, appearing with head holding at about 3 to 4 months, and lumbar, appearing with standing and walking at about 12 to 18 months.
  • Abnormal curves are excessive kyphosis (osteoporotic fractures, Pott disease, which causes an angular gibbus), excessive lordosis (pregnancy, obesity) and scoliosis (lateral curvature with rotation).
• Joints:
  • The intervertebral discs are secondary cartilaginous joints. The outer annulus fibrosus (fibrocartilage) surrounds the inner nucleus pulposus (notochord remnant).
  • The zygapophyseal (facet) joints are synovial.
  • Ligaments: the anterior longitudinal ligament (strong, limits extension), the posterior longitudinal ligament (narrow, weakest posterolaterally, which explains posterolateral herniation), the ligamentum flavum (elastic, pierced during lumbar puncture), and the interspinous and supraspinous ligaments.
  • The atlanto-occipital joint allows nodding ("yes"); the atlanto-axial joint allows rotation ("no"). The transverse ligament of the atlas holds the dens; its laxity in rheumatoid arthritis and Down syndrome risks cord compression.
• Ossification: each vertebra has three primary centers (one for the body, or centrum, and one for each half of the arch), and secondary centers appear at puberty. Failure of the arch halves to fuse causes spina bifida, ranging from occulta (a tuft of hair or dimple) to meningocele and myelomeningocele (a neural tube defect, lowered by periconceptional folic acid).

Disc herniation. Most herniations are posterolateral at L4-L5 or L5-S1. They compress the root that exits below that level:

Cauda equina syndrome is a surgical emergency: bilateral leg weakness, saddle anesthesia, urinary retention or incontinence, and reduced anal tone.

Muscles of the back

• Superficial (extrinsic) group, which moves the upper limb: trapezius (CN XI), latissimus dorsi (thoracodorsal nerve), rhomboids and levator scapulae (dorsal scapular nerve).
• Intermediate group (respiratory, minor): serratus posterior superior and inferior.
• Deep (intrinsic) group, innervated by the dorsal rami of spinal nerves:
  • splenius capitis and cervicis;
  • erector spinae (iliocostalis, longissimus and spinalis, from lateral to medial), the main extensors;
  • the transversospinalis group (semispinalis, multifidus, rotatores);
  • interspinales and intertransversarii.
• Suboccipital triangle: bounded by the rectus capitis posterior major, obliquus capitis superior and obliquus capitis inferior. It contains the vertebral artery and the suboccipital nerve (dorsal ramus of C1). The greater occipital nerve (C2) emerges below the obliquus capitis inferior.
• Deep neck (prevertebral) muscles: longus colli and longus capitis.

Spinal cord, spinal nerves and meninges

• The spinal cord runs from the foramen magnum and ends as the conus medullaris at about L1 to L2 in adults and L3 in newborns. The cauda equina consists of the lower nerve roots. The filum terminale anchors the cord to the coccyx.
• There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Each is formed by a dorsal (sensory, with the dorsal root ganglion) and a ventral (motor) root, and divides into a dorsal ramus (back muscles and skin) and a ventral ramus (limbs, trunk, plexuses). C1 to C7 nerves exit above their vertebra, and C8 and below exit below.
• Meninges and spaces:
  • The dura mater forms the dural sac to S2, and the epidural space lies outside it, containing fat and the internal vertebral (Batson) venous plexus. Batson's valveless plexus allows prostate and breast cancers to spread to the vertebrae.
  • The arachnoid encloses the subarachnoid space, which contains CSF; it enlarges below the conus into the lumbar cistern.
  • The pia mater forms the denticulate ligaments and the filum terminale.
• Lumbar puncture:
  • done at L3-L4 or L4-L5 with the patient flexed;
  • the needle passes skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum (a "pop"), epidural space, dura and arachnoid to reach the subarachnoid space;
  • it is contraindicated with raised intracranial pressure from a mass (herniation risk), local infection and coagulopathy.
• Blood supply of the cord:
  • one anterior spinal artery (from the vertebral arteries) supplies the anterior two-thirds;
  • two posterior spinal arteries supply the dorsal columns;
  • segmental (radicular) arteries reinforce them, the largest being the great radicular artery (of Adamkiewicz), usually on the left at T9 to T12. Injury during aortic surgery can cause anterior cord infarction and paraplegia.

Spinal Cord Syndromes

• Brown-Séquard Syndrome (Spinal Cord Hemisection): Ipsilateral spastic paralysis below the lesion (lateral corticospinal tract); ipsilateral loss of proprioception and vibration below the lesion (dorsal column); contralateral loss of pain and temperature sensation beginning 1 to 2 dermatomes below the lesion (lateral spinothalamic tract, which decussates in the anterior white commissure).
• Syringomyelia: Fluid-filled cavitation expanding within the central canal of the spinal cord (frequently associated with Chiari I malformation); initial disruption of crossing decussating fibers in the anterior white commissure produces a bilateral, "cape-like" loss of pain and temperature sensation across the shoulders and upper extremities with preserved light touch and proprioception (dissociated sensory loss).
• Tabes Dorsalis (Neurosyphilis): Slow degeneration of the dorsal columns and dorsal nerve roots; bilateral loss of vibration, conscious proprioception, and position sense; severe sensory ataxia with a positive Romberg sign; lancinating pains; associated with Argyll Robertson pupils (accommodate to near vision but do not react to light).
• Anterior Spinal Artery (ASA) Occlusion: Infarction of the anterior two-thirds of the spinal cord; bilateral flaccid paraplegia progressing to spastic paraplegia below the lesion (bilateral corticospinal tracts); bilateral loss of pain and temperature sensation below the lesion (bilateral spinothalamic tracts); dorsal columns are spared (intact vibration and proprioception, supplied by intact posterior spinal arteries).