AAOS Comprehensive Orthopaedic Review

Section 7 - Spine

Chapter 62. Anatomy of the Spine

I. Nervous System/Spinal Cord Anatomy

A. Nervous system

 

1. The central nervous system includes the brain and spinal cord.

 

2. The peripheral nervous system includes the cranial and spinal nerves.

 

B. Early embryologic development (

Figure 1)

 

1. During the third week, the embryo assumes a planar structure before true development of the central nervous system.

 

2. The primitive streak deepens to form the primitive groove. This midsagittal groove deepens within the ectoderm and begins to fold onto itself, forming a neural tube.

 

3. As it closes, the neural crest forms dorsal to the neural tube, whereas the notochord remains ventral.

 

a. The neural crest forms the peripheral nervous system (also pia mater, spinal ganglia, and sympathetic trunk).

 

b. The neural tube forms the spinal cord, and the notochord forms the anterior vertebral bodies and intervertebral disks.

 

4. Failure of the ends of the neural tube to close

 

a. Cranially—May cause anencephaly

 

b. Caudally—May cause spina bifida occulta, meningocele, myelomeningocele, or myeloschisis

 

c. Diastomatomyelia is believed to be caused by persistence of the neuroenteric canal, which is present during the third and fourth weeks of gestation.

 

5. The spinal cord changes position with growth. At birth, the conus lies at the level of L3 and moves to L1-2 by skeletal maturity.

 

C. Spine development

 

1. Vertebrae develop from the somites, which surround the notochord and neural tube. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 or 5 coccygeal somites, similar to the complement of adult vertebrae.

 

2. All vertebrae have three primary ossification centers—The centrum (anterior vertebral body), neural arch (posterior elements, pedicles, and a small portion of the anterior vertebra), and a costal element (anterior part of lateral mass, transverse process, or rib).

 

3. The nucleus pulposus develops from cells of the notochord, the anulus from sclerotomal cells associated with resegmentation.

 

4. Failure of formation of these structures may lead to the development of hemivertebrae, whereas failure of the somites to segment may result in unsegmented bars or block vertebrae.

 

a. The most aggressive congenital scoliosis is associated with a hemivertebra on one side and an unsegmented bar on the other.

 

b. Another defect of segmentation is Klippel-Feil syndrome (congenital brevicollis).

 

D. Spinal cord structure (

Figure 2)

 

1. During neural tube closure, the dorsal cells (alar laminae) become primarily sensory (afferent), and the ventral cells (basal laminae) become primarily motor (efferent) in function.

 

a. The dorsal columns transfer vibration, deep touch sensation, and proprioception.

 

b. Anterolaterally, the lateral spinothalamic tract transmits pain and temperature sensation.

 

c. The ventral spinothalamic tract transmits light touch sensation.

 

d. Voluntary motor function is transmitted by the lateral corticospinal tracts, with the tracts for the lower extremities, torso, and upper extremities running superficial to deep.

 

2. There are 31 paired spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

 

E. Autonomic nervous system

 

1. Sympathetic—There are 22 ganglia—3 cervical, 11 thoracic, 4 lumbar, and 4 sacral.

 

[Figure 1. The midsagittal groove deepens within the ectoderm and begins to fold onto itself, creating the neural tube.]

a. In the cervical spine, the ganglia lie posterior to the carotid sheath on the longus capitus muscle and transverse processes.

 

b. There are stellate, middle, and superior ganglions. The middle ganglion is the most surgically at risk at C6, where it is close to the medial border of the longus coli.

 

c. Damage to the sympathetic nervous system may cause Horner syndrome.

 

2. Parasympathetic

 

a. In the lumbar spine anterior to the lower lumbar vertebra, the parasympathetic fibers from S2, S3, and S4 levels for the pelvic splanchnic nerves combine with the sympathetic lumbar splanchnic nerves to form the hypogastric plexus.

 

b. Tissue dissection anterior to the lumbar spine should proceed left to right and minimize use of electrocautery to decrease risk of injury to the hypogastric plexus and subsequent retrograde ejaculation.



II. Structure of the Spine

A. Osseous components of the spinal column: 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae, 4 or 5 coccygeal vertebrae (

Figure 3)

 

B. Spinal ligaments (

Figure 4)

 

1. Three sets of ligaments are present throughout the spine, all of which contribute to the static stability of the spine.

 

a. Anterior longitudinal ligament (ALL). The ALL is thicker centrally.

 

b. Posterior longitudinal ligament (PLL). The PLL is thicker over the body and thinner over the disks.

 

c. Facet joint capsule/ligaments

 

2. The ALL is generally thicker than the PLL.

 

3. The facet joint capsule is thought to be innervated by the nerve to the facet capsule, the ALL, PLL, and disk by the sinuvertebral nerve. All of these are thought to contribute to discogenic/degenerative pain syndromes.

 

4. A particular arrangement of three ligaments forms the transverse apical alar ligament complex at the C1-2 articulation (

Figure 5).

 

a. The transverse atlantal ligament runs horizontally behind the dens and is the major stabilizer of the C1-C2 segment.

 

[Figure 2. Drawings of the cross-sectional anatomy of the cervical spinal cord. S = sacral, L = lumbar, T = thoracic, C = cervical, 1 = fasciculus gracilis, 2 = fasciculus cuneatus, 3 = dorsal spinocerebellar tract, 4 = ventral spinocerebellar tract, 5 = lateral spinothalamic tract, 6 = spino-olivary tract, 7 = ventral corticospinal tract, 8 = tectospinal tract, 9 = vestibulospinal tract, 10 = olivospinal tract, 11 = propriospinal tract, 12 = lateral corticospinal tract.]

[Figure 3. Sagittal and coronal views of the spine.]

   

 

[Figure 4. The anatomic structures comprising the three longitudinal columns of stability in the thoracolumbar spine: the anterior column (anterior two thirds of the vertebral body, anterior part of the anulus fibrosus, and anterior longitudinal ligament), middle column (posterior third of the vertebral body, posterior part of the anulus fibrosus, and posterior longitudinal ligament), and posterior column (facet joint capsules, ligamentum flavum, bony neural arch, supraspinous ligament, interspinous ligament, and articular processes).]

b. It is crossed (hence cruciate) anteriorly by the vertically oriented apical ligament.

 

c. Finally, a paired set of obliquely oriented alar ligaments adds support to the articulation.

 

C. Alignment

 

1. Normal thoracic kyphosis averages 35° (range, 20° to 50°).

 

2. Normal lumbar lordosis averages 60° (range, 20° to 80°).

 

a. Up to 75% of the lumbar lordosis occurs at L4 through S1, and up to 47% may occur at L5-S1.

 

b. Men with low back pain tend to have decreased lumbar lordosis.

 

D. Cervical vertebrae

 

1. Spinous processes are bifid (except C7 and sometimes C6).

 

2. The C1 (atlas) has no body or spinous process, instead having a posterior tubercle and two lateral masses.

 

3. The C2 vertebra has the odontoid process (dens), which articulates with the anterior ring of C1.

 

E. Thoracic vertebrae

 

1. The T1 vertebra has a long prominent spinous process (vertebra prominens).

 

2. Angled spinous processes overlap significantly.

 

3. Costal facets (articulations of the ribs with the vertebral segments) are present on all vertebral bodies and transverse processes (TPs) of T1 through T9.

 

F. Lumbar vertebrae

 

1. The lumbar vertebrae have large bodies with rectangular, nonoverlapping spinous processes.

 

2. These vertebrae have very large pedicles with similar transpedicular screw starting points. L1 and L2 typically have smaller pedicles than T11 and T12.

 

G. Sacrum

 

1. The sacrum is a single structure formed of the five fused embryologic sacral vertebrae. It has four sacral foramina, through which run the S1 through S4 nerve roots.

 

2. The S5 roots run inferiorly through the sacral hiatus.

 

3. The broad superior sacral ala is similar to the TP of the more superior spine and is a major target for lumbosacral fusion graft material.

 

H. Intervertebral disk complex

 

1. The intervertebral disks and vertebral bodies support more than 80% of the axial load transmitted through the spine.

 

2. The disks are central to the functional spinal unit (FSU), which comprises the vertebrae above and below the disk and the associated paired facet joints at that level.

 

3. The disk consists of a fibrous outer anulus fibrosus with obliquely oriented collagen I molecules, and a softer inner core called the nucleus pulposus, which cushions force with predominantly type II collagen molecules.

 

I. Facet joints

 

1. Also called the zygapophyseal joints, the facet joints are the articulating components of the FSU.

 

2. The orientation of these joints varies throughout the spine in accordance with the predominant direction of allowed motion at that level. While there is a smooth transition throughout the spine, the motion allowed in each spine region is roughly as follows:

 

a. Cervical: 0° coronal, 45° sagittal

 

b. Thoracic: 20° coronal, 55° sagittal

 

c. Lumbar: 50° coronal, 90° sagittal

 

3. The superior tip of the inferior articulating process is a major offending structure in lumbar foraminal stenosis.



III. Surgical Approaches

A. Anterior cervical approach

 

1. This approach uses a plane between the strap muscles, trachea, and esophagus medially and the sternocleidomastoid and carotid sheath laterally.

 

2. After sharp or blunt dissection first through the pretracheal fascia and then the prevertebral fascia, the midpoint of the anterior spine is identified between the paired longus coli muscles that must be dissected medial to lateral.

 

[Figure 5. Drawing of the ligamentous stabilizers of the atlantoaxial segment showing the relationship among the transverse (TR), alar (AL), and atlantodens (AD) ligaments.]

3. The recurrent and superior laryngeal nerves are particularly at risk for injury during this approach. Injury may lead to temporary or permanent vocal cord paralysis.

 

a. The recurrent laryngeal nerve is a branch of the vagus nerve (cranial nerve X [CNX]).

 

b. On the left, it turns beneath the aortic arch and ascends in the tracheoesophageal groove.

 

c. On the right, it variably crosses under the subclavian artery and is usually in the surgical field, often (50% of the time) crossing to the tracheoesophageal groove at C5-6.

 

d. Injury to the superior laryngeal nerve may damage the mucosal sensory reflex, which prevents aspiration.

 

4. Cutaneous landmarks for the anterior approach

 

a. The cricoid membrane at C5-6

 

b. The prominence of the thyroid cartilage overlying C4-5

 

c. The hyoid bone at the C3 level

 

d. The angle of the mandible at the C2 level

 

B. Posterior cervical approach

 

1. A longitudinal incision is made through the skin and down to the ligamentum nuchae/supraspinous ligament.

 

2. Following the bone of the spinous process, one can dissect down to the cervical lamina and out onto the lateral masses as needed.

 

3. Dissection anterolateral to the posterolateral margin of the lateral masses places the vertebral artery at risk, especially proximal to the C2 lateral mass or in the case of aberrant vascular anatomy.

 

C. Anterolateral thoracolumbar approach

 

1. The rib two higher than the vertebral body to be approached is selected. The skin is incised along with the posterior musculature, being careful to tag the matching fascia, and the dissection is carried down to the rib itself.

 

2. A subperiosteal dissection is performed around the rib, carefully avoiding the neurovascular bundle on the inferior surface of each rib. The rib is then resected and can be used for bone graft.

 

3. Incision through the anterior periosteum and parietal pleura allows access to the thoracic cavity.

 

4. A right-sided approach is preferred, allowing for more complete retraction of the lung (heart on left) and avoiding the aorta, thoracic duct, and the large segmental artery of Adamkiewicz (around T8), which can cause cord infarction.

 

5. The azygous system is dissected anteriorly with the vena cava by subpleural dissection on the lateral vertebral bodies.

 

6. The segmental arteries must be identified at the waist of the midbody between the intervertebral disks. These can be ligated after identification.

 

7. Resection of the rib head allows visualization of the foramina, pedicle, and demarcation of the depth of the posterior vertebral body.

 

8. More inferiorly, the diaphragm may be taken down by incising through its tendon 1 to 2 cm from the lateral insertion.

 

a. This must be tagged and carefully repaired.

 

b. More medial incision risks damage to the phrenic nerve and muscle of the diaphragm.

 

9. The peritoneum is swept anteriorly and the psoas muscle is retracted posterolaterally.

 

a. The lumbar plexus runs through the posterior two thirds and around the psoas muscle with the genitofemoral nerve directly anterior.

 

b. These structures are at risk, as are the ureter and, more distally, the iliac vessels.

 

10. If a direct anterior exposure to the L4-5 or L5-S1 levels is performed, the median sacral artery anterior to the sacral promontory (L5-S1) or the iliolumbar vein (L4-5) may need to be identified and carefully ligated.

 

D. Posterior thoracic/lumbar approach

 

1. The patient is positioned prone and carefully padded. Avoiding external pressure on the abdominal contents will minimize intraoperative bleeding.

 

2. The most common palpable landmark is the superior margin of the iliac crest, which is usually at the L4-5 level.

 

3. Dissection is carried out down the spinous processes, an internervous plane, to remove the insertions of the paraspinal muscles.

 

4. The TP of the lower vertebra is at the level of each facet joint (eg, the L4 TP is at the level of the L3-4 facet joint).



IV. Instrumentation

A. Halo

 

1. The outer surface of the skull hardens quickly after birth, but the inner tables harden only after remodeling and growth. Thus children younger than 10 years are at risk for inner table penetration when halo pins are placed.

 

2. Normal pins are tightened to 8 inch-pounds.

 

3. Multiple pins (10 to 12) with lower torque may be used in children.

 

4. Anterior pins should be placed in the safe zone—1 cm above the supraorbital ridge, below the equator, and over the lateral two thirds of the orbit.

 

5. Halo placement or Gardner-Wells tongs should include pins located laterally or posterolaterally above the pinna of the ear and below the equator of the skull.

 

B. Osseous anatomy for anterior cervical fixation

 

1. From C2 to C6, vertebral depth averages 15 mm to 17 mm, increasing distally. The usual length of anterior cervical discectomy fusion (ACDF) plate screws is 14 mm, although clinical correlation is necessary for each individual patient.

 

2. The sagittal diameter of the canal is 17 to 18 mm at C3 to C6 and decreases to 15 mm at C7. Correspondingly, the distance between the vertebral arteries decreases cephalad to caudad.

 

3. In 95% of individuals the vertebral arteries enter the transverse foramen at C6; in the other 5%, they enter at C7.

 

4. The safest approach to the anterior vertebral body is at the level of the superior end plate/uncinate process, because the anterior transverse

 

[

Figure 6. Suggested starting points for thoracic (A) and lumbar (B) pedicle screws based on the center of the pedicle axis.]

   process protects the vertebral artery. There is no uncinate process at C7-T1.

 

5. Plates should be as short as possible for secure fixation to avoid damage to the adjacent FSU and disk space.

 

C. Osseous anatomy for posterior cervical fixation

 

1. The vertebral artery is at risk with dissection of C1 laterally beyond 12 mm (cephalad).

 

2. The internal carotid artery lies anteriorly within 1 mm of the ideal exit point of a bicortical C1 lateral mass screw or C1-2 transarticular screw. Despite the popularity of this fact as an anatomic test question, few complications have been reported.

 

3. The C1 lateral mass is 9 to 15 mm wide, 17 to 19 mm deep, and 10 to 15 mm high.

 

4. At C2, a unilateral anomalous vertebral artery anatomy resulting in narrowing of the C2 pedicle and lateral mass is present in 18% of individuals.

 

a. Careful examination of preoperative CT is necessary to determine whether C2 pedicle or transarticular C1-2 fixation is feasible.

 

b. The width of the inferior surface of the isthmus is the most important measurement for avoidance of the vertebral artery.

 

5. From C3 to C6, ideal lateral mass screw placement is 30° lateral and 15° cephalad (Magerl technique), starting just inside the inferior medial quadrant of the lateral mass. The usual screw length is 12 to 14 mm, although probing for depth is advised.

 

6. Pedicle screws are often used at C7 and T1 because the lateral mass of C7 can be quite small and the pedicles of T1 can be quite large, with medial angulations of 34° and 30°, respectively. The exiting nerve is closer to the superior surface of the pedicle throughout the cervical spine.

 

D. Osseous anatomy for occipital fixation

 

1. In males, the thickest bone in the external occipital protuberance averages 13.3 mm; in women, it averages 10.9 mm.

 

2. Screw placement is recommended within 2 cm of the midline (bone thickness decreases laterally) and 2 cm inferior to the confluence of sinuses, which corresponds with the superior nuchal line.

 

E. Osseous anatomy for thoracic fixation

 

1. The pedicle wall is two to three times thicker medially than laterally.

 

2. On average, T5 has the narrowest pedicle diameter; in scoliotic spines, T7 on the concave side of the curve can be abnormally narrow.

 

3. Screw lengths gradually decrease from 20 mm at T1 to 14 mm at T4 and gradually increase again to 20 mm at T10.

 

a. The nerve root is roughly equidistant between the pedicles.

 

b. The mean distance between the medial pedicle wall and the dura is 1.5 mm.

 

F. Osseous anatomy for lumbar and sacral fixation

 

1. The midpoint of the transverse process is usually at the superior-inferior midpoint of the pedicle (Figure 6).

 

2. From L2 to L4, the medial border of the pedicle is in line with the lateral border of the pars.

 

3. At L5, the pars is closer to the center of the pedicle.

 

4. Pedicle angulations increase from 12° at L1 to 30° at L5.

 

5. The nerve root is at greatest risk of injury inferiorly and medial to the pedicle.

 

6. The S1 pedicle is quite broad (19 mm) and angles 39°.

 

a. Bicortical transpedicular fixation is common at S1.

 

b. Sacralization of lumbar vertebrae is more common with cervical ribs.



Top Testing Facts

1. All vertebrae have three primary ossification centers: the centrum (anterior vertebral body), the neural arch (posterior elements, pedicles, and a small portion of the anterior vertebra), and a costal element (anterior part of lateral mass, transverse process, or rib).

 

2. The most aggressive congenital scoliosis combines failures of segmentation and failures of formation and is associated with a hemivertebra on one side and an unsegmented bar on the other.

 

3. A particular arrangement of three ligaments forms the transverse apical alar ligament complex at the C1-2 articulation. The transverse atlantal ligament runs horizontally behind the dens and is the major stabilizer. It is crossed (hence cruciate) anteriorly by the vertically oriented apical ligament. Finally, a paired set of obliquely oriented alar ligaments adds additional support to the articulation.

 

4. The disk consists of a fibrous outer anulus fibrosus with obliquely oriented collagen I molecules, and a softer, crabmeat-consistency nucleus pulposus, which cushions force with predominantly type II collagen molecules.

 

5. The superior tip of the inferior articulating process is a major offending structure in lumbar foraminal stenosis.

 

6. The recurrent and superior laryngeal nerves are particularly at risk for injury during the anterior cervical approach. Injury may lead to temporary or permanent vocal cord paralysis.

 

7. In the thoracic spine on the left, usually around T8, is the large segmental artery of Adamkiewicz, which can cause cord infarction if injured.

 

8. The lumbar plexus runs through the posterior two thirds and around the psoas muscle laterally, with the genitofemoral nerve directly anterior to the muscle.

 

9. The lumbar pedicle screw starting point is at the intersection of two lines, one drawn medial to lateral through the midpoint of the TP, the other superior to inferior on the lateral edge of the pars interarticularis.

 

10. The internal carotid artery lies anteriorly within 1 mm of the ideal exit point of a bicortical C1 lateral mass screw or C1-2 transarticular screw.



Bibliography

Chin CR: Anatomy, in Spivak JM, Connolly, PJ (eds): Orthopaedic Knowledge Update: Spine 3. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2006, pp 15-24.

Fischgrund JS (ed): Section 6 (Spine), in Vaccaro AR (ed): Orthopaedic Knowledge Update 8. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2005, pp 509-616.

Frymoyer JW, Wiesel SW: The Adult and Pediatric Spine, ed 3. Philadelphia, PA, Lippincott Williams & Wilkins, 2004.

Hoppenfeld S, deBoer P: Surgical Exposures in Orthopaedics: The Anatomic Approach. Philadelphia, PA, J.B. Lippincott Co, 1984.

Netter FH: Atlas of Human Anatomy, ed 2. Summit, NJ, Novartis Pharmaceuticals Corp, 1997.

Rinella A: Human embryology emphasizing spinal and neural development, in Spivak JM, Connolly PJ (eds): Orthopaedic Knowledge Update: Spine 3. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2006, pp 3-13.