Page 353 - Kinesiology of the musculoskeletal system foundations for physical rehabilitation

Chapter 9 Axial Skeleton: Osteology and Arthrology 329
SPECIAL FOCUS 9-4 Nucleus pulposus

Intra-articular Structures Located Annulus fibrosus Vertebral endplate
within Apophyseal Joints
FIGURE 9-33. The intervertebral disc is shown lifted away from the
S mall and inconsistently formed accessory structures (inclu- underlying vertebral endplate. (Modified from Kapandji IA: The
sions) are typically found around the margins of apophyseal physiology of joints, vol 3, New York, 1974, Churchill Livingstone.)
joints, most frequently described in the upper cervical and the
lumbar regions.23,129 In the lumbar spine, Bogduk describes two FIGURE 9-34. The detailed organization of the annulus fibrosus
primary types of accessory structures: subcapsular fat pads and shown with the nucleus pulposus removed. Collagen fibers are
fibro-adipose meniscoids.23 Subcapsular fat pads fill small crev- arranged in multiple concentric layers, with fibers in every other
ices formed between the capsule and the underlying synovial layer running in identical directions. The orientation of each col-
membrane, typically at the superior and inferior margins of the lagen fiber (depicted as θ) is about 65 degrees from the vertical.
joint. The subcapsular fat pads may extend outside the joint (Modified from Bogduk N: Clinical anatomy of the lumbar spine, ed 4,
through very small crevices in the capsule. When fully formed, New York, 2005, Churchill Livingstone.)
larger extracapsular fat pads within the lumbar region fill part of
the space between the lamina and the overlying multifidi muscles.

Fibro-adipose meniscoids are another set of connective
tissue found at the periphery of apophyseal joints. These struc-
tures range from thickenings or “pleats” of connective tissue
variously placed along the internal surface of the joint capsule,
to folds of synovium that encapsulate small fat pads, collagen
fibers, and blood vessels. The larger fibro-adipose meniscoids
can extend several millimeters into the apophyseal joint.23

The function of intra-articular inclusions within apophyseal
joints is controversial. Some authors have described them as
deformable spacers that help dissipate compression forces
within the joint.73,129 Others have speculated that the structures
are designed to partially cover the articular cartilage that
becomes exposed at the extremes of motion.23 This transient
coverage may protect and lubricate the exposed surfaces until
the joint is returned to its neutral position. Although opinions
vary, the intra-articular inclusions may have important clinical
relevance. The larger fibro-adipose meniscoids in cervical
regions may become impinged as the apophyseal joints force-
fully hyperextend, such as during a cervical whiplash injury.99
Meniscoids may proliferate after long-term immobilization and
restrict spinal movement. Because these tissues are innervated,
they may be a source of pain.74

proteoglycan is an aggregate of many water-binding glycos- 15% to 20% in the nucleus pulposus.23 Abundant elastin
aminoglycans linked to core proteins (see Chapter 2).6,66 Dis- protein is interspersed in parallel to the rings of collagen,
persed throughout the hydrated proteoglycan mixture are bestowing an element of circumferential elasticity to the
thin type II collagen fibers, elastin fibers, and other proteins. annulus fibrosus.225
The collagen forms an infrastructure that helps support the
proteoglycan network. Very small numbers of chondrocytes The outermost or peripheral layers of the annulus fibrosus
and fibrocytes are interspersed throughout the nucleus, ulti- consist primarily of type I and type II collagen.35 This arrange-
mately responsible for the synthesis and regulation of the ment provides circumferential strength and flexibility to the
proteins and proteoglycans. In the very young, the nucleus disc, as well as a means to bond the annulus to the anterior
pulposus contains a few chondrocytes that are remnants of and posterior longitudinal ligaments and to the adjacent rim
the primitive notochord.169,188 of the vertebral bodies and endplates. (The outer layers of the
annulus fibrosus contain the disc’s only sensory nerves; see
The annulus fibrosus in the lumbar discs consists primarily innervation of the disc, Chapter 10). The deeper, internal
of 15 to 25 concentric layers, or rings, of collagen fibers.23 layers of the annulus contain less type I collagen and more
Like dough surrounding jelly in a doughnut, the collagen water—gradually transforming into tissue with characteristics
rings encase and physically entrap the liquid-based central similar to those of the centrally located nucleus pulposus.228
nucleus. The annulus fibrosus contains material and cells
similar to what is found in the nucleus pulposus, differing Normally, compression forces acting on the disc increase
mainly in proportion. In the annulus, collagen makes up the hydrostatic pressure within the water-logged nucleus
about 50% to 60% of the dry weight, as compared with only pulposus. This rise in and containment of hydrostatic pres-
sure ultimately absorb and evenly distribute loads across the
entire intervertebral junction. Fully hydrated and pressurized
discs protect not only the interbody joints, but also, indi-