Anatomy and Mobility of the Spine

Figure 9.1

All joints are trade-off between mobility and stability

Factors that influence mobility and stability of a joint

• Articulating surface and surrounding bones– Determines types of movements possible– Bone-hitting-bone can limit movements

• Joint capsule and ligaments– Can restrict or permit movements

• Muscle that crosses joint or is near joint– Can stabilize joints– Can limit movement

• E.g., hip and knee flex more passively than actively

Spine:

A series of 24 individual vertebrae + sacrum (5 fused vertebrae) + coccyx (~4 fused vertebrae).

Functionally:

• bears and transmits upper body weight onto the pelvis while permitting movement.

– Accomplished by being made of many individual bones stacked on one another

– Small movement between each vert considerable mobility with decent stability

• protects the spinal cord

Regions• Cervical — secondary curve—created and

maintained by muscle use associated with upright posture

• Thoracic—primary curve—present at birth and maintained by associated bones

• Lumbar — secondary curve—created and maintained by muscle use associated with upright posture

• Sacrococcygeal—primary curve—present at birth and maintained by associated bones

• Presence of Curves places body weight in the correct place for upright posture.

• Primary curvatures are less mobile, but more stable and less prone to injury.

Vertebral Anatomy

• Body is the weight bearing portion• Processes are sites of muscle and ligament attachment• the pedicle and lamina help create the vertebral foramen through

which passes the spinal cord (also protects the cord)• Articulating processes help define movements that are possible

between adjacent vertebrae

Intervertebral Joints

There are two joints between individual vertebrae1. Joints between bodies: symphysis—intervertebral discs

– disc both absorbs shock and acts as a flexible spacer that allows intervertebral movement.

2. Joint between superior and inferior articulating processes (facets): synovial joint

– direction/orientation and shape of the articulating processes defines/determines what kind of movements are possible between vertebra—especially the degree of rotation possible

nucleus pulposus

annulus fibrosus

intervertebral disk--symphisis

synovial joint –plane-- between articulating processes

Joints of the spine

Spinal nerves pass through this opening

• Refer to lecture notes for details on the structure and function of intervertebral discs

Herniation of disc (rupturing and protrusion of nucleus pulposus) can pinch nerves between the expanded disc and nearby bone structures which creates pressure on the nerve and thus pain and dysfunction.

Joint Mobility

Mobility (types and range of motion) at a joint can be influenced by:

1. shape of articulating bone surfaces and boney structures interfering with joint movement (bone-bone interference)

2. ligaments becoming tight and braking/stopping further movement in a particular direction

3. the compression of soft tissue (e.g., muscle, adipose) preventing further joint movement.

Ligaments of the spine

anterior ligaments

Posterior ligaments

lateral ligaments

Ligaments of the Spine:Simplified Scheme

Bones are held in place and supported in much the same way that a pole is supported/held in place by guy-lines/ropes anchoring them into the ground.

Bone—which structurally bears/transmits compressive force is supported (i.e., held in place) by fibrous tissue and muscle—tensile structures (the tensegrity model)

SPINAL EXTENSION

In extension/hyperextension:• spinous processes move toward one another and can limit

movement on the side being moved to • Anterior ligaments become tight and limit/brake movement• Note: nucleus pulposus is being pushed anteriorly

SPINAL FLEXION

In Flexion:• There are no boney structures of the vertebrae to limit

flexion• Posterior ligaments become tight and act as a brake to

flexion• Note: the disc is stressed toward the posterior.

LATERAL FLEXION (I.E., SIDE BENDING)

Lateral flexion• The transverse processes on the side being flexed

toward (ipsilateral) move toward one another and will limit movement

• the lateral ligament on the opposite side (contralateral) will become tight to limit movement

• Note: disc being stressed toward the side being bent away from (contralateral).

ROTATION

Largely influenced by orientation of articulating processes and presence/absence of ribs