PHED 386

Biomechanics of the Spine

Today’s objectives…

Analyze structure vs. function of the spine Identify factors influencing mobility & stability of

regions of the spine Identify the relationship between muscle location

& its effectiveness on the trunk List mechanical contributions to common injury

Structure of the spine

Vertebral column Curved stack of 33 vertebrae Movements: motion segments Joints of the movement segment

Symphysis: bodies & discs Gliding (diarthroses): facet joints

Structure of the Spine

Vertebrae Body, neural ring, processes Processes provide MA for mms Body size-function relationship Spines & facet joint relationship Facet joints assist in loading Facet joint compression

Structure of the spine

Intervertebral discs Composition

Annulus fibrosus: collagen bands cross vertically at 30° to each other

Nucleus pulposus: 90% water + collagen & proteoglycans

Function Bind vertebrae together Allow for movement Bear & distribute loads

Structure of the spine

Mechanics of discs Flexion, extension & lateral flexion forces Rotation forces

Structure of the spine

Compression forces Water loss & sodium /potassium gain

Results in loss of ~2cm of height Within first 30 minutes Increased volume stiffness Theory: risk of disc injury early in day

Disc nutrition

Blood supply to age 8 Pumping action from

position change Negative aspect of fixed

position Affect of injury & aging

Discs and aging

• Reduced water absorption• Decreased shock absorbing• Abnormal movements between vertebrae• Loads assumed by other structures

Reduced spinal column height Postural alterations

Ligaments of the spine

Anterior longitudinal ligament Posterior longitudinal ligament Supraspinous ligament (ligamentum nuchae) Short ligaments

Interspinous Intertransverse Ligamentum flavum**

Spinal curves

Primary Thoracic & sacral

Secondary Cervical & lumbar

Life changes

Postural & mechanical changes

?? Wolff’s law Lordosis

Associated w/weak abdominal mm’s & anterior tilt Causes

Kyphosis Cause (Scheuermann’s disease) Population/s

Scoliosis Lateral/rotational deformity Causes

Normal Hypolordotic Military Kyphotic S-Shaped

Neck

Lower Back

Movements of the spine

Flexion, extension, hyperextension Greatest in cervical & lumbar regions

Lateral flexion & rotation Greatest in cervical Least in lumbar

Muscles of the spine

Complex arrangement Naming

Pairs Unilaterally or bilaterally

Muscles of the spine

Anterior (cervical region) Pre-vertebral muscles

Rectus capitus anterior & lateralis Longus capitus Longus colli

Hyoid muscles – 8 pairs Bilateral tension: flexion of the head Unilateral: lateral flexion or rotation

Muscles of the spine

Anterior Abdominals

Rectus abdominus External obliques Internal obliques

Bilateral tension: flex spine, reduce anterior tilt Unilateral tension: lateral flexion, rotaion Abdominal wall function

Muscles of the spine

Posterior Cervical

Spenius capitus Splenius cervicis Assisted by smaller/deeper mms

Thoracic & lumbar Errector spinae

Sacrospinalis, semispinalis, Spinalis, longissimus, iliocostalis

Deep spinal mms

Primary extensors

Muscles of the spine

Lateral Sternocleidomastoid (antagonistic) Scalenes

Lumbar region Quadratus lumborum Psoas major

Loads on the spine

Forces acting on Body weight Tension in ligaments Muscles tension Intra-abdominal pressure External loads

Axial loading Spinal compression

Loads on the spine

Effect of center of gravity Role of spinal extensor mms

Effect of actual forward bending or arm position Spinal mms have very small moment arms

must generate large forces

Increased compression forces on lumbar spine

Loads on the spine

Shear forces on spine On lumbar spine in standing position Increases during flexion & hyperextension Contributor to disc herniation

Loads on the spine

Flexion relaxation phenomenon Tension in extensors increases w/flexion At full flexion – extensor tension disappears

Flexion torque supported by posterior spinal ligaments But increased shear from tension in interspinous ligaments Also increased facet loading

Loads on the spine

Lateral flexion and rotation

Role of movement speed

Common injuries

Low back pain 75-80% of population Second only to common cold Most frequent/expensive of workman’s comp claims More prevalent in males Causes:

Children – strains & sprains Adults –

Sitting for prolonged periods/unable to sit at all Unnatural postures Sudden unexpected motions Weak abdominal muscles

Common injuries

Soft tissue Contact or overload Spasm response

Acute fractures: forceful contraction of mms Contact: direct or indirect Large compressive loads

Common injuries

Stress fractures Spondylolysis Spondylolisthesis From repeated axial loading while hyperextended

Common injuries

Disc herniations Bulging, slipped, ruptured Protrusion of pulposus from

annulus Common locations cervical &

lumbar