Week 7: Pelvis, Thoracic + lumbar spine

Anatomical definition of the pelvis

A bony ring located between L5 and the femoral heads.

Three bones forming each innominate bone

Ilium, ischium, and pubis.

Key biomechanical and anatomical roles of the pelvis

Transfer forces between spine and lower limbs, protect pelvic organs, provide muscle attachment, assist shock absorption, and influence lumbar/hip alignment.

Normal sacral angle

$$30^\circ$$

Effect of increased sacral angle on lumbar lordosis

Lumbar lordosis tends to increase.

Effect of decreased sacral angle on lumbar lordosis

Lumbar lordosis tends to decrease.

Normal movement range within the pelvis/SIJ

Very little: about $2\text{--}6^\circ$ rotation and around $2\,\text{mm}$ translation.

Three main motions of the pelvis

Anterior/posterior tilt in the sagittal plane, lateral tilt in the coronal plane, and axial rotation in the transverse plane.

Anterior pelvic tilt definition

Movement where the front of the pelvis rotates down and forward, usually increasing lumbar lordosis.

Posterior pelvic tilt definition

Movement where the front of the pelvis rotates up and back, usually flattening lumbar lordosis.

Factors providing inherent stability to the sacroiliac joint (SIJ)

Wedge shape of the sacrum between innominates, irregular joint surfaces, and strong ligamentous support.

Structural joint classification of the sacroiliac joint (SIJ)

Diarthrodial synovial joint.

Cartilage type covering the sacral SIJ surface

Hyaline cartilage.

Cartilage type covering the iliac SIJ surface

Fibrocartilage.

Sacral nutation

Forward nodding of the sacrum relative to the ilium.

Sacral counternutation

Backward nodding of the sacrum relative to the ilium.

Comparison of SIJ stability: nutation vs. counternutation

Nutation provides greater joint stability.

Ligaments limiting sacral nutation

Interosseous and sacrotuberous ligaments.

Ligament limiting sacral counternutation

Long dorsal sacroiliac ligament.

Form closure of the SIJ

Passive joint stability provided by closely fitting joint surfaces, cartilage shape, ligaments, friction, and sacral wedge geometry.

Force closure of the SIJ

Dynamic stability produced by muscle and fascial forces compressing the joint.

Direct muscle attachment to the SIJ

No muscles attach directly to the SIJ, though surrounding muscles and fascia cross and compress it.

Posterior muscles contributing to SIJ force closure

Erector spinae, gluteus maximus, latissimus dorsi, and biceps femoris.

Primary biomechanical role of the thoracic spine (mobility vs. stability)

Stability.

Factors causing high stiffness in the thoracic spine

Attachment to the rib cage and relatively thin intervertebral discs.

Secondary functions of the thoracic spine

Protection of vital organs, respiratory function, and provision of muscle attachment sites.

Number of rib pairs

Twelve pairs.

True ribs definition and rib numbers

Ribs 1–7; they attach directly to the sternum via their own costal cartilages.

False ribs definition and rib numbers

Ribs 8–12; they do not attach directly to the sternum.

Floating ribs definition and rib numbers

Ribs 11–12; they have no anterior attachment to the sternum or costal cartilage.

Surface landmark corresponding to the second rib

Sternal angle.

Vertebral level corresponding to the spine of the scapula

T3.

Vertebral level corresponding to the inferior angle of the scapula

T7.

Approximate overall thoracic range of motion (ROM)

Flexion $45^\circ$, extension $25^\circ$, lateral flexion $20^\circ$ per side, rotation $35^\circ$ per side.

Structures limiting thoracic flexion

Posterior ligaments and muscles, joint capsules, disc size, and anterior vertebral body shape.

Structures limiting thoracic extension

Bony approximation, anterior longitudinal ligament (ALL), capsular tension, disc size, and abdominal muscle tension.

Structures limiting thoracic lateral flexion

Facet approximation on the concave side, tension on the convex side, and the rib cage.

Primary structures limiting thoracic rotation

Facet joints, intervertebral discs, ligaments, and particularly the rib cage.

Rib movement during thoracic flexion

Anterior rotation.

Right rib movement during right thoracic rotation

Posterior rotation.

Right rib movement during right thoracic lateral flexion

Anterior rotation.

Main functional challenge of the lumbar spine

Balancing mobility with sufficient stability to support upper-body load.

Functional adaptation of large lumbar vertebral bodies

To transfer and support high loads imposed by the upper body.

Facet joint orientation in the lumbar spine and supported motion

Vertical/sagittal orientation allows flexion and extension while restricting axial rotation.

Anatomical reason for restricted lumbar axial rotation

J-shaped, sagittally oriented facet surfaces restrict rotation.

Approximate overall lumbar range of motion (ROM)

Flexion $50\text{--}60^\circ$, extension $20\text{--}35^\circ$, lateral flexion $20^\circ$ per side, rotation $5^\circ$ per side.

Normal lumbosacral angle

Approximately $140^\circ$.

Normal pelvic tilt angle (sacral promontory–pubic symphysis method)

Approximately $60^\circ$.

Lumbar-pelvic rhythm

Coordinated movement between lumbar flexion/extension and pelvic rotation during trunk bending.

Behavior of lumbar lordosis during forward bending

Flattens initially, then reverses into flexion.

Segmental movements occurring during lumbar flexion

Anterior sagittal rotation and anterior translation.

Segmental movements occurring during lumbar extension

Posterior sagittal rotation and posterior translation.

Structures limiting lumbar flexion

Posterior ligaments, posterior disc tension, facet capsules, facet impaction, and erector spinae length.

Structures limiting lumbar extension

Bony impaction, anterior longitudinal ligament (ALL)/anterior disc tension, and anterior muscle tension.

Structures limiting lumbar lateral flexion

Facet joints, intertransverse and iliolumbar ligaments, disc fibres on the convex side, and lateral muscles.

Structures limiting lumbar axial rotation

Facet impaction, posterior ligament and disc-fibre tension, and surrounding muscles.

Lumbar coupling patterns

Variable overall: upper lumbar levels couple lateral flexion with contralateral rotation; L5/S1 couples lateral flexion with ipsilateral rotation.