Biomechanics of Human Spine Introduction n n n

Biomechanics of Human Spine

Introduction n n n Protection of spinal cord Transference of load Motion in three planes Intrinsic stability= disc and ligaments Extrinsic stability= Muscles Significantly greater loads in lumbar spine

Spine

Cervical Spine n n n Seven vertebrae C 1 -7 More flexible Supports the head Wide range of motion n Rotation to left and right n Flexion extension n Up and down Peripheral nerves

Thoracic Spine n n n Mid-back or dorsal region Twelve vertebrae n T 1 -12 Ribs attached to vertebrae Relatively immobile Peripheral nerves n Intercostal

Lumbar Spine n n Lower back Five vertebrae L 1 -5 Carries the weight of the upper body n n Larger, broader Peripheral nerves

Sacral and Coccygeal region n n Sacrum n Triangular structure n Base of the spine n Connects spine to pelvis n Nerves to pelvic organs Coccyx n n Few small bones Remnant of tail

Lordosis n In the sagittal plane n n As a small child(c shaped) n n n ‘S’ shape When starts to sit Cervical Lordosis Toddler and adult n n n When starts to stand Lumbar Lordosis Allows spring-like action

Orientation of facet joints with respect to transverse plane

Compressive Strength of Spine

Major Lumbar Ligaments ALL: Anterior Longitudinal Ligament PLL: Posterior Longitudinal Ligament LF: Ligamentum Flavum ISF: Inter-Spinous Ligament SSL: Supra-Spinous Ligament Herzog Fig 2 -13 17

Failure Strength of Spinal Ligaments

Motion Segment n n n Two adjacent vertebrae Intervertebral disc Six degrees of freedom n n n Flexion-extension Lateral flexion Axial rotation

Types of motion

Motion Segment

Motion of Entire Spine

Weight bearing properties of motion segment unit

Intervertebral Disc n Soft fibro-cartilaginous cushions n n n n Between two vertebra Allows some motion Serve as shock absorbers Total – 23 discs ¼ th of the spinal column's length Avascular Nutrients diffuse through end plates

Intervertebral Disc Functions n Movement of fluid within the nucleus n n Allows vertebrae to rock back and forth Flexibility Act to pad and maintain the space between the twenty-four movable vertebrae Act as shock absorbers

Intervertebral Disc Anatomy n Spongy center n n Nucleus pulposus Surrounded by a tougher outer fibrous ring n Annulus fibrosus

Annulus Fibrosus n n n Strong radial tire–like structure Series of lamellae Concentric sheets of collagen fibers n n n Orientated at various angles Connected to end plates Encloses nucleus pulposus

Annulus n In Bending (flexion) n n n Increased tensile force posteriorly Increased compressive force anteriorly (vice versa) In Rotation n Reorientation of collagenous fibers Tightening of fibers in one direction Loosening of fibers in opposite direction

Nucleus Pulposus n n Has more water and PGs PG are macro-molecules n n Attract and retain water Hydrophilic gel–like matter n n Resists compression Amount of water n n Activity related Varies throughout the day

Theory of weight bearing n n n Nucleus pulpous imbibes water Develops internal pressure Pressure exerted in all directions n Lateral forces n n Superiorly and inferiorly directed forces n n Against annulus Against end plates Increases stiffness n Of end plate and annulus fibrosus

Theory of weight bearing (cont’d)

Creep Characteristics Grade 0 - Non-degenerative disc ( more viscoelastic) Grade 2 – Mild degenerative disc (less sustenance) Grade 3 – Severe degenerative disc ( more deformation)

Shear & Tensile Characteristics n In direct shear tests n Shear stiffness in horizontal direction n 260 N/mm 2 Spine rarely fails in pure shear Similarly under normal physiologic activities n n Pure tensile loading doesn’t occur But annulus undergoes tensile loading during n n n Bending Axial rotation Extension

Compressive load characteristics n Cancellous bone n Large deformation n n Up to 9. 5% before failure Cortical bone n Small deformation n Up to 2% before failure

Measurements of In vivo Loads n n Needle pressure transducer Calibrated n n Introduced into nucleus pulpous of cadaveric functional unit Inserted in vivo in L 3 -4 disc

Pathology of Intervertebral Disc Injury Disc Bulge n Extrusion n n Fragmentation of nucleus pulposus Nuclear material dissects its way through breaches in annulus fibrosus