Cervical Biomechanics PDF

Cervical biomechanics Vertebrae Vertebrae The smallest and most mobile. Unique features Transverse foramina: for vertebral artery Uncinate joints Spinous processes are short and bifid Neural canal volume Greatest in full flexion and least in full extension....

Cervical biomechanics Vertebrae Vertebrae The smallest and most mobile. Unique features Transverse foramina: for vertebral artery Uncinate joints Spinous processes are short and bifid Neural canal volume Greatest in full flexion and least in full extension. Person with neural canal stenosis are more vulnerable to spinal cord injury and myelopathy (myelo, mean spinal cord, and pathos, pathology), especially during extension. Atlas (C1) Function: support the head. It doesn’t have body, pedicle, lamina, or spinous process, Consist of two large lateral masses joined by anterior and posterior arches The short anterior arch give attachment to anterior longitudinal ligament. The larger posterior arch concave superior articular facets face cranially, to accept convex occipital condyles inferior articular facets are flat to slightly concave sloped downward by 20 degrees from horizontal has it has large, most prominent and palpable transverse processes Axis (C2) The axis has a large, tall body that serves as a base for the upwardly projecting dens (odontoid process) Part of the elongated body is formed from remnants of the body of the atlas and the intervening disc. The dens provides a rigid vertical axis of rotation for the atlas and head (Fig. 9.21). Projecting laterally from the body is a pair of superior articular processes (see Fig. 9.20A). These large processes have slightly convex superior articular facets that are oriented about 20 degrees from the horizontal plane, matching the slope of the inferior articular facets of the atlas. Projecting from the prominent superior articular processes of the axis are a pair of stout pedicles and a pair of very short transverse processes (see Fig. 9.20B). A pair of inferior articular processes projects inferiorly from the pedicles, with inferior articular facets facing anteriorly and inferiorly (see Fig. 9.18). The spinous process of the axis is bifid and very broad. The palpable spinous process Vertebra Prominens (C7) it is the largest cervical vertebrae, has many characteristics of thoracic vertebrae as: large transverse processes large palpable spinous process so the name vertebra prominens hypertrophic anterior tubercle on transverse process may sprout an extra cervical rib, which may impinge brachial plexus. Vertebral junction Vertebral junction Uncovertebral joint Uncovertebral joint s (uncus means “hook”) Another name: luschka From C2–3 to C6–7. Cannot tolerate repetitive or large forces due to thin articular cartilage Discs acts as spacers between vertebrae to unload it. Increased loading stimulates osteophyte growth which compresses nerve root Function Maintain central position of disc during rotation. Mechanical “coupling” that naturally occurs between axial rotation and lateral flexion Apophyseal joints Apophyseal joints Apophysis means “outgrowth,” emphasizing the protruding nature Close-packed position: extension Loose packed position: slight flexion 24 pairs. Synovial, plane (flat) joints Surface is slightly curved in upper cervical and lumbar regions. Orientation Near–vertically in lower thoracic and lumbar regions block excessive anterior translation of vertebra (spondylolisthesis) Horizontal in upper cervical, favor axial rotation, Accessory structures of apophyseal joints, In upper cervical and lumbar regions. In lumbar spine accessory structures: 1. Subcapsular fat pads: Fill spaces between capsule and synovial membrane It may extend outside joint. In lumbar, it fills space between lamina and multifidi muscles. 2. Fibro-adipose meniscoids Folds of synovium from internal surface of capsule, that encapsulate fat pads, and extend into apophyseal joint. Function Deformable spacers that dissipate compression forces. Cover articular cartilage that becomes exposed at extremes of motion. To protect and lubricate it until joint is returned to natural position. Clinical points In cervical regions, meniscoids may be impinged during forcefully hyperextend (whiplash injury). Because these tissues are highly innervated, they may be a source of pain. Meniscoids may proliferate to a point that restricts or lock joint. Discs Discs nucleus pulposus consists of 70% to 90% water. function as a modified hydraulic shock absorption system Proteoglycans It is aggregate of water-binding glycosaminoglycans that provide gel-like consistency type II collagen fibers (15% to 20%) to support proteoglycan network. small numbers of chondrocytes and fibrocytes for synthesis and regulation of proteoglycans. The annulus fibrosus I consists of 15 to 25 concentric layers. collagen 50% to 60% elastin protein outermost layers consist of type I and type II collagen. to provide strength and flexibility Enhance merging with anterior and posterior longitudinal ligaments and rim of vertebral bodies The only innervated part of disc inner layers contain less type I collagen and more water, for gradual transforming into characteristics of nucleus pulposus. dehydrated and thinned disc increases compression on apophyseal joints that increase risk of arthritis In lumbar region, collagen rings are Oriented 65 degrees from vertical, To distribute force in to compression and shear force Fibers of adjacent layers traveling in opposite directions. During rotation, only layers oriented in the direction of rotation become taut; fibers in every other layer slacken (only half of layers are in optimal position). Repetitive and forceful rotation is risk factor for disc injury. Movement  Movement Flexion and extension Resting position: 30° to 35° of extension (lordosis) Extension: 75 to 80° Flexion: 45 to 50° 25% occurs at atlanto-occipital and atlanto-axial joints End feel: Flexion: Stretch of ligamentum nuchae and interspinous ligaments) Compression of anterior annulus fibrosus Extension Approximation of apophyseal joints Compression of posterior annulus fibrosus. Protraction and retraction Resting position: 35% forward from fully retracted position. Protraction 6.23 cm Flexes lower-to-mid spine and extends upper cervical Retraction 3.34 cm Extends lower-to-mid spine and flexes upper cervical Prolonged protraction leads to chronic forward head posture and strain of extensor muscles. Rotation Range Children of 3.5 and 5 y: 100° to each side. Young adult : 80° to each side 65 to 75° to each side, With 160 to 170° of eyes movement ,visual field reach 330° without trunk movement. 50–60% occurs at atlanto-axial joint, Full rotation stretches both vertebral arteries Coupling between side bending and rotation In mid-and-low cervical, right side bending occurs with right rotation (ipsilateral coupling). This rotation is compensated by contralateral coupling (left rotation) of atlantoaxial joint That minimizes the overall rotation of head, to help eyes fixation on a stationary object during side bending of neck. Superficial layer of back muscles: Upper and lower trapezius rotate cervical and upper thoracic spine to left, with right upper limb movement “contralateral coupling” Trapezius (and rhomboids) stabilizes scapula against pull of deltoid. As muscle pulls spinous process to right, anterior side of vertebra is rotated to left. Muscles  Muscles Sternocleidomastoid Origin: two heads: medial (sternal) and lateral clavicular) Insertion: between mastoid process and lateral half of superior nuchae line. Action Unilaterally: side bending and rotation to opposite side Bilaterally: flexion to mid-to-lower cervical and extension to upper cervical This action favors forward head posture This action affected by initial posture cervical flexion of mid-to-lower cervical spine (forward head posture) doubles muscle’s flexion torque by increasing moment arm that exaggerate forward head posture Scalenes scalene mean unequal sides triangle. Origin: transverse process tubercles of middle to lower vertebrae Insertion: first two ribs scalenus anterior, anteriorlateral region of first rib, (the greatest moment) brachial plexus courses between scalenus anterior and medius. Hypertrophy, spasm can compress brachial plexus. function Origin on insertion or insertion on origin With stabilized cervical spine, it raise ribs to assist inspiration with stabilized first two ribs, it moves cervical spine. unilaterally, side bending and ipsilateral rotation torque is dependent on initial posture of spinel Bilaterally: return of cervical region to neutral position from rotated position. Dr. Mahmoud nabhan