1. BIOMECHANICS OF CERVICAL SPINE DR. FATIMA ZEHRA (Physical Therapist) School of Physiotherapy, IPMR Dow University of Health Sciences

2. Anatomy of spine Spine has 33 vertebrae; 7 Cervical 12 Thoracic 5 Lumbar 5 SACRAL 4 Coccygeal

3. MOTION SEGMENT The functional unit of spine is called motion segment of the spine It consists of 2 adjacent vertebrae & associated soft tissues (IVD & ligaments b/w the vertebrae) Ligaments – Anterior and posterior longitudinal ligaments, intertransvers, interpinous, supraspinous and the facet capsular ligaments

4. Ligaments

5. Anatomy of the cervical spine C1 - C2 Complex – Comprises of upper cervical spine – Responsible of approx. 40 % of cervical flexion & 60 % of cervical rotation

6. C1 - C2 Complex Atlanto – Occipital Joint (O-C1 Joint) permits primarily flexion and extension Atlanto – Axial joint ( C1-C2 Joint) is primarily responsible for rotation in the cervical spine

7. First Cervical vertebra(Atlas) - A bony ring - Consists of an anterior & posterior arches and 2 lateral masses Anterior Arch Anterior tubercle for Longus colli muscle’s attachment Posterior Arch Grooves for passage of vertebral arteries on superior surface Anatomy of the cervical spine

8.  2 lateral masses  Face cranially & inward  Forms articulation with occipital condyles called Occipitocervical joint (OC1 Joint)  Occipital condyles face caudally & outward Anatomy of the cervical spine

9.  OC1 Joint permits primarily flexion & extension  Extension is limited by bony anatomy & Flexion is limited by ligaments and tectoral membrane  No intervertebral disc at OC1 Junction  Stability BY intact bones & ligamentous Anatomy of the cervical spine

10.  Second cervical vertebra (C2)  Odontoid process from superior surface of body  Restrain in a socket formed by transverse ligament & anterior arch of C1 which prevent anterior translation of C1 on C2  Forms articulation with C1 called Atlanto-axial joint (C1-C2 joint)  C1-C2 joint is primarily specialized for Rotation  Other ligaments at C1-C2 joint are alar, apical & accessory alar Anatomy of the cervical spine

11.  Alar ligaments  Symmetrically attach the dens to the occiput  Prevent excessive Rotation (Right & left Rotation)  To some extent act to limit side bending as well  Apical ligament  Also connects dense to occiput Anatomy of the cervical spine

12. 3 rd to 6 th cervical vertebrae body2 pedicles2 lateral masses 2 laminae spinous process Consist of a body, 2 transverse processes, 2 pedicles, 2 lateral masses, 2 laminae and a spinous process Body – Oval-shaped – Wider Medio-lateral than Antero-posterior Transverse Processes – Foramen for vertebral artery – Anterior & Post. tubercles for muscles attachment – Groove for nerve root on Superior surface

13. 3 rd to 6 th cervical vertebrae

14. Pedicles connects vertebral body to lateral mass Laminae connect spinous process to lateral masses Facet joints Lateral Masses have Inf. & Sup. facets which form Facet joints – Cervical Facet Joints – Cervical Facet Joints play a critical role in spinal stability Oriented approx. 45˚ to the coronal plane Located in the sagittal plane shear forces compressive forces Resists most shear forces & approximately 16% of compressive forces acting on the spine

15. Inter-Vertebral discs Contribute up to 1/3 of the height of vertebral column Withstand greater than normal loads when compressive forces are applied – Short-duration/High-amplitude loads by activities such as running & jumping – Long-duration/ Low-amplitude Loads by normal physical activities and upright stance Disc consists of nucleus pulposus & annulus fibrosus

17. Intervertebral discs Nucleus Pulposus Nucleus Pulposus – Central part of disc – Comprises of water, proteoglycan & type II collagen – Water (90% in young individuals & decreases 70% as the disc degenerates with age) – Proteoglycan decreases with aging & disc degeneration – Type-II collagen fibers absorb more compressive forces than Type-I collagen fibers

18. Intervertebral discs Annulus Fibrosus – Outer portion of the disc – Consists of collagen with water content – Water (approx. 78% in young individuals & decreases 70% with age in older persons) – Collagen fibers Arrange in approx. 90 concentric lamellar bands Arrange in approx. 90 concentric lamellar bands Having Considerable strength & some flexibility Having Considerable strength & some flexibility 60% Type-II Collagen & 40% Type-I collagen 60% Type-II Collagen & 40% Type-I collagen With age Type-II Collagen are replaced by Type-I With age Type-II Collagen are replaced by Type-I

19. Mechanical properties of vertebrae Vertebrae – Strength and stiffness, stress & strain relationship – Cancellous bone withstands greater stress before fracture than cortical bone – Vertebral compression strength increases from the upper cervical to the lower lumber levels – Bone strength decreases with age because of constant decrease in mineral contents – Cortical shell is responsible for only 10% of its strength during compressive loading

20. Mechanical properties of Vertebrae Contraction of attached Muscles can alter the stress distribution in the vertebrae During flexion of spine, tensile stresses on post. Cortex & compressive stresses to on ant. cortex of the vertebral body Bone Failure in tension first & then bone failure in compression (in weaker bone) Bone failure is first & then disc damage during compressive loading

21. Mechanical properties of IV Discs Visco-elastic Properties (Creep & relaxation) and Hysteresis Creep ? Relaxation ? Hysteresis ? Creep occur more slowly in healthy discs than in degenerated or herniated discs

22. Mechanical properties of Ligaments Spinal stability is primarily maintained by ligaments strength & limited extensibility especially at OC1 junction Spinal ligaments are functional mainly during distraction Alar ligament strength = 200N Transverse ligament strength = 350 N (Dvorak et al., 1988a)

23. Mechanical properties of Ligaments All ligaments have high collagen contents except ligamentum flavum Ligamentum flavum is under tension even in a neutral position or somewhat extended position Thus, Pre-stress the disc and provide intrinsic stability to the spine

24. Mechanical properties of spinal muscles Muscle strength & control – Maintain head/neck balance (postural homeostasis) – Reduce stresses on bones During Cervical flexion – Tensile stresses on post. cortex & Compressive stresses on anterior cortex – Considerable load on vertebral bodies especially in the lower cervical spine

25. Mechanical properties of spinal muscles Load on OC1 Junction (Harms-Ringdahl, 1986) – Lowest during extreme neck extension – Highest during extreme neck flexion – Slight increase with neck in neutral position Load on C7-T1 motion segment (Harms-Ringdahl, 1986) – Low with the neck in neutral position – Lower with the head in upright/chin tucked – Increased/greatest during extreme extension – Considerable during slight flexion

26. Mechanical properties of spinal muscles Harms-Ringdahl, 1986 – Very low level of activity erector spinae muscles during full flexion, slight flexion, neutral, head up-right with the chin tucked in and full extension Fountain et al., 1996; Takebe et al., 1974 – Flexion momentum is balanced by passive connective tissues such as capsules and ligaments

27. Mechanical properties of neural elements Significant length changes of cervical spine during cervical flexion & extension Spinal cord tolerates axial translation poorly Translatory forces typically result in neurological injury Compressive tolerance of adult cervical spine is 2.75 to 3.44 kN before significant neurological injury

28. Mechanical properties of neural elements SCI can result from extreme or sudden flexion-extension movements Isolated SC resists tension poorly Lower forces may results in direct neural injury or vascular disruption SCIWORA (Spinal cord injury without radiographic abnormality) – Unknown etiology – Mechanism may be longitudinal traction

29. Kinematics Motion segment … the tradition unit of study in kinematics – Application of forces to vertebral body & the subsequent measurement of occurring movement (rotational or translational) Basic Biomechanical testing Degrees of freedom of vertebral body – 3 translational & 3 rotational planes

30. Kinematics Axial rotation to one side at C1-C2 (Dvorak et al., 1987 & 1988b; Penning & Wilmink et al., 1987) – Active … 27˚ to 49˚ (mean = 39˚) – Passive … 29˚ to 46˚ (mean = 41˚) – Approx. 50% of the total cervical rotation Axial rotation b/w O - C7 vertebra (Mimura et al., 1989) – Mean = 105˚ – 70% of the total occurred b/w O – C2 – 4˚- 8˚ at each motion segment from C2 to C7

31. Kinematics Axial rotation at C3 to C7 – Approx. 90˚ ( 45˚ to each side of the neutral) Lateral flexion – Approx. 98˚ ( 49˚ to each side of the neutral) Flexion and extension – Total approx. 64˚ – About Extension = 24˚, flexion = 40˚

32. Kinematics Cervical spine Active ROMs in ADLs (Bennett et al., 2000) – Tying- shoes (Flexion - Extension 66.7˚) – Backing up a car (Rot. 66.7˚) – Washing hairs in the shower (Flex.-Ext. 42.9) – Crossing the street (Head Rot. to Left 31.7˚ & to Right 54.3˚) – Reading a newspaper (flex.-Ext. 19.9˚) – Writing at a table (flex.-Ext. 26.2˚) – Side bending is coupled with rotation

33. Kinematics Surface Joint Motion of Cervical Spine – By instant centre technique of Reuleaux – Gliding b/w facet joint during cervical Flexion and Extension – IVF Diameter Increases with Flex. & decreases with Ext. 10 % & 13% reduction during 20˚ & 30˚ Extension respectively

34. Clinical Consideration – Use of Cervical collar for neck pain Soft collar may aggravate the symptoms as it place the neck in slight extension Foam collar with narrow part anterior may relieve the symptoms as it place the neck in slight flexion – Disc degeneration & Ligament Impairment Distraction & Compression of facet joints during flexion–extension instead of gliding

35. Coupled Motion of the Cervical Spine Atlanto – Axial Segment (C1-C2) – Extremely mobile area of the neck – sliding and rolling during flexion and extension – Rotation at C1-C2 is coupled with vertical translation & a degree of AP displacement – C1-C2 joint is most stable in neutral position

36. Coupled Motion of the Cervical Spine Sub-Axial Spine (C3 to C7) – Left Lat. Bending … SP moves to right – Right Lat. Bending … SP moves to left – 2˚ Axial Rot. : Every 3˚ of lateral bending – 1˚ Axial Rot. : Every 7.5˚ of lateral bending at C7 – During flexion, vertebral body normally shifts forward; the facets glide up and over one another

37. Abnormal Kinematics “ Excessive motion within the spinal unit or Atypical pattern of motion such as abnormal coupling /paradoxical motion” Paradoxical Motion – When the overall pattern of motion of one aspect of the spine is in one direction while the local pattern is the opposite Such as Paradoxical Flexion ( when flex. occurs at a single functional unit, although the spine as a whole is extended) – Describe as Cervical Instability

38. Spinal Stability Clinical Definition – “ The loss of the ability of the spine under physiological loads to maintain its pattern of displacement so that there is no initial or additional neurological deficit, no major deformity, & no incapacitating (unbearable) pain.” (white & Panjabi, 1990) – Physiological loads acquire during normal activities

39. Conceptual Types of Spinal Instability Kinematic, Component & Combined Instability Kinematic Instability – Alter Quantity of Motion Hypomobility or Hypermobility – Alter Quality of Motion ( Alter normal pattern) Coupling characteristic changed Paradoxical movement present

40. Conceptual Types of Spinal Instability Component Instability – Loss or Alteration in various anatomical portions – Two column concept (Sir Frank Holdsworth, 1963) 1.Stability of IVB joints (Synarthroses) by strong Annulus Fibrosus. 2.Apophyseal (Diarthrodial) joints stability by capsule & ligaments (post. Lig. Copmplex: Interspinous, Supraspinus & ligamenta Flava)

41. Spinal Instability Three column concepts (Denis, 1983) 1. Anterior Column: Ant. Longitudinal Lig., Ant. Annulus fibrosus, Ant. Half of Vertebral Body 2. Middle Column: Post. Longitudinal Lig., Post. Annulus fibrosus, Post. Half of Vertebral Body 3. Posterior Column: Pedicles, Facet Joints, Lamina, Spinous Processes, Interaspinous & Supraspinous Ligaments Note: AC & MC are weight bearing and PC is the guiding & stabilizing column

42. Spinal Stability O-C1-C2 Complex – Transverse ligament allows dens to rotate but limit its anterior translation – Ant. Translation is assessed radiographically by measuring ADI – 3mm ADI in adult & 4mm ADI in children … Normal – 3 to 5 mm ADI … Rupture of transverse ligament 5 to 10mm ADI … Accessory ligament damage More than 10mm ADI … Rupture of all ligaments

43. Applied biomechanics Decompressive cervical Laminectomy – Development of Post-laminectomy kyphosis – Resectioning of 1 or more SPs and Post. ligaments (ligamentum flavum, IS or SS ligaments) results in tensile forces to become unbalanced & place extra stress on facet joints Full Facetectomy – Significant decrease in coupled motions Partial Facetectomy ( ˂ 50%) – No significant alteration in Flex.– Ext. movements

44. Cervical Arthrodesis – Increases the motion at nearby unfused levels in fairly uniform manner – Increased potential for degenerative changes – By bone graft (fibula or iliac crest) or Internal fixation – Post arthrodeses for trauma, degenerative, inflammatory or neoplastic conditions – Anterior fixation devices in subaxial cervical spine

45. Spinal Stability Weakness of transverse ligament – Rheumatoid Arthritis – Down Syndrome Steel’s “Rule of Thirds” (Steel, 1968) – Total internal AP diameter of atlas = 3 cm approx. – Dens occupies = 1 cm approx. – Spinal cord occupies = 1 cm approx. – Remaining space for Soft tissue / movement = 1 cm approx.