Biomechanics of the Spine lecture

1. Biomechanics of the Spine
Richard C. Rooney, MD, FACS
www.seattlespinegroup.com

2. Biomechanics of the Spine
• Definitions and Fundamental Biomechanics
• Kinematics
• Stability and Instability
• Biomechanics of Fusion and Instrumentation

3. Definitions and Fundamental
Biomechanics
• Vector-force that has direction and magnitude
• Moment Arm-force vector acting upon a lever
creates a bending moment
• Instantaneous Axis of Rotation-(IAR) the axis or
fulcrum about which a bending moment causes
rotation
• Centrode-space defined by the IAR’s through a
range of motion
Gertzbein ‘85

4. Definitions and Fundamental
Biomechanics
• Strain-measure of deformation
• Stress-force per unit area
• Elastic Modulus (E)-stress/strain
• Hooke’s Law-the size of the deformation of a
body is proportional to the size of the deforming
force
• Poisson’s ratio
– Ratio of lateral expansion or contraction ┴ to
longitudinal expansion or contraction

5. Definitions and Fundamental
Biomechanics
• Elastic deformation-deformation that disappears
when stress is removed
• Plastic deformation-permanent deformation of
an object even after removal of stress
• Elastic limit-point at which any further
deformation is no longer purely elastic
• Endurance limit-repetitive stress that can be
endured indefinitely by a material
• Ultimate strength - point of failure

6. Definitions and Fundamental
Biomechanics
• Section modulus
– Strength of an object is a property of its intrinsic
geometry
– An implant fails at the point of maximum stress
application
• Strength of rod = πD3
/32
• Stiffness of rod = πD4
/16

7. Definitions and Fundamental
Biomechanics
• Elastic zone
• Plastic zone

8. AB - neutral zone
BC - elastic zone
CD - plastic zone
D - failure

9. Kinematics
• Kinematics - Description of motion,
regardless of how the motion came about
• Kinetics - Study of forces that bring about
motion

10. Kinematics
Motion
• 2 types (translation and rotation)
• 3 planes (x,y,z)
• 6 degrees of freedom
• *main determinant of spinal range of motion is
the orientation of the disc and facets

12. Kinematics
Motion Measurement
• Clinical measurement is difficult
• Dual inclinometer method
– Measure tangents of the curve
Ng ‘01
• Rotation is difficult
– Biplanar or stereotactic X-rays, percutaneous pins

13. Kinematics
Region specific Motion
• C-spine
– Flex-ext in sub-axial spine (C5-6)
– 50% rotation at C1-C2
Multiple authors
• T-spine
– Little motion due to rib cage
• L-spine
– Considerable lateral bending in mid L-spine
– Flex-ext greatest at L-S junction
– Rotation minimal due to sagittal facet orientation

14. Kinematics
The Motion Segment
• 2 adjacent vertebral bodies and intervening soft
tissues
• Functional spinal unit (FSU)
• 6 articulate faces and multiple ligamentous
attachments
• 3 translations and 3 rotations
• 6 degrees of freedom

15. Kinematics
The Motion Segment
• Motions are complex
• Multiplanar or multiaxial
• Coupled motion
– Movement about one axis obligates movement about
a second axis
• Varies from individual to individual

16. Coupled motion

17. Kinematics
The Intervertebral Disc
• Primary articulation
• Viscoelastic
• Absorbs energy
• Major constraint to motion
• Limited fatigue tolerance
• Nucleus
– 90% H2O
– Type II collagen and
proteoglycans
• Annulus
– ≈90 type I collagen
sheets at 30o

18. Kinematics
The Intervertebral Disc
• Designed to handle pure axial loads
• Asymmetric loading and cyclical loading (5 Hz)
doesn’t allow nuclear engorgement and optimal
viscoelastic characteristics

20. Kinematics
The Intervertebral Disc
• NORMAL-↑ intradiscal pressure with load which
↑ annular tension which ↑ stiffness
• ABNORMAL- annulus can’t become as tense so
↓ stiffness leads to bulging, collapse and ↑
motion

23. Kinematics
The Vertebral Body
• Body shaped for weight-bearing
• Trabecular patterns arranged for pure axial
loading

25. Kinematics
The Posterior Elements
• All muscles that act on the spine attach to the
posterior elements.
• All forces transmitted through the lamina and
pedicle to the body
• Pars interarticularis sustains bending forces that
put it at risk for fatigue failure

26. Kinematics
The Facet Joints
• Fundamental contribution to resistance of shear
and torsion
• Axial load resistance is flexion-extension specific
– Flexion unloads facets, extension loads facets
– 20% of axial load upright
Yoganandan ‘88
– Up to 70% of axial load depending on extent of disc
degeneration
Adams ‘80

27. Kinematics
The Facet Joints

28. Kinematics
Ligaments
• ALL twice as strong as PLL
• Interspinous
• Intertransverse
• Ligamentum flavum
• Capsular

29. Kinematics
Regional Anatomic Specialization
• Cervical spine
– Facets ┴ sagittal plane
– 60% rotation at occ-C1-C2
– Majority of flex/ext C3-6
• Thoracic spine
– Costovertebral articulation
• Lumbar spine
– Flex/ext disc dependant, minimal facet
constraint

30. Kinematics
Regional Anatomic Specialization
• Cervical facet orientation
– Diminished ability to resist
• flexion/extension
• Rotation
• lateral bending
• Lumbar facet orientation
– Sagittal orientation becomes coronal at L5-S1 which
provides more resistance to translation, thus L4-5 is the
most commonly seen degenerative spondylolisthetic
segment

32. Kinematics
Degenerative Cause and Effect
• Degenerative discs have less ability to absorb
shock and deform more easily
Hirsch & Nachemson ‘54
• Stiffness decreases as DDD increases
Nachemson ‘79
• Decreased stiffness with loading in DDD
Keller ‘87
• Prolonged exposure to body’s resonant
frequency (4-5 Hz) may increase spinal
degeneration
Pope ‘89

34. Kinematics
Degenerative Cause and Effect
• Only experimental situation that has produced
traumatic disc herniation in an otherwise healthy
disc is flexion and lateral bending as a preload,
followed by a high-amplitude rapid compression
load.
Adams ’81

35. Kinematics
Ergonomics - sitting
• 40% increase in intradiscal pressure when
seated
Nachemson ’64, ‘81
• Flattening of the L-spine rotates pelvis on ischial
tuberosities increasing lever arm and magnifying
vibration effects
Chaffin ‘84
• Increase posterior disc height stretches posterior
annular fibers
Farfan ’73, Krag ‘87

36. Diagrammatic comparison of in vivo loads (disc pressures) in the third lumbar disc during various
activities. Note that sitting pressures are greater than standing pressures.
(From Nachemson AL: The lumbar spine, an orthopaedic challenge. Spine 1:59, 1976.)

37. Kinematics
Ergonomics - sitting
• Disengage facets allowing increase in shear
forces on disc
Panjabi ’77, Tencer ‘82
• Myolectric activity and disc pressure decreased
when back supported
– 20o
backwards tilt, 4 cm lumbar support ideal
Anderrson ‘74

38. Stability and Instability
The ability of the spine under normal physiologic
loads to limit patterns of displacement so as not
to damage or irritate the spinal cord or nerve
roots and, in addition, to prevent incapacitating
deformity or pain caused by structural changes.
White and Panjabi 1990

39. Stability and Instability
In vivo measurement methods
• Controversial
• Flex/ext X-rays
– Coupled motion poorly represented
– > 6 mm translation, > 20o
angular motion
Shaffer ‘90
• No good 3-D data collection
• Multiple grading schemes
• Must correlate static and dynamic imaging
Bendo ‘01

40. One side C0-C1 axial rotation > 8o
C0-C1 translation > 1 mm increase with flexion/extension
Overhang C1-C2 (total right and left) > 7 mm
One side C1-C2 axial rotation > 45o
C1-C 2 translation > 4 mm
Distance between the posterior body of C2 and the posterior ring C1 < 13 mm
Stability and Instability
White and Panjabi – C0-C1-C2 Instability

41. Stability and Instability
White and Panjabi – Middle and lower cervical instability
Element Point Value
Anterior elements destroyed or unable to function 2
Posterior elements destroyed or unable to function 2
Positive stretch test 2
Radiographic criteria 4
A. Flexion/extension x-rays
1. Sagittal translation > 3.5 mm or 20% (2pts)
2. Sagittal plane rotation >20 (2pts)
OR
B. Resting x-rays
1. Sagittal plane displacement > 3.5 mm or 20% (2pts)
2. Relative sagittal plane angulation > 11 (2pts)
Abnormal disc narrowing 1
Developmentally narrow spinal canal 1
1. Sagittal diameter < 13 mm
OR
2. Pavlov’s ratio < 0.8
Spinal cord damage 2
Nerve root damage 1
Dangerous loading anticipated 1
Total of 5 points or more = unstable

42. Element Point Value
Anterior elements destroyed or unable to function 2
Posterior elements destroyed or unable to function 2
Disruption of Costovertebral articulation 1
Radiographic criteria 4
1. Sagittal plane displacement > 2.5 mm (2 pts)
2. Relative sagittal plane angulation > 5o
(2pts)
Spinal cord of cauda equina damage 2
Dangerous loading anticipated 1
Total of 5 points or more = unstable
Stability and Instability
White and Panjabi – Thoracic and thoracolumbar instability

43. Stability and Instability
Effects of decompression on stability
• Hypermobility at L4-5 after discectomy
» Frymoyer ’79, ’85
» Spengler ‘82
• Post-laminectomy kyphosis
» Haft ’59
» Lonstein ’76
» Yasouka ’81
» Papagelopoulos ‘97
• Facet resection
– < 50% stable Zdeblick ’92, ’93
– > 50% resection = pathologic motion Ng ’04
• Motion correlates with extent of laminectomy
» Detwiler ‘03

44. Stability and Instability
Effects of decompression on stability
• Fusion after decompression
< 75 years
> 50% total facet resection at 1 level
> 30-40% of annulus resected
White and Panjabi ‘90

45. Stability and Instability
IAR migration in extension
Anterior and inferior migration with posterior
and posterior-middle column destruction

46. Stability and Instability
IAR migration in flexion
Posterior migration with anterior and
anterior-middle column destruction

47. Biomechanics of Fusion and
Instrumentation
• Section modulus
– Strength of an object is a property of its intrinsic
geometry
– An implant fails at the point of maximum stress
application
• Strength = πD3
/32
• Stiffness = πD4
/16

49. Biomechanics of Fusion and
Instrumentation
• Abutting Implant-Bone interfaces
– Closeness of fit
• Concentration of forces = excessive subsidence
– Surface area of contact
• Extent of subsidence proportional to surface area of contact
• > 30 % covered to avoid subsidence
Closkey ‘93
– Quality of contact surfaces
• Extent of endplate preparation
• Proximity to edge of vertebral body
Steffen ‘00

50. Biomechanics of Fusion and
Instrumentation
• Screw – bone interface
• Pedicle - 80% stiffness, 60% pullout strength
Lehman ‘02

51. Biomechanics of Fusion and
Instrumentation
• Screw anatomy
– Head
– Core
• Strength and stiffness function of inner diameter
– Thread
• Pitch, depth, and shape
– Tip
• Tapping or non-tapping

53. Biomechanics of Fusion and
Instrumentation
• Pullout resistance
– Proportional to insertional torque and BMD
Ryken ’95, Peiffer ’96, ’97, Hitchon ‘03
– Proportional to volume of bone between threads
Chapman ‘96
– Thread depth and outer diameter most important
– Conical screws with constant major diameter
Abshire ’01, Choi ‘02
– Triangulation
Ruland ’91, Suzuki ’01, Huang ‘03

55. Biomechanics of Fusion and
Instrumentation
Critical Insertional Torque?
4 in/lbs
Zdeblick ‘93
Critical BMD?
.7g/cm2
Ito ‘02
.22 g/cm2
Knöller ‘05
.45 g/cm2
Lim ‘95
.674 g/cm2
Bühler ‘98

56. Pull out strength proportional to volume of bone between threads

57. Biomechanics of Fusion and
Instrumentation
• 10% body weight above T1
• 50% body weight about T12
Henzel ‘68
• L spine compressive loads
– 400 N standing
– 7000 N lifting
McGill ’86, Schultz ‘87

58. Biomechanics of Fusion and
Instrumentation
• ASIS 430 to 8112 N
• PSIS 350 to 4639 N
Smith ’93
• Cortical bone 10-31,000 N
Bianchi ‘99
• BMD correlates with strength
Smith ’93, An ’94, Toth ’94
• Ceramic strength correlates with porosity
Toth ’94

59. Biomechanics of Fusion and
Instrumentation
• Upper Cervical spine
– Wiring < Halifax clamp < trans-articular screws =
pedicle screws
• Lower Cervical spine
– Posterior
• Wiring < lateral mass screws
– Anterior plating
• Dynamic < fixed
• Lumbar spine
– Cage < cage/facet screw < cage/pedicle screws
Multiple authors

60. Biomechanics of Fusion and
Instrumentation
Biomechanical Testing
• Biomechanical Testing Techniques
• Experimental Design
• Limitations and Sources of Error

61. Biomechanics of Fusion and
Instrumentation
Biomechanical Testing
• Biomechanical Testing Techniques
– Test Type
• Static or dynamic
• Strength, stability or fatigue
– Load vs Displacement Control
• Flexibility (fixed load)
• Stiffness (fixed displacement)
– Cyclic Loading
• 0.5 – 25 Hz is physiologic range
• 5 million cycles is FDA standard
– Mathematic Testing (finite element analysis)

62. Biomechanics of Fusion and
Instrumentation
Biomechanical Testing
• Experimental Design
– Specimen selection
• Human, animal, synthetic
• Freezing doesn’t alter physical properties
Panjabi ’85, Callaghan ‘95
– Specimen preparation
– Specimen fixation
• potting
– Environmental conditions
• 100% humidity during storage and testing
– Preloading
• May provide a closer approximation to in vivo testing

63. Biomechanics of Fusion and
Instrumentation
Biomechanical Testing
• Limitations and Sources of Error
– Specimen variability
• BMD, disc degeneration
– Extrapolating from cadaveric and animal data
– Effect of musculature
– Creep and the viscoelastic response
– Data interpretation

64. Biomechanics of the Spine
• Conclusion