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8/1/13 1 Clinical Biomechanics in Spinal Surgery Joseph S. Cheng, M.D., M.S. Associate Professor of Neurological Surgery and Orthopedic Surgery Director, Neurosurgery Spine Program Disclosure I have no relevant financial relationships with the manufacturer(s) of any commercial product (s) and/or provider of commercial services discussed in this CME activity. •I will discuss an unapproved or investigative use of a commercial product or device in my presentation. – Cervical lateral mass screws. Importance of Biomechanics in Our “Routine” Practice June 2005

Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Page 1: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Clinical Biomechanics in Spinal Surgery

Joseph S. Cheng, M.D., M.S. Associate Professor of Neurological Surgery

and Orthopedic Surgery Director, Neurosurgery Spine Program

Disclosure

•  I have no relevant financial relationships with the manufacturer(s) of any commercial product(s) and/or provider of commercial services discussed in this CME activity.

•  I will discuss an unapproved or investigative use of a commercial product or device in my presentation. – Cervical lateral mass screws.

Importance of Biomechanics in Our “Routine” Practice

June 2005

Page 2: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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July 2005 Oct 2006

Nov 2006 Aug 2007

Page 3: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Dec 2007

So How Do We Avoid Biomechanical Misadventures

in Spinal Surgery?

Understanding Biomechanical Comorbidities

•  Forces – Compression – Distraction – Shear

•  Rotation (Load) – Moment – Torque

•  Displacement – Linear – Angular

•  Material Properties – Stress, Strain

•  Energy – Work – Potential, Kinetic

Complications of Forces

Page 4: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Linear Spinal Forces •  Force

– Linear load vector –  “Through” or “transmitted” – F = m * a – N = kg * (m/s2) –  lbf = lbm * (ft/s2)

•  Gravity as acceleration is constant

•  Units –  lbf (Pounds force) – N (Newtons)

Source: http://en.wikipedia.org/wiki/File:Body_mass_index_chart.svg

Quasi-Static Loading in Compression

Dynamic Loading in Compression

Page 5: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Complications of Forces Distraction VB 1

VB 2

Complications of Tensile Force Shear Translational

VB 1

VB 2

Page 6: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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VB 1

VB 2

How About Rotational?

Can there be a “curved” force vector?

Post-laminectomy Kyphosis

•  Lizuka (Spine, 2001) –  Semispinalis cervicis 37%

of extension –  Removal of semispinalis

attachments on C2 results in loss of cervical alignment

–  Repair more difficult in elderly woman

Concept of Spinal Moments •  Moment

– Force applied at a distance

– Mx, My, Mz – Units: ft-lbs, N-m

•  Torque – Spinal motion is

coupled – Rotational

displacement from a force couple

Page 7: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Concept of Spinal Moments

Balance Is Minimizing Moments!

•  Dubousset –  “Cone of Economical

Function” •  Periphery has increased

effort of musculature for posture – Supraphysiologic energy – Causing fatigue and pain

C2-C3 Autofused

Page 8: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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What Biomechanics Did We Overlook?

Different Pathomechanics, Different Tissue Injury

Material Properties

•  Stress –  Internal material

resistance to the force – σ = F/A

•  Strain – Dimensional changes

under the action of a force

– ε = ΔL/L

Real Anatomy Heterogenous •  Discs

–  Axial load absorption, main rotatory stabilizers

•  Ligaments –  Small surface area to distribute force makes them susceptible to rapid

deceleration disruption

•  Thoracic spine –  Paraspinal muscles and rib cage stress shield –  Kyphosis of upper thoracic spine predisposes to injury

•  Lumbar spine –  Large VB surface area to distribute force vectors load –  Lordosis subjects spine to shear injury

Page 9: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Material Versus Structural Properties

What Biomechanics Did We Overlook?

Miscalculation of Injury

So Spinal Biomechanics Is More Than Simple Linear Quasistatic

Force Vectors?

Page 10: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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It Is ALL About Energy!

•  Area under the force-deformation response

•  Point of failure describes the structure's total energy-absorbing ability

It Is ALL About Energy!

Energy Theorom

•  Trauma – Potential Energy = mgh

•  Force=mg (Newtons) •  Distance=h (meters)

– Kinetic Energy = ½ mv2

•  Tissue Injury – Due to inability to absorb

transferred energy

Biomechanical Pain Hypothesis •  Surgery should be load

sparing, not motion sparing. – Pain is physiological

response to tissue damage – Tissue damage from inability

to absorb energy – Mechanical pain can be

reduced by minimizing excessive energy to the bone and soft tissues

Page 11: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Regional Balance

•  Cobb Angle –  Angle formed by the

endplates of the 2 most tilted levels

–  Coronal plane >10° –  Intra-observer variability

of +/- 3-6° –  Curve progression > 6°

per year is significant

Pelvic Incidence (PI) •  Sacral Slope (SS)

–  ∠ of sacral plate wrt horiz •  Pelvic Tilt (PT)

–  ∠ midpt sacral line to fem head wrt vertical

•  PI = SS + PT –  ∠ sacral perpendicular wrt

line to femoral head –  Constant and specific –  Independent of 3D

orientation of pelvis

Page 12: Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector? Post-laminectomy Kyphosis • Lizuka (Spine, 2001) – Semispinalis cervicis 37%

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Biomechanical Errors To Avoid

•  Over correction of a curve creating a new imbalance.

•  Fusion to the sacrum with poor balance above.

•  Residual trunk shift. •  Introduction of shoulder

imbalance.

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“Patients will always benefit more from sloppy surgical

technique with the right indications, than a perfect

surgical technique with wrong indications.”

Conclusion

•  Applying concept of biomechanics is important in reconstructive spinal surgery

•  More translational research is needed to bridge the gap from the biomechanics lab to a clinic setting

•  Diagnostic assessment for biomechanical comorbidities are key!

Thank You!

Joseph S. Cheng, M.D., M.S. Associate Professor of Neurological Surgery, Orthopedic

Surgery, and Rehabilitation Director, Neurosurgery Spine Program