Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector?...
Transcript of Clinical Biomechanics in Spinal Surgery - UCSF CME · 2013. 9. 27. · “curved” force vector?...
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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
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July 2005 Oct 2006
Nov 2006 Aug 2007
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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
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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
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Complications of Forces Distraction VB 1
VB 2
Complications of Tensile Force Shear Translational
VB 1
VB 2
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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
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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
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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
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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?
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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
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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
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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