Disclosure Clinical Biomechanics in Spinal Surgery Joseph S. Cheng, M.D., M.S. Associate Professor of Neurological Surgery and Orthopedic Surgery Director, Neurosurgery Spine Program 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 1
July 2005 Oct 2006 Nov 2006 Aug 2007 2
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 3
Linear Spinal Forces Force Linear load vector Through or transmitted F = m * a N = kg * (m/s 2 ) lb f = lb m * (ft/s 2 ) Gravity as acceleration is constant Units lb f (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 4
Complications of Forces VB 1 Distraction VB 2 Complications of Tensile Force Shear Translational VB 1 VB 2 5
VB 1 How About Rotational? Can there be a curved force vector? VB 2 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 6
Concept of Spinal Moments Balance Is Minimizing Moments! C2-C3 Autofused Dubousset Cone of Economical Function Periphery has increased effort of musculature for posture Supraphysiologic energy Causing fatigue and pain 7
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 8
Material Versus Structural Properties What Biomechanics Did We Overlook? Miscalculation of Injury So Spinal Biomechanics Is More Than Simple Linear Quasistatic Force Vectors? 9
It Is ALL About Energy! It Is ALL About Energy! Area under the force-deformation response Point of failure describes the structure's total energy-absorbing ability Energy Theorom Trauma Potential Energy = mgh Force=mg (Newtons) Distance=h (meters) Kinetic Energy = ½ mv 2 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 10
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 Regional Balance 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 11
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. 12
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 13