PowerPoint Presentation€¢Growth factors such as BMP, TGF-B –promote bone formation 3....

6/24/2018

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Francis H. Shen, MDWarren G. Stamp Endowed Professor

Division Head, Spine DivisionCo-Director, Spine Center

Department of Orthopaedic SurgeryUniversity of Virginia

University of Virginia Orthopaedic Surgery

Key Points

1. Understand body weight line / sagittal balance.

2. Understand theory of anterior column support and posterior tension band

3. Understand behavior of uninstrumented/instrumented strut graft

4. Understand implications of shear at the LS junction

5. Understand why distraction is bad in the L-spine

6. Understand basic of properties of bone graft materials

7. Understand intervertebral disc and the basics changes with aging


Concepts from Spine Trauma

Spinal Stability

Mechanical

Stability

Neurologic

Stability

Mechanical and

Neurologic Stability

Neurologic

StabilityUniversity of Virginia Orthopaedic Surgery

Spinal Biomechanics

• Body weight line is anterior to spinal column

• Vertebral body

– Anterior column support

• Posterior elements

– Tension band

• Sagittal balance is important

Body WtLineSagittal Balance


Lose anterior

column support

Junctional Kyphosis

Harms J, Tabasso G. Instrumented Spine. Thieme. 1999

Spinal Biomechanics


Lose posterior

tension band

Junctional Kyphosis


Spinal Biomechanics

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Spine Biomechanics


Spinal Biomechanics

• Goal

– Restore:

anterior column support

– Restore:

posterior tension band


CTQ


Spine Biomechanics

Harms J, Tabasso G. Instrumented Spine. Thieme. 1999University of Virginia Orthopaedic Surgery

Spine Biomechanics



Spine Biomechanics


• Structural Support

– Bone graft

• Autograft / Allograft

– Cages

– Arthroplasty

• Cantilever Beams

– Plates

– Rods

Common Anterior Column Devices

Interbody Devicesapplied between vertebral bodies

Tension Bandapplied on

vertebral bodies

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Anterior Column Interbody Support

• Axial compression

– Loads graft

• Flexion

– Loads graft

• Extension

– Unloads graft

• Translation

– Unstable

– “Pole vaulting”

Benzel EC. Biomechanics of Spine Stabilization. Thieme. 2001.University of Virginia Orthopaedic Surgery



– Loads graft

• Flexion

– Loads graft

• Extension

– Unloads graft

• Translation

– Unstable


X

Benzel EC. Biomechanics of Spine Stabilization. Thieme. 2001.




– Loads graft

• Flexion

– Loads graft

• Extension

– Unloads graft

• Translation

– Unstable


X

Benzel EC. Biomechanics of Spine Stabilization. Thieme. 2001.University of Virginia Orthopaedic Surgery



– Loads graft

• Flexion

– Loads graft

• Extension

– Unloads graft

• Translation

– Unstable





– Loads graft

• Flexion

– Loads graft

• Extension

– Unloads graft

• Translation

– Unstable





Isolated Anterior Plating

Moves

axis of rotation

anterior

X

X

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Moves

axis of rotation

anteriorX



Isolated Anterior Fixation loads the graft in extension X



X

Moves

axis of rotation

anterior



X

Isolated Anterior Fixation unloads the

graft in flexion


• Structural Support

• Clinically not used much

• ?? Brooks C1-C2 fusion

• Cantilever Beams

• Plates / Rods

• Pedicle and lateral mass fixation

• Spinous process wiring

Common Posterior Column Devices

“Interprocess Devices”applied between spinous process

Tension Bandapplied on

spinous process


Modified Gallie Fusion Brooks Fusion

Transarticular Screws

Posterior

C1-C2

Fusions

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Biomechanical C1-C2 Posterior Techniques

Flexion Extension Rotation

Modified Gallie Good Poor Poor

Brooks Good Better Better

Transarticular

ScrewsBetter Better Better

C1 Lateral Mass

C2 PedicleBest Best Best

CTQUniversity of Virginia Orthopaedic Surgery

Modified Gallie

Tension band construct helps provide resistance against

FLEXION . . .

but, provides little resistance against

EXTENSION moment


Bone graft

helps provide resistance

against Extension

Tension band construct

helps provide resistance

against Flexion

Brooks


Transarticular Screws

• Immediate internal fixation

• Rigidly couples facets of C1-C2

• Combined with interspinous wiring and bone grafting provides three-point atlantoaxial fixation


C1 Lateral Mass and C2 Pedicle Screws


Oh, by the way…

2 Random Lumbar Concepts

for the Boards

Anterior: Shear at L5-S1is bad

Posterior: Lumbar distraction is bad

CTQ

6/24/2018

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Lumbosacral Anatomy and Fusion

• Interbody implants work best in compression

• Fusion is best in compression—Wolff’s Law

• In an upright posture, the low lumbar / lumbosacral interspace can experience significant shear forces

Anterior


CTQ


• Q: Choosing between L5-S1 vs. L4-S1• A: In low grade slips

– Grade I / II (0-50%) slip– Fairly horizontal L5-S1– Fuse from L5-S1 only

• A: In high grade slips – Grade III/IV (51-100%) slip– L5-S1 often failry vertical – Fuse from L4-S1

L5-S1 Spondylolisthesis

Spinal Deformity

CTQ


Lumbar Distraction

• Lumbar distraction

• Flattening of normal lordotic curve

• Flat back syndrome

• Positive sagittal alignment

Bad, bad, bad,

and bad

Benzel EC. Biomechanics of Spine Stabilization. 2001. University of Virginia Orthopaedic Surgery

Spinal Instability

Does a Laminectomy

meet these goals?

NO !


High Yield

Basic Science

Concepts

in

Spine

Courtesy of Kris Radcliff, MD


Properties of Ideal Graft Material

1. Osteoconductive matrix

• Scaffold or framework into which bone growth occur

2. Osteoinductive factors

• Growth factors such as BMP, TGF-B – promote bone formation

3. Osteogenic cells

• Mesenchymal cells, osteoblasts, and osteocytes

CTQ


6/24/2018

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Allografts

• Fresh – Increased antigenicity

• Fresh-Frozen– less immunogenic– BMP preserved

• Freeze Dried – Loses structural integrity – Depletes BMP– Least immunogenic– Purely osteoconductive



• Cortical Bone Graft

– Slow incorporation

– Remodeling of existing Haversian systems

– via resorption followed by deposition of new bone

– Weak during resorption phase (fatigue Fracture)

• Cancellous Bone Graft

– Revascularizes quickly

– Osteoblasts lay down new bone on old trabeculae

– CREEPING SUBSTITUTION

CTQCourtesy of Kris Radcliff, MD


Bone Morphogenetic Protein (BMP)

• Part of the TGF-beta superfamily

• Naturally occurring protein in human bone

• Signaling molecule

• Must be placed on carrier in order to stay in region of repair and influence skeletal formation

• Over 20 structurally related BMP and BMP-receptors identified and described

– rhBMP2- Infuse

– rhBMP7- OP1



Growth Factors in Fx Healing

• BMP2 (Bone Morphogenetic Protein)

– Osteoinductive

– Causes mesenchymal cell differentiation to osteoblasts

– Target cell for BMP ---

• Undifferentiated Perivascular Mesenchymal Cell

• Osteoblast Progenitor Cell



Other modulators of bone healing

• COX2

– Required For Normal Endochondral Ossification.

– NSAIDs inhibit

• Quinolone antibiotics

– Chondrotoxic and inhibit fracture repair

CTQ



Intevertebral Discs

• Annulus Fibrosis– High collagen/Low GAG

– Extensibility and tensile strength

– Type I collagen

– Nerve fibers present

• Central Nucleus Pulposus – High GAG/Low collagen

– Allows for compressibility

– Type II collagen

CTQ


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Aging Disc

• Decreased– Water content– GAG– Proteoglycan content– Chondrotin sulfate

• Increased – Collagen Diameter – Keratin Sulfate concentration– Noncollagen glycoprotein

CTQ



THANK

YOU!

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