Practical Spine Biomechanics for

Clinical Practice

2012 CNS Annual Meeting October 6-10 in Chicago, Illinois.

1

Biomechanics and

Surgical Decision Making

Present and Future

Edward C Benzel Cleveland Clinic

DePuy OrthoMEMS

AxioMed

BIOMECHANICS OF

SPINAL COLUMN FAILURE

M = F x D

2

BIOMECHANICS OF

SPINE INSTRUMENTATION

SIX DEFORMATION RESISTANCE MECHANISMS Distraction

Three-Point Bending Tension-Band Fixation

Fixed Moment Arm Cantilever Beam Fixation Non-Fixed Moment Arm Cantilever Beam Fixation

Applied Moment Arm Cantilever Beam Fixation

3

PRINCIPLES

Use as Many Mechanisms as Possible to

Resist Deformation

Implants Function Uniquely

Under Different Loading Conditions

DEFORMATION RESISTANCE MECHANISMS

Distraction Tension-Band Fixation Three-Point Bending

Cantilevers

4

Understand Limitations

of

Strategy

5

Dynamic Fixation

Wolff’s Law

Every change in the form and function of a bone, or of

function alone, is followed by specific definitive change in its

internal architecture and equally definitive secondary

changes in its external configuration, in accordance with

mathematical laws.

“Structure is nothing else than the physical expression of

function… under pathologic conditions the structure and

form of the parts change according to the abnormal

conditions of force transmission”

6

LOADING THE BONE GRAFT

Significant Loading and Unloading in

Flexion and

Extension

Buffered by Dynamic Implant

DiAngelo and Foley

7

CONSTRUCT FAILURE

MODES OF CONSTRAINED CONSTRUCT FAILURE

Construct Failure Implant Failure Stress Shielding

8

LOAD SHARING versus

LOAD BEARING

THE BIOMECHANICS

OF FAILURE

9

10

11

12

Cervical Spondylosis

Myelopathy Deformity

ENCROACHMENT TETHERING

REPETETIVE TRAUMA

SURGICAL

STRATEGIES

13

Focus on Deformity

Why?

Neck Pain Myelopathy

Improve Short-Term Success Decrease End Fusion Changes Improve Long-Term Success

Kyphosis Trapezius Sign

Intra-Operative Deformity Correction

Ventral

vs Dorsal

14

Its all about the leverage!!!

Dorsally, leverage is VEEEERRRYYYY

difficult to achieve!!!

Polyaxial Screws

Exception

Capital Flexion and

Extension

15

16

Gravity

Could Correct Ventrally

Because Facets not Ankylosed

17

Its all about leverage!!!

18

Fixation Follows!!!

19

CSM

Repetitive Trauma and

Tethering / Distraction

20

Could Correct Dorsally

Because Relaxed/Released

21

22

Correct at

Another Level

23

24

STAGING?

25

26

Dorsal Releasing (eg Osteotomy)

Fixation

Ventral Releasing Fixation

Correction

Posture Posture

Back Pain

Back Pain

Fundamentals Fundamentals

27

Rocket Science Rocket Science Innovation Drives Research

Innovation Drives Research

Research Drives Innovation Research Drives Innovation

Fusion

Motion Preservation Technologies

Assessment Strategies

Fusion

Motion Preservation Technologies

Assessment Strategies

“The treatment of back pain falls primarily in the non-operative domain. Surgical management for such a malady, therefore, should be a relative exception.”

Deep, Agonizing Pain Worsened with Loading

Improved with Unloading

28

MOTION

PRESERVATION

Interspinous Spacers (Motion Preservation)

Interspinous Spacers (Motion Preservation)

X-Stop (St. Francis Medical) X-Stop (St. Francis Medical)

Wallis (Spine Next, Bordeaux) Wallis (Spine Next, Bordeaux)

Diam (Medtronic) Diam (Medtronic)

Dynamic Stabilization (Motion Preservation or Fusion Enhancement)

Dynamic Stabilization (Motion Preservation or Fusion Enhancement)

29

TDA

Pain Generator ???????????

30

DISCOGENIC BACK PAIN

?

DYSFUNCTIONAL MOTION SEGMENT

Mechanical BACK PAIN

31

Deep, Agonizing Pain Worsened with Loading

Improved with Unloading

32

ATTRIBUTES

Replicates Anatomy Replicates Motion

Replicates Mechanics Minimal Complications

Acceptable Revision Strategies Longevity

Retardation of Degenerative Changes Symptom Relief

REPLICATES ANATOMY

Disc Interspace Height Disc Interspace Angle

REPLICATES MOTION

Translation Angular

Axial

Coronal

Sagittal

In Which Plane?

Axial? Translational?

Bending?

Along Which Axis? Axial

Coronal Sagittal

REPLICATES MECHANICS

Stiffness Shock Absorption

Creep

33

Stiffness

Unconstrained Semiconstrained

Constrained

In Which Plane?

Axial?

Translational?

Bending?

Motion Segment Stiffness

Unconstrained Semiconstrained Constrained

Disengagement Engagement

Clinical

Application

Clinical

Application

34

Structural Pathology

Anatomic

Mechanical

Structural Pathology

Anatomic

Mechanical

The

Biomechanical Correlate

of

Mechanical Back Pain

Widened Neutral Zone

The

Biomechanical Correlate

of

Mechanical Back Pain

Widened Neutral Zone

FLEXION AND EXTENSION Unconstrained Semiconstrained Constrained

Disengagement Engagement

FLEXION AND EXTENSION Unconstrained Semiconstrained Constrained

Disengagement Engagement

Mechanical Back Pain

Begins as Biochemical / Nutrient – Related

then

Becomes Structural

Mechanical Back Pain

Begins as Biochemical / Nutrient – Related

then

Becomes Structural

35

Mechanical Surgery ~

Mechanical Pathology

Mechanical BACK PAIN

Deep, Agonizing Pain Worsened with Loading

Improved with Unloading

36

IAR

1.

Appropriate Loading

Bone Supporting Soft Tissues

37

MINIMAL COMPLICATIONS

Short Term Vascular

Neurological Expulsion

Long Term

Expulsion Subsidence

Failure of Ingrowth Osteophyte Formation

Osteointegration

Bony Ingrowth

M

a

t

u

r

e

b

o

e

w

/

o

s

t

e

o

c

y

t

e

s

O

s

t

e

o

n

w

/

e

m

e

n

t

l

i

n

e

s

Paragon

X20

78 Week

PP-PMMA PP-PMMA

38

:

Large vs Small Pore Size Short - Long Term Fixation

Short Term Fixation

Intermediate Term Fixation

Long Term Fixation

ACCEPTABLE REVISION STRATEGIES

Dorsal Fusion and Instrumentation Removal?

LONGEVITY ?

RETARDATION OF DEGENERATIVE CHANGES ?

39

SYMPTOM RELIEF

Anecdotal Information Non-Inferiority ~ Inferior Device

Bias Investigator Bias

Patient Selection Bias Winner-Loser Bias

Case Against

Cervical

TDA

Case Against

Cervical

TDA

Adjacent Segment

Degeneration

Adjacent Segment

Degeneration

Adjacent Segment

Disease

Adjacent Segment

Disease

ASDeg

vs

ASDis

ASDeg

vs

ASDis

40

3 Studies – Ave Followup 4.5 Years

Prevelance of ASDis – 9-17%

Prevelance / years followed Annual Incidence of ASDis Requiring Surgery

1.5 - 4% / year

Hilibrand AS, Robbins M. Adjacent segment degeneration and adjacent segment disease: the

consequences of spinal fusion? The Spine Journal 4 (2004) 190S-194S

3 Studies – Ave Followup 4.5 Years

Prevelance of ASDis – 9-17%

Prevelance / years followed Annual Incidence of ASDis Requiring Surgery

1.5 - 4% / year

Hilibrand AS, Robbins M. Adjacent segment degeneration and adjacent segment disease: the

consequences of spinal fusion? The Spine Journal 4 (2004) 190S-194S

846 Patients – PLF – f/u 2.8 yrs

Prevalance ASDis - 9%

Annual Incidence ASDis - 3%

Henderson CM, Hennessy RG, Shuey HM, Shackelford EG. posterior lateral foraminotomy as an

exclusive operative technique for cervical radiculopathy: a review of 846 consecutively operated

cases. Neurosurgery 1983: 13(5):504-12

846 Patients – PLF – f/u 2.8 yrs

Prevalance ASDis - 9%

Annual Incidence ASDis - 3%

Henderson CM, Hennessy RG, Shuey HM, Shackelford EG. posterior lateral foraminotomy as an

exclusive operative technique for cervical radiculopathy: a review of 846 consecutively operated

cases. Neurosurgery 1983: 13(5):504-12

253 Patients ACD w and w/o F

f/u 3 years

Prevelance ASDis – 7%

Annual Incidence ASDis – 2.5%

No Difference - w and w/o

Lunsford LD, Bissonette DJ,Jannetta PJ, Sheptak PE, Zorub DS. Anterior surgery for

cervical disc disease, part 1: treatment of lateral cervical disc herniation in 253 cases. J

Neurosurg 1980:53:1-11

253 Patients ACD w and w/o F

f/u 3 years

Prevelance ASDis – 7%

Annual Incidence ASDis – 2.5%

No Difference - w and w/o

Lunsford LD, Bissonette DJ,Jannetta PJ, Sheptak PE, Zorub DS. Anterior surgery for

cervical disc disease, part 1: treatment of lateral cervical disc herniation in 253 cases. J

Neurosurg 1980:53:1-11

Hilibrand AS, Carson GD, Palumbo MA, Jones PK, Bohlman

HH. Radiculopathy and myelopathy at segments adjacent to

the site of a previous anterior cervical arthrodesis. J Bone Joint

Surg 1999; 81A(4):519-28

Hilibrand AS, Carson GD, Palumbo MA, Jones PK, Bohlman

HH. Radiculopathy and myelopathy at segments adjacent to

the site of a previous anterior cervical arthrodesis. J Bone Joint

Surg 1999; 81A(4):519-28

409 ACDF f/u 2-21 years

Prevelance ASDis – 14%

Annual Incidence ASDis – 3%

Risk Factors Neural Element Compression at Adjacent Levels

Surgery Adjacent to C56 or C67

Multilevel ACDF Lower Incidence of ASDis

(12% vs 18%, p<0.001)

409 ACDF f/u 2-21 years

Prevelance ASDis – 14%

Annual Incidence ASDis – 3%

Risk Factors Neural Element Compression at Adjacent Levels

Surgery Adjacent to C56 or C67

Multilevel ACDF Lower Incidence of ASDis

(12% vs 18%, p<0.001)

41

Back to the Basics

42

What Happens in

10, 15, 20 ….. Years?