BIOMECHANICS OF
SPINAL INSTRUMENTATION
SPINAL INSTRUMENTATION
Goal:
–
To maintain anatomic alignment of injured spinal segments by
sharing the loads acting on the spine until a solid biologic
fusion takes place
Prerequisite Understandings:
–
–
–
Pathology
Spine Biomechanics
Biomechanics of implant constructs
SPINAL INSTRUMENTATION
ADVANTAGES
- Promote fusion
- Enhance early mobilization
DISADVANTAGES
- Adverse effects on the adjacent segments
- Stress-shielding effects on the stabilized segment
- Hardware Failure
HARDWARE FAILURE
Breakage of Rods or Screws
Hook Dislodgement
Loosening
Screw Pull-out
in cases of extreme Osteoporosis
Biomechanical Consideration Factors
Biomechanical Strength (Static and Dynamic)
–
–
–
–
Impact strength
Surgical construct strength
Metal-metal interface strength
Bone-metal interface strength
Stability
–
Segmental stiffness/flexibility
Load Sharing
–
–
–
Implant survival
Stress shielding
Fusion rate and quality
Implant Design Factors
Mechanical strength:
Profile:
–
Low profile preferred particularly in anterior instrumentation
User Friendliness:
–
–
–
–
Top loading
Poly-axial screw insertion
Lateral-medial ajustment
Easy-to-use surgical insruments
Versatility:
Biomechanical Consideration Factors
Fretting Corrosion:
–
Any damage to the material that takes place at the edge of
contacting parts or within the local contact area
• Associated with wear, surface damage and accumulated debris
• The rate of fretting corrosion is mostly governed by micromotion between
component surfaces within an interconnection
–
–
Cause of late infection and loosening of interconnections
Greater tendency for:
• designs with lower strength interconnections
• Improper assembly such as misalignment and incomplete seating
• Higher applied loads
PULL-OUT AND CYCLIC FAILURES OF
THE ANTERIOR VERTEBRAL SCREW
FIXATION IN RELATION TO BONE
MINERAL DENSITY
Howard S. An, M.D.
Tae-Hong Lim, Ph.D.
Christopher Evanich, M.D.
Toru Hasegawa, M.D.
Kaya Y. Hasanoglu, M.D.
Linda McGrady, B.S.
Anterior Spinal Instrumentation
Fractures
Tumors
Correction of Deformities
Hardware Failure
Breakage of rods or screws
Screw loosening
Screw pullout
in cases of extreme Osteoporosis
PREVIOUS STUDIES
Screw Pullout Strength in relation to Design
Variables and Insertion Methods of Pedicle
Screw
- Zindrick et al., 1986; Krag et al., 1986; Skinner et al., 1990; Daftari et al., 1992
Screw Pullout Strength in relation to Bone
Mineral Density (BMD)
- Coe et al., 1990; Soshi et al., 1991; Yamagata et al.,
1992; An et al., 1993
PREVIOUS STUDIES
Loosening of the Pedicles Screw in relation to
Design Variables and Insertion Methods of
Pedicle Screw
- Zindrick et al., 1986
No BMD Measurement
Correlation between
Anterior Vertebral Screw Fixation Failure
And
Bone Mineral Density (BMD)
Of the Verebral Body
PURPOSE OF THE STUDY
Relationships between BMD and:
- Pull-out Strength
- Cyclic Screw Loosening
PART I: Pull-out Test
(6 fresh-frozen lumbar spines)
PART II: Cyclic Loosening Test
(5 fresh-frozen lumbar spines)
MEASUREMENT OF
BMD (g/cm2)
Dual Energy X-ray Absorptiometry (DEXA)
- Scan Speed: 60 mm/sec
- Resolution: 1 x 1 mm
Lateral view of each lumbar spine
Kaneda Screw
(6.5 mm diameter; 55 mm long)
Measurement of Torque (Nm) and
Vertebral Width (mm) for screw insertion
Pull-out Strength Test
Screw was pulled out
along its long axis.
Loading Rate: 10 mm/min
(Displacement Control)
DATA ANALYSIS
(Pull-out Strength Test)
Pull-out Strength
– Maximum
pull-out force in the load
displacement curve
Regression Analyses
Cyclic Loading Test
to Induce Screw Loosening
Cyclic loading was applied to the screw in the
cephalad-caudal direction using an MTS
machine.
Loading Frequnecy: 0.5 Hz
Loading Amplitude: 200 N 
100 N 
DATA ANALYSIS
(Cyclic Loosening Test)
Number of Loading Cycles (NLC) to Induce the
Screw Loosening:
–
when the displacement > the displacement at the first peak
load + 1 mm.
Regression analysis
NLCs for
–
specimens with BMD < 0.45 vs. BMD > 0.45.
RESULTS
Means (SD) of
Measured Parameters
(Pull-out Test)
BMD:
Torque:
Depth:
Pull-out Strength:
0.58 g/cm2 (STD 0.14)
0.86 Nm (STD 0.2)
41.0 mm (STD 3.28)
211 N (STD 124)
CORRELATION COEFFICIENTS
(Pull-out Test)
Pull-out
Strength
BMD
Torque (T)
1.0
0.85
0.47
No relation
BMD
-
1.0
0.58
No relation
Torque (T)
-
-
1.0
No relation
Width (W)
-
-
-
1.0
Pull-out
Strength
Width (W)
CORRELATION
Pull-out Strength vs. BMD
R = 0.83 (p < 0.0002)
Pull-out Strength = -226 + 774 x BMD
Pull-out Strength vs. BMD
Pull-out Force (N)
500
400
300
200
100
0
0.3
0.4
0.5
0.6
0.7
0.8
BMD (g/cm2)
Means (SD) of
Measured Parameters
(Cyclic Loosening Test)
BMD:
Torque:
NLC:
0.32 g/cm2 (STD 0.10)
6.9 Kg-cm (STD 2.8)
149 (STD 234)
(5 to 960)
CORRELATION
(Cyclic Loosening Test)
NLC vs. BMD
Second Order Polynomial Relationship
2
NLC = -1190 BMD + 3168 BMD
R = 0.80 (p < 0.01)
NLC vs. BMD
1000
NLC
800
600
400
200
0
0.3
0.4
0.5
BMD (g/cm/cm)
0.6
0.7
CORRELATIONS
(Cyclic Loosening Test)
NLC vs. Screw Insertion Torque:
Second Order Polynomial
R = 0.68 (p < 0.01)
BMD vs. Screw Insertion Torque:
Linear
R = 0.51 (p < 0.02)
MEAN NLCs (SD)
12 Specimens with BMD<0.45:
18.0 (20.1)
11 Specimens with BMD>0.45:
270.8 (278.8)
These were significantly different.
(p=0.003)
BMD:
–
–
correlation with pull-out strength as well as cyclic loosening
failure of the anterior vertebral screw
a useful means to evaluate the severity of osteoporosis
DEXA: measurement of BMD
- Low-radiation dose
- Short scanning time
- Increased image resolution
- Improved precision
CONCLUSION
Quantitative assessment of BMD using a
DEXA unit may be a good predictor of
anterior vertebral screw fixation failure.
BMD < 0.45 g/cm2 may be the critical
value for osteoporosis in screw loosening.
PREDICTION OF FATIGUE
LOOSENING FAILURE OF THE
PEDICLE SCREW FIXATION
Tae-Hong Lim, Ph.D.
Lee H. Riley, III, M.D.
Howard S. An, M.D.
Linda M. McGrady, B.S.
John Klein, M.S.
Pullout Strength of the
Pedicle Screw
In relation to Pedicle Screw Design Variables
- Zindrick et al., 1986; Krag et al., 1986; Skinner -et al.,
1990; Daftari et al., 1992
Effects of screw insertion torque, screw hole
preparation method, and screw angulation
- Skinner et al. 1991; Daftari et al. , 1992; Zdeblick et al. ,
1993
In relation to Bone Mineral Density (BMD)
- Coe et al., 1990; Soshi et al., 1991; Yamagata et al., 1992
Loosening Failure of the
Screw Fixation
Pedicles Screw Loosening in relation to Design
Variables and Insertion Methods of Pedicle Screw
- Zindrick et al., 1986
Anterior Vertebral Screw Loosening in relation to the
Bone Mineral Density of the Vertebral Body
- Lim et al., 1994
PURPOSE
Fatigue Loosening
of the Pedicle Screw
in relation to
Bone Mineral Density
of the Vertebral Body
PURPOSE
Fatigue Loosening
of the Pedicle Screw
in relation to
Pedicle Size and Screw Insertion Torque
MATERIALS
Three fresh frozen human lumbar
spines (L1 - L5) were used in this
study.
Anterior-posterior and lateral
radiographs were taken to exclude
spines with gross pathology.
MEASUREMENT OF
2
BMD (g/cm )
Dual Energy X-ray Absorptiometry
(DEXA)
- Scan Speed: 60 mm/sec
- Resolution: 1 x 1 mm
Lateral view of each lumbar spine
Pedicle Size Measurement
– Pedicle Height (PH): Long Axis
(cephalad-caudal direction)
– Pedicle Width (PW): Short Axis (mediallateral direction)
Pedicle Screw Placement
– A 3.5 mm drill hole and 6.25 mm tapper
– A 6.25 mm Steffee Screw (40 mm long)
Screw Insertion Torque (TQ)
– Measured using a Torque Wrench
Cyclic Loading Test
to Induce Screw Loosening
Cyclic loading was applied to the screw
in the cephalad-caudal direction using
an MTS machine.
Loading Frequency: 0.5 Hz

100 N 
Loading Amplitude: 200 N
Cyclic Test Set-up
MTS
Load Cell
MMA
Fixture
RAM
DATA ANALYSIS
Number of Loading Cycles (NLC) to
Induce the Screw Loosening:
–
when the displacement > the displacement at the first
peak load + 1 mm.
Regression analysis
–
Relationships between NLC, BMD, PH, PW, and TQ
Means (SDs) of
the Measured Parameters
BMD:0.465 g/cm2 (0.187)
PH: 15.57 mm (4.97)
PW: 12.08 mm (2.44)
TQ: 9.25 Kg-cm (3.14)
NLC: 300 (377)
Correlation between the
Measured Parameters
BMD
PH
PW
TQ
NLC
BMD
1.0
-
-
-
-
PH
0.54
1.0
-
-
-
PW
*0.07
0.51
1.0
-
-
TQ
*0.44
0.59
*0.11
1.0
-
0.58
0.72
*0.42
0.50
1.0
NLC
* indicates relationship with no statistical significance (p > 0.05).
Linear Relationship between NLC
and BMD
1200
1000
800
NLC
600
400
200
0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
BMD (g/cm/cm)
0.9
Linear Relationship between NLC and PH
1200
1000
800
NLC
600
400
200
0
5
10
15
20
PH (mm)
25
Multiple regression analysis demonstrated a
significant correlation between NLC and
other measured parameters.
(R = 0.79, p = 0.02)
NLC = 775.8 x BMD + 22.4 x PH 44.3 PW + 15.5 TQ - 17.2
The stepwise regression analysis
revealed that PH was the most significant
predictor of pedicle screw loosening.
DISCUSSION
Screw loosening: a significant complication in
spinal fixation.
Pedicle screw loosening was experimentally
induced under cyclic loading in this study.
Application of the cyclic load in the cephaladcaudal direction on the screw
–
in order to simulate the load transferred to the screw by the connecting rod or
plate
DISCUSSION
NLC was significantly correlated with BMD, PH, and
TQ, while PH was the most significant predictor of
screw loosening failure.
–
Honl et al. also found that the amount of loosening correlated more with
pedicle geometry than BMD. (Proceedings, 2nd World Congress of
Biomechanics, 1994)
Limitations:
–
–
Small number of Specimens;
No studies on the optimal ratios of the screw diameter to PH for the best
fixation strength
A significant relationship between the
fatigue loosening of the pedicle screw and
PH and BMD was found.
The assessment of the pedicle geometry as
well as the vertebral body BMD may
provide valuable preoperative information
to predict the early loosening failure of the
pedicle screw fixation.
BIOMECHANICAL EVALUATION
OF ANTERIOR SPINAL
INSTRUMENTATION
Jae Won You, M.D., Ph.D.
Tae-Hong Lim, Ph.D.
Howard S. An, M.D.
Jung Hwa Hong, M.S.
Jason M. Eck, B.S.
Linda McGrady, B.S.
Anterior Spinal Instrumentation
Plate Type
–
Syracuse I-Plate, CASP, Z-Plate, University Plate, etc.
Rod Type
–
Zielke, Kaneda, TSRH instrumentation, etc.
Purpose of the Study
Compare the biomechanical Stability
of anterior fixation constructs,
particularly comparing the use of
plate vs. rod type anterior fixators.
METHODS
Specimen Preparation
20 Fresh Calf Lumbar Spines (L2-L5)
–
The lumbar spine dissected free of muscles, leaving ligaments, capsules,
and disc intact
L5 fixed to the loading rig, and L2 mounted in
an unconstrained loading setup.
3 markers reflecting infra-red light attached to
L3 and L4 vertebral body.
Loading Setup
Pure Moments:
–
using dead weights, unconstrained
6 Directions:
–
FLX, EXT, RLB, LLB, RAR, and LAR
Incremental Loading:
–
to a maximum of 6.4 Nm in 6 steps
Motion Measurements
3-D Motion Analysis System:
3 Vicon Cameras (Oxford, England)
– Micro-Vax Workstation (DEC, Maynard, MA)
–
Marker position data were transformed to
rotational angles in FLX/EXT, LBs, and
ARs.
Testing Constructs
Intact Spine
Anterior fixation with an interbody graft
following total discectomy and endplate
excision of L3-L4
–
PMMA block was used as a bone graft
Anterior fixation only
Tested Anterior Fixators
Plate Type:
–
–
University Anterior Plating System (UNIV): AcroMed Corp.
Z-Plate (ZP): Danek, Inc.
Rod Type:
–
–
Kaneda device with transfixator (KAN): AcroMed, Corp
TSRH vertebral body screw construct: Danek, Inc.
Data Analyses
Stabilizing Effect:
–
% change of motion as compared with the intact motion
Role of Graft:
–
% change of motion between with and without graft cases
ANOVA with Tukey’s multiple mean
comparison
Results
Stabilizing Effect compared with Intact
Motion (Anterior Fixation with Graft)
In Axial Rotation:
–
–
Similar to the Intact Motion in UNIV, ZP, TSRH Fixation
Significantly stabilized by KAN (p < 0.05)
In Lateral Bending and Flexion:
–
–
Singnificantly stabilized by all tested devices (p < 0.05)
No significant difference between devices
Stabilizing Effect compared with Intact
Motion (Anterior Fixation with Graft)
*
Stabilizing Effect compared with Intact
Motion (Anterior Fixation with Graft)
In Extension:
–
–
Singnificantly stabilized by all tested devices (p < 0.05)
Stabilizing effect of KAN > ZP (p = 0.05), but no
difference between any other devices
Stabilizing Effect compared with Intact
Motion (Anterior Fixation with Graft)
*
Stabilizing Effect compared with Intact
Motion (Anterior Fixation only)
In Axial Rotation:
–
–
Similar to the intact motion in UNIV, ZP, TSRH Fixation
Significantly stabilized by KAN (p < 0.05)
In Lateral Bending:
–
–
Singnificantly stabilized by all tested devices (p < 0.05)
No difference between devices (p > 0.05)
Stabilizing Effect compared with Intact
Motion (Anterior Fixation only)
In Flexion:
–
–
Significantly stabilized by UNIV, KAN, TSRH(p < 0.05), but
not in ZP fixation
Stabilizing Effect of ZP < KAN (p = 0.01) and TSRH (p = 0.05)
In Extension:
–
–
Singnificantly stabilized by KAN (p < 0.05)
Stabilizing Effect of KAN > TSRH (p = 0.005)
Stabilizing Role of Interbody Graft
In Axial Rotation:
–
No significant stabilizing effect
In Lateral Bending:
–
Significantly increase the stabilizing effect in all tested cases
In Flexion and Extension:
–
Significantly increase the stabilizing effect in ZP fixation only
Stabilizing Role of Interbody Graft
DISCUSSION
Biomechanical Spinal Implant
Testing Protocols
Measuring Construct Stiffness in Axial
Load, Torsion, Flexion, and Extension in
nondestructive manner
–
Ashmann et al., 1989
Measuring flexibility in terms of uncostrained
motion under pure moment application
–
Panjabi, 1988
Previous Studies
Abumi et al. 1989:
–
Kneda device provides good stability in FLX and EXT, but
not in AR.
Gaines et al., 1991:
–
Kaneda device provided good stabilization and resistance to
torsional and lateral bending loads.
Zdeblick et al., 1993:
–
Kaneda and TSRH devices are effective in restoring acute
stability to the lumbar spine after corpectomy.
In this study
Measure the unconstrained motion under pure
moments application.
Findings:
–
–
Kaneda device provides effective stabilization in all directions,
particularly with the use of interbody graft.
UNIV, ZP, TSRH are also effective in stabilizing LB, FLX, and
EXT with an interbody graft, although they restore to the
intact motion in AR.
CONCLUSION
Modern plate and rod type anterior
fixation devices are effective in restoring
stability of anterior and middle column
defects in flexion, extension, and lateral
bending beyond the intact specimen.
CONCLUSION
Interbody grafting may be important in
anterior instrumentation for effective
stabilization immediately after surgery.
Anterior fixation restores torsional stability
relatively less, although Kaneda device was
the best among the tested devices.
Percentage AR Motion Changes
compared to the Intact Case
Percentage LB Motion Changes
compared to the Intact Case
Percentage FLX Motion Changes
compared to the Intact Case
Percentage EXT Motion Changes
compared to the Intact Case
Effects of Crosslinking Devices
in Pedicle Screw
Instrumentation:
A Biomechanical and Finite Element
Modeling Study
Tae-Hong Lim, Ph.D.
Howard S. An, M.D.
Jason C. Eck, B.S.
Jae Y. Ahn, M.D.
PURPOSE
To evaluate the stabilizing effect of
transfixation in flexion, extension, lateral
bending, and axial rotation modes
To determine the optimal position of
transfixation to achieve greater stabilizing
effect
Flexibility tests
Unstable Calf Spine Model
Finite element studies
FLEXIBILITY TESTS
10 Ligamentous calf spines
- 5 spines for L3-L4 stabilization
- 5 spines for L2-L4 stabilization
Maximum pure moment of 6.4 Nm in FLX,
EXT, LB, and AR
Vicon 3-D motion analysis system was used
to measure the resultant segmental
motions.
Tested Constructs
One Segment (L3-L4) Stabilization
- Intact
- Instrumentation without Transfixation after total discectomy
- Instrumentation with Single Transfixation
Tested Constructs
Two Segment (L2-L4) Stabilization
- Intact
- Instrumentation without Transfixation after L3
corpectomy
- Instrumentation with Single Transfixation at the
Middle Points
- Instrumentation with Double Transfixation at the
proximal and distal 1/3 points
Finite Element Studies
To determine the optimal position of
transfixation
Boundary and Loading Conditions:
- Nodes in lower vertebra were held fixed.
- Axial compression (445 N), FLX, EXT, LB, and AR Moments (5
Nm) at the middle point of the vertebra element
MENTAT II Finite Element Analysis
Package (Marc Analysis Research Corp.)
Finite Element Models
Axial Compression force
and Moment
ISOLA System
Transfixators
Vertebrae
Fixed Nodes
One Segment Stabilization
Two Segment Stabilization
RESULTS
Rotational Motions (deg) responding to
Applied Moments of 6.4 Nm in One Segment
Stabilization
p<.01
Rotational Motions (deg) responding to
Applied Moments of 6.4 Nm in Two Segment
Stabilization
p = .01
p = .06
Single Transfixation
(FE Model Predictions)
Compared with no TF case:
No additional stabilizing effect in axial
compression, FLX, and EXT
In one segment stabilization:
- Up to 10.8% LB motion decrease
- Up to 25.0% AR motion decrease
In two segment stabilization:
- Up to 13.0% LB motion decrease
- Up to 18.2% AR Motion decrease
Stabilizing Effect of 1 TF in AR Mode with
respect to the implanting Positions
2 Segment Stabilization
1 segment stabilization
Double Transfixation
(FE Model Predictions)
Compared with no TF case:
No additional stabilizing effect in axial
compression, FLX, and EXT
Up to 14.6% LB motion decrease
Up to 30.3% AR motion decrease
Stabilizing Effect of 2 TF in AR Mode with
respect to the Implanting Positions
CONCLUSION
Flexibility Tests:
Significant stabilizing effect of TF in AR
only as shown in previous studies
2 TFs can provide more AR stability than
one TF.
With 2 TFs, the restored AR stability is
similar to the intact specimen.
CONCLUSION
Finite Element Model Predictions:
Model prediction correlated well with the
experimental results.
The greatest AR stability can be obtained
by implanting two TFs, one at the proximal
1/8 points and the other at the midpoints of
the longitudinal rods in two level
stabilization.
A BIOMECHANICAL COMPARISON
BETWEEN MODERN ANTERIOR
VERSUS POSTERIOR PLATE
FIXATION OF UNSTABLE CERVICAL
SPINE INJURIES
Tae-Hong Lim, Ph.D.
Howard S. An, M.D.
Young Do Koh, M.D.
Linda M. McGrady, B.S.*
Unstable Cervical Spine
Injuries
Flexion-distraction Injury
–
–
Three column injury
Unilateral or bilateral facet dislocation
Burst Fracture of the Vertebral Body
–
produce severe instability by destroying the anterior and middle column of
the cervical spine
Surgical Treatments
Closed or open reduction and fusion with
Anterior Fixation
–
anterior plate
Posterior Fixation
–
–
Wiring
Posterior lateral mass screw system
Combined Fixation
Surgical Treatment Goals
Reduction
Maintenance of alignment
Early rehabilitation
Enhancement of fusion
Decreased use of external orthoses
Biomechanical stability provided by various fixation
methods is an important factor to achieve these goals.
Purpose of the Study
To determine and compare the
biomechanical stability provided by
modern posterior, anterior, and
combined screw-plate fixation in
flexion-distraction and corpectomy
models simulated in human
cadaveric cervical spines.
METHODS
Flexibility Tests
10 cadaveric cervical spines (C2-T1)
–
–
Group I (n = 5): One-level 3-column injury at c4-5
Group II (n =5): Corpectomy of C5 vertebral body
Pure moment of 2.45 Nm was achieved in five steps using
dead weights in FLX, EXT, LB, and AR directions.
Vicon 3-D motion analysis system was used to measure
the resultant segmental motions.
Stability was quantified in terms of the percent changes in
rotational motions of the stabilized segment compared
with those in the intact segment.
Tested Constructs
Group I (Flexion Distraction Model)
–
–
–
–
–
Intact
Posterior fixation without interbody grafting after 3-column injury
Posterior fixation + an interbody graft (PMMA block)
Combined anterior and posterior fixation + PMMA block
Anterior fixation + PMMA block
3-Column injury was made by cutting
supraspinous, interspinous, capsular and
posterior longitudinal ligaments, ligamentum
flavum, and the posterior half of the
intervertebral disc.
Tested Constructs
Group II (Corpectomy Model)
–
–
–
–
–
Intact
Posterior fixation without interbody grafting after corpectomy
Posterior fixation + an interbody graft (PMMA block)
Combined anterior and posterior fixation + PMMA block
Anterior fixation + PMMA block
C5 vertebral body was removed (corpectomy) to simulate
a very unstable burst fracture.
Axis plate and Orion plate systems (Danek Inc.,
Memphis, TN) were used for posterior and anterior
fixation, respectively.
Axial Rotation
(Flexion-Distraction Model)
The posterior fixation (cases 1, 2, and
3) decreased the motion significantly.
Anterior fixation alone provided the
rigidity similar to the intact specimen.
A
x
i
a
l
R
o
t
a
t
i
o
n
3
0
0
(fracture-dislocation)
2
0
0
1
0
0
%ChangeofMti
0
1
0
0
1
2
3
I
n
s
t
r
u
m
e
n
t
a
t
i
o
n
4
Lateral Bending
(Flexion-Distraction Model)
Posterior fixation with or without PMMA block
reduced motion as compared with the intact case,
although the differences were not statistically
significant.
When anterior fixation was used alone, the
lateral bending became significantly greater than
the intact motion (p < 0.05).
Lateral Bending
(fracture-dislocation)
% Change of Motion
300
200
100
0
-100
1
2
Instrumentation
3
4
Flexion
(Flexion-Distraction Model)
The posterior fixation (cases 1, 2, and 3) significantly
reduced the flexion motion from the intact case.
No significant difference was found between the
posterior fixation constructs.
In case of anterior fixation (case 4), the flexion motion
was significantly larger than the intact motion (128%, p
< 0.01).
F
l
e
x
i
o
n
3
0
0
(fracture-dislocation)
2
0
0
1
0
0
%ChangeofMti
0
1
0
0
1
2
3
I
n
s
t
r
u
m
e
n
t
a
t
i
o
n
4
Extension
(Flexion-Distraction Model)
Extension motion was significantly less than the intact
motion in the combined anterior posterior fixation only
(case 3).
There were no significant differences between the posterior
constructs with and without PMMA block.
Extension motion was also not significantly different
between the posterior and anterior fixation constructs.
E
x
t
e
n
s
i
o
n
3
0
0
(fracture-dislocation)
2
0
0
1
0
0
%ChangeofMti
0
1
0
0
1
2
3
I
n
s
t
r
u
m
e
n
t
a
t
i
o
n
4
Axial Rotation
(Corpectomy Model)
Axial rotational motion was significantly
reduced from the intact motion only when the
combined anterior and posterior fixation was
used (case 3).
Axial rotational motions in other constructs were
not significantly different from the intact motion.
Axial Rotation
(Corpectomy)
90
70
50
30
10
-10
-30
-50
-70
-90
-110
-130
1
2
3
Instrumentation
4
Lateral Bending
(Corpectomy Model)
Lateral bending motions in posterior fixation
cases 1, 2, and 3 were significantly less than the
intact motion.
Anterior fixation alone (case 4) did not provide
the rigidity beyond the intact specimen even with
the anterior grafting material.
L
a
t
e
r
a
l
B
e
n
d
i
n
g
(corpectomy)
%ChangeofMti
9
0
7
0
5
0
3
0
1
0
1
0
3
0
5
0
7
0
9
0
1
1
0
1
3
0
1
2
3
I
n
s
t
r
u
m
e
n
t
a
t
i
o
n
4
Flexion
(Corpectomy Model)
Flexion motion was significantly reduced from
the intact motion in all tested constructs (p <
0.05).
The combined anterior and posterior fixation
provided most rigid fixation than the other
constructs.
F
l
e
x
i
o
n
(corpectomy)
%ChangeofMti
9
0
7
0
5
0
3
0
1
0
1
0
3
0
5
0
7
0
9
0
1
1
0
1
3
0
1
2
3
I
n
s
t
r
u
m
e
n
t
a
t
i
o
n
4
Extension
(Corpectomy Model)
Extension motion was significantly less
than the intact motion in all tested
constructs except for the posterior
fixation with PMMA block (case 2).
E
x
t
e
n
s
i
o
n
(corpectomy)
%ChangeofMti
9
0
7
0
5
0
3
0
1
0
1
0
3
0
5
0
7
0
9
0
1
1
0
1
3
0
1
2
3
I
n
s
t
r
u
m
e
n
t
a
t
i
o
n
4
DISCUSSION
Human cadaveric cervical spines were used to minimize
the anatomical difference between the in vitro and in vivo
cases.
OrionTM and AxisTM plate system were chosen for testing
because they represent relatively rigid modern anterior
posterior plates constructs in cervical spine fixation.
A PMMA block was used to simulate the interbody
grafting technique while eliminating potential variation
in inherent material properties of bone graft.
Limitations
–
–
old age specimens and Bone quality variations
Exclusion of supporting structures such as muscle, fascia, and ligamentum nuche
SUMMARY
Anterior fixation system is biomechanically inferior to
the posterior lateral mass screw-plate fixation,
particularly in flexion-distraction injury in which
posterior ligamentous structures are disrupted.
The anterior fixation seems to be more suitable for
anterior middle column injury where the posterior
ligamentous elements are intact.
Postoperative use of external orthoses should be
considered when the anterior plate is used alone for the
treatment of unstable cervical spine injuries with
disruption of posterior stabilizing elements.
SUMMARY
Anterior fixation was particularly ineffective to prevent
the flexion motion in the flexion-distraction injury model.
This seems to occur mostly due to no rigid connection
between the plate and screws. Rigid connection at this
junction may significantly improve the fixation.
Combined fixation seemed to improve the stability as
compared with anterior or posterior fixation alone.
However, the difference in stability was not always
significant and the combined fixation requires additional
incision. These suggest that a combined anterior and
posterior fixation should be carefully indicated.
Findings of Finite Element Studies
Stress-shielding effects exist in the surgical
construct due to the presence of spinal fixation
devices and healed bone mass.
The fixation device may transmit 9 to 40% of the
applied compression load depending upon the
stabilized technique used.
Semi-rigid fixation may reduce the stressshielding effect and incidence of hardware
failure.
Finite Element Models
Intact L3-L4 (INT)
Bilateral Fixation
(STVSP & PVSP)
Unilateral Fixation
(UVSP)
Important Factors in Finite
Element Modeling
Adequate Assumptions
Use of Accurate Input Data
–
Geometry; material properties; Element types
Proper Solution Procedures
–
Linear or nonlinear analysis; viscoelasticity; poroelasticity,
etc.
Complete Understanding of Results
EFFECT OF INSTRUMENTED
FUSION ON THE BIOMECHANICS
OF ADJACENT SEGMENT:
AN IN VIVO CANINE STUDY
Tae-Hong Lim, PhD, Avinash G. Patwardhan, PhD,*
Jung Hwa Hong, MS, Howard S. An, MD,
Lee H. Riley III, MD, Scott Hodge, MD,* Jason Eck, BS,
Michael M. Zindrick, MD*
Complications in the Segments
Adjacent to Fusion
Degenerated Disc
Stenosis
Segmental Instability
Osteoarthritis
Previous Studies
Ex-vivo and In-vivo Studies of Post-fusion
Mechanics:
–
Increased motion and loads at the adjacent segment.
In-vivo Animal Studies:
–
–
–
Hypoactive metabolism in the adjacent discs;
Significant biochemical changes indicating a degeneration
process.
Changes in the biomechanical response of the adjacent
segment resulting from theses alterations in the disc were not
investigated.
Previous Studies
It is believed that a concentration of load
can cause degeneration at the adjacent
segment.
There are little data on the long-term changes in
the biomechanical properties of the adjacent
segment.
OBJECTIVE
Quantify the long-term changes in the
flexibility and viscoelastic properties of
the intervertebral disc at the adjacent
segment due to the instrumented
lumbar spinal fusion in a canine
model.
In-vivo Canine Model
Flexibility Tests
Relaxation and Cyclic Loading Tests
In-vivo Canine Model Development
Control Group:
–
–
5 adult mongrel dogs (age: 2 yr and weight: 25-30 kg)
Euthanized at the beginning of the study
Experimental Group:
–
–
–
5 adult mongrel dogs (age: 1.5 yr and weight: 25-30 kg)
Posterior instrumented fusion surgery across L5-S1 levels
using ISOLA system (AcroMed, Cleveland, OH)
Follow-up Period: 30 weeks
Flexibility Tests
Ligamentous lumbar spines (L2-S1)
Maximum pure moment of 2.0 Nm applied in
FLX, EXT, LB, and AR
Vicon 3-D motion analysis system was used to
measure the resultant segmental motions at L45, L5-L7, and L7-S1 levels.
Flexibility Tests
Control Group
Control-Intact
– Control-Implant
–
Isola instrumentation following a partial dissection
of the L5-6, L6-7, and L7-S1 facets
Experimental Group
Fusion + Instrumentation
– Fusion Alone
–
After removing Isola instrumentation
Viscoelastic Property Measurement
MTS Load
Cell
Relaxation Test
–A constant
L4-5
VBDisc - VB
MTS Ram
strain of 8% were
applied using a ramp function
in 1 minute.
–Relaxation Period: 1 hour
Viscoelastic Property Measurement
Cyclic Deformation Tests in Axial Compression
Test Parameters:
Test Step
Control
Type
Loading
Frequency
1
Strain
Control
0.1 Hz
2
PreStrain
Maximum
Strain
# of
Cycles
8%
12%
50
1.0 Hz
Time
Measured Parameters
Disc Height (mm)
–
using AP and Lateral radiographs before relaxation and cyclic
deformation tests
Gross Anatomic or Degenerative Changes in the L4-5
Discs
–
dissection of L4-5 VB-Disc-VB unit after mechancial tests
Disc Cross-sectional Area (mm2)
–
using an image processor
3-D angular displacements of L4 relative to L5, L5
relative to L7, and L7 relative to S1 in response to a 2.0
Nm
Measured Parameters
E1 (equilibrium modulus); E2 (instantaneous modulus);
and  (damping coefficient) using a three-parameter
standard linear solid (SLS) model
Relaxation Time Constant ( = /(E1 +E2))
Dynamic Stiffness (MN/m): peak-to-peak load/peak-topeak deformation
–
from the last three load-unload cycles
Hysteresis
–
from the last three load-unload cycles
RESULTS
General Observations
No complications were observed in the
experimental dogs during the follow-up
period.
At sacrifice, the loosening of the rod-screw
connection was observed at the sacrum
level in four out of five experimental dogs.
Flexibility Testing Results
Significant acute stabilization in all loading modes in
the control-implant group.
FLX/EXT motion of the experimental groups was
statistically smaller than the intact control group, but it
was nearly 12 degrees of motion.
No significant difference in LB and AR motion between
the experimental groups and the intact control group.
No solid fusion was achieved across the L7-sacrum
level.
Rotation Angles (deg)
L7-S1 Motions
Control-Intact
28
Control-Implant
21
Fusion alone
Fusion+Implant
14
7
0
FLX
EXT
LB
AR
Flexibility Testing Results
Significant acute stabilization in all loading modes in
the control-implant group.
As compared to the intact control group, motions of the
experimental groups were significantly reduced in all
loading modes.
Solid fusion across the L5-L7 segments was achieved at
the end of 30 weeks of follow-up.
Rotation Angles (deg)
L5-L7 Motions
Control-Intact
16
Control-Implant
Fusion alone
12
Fusion+Implant
8
4
0
FLX
EXT
LB
AR
Flexibility Testing Results
No significant L4-L5 motion changes were found
among the tested groups in all loading modes.
The flexibility of the L4-5 segment (adjacent to
instrumentation and fusion) was affected either
immediately following instrumentation across
L5-sacrum or after the 30 weeks follow-up.
Rotation Angles (deg)
L4-L5 Motions
(Adjacent to Fusion)
Control-Intact
12
Control-Implant
9
Fusion alone
Fusion+Implant
6
3
0
FLX
EXT
LB
AR
SLS Model Parameters
(mean ± SD)
Parameters
Control Group
(n=5)
Experimental Group
(n=5)
E1 (MPa)
1.69 ± 0.38
1.46 ± 0.82
E2 (MPa)
3.40 ± 1.59
3.21 ± 1.99
 (MPa-min)
47.5 ± 22.3
39.7 ± 22.1
/(E1+E2)
9.14 ± 2.55
8.99 ± 1.25
No significant differences in SLS model parameters between the
control and experimental groups
Relaxation of the L4-5 Disc
Cyclic Load-Displacement Testing Results
(mean ± SD)
Measured
Quantity
1 Hz
0.1 Hz
Experimental
Control
Experimental
1.46 ± 0.89
0.98 ± 0.54
1.65 ± 1.33
1.46 ± 0.54
Hysteresis 13.4 ± 4.22
(%)
18.0 ± 8.75
14.4 ± 5.29
14.3 ± 5.34
Dynamic
Stiffness
(MN/mm)
Control
No significant changes in measured quantities between
the control and experimental groups.
Steady State Load-Unload Curves
Observations of L4-5 Discs
No morphological changes as a result of
instrumentation and fusion across L5-sacrum.
Similar Disc Heights:
–
–
3.21 (SD 0.4) mm for the control group
3.17 (SD 0.5) mm for the experimental group
Similar cross-sectional areas
–
–
303.5 (SD 53.2) mm2 for the control group
335.1 (SD 24.3) mm2 for the experimental group
Observations of L4-5 Discs
No visual signs of disc degeneration were
observed in both the control and
experimental L4-5 intervertebral discs.
Discussion
An in-vivo canine model:
–
It has been frequently used for the studies of spinal
instrumentation.
In the dog spine, the L7-sacrum segment is most
mobile in FLX-EXT as compared to the other
lumbar segments. This may have contributed to
the rod loosening at the sacrum level and the
subsequent development of nonunion at the
lumbosacral junction.
Discussion
Results of this study indicate that the solid fusion
across the L5-L7 segments did not alter the
biomechanical properties of the adjacent
segment at 30 weeks postoperatively in the
canine spine.
Fusion may induce changes in the biochemical
and nutritional environments, that may indicate
the degeneration process. However, no
biochemical analysis was performed in this
study.
What causes degeneration at the
adjacent segment?
Increased motion and loads at the the adjacent
segment due to:
–
–
Solid fusion at the lumbosacral junction (Elimination of the
most mobile segment) vs. floating fusion
Loss of lordotic curves due to instrumentation
Metabolic changes due to the presence of screws
in the spine
Genetic factors?
Other factors?
CONCLUSION
Solid fusion across the L5-L7 segments but not solid
fusion across the lumbosacral junction could be
achieved in the dog model even using a rigid Isola
pedicle screw instrumentation across the L5-sacrum
levels.
The flexibility of the L4-5 segment was not changed due
to the instrumentation across L5-sacrum in the control
as well as the experimental dogs.
Fusion of the L5-L7 segments did not significantly
affect the viscoelastic properties of the adjacent disc at
the end of 30 weeks.
Current Findings of Spinal
Instrumentation
Rigid Spinal Instrumentation can
–
–
enhance the solid fusion rate
strong enough to allow early mobilization without serious hardware problems
Construct stability (Anterior vs. Posterior Fixation)
–
–
Posterior fixation is superior to anterior fixation in general.
Both fixation systems can not provide the axial rotational stability beyond the
intact AR stability in most cases.
Need to determine the optimum stability of the
surgical construct
–
Too much rigid fixation may cause various complications particularly at the
adjacent level.
Current Concepts
Minimal Invasive Surgery
–
Laprascopic surgery techniques
Solid Fusion with minimum use of spinal
implants
–
–
–
–
BAK screw system,
Metal cages
Artificial bone graft
use of BMP
Ongoing Debates
Use of rigid or semirigid fixation
Fixation in more or less lordosis
Combination of posterior and anterior
fusion
Others
Design Factors for Improvement
User friendliness
–
fixing all components posteriorly
Rigid Fixation between Components
–
rigid connection between screw and rod (or plate),
particularly important in anterior cervical fixation
Adjustable Connection between
Components
–
–
Poly-axial screw insertion
Allow adjustment in medial-lateral direction as well as in
vertical direction
Devices needs to be developed
Anterior graft device (Cage, Ceramic, etc.)
Biological enhancement such as BMP
Cervical spine fixation device
Instrument for larprascopic surgery
Computer-aided surgery techniques
Scoliosis reduction system
Artificial intervertebral joint
EFFECT OF INTERVERTEBRAL JOINT
STIFFNESS CHANGES ON THE LOAD
SHARING CHARACTERISTICS IN THE
STABILIZED LUMBAR SEGMENT
Tae-Hong Lim, Ph.D.
Department of Orthopaedic Surgery
Rush-Presbyerian-St. Luke’s Medical Center
Chicago, Illinois
Vijay K. Goel, Ph.D.
Department of Biomedical Engineering
The University of Iowa
Iowa City, Iowa
Load Sharing
Ability of the instrumented segment to resist a
fraction of the externally applied load
Clinical Relevance of Load Sharing:
–
–
–
Implant survival
Stress shielding of the instrumented segment
Fusion/healing rate and quality
Load sharing characteristics varies as a function
of:
–
–
Spinal column stiffness
Implant stiffness
LOAD SHARING MECHANISM
F
Spinal
Instrumentation
Fd
Spinal Segment
FP
When stiffnesses of
the spinal column and
the screws are not changed;
Plate Stiffness
Fd
FP
Previous Studies
20% of the axial load through the VSP
plate in case of PLIF:
–
Goel et al., 1988
10%, 20% and 23% of the axial load
through the 4.76mm rods, 6.35mm rods,
and VSP plates
–
Duffield et al., 1993
Spinal Column Stiffness Changes
In STVSP, Edisc of the elements in dashed
area was changed;
– Edisc= 0.0 MPa (Total Nucleotomy)
– Edisc= 4.2 MPa
– Edisc= 8.4 MPa
– Edisc= 1,000 MPa (PLIF)
– Edisc= 2,000 MPa (PLIF)
– Edisc= 3,000 MPa (PLIF)
Stabilization with stainless steel VSP
system was maintained.
Purpose of This Study
To investigate the effects of variations in
the intervertebral joint stiffness as well
as the implant stiffness on the load
sharing characteristics across the
stabilized motion segment
METHODS
Finite Element Analysis
Finite Element Models
Intact L3-L4 (INT)
Bilateral Fixation
(STVSP & PVSP)
Unilateral Fixation
(UVSP)
Implant Stiffness Changes
While keeping a degenerated intact disc:
STVSP:
–
L3-L4 motion segment stabilized by VSP system bilaterally
PVSP:
–
L3-L4 motion segment stabilized by non-metal plates and metal screws bilaterally
Unilateral:
–
L3-L4 motion segment stabilized by VSP system unilaterally
Boundary & Loading Conditions
Boundary Conditions:
–
–
All nodes in the midsagittal plane were not allowed to move in lateral
direction because of assumed midsagittal symmetry, while these
constraints were not imposed on UVSP.
Nodes in the inferior most surface of the VB were constrained not to move
in any direction.
Loading Conditions:
–
–
Axial compressive loads of 200, 413, and 700N
Simulated by a uniform distribution of loads on the superior most surface
of VB and superior facets of L3
Data Analysis
FE models were executed using ANSYS, and
solutions were obtained in an iterative manner.
Output was processed to obtain:
–
–
–
Stresses in various components (axial component of facet contact force)
Axial forces transmitted through facets and VSP plate
Fp = Plate x-are x sum of axial stresses in the middle of the plate
Axial Forces across the VSP Plates
in STVSP Model
Axial Forces (N)
300
250
200
150
100
50
0
0
200
413
700
Applied Axial Compression Load (N)
Axial Forces (N) across the VSP Plates in
Case of Varying Spinal Column Stiffness
450
100%
Axial Force (N)
400
350
300
250
43%
200
38%
150
10.9%
100
10.1%
9.1%
50
0
0 (SD)
4.2
8.4
1000
(PLIF)
Edisc Variations
2000
(PLIF)
3500
(PLIF)
Axial Force (N)
Axial Forces (N) Across the VSP Plates
in Case of Varying Implant Stiffness
180
160
140
120
100
80
60
40
20
0
38 %
24 %
17 %
8%
on facets
Intact
VSP
PVSP
UVSP
Average von-Mises Stresses (MPa)
in Spinal Segment
INT
Cortical Bone
VSP
PVSP
UVSP
1.83 (1.98) 1.51 (2.04) 1.91 (2.21) 1.50 (2.03)
Cancellous Bone 0.11 (0.19) 0.07 (0.15) 0.10 (0.18) 0.08 (0.18)
Disc Annulus
0.21 (0.29) 0.12 (0.18) 0.16 (0.23) 0.15 (0.25)
*Maximum stresses are listed in the parentheses.
Maximum von-Mises Stresses in the
VSP System
Stress (MPa)
VSP
PVSP
UVSP
50
45
40
35
30
25
20
15
10
5
0
Superior Screw
Inferior Screw
Plate
A parametric study was conducted to
investigate the effect of variations in the
intervertebral joint stiffness as well as the
implat stiffness on load sharing
characteristics across the stabilized motion
segment using finite element method.
Limitations:
–
–
Rigid connection was assumed at bone-screw interface and
metal-to-metal interfaces.
Only a few limited range of variations in the spinal column
and implant stiffnesses was simulated without modeling the
property changes over the healing process.
Implications of Model Predictions
Model Predictions for Spinal Column Stiffness
Variations:
–
Importance of preserving the IVD to keep the load on the implants low for
reducing the incidence of hardware failure, particularly in case of severe
discectomy
–
Benefit of using an interbody graft from the perspective of load sharing
–
Minimal adverse effect of a slight decrease in the graft stiffness on the load
sharing when using posterior fixation
–
High potential for subsidence in case of interbody fusion with a too stiff graft even
with rigid fixation
Implications of Model Predictions
Model Predictions for Implant Stiffness Variations:
–
Greater load on the spinal column in case of using a less rigid fixation
–
Significant stress reduction in the implant components and less stress-shielding
effect with decreasing implant stiffness
–
Further studies are required to address how to maintain the surgical construct
with the use of less rigid fixation.