SPINAL FUSION
AND
INSTRUMENTATION
Tae-Hong Lim, Ph.D.
Department of Biomedical Engineering
The University of Iowa
Iowa City, Iowa
Normal Function of the Spine
 Protect
spinal cord and nerves
 Support
load
–
the body weight and external
Stability
 Allow
motion of the body for various
activities
–
Flexibility
Spinal Disorders
 Trauma
–
Fractures, Whiplash injury, etc.
 Tumor
 Infection
& Inflammatory Disease
 Deformity
–
Scoliosis, spondylolisthesis, degenerative lumbar kyphosis, etc.
 Cervical
–
& Low-back Pain
Degenerative disease, such as disc herniation, stenosis,
spondylolisthesis, etc.
Treatment of Spinal
Disorders
 Conservative
–
–
–
Degenerative disease
Stable fracture
Mild deformity
 Surgical
–
–
–
Treatment
Treatment
Failed conservative treatment
Unstable fracture (dislocation)
Progressive deformity
Goals of Spine Surgery
 Relieve
pain by eliminating the source
of problems (decompression)
 Stabilize the spinal segments after
decompression
–
–
–
Restore the structural integrity of the spine (almost
normal mechanical function of the spine)
Maintain the correction
Prevent the progression of deformity of the spine
Spinal Fusion
 Elimination
of segmental movement
across an intervertebral segment by
bone union
–
–
One of the most commonly performed, yet
incompletely understood procedures in spine surgery
Non-union rate: 5 to 35 %
Types of Fusion
Factors for Consideration
in Spine Fusion
 Biologic
–
Factors
Local Factors:
• Soft tissue bed, Graft recipient site preparation, Radiation, Tumor and bone
disease, Growth factors, Electrical or ultrasonic stimulation
–
Systematic Factors:
• Osteoporosis, Hormones, Nutrition, Drugs, Smoking
 Graft
–
Factors
Material, Mechanical strength, Size, Location
 Biomechanical
–
Stability, Loading
Factors
Properties of Graft Materials
Graft
Materials
Osteogenic
Potential
Osetoinduction
Osteoconduction
Autogenous bone
o
o
o
Bone marrow cells
o
?
x
Allograft Bone
x
?
o
Xenograft bone
x
x
o
DBM
x
o
o
BMPs
x
o
x
Ceramics
x
x
o
DBM = Demineralized bone matrix; BMP = Bone morphogenetic proteins
Spinal Instrumentation
 Goals
–
–
–
–
of Spinal Instrumentation:
Correction of deformities or misaligned segments;
Enhancement of solid fusion;
Maintain anatomic alignment until a solid fusion takes
place; and
Allow early mobilization of patients
by providing an immediate stability
Spinal Instrumentation Types
 Implantation
Vertebra
Method:
–
Graft
–
Wiring, Hooks, Screws
Rods vs. Plates
 Spinal
Vertebra
Pedicle screw instrumentation
–
Level:
Cervical, Thoracolumbar
 Position:
–
Anterior vs. Posterior
Instrumentation
Cervical Spine Instrumentation
Cervical Spine Instrumentation
Thoracolumbar Spine Instrumentation
Z-plate (Danek)
Kaneda (AcroMed)
Thoracolumbar Spine Instrumentation
Thoracolumbar Spine
Instrumentation
Operative Techniques
 Patient
–
Positioning:
The intra-abdominal pressure must be minimized to
avoid venous congestion and excess intraoperative
bleeding, while allowing adequate ventilation of the
anesthetized patient.
 Surgical
spine:
–
exposure of the lumbar
Midline incision extended to an additional level
GSFS Implantation Procedure
Screw Hole Preparation


Exposure of the junction
between the pars interarticularis
and transeverse processes
Pedicle entrance point is at the
crossing of two lines
–
–
–
Vertical line: 2-3 mm lateral from the
pars and slants slightly from L4 to S1.
Horizontal line passes through the
middle of the insertion of the
transverse processes or 1-2 mm below
the joint line.
1-2 mm lateral from the center of the
pedicle to insert the screw without
disturbing the facet joint above and to
medialize the screw for better fixation.
GSFS Implantation Procedure
Screw Hole Preparation
Angle and depth of the screw holes?
Direction and Depth
of the Screw
GSFS Implantation Procedure
Preparation of Fusion Bed and Grafting
 Decortication
 Marking
holes
 Grafting
screw
GSFS Implantation Procedure
Screw Selection and Insertion
 Screw
–
–
approx. 80% of the medial
diameter of the pedicle
Perforation of the pedicle into
the medial or inferior side has
higher chance of nerve root
injury.
 Screw
–
–
Diameter:
Length:
Long enough to pass the half of
the vertebral body but
Short enough not to penetrate
the anterior cortex
Screw Length
For GSFS
GSFS Implantation Procedure
Rod-Connector-Screw Assembly
 Rod
Length:
- Rod length must not be too long so that the proximal tip of
the rod do
not touch the inferior facet of the upper vertebra.
 Rod
Bending
 Connector Selection
 Rod-Connector Assembly
 Screw-Connector-Rod Assembly
 Tightening the nuts and set screws
Rod-Screw Assembly
Rod-Screw Assembly
Rod-Screw Assembly
Medial-lateral adjustability can
eliminate:
1) The use of additional
components; and
2) Application of force in mediallateral directions or additional
rod bending
In order to make the rod-screw
connection
GSFS Implantation Procedure
Rod-Connector-Screw Assembly
GSFS:
- Screw-Connector: Polyaxial
- Connector Length: M-L
Adjustment
*No precise rod-bending is
required.
*Screw alignment is not as critical.
GSFS Implantation Procedure
Rod-Connector-Screw Assembly
Rod-bending;
 Insert the rod to the connectors;
 Temporary tightening of set screws of the
proximal and distal most connectors;
 Place the rod-connector assembly on the screws;
 Tightening the screw caps and set-screws in the
proximal and distal most connectors while
holding the rod in a desired shape; and
 Fix the other screw caps and set-screws in the
mid-portion.

Ideal Features
 The
–
–
–
–
use of connectors:
Polyaxial and medial-lateral adjustability;
No need for precise rod bending
Easy screw-rod connection without a good alignment of screw heads
Screw insertion according to the best possible anatomic conditions
 Rigid
connection at rod-connector and screwcap connection:
–
–
Strong maintenance of correction
Better mechanical environment to enhance bone healing (fusion)
 Top-tightening:
 Low Assembly
profile:
Consideration Factors in
Spinal Instrumentation

Materials:
–
–
–

Implant Strength:
–
–
–
–

Bio-compatibility and Imaging compatibility
Stiffness (or elasticity) and strength
Corrosion
Component (screw, rod, plate, wire, etc.) strength
Metal-metal interface strength
Construct strength
Bone-metal interface strength: Bone–wire, -hook, and -screws
Construct Stability:
–
Segmental stiffness or flexibility
Profile:
 Ease of Use:

Spinal Implant Materials
 316L Stainless
–
–
–
steel:
Biocompatible
Strong and stiff
Poor imaging compatibility: artifact to CT and MRI
 Titanium Alloy
–
–
–
–
(Ti6Al4V ELI):
Biocompatible
No artifacts during CT and MRI
Excellent fatigue strength, high strength, high elasticity
High resistance to fretting corrosion and wear (surface
treatments)
Spinal Implant Strength
 Static
–
and Fatigue Strength of Components:
Depends on the material properties, size and shape of the components
 Metal-metal
–
–
Rod-screw connections
GSFS (rod-connector and screw-connector interfaces): excellent
 Construct
–
Interface Strength:
Strength:
Excellent in GSFS
Bone-Metal Interface Strength
 Pedicle
screws are known to provide the
strongest bone purchase compared to wires,
hooks, and vertebral screws.
 Screw Pullout Strength:
–
–
–
–
Affected by major diameter and bone quality (BMD) but not by
minor diameter, thread type, and thread size.
Insertion depth is not critical.
Screw insertion torque was known to have relationship with screw
pullout strength.
Conical screws showed similar pullout strength to that of the
cylindrical screws.
Surgical Construct Stability

Construct stability varies depending on the size of
the screws and rods (plates).
–
Recommended rod diameter is 6 mm or ¼ inch in adult spine surgery.

Preservation of more than 70% of the disc or
meticulous anterior grafting is critical to obtain
stable construct with no hardware failure (screw or
rod breakage).

Modern spinal fixation systems, regardless of
anterior or posterior fixation, similarly significant
stability in flexion, extension, and lateral bending,
but not effective in preventing axial rotational (AR)
motion.
–
Use of a crosslink (DDT) is recommended to improve the AR stability,
particularly in the fixation of long segments (more than 2 levels).
Surgical Construct Stability
EXT
 Ligamentous
spines
 Pure moment
–
–
AR
LB
FLX
FLX
L2
AR
in FLX, EXT, LB, and AR
Maximum 8.2 Nm
EXT
motion
analysis system
LB
 3-D
L5
Implant Assembly Profile
 Anterior
–
–
Critical in anterior plating of the cervical spine, and the profile must
be less than 3 mm.
Lower profile is recommended in the anterior fixation of the
thoracolumbar spine.
 Posterior
–
Instrumentation:
Instrumentation:
Assembly profile is not as critical as in anterior fixation, but lower
profile is recommended because a high profile may cause a surgery
for implant removal due to patients’ uncomfortness.
Ease of System Assembly
 Screw
–
Insertion:
Screw insertion according to the best possible anatomic
orientation and location
 Adjustment
–
–
–
–
in Screw-Rod Assembly:
Rod bending
Angular adjustment
Medial-lateral adjustment
Polyaxial screw head vs. Connector
 Top-tightening
–
All assembly procedures can be made from the top.
BIOMECHANICAL EVALUATION
OF DIAGONAL TRANSFIXATION
IN PEDICLE SCREW
INSTRUMENTATION
Tae-Hong Lim, Ph.D.
Atsushi Fujiwara, M.D.
Jesse Kim, B.S.
Timothy T. Yoon
Sung-Chul Lee, M.D.
Howard S. An, M.D.
Horizontal Transfixation
(HTF)

Construct stability
–
–
–
No improvement in FLX and EXT
Some improvement in LB
Significant improvement in AR
Transfixator (TF)
Increased AR stability when
using 2 transfixators
 Optimum position for TF
VB

–
–
Proximal 1/4 points for 1 TF
Proximal 1/8 and middle points for 2 TF
Lim et al. 1995
VB
Pedicle screw instrumentation
Diagonal Transfixation
(DTF)

Construct stability
–
–
Transfixator (TF)
No changes in FLX (Texada et al, 1999)
Significant improvement in LB and AR
(Texada et al., 1999; McLain et al. 1999)
VB
VB
Pedicle screw instrumentation
Diagonal Transfixation
(DTF)
 Clinical
application of DTF using 2
TFs may not be practical.
–
–
Limited space
Higher construct profile
 DTF using
1 TF is feasible, but its
effect has not been investigated yet.
PURPOSE
 To
evaluate the effect of diagonal
transfixation (DTF) on the construct
stability and the corresponding stress
changes in the pedicle screw in
comparison with the effect of
horizontal transfixation (HTF)
MATERIALS
and
METHODS
Flexibility tests
Unstable Calf Spine Model
Finite element studies
FLEXIBILITY TESTS
Ligamentous
calf spines (L2L5)
 Pure moment
EXT
 10
–
–
in FLX, EXT, LB, and AR
Maximum 8.2 Nm
 3-D
motion
analysis system
AR
LB
FLX
FLX
L2
LB
AR
EXT
L5
Tested Constructs
- Intact
- Instrumentation without TF after
total
discectomy (no TF)
- Instrumentation with HTF using 1
TF (HTF)
- Instrumentation with DTF using 1
TF (DTF)
Diapason Spinale Fixation System (Stryker, Allendale, NJ: 6.5
mm screws and 6 mm rods and TF)
Finite Element Studies
 To
investigate the stress changes in
the pedicle screws due to HTF and
DTF.
 Boundary
–
–
and Loading Conditions:
Nodes in lower vertebra were held fixed.
FLX, EXT, LB, and AR Moments (8.2 Nm) at the middle point of the
vertebra element
 ADINA Finite
Element Analysis S/W
Finite Element Models
Moment
Moment
Vertebrae
Transfixators
Fixed Nodes
(A) Horizontal transfixation (HTF)
(B) Diagonal transfixation (DTF)
Data Analysis
 Rotational
motion of L3 with respect
to L4 in response to 8.2 Nm
 Rate of motion change with respect to
–
–
Intact case
No TF case
 Total
–
load = [Mx2 + My2 + Mz2]1/2
Mx = Torsional moment; My & Mz = Bending moments
 Stress
change  changes in total load
RESULTS
Rotational Motions (deg) responding to
Applied Moments of 8.2 Nm
INT
no TF
HTF
DTF
9
Rotational Angle (deg)
8
7
6
5
4
3
2
1
0
Flexion
Extension
Lateral
Bending
Axial Rotation
Loading Directions
Mean Rate of Motion Change from Intact
Case
0.4
no TF
HTF
DTF
Rate of Motion Change
0.2
0.0
*
*
*
-0.2
-0.4
-0.6
-0.8
-1.0
Flexion
Extension
Lateral Bending Axial Rotation
Mean Rate of Motion Change from no TF
Case
0.2
Rate of Motion Change
HTF
DTF
0.1
0.0
-0.1
-0.2
*
*
*
-0.3
*
-0.4
Flexion
Extension
Lateral Bending Axial Rotation
Loading Modes
Rate of Motion Change with respect to
no TF Case
(FE Model Predictions)
0
-0.05
-0.1
-0.15
HTF
DTF
-0.2
-0.25
-0.3
Flexion
Lateral
Bending
Rate of Total Load (Stress) Changes
in Pedicle Screws
(FE Model Predictions)
0.6
0.4
0.2
0
Flexion/Extension
-0.2
-0.4
-0.6
-0.8
Lateral Bending
Axial Rotation
HTF_Left
HTF_Right
DTF_Left
DTF_Right
DISCUSSION
 The
effect of DTF using 1 crosslinking
device on the construct stability and the
corresponding stress changes in the pedicle
screws was investigated using flexibility
tests and finite element techniques.
 In flexibility tests:
–
–
–
Calf spines were used to reduce inter-specimen variability.
Most unstable model was made by performing total discectomy to highlight the
stabilizing effect of pedicle screw instrumentation.
Motion data were normalized by those of the intact and no TF case to emphasize
the effect of TF.
 For
–
–
–
FE studies:
Beam element was used for modeling for simplification.
Predicted motion changes showed a good agreement with measured data.
Stress changes were represented by the changes in total load in screws because of
linear nature of the model.
Summary of Findings
in Comparison with no TF Case
HTF
 Construct stability:
–
–
no improvement in FLX/EXT
Significant improvement in LB
and AR
 Stress
in the
screws:
DTF
 Construct stability:
–
–
Significant improvement in
FLX/EXT
no improvement in LB and AR
 Stress
in the
screws:
–
No increase in FLX/EXT
–
–
28% increase in LB
–
–
58% decrease in AR
–
12% in left screw & 11% in
right screw in FLX/EXT
44% in left screw & 7% in right
screw in LB
8% in left screw & 18% in right
screw in AR
CONCLUSION
 DTF provides
more rigid fixation in
FLX and EXT but less in LB and AR
as compared with HTF case.
 Pedicle screws may experience greater
stresses in DTF than in HTF.
 These
limitations of DTF should be
considered for clinical application.