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 the body weight and external load

– 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 Treatment– Degenerative disease– Stable fracture– Mild deformity

Surgical 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 Considerationin 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 Factors– Stability, Loading

Properties of Graft MaterialsGraft Osteogenic Oseto- Osteo- Materials

Potential induction conduction

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 Method:

– Wiring, Hooks, Screws– Rods vs. Plates

Spinal Level:– Cervical, Thoracolumbar

Position:– Anterior vs. Posterior

Instrumentation

Vertebra

Pedicle screw instrumentation

Vertebra

Graft

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 exposure of the lumbar spine:

– Midline incision extended to an additional level

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 PreparationGSFS Implantation Procedure

Angle and depth of the screw holes?

Direction and Depth of the Screw

Decortication

Marking screw holes

Grafting

Preparation of Fusion Bed and Grafting

GSFS Implantation Procedure

Screw Diameter: – 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 Length:– Long enough to pass the half of the

vertebral body but – Short enough not to penetrate the

anterior cortex

Screw Selection and InsertionGSFS Implantation Procedure

Screw LengthFor GSFS

Rod-Connector-Screw Assembly

GSFS Implantation Procedure

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 medial-lateral directions or additional rod bending

In order to make the rod-screw connection

Rod-Connector-Screw Assembly

GSFS Implantation Procedure

GSFS: - Screw-Connector: Polyaxial - Connector Length: M-L Adjustment

*No precise rod-bending is required.*Screw alignment is not as critical.

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.

Rod-Connector-Screw Assembly

GSFS Implantation Procedure

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 screw-cap connection:

– Strong maintenance of correction– Better mechanical environment to enhance bone healing (fusion)

Top-tightening: Low Assembly profile:

Consideration Factors in Spinal Instrumentation Materials:

– Bio-compatibility and Imaging compatibility– Stiffness (or elasticity) and strength– Corrosion

Implant Strength:– 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 Interface Strength:– Rod-screw connections– GSFS (rod-connector and screw-connector interfaces): excellent

Construct 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

Ligamentous spines

Pure moment – in FLX, EXT, LB, and AR– Maximum 8.2 Nm

3-D motion analysis system

L2

L5

EXT

EXT

LBAR

AR

FLX

FLX LB

Implant Assembly Profile

Anterior Instrumentation:– 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:– 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

Increased AR stability when using 2 transfixators

Optimum position for TF– Proximal 1/4 points for 1 TF– Proximal 1/8 and middle points for 2 TF

Lim et al. 1995

VB

VB

Pedicle screw instrumentation

Transfixator (TF)

Diagonal Transfixation (DTF)

Construct stability– 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

Transfixator (TF)

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.

PURPOSETo 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)

MATERIALSand

METHODS

Flexibility tests Unstable Calf Spine Model

Finite element studies

FLEXIBILITY TESTS10 Ligamentous

calf spines (L2-L5)

Pure moment – in FLX, EXT, LB, and AR– Maximum 8.2 Nm

3-D motion analysis system

L2

L5

EXT

EXT

LBAR

AR

FLX

FLX LB

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

Vertebrae

Transfixators

Fixed Nodes

Moment Moment

(A) Horizontal transfixation (HTF) (B) Diagonal transfixation (DTF)

Data AnalysisRotational motion of L3 with respect

to L4 in response to 8.2 NmRate of motion change with respect to

– Intact case– No TF case

Total load = [Mx2 + My

2 + 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

Loading Directions

Rot

atio

nal

An

gle

(deg

)

0

1

2

3

4

5

6

7

8

9

INT no TF HTF DTF

Flexion Extension Lateral Bending

Axial Rotation

Mean Rate of Motion Change from Intact Case

Rat

e of

Mot

ion

Ch

ange

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4 no TF HTF DTF

Flexion Extension Lateral Bending Axial Rotation

* * *

Mean Rate of Motion Change from no TF Case

Loading Modes

Rat

e of

Mot

ion

Ch

ange

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

HTF DTF

Flexion Extension Lateral Bending Axial Rotation

* **

*

Rate of Motion Change with respect to no TF Case

(FE Model Predictions)

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

Flexion LateralBending

HTFDTF

Rate of Total Load (Stress) Changes in Pedicle Screws

(FE Model Predictions)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Flexion/Extension 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:– No increase in FLX/EXT

– 28% increase in LB

– 58% decrease in AR

DTF Construct stability:

– Significant improvement in FLX/EXT

– no improvement in LB and AR

Stress in the screws:– 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

CONCLUSIONDTF 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.