1. SPINAL FUSION AND INSTRUMENTATION
Tae-Hong Lim, Ph.D.
Department of Biomedical Engineering
The University of Iowa
Iowa City, Iowa

2. 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

3. Spinal Disorders Trauma Tumor Infection & Inflammatory Disease
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.

4. Treatment of Spinal Disorders
Conservative Treatment
Degenerative disease
Stable fracture
Mild deformity
Surgical Treatment
Failed conservative treatment
Unstable fracture (dislocation)
Progressive deformity

5. 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

6. 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 %

7. Types of Fusion

8. 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 Factors
Stability, Loading

9. Properties of Graft Materials
Graft 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

10. 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

11. Spinal Instrumentation Types
Implantation Method:
Wiring, Hooks, Screws
Rods vs. Plates
Spinal Level:
Cervical, Thoracolumbar
Position:
Anterior vs. Posterior Instrumentation
Vertebra
Graft
Vertebra
Pedicle screw instrumentation

12. Cervical Spine Instrumentation

13. Cervical Spine Instrumentation

14. Thoracolumbar Spine Instrumentation
Z-plate (Danek)
Kaneda (AcroMed)

15. Thoracolumbar Spine Instrumentation

16. Thoracolumbar Spine Instrumentation

17. 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

18. Screw Hole Preparation
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.

19. Screw Hole Preparation
GSFS Implantation Procedure
Screw Hole Preparation
Angle and depth of the screw holes?

20. Direction and Depth of the Screw

21. Preparation of Fusion Bed and Grafting
GSFS Implantation Procedure
Preparation of Fusion Bed and Grafting
Decortication
Marking screw holes
Grafting

22. Screw Selection and Insertion
GSFS Implantation Procedure
Screw Selection and Insertion
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 Length
For GSFS

23. Rod-Connector-Screw Assembly
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

24. Rod-Screw Assembly

25. Rod-Screw Assembly

26. Rod-Screw Assembly Medial-lateral adjustability can eliminate:
The use of additional components; and
Application of force in medial-lateral directions or additional rod bending
In order to make the rod-screw connection

27. Rod-Connector-Screw Assembly
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.

28. Rod-Connector-Screw Assembly
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.

29. 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:

30. 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:

31. Spinal Implant Materials
316L Stainless steel:
Biocompatible
Strong and stiff
Poor imaging compatibility: artifact to CT and MRI
Titanium Alloy (Ti6Al4V ELI):
No artifacts during CT and MRI
Excellent fatigue strength, high strength, high elasticity
High resistance to fretting corrosion and wear (surface treatments)

32. 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

33. 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.

34. 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).

35. Surgical Construct Stability
Ligamentous spines
Pure moment
in FLX, EXT, LB, and AR
Maximum 8.2 Nm
3-D motion analysis system
EXT AR LB
FLX
FLX L2 LB AR
EXT L5

36. 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.

37. 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.

38. 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.

39. 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
Transfixator (TF) VB VB
Pedicle screw instrumentation

40. 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)
Transfixator (TF) VB VB
Pedicle screw instrumentation

41. 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.

42. 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)

43. MATERIALS and METHODS

44. Finite element studies
Flexibility tests
Unstable Calf Spine Model
Finite element studies

45. FLEXIBILITY TESTS 10 Ligamentous calf spines (L2-L5) Pure moment
in FLX, EXT, LB, and AR
Maximum 8.2 Nm
3-D motion analysis system
EXT AR LB
FLX
FLX L2 LB AR
EXT L5

46. 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)

47. 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

48. Finite Element Models Moment Moment Vertebrae Transfixators
Fixed Nodes
(A) Horizontal transfixation (HTF)
(B) Diagonal transfixation (DTF)

49. 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

50. RESULTS

51. Rotational Motions (deg) responding to Applied Moments of 8.2 Nm
INT
no TF
HTF
DTF 9 8 7 6 5
Rotational Angle (deg) 4 3 2 1
Flexion
Extension
Lateral
Bending
Axial Rotation
Loading Directions

52. Mean Rate of Motion Change from Intact Case
0.4
no TF
HTF
0.2
DTF * * *
0.0
Rate of Motion Change
-0.2
-0.4
-0.6
-0.8
-1.0
Flexion
Extension
Lateral Bending
Axial Rotation

53. Mean Rate of Motion Change from no TF Case
0.2
HTF
DTF
0.1
0.0
Rate of Motion Change
-0.1
-0.2 * * *
-0.3 *
-0.4
Flexion
Extension
Lateral Bending
Axial Rotation
Loading Modes

54. Rate of Motion Change with respect to no TF Case (FE Model Predictions)

55. Rate of Total Load (Stress) Changes in Pedicle Screws (FE Model Predictions)

56. DISCUSSION

57. 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.

58. Summary of Findings in Comparison with no TF Case
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
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

59. 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.