1. BIOMECHANICS OF FUSION

2. Spinal Disorders Trauma Tumor Infection & Inflammatory Disease
Fractures, Whiplash injury, etc.
Tumor
Infection & Inflammatory Disease
Deformity
Cervical & Low-back Pain
Degenerative disease, such as disc herniation, stenosis, spondylolisthesis, etc.

3. Goals of Spine Surgery
Relieve pain by eliminating the source of problems
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

4. Spinal Fusion
Elimination of 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 %

5. Types of Fusion

6. 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

7. Properties of Graft Materials
Graft Materials Osteogenic Potential Osteoinduction 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

8. Bone Morphogenetic Protein
Carriers:
Collagen, DBM, HA or HA/TCP Ceramics
Animal studies:
Dog: 100% bilateral fusion (Sandhu et al.)
Rabbit & Baboon: 100% bilateral fusion (Boden et al.)
Goat: No enhancement in cervical interbody fusion (Toth et al.)
Perspectives:
Use of osteoinductive proteins may result in a more rapid, more reliable and more biomechanically sound fusion than the autograft gold standard.
Issues need to be addressed:
Dosage, Carrier, Mechanical environment, Technology for easier application method
Prospective, blinded, and randomized clinical trials required

9. Mechanical Strength of Graft
Important particularly in interbody fusion
50% of body weight on the lumbar spine
Axial compressive load ranging from 400N during quiet standing to as high as 7000N during lifting
Graft Materials:
Autograft obtained from illiac crest
HA/TCP Ceramics with various porosities

12. Compressive Strength (MPa) of the Illiac Bone Graft and HA/TCP Ceramics with varying Porosity

13. Graft Size Effect Height: Cross-sectional Area:
Enlarge the foraminal area (FA) as well as the IVD height
In C-spine fusion:
2mm+baseline disc height of 3.5 to 6.0 mm
Thicker graft for smaller IVDH & Thinner graft for IVDH > 7 mm
In lumbar fusion:
BAK threaded cage (13, 15, 17 mm) increased the FH significantly with minimal changes in lordosis (upto 29% in L4-5 & 33.8% in L5-S1).
Cross-sectional Area:
Too small x-area may increase the incidence of subsidence
> 30% of the x-area of the vertebral body to carry minimum thoracic physiologic loads without trabecular subsidence.

14. Graft Location Three Types of Fusion: Investigated Parameters:
Interbody fusion, Bilateral fusion, Posterior fusion
Investigated Parameters:
Axial stiffness
Axial compression load, Bending moment, and Motion at the adjacent levels while apply 20 deg FLX and EXT at L3
Load on the facets
Bilateral fusion
presented least amount of alteration in the mechanical properties of the adjacent, unfused segment, while providing good stabilization on the fused segment

15. Complications of Spinal Fusion
Spinal Stenosis:
11 to 41% (Brodsky, 1970; Macnab, 1971)
Complications at the Donor Site
Pain, morbidity, sepsis, reduced structural integrity, etc.
>20% (Younger and Chapman, 1989)
Junctional Degeneration
Degenerated or prolapsed disc
Stenosis
Osteoarthritis of facets
Segmental instability

17. Future Studies Healing Type: Ideal Mechanical Environment:
What type of healing occurs during fusion consolidation? Membranous bone formation, endochondral ossification, or both?
Ideal Mechanical Environment:
What is the ideal rigidity required?
Molecular Biology of the Spine Fusion
What triggers bone induction?
What is the sequence of gene expression occurring?
Synthetic Graft Materials:
DBM, HA/TCP Ceramics with Growth Factor