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BIOMECHANICS OF FUSION

BIOMECHANICS OF FUSION. Spinal Disorders. Trauma Fractures, Whiplash injury, etc. Tumor Infection & Inflammatory Disease Deformity Cervical & Low-back Pain Degenerative disease, such as disc herniation, stenosis, spondylolisthesis, etc. Goals of Spine Surgery.

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BIOMECHANICS OF FUSION

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  1. BIOMECHANICS OFFUSION

  2. Spinal Disorders • Trauma • 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 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

  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

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

  11. Graft Size Effect • Height: • 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.

  12. Graft Location • Three Types of Fusion: • 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

  13. 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

  14. Future Studies • Healing Type: • 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

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