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BIOMECHANICS OFFUSION
Spinal DisordersTrauma
– Fractures, Whiplash injury, etc.
TumorInfection & Inflammatory DiseaseDeformityCervical & Low-back Pain
– Degenerative disease, such as disc herniation, stenosis, spondylolisthesis, etc.
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
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 %
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 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
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
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
Compressive Strength (MPa) of the Illiac Bone Graft and HA/TCP Ceramics with varying Porosity
Graft Size EffectHeight:
– 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.
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
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
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