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