BIOMECHANICS OF FUSION

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.

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 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

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

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 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.

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

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: 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