Terminology

Imaging

Clinical Issues

Abbreviations

• Bone morphogenetic protein (BMP)
• Polymethylmethacrylate (PMMA), a.k.a. bone cement

Definitions

• Various materials and techniques used to fill defects, restore structure and strength, provide scaffold for bone ingrowth
• Ceramic: inorganic material transformed during process of heating and cooling, often has crystalline structure
• Bone graft
  • Autograft: harvested from patient, contains marrow elements leading to osteogenesis
  • Allograft: derived from different individual
  • Xenograft: derived from different species
  • Onlay graft: placed along surface of bone
  • Strut graft: spans defect in bone or segment of spine
  • Vascularized graft: graft with blood supply, usually reestablished through microvascular techniques; fibula most common source, typically autograft
    • Sources: iliac crest, fibula, rib, mandible
• Biologic activity of grafts
  • Osteoinductive: recruits and then stimulates osteoprogenitor cells and undifferentiated stems cells to form osteoblasts
  • Osteoconductive: scaffolding for bone ingrowth
  • Osteogenic: stimulates new bone formation via implanted cells within graft, primarily autograft
    • Cancellous graft greater potential than cortical graft

Bone Graft

• Autograft
  • Additional morbidity, especially pain, occurs at harvest site (especially iliac crest)
  • Cortical: provides strength of cortical bone
    • Often used as onlay graft
    • Minimal marrow elements, so incorporation requires long period of time
  • Cancellous: offers no structural support
    • Provides marrow elements stimulating bone ingrowth and more rapid incorporation
  • Corticocancellous
    • Provides structural support of cortical bone, combined with marrow elements with their osteogenic properties
  • Vascular grafts
    • Blood supply and marrow offer all elements needed for graft incorporation
• Allograft
  • Only contains mineralized component of bone; lacks marrow elements
  • Little risk of disease transmission
  • Frozen or freeze-dried; techniques reduce strength
  • Higher rate of complications
    • Nonunion, fracture, infection
    • Lacks ability to repair following damage
  • Incorporation requires balance between bone resorption and bone deposition

Bone Morphogenetic Protein

• Group of 6 different proteins (BMP-2 through BMP-7) that stimulate formation of bone and cartilage
  • BMP-7, a.k.a. OP-1 (osteogenic protein-1)
• Derived from recombinant DNA techniques
• Requires carrier, usually mixed with bone graft or bone graft substitutes
• Provides osteoinductive influence
• Offers no structural support
• Use in spine may be associated with aggressive bone resorption, mimicking infection
  • → caution warranted when interpreting images

Ceramics

• Majority are calcium phosphate based
  • Hydroxyapatite forms most common
    • Mimics structure of calcium within bone
    • Available in pastes, powder, granular, block forms
    • Poor mechanical properties
    • Coralline hydroxyapatite derived from sea coral; others synthetic
  • Other less commonly used chemical forms: tricalcium phosphate, calcium sulfate
• Density similar to or greater than bone
• Fills defects, provides scaffolding for new bone
  • Resorbs over long periods of time as new bone is incorporated: rapid resorption indicates failure of incorporation or tumor recurrence
• Biologic activity
  • Osteointegrative (new bone binds to graft)
  • Osteoconductive
  • May be osteoinductive
  • Inert material, no toxicity
• Bioactive glass: brittle, usually combined with another agent, such as PMMA

Demineralized Bone Matrix

• Type of allograft
• Useful because releases BMP
• No structural properties
• Often mixed with other bone fillers to introduce its osteoinductive properties
• Demineralized, therefore radiolucent; carrier usually has density

Injectable Cements

• Useful because of mechanical properties
  • Structural characteristics mimic bone
• Also provide 3D scaffold for bone ingrowth
• Polymer based; no biologic activity

Polymethylmethacrylate

• Same material used to make acrylic
• Supplied as liquid monomer and powder polymer
  • Monomer: stabilizer, activator
  • Powder: includes polymerization initiator
  • When mixed, creates exothermic reaction
  • Phases of polymerization
    • Wetting phase: all powder is wet
    • Dough time: cement pulls away from surface
    • Setting time: maximum temperature rise
    • Working time: interval from dough time to setting time
      • Desirable phase for joint implants
      • Too thick for injection via cannula for vertebral augmentation
• Uses
  • Fix joint implants to bone, sometimes pedicle screws
  • Fill defects in bone after curettage for tumor
  • May be mixed with slow-release antibiotics and placed in infected bone defects
    • Most frequently following explant of infected implant
  • Treatment of painful vertebra and sacrum fractures
• Low viscosity cements preferred for vertebral augmentation
• Function → load sharing
  • More flexible than cortical bone, less flexible than cancellous
  • Strength is in compression; fails along lines of shear
• No biologic activity
• Barium added to ↑ density
• Toxicity to user
  • Vapors permeate contact lenses
  • Mucous membrane irritation
  • Contact dermatitis, numbness, paresthesias
• Excess monomer may be toxic to patient
  • Small amounts always exist in tissues
  • Earlier vertebroplasties had different monomer:powder ratio resulting in ↑ amounts of free monomer; this caused damage to lungs and liver

Imaging Recommendations

Radiographic and CT Findings

MR Findings

Microscopic Features

Natural History & Prognosis