Updated: 9/6/2014

Bioabsorbable Materials

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Introduction
  • Bioabsorbable materials were invented to address issues with synthetic implants including
    • migration
    • growth disturbance
    • rigidity
    • radioopacity
    • infection
    • need for implant removal operations
  • Indications include but are not limited to
    • pediatric orthopaedics
      • transphyseal SR PLGA 80/20 screws only cause temporary growth arrest in rabbits (unlike nonbioabsorbable implants)
    • osteomyelitis
      • antibiotic eluting PLA 
    • carriers for growth factors
      • rhBMP2 and rhBMP7
    • augmentation of bone healing at iliac crest bone harvest site
Types of Bioabsorbable Materials
  • Polyglycolic acid (PGA)
    • hydrophilic, crystallic
    • glass transition temperature 36degC 
      • becomes malleable if this temperature is exceeded
    • disadvantages
      • early degradation and strength loss
        • potential postop complications
      • intraoperatively, must be heated to adapt to implantation surface, and cooled
        • increased intraoperative time consumption
  • Polylactic acid (PLA)
    • more hydrophobic than PGA
      • L-isomer or poly-L-lactic acid (PLLA)
        • hydrophobic and crystallic
        • has prolonged degradation time (several years)
          • late adverse reactions in the final stages of polymer degradation
        • glass transition temperature 57degC
      • D-isomer
        • amorphous, less stable
        • useful for building co-polymers
  • Co-polymers
    • P(L/D)LA copolymers
      • mixture of D- and L-isomers of PLA
      • hydrophobic and crystallic
      • resistant to hydrolysis and degradation
      • adding D-isomers results in less tightly packed polymer chains
        • less crystallic and more rapidly degraded than PLLA alone
      • example is P(L/D)LA 70/30 in oral-maxillofacial surgery
      • simple and self-reinforced forms
    • PLGA copolymers
      • combination of PLA and PGA
      • low crystallinity
      • used in oral-maxillofacial surgery
      • simple and self-reinforced forms
  • Self reinforcing (SR)
    • composite structure made from partially crystalline/amorphous material made of orientated fibers/fibrils and binding matrix
    • better biomechanical properties
      • improved rigidity and strength along longitudinal axis
      • malleable at room temperature
        • no need for heating-cooling
      • can withstand 4 times bending
      • minimal "memory" (tendency to return to previous shape after bending)
      • can be sterilized by gamma irradiation
        • gamma irradiation cannot be used with non-reinforced materials 
          • will reduce its molecular weight and adversely affect the mechanical properties of the implant
 
Material Absorption Time
SR PLLA >5-6 years
PLLA >5 years
P(D/L)LA 70/30 2-3 years
PLA/PGA (PLGA) 80/20 1-2 years
P(D/L)LA 96/4 years
SR PGA 0.5-1 years
PDS 2 months
PGA 1-2 months
 
Biodegradation
  • Primary mechanisms of biodegradation
    • poly-hydroxy-acid degradation  
      • breakdown is by random hydrolysis of ester bonds, which leads to
        • reduction of molecular weight
        • loss of mechanical properties
        • final products are CO2, H2O, and products of TCA (tricarboxylic acid, Krebs) cycle
    • kidney excretion
      • PDS and PGA products can be excreted by the kidney 
    • enzyme breakdown
      • enzymes are involved with PLA and PGA degradation
    • lowered pH
      • polymer breakage produces products that lower pH
      • accelerating the breakdown
    • material crystallinity
      • determines hydrophobicity and degradation speed
      • amorphous and hydrophillic materials degrade faster
        • more contact with water molecules
      • crystalline and hydrophobic materials degrade slower
        • less contact with water molecules
  • Additional variable that affect degradation
    • chemical composition and molecular weight
    • fiber orientation (SR or simple)
    • monomer concentration (in polymers)
    • stereoisomerism and conformation
    • pores and surface area/volume ratio
    • pores and surface area/volume ratio
    • sterilization method (gamma irradiation vs others)Degradation method (enzymatic vs hydrolysis)
Histopathology
  • Granulomatous inflammation
    • cellular reactions around bioabsorbale implants are characterized by
      • T lymphocytes (CD4>CD8)
      • plasma cells
      • endothelial cells
      • birefringent polymer debris
      • thin macrophage layer
      • multinucleated giant cells
  • Capsule formation
    • a capsule forms around implants that consists of
      • internal cell layer 
        • 2-3 cells thick
        • type III collagen predominance
      • external fibrous layer
        • few spindle shaped cells
        • type I collagen predominance
  • Stages
    • begins with infiltration of neutrophils 
      • tissue reaction to trauma
    • followed by CD4 T lymphocytes infiltration
    • macrophages infiltration is last
Adverse Tissue Reactions
  • Incidence
    • 3% in pediatrics
    • up to 60% in adults (more common in adults)
  • Presentation
    • symptoms
      • fluid accumulation, fluctuant papules
        • when implant degradation exceeds debris removal rate, fluid accumulates
      • symptoms appear late in materials with low degradation rate 
        • e.g. PLLA at 5 years postop
    • physical exam
      • synovitis
      • discharging sinus
  • Labs
    • fluid cultures are sterile 
      • unless there is secondary bacterial infection after bursting
  • Radiography
    • osteolysis is seen in up to 60% of cases
  • Treatment
    • nonoperative
      • observation
        • healing without active treatment
    • operative
      • aspiration and/or surgical debridement
      • implant removal
        • indicated if there is sterile implant failure
        • or if there is secondary bacterial infection
      • arthrodesis
        • if there is severe osteoarthritis
 

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