Updated: 6/17/2021

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
          • Absorption time of various materials 
            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
            2 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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