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 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