Bioabsorbable Implants For Flatfoot: Can They Work?
A Guide To The Evolution Of Bioabsorbable Materials
Foot and ankle surgeons have used bioabsorbables for decades. Reports of absorbable suture, suture anchors, interference screws, screws and pins have all been published in the literature. The ideal absorbable implant should have strength comparable to metallic implants, should remain inert throughout the degradation process and should gradually transfer load to healing bone and soft tissues.9 While companies are still striving for this ideal implant, current materials continue to evolve and gain widespread acceptance within the podiatric community. Absorbable materials have been available for centuries but synthetic absorbables made their debut in 1962 with the advent of Dexon. Vicryl suture, a co-polymer of polyglycolide (PGA) and polylactide (PLA), soon followed in 1975. Widespread commercial production of absorbable fixation devices began in the late 1980s with the earliest device constructed of polydiaxanone (PDS).10,11 All of the bioabsorbable polymers exhibit unique mechanical properties. These properties are dependent on many variables, including the polymer’s molecular weight, chemical arrangement, structural organization, porosity and purity. The strongest polymers are those which are highly crystalline in structure and have a higher molecular weight.9 When one uses these polymers as orthopedic implants, they do exhibit viscoelasticity. In other words, these implants have dynamic physical properties, which are largely dependent upon load application and time from implantation.12 Early mass production of bioabsorbable materials involved casting the polymers into long, flat films. Today, the implants are made through a process of melt-molding and pouring into casts.13 This process provided the necessary stability and rigidity required for the use of bioabsorbable polymers as fixation devices in lieu of metal implants.10 Another important advancement in the production of bioabsorbable implants was the development of self-reinforced composites. These composites retained the strength characteristics required for rigid fixation while providing the flexibility to closely mirror bone’s modulus of elasticity.10 The biodegradation rate is based upon numerous factors with implant size, implant site, porosity and composition all playing roles. However, the two most important factors in the rate of degradation are a polymer’s hydrophilic properties and crystallinity.12,14 Those materials with hydrophilic components degrade at a quicker rate versus those with hydrophobic components. Also be aware that those with a higher degree of structural organization will degrade at a slower rate than those with decreased crystalline structure.11 The degradation process begins with a decrease in molecular weight. This is followed by decreased strength and ultimately decreased mass.12 Hydrolysis begins the process for PGA, PLA and PDS materials. This causes a decrease in the size of the polymeric chains into smaller byproducts. The byproducts from PGA and PLA (glycolic acid and lactic acid respectively) are partially excreted in the urine. These byproducts are also transformed into pyruvic acid, subsequently used in the Kreb’s cycle, and are ultimately excreted as carbon dioxide and water.9 The byproducts of PDS hydrolysis are mainly excreted via the kidneys with a small amount released as carbon dioxide and in feces.10