Bioabsorbable Implants For Flatfoot: Can They Work?
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
Key Pointers On Bioabsorbable Materials
The three most widely used materials today are PGA, PLLA, and PDS, all of which are alpha-polyesters.12 There are also numerous co-polymers with various combinations of the three polymers.
PGA offers moderately high crystalline structure. It is the most hydrophilic and the stiffest polymer of the three materials. It is also the fastest to degenerate. It loses half its strength within two weeks and complete resorption takes one year.9,11 Researchers have blamed the material’s rapid rate of degeneration for the plethora of synovitic reactions associated with PGA.12
The polymerization of lactic acid can take two forms: dextrorotatory (D) and levorotatory (L). The L form, polymerized L-lactic acid, is highly crystalline and represents the biologically active form.9 Compared with PGA, PLA is more hydrophobic and accordingly has a slower degradation rate. PLLA implants lose half their strength at around 12 weeks.11 Some authors have documented that it may take up to six years for complete degradation of PLLA implants.10
PDS is produced by polymerization of para-dioxanone. While it remains popular as a suture material, it has fallen out of favor as a biomaterial for fixation devices.12
Reviewing The Pros And Cons Of Bioabsorbable Implants
Bioabsorbable implants have gained popularity for a number of reasons. Perhaps the leading reason is the fact that they do not require a second procedure for removal. Metallic fixation can be problematic due to corrosion, biomechanical stiffness and, most importantly, stress shielding.13 Since the absorbable implants exhibit rigidity closer to that of bone, there is a significant decrease in stress shielding.5 The initial stability is adequate for healing but the implant gradually degrades and the stresses are transferred to the surrounding tissues.13 Another benefit of bioabsorbables is their radiolucency. They will not produce the X-ray scatter on magnetic resonance images or computed tomographic scans associated with metal fixation.15
The most cited complications associated with bioabsorbable implants are sterile sinus tracts, osteolysis at the site of insertion and late foreign body reactions. Other negative attributes include a finite life span, a diminishing strength profile over time and increased cost.9
Out of the aforementioned problems, adverse tissue reactions are of most concern to the surgeon. Böstman and Pihlajamäki reported that 4.3 percent of 2,528 patients experienced a clinically significant local inflammatory, sterile tissue reaction.16 Upon further analysis, the rate of reaction to PGA implants in the study was 5.3 percent versus 0.2 percent for PLA implants. Also, PGA reactions appeared at an average of 79 days versus 4.3 years for the lone PLA reaction.