Emerging Insights On Fixation For Austin/Chevron Bunionectomies

Start Page: 56
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Author(s): 
Gary M. Lepow, DPM, MS, FACFAS, and Brian D. Lepow, DPM, AACFAS

What You Should Know About Bioabsorbable Implants

Johnson and Johnson introduced the first commercially available resorbable pin, OrthoSorb (poly-p-dioxanone) and in the late 1970s, it became a very popular subsititue for metal pins. Bionex (polyglycolic acid) then joined Orthosorb. Bionex had a difficult introduction to the market due to adverse patient reaction, which an investigation later determined to have been caused by a dye agent in the material. Other materials and designs for pins and resorbable screws from several manufacturers such as Bionx, Arthrex, Biomet and Tornier began to enter the market with wider acceptance due to increased strength and resistance to movement and migration.5

   The rate of degradation has long been considered a potentially important factor in the development of adverse tissue reactions. Differences in degradation depend on a number of factors involving the specific polymer being used. In addition to molecular weight, mass and strength, the porosity, crystallinity, hydrophobicity or hydrophilicity and thermal history also influence degradation.1 The volume of breakdown in products per unit of time in local surrounding tissues is greater for an implant that degrades rapidly, such as a polyglycolic acid device, than for an implant of the same size that degrades more slowly, such as one made of polylactic acid.1 Polyglycolic acid degrades over a period of a few months whereas polylactic acid completely resorbs over a period of one and a half to four years.1,6 Poly-p-dioxanone degradation is slightly longer than that for comparably sized polyglycolic acid implants and loses its strength within two months.7

   Resorption of polymers generally occurs in two phases.8 In phase one, the polymer chains break down through hydrolysis. In this particular phase, the molecular weight drops first, is followed by mechanical strength loss and finally by a loss of mass.9 In phase two, the implant loses its form and breaks down into particles, which macrophages then attack. Depending on the size of the particulates, they are phagocytosed and the kidneys and lungs excrete the byproducts.10 The corresponding biological response to the degradating polymer is thought to happen as a result of either a buildup of acidic degradation products or as a response to the particulates of the polymer.11

   The most commonly cited advantages of resorbable implants include reduced stress shielding since the implants bear less load initially and transfer the load as they degrade.10 The polymers can be engineered to provide the optimum degradation profile for a specific application.10 Additionally, there is also elimination of the need for removal of hardware (and its related cost), reducing additional patient inconvenience as well as the potential for pin tract infection.1 Disadvantages of the use of resorbables include a lower strength and higher cost as well as the potential for the development of a sinus tract with sterile discharge.10 Studies have also documented synovitis and osteolytic changes in the bone, migration and breakage of the implant.12-14

How Human Allograft Can Be Advantageous In Fixation

Human allograft bone is harvested from an individual other than the one receiving the graft. Allograft bone is generally harvested from cadavers and is typically sourced from a bone bank.15

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BRIANsays: February 6, 2013 at 9:27 am

"There is no argument that the use of rigid internal fixation with metal screws, staples and/or plates has been the gold standard for bone fixation."

Could not agree more.

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