A Study of Tensile Degradation of Bioresorbable Materials Used for ...

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Degradation of Bioresorbable Tensile Bars Breakdown of Bioresorbable Polymers The breakdown of bioresorbable polymers is a complex process that takes place in the body subsequent to the healing of the bone fracture. The resorbable materials dissolve in the body at the same rate that the surrounding tissue regenerates. When the biomaterial is completely dissolved, the space it occupied will be filled with the new tissue (Barden, 2009, slide 19). The PLA/PGA compounds break down into carbon dioxide and water components. These components have no ill effects on the body because they are found in nature and used by the body daily. There are three different terms used to classify the breaking down of biomaterials. Biodegradation is a term used to describe how an enzyme, cell, or microbe in the body breaks down the polymer. The actual amount and speed of the components that make up the polymer being absorbed is referred to as bioresorption. The physical and chemical properties of the degradation of the polymer are defined as bioerosion (Barden, 2009, slide 25). The effects of bioerosion will be studied in this experiment. Bioerosion can cause multiple reactions in the bioresorbable polymers. For example, the polymer might suffer a change in physical appearance or chemical properties. The weight of a polymer could decrease causing the material to lose its ability to function for its original purpose. If the polymer suffers from deformation as a result of the bioerosion, the entire shape of the biomaterial may be changed. This would force the biomaterial to be unable to function well in its previous location. Inflation or deflation of the bioabsorbable material could also be a physical change that occurs during bioerosion. Christopher Barden, a project engineer at MicroPEP, reported that the dissolving of the polymer is the most well-known effect of bioerosion because that is what biomaterials are meant to do (personal communication). Hydrolysis is the typical method by which the breakdown of polymers occurs. The bond between most PLA/PGA compounds is an ester bond. When an ester bond is broken by hydrolysis, acid and alcohol are formed. Hydrolysis occurs when the polymers or molecules react with water. The water enters the polymer or molecules and breaks it down from the inside causing hydrolic degradation (Barden, 2009, slide 15). Figure 1 shows the breakdown of an ester bond by hydrolysis. Figure 1. Breakdown of an ester bond by the process of hydrolysis. In the presence of water, certain molecules are broken down by hydrolysis. This process results in the formation of an acid and an alcohol when applied to an ester bond. The opposite process that occurs in reaction to hydrolysis is condensation (Barden, 2009, slide 15). Processing Bioresorbable Materials As with all materials used in medical applications, biodegradable materials must be tested for their ability to be processed in order to make it easier for manufacturers and surgeons to handle the products. The characterization of a polymer in regards to its weight, stability, and ability to be processed can be determined using an IV (Intrinsic Viscosity) test. Viscosity refers to the thickness and tackiness of the solution surrounding the polymer. The IV test measures a 7
Degradation of Bioresorbable Tensile Bars polymers capability to increase the viscosity of the solution it resides in. During the testing process of the durability of the polymer, the viscosity is established at various concentrations of solution. By performing the IV test at various concentrations, the viscosity at a concentration of zero units of solution surrounding the polymer can be determined. The IV test must also be conducted at numerous stages throughout the molding process of the polymers. This ensures that the viscosity level will be correct for the application and location in the body that each specific polymer mold will be used for. If the biomaterial is processed incorrectly with the wrong IV, the polymer could prematurely degrade in the body causing a failed bone union. Processing the materials correctly without exposing them to heat and moisture is important in the success of the application (Bibber, 2009, p. 1). Because of the several complexities when handling biomaterials, the handling process is the area most susceptible to error. The polymers are extremely sensitive to heat and moisture and must be stored in the correct environment which is typically a freezer. The temperature must be exact, and the biomaterials must always be kept in their nitrogen sealed containers. Extremely small machines must be used to process the biomaterials because even minute amount of offtemperature air could cause premature degradation. Furthermore, the venting system of the molds for polymers must never become clogged because backed up vents will also dissolve the materials prematurely. The processing of bioresorbable materials involves potential problems due to the numerous minute details in handling techniques (Bibber, 2009, p. 2). Uses of Bioresorbables Bioresorbable materials are useful in several medical applications. These polymers can be molded in a variety of shapes depending on how they will be used in the body. For example, specific molds of biomaterials can be tensile screws, plates, or mesh. Bioresorbables are best suited for applications in which they are only needed in vitro for a short period of time (Bibber, 2009, p. 1). Most facial implants for aesthetic reasons use resorbable materials because they will not stay in the patient’s face forever. Bioresorbables are also used as bone screws when they bone in question does not have a long healing process. Once bones are healed using titanium screws, bioresorbable polymers can replace the titanium screws. After the bone no longer needs the stability of the tensile bar, it will be replaced with biomaterials that will dissolve into the body. Another use for biomaterials is a method of drug release. If the biomaterial has antibiotic medicine inside it, the medicine will be time released directly into the area of the body that needs it as the polymer is dissolving (Barden, 2009, slide 29). On a smaller scale, resorbable materials can be used as sutures. To patients these are known as dissolvable stitches. Bioresorbables can be used for many applications and as technology and research increases, more uses will be found. Bioresorbable Fixation Plates as an Alternative to Titanium in Mandibular Fractures Several recent studies of bioresorbable fixation have focused on biomaterials used in the fixation of human mandibular fractures. A study executed by Krach, Wagner, Yerit, Rothlisberger, and MCiller concentrated on whether bioresorbable fixation plates would be a successful alternative to titanium in human mandibular fractures (Krach et al., 2008, p. 59). The necessary data was collected using CT information. Characteristics were attributed to the materials using actual bone density values of the human mandible. Muscular forces during biting, such as direction, location, and height, were used to determine loading conditions of the mandible. There are four common fracture planes in the mandible: a median, paramedian, angle, and column. All four of these planes were considered during the study. The fractures were first analyzed assuming titanium fixation plates were used. This data showed that the plates will 8
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