ABRAHAM Gustavo Abel
capítulos de libros
Totally and partially biodegradable self-polymerizing composites for orthopedic surgery and dental applications
Biodegradable Systems in Tissue Engineering and Regenerative Medicine
CRC Press
Lugar: Boca Raton, FL, USA; Año: 2005; p. 39 - 51
Self-polymerising resins are obtained at room temperature by the free radical polymerisation reaction of an acrylic monomer in presence of a prepolymerised component. The process is initiated by the decomposition of a small amount of organic peroxides (1-3%), usually benzoyl peroxide (BPO) activated by the redox reaction with tertiary amines. Aromatic amines are the most effective compounds to generate the primary radicals and N,N’-dimethyl-4-toluidine (DMT) is the most frequently used. Self-curing acrylic resins based on blends of poly(methyl methacrylate) (PMMA) particles and methyl methacrylate monomer, or copolymers of methyl methacrylate with styrene or other acrylic monomers, including ethyl or butyl methacrylate, are used in dentistry and orthopaedic surgery for the fixation of endoprosthesis (i.e., total hip replacements and knee joints). This system has also been the primary constituent of the vast majority of artificial dentures for at least 50 years, and has been used in medicine by neurosurgeons for replacements of skull defects. The blend of polymer/copolymer and monomer (or a mixture of acrylic monomers) forms slurry of relatively high viscosity which can be manipulated easily or injected into the intraosseous cavity. The blend hardens in the physiological medium by the free radical polymerisation of the monomeric components at physiological temperature. Self-polymerising biodegradable bone cements present many advantages over poly(methyl methacrylate) bone cements. These cements besides of being mouldable into bone interstices, and capable of hardening in situ with minimal release of residual monomer, they are biodegradable with a controlled rate of resorption conducting to eventual ingrowth and replacement by new bone. Then, they can provide initial structural support for what they should exhibit mechanical properties within the range of normal trabecular bone (5-12 MPa compression strength and 0.02-0.50 GPa Young’s modulus) and also provide sufficient rigidity and strength during degradation. Ideally, the degradation rate should couple to the rate of tissue formation, and the amount of void space should be adequate to allow tissue vascularization. Potential uses for such biodegradable cements include structural reinforcement of age-related fractures or as an artificial bone substitute.             Another important application of self-polymerising composites is focused on the field of controlled delivery systems. In orthopaedics, PMMA beads and antibiotic impregnated acrylic bone cements have been used in the treatment of chronic osteomyelitis and other musculoskeletal infections for many years due to the inefficiency of the systemic administration. Parenteral administration of antibiotics is an inefficient method to achieve a high local tissue drug concentration, partially due to the poor blood circulation of the osseous tissue. If high systemic levels are maintained, then there is a potential risk for organ toxicity such as hearing and kidney damage. However, as poly(methyl methacrylate) beads are inert, they must be removed after approximately two weeks, otherwise bacteria can adhere and grow on their surface, and potentially develop antibiotic resistance. As an example of this, an exceptional case was found in which these beads were left in situ for 5 years. Studies carried out on these beads revealed residual antibiotic release after being 5 years in situ and also recovery of gentamicin-resistant staphylococcal strain for the surface of the beads. This case emphasizes the importance of developing biodegradable antibiotic-loaded beads as antibiotic delivery systems. The biodegradable cements have the advantage of releasing their entire load of drug as they degrade in contrast to the limited amount that can be diffused out of an inert matrix such as that of PMMA.