IntroductionBisphenol-a-glycidyl dimethacrylate (bis-GMA) resins have been studied in orthopedic applications since the late 1970s [21]. Various formulations were developed in efforts to reduce the amount of leachable unreacted monomer, to avoid high local temperatures during polymerization, to improve mechanical properties, and to enhance direct bone contact [4,19]. Current formulations of bis-GMA resins have achieved these goals of improved mechanical, chemical, and biologic properties [14,18]. One such formulation is Cortoss synthetic cortical bone void filler, a biocompatible, bone-bonding, terpolymer cortical bone substitute containing bisphenol-a-glycidyl dimethacrylate (bis-GMA), bisphenol-a-ethoxy dimethacrylate (bis-EMA), and triethylene glycol dimethacrylate (TEGDMA) resins reinforced with synthetic combeite glass-ceramic particles to stimulate bone apposition at the interface, barium boroaluminosilicate glass for radiopacity and strength, and silica for improved viscosity.For more than 20 years, polymethyl methacrylate (PMMA) bone cement has been used to reconstruct vertebral bodies that have collapsed as a result of trauma or tumor invasion [8,9,12]. Long-term studies report that these treatments are highly durable and that significant failAbstract A newly formulated and reinforced bisphenol-a-glycidyl dimethacrylate (bis-GMA) resin (Cortoss/Orthovita, Malvern, Pa.) was compared with Simplex P polymethyl methacrylate (Stryker Howmedica Osteonics, East Rutherford, N.J.) in rabbits for up to 52 weeks and in sheep for up to 78 weeks. As seen in scanning electron microscopy and histology examinations, both implant materials were surrounded by bone at late time periods, with fibrous layers of connective tissue seen in half the Simplex P specimens. No clinically significant safety differences between implant materials were apparent. Interfacial bond strengths between the implant and bone generally increased with time, but were 4.5-fold greater with Cortoss than Simplex P at 24 weeks, and 100-fold greater at 52 weeks. Forces required to displace 316SS rods held in place with Cortoss were consistently greater than forces to displace rods held in place with Simplex P. No statistically significant differences in displacement forces were found between rods held in place with Cortoss polymerized in situ and rods held with prepolymerized Cortoss. Interfacial bond strengths were greater for Simplex P that was polymerized in situ than for prepolymerized polymethyl methacrylate specimens. Cortoss synthetic cortical bone void filler is a good candidate material to fix implants in bone. It has characteristics consistent with longterm safety and has a better ability to bond to bone than Simplex P.

We recently evaluated the peak pullout loads for anchors made from our new copolymeric swelling-type material compared with anchors made of a nonswelling material. In vitro and in vivo peak pullout loads of these anchors were evaluated after different intervals of implantation in the lateral femoral condyles of New Zealand White rabbits. Scanning electron microscopy and energy dispersive x-ray analyses were additionally performed on selected retrieved samples after pullout to examine the characteristics of bone attachment to the implant. The mean peak pullout load was greater for the swelling anchors than for the nonswelling anchors after 48 hours in vitro (46.0 +/- 15.8 compared with 10.8 +/- 9.1 N, p = 0.0541). After 2 weeks in vivo, it was significantly greater for the swelling anchors than for the nonswelling controls (177.7 +/- 41.3 compared with 53.7 +/- 17.5 N, p = 0.0024). The peak pullout load was also greater for the swelling anchors after 8 weeks in vivo; however, this difference was less pronounced than at 2 weeks (101.8 +/- 35.0 compared with 58.9 +/- 9.7 N, p = 0.0508). Furthermore, the swelling implants tended to induce bone deposition at the bone-implant interface. Results from this investigation reveal that the new family of dynamic implants has potential for applications requiring fixation to cancellous or osteoporotic bone.

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IntroductionBisphenol-a-glycidyl dimethacrylate (bis-GMA) resins have been studied in orthopedic applications since the late 1970s [21]. Various formulations were developed in efforts to reduce the amount of leachable unreacted monomer, to avoid high local temperatures during polymerization, to improve mechanical properties, and to enhance direct bone contact [4,19]. Current formulations of bis-GMA resins have achieved these goals of improved mechanical, chemical, and biologic properties [14,18]. One such formulation is Cortoss synthetic cortical bone void filler, a biocompatible, bone-bonding, terpolymer cortical bone substitute containing bisphenol-a-glycidyl dimethacrylate (bis-GMA), bisphenol-a-ethoxy dimethacrylate (bis-EMA), and triethylene glycol dimethacrylate (TEGDMA) resins reinforced with synthetic combeite glass-ceramic particles to stimulate bone apposition at the interface, barium boroaluminosilicate glass for radiopacity and strength, and silica for improved viscosity.For more than 20 years, polymethyl methacrylate (PMMA) bone cement has been used to reconstruct vertebral bodies that have collapsed as a result of trauma or tumor invasion [8,9,12]. Long-term studies report that these treatments are highly durable and that significant failAbstract A newly formulated and reinforced bisphenol-a-glycidyl dimethacrylate (bis-GMA) resin (Cortoss/Orthovita, Malvern, Pa.) was compared with Simplex P polymethyl methacrylate (Stryker Howmedica Osteonics, East Rutherford, N.J.) in rabbits for up to 52 weeks and in sheep for up to 78 weeks. As seen in scanning electron microscopy and histology examinations, both implant materials were surrounded by bone at late time periods, with fibrous layers of connective tissue seen in half the Simplex P specimens. No clinically significant safety differences between implant materials were apparent. Interfacial bond strengths between the implant and bone generally increased with time, but were 4.5-fold greater with Cortoss than Simplex P at 24 weeks, and 100-fold greater at 52 weeks. Forces required to displace 316SS rods held in place with Cortoss were consistently greater than forces to displace rods held in place with Simplex P. No statistically significant differences in displacement forces were found between rods held in place with Cortoss polymerized in situ and rods held with prepolymerized Cortoss. Interfacial bond strengths were greater for Simplex P that was polymerized in situ than for prepolymerized polymethyl methacrylate specimens. Cortoss synthetic cortical bone void filler is a good candidate material to fix implants in bone. It has characteristics consistent with longterm safety and has a better ability to bond to bone than Simplex P.

In recent articles, our research group explored the use of crosslinked Poly(methylmethacrylate-acrylic acid) and composites based on this copolymer for bone implant applications such as suture anchors. The swelling response of this system was studied first in vitro, using a 0.85 g/100-mL saline solution (chosen because it simulates well the in vivo environment), and later in vivo by using samples implanted for various time periods in the lateral femoral condyles of New Zealand white rabbits. It was found that the swelling response of the crosslinked copolymer in vivo was much greater than that in the saline solution. The present investigation was conducted to determine the mechanism of excessive swelling in the in vivo tests. The approach used was to establish the changes occurring in the chemical structure of the copolymer due to immersion in serum. A number of hypotheses that can potentially explain the observed excessive swelling in serum were investigated and are discussed in this article. The results of this study indicate that the mechanism of excessive swelling in serum was the neutralization of OCOOH groups in the copolymer to produce salts of acrylic acid, which are known to result in greater swell due to their higher degree of dissociation compared to free acid. It was also found that, for compositions containing the acrylic salts (produced by preswelling in high pH solutions and drying), the swelling behavior in serum was similar to that in saline solution, and more importantly, equilibrium swelling was reached in a relatively shorter time period, which has several practical advantages for bioimplant applications.