An Er:YAG laser coupled with a cooling stream of water effectively removes dental hard tissues. However, before such a system can be deemed clinically viable, some safety and efficacy issues must be addressed. We compared the bonding of composite to dentin following the preparation of the dentinal surface with either an Er:YAG laser (lambda = 2.94 microns) or a standard dental bur and with and without a subsequent acid-etching treatment. The crowns of extracted human molars were removed, revealing the underlying dentin. We removed an additional thickness of material with either a dental handpiece or an Er:YAG laser (350 mJ/pulse at 6 Hz) by raster-scanning the samples under a fixed handpiece or laser. Comparable surface roughnesses were obtained. Several samples from each group received an acid-conditioning treatment. A cylinder of composite was bonded onto the prepared surfaces. The dentin-composite bond was then shear-stressed to failure on a universal testing apparatus. The results indicate that laser-irradiated samples had improved bond strengths compared with acid-etched and handpiece controls. SEM photographs of the surfaces show exposed tubules following the laser treatment: tubules could also be exposed with acid etching. We conclude that Er:YAG laser preparation of dentin leaves a suitable surface for strong bonding or an applied composite material.
Loosening remains an impediment to the long-term success of total hip replacements despite numerous improvements in the materials used. In cemented prostheses, fatigue and fracture of bone cement have been implicated in the failure of these devices. A new material, self-reinforced composite poly(methyl methacrylate). (SRC-PMMA), has been developed. SRC-PMMA is formed by a novel processing method that will be described. The composite consists of high strength, highly oriented PMMA fibers embedded in a matrix of PMMA. Using a woven form of SRC-PMMA, an in vitro physical and mechanical evaluation was performed to assess the feasibility of its use in an orthopedic prosthesis. Three different weaves of SRC-PMMA were evaluated in bending and fracture toughness in air, after immersion for 30 days in 37 degrees C saline, and after gamma irradiation followed by immersion. Bending modulus and strength were decreased by gamma irradiation followed by saline immersion. The effect of saline immersion alone on bending strength and modulus was negligible. Saline immersion and gamma irradiation followed by saline immersion was shown to have little or no effect on the fracture toughness of woven SRC-PMMA. Differences in the fracture processes of the different weaves were found and can be related to the differing orientation of fibers to the fracture toughness pre-crack. Optimally incorporated SRC-PMMA absorbs the same amount of water as bone cement. Comparison to previous and current work with bone cement controls shows that SRC-PMMA is a material equal to or better than bone cement in all tests performed. It deserves further consideration as a candidate biomaterial.
Brittle failure of bone cement remains a problem for the long-term stability of hip prostheses. Fibers have been developed from poly(methyl methacrylate) (PMMA) that retain the chemistry of bone cement, but improve the mechanical properties greatly. To fabricate the fibers, a polymer melt is extruded out of a small die (spinneret) and pulled onto a take-up wheel. Varying the speed of extrusion and temperature of the melt can control the viscosity of the molten polymer. This study examines the effect of melt viscosity on the resultant properties of fibers fabricated from PMMA. The goals are to optimize fiber processing and determine processing-structure--property relationships. Resultant fibers had moderate to high levels of retained molecular orientation, and ultimate tensile strengths (UTS) ranging from 60 to 225 MPa, moduli from 1.5 to 3.5 GPa, and strain to failure from 10 to 40%. Fibers fabricated at a constant viscosity and draw velocity had identical properties, whereas decreasing the viscosity generally increased the mechanical properties and retained orientation. Linear regression models were constructed to predict how the processing variables affect the structure (orientation) of the fiber and how the structure affects the UTS. This can be used to design efficient processing methods for PMMA fibers.
Total joint prostheses are often fixed in the bone using bone cement. The cement mantle, however, is prone to fatigue fracture that can lead to failure of the mantle, evolution of bone cement particles, and eventual loosening and failure of the prosthesis. A new material, self-reinforced composite poly(methyl methacrylate) (SRC-PMMA) was developed previously by the authors. This material has a similar chemical composition to bone cement, with the matrix and reinforcing fibers both fabricated from PMMA. One potential use for this material is as a precoat for hip prostheses or other stemmed prostheses. This study sought to examine the strength of the bonds that SRC-PMMA forms with simulated prostheses and bone cement. SRC-PMMA was woven about Co-Cr rods and push out tests were performed. Samples were tested in air as processed or after immersion in saline for 30 days at 37 degrees C. Three different weaves were investigated and compared to bone cement. Bone cement and SRC-PMMA formed interfacial bonds with Co-Cr rods that failed at an average load (stress) of 980 N (2.0 MPa). After saline immersion, the bone cement's interfacial bond strength was 642 N (1.23 MPa) and the tight weave SRC-PMMA was statistically stronger at 973 N (1.86 MPa). The shear strength within bone cement alone as measured by push out tests was an order of magnitude higher at 9210 N (15.2 MPa) in air and 9900 N (15.7 MPa) after saline immersion. The bond between SRC-PMMA and bone cement was 10,900 N (17.9 MPa) in air and 9610 N (15.8 MPa) after immersion in saline. Woven SRC-PMMA performed as well or better than bone cement in these push out tests.
Loosening remains an impediment to the long-The effect of saline immersion alone on bending strength term success of total hip replacements despite numerous and modulus was negligible. Saline immersion and gamma improvements in the materials used. In cemented prostheses, irradiation followed by saline immersion was shown to have fatigue and fracture of bone cement have been implicated in little or no effect on the fracture toughness of woven SRCthe failure of these devices. A new material, self-reinforced PMMA. Differences in the fracture processes of the different composite poly(methyl methacrylate) (SRC-PMMA), has weaves were found and can be related to the differing orienbeen developed. SRC-PMMA is formed by a novel processing tation of fibers to the fracture toughness pre-crack. Optimally method that will be described. The composite consists of incorporated SRC-PMMA absorbs the same amount of water high strength, highly oriented PMMA fibers embedded in a as bone cement. Comparison to previous and current work matrix of PMMA. Using a woven form of SRC-PMMA, an with bone cement controls shows that SRC-PMMA is a matein vitro physical and mechanical evaluation was performed rial equal to or better than bone cement in all tests performed. to assess the feasibility of its use in an orthopedic prosthesis.It deserves further consideration as a candidate biomaterial. Three different weaves of SRC-PMMA were evaluated in © 1997 John Wiley & Sons, Inc. J Biomed Mater Res, 36, 441-bending and fracture toughness in air, after immersion for 453, 1997. 30 days in 37ЊC saline, and after gamma irradiation followed by immersion. Bending modulus and strength were de-
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