Studies of the mechanism by which the mechanical failure of polymethylmethacrylate leads to bone resorption.

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Abstract
The purpose of this study was to examine the relationship between the mechanical failure of polymethylmethacrylate and bone resorption at the bone-cement interface of a prosthesis. Evaluation of tissue that had been retrieved from the cement-bone interface of eighteen femoral components of total hip prostheses that were loose without associated infection revealed that a critical factor associated with bone resorption was the presence of particles that were small enough (one to twelve micrometers) to be phagocytized by macrophages. To study this phenomenon in vitro, macrophages in tissue culture were exposed to three preparations of polymethylmethacrylate cement. A novel method of cement preparation was used with control for solid and soluble contaminants, which provided a sensitive and specific technique for the determination of which mediators were released from the macrophages. Electron microscopy demonstrated phagocytosis of particles of less than twelve micrometers in size, regardless of the type of cement preparation. Exposure to all three cement preparations resulted in toxicity, as reflected by inhibition of 3H-thymidine incorporation. Exposure also led to increased release of tumor necrosis factor, but none of the three preparations resulted in release of prostaglandin E2. Division of the cement preparations into two groups on the basis of the size of the particles demonstrated that exposure to particles that were small enough to be phagocytized led to inhibition of 3H-thymidine incorporation and release of tumor necrosis factor, while exposure to particles that were too large to be phagocytized did not. Neither exposure to small particles nor exposure to large particles of cement led to release of prostaglandin E2. Our results show that when the mechanical failure of cement produces particles that are small enough to be phagocytized, phagocytosis of the particles results in the increased production of tumor necrosis factor by the macrophages, which may in turn lead to bone resorption and prosthetic loosening. These small particles also decrease 3H-thymidine uptake by the macrophages.