Loosening is a major cause of failure in total arthroplasties. The efficacy of the fixation systems depends not only on the bulk properties of the components but also on the interfaces through which they interact. This study was initiated to examine the implant/bone-cement interface for four of the most commonly used implant materials, Co-Cr-Mo, Ti-6A1-4V, 316SSLVM and ultrahigh molecular weight polyethylene (UHMWPE). The surface preparation, specimen design, joining, and testing techniques were studied and then standardized in a manner which accurately represents current clinical procedures. The interfaces were tested for both their quasistatic and fatigue properties. Finite-element and fracture toughness analyses of the quasistatic shear specimens were performed in order to provide results of an absolute nature which could be subsequently compared to bulk material properties of bone cement. The interfaces were tested "dry" (i.e., at room temperature and 50% R.H.) and in physiological saline at 37°C. The interfaces demonstrated both fracture toughness and fatigue properties far inferior to those of bone cement. A predominantly interfacial type failure was observed using SEM fractography. The ultimate compressive strength (U.C.S.) of bone cement was measured after prolonged exposure to saline at 37°C and showed no decrease in U.C.S. suggesting that the reasons for the interface strength reductions were interfacial rather than bulk in nature. The "wetting" ability of bone cement was measured using contact angles at various cure times on the four implant materials. These measurements showed that intimate interfacial contact is impossible with current clinical methods. This study indicates that failure of the implantibonecement interface is likely only a short period after implantation and therefore may be a major contributor to implant loosening.

It has been argued that various specific requirements based on known principles of good adhesion are not being met in the current procedures of formation of the implant-bone cement interface. It has been shown that an annealed thin film PMMA precoating, applied in a low-contact-angle form to surgical alloy surfaces devoid of weak boundary layers, satisfies the majority of the requirements during interface formation. Techniques for the application of the precoating have been developed for SS316LVM, Co-Cr-Mo, and Ti-6A1-4V based on fracture toughness and fatigue tests, and fractography of the interface. Implant surface preparation methods have been established to yield surfaces amenable to adhesive bonding. The composition of the coatings have been studied from the point of view of implant surface wetting, coating roughness and thickness, and interface strength. A biocompatible silane coupling agent (A-174), currently used in orthodontics, has been introduced to provide saline resistant interfaces. The final precoated metal-bone cement interfaces have demonstrated fracture toughnesses in excess of that of bone cement even after prolonged exposure to 37°C physiological saline. Fatigue tests have shown that the fatigue lives (6.5 MPa) of the precoated metal interfaces in saline are at least twice, and in one case several orders of magnitude greater than, that of the uncoated ones even when the latter are tested dry. Fractography of the interfaces show failures that are entirely cohesive in nature. For the UHMWPE (ultrahigh molecular weight polyethylene)-bone cement interface, similar improvement with precoating, however, could not be attained. Finally, the coated metal-bone cement interfaces have been tested as a function of some clinical variables such as cement type, joining time, clinical contamination, and sterilization procedure. Results show that coated metals exhibit a certain level of insensitivity to these variables and retain their performance under all conditions except after particular repeated sterilizations.

This study was conducted to investigate some fundamental fatigue testing variables as they apply to the response characteristics of acrylic bone cement. Cyclic loading under load control was conducted at frequencies of 1, 2, 5, 10, and 20 Hz in air at room temperature. At a tensile stress range of 0.3-20.0 MPa the fatigue life increased linearly with logarithmic frequency. The effect of conditioning and testing in saline at both room temperature and 37 degrees C at similar stress levels and a frequency of 10 Hz were also examined. When compared to dry testing at room temperature, testing in saline at 37 degrees C resulted in a reduction in fatigue life while testing in saline at room temperature produced an increase in fatigue life. Of a number of statistical distributions considered, the Weibull was found to be the most appropriate in documenting the findings of this investigation. A companion fractographic investigation of the failure surfaces demonstrated distinct regions of crack growth and fast fracture.

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