Appropriateness of plane pressure-sensitive film calibration for contact stress measurements in articular joints

Objective. To examine how accurately calibration stains from pressure-sensitive film samples compressed between perfectly flat surfaces represent contact pressures measured with the film inserted between curved joint surfaces. Design. Experimental study. Background. Contact pressure distributions and areas in joints are often quantified using pressure-sensitive film inserted between the articular surfaces. Absolute pressure values are recovered from the film through a calibration procedure. Since the calibration is typically performed in a different environment from the experimental measurements, a systematic error in the pressure magnitudes may be introduced. Methods. Calibration stains were produced with an Instron machine using a flat and a slightly curved indenter. Calibration curves were fitted to the raw data of both indenters using a model representing the functional characteristics of the pressure-sensitive film. Results. No systematic differences between the calibration curves from the flat and the curved indenter were observed. Conclusions. It appears that calibration with the film in a flat configuration is adequate for contact pressure measurements between moderately curved articular surfaces.