Articular cartilage deformation under physiological cyclic loading--apparatus and measurement technique.

In locomotive activities, areas of cartilage in both the hip, and especially the knee, experience periods of complete unloading between loading cycles as contact is lost between opposing articulating surfaces. During these periods these cartilage sites experience load-free recovery. Therefore, it was decided to model as closely as possible the physiological situation described, in order to study the deformation response of articular cartilage to physiological cyclic loading. For this it was necessary to design an apparatus in order to overcome some of the difficulties experienced with servo-hydraulic materials testing apparatus when specifying a lower load limit of zero. Cyclic loading in the frequency range 0-2.5 Hz was controlled by a cam and follower assembly driven by a stepper motor. The ratio of loading to recovery duration per cycle could be adjusted using a two-plate cam design enabling cartilage loading to occur for a duration as short as 20 ms within a 1 Hz cycle. Interchangeable impervious, porous, hemispherical or plane-ended indenters could be used. Load amplitude was controlled by compression of a spring giving a wide range of contact stresses (0.04-7.0 MPa). Load rise times were controlled by the spring in conjunction with a dashpot, enabling critically damped loads to reach their maximum value within an interval as short as 15 ms. The measurement technique and subsequent analysis relied on simultaneous recording of indenter load and vertical displacement at a sampling frequency of 5 kHz.